EP0112858B1 - Method and device producing molecular beams and use of this method - Google Patents

Abstract

Method and device for producing molecular beams comprising large thermally unstable molecules, wherein the molecules pass from the non-gaseous phase to the gaseous phase by supply of energy and are mixed to a carrier gas and cooled by adiabatic expansion with the carrier gas. The large molecules pass from the non-gaseous phase to the gaseous phase at such a temperature that their evaporation rate is higher than their decomposition rate. The energy required to pass the large thermally unstable molecules from the non-volatile phase to the gaseous phase is supplied so rapidly that the large thermally unstable molecules pass, at a temperature higher than the decomposition temperature, from the non-volatile phase to the gaseous phase wherein their evaporation rate is higher than the decomposition rate. Those molecules are brought, in a gaseous state, to a carrier gas stream region wherein they start to expand, the temperature of such gas stream being substantially lower than the evaporation respectively decomposition temperature of the large thermally unstable molecules.

Description

The invention is based on a method for generating pulsed molecular beams, in which a sample substance is converted from the non-gaseous phase into the gaseous phase by supplying energy, the free molecules resulting from the transfer are mixed with a carrier gas and the carrier with the molecules of the Sample substance is cooled adiabatically by expansion of a beam of the carrier gas.

The invention further relates to a device for performing the aforementioned method, comprising a gas jet nozzle for generating a carrier gas jet, a carrier gas supply device for supplying the carrier gas to the gas jet nozzle, an evaporation and mixing chamber for transferring a sample substance from the non-gaseous to the gaseous phase and for admixing this phase to the carrier gas, and an energy supply device for supplying evaporation energy to the evaporation and mixing chamber.

Such a method and such a device are known, for example from US-Z Chemical Physics Letters, Vol. 77, Wr. 3, pages 448 to 451.

According to the current state of the art, molecular beams can be generated which consist of a carrier gas, which is usually a noble gas, such as argon, and molecules added to the carrier gas, provided they are molecules of a volatile substance that can be vaporized at a temperature in which the molecules do not decompose at all, i.e. H. that is, provided that the molecules are thermally stable.

Even if the mixture consisting of carrier gas and vaporized molecules, as known from "Accounts of Chemical Research", vol. 10 (1977), pages 139-145, is cooled adiabatically by expansion after the formation of a molecular beam by means of a gas jet nozzle, so the fact that it thereby obtains a very low temperature, which is particularly suitable for examining the molecules by means of mass spectroscopy or other molecular or ion examination methods, means that the molecules evaporate before the mixture passes through the gas jet nozzle, i.e. at relatively high temperatures and pressures, and it the substances must have a sufficiently high vapor pressure so that they do not decompose under the conditions in front of the steam jet nozzle.

Usually, the molecules to be mixed with the carrier gas are evaporated in the evaporation and mixing chamber provided upstream of the gas jet nozzle by simple thermal heating, provided that the volatile substance, which consists of the molecules to be examined or contains these molecules, does not by itself due to its vapor pressure at room temperature evaporates.

Preferred examination methods, by means of which such molecular beams are examined, are examination by means of tunable laser radiation either in fluorescence or by multiphoton loinization (MPI) by mass spectroscopy.

The above-mentioned procedure and the device for carrying it out are not suitable for thermally unstable molecules, that is to say for non-volatile molecules which decompose on heating before they have reached a sufficient vapor pressure.

From US-A-4 259 572 it is known for the investigation of large, thermally sensitive molecules by means of mass spectroscopy to transport the substance to be examined by means of a belt or the like into the ionization chamber of the mass spectrometer and to irradiate it therein by means of very short laser pulses. This is to avoid the decomposition of sensitive molecules that would otherwise occur during evaporation. The irradiation with the laser pulses evaporates and ionizes in one step. The majority of the molecules are also destroyed in the process, but a certain amount of usable ions remains, which can directly enter the electromagnetic field of the mass spectrometer. The generation of a carrier gas jet with admixed, almost exclusively non-ionized molecules does not take place in the known method and is also not possible according to the known method. On the contrary, this process is intended to avoid vaporization before ionization, which is so harmful for sensitive molecules.

For the evaporation and simultaneous ionization of thermally unstable molecules, there are a number of other recently developed methods with which these thermally unstable molecules can be evaporated directly under vacuum into vacuum and examined by mass spectrometry.

The most important methods here are, for example, "flash" pyrolysis, the spraying of a solution ("electrospray") with immediate evaporation of the solvent, field desorption and ionization, chemical ionization and bombardment with fast atoms. All of these methods have so far been used with varying degrees of success for the direct ionization of thermally unstable molecules. They have the considerable disadvantage that ions are generated and that these ions are partly fragments and partly add-on ions, with "cationized" mother molecules very often occurring, ie mother ions to which another ion, for example a proton or alkali ion, is additionally attached. is attached. In addition, the same fragments and attachment complexes do not always form, so that conclusions can be drawn the undecomposed molecules that are supposed to be used to obtain test results are difficult and not safe and sometimes impossible. It may be that, in most of the methods mentioned, neutral molecules in the undecomposed state may also be evaporated, especially in the case of laser evaporation, but it has not yet been possible to directly detect such thermally unstable molecules, especially since these molecules are not present as a molecular beam and be decomposed before the actual detection, for example by ionization.

From "Zeitschrift für Naturforschung", volume 36a, 1981, pages 1338 to 1339 and volume 35a, 1980, pages 1429 to 1430 and from the journal "Chemical Physics Letters", Vol. 77 No. 3, pages 448 to 451 in each case a method for generating pulsed molecular beams is known, but the molecular beams generated by this particular method do not contain any thermally unstable molecules.

Specifically, the process described in the "Zeitschrift für Naturforschung", vol. 36a, 1981, pages 1338 to 1339 is the evaporation of a volatile substance which has a high vapor pressure even at room temperature, namely the evaporation of benzene. This substance is evaporated into the carrier gas, which in the present case is argon, and then a molecular beam is formed from the mixture thus obtained, with no problems whatsoever, as has already been mentioned above. The same applies to the process described in the journal "Chemical Physics Letters", Vol. 77, No. 3, pages 448 to 451, only one other volatile substance, namely diacetyl, being used here; In addition, the pulsed molecular beam is generated by means of a solenoid valve in this process. In principle, the same also applies to the method described in the "Zeitschrift für Naturforschung", volume 35a, 1980, pages 1429 to 1430, but with the difference that here the substance by means of which a molecular beam is to be formed is heated , in order to convert this substance, which is anthracene in the present case, into the gas phase to a temperature which is far below the decomposition temperature.

When a molecular beam is formed from the substance vapor / carrier gas mixture, the processes described in the "Zeitschrift für Naturforschung", volume 36a, 1981, pages 1338 to 1339 and volume 35a, 1980, pages 1429 to 1430 result in the processes , only after the molecular beam has been generated, this molecular beam as by-products complexes of the substance mentioned with the carrier gas. These complexes are not present at all before the molecular beam is formed, in particular not before cooling due to the expansion of the carrier gas jet, but they only form as a result of this strong cooling. These complexes, which are not large, thermally unstable molecules, do not exist under normal conditions and are only interesting in that very weak Van der Waals interaction forces can be investigated. In contrast, the present invention is intended to generate molecular beams with large, thermally unstable molecules which exist under normal conditions. Such molecules can never be formed in the molecular beam by mere addition, as is the case with the aforementioned complexes, but must be introduced as such into a molecular beam. Finally, US-A-4 091 256 describes a method and a device in which so much energy is supplied to a substance that the substance decomposes down to the individual atoms and thus provides a beam of neutral atoms. Large, thermally unstable molecules cannot be converted into the gas phase at all without decomposition using this method and this device.

A non-volatile substance should in particular be understood to mean a substance which is non-volatile under normal conditions (20 ° C. and 1 bar).

At the temperature at which the anthracene is evaporated according to the method described in the "Zeitschrift für Naturforschung", volume 36a, 1981, pages 1338 to 1339, there is a balance of solid anthracene and anthracene vapor.

In contrast, the invention is based on the object of further developing a method of the type mentioned at the beginning in such a way that it is possible to bring large, thermally unstable molecules, i.e. molecules of substances which can be vaporized without decomposition, undamaged and without being ionized into a molecular beam which is usable for spectroscopic purposes, in particular mass spectroscopy.

This object is achieved by the aforementioned method in that the energy is supplied in pulses at such a height that the sample substance evaporates faster than decomposes to generate molecular beams with large, thermally unstable, almost exclusively non-ionized molecules that the temperature of the Carrier gas jet in an area of the jet in which it begins to expand is set significantly lower than the decomposition temperature of the sample substance and that the released molecules of the sample substance are brought directly into this area of the carrier gas jet.

The underlying task is further solved according to the above-mentioned device in that, in order to generate a molecular beam which contains large, thermally unstable, almost exclusively non-ionized molecules, the carrier gas supply device supplies the carrier gas to the gas jet nozzle upstream of the latter at a temperature which is substantially lower than the decomposition temperature of the sample substance that the evaporation and mixing chamber is arranged at least with the part in which the molecules are mixed into the carrier gas, downstream of the outlet opening of the gas jet nozzle, where the carrier jet begins to expand, and adjacent to it, and that the energy supply device provided for the sample evaporation is arranged emits such high energy in pulse mode that more sample substance is evaporated than is decomposed during the pulse duration.

In the context of the present invention, thermally unstable molecules are to be understood to mean molecules of those substances whose molecules already break down at temperatures which are far below the evaporation temperature. These substances therefore have a vapor pressure that is far below the vapor pressure that would be required for the molecules to go into the gas phase at room temperature. In the context of the invention, “large” molecules are to be understood in particular as molecules whose molecular weight is 100 or more.

In the process according to the invention, the molecules are evaporated off under conditions in which there are more undecomposed molecules in the gas phase than corresponds to the thermodynamic equilibrium at the prevailing temperature, while in the state of thermodynamic equilibrium no undecomposed molecules in the case of substances, consisting of large, thermally unstable molecules.

• (1) Es wird eine sehr schnelle Verdampfung der Substanz durchgeführt, welche aus den großen, thermisch instabilen Molekülen besteht oder diese Moleküle enthält, und zwar erfolgt die Verdampfung so schnell, daß sich der Hauptteil der Moleküle hierbei nicht zersetzen kann. Ein solches sehr schnelles Verdampfen ist, wie weiter unten unter Bezugnahme auf die Figur 1 näher erläutert werden wird, an sich bekannt; es eröffnet jedoch allein noch nicht die Möglichkeit, die gewünschten Molekularstrahlen, welche die großen, thermisch instabilen Moleküle unzersetzt enthalten, zu erzeugen, weil die verdampften Moleküle infolge der bei der Verdampfung aufgenommenen Energie kurz nach der Verdampfung in der Gasphase zerfallen.
• (2) Der zweite Vorgang besteht darin, daß die verdampften, thermisch instabilen Moleküle sofort nach ihrer Verdampfung einer Wärmeabführung im Trägergasstrahl ausgesetzt werden, indem sie dem expandierenden Trägergasstrahl, dessen Temperatur wesentlich geringer als die Abdampfungs- bzw. Zersetzungstemperatur der großen, thermisch instabilen Moleküle ist, beigemischt werden, und zwar werden sie in denjenigen Bereich des "kühlen" Trägergasstrahls eingeführt, in welchem dieser zu expandieren beginnt. Dadurch wird verhindert, daß diese thermisch instabilen Moleküle nicht nachträglich nach dem Übergang in die Gasphase zerfallen können. Die hierbei erfolgende "Stabilisierungskühlung" ist zu unterscheiden von der adiabatischen Kühlung, die erst später erfolgt und den ganz anderen Zweck hat, die zu untersuchenden Moleküle auf eine Untersuchungstemperatur von wenigen Grad Kelvin abzukühlen.

In the method according to the invention and in the device according to the invention, two essential processes take place in combination due to the conception of the invention, which enables the generation of molecular beams with undecomposed large, thermally unstable molecules:

• (1) The substance, which consists of the large, thermally unstable molecules or contains these molecules, is evaporated very quickly, and the evaporation takes place so quickly that the majority of the molecules cannot decompose. Such a very rapid evaporation is known per se, as will be explained in more detail below with reference to FIG. 1; however, it alone does not open up the possibility of generating the desired molecular beams, which contain the large, thermally unstable molecules without decomposition, because the evaporated molecules decay shortly after the evaporation in the gas phase as a result of the energy absorbed during the evaporation.
• (2) The second process consists in exposing the evaporated, thermally unstable molecules to heat dissipation in the carrier gas jet immediately after their evaporation, by exposing the expanding carrier gas jet to a temperature which is substantially lower than the evaporation or decomposition temperature of the large, thermally unstable molecules is admixed, namely they are introduced into the area of the "cool" carrier gas jet in which this begins to expand. This prevents these thermally unstable molecules from disintegrating after the transition to the gas phase. The "stabilization cooling" that occurs here is to be distinguished from the adiabatic cooling, which takes place only later and has the completely different purpose of cooling the molecules to be examined to an examination temperature of a few degrees Kelvin.

It appears important at this point to point out that according to the prior art, in which, as already mentioned above, the molecules in the carrier gas atmosphere, for example in an argon atmosphere, are evaporated upstream of the gas jet nozzle, the carrier gas as a result of the The fact that during the process of thermal evaporation of the molecules to be examined is heated together with them, is "hot" and thus contributes to the decomposition of the thermally unstable molecules. Only after the carrier gas and the evaporated molecules have mixed in the evaporation and mixing chamber in the case of thermal evaporation at a relatively high temperature, during which mixing, that is, thermally unstable molecules disintegrate, even if a certain percentage should have evaporated without being decomposed Molecules expanded together with the carrier gas through the gas jet nozzle. As a result, it is actually not possible in the prior art to generate molecular beams that contain undecomposed large, thermally unstable molecules.

In contrast to this, the invention makes it possible to generate MMolecular beams in which the large, thermally unstable molecules are available undamaged for a wide range of tests, in particular for optical spectroscopy, for reaction kinetics, in which molecular beams are known to be widely used for test purposes be, as well as for mass spectrometry. In the context of the invention, “evaporation” or “evaporation” or “evaporation” is to be understood to mean all types of converting molecules into the gas phase; this transfer can thus either from the solid substance containing the molecules to be examined or from These molecules exist, as well as from a surface on which the molecules are attached or adsorbed. The large, thermally unstable molecules are preferably vaporized by means of laser radiation; This type of evaporation has the advantage that it is particularly well possible to carry out the very rapid, relatively high temperature evaporation of the large, thermally unstable molecules.

Another preferred embodiment of the method according to the invention is characterized in that the large, thermally unstable molecules are each evaporated into a carrier gas jet pulse by means of evaporation pulses. In this way, the molecules can be well examined without the substance from which they are evaporated being continuously heated and thus largely decomposed; rather, only the top layer of the substance is always heated to the high evaporation temperature which serves for rapid evaporation.

So that the large, thermally unstable molecules get directly into the relatively cool carrier gas jet at the beginning of its expansion after they have passed into the gas phase, they are preferably vaporized by a sample surface which runs essentially parallel to the axis of the carrier gas jet and is adjacent to the outlet opening of the gas jet nozzle , whereby this sample surface does not protrude into the carrier gas jet, so that an undisturbed expansion of the carrier gas jet is made possible.

The device for carrying out the method according to the invention, which has already been specified above in its basic structure, is preferably designed such that the large, thermally unstable molecules are exposed to the stabilization cooling mentioned immediately after their entry into the gas phase, in such a way that the evaporation point in from the outlet opening of the gas jet nozzle a longitudinal distance, which is smaller than or equal to 20 times the effective diameter of the outlet opening, is arranged laterally from the outlet opening, the longitudinal distance being that along the axis of the gas jet nozzle and the effective diameter being the diameter corresponding to a circular outlet opening. If the outlet opening of the gas jet nozzle has an annular cross section, then the effective diameter in the above sense is to be understood as the diameter of a circular outlet opening which has the same outflow cross section in terms of area as the annular outlet opening.

Furthermore, likewise for the purpose of ensuring stabilization cooling as immediately as possible, the evaporation point is preferably arranged at a transverse distance from the axis of the gas jet nozzle which is less than or equal to 20 times, preferably less than or equal to 10 times, the effective diameter the outlet opening is.

It is particularly preferred that the transverse distance be less than half the longitudinal distance.

The device can furthermore be designed such that the evaporation and mixing chamber has a preferably cylindrical expansion channel for the carrier gas jet, downstream of the outlet opening of the gas jet nozzle, on or in the wall of which the evaporation point is provided. This evaporation point can in particular be provided in an obliquely, preferably perpendicularly, to the axis of the gas jet nozzle arranged sample channel which is formed in the side wall of the expansion channel. With this construction, if the evaporation takes place by means of laser radiation, a laser beam channel lying in the axial extension of the sample channel can be formed in the side wall of the expansion channel. The distance of the sample surface from the side wall of the expansion channel is preferably less than or equal to the diameter of the sample channel, the sample diameter preferably being equal to the diameter of the sample channel.

• Figur 1 eine graphische Darstellung zur Erläuterung des Zusammenhangs zwischen Verdampfung und Zersetzung von großen, thermisch instabilen Molekülen;
• Figur 2 einen Längsschnitt durch eine Gasstrahldüse und einen Teil einer Abdampfungs- und Mischkammer, wie sie vorzugsweise zur Durchführung des Verfahrens nach der Erfindung verwendet werden;
• Figur 3 eine Darstellung einer bevorzugten Ausführungsform einer Einrichtung nach der Erfindung mit den daran anschließenden Feldplatten eines Massenspektrometers, jedoch ohne die Einrichtung, mit welcher die Abdampfungsenergie zu der in der Abdampfungs- und Mischkammer befindlichen Probe zugeführt wird;
• Figur 4 eine Ausführungsform der Einrichtung, mit der die Abdampfungsenergie erzeugt und zur Probe zugeführt wird; und
• Figuren 5 bis 12 Untersuchungsergebnisse in Kurvenform, wie sie mittels des Verfahrens und der Einrichtung nach der Erfindung gewonnen worden sind.

The invention is explained in more detail below with reference to FIGS. 1 to 12 of the drawing using a particularly preferred embodiment and the test results achieved thereby; show it:

• Figure 1 is a graphical representation to explain the relationship between evaporation and decomposition of large, thermally unstable molecules;
• FIG. 2 shows a longitudinal section through a gas jet nozzle and part of an evaporation and mixing chamber, as are preferably used to carry out the method according to the invention;
• FIG. 3 shows a preferred embodiment of a device according to the invention with the adjoining field plates of a mass spectrometer, but without the device with which the evaporation energy is supplied to the sample located in the evaporation and mixing chamber;
• Figure 4 shows an embodiment of the device with which the evaporation energy is generated and supplied to the sample; and
• Figures 5 to 12 examination results in the form of curves, as they have been obtained by means of the method and the device according to the invention.

Reference is first made to FIG. 1, in which the natural logarithm of the reaction constant k for the decomposition (straight line 1) and for the evaporation (straight line 11) is schematically dependent on the reciprocal of the absolute temperature T are shown. After that, it's like RJ Cotter's "Mass Spectrometry of Nonvolatile Compounds" in "Anal. Chem". 52 (1980) 1589A, so that for large, thermally unstable molecules, the evaporation has a higher activation energy than the decomposition, so that the rate of evaporation increases with temperature faster than that of the decomposition. Thus, from a certain temperature, the evaporation rate becomes greater than the decomposition rate, namely from the intersection of the two straight lines I and 11 to higher temperatures.

If you go very quickly to a very high temperature, most of the molecules will evaporate before enough energy has accumulated in the internal vibration modes that can lead to dissociation. In addition to these two neutral pathways, there are various ionization pathways that include lead to the energetically favorable "cationized" species already mentioned.

The evaporation or evaporation of thermally unstable molecules can be done by extremely rapid heating, as is caused by a very short laser pulse with a high power density. The distribution of energy over the three evaporation processes, evaporation, decomposition and ionization, depends mainly on the following factors when bombarded with laser radiation: laser energy density, pulse duration and nature of the sample. The influence of the laser wavelength on the evaporation process seems to be of minor importance; However, it cannot be ruled out that particularly high evaporation rates can be achieved with specific wavelengths, for example in the infrared (resonance desorption). From this point of view, the C0 ₂ laser (wavelength 10.6 µm) should be preferred to the alternative neodymium YAG laser (wavelength 1.06 µm), because at 10.6 µm most large organic molecules have vibrational bands , which is not the case at 1.06 µm.

A molecular beam is a bundled stream of molecules that move in a preferred direction essentially without jolts. The freedom from bumps is also given for free expansion into a vacuum, but the preferred direction is generally missing here. In the method according to the invention, the vaporized molecules are admixed to a carrier gas jet immediately after it emerges from a pulsed gas jet nozzle. The "hot" molecules initially experience collisions with the carrier gas atoms and are thus deactivated by "stabilization cooling", so that the probability of a subsequent unimolecular decay is greatly reduced. Through further adiabatic expansion, the initial nozzle jet changes into a molecular jet after a short running distance.

• (1) eine Gasstrahldüse 1, die vorliegend in ihrer bevorzugten Ausführungsform als elektromagnetisch gepulstes Düsenventil ausgebildet ist und zum Erzeugen eines Trägergasstrahls, zum Beispiel eines Argonstrahls, dient;
• (2) eine Abdampfungs- und Mischkammer 2 zur Beimischung der von einer Probe abgedampften Moleküle zu dem Trägergasstrahl; und
• (3) eine Energiezuführungseinrichtung 3 zum Zuführen von Abdampfungsenergie zu der Abdampfungs-und Mischkammer 2, deren Hauptbauteil ein gepulster C0₂-Laser (TEA-Laser) ist; sowie
• (4) eine Trägergaszuführungsvorrichtung 4 zum Zuführen des Trägergases zu der Gasstrahldüse 1; von dieser Trägergaszuführungsvorrichtung 4 ist in der Zeichnung, nämlich in Figur 3, nur der Anschlußstutzen dargestellt, über den das Trägergas zu der Gasstrahldüse 1 zugeführt wird.

A particularly preferred embodiment of a device according to the invention will now be explained in more detail with reference to FIGS. 2 to 4. This device for generating a pulsed, doped molecular beam has the following four main components:

• (1) a gas jet nozzle 1, which in the present embodiment is designed as an electromagnetically pulsed nozzle valve and is used to generate a carrier gas jet, for example an argon jet;
• (2) an evaporation and mixing chamber 2 for admixing the molecules evaporated from a sample to the carrier gas jet; and
• (3) an energy supply device 3 for supplying evaporation energy to the evaporation and mixing chamber 2, the main component of which is a pulsed C0 ₂ laser (TEA laser); such as
• (4) a carrier gas supply device 4 for supplying the carrier gas to the gas jet nozzle 1; of this carrier gas supply device 4, only the connection piece is shown in the drawing, namely in FIG. 3, via which the carrier gas is supplied to the gas jet nozzle 1.

Using these four structural units, the preferred embodiments of which are illustrated in more detail below, a molecular beam is generated, of which only the axis 5 is shown, which at the same time the axis of the gas jet nozzle 1 and accordingly also the carrier gas jet emerging therefrom and the axis of the carrier gas jet and the mixed gas jet existing in these evaporated molecules, which after adiabatic expansion becomes the molecular jet.

To demonstrate the properties of the molecular beam, a time-of-flight mass spectrometer with laser multiphoton ionization is provided in the present case, of which some field plates and part of the drift tube are indicated in the lower part of FIG. 3. The ionizing laser is a tunable dye laser that is pumped by a Q-switched neodymium-YAG laser.

The impulse operation of the nozzle jet (this is how the entire jet is called here, which becomes the carrier gas jet via the mixed gas jet to the molecular jet) is not an absolutely necessary feature of the method according to the invention, but it is of extremely important practical importance for maintaining a sufficient vacuum with a reasonable pumping effort. A continuous jet of carrier gas, for example made of argon, would have no practical meaning here anyway because of the pulsed evaporation. Important for the functioning of the method is an exact temporal correlation of the nozzle jet, the evaporation laser pulse and the ionization laser pulse, which by electronic Standard circuits are carried out.

The individual structural units are now explained in more detail below and, for example, structural dimensions and preferred optimized operating data are then given.

The gas jet nozzle is first described in detail with reference to FIGS. 2 and 3:

The gas jet nozzle 1 is designed as a nozzle valve, and in the present case it is a commercial electromagnetically operated valve from Bosch, which was originally intended for the operation of fuel injection engines. This nozzle valve has an annular outlet opening 6 which is delimited on the inside by a cylindrical end of a valve tappet 7 and on the outside by a cylindrical opening of a valve seat cylinder 8. After the interior of the gas jet nozzle 1, a conical valve surface 9 adjoins the cylindrical end of the valve tappet 7, which cooperates with a complementary conical valve seat surface 10 which adjoins the cylindrical opening of the valve seat cylinder 8. The nozzle valve is reworked so that the valve seat cylinder 8 is freely accessible and is provided with an external thread 11 for screwing on the evaporation and mixing chamber 2.

In the present embodiment, the annular outlet opening 6 has a radial ring width of approximately 0.1 mm and an outer ring diameter of approximately 1 mm, so that a corresponding ring-shaped carrier gas jet is generated. The distance between the valve surface 9 and the valve seat surface 10 when the nozzle valve is open is approximately 0.1 mm. The nozzle valve is operated electromagnetically so that a carrier gas pulse of approximately 1 msec duration with rising and falling edges of approximately 200 µsec is produced. This is achieved by electrical impulses of 500 _JlS ec, which are applied to the magnetic winding of the nozzle valve.

Next, the evaporation and mixing chamber 2 will also be explained in more detail with reference to FIGS. 2 and 3, which, at least with regard to its essential part in which the evaporation and mixing takes place, is arranged downstream of the outlet opening 6 of the gas jet nozzle 1 and adjacent to this outlet opening:

The evaporation and mixing chamber 2 has a cylindrical expansion channel 12 for the carrier gas jet, the axis of which coincides with the axis 5 of the nozzle jet and which forms an enlarged, downstream extension of the outlet opening 6 of the gas jet nozzle 1 and connects one end directly to the outlet opening 6 . The other end of the expansion channel 12 merges into a vacuum for further expansion of the jet.

On or in the wall of the expansion channel 12, the evaporation point 13 is provided, which in the present case is formed by the surface of a sample 14 pressed into a pill. This evaporation point 13 is provided in a sample channel 15 which is arranged perpendicular to the axis 5 of the gas jet nozzle 1 and is formed in the lateral wall of the expansion channel 12.

Furthermore, the evaporation and mixing chamber 2 has a laser beam channel 16, which is likewise formed in the lateral wall of the expansion channel 12, specifically in the axial extension of the sample channel 15 on the side of the expansion channel 12 opposite the latter is explained below with reference to Figure 4, the evaporation energy supplied.

• (1) Der Längsabstand a der Abdampfungsstelle 13 von der Austrittsöffnung 6 der Gasstrahldüse 1 ist kleiner als der oder gleich dem 20-fachen effektiven Durchmesser der Austrittsöffnung 6. Hierbei ist unter dem Längsabstand a der Abstand zwischen der Austrittsöffnung 6 und dem Projektionspunkt P der Mitte M der Abdampfungsstelle 13 auf die Achse 5 des Düsenstrahls zu verstehen. Der Begriff des effektiven Durchmessers der Austrittsöffnung 6 wurde weiter oben bereits erläutert.
• (2) Der Querabstand b der Abdampfungsstelle 13 von der Achse 5 des Düsenstrahls ist kleiner als der oder gleich dem 20-fachen, vorzugsweise kleiner als der oder gleich dem 10-fachen, effektiven Durchmesser der Austrittsöffnung 6.
• (3) Der Querabstand b ist vorzugsweise kleiner als die Hälfte des Längsabstands a.
• (4) Die Entfernung c der Abdampfungsstelle 13 von der seitlichen Wandung des Expansionskanals 12 ist kleiner als der oder gleich dem Durchmesser d des Probenkanals 15, der vorzugsweise im gesamten Querschnitt von der Probe 13 ausgefüllt wird.

So that an undisturbed expansion of the carrier gas jet can take place in the expansion channel 12, the diameter of the latter is substantially larger than the outer diameter of the outlet opening 6. So that the molecules from the sample 14 for stabilization cooling are as directly as possible into the expanding carrier gas jet and into an area of the latter , which is as close as possible to the beginning of the expansion, the following conditions are preferably met:

• (1) The longitudinal distance a of the evaporation point 13 from the outlet opening 6 of the gas jet nozzle 1 is less than or equal to 20 times the effective diameter of the outlet opening 6. Here, the distance between the outlet opening 6 and the projection point P is the center under the longitudinal distance a M to understand the evaporation point 13 on the axis 5 of the jet. The concept of the effective diameter of the outlet opening 6 has already been explained above.
• (2) The transverse distance b of the evaporation point 13 from the axis 5 of the jet is less than or equal to 20 times, preferably less than or equal to 10 times the effective diameter of the outlet opening 6.
• (3) The transverse distance b is preferably less than half the longitudinal distance a.
• (4) The distance c of the evaporation point 13 from the side wall of the expansion channel 12 is less than or equal to the diameter d of the sample channel 15, which is preferably filled in by the sample 13 in its entire cross section.

In the present embodiment, the evaporation and mixing chamber 2 consists of a cylindrical block made of stainless steel, which has a threaded bore 17 which is concentric with the expansion channel 12 and by means of which it is screwed onto the external thread 11 of the valve seat cylinder 8. Preferred dimensions of this cylindrical block, of the expansion channel 12 and of the sample channel 15 and laser beam channel 16 passing through from the expansion channel on the outside of the cylindrical block are as follows:

It should be noted that the evaporation and mixing chamber 2 can be modified in such a way that, at the location of the pressed sample 14, a strip coated with the sample substance, which can be made of copper or Teflon, etc., on which the Evaporation laser beam 18 acting on the laser beam channel 16 (see FIG. 4) is guided past, so that the surface exposed to the evaporation laser beam can thereby be constantly renewed by continuously or stepwise moving the belt; however, this embodiment is not shown in the drawing.

The energy supply device 3, with which the large, thermally unstable molecules are vaporized at a temperature at which the vaporization rate of these molecules is greater than their decomposition rate, will now be described in more detail:

This energy supply device 3 comprises as energy source a laser 19, which in the present case is a pulsed CO ₂ TEA laser, which has a laser cross-section of 0.3 J / cm ² and 1 with a beam cross section of 2.3 x 2.5 cm µsec duration delivers. The repetition frequency is variable in the range from 0 to 10 pulses / second. The evaporation laser beam 18, as shown in the not to scale drawing of FIG. 4, is deflected by 90 ° immediately after it emerges from the laser 19 by a first gold-coated, flat deflecting mirror 20 and via a first iris diaphragm 21 to a second flat gold-coated Deflecting mirror 22 and a second iris diaphragm 23 and a third flat gold-coated deflecting mirror 24 are directed onto a concave mirror 25, which is also gold-coated. The two variable iris diaphragms 21 and 23 are provided to attenuate the evaporation laser beam 18, namely, as shown in FIG. 4, an iris diaphragm 21 is attached directly to the output of the laser 19 and the other iris diaphragm 23 is located in the vicinity of the third deflection mirror 24.

By means of the concave mirror 25, the evaporation laser beam 18 is directed through a window 26 into the interior of the vacuum space 27 containing the evaporation and mixing chamber and through the laser beam channel 16 onto the evaporation point 13, i.e. concentrated on the surface of the sample 14. However, it should be noted that the sample 14 is not exactly in the focal point of the concave mirror 25. Rather, the distance of the concave mirror 25 from the evaporation point 13 can be changed, and this change allows the energy density of the evaporation laser beam 18 impinging on the surface of the sample 14 to be varied in a simple manner.

Preferred data of the energy supply device according to FIG. 4 are given below:

In order to examine the molecular beam obtained in the already mentioned time-of-flight mass spectrometer, it has to be ionized, which is done at the point designated by A in FIG. 3 by means of an ionization laser beam 28, which is indicated in the plane of the drawing in FIG runs to the drawing level. A neodymium-YAG dye laser system from Quanta Ray is used to generate this ionization laser beam 28. This laser system works optimally at a pulse repetition frequency of 10 Hertz and delivers pulses of approx. 10 nsec duration. In addition to the basic wavelength of the YAG laser, which is at 1064 nm and its harmonics, which go up to the fourth harmonic, which is at 266 nm, the entire range from approx. 800 to 240 nm can be covered by suitable choice of dye as well as doubling and frequency mixing . The ionization laser beam 28 is focused on the intersection A of the molecular beam with the ion-optical axis 29 of the time-of-flight mass spectrometer by means of a lens, not shown, which preferably has a focal length of 20 cm or 50 cm. In the present embodiment, this intersection point A is at a distance r of 2.7 cm from the outlet opening 6 for the carrier gas jet.

In Figure 3, the field plates 30 to 33 and a part of the drift tube 34 of a time-of-flight mass spectrometer of conventional design are indicated, which form a pull field for extracting the ions generated at the intersection point A, a single lens and a drift space, the latter through pinholes 35, 36 and 37, which are provided in the field plates 31, 32 and 33, of the Molecular beam is separated. The perforated diaphragms 35, 36 and 37 have a diameter s of 5 mm, for example. At the end of the drift path, which is, for example, 25 cm, there is a secondary electron multiplier (not shown). The ions are detected via a preamplifier either on a fast oscillograph or on a TRANSIENT DIGITIZER (Tectronix), which is a device that registers and digitizes very fast processes (nanoseconds to picoseconds).

The molecular beam space has a buffer volume of approx. 6 l, so that the chamber pressure does not increase so much at the moment with each gas pulse; it is kept at a mean pressure of approx. 1.3 µbar with a Roots pump with a suction capacity of 1000 m ³ / h and with a suitable backing pump. The pressure in the drift chamber is kept below 0.013 ubar by a diffusion pump.

As FIG. 3 also shows, the gas jet nozzle 1 is screwed as a unit into an essentially hollow cylindrical socket 38, which in turn is attached to a larger flange 43 which is attached to a larger flange 43 and a spacer ring 40 and seals 41, 42 tubular part 44 (see Figure 4) is provided. This tubular part, which is not shown in FIG. 3, is located laterally at a distance from the evaporation and mixing chamber 2 and supports the window 26 via a corresponding holder 45.

Finally, the following are given below with reference to FIGS. 5 to 12 test tests which were carried out with the method and the device according to the invention and the results thereof:

• (1) Anthrazen
• (2) Retinal (Vitamin A-Aldehyd) und
• (3) Tryptophan (eine Aminosäure)

The generation of a molecular beam has been demonstrated with three different substances:

• (1) anthracene
• (2) retinal (vitamin A aldehyde) and
• (3) tryptophan (an amino acid)

The measurement curves in FIGS. 7 to 12 are proof that thermally unstable molecules could actually be converted into a molecular beam without being destroyed.

The anthracene was detected in the molecular beam by both fluorescence and a mass spectrum.

FIG. 5 shows a resonance fluorescence spectrum in the 0-0 transition to the first excited electron state of the anthracene.

Typical test conditions were:

Delay between valve opening and YAG laser: 1.3 msec, delay between C0 ₂ laser and YAG laser: 500 µsec, argon pressure 0.5 bar, photon multiplier voltage: 1800 V, power density of the C0 ₂ laser: approx. 1 MW / cm ² . The narrow band - approx. 0.1 nm wide - is characteristic of a molecule cooled to a few K.

The following figures (Figures 6 to 11) show mass spectra that were recorded under the following conditions:

Delay between valve opening and YAG laser: 880 µsec, delay between C0 ₂ laser and YAG laser: 350 µsec, power density of the C0 ₂ laser on the sample: approx. 1.2 MW / cm ² , argon pressure 0.3 - 0.4 bar, electron multiplier: 3000 V.

The substance was introduced as a compact in the sample channel of the evaporation chamber.

Figure 6 shows a mass spectrum of anthracene. It essentially contains the mother mass and a calibration peak of toluene, which has been mixed in trace amounts with the argon. The wavelength of the ionization laser was 266 nm. It should be noted that an anthracene molecule is not a thermally unstable molecule per se, rather anthracene has only been chosen as an example for the functioning of adiabatic cooling; It was found that this adiabatic cooling works even when the molecules are introduced into the carrier gas downstream of the gas jet nozzle.

The mother masses of retinal, anthracene and toluene also appear in FIG. The latter two were still present from the previous attempt. Next to it, a peak appears at mass 35, probably due to fragmentation of the retinal. The wavelength of the ionization laser was 266 nm.

Figure 8 shows a spectrum in which almost only the mother ion of the retinal appears. Here, the evaporation chamber had been thoroughly cleaned and filled with a fresh retinal sample. The incident wavelength was again 266 nm, but with a lower intensity than in FIG. 7.

The retinal spectra in the following two figures were obtained with an ionization wavelength of 355 nm, specifically in FIG. 9 at high energy density and in FIG. 10 at low energy density. It can be seen that the fragmentation depends strongly on the intensity and does not result from the evaporation process.

The last two pictures show mass spectra of tryptophan. The ionization wavelength is 266 nm. The spectrum of FIG. 11 is recorded with a compact, that of FIG. 12 with a tryptophan coating on copper tape. The other conditions are the same.

The results clearly document that molecular beams of argon with admixed thermally unstable molecules, for example retinal and tryptophan, can be produced with the stated method.

In conclusion, it should be pointed out that molecular beams are widely used in chemistry to elucidate molecular structures and reaction mechanisms, in spectroscopy and mass spectroscopy, etc. Your The more chemically and biologically interesting molecules that can be brought into the beam, the greater the potential for application. This number has been expanded by the device according to the invention and the method according to the invention by a considerable, previously inaccessible class of molecules. This leads to new applications for molecular beams in the scientific, analytical and technical fields, both in chemistry and in biology, medicine and related sciences.

Claims (16)

1. A method of producing pulsed molecular beams wherein:

a) a sample substance is converted from the non-gaseous to the gaseous phase by the supply of energy;

b) the free molecules of the sample substance, which are produced in the conversion step, are mixed with a carrier gas;

c) the carrier gas, with the molecules of the sample substance, is coled adiabatically by expansion of a jet of the carrier gas;

characterised in that to produce molecular beams with large thermally unstable and almost exclusively non-ionised molecules:

d) the energy is supplied to the sample substance in pulse form at such a level that the sample substance vaporises more quickly than it decomposes;

e) the temperature of the carrier gas jet in a region of the jet in which it begins to expand is set substantially lower than the decomposition temperature of the sample substance;

f) the liberated molecules of the sample substance are directly supplied first in that region to the carrier gas jet.

2. A method according to claim 1 characterised in that the sample substance is vaporised by means of pulsed laser radiation.

3. A method according to claim 1 or claim 2 characterised in that the carrier gas jet is produced in pulsed form.

4. A method according to one of the preceding claims characterised in that the molecules are vaporised from a sample surface, as the vaporisation location (13), which extends substantially parallel to the axis of the carrier gas jet and which is adjacent to the outlet opening (6) of a gas jet nozzle (1) for producing the carrier gas jet.

5. Apparatus for carrying out the method according to one of the preceding claims including a gas jet nozzle (1) for producing a carrier gas jet, a carrier gas supply means (4) for supplying the carrier gas to the gas jet nozzle (1 a vaporisation and mixing chamber (2) for converting a sample substance from the non-gaseous to the gaseous phase and for mixing said phase with the carrier gas, and an energy supply means (3) for supplying vaporisation energy to the vaporisation and mixing chamber (2) characterised in that to produce a molecular beam which contains large, thermally unstable, almost exclusively non-ionised molecules:

a) the carrier gas supply means (4) supplies the carrier gas to the gas jet nozzle (1) upstream of the latter at a temperature which is substantially lower than the decomposition temperature of the sample substance;

b) the vaporisation and mixing chamber (2), at least with the portion in which mixing of the molecules with the carrier gas occurs, is arranged downstream of the outlet opening (6) of the gas jet nozzle (1) where the carrier jet begins to expand, and adjacent to said nozzle, and

c) the energy supply means (3) which is provided for sample vaporisation produces such a high level of energy in the pulse mode that more sample substance is vaporised than is decomposed during the pulse duration.

6. Apparatus according to claim 5 characterised in that the vaporisation location (13) of the large thermally unstable molecules is arranged laterally of the outlet opening (6) of the gas jet nozzle (1), at a longitudinal spacing (a) therefrom which is smaller than or equal to 20 times the effective diameter of the outlet opening (6), wherein the longitudinal spacing (a) is the spacing along the axis (5) of the gas jet nozzle (1) and the effective diameter is the diameter corresponding to a circular outlet opening.

7. Apparatus according to claim 5 or claim 6 characterised in that the vaporisation location (13) is arranged at a transverse spacing (b) from the axis (5) of the gas jet nozzle (1), which is less than or equal to 20 times and preferably less than or equal to 10 times the effective diameter of the outlet opening (6).

8. Apparatus according to claim 6 and claim 7 characterised in that the transverse spacing (b) is less than half the longitudinal spacing (a).

9. Apparatus according to one of claims 5 to 8 characterised in that the vaporisation and mixing chamber (2) has a preferably cylindrical expansion passage (12) for the carrier gas jet, said passage adjoining the outlet opening (6) of the gas jet nozzle (1) downstream thereof, the vaporisation location (13) being provided at or in the wall of the expansion passage.

10. Apparatus according to claim 9 characterised in that the vaporisation location (13) is provided in a sample passage (15) which is arranged obliquely and preferably perpendicularly to the axis (5) of the gas jet nozzle (1) and which is provided in the side wall of the expansion passage (12).

11. Apparatus according to claim 9 characterised in that a laser beam passage (16) which is disposed in such a position as axially to extend the sample passage (15) is provided in the side wall of the expansion passage (12).

12. Apparatus according to claim 10 or claim 11 characterised in that the distance (c) of the vaporisation location (13) from the side wall of the expansion passage (12) is smaller than or equal to the diameter (d) of the sample passage (15), wherein the sample diameter is preferably equal to the latter.

13. Apparatus according to one of claims 5 to 12 characterised in that the gas jet nozzle (1) is in the form of an electromagnetically actuable nozzle valve.

14. Use of the method according to one of claims 1 to 4 and/or the apparatus according to one of claims 5 to 13 for identifying the properties of the molecules by means of laser multiphoton ionisation.

15. Use of the method according to one of claims 1 to 4 and/or the apparatus according to one of claims 5 to 13 for identifying the properties of the molecules by means of resonance fluorescence.

16. Use according to claim 14 utilising a mass spectrometer.

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