JPH1114571A - Photoionization mass spectrometer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance the total detecting efficiency of a photoionization mass spectrometer. SOLUTION: A photoionization mass spectrometer is composed of an elliptical spherical mirror 10 which owns geometrical focuses jointly, of a laser device 6 which is arranged in such a way that a pulsed laser beam 7 is condensed on one of the geometrical focuses 11, of a lens 12, of an ion gun 2 which shines a pulsed laser beam 3 at a sample, of a mass spectrometer 9 and of the like. Pulses of a laser beam are passed repeatedly through an identical position on the sample, a laser intensity due to the overlap of the pulses is increased, and an ionization efficiency is increased.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a photoionization mass spectrometer.

[0002]

2. Description of the Related Art Conventionally, a mass spectrometer has been used as a measuring device for determining the type of an element in a sample, and neutral particles sputtered by irradiating the sample with an ion beam are irradiated with a laser. 2. Description of the Related Art A photoionization mass spectrometer that ionizes with light and performs mass analysis of generated ions is known. In this photoionization mass spectrometry, when a sample is irradiated with an ion beam, the number of neutral particles emitted from the sample is at least a few digits larger than the number of secondary ions emitted simultaneously from the sample. It is expected that the sensitivity will be significantly improved as compared with the secondary ion mass spectrometry in which the type of element is determined by measuring the mass spectrum of the ion.

FIG. 2 is a view for explaining the principle of a conventional photoionization mass spectrometer. The principle will be described below. An ion beam 3 emitted from an ion gun 2 to an analysis sample 1
, Neutral particles 4 and secondary ions 5 are released from the analysis sample 1. When the neutral particles 4 are irradiated with the laser beam 7 from the laser device 6, the neutral particles 4 are ionized by photoexcitation, and photo ions 8 are generated. The sample 1 is subjected to mass analysis by introducing the photoion 8 into the mass spectrometer 9 and measuring the mass spectrum.

In photoionization mass spectrometry, sputtered neutral particles are irradiated with a laser beam to perform ionization. However, neutralizing particles are ionized with only one photon for high-sensitivity analysis and are currently commercially available. Insufficient power is output by the laser. Therefore, a method has been adopted in which neutral particles are stepwise ionized by laser light through one or more excited states, that is, the neutral particles are multiphoton ionized. At this time, in order to obtain a laser output expected to enable high-sensitivity analysis, a continuous oscillation laser has no sufficient output. Therefore, a pulse oscillation laser having a high peak output is used.

In the photoionization mass spectrometry, the detection efficiency is determined by the ratio of the neutral particles emitted from the sample surface to the laser irradiation (laser irradiation rate) and the ratio of the neutral particles irradiated by the laser to the laser. It is given by the product of the ratio of ionization by light (ionization efficiency) and the ratio of the ionized neutral particles detected by the mass spectrometer. When a pulsed laser is used, the laser irradiation rate increases as the diameter of the laser beam increases, but the ionization efficiency decreases due to the decrease in the output density of the laser beam. It is known that efficiency is maximized. However, depending on the type of element to be analyzed, it is difficult to obtain high detection efficiency because the output is still insufficient even if the laser beam diameter is optimized using a currently available pulse laser with the highest output. is there.

The following three methods have been devised to solve the above problems.

The first method (Japanese Patent Application Laid-Open No. Sho 62-170841) will be described with reference to FIG. 3. Neutral particles 4 emitted from a sample 1 are passed through an ionization chamber 16 whose inner wall is coated with a substance having a high reflectance. Then, the laser beam 7 is made to enter from the entrance hole 17 of the ionization chamber 16. In this method, since the laser light is repeatedly reflected by the inner wall of the ionization chamber 16, the number of neutral particles to be irradiated with the laser is increased, and the ratio of generated photoions is increased.

The second method (Japanese Patent Laid-Open No. 2-119040) will be described with reference to FIG. 4. A photoionization region 20 is provided between two total reflection mirrors 25 and 27 constituting a laser resonator.
The laser light 7 inside the laser resonator is used for photoionization. In this method, compared with the conventional method in which laser light is extracted from a laser resonator including one total reflection mirror and one partial transmission mirror through a partial transmission mirror and used for photoionization, Light intensity is extremely high. Further, since the loss due to the resonator mirror is small, the laser light is not easily attenuated by a plurality of round trips of the laser light, so that the pulse width is long. Therefore, the number of generated photoions increases.

Referring to FIG. 5, a third method (Japanese Patent Laid-Open Publication No. HEI 3-16547) will be described. A half mirror between the laser device 6 and the chamber 29 transmits light from the front surface and reflects light from the back surface. 28 is arranged so that the surface faces the laser device 6, and the reflection mirror 27 is arranged to face the half mirror 28 with the chamber 29 interposed therebetween. In this method, since the laser light repeatedly reciprocates between the half mirror 28 and the reflection mirror 27, the ratio of the neutral particles receiving the laser irradiation increases, and the number of generated photoions increases.

[0010]

The above-mentioned conventional examples shown in FIGS. 3, 4 and 5 for improving the detection efficiency have the following problems.

In the conventional example shown in FIG. 3, the laser beam passes through a different path each time it is reflected by the inner wall, and the laser beam passes through a wide spatial area over a wide time width. Therefore, when using a time-of-flight mass spectrometer with the highest ion transmittance among mass spectrometers, the ion generation time width is widened and the ionization region is wide, so the flight time of the generated ions The dispersion becomes large and the mass resolution becomes extremely poor.

In the conventional example shown in FIG. 4, it is not easy to incorporate a photoionization mass spectrometer into a laser device, and it is not practical.

In the conventional example shown in FIG. 5, when the half mirror 28 and the reflection mirror 27 are arranged perpendicular to the optical axis of the laser beam,
Although the passing position of the laser beam is constant, there is a problem that the laser device 6 is damaged by the return light from the half mirror 28. If the half mirror 28 is arranged so as not to be perpendicular to the optical axis of the laser beam, the above problem does not occur, but the ion generation time width is widened, and the laser beam passage position is not constant, and the ionization is not performed. When the time-of-flight mass spectrometer is used, the mass resolution is extremely poor.

The present invention has been devised in order to solve the above-mentioned problems, and does not have the above-mentioned problems caused by reciprocating a laser beam in a straight line, and can improve the detection efficiency by photoionization. It is an object to provide a mass spectrometer.

[0015]

SUMMARY OF THE INVENTION It is an object of the present invention to irradiate a laser beam from a laser device to a neutral particle generated by irradiating a sample with an ion beam to ionize the neutral particle and generate the ion. In a photoionization mass spectrometer that conducts mass spectrometry to a mass spectrometer and achieves mass spectrometry, the optical system is configured so that the laser light repeatedly passes over the sample by reflection from an elliptic cylindrical mirror that shares a geometrical focus. Is done.

[0016]

FIG. 1 is a diagram showing the configuration of a photoionization mass spectrometer according to a first embodiment of the present invention. An ion gun 2 for irradiating the surface of the sample 1 with a pulsed primary ion beam 3 for sputtering is provided obliquely above the sample 1.
However, immediately above the sample 1, a mass spectrometer 9 for detecting the photoion 8 is provided. Numeral 4 denotes neutral particles emitted from the surface of the sample 1 when the surface of the sample 1 is irradiated with the pulsed primary ion beam 3, and numeral 5 denotes secondary ions emitted simultaneously. A laser device 6 emits a pulse laser beam 7. Reference numeral 10 denotes an elliptical spherical mirror that shares a geometric focal point 11. Reference numeral 12 denotes a lens, and the laser device 6
Is focused on one of the geometrical focal points 11, and the light reflected by the elliptical spherical mirror 10 is disposed so as to repeatedly pass immediately above the sample 1.

The operation of the first embodiment configured as described above will be described with reference to FIG.

An ellipse has the property that a straight line obtained by reflection of a straight line passing through one of the elliptical geometric focal points on the elliptic curve necessarily passes through another elliptical geometric focal point. Therefore, in the geometrical optics, if the laser beam 7 is incident on one of the geometrical focal points 11 of the elliptical spherical mirror 10 by a lens or the like so as to be condensed, any light in the laser beam 7 Any reflected light of each light ray by the ellipsoidal spherical mirror 10 passes through the geometrical focal point 11, and the other geometrical focus 1 of the elliptical spherical mirror 10.
Go through 1. That is, the beam reflected by the elliptical spherical mirror 10 is also focused on another geometrical focal point 11. The reflected beam is also reflected by the elliptical spherical mirror 10 and focused on the first geometric focus 11. This reflection is repeated, and the laser beam 7 propagates so as to be focused on the two geometrical focal points 11 of the elliptical sphere regardless of the number of reflections. Moreover, since the straight line passing through the geometrical focal point has a geometrical property that it gradually approaches the straight line passing through the two geometrical focal points for each reflection by the elliptic curved surface, the laser beam 7 After the first reflection, the light beam reciprocates on a straight line passing through the two geometrical focal points 11.

As described above, due to the geometric effect, the laser beam 7 causes the two geometrical focal points 11 between the two elliptical spherical mirrors 10 after several reflections by the elliptical spherical mirror 10. It reciprocates on a straight line that passes. Therefore, if the distance between the two elliptical spherical mirrors 10 is shorter than the distance traveled by light within the time of the pulse width of the laser, one pulse overlaps on a straight line passing through the two geometrical focal points 11. . This means that the light intensity is higher than in the conventional case where pulses are not superimposed, so that the number of photo ions generated per unit time is larger than in the conventional case. Therefore, the detection efficiency is greatly improved, and the photoion generation position is almost constant unlike the conventional case.When using a time-of-flight mass spectrometer as the mass spectrometer, the mass resolution is improved compared to the conventional case. Will do. Further, since an elliptical spherical mirror is used, there is no return light to the laser device 6.

In the above description, the lens 12 is used to focus the laser beam on the geometrical focal point of the ellipsoidal sphere, but the laser beam is reflected not by the lens but by a spherical concave mirror. The light may be collected at the geometrical focal point of the spherical surface. Although two elliptical spherical mirrors are used,
As long as the laser beam can be reciprocated on a straight line as described above, one partially elliptic cylindrical mirror or a plurality of elliptical spherical mirrors may be used.

The second embodiment is shown in FIG. 7, and is different from the first embodiment in that an elliptic cylindrical mirror 13 is used instead of the elliptical spherical mirror. As shown in FIG. 8, the elliptical cylindrical mirror 13 is one whose vertical cross section becomes two parts of an elliptic curve sharing a geometrical focal point 11, and the laser beam 7
3 so that it converges to one point 11 on a line segment 14 consisting of the geometrical focal points of all vertical sections 15
It is incident so as to propagate inside. In this embodiment, too, as shown in FIG. 8, the laser beam 7 is reflected several times by the elliptical cylindrical mirror 13 and then the geometrical focus 11
Will reciprocate on a straight line passing through. In addition, when the distance between the two elliptical cylindrical mirrors 13 is shorter than the distance that light travels within the time of the pulse width of the laser, one pulse corresponds to a straight line passing through the two geometrical focal points 11. Since it overlaps above, it has the same operation as the first embodiment.

As in the first embodiment, the laser beam may be reflected by the spherical concave mirror instead of the lens 12 and focused on the geometrical focal point 11 of the elliptic cylindrical mirror 13. If the laser beam 7 reciprocates on a straight line passing through the geometrical focal point 11, the number of the elliptic cylindrical mirrors need not be limited to two, but may be one partially applied or a plurality of partially applied elliptic mirrors. In the above description, the laser beam is incident so as to propagate in one vertical section 15 of the elliptical cylinder.
If the laser beam 7 is incident so as to converge on one point of the line segment 14 consisting of a geometrical focus, the vertical section 15
It does not have to be incident parallel to. In this case, however, the laser beam 7 does not reciprocate on a straight line due to several reflections by the elliptic cylindrical mirror 13,
The laser beam propagates in such a manner as to scan in a plane including the two line segments 14 composed of the geometrical focal point 11, thereby increasing the laser irradiation rate and improving the detection efficiency.

[0023]

As described above, the pulsed laser beam emitted from the laser device repeatedly passes through the same position on the sample, and the intensity of light increases due to the superposition of the pulses to increase the ionization efficiency. The overall detection efficiency is improved and the analysis sensitivity is improved depending on whether the laser irradiation rate increases as the laser beam passes while scanning the same plane.

[Brief description of the drawings]

FIG. 1 is an apparatus configuration diagram showing an embodiment of the present invention.

FIG. 2 is an explanatory diagram of the principle of a photoionization mass spectrometer.

FIG. 3 is a configuration diagram showing a conventional example in which an attempt is made to increase the number of photoions.

FIG. 4 is a configuration diagram showing a conventional example in which an attempt is made to increase the number of photoions.

FIG. 5 is a configuration diagram showing a conventional example in which an attempt is made to increase the number of photoions.

FIG. 6 is a diagram illustrating light reflection by an elliptic curve.

FIG. 7 is an apparatus configuration diagram showing one embodiment of the present invention.

FIG. 8 is an explanatory diagram of a method of making a laser beam incident on an elliptic cylindrical mirror.

[Explanation of symbols]

1 ... sample, 2 ... ion gun, 3 ... primary ion beam, 4
... neutral particles, 5 ... secondary ions, 6 ... laser device, 7 ... laser beam, 8 ... photoion, 9 ... mass spectrometer, 10 ... elliptical spherical mirror, 11 ... geometrical focus, 12 ... lens, 1
3 ... elliptical cylindrical mirror, 14 ... line segment consisting of geometrical focus, 15 ... vertical cross section of one elliptical cylinder, 16 ... ionization chamber, 1
7 ... laser entrance hole, 18 ... converging lens, 19 ... electrode, 2
0: ionization region, 21: photoion, 22: extraction electrode, 23: converging lens, 24: ion detector, 25: total reflection mirror, 26: laser medium, 27: total reflection mirror,
28: half mirror, 29: vacuum chamber, 30: window for laser incidence.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI H01J 49/10 H01J 49/10

Claims (5)

[Claims] 1. A method for irradiating a laser beam from a laser device to a neutral particle generated by irradiating a sample with an ion beam to ionize the neutral particle, guide the generated ion to a mass spectrometer, and perform mass analysis. In the photoionization mass spectrometer, at least one or more elliptical spherical mirrors sharing a geometrical focus, and an optical element for condensing the laser beam to one of the geometrical focuses, A photoionization mass spectrometer, wherein an ionization region is provided between the two geometrical focal points of the ellipsoidal sphere. 2. Neutral particles generated by irradiating a sample with an ion beam are irradiated with laser light from a laser device to ionize the neutral particles, and the generated ions are guided to a mass spectrometer for mass analysis. In the photoionization mass spectrometer, at least one or more elliptic cylinder mirrors whose vertical cross section is an elliptic curve elliptic curve or a plurality of elliptic curve mirrors sharing a geometrical focus, and the laser light An optical element for converging light to an arbitrary point on two line segments composed of geometrical focal points of all the vertical cross sections of the elliptic cylinder surface, and between the two line segments. A photoionization mass spectrometer characterized by providing an ionization region. 3. The photoionization mass spectrometer according to claim 1, further comprising a lens for converging said laser beam to one of said geometrical focal points. 4. The photoionization mass spectrometer according to claim 2, further comprising a lens for converging the laser beam to an arbitrary point on the line segment. 5. The photoionization mass spectrometer according to claim 2, wherein the laser beam is incident so that the laser beam propagates in one vertical section of the elliptic cylinder. .