CN1975387A - Alkaline earth metal atom effective detecting method - Google Patents

Abstract

The invention discloses a high-efficiency detection method for alkaline earth metal atoms. The method utilizes resonance ionization to detect alkaline earth metal atoms in a highly excited state. Use one Nd:YAG laser to pump three dye lasers at the same time, first use two dye lasers to irradiate alkaline earth metal atomic beams to prepare atomic samples in highly excited states with different n and l values, and then use the third dye laser The laser ionizes the atoms through the process of autoionization. High-efficiency detection of highly excited atoms is achieved by taking advantage of the much greater ionization probability of auto-ionization than conventional photo-ionization. The present invention is a very effective detection means suitable for highly excited alkaline earth metal atoms, it overcomes the defects of the electric field ionization detection method and the photoionization detection method, and thus has high detection efficiency. Compared with other conventional detection methods, self-ionization detection method has obvious advantages. This detection method is applicable to all alkaline earth metal atoms.

Description

High Efficiency Detection Method for Alkaline Earth Metal Atoms

【Technical field】：

The patent of the present invention relates to a detection technology, especially a high-efficiency detection method for alkaline earth metal atoms.

【technical background】:

With the application of laser spectroscopy in the fields of photochemistry, isotope separation, and atmospheric monitoring, it is necessary to detect atoms and molecules in highly excited states, and the resolution and sensitivity of detection are increasingly required. Traditional detection methods (such as absorption spectroscopy, resonance fluorescence spectroscopy, photoacoustic spectroscopy, photocurrent spectroscopy, and electric field ionization spectroscopy) cannot meet the needs of highly excited state research due to constraints in detection efficiency and resolution. For the detection of some atoms and molecules in a low excited state, the more effective way is to use the fluorescence detection method to collect the radiated photon signal, while for the atoms and molecules in a high excited state, due to their long lifetime, the rate of radiated photons are lower, so it is more reasonable to collect their ionization products—electrons and ions. The principle is to excite the alkaline earth metal atoms with a laser to prepare an atomic sample in the Rydberg state, and then use an appropriate method to ionize the atoms. By detecting ions or electrons generated by ionization, important information about photoexcitation and atomic energy states can be obtained. Because laser excitation can ensure the sensitivity and selectivity of detection, and the electron multiplier as a detector has a high gain, so ions or electrons can be collected and detected with high efficiency, so resonance excitation and electric power technology have extremely high Detection efficiency, enabling single-atom detection.

Commonly used ionization methods include field ionization and photoionization. Both have their own advantages and disadvantages. The applied electric field ionization is suitable for the highly excited Rydberg state with a longer lifetime. The disadvantage is that for atoms in the lower Rydberg state, a very high electric field is required to ionize them. The lower Rydberg states that cannot be detected by field ionization can be detected by photoionization method, but the conventional direct photoionization process is a non-resonant transition, and its excitation cross section is only 10 ^-20 cm ² , resulting in low detection efficiency. In order to improve the detection efficiency, a strong laser energy must be provided.

【Invention content】：

The purpose of the present invention is to solve the shortcomings and problems of the existing technology for detecting atoms, such as low efficiency, narrow adaptability, not easy to operate, and high requirements on the power level of the laser, and propose a new technology developed by using the concept of resonance ionization. High-efficiency detection method for highly excited alkaline earth metal atoms.

To achieve the purpose of the invention, the invention discloses a high-efficiency detection method for alkaline earth metal atoms. It is characterized in that it comprises the following steps:

(1) The high-purity alkaline earth metal placed in the crucible is heated to 580-630 degrees Celsius in a vacuum environment through the Joule heating method of resistance, so that the metal vapor is ejected from the small hole with a diameter of 1-2mm in the crucible, and after two The collimation of two apertures makes the metal vapor become an atomic beam with a parallelism between 8-12 degrees;

(2) Use the first dye laser pumped by a Nd:YAG laser to irradiate the atomic beam, and tune its wavelength to make the atoms transition from the ground state ms ^{2 1} S ₀ to the msmp ¹ P ₁ state; for magnesium, calcium, For strontium and barium metal atoms, the values of m are 3, 4, 5, and 6 respectively (all the following are true);

(3) Use the second dye laser pumped by the same Nd:YAG laser to irradiate the atomic beam, and scan its wavelength between 750-420nm to excite the atoms from msmp ¹ P ₁ state to msnlRydberg state respectively, where: n=7-50, l=0 or 2, prepare a sample of highly excited atoms in the corresponding atomic state;

(4) Use the third dye laser pumped by the same Nd:YAG laser to irradiate the atomic beam, and further resonantly excite the atoms from the msnl state to the mp _3/2 nl autoionization state or mp _1/2 nl autoionization state;

(5) Use a pulsed electric field with an electric field strength of 200V/cm to collect the ions produced by the self-ionization process, and import the ions into a microchannel plate (MCP) detector with a gain of ¹⁰⁶ for reception and amplification;

(6) The output signal of the detector is respectively input into an oscilloscope and a Boxcar averaging integrator. The former is used to observe the size and position of the signal, and the latter samples and averages 10-20 pulse signals, and after analog-to-digital conversion, it is input to the computer for real-time display and analysis.

Beneficial effects of the present invention: the present invention is a very effective detection method suitable for highly excited atoms. Because it overcomes the defects of the electric field ionization detection method and the photoionization detection method at the same time, it has high detection efficiency no matter for high or low Rydberg atoms. Compared with other conventional detection methods, self-ionization detection method has obvious advantages. This detection method is applicable to all alkaline earth metal atoms.

The invention uses the method of resonance ionization to detect atoms in a highly excited state. It is excited by three-step resonance, so that the atoms can reach the mpnl self-ionization state, and its excitation cross section can reach 10 ^-14 cm ² . Atoms in the self-ionized state rapidly decay into ions through the collision and energy exchange between two excited valence electrons, so as to realize the detection of atoms and obtain the information of the microscopic world.

【Description of drawings】

Fig. 1 is the spectrum of barium atom 6p _1/2 17d self-ionization state;

Figure 2 is the spectrum of multiple 6p _1/2 ns self-ionization states;

Figure 3 is the Rydberg state spectra of Ba atoms obtained by two different photoionization detection methods;

Figure 4 is the spectrum of the Rydberg state detected by the self-ionization detection method.

【Detailed ways】

The equipment used in the present invention includes: a Nd:YAG pulsed laser used for pumping dye lasers, three tunable dye lasers whose wavelength is tunable and stable on the atomic absorption peak, an atomic heating vacuum furnace that can accurately control the temperature, An atomic beam system capable of producing high collimation, a set of micro-channel plate detectors with fast response and high sensitivity to charged particles, an optical delayer and electrical pulses that can precisely control the time sequence and delay of optical pulses and electrical pulses One delay device, one average integrator capable of integrating and averaging pulse signals, one set of computer data acquisition and analysis system that controls and scans all electro-optic devices and realizes data communication.

The technologies involved in the present invention include: laser debugging technology, laser frequency doubling and sum frequency technology, atomic vapor beam forming technology, isolated real excitation technology of atoms, sample preparation technology of highly excited atoms, time sequence of optical pulses and electrical pulses and delay precise control technology, beam combination and collimation technology, control technology to improve the signal-to-noise ratio of the microchannel plate detector, etc.

The atomic autoionization spectrum used in the present invention: if the isolated real excitation technique is not used, the atomic autoionization spectrum presents an asymmetrical Fano line shape, and its peak position and width are difficult to determine. If the isolated real excitation technology is used, the excitation interference phenomenon between the two valence electrons of the atom is avoided, so that the self-ionization spectrum of the atom presents a symmetrical Lorentz line shape, and its peak position and width are easy to determine, thus facilitating the third step of excitation. The location of the wavelength, when the excitation efficiency reaches the maximum, corresponds to the highest efficiency of atomic detection.

The excitation technology of the present invention is a multi-step isolated real excitation technology: tune the wavelength of the first laser to make the atoms transition from the ground state ms ^{2 1} S ₀ to the msmp ¹ P ₁ state, and then use the second dye laser to convert the atoms from msmp ¹ P _{The 1} state is excited to the msnl (l=0,2) Rydberg state, where n=7-50, and the corresponding laser wavelength is between 750-420nm. Since the nl electron is in the large orbit, it is far away from the ms electron which is still in the small orbit, so the atom has actually been isolated. A third dye laser is used to further resonantly excite the atoms into mp _3/2 nl or mp _1/2 nl autoionized states. These two different self-ionization states provide two equivalent excitation schemes. For barium atoms, the corresponding wavelength of the third laser is two ranges: 455-457nm or 493-495nm. Isolated real excitation technology is the basis of high-efficiency detection technology for highly excited atoms, and plays an important role in the overall detection efficiency.

The preparation of highly excited atomic samples in the present invention: it is necessary to accurately control the temperature of the heating crucible, and heat the metal with high purity (99.5% or more) to 580-630 degrees Celsius in a high vacuum (vacuum degree is ^10-4 Pa) environment, so that the metal The vapor is ejected from small holes with a diameter of 2 mm in the side of the crucible. The collimation through two apertures makes the metal vapor into a highly collimated (parallelism between 8-12 degrees) atomic beam. The beams of the first laser and the second laser are parallel and incident from the front window of the vacuum chamber; the beams of the third laser are antiparallel to them and incident from the rear window; perpendicular to the beam direction. Controlling the orthogonality between the atomic beam and the three laser beams is to basically eliminate the Doppler broadening effect and ensure high-resolution detection of highly excited states of atoms. When the atomic beam is sequentially irradiated by the first two laser beams, a sample with highly excited atoms is prepared.

Self-ionization detection of atoms utilized by the present invention

Use the third dye laser to resonate the prepared highly excited atoms in msnl state to mp _3/2 nl or mp _1/2 nl autoionized state. Atoms will rapidly become ions by self-ionization, which will be detected by the microchannel plate detector. Atoms can achieve high-efficiency ionization through the above two different self-ionization states, thus providing two equivalent excitation schemes. Corresponding to these two excitation schemes, the wavelength of the third laser can be selected in two ranges. For barium atoms they are: 455-457nm or 493-495nm.

The time delay control of the three laser pulses in the present invention: precisely controlling the time sequence of the three laser pulses is to ensure that the atoms are sequentially excited to the designated atomic states. The relevant technical parameters are: the optical pulse of the second laser is delayed by 8ns relative to the optical pulse of the first laser, and the optical pulse of the third laser is delayed by 10ns relative to the optical pulse of the second laser. This delay can be monitored with a fast photodiode and a broadband digital storage oscilloscope to maintain detection efficiency at an optimum level.

In the present invention, the pulse of the electric field is collected relative to the time delay control of the laser pulse: after atoms decay into ions through self-ionization, the charged particles are collected by the pulse electric field. This pulsed electric field must be applied after the light pulse has completely disappeared to avoid the Stark effect on the highly excited states of the atoms. The specific method is to use the pulse signal of the Nd:YAG pumping laser to output the pulse power supply synchronously, and delay the electrical pulse by 500ns relative to the optical pulse.

The invention transforms the research of self-ionization spectrum into a spectrum detection method with extremely high sensitivity and resolution. The self-ionization state is a quasi-bound state embedded in a continuous state above the ionization threshold of the atom when two electrons in the atom are excited at the same time. Due to its high ionization rate and efficiency, the study of autoionization spectroscopy has attracted great attention in this research field. The invention proves the effect of the detection method through experiments on the self-ionization spectra of various alkaline earth and rare earth metal atoms. Specific methods of detection include:

(1) In a vacuum environment with a vacuum degree of 10 ^-4 Pa, the high-purity metal (99.5%) placed in the crucible is heated to a temperature of 580-630 degrees Celsius by the Joule heating method of resistance, and the atomic gas passes through the crucible. The jet is ejected from a small hole with a diameter of 2mm, collimated by two apertures with a diameter of 1cm and a distance of 10cm, and enters the active area. At this time, the atomic number density is about 10 ⁹ cm ^-3 .

(2) Use a Nd:YAG laser to pump the first dye laser to excite the atoms in the ground state ms ^{2 1} S ₀ to the msmp ¹ P ₁ state.

(3) Use the same Nd:YAG laser to pump the second dye laser and further excite the atoms to the msnl (l=0,2) excited state to be measured, thereby preparing highly excited states in various corresponding atomic states Atomic samples. When n=7-50, the corresponding laser wavelength is between 750-420nm. In the experiment, it is necessary to replace a variety of different laser dyes to completely cover such a wide scanning band, for example: Stilbene 3 dye can cover the 415-445nm band; Coumarin460 dye can cover the 446-478nm band.

(4) Use the same Nd:YAG laser to pump the third dye laser as the probe light, and further resonately excite the atoms from msnl (l=0, 2) Rydberg state to mp _j nl (l=0, 2 and j =3/2, 1/2) on the self-ionization state and rapidly ionized, thereby constituting a high-efficiency self-ionization detection method. For barium atoms, the wavelength of the laser can be set in the two ranges of 455-457nm and 493-495nm, corresponding to j=3/2 and 1/2 self-ionization state in 6p _j nl respectively, these two wavelengths The detection effect produced by the selection is the same.

(5) When the wavelengths of the first-step and third-step lasers are respectively fixed at the above-mentioned wavelengths, scan the wavelength of the second-step laser, and arrange the metal atoms on different msnl (l=0,2) Rydberg states. The sequential excitation of the atoms by the three lasers completes the three-step isolated real excitation process of ms ² -msmp-msnl-mp _j nl, thus realizing the purpose of detecting highly excited atoms in the msnl state.

(6) Control the direction of the atomic beam to be perpendicular to the direction of the three laser beams, and make the atomic beams irradiated by the three laser beams in sequence, that is, the optical pulse of the second laser is delayed by 8 ns relative to the optical pulse of the first laser, The optical pulse of the third laser is delayed by 10 ns relative to the optical pulse of the second laser.

(7) After all the laser pulses are completely over, the ions generated by the self-ionization process can be collected by applying a pulsed electric field of 200V through two parallel metal plates located in the atom-photon interaction area, accelerated and reached the detector. The pulse signal output of the Nd:YAG pumping laser is used to trigger the pulse power supply synchronously, and the electrical pulse is delayed by 500ns relative to the optical pulse.

(8) A microchannel plate (MCP) detector perpendicular to the direction of the laser beam and the atomic beam is used to receive and amplify the ion signal. The effective aperture of the detector is 2.5cm. Drive the detector with a DC power supply. When the operating voltage is 1.7kV and the collection voltage is 300V, the gain is about 10 ⁶ .

(9) The signal output by the detector is still in the form of a pulse, which needs to be sampled and averaged by the Boxcar to become a DC analog signal. The digital signal obtained through analog-to-digital conversion is input to the computer for real-time display and analysis.

This detection method is applicable to all alkaline earth metal atoms. However, the wavelength selection of the three laser beams should be different for different atoms. For example: for a magnesium atom, m=3. The three-step excitation process is: 3s ² -3s3p-3snl-3p _j nl, the wavelengths of the corresponding three lasers are 285.3nm, 395-420nm and 280-285nm respectively; for calcium atoms, m=4. The three-step excitation process is: 4s ² -4s4p-4snl-4p _j nl, the wavelengths of the corresponding three lasers are 423.7nm, 390-550nm and 390-410nm respectively; for strontium atoms, m=5. The three-step excitation process is: 5s ² -5s5p-5snl-5p _j nl, the wavelengths of the corresponding three lasers are 460.9nm, 400-620nm and 420-455nm respectively; for barium atoms, m=6. The three-step excitation process is: 6s ² -6s6p-6snl-6p _j nl, and the wavelengths of the corresponding three lasers are 553.9nm, 420-750nm and 455-495nm respectively.

Here we only take the research results of barium atoms as an example to illustrate the principle and method of this detection method in detail.

Example 1: Detection of barium atoms in the 6s17d state.

Fig. 1 shows the spectrum of the barium atom 6p _1/2 17d self-ionization state measured in the present invention. It can be seen from Figure 1 that the resonance peak of the self-ionization state is located near the wavelength of 493.7nm. Therefore, when detecting atoms in the 6snd highly excited state, the highest detection efficiency can be obtained only by fixing the wavelength of the third-step laser at around 493.7nm.

Example 2: Probing barium atoms in multiple 6sns states.

Fig. 2 shows the spectra of multiple 6p _1/2 ns self-ionization states measured by the present invention, where n=10-20. Therefore, as long as the photon energy of the third-step laser is fixed at around 20250cm ^-1 (wavelength 493.7nm), the highest detection efficiency can be used to detect barium atoms in many 6sns states, where n=10-20.

It is worth pointing out that generally the autoionized state has a large width and increases with the decrease of n. Therefore, when detecting Rydberg atoms, all the atoms in the 6snl state can obtain better detection results by fixing the wavelength of the excitation light in the third step near the above wavelength, thus overcoming the conventional direct photoionization method. In the detection mode, the detection efficiency is highly sensitive to the laser wavelength. In conclusion, the present invention is both efficient and easy to use.

Example 3: Comparison of the effects of self-ionization detection and direct photoionization detection.

Figure 3(a) and 3(b) show the Rydberg spectra of Ba atoms obtained from self-ionization detection and direct photoionization detection, respectively. Experiments have found that when direct photoionization is used, the required laser intensity is dozens of times that of self-ionization detection. On the other hand, it can be seen from Figure 3 that the detection efficiency of direct photoionization decreases rapidly as the energy of the Rydberg state increases. When n>30, direct photoionization detection has basically failed. When using auto-ionization technology for detection, it can give high-efficiency detection for all states. Especially for those atoms in states with higher n values (n>20), self-ionization detection has more obvious advantages. According to the above analysis, the efficiency of detecting the highly excited states of atoms by using the self-ionization detection method is much higher than that of the direct photoionization detection.

Example 4: Detection of barium atoms in the 6snl (l=0,2) state.

Compared with electric field ionization detection, the present invention also has obvious advantages. It is mainly manifested in the detection of lower excited states. If the electric field ionization detection method is used, it is not suitable for detecting the lower Rydberg atoms. The threshold for electric field ionization of atoms is F∝n ^-4 ×10 ⁹ (V/cm). Therefore, in order to detect atoms in a very low Rydberg state, the electric field strength must be greatly increased. As shown in Fig. 4, the lowest 6s8s state can be detected with self-ionization detection. The field ionization threshold of this state is about 100kV/cm electric field intensity, such a high electric field intensity is difficult to achieve under experimental conditions.

Claims (9)

1. A high-efficiency detection method for alkaline earth metal atoms, characterized in that it comprises the following steps: (1) The high-purity alkaline earth metal placed in the crucible is heated to 580-630 degrees Celsius in a vacuum environment through the Joule heating method of resistance, so that the metal vapor is ejected from the small hole with a diameter of 1-2mm in the crucible, and after two The collimation of two apertures makes the metal vapor become an atomic beam with a parallelism between 8-12 degrees; (2) Use the first dye laser pumped by a Nd:YAG laser to irradiate the atomic beam, and tune its wavelength to make the atoms transition from the ground state ms ²¹ S ₀ to the msmp ¹ P ₁ state; (3) Use the second dye laser pumped by the same Nd:YAG laser to irradiate the atomic beam, and scan its wavelength between 750-420nm to excite the atoms from msmp ¹ P ₁ state to msnlRydberg state respectively, where: n=7-50, l=0 or 2, prepare a sample of highly excited atoms in the corresponding atomic state; (4) Use the third dye laser pumped by the same Nd:YAG laser to irradiate the atomic beam, and further resonantly excite the atoms from the msnl state to the mp _3/2 nl autoionization state or mp _1/2 nl autoionization state; (5) Use a pulsed electric field with an electric field strength of 200V/cm to collect the ions generated by the self-ionization process, and import the ions into a microchannel plate (MCP) detector with a gain of ¹⁰⁶ for reception and amplification; (6) The output signal of the detector is respectively input into an oscilloscope and a Boxcar averaging integrator. The former is used to observe the size and position of the signal, and the latter samples and averages 10-20 pulse signals, and after analog-to-digital conversion, it is input to the computer for real-time display and analysis. 2. The high-efficiency detection method for atoms according to claim 1, characterized in that the vacuum degree of the vacuum environment in the step (1) is 10 ^-4 Pa. 3. The high-efficiency detection method for atoms according to claim 1 or 2, characterized in that the apertures of the two apertures in the step (1) are 1 cm, and the distance between them is 10 cm. 4. The high-efficiency detection method of alkaline earth metal atoms according to claim 1, characterized in that in said steps (2), (3) and (4), the light pulse of the second laser is relatively The optical pulse of the laser is delayed by 8 ns, and the optical pulse of the third laser is delayed by 10 ns relative to the optical pulse of the second laser. 5. The high-efficiency detection method of alkaline earth metal atoms according to claim 1, characterized in that in said steps (2), (3) and (4), the light beams of the first laser and the second laser are Parallel and incident from the front window of the vacuum chamber; the beam of the third laser is antiparallel to them and incident from the rear window; the three beams are on the same straight line and perpendicular to the direction of the atomic beam. 6. The high-efficiency detection method of alkaline earth metal atoms according to claim 1, characterized in that in said steps (2), (3) and (4), the wavelength of a Nd:YAG laser is 532nm The first dye laser is pumped with a doubled output, and the second and third dye lasers are pumped with a tripled output of the same Nd:YAG laser at a wavelength of 355 nm. 7. The high-efficiency detection method for alkaline earth metal atoms according to claim 1 or 6, characterized in that in the step (4), the wavelength range of the third dye laser is: 455-457nm or 493-495nm. 8. The high-efficiency detection method for alkaline earth metal atoms according to claim 1, characterized in that in said step (5), after the light pulse ends, a pulsed electric field is applied to introduce ions into one direction and the laser beam and the atom The beams are vertical inside the microchannel plate detector. The diameter of the detection surface of the detector is 2.5cm. When the working voltage is 1.7Kv and the collection voltage is 300V, the gain is about 10 ⁶ . 9. The high-efficiency detection method for alkaline earth metal atoms according to claim 1 or 8, characterized in that in said steps (5) and (6), the pulse output of the Q switch of the Nd:YAG laser is used to trigger synchronously Oscilloscope, Boxcar averaging integrator and pulse power supply, and delay the electric field pulse relative to the laser pulse by 500ns.

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