亚声速NH3分子束静电Stark减速的理论研究

本文基于自行研制的第二代(180级)静电Stark减速器, 展开了对NH₃的有效减速与冷却的理论研究. 首先, 计算了NH₃分子在|J=1, K=1>量子态的Stark分裂, 研究了不同的同步相位角下, 减速器中NH₃分子的纵向相空间稳定区域; 接着, 采用Monte-Carlo方法研究了该分子在传统工作模式下的减速效果, 并讨论了该减速模式下多个参数(包括每级损失动能、分子波包末速度和相对减速效率)与同步相位角的依赖关系, 以及减速波包末速度与减速电压的关系, 研究发现: 采用传统的Stark减速模式, 当减速电压为±13 kV、同步相位角φ₀=26.08°时, 即可实现NH₃从280 m/s到6.7 m/s的有效减速, 对应平动动能减少了99.9%, 其波包温度由1.34 K降至80 mK; 最后, 研究了先聚束后减速模式下NH₃分子的减速效果, 以及该减速模式下减速波包末速度与同步相位角的依赖关系, 结果表明: 当减速电压为± 6.5 kV, 采用前15级电极作为聚束电极, 后165级作为减速电极时, 可将NH₃分子波包的中心速度由280 m/s减至20.7 m/s, 平动动能减少了99.4%, 温度由1.34 K降至1.6 mK, 与传统减速模式相比, 冷分子波包温度降低至1/50. 由此可见, 采用180级的传统Stark减速器完全可以实现具有较低Stark势能的NH₃分子的有效减速与冷却, 并获得温度约为1 mK的冷分子波包, 为进一步的实验研究提供了可靠的理论依据.     

Evidence for Stark induced frequency shift in a CH3F fir laser

We report the first direct measurement of the frequency shift induced by the application of an electric field to an optically pumped CH₃F far infrared laser. These shifts are obtained only if the laser resonator is slightly detuned, so that the electric field enhances the cavity pulling effects. The maximum Stark coefficient we observed is about 2 kHz/(Volt/cm), fairly sufficient to allow FM modulation or fast laser frequency stabilization.     

Generation of CW cold CH3CN molecular beam by a bent electostatic quadrupole guiding: Monte-Carlo study

A new kind of continuous-wave (CW) cold molecular beam, methyl cyanide (CH₃CN) beam, is generated by a bent electrostatic quadrupole guiding. The Stark shift of rotational energy levels of CH₃CN molecule and its population distribution are calculated, and the dynamic processes of electrostatic guiding and energy filtering of CH₃CN molecules from a gas source with room temperature (300 K) are simulated by Monte Carlo Method. The study showed that the longitudinal and transversal temperatures of output cold CH3CN beam could be about ∼2 K and ∼ 420 mK, and the corresponding guiding efficiency was about 10⁻⁵ as the guiding voltage was 3 kV. Furthermore, the temperature of the guided molecules and its guiding efficiency can be controlled by adjusting the guiding voltages applied on electrodes.      

Mitigation of loss within a molecular Stark decelerator

The transverse motion inside a Stark decelerator plays a large role in the total efficiency of deceleration. We differentiate between two separate regimes of molecule loss during the slowing process. The first mechanism involves distributed loss due to coupling of transverse and longitudinal motion, while the second is a result of the rapid decrease of the molecular velocity within the final few stages. In this work, we describe these effects and present means for overcoming them. Solutions based on modified switching time sequences with the existing decelerator geometry lead to a large gain of stable molecules in the intermediate velocity regime, but fail to address the loss at very low final velocities. We propose a new decelerator design, the quadrupole-guiding decelerator, which eliminates distributed loss due to transverse/longitudinal couplings throughout the slowing process and also exhibits gain over normal deceleration to the lowest velocities.     

Far-Infrared Laser Stark Spectroscopy of CH3OD

The high resolution Stark spectra of the singly deuterated methanol isotope, CH₃OD, have been studied using the HCN laser with Stark fields up to approximately 60 000 V/cm. Numerous families of absorption lines have been observed in both parallel and perpendicular polarizations, resulting in the following assignments: with the 311 m line – J _K = 18₁ 18₀ E ₂, _t = 1; and with the 337 m line – J _k = 6₄ 5₃ E ₂, _t = 0 and J _K = 14₆ 13₅ A, _t = 1. Zero-field frequencies for the assigned transitions are in agreement with Fourier transform measurements and those calculated from the available molecular constants.     

P-branch operation of CD3F and CH3F Raman FIR lasers

Multi-kolowatt pulses with limited wavelength tunability have been produced in several bands between 186 and 817 m by pumping v3 P-branch transitions in¹²CH₃F and¹²CD₃F with the 10.6 m band of a ten-atmosphere CO₂ laser. The FIR emissions are all restricted to the immediate neighborhood of the resonances associated with successive J-numbers, but there is some evidence that additional off-resonant emission could be produced with stronger pumping. The optimum operating pressure for the various molecule/J-number combinations range from 7 to 75 torr and increase rapidly with increasing J-number.     

Surface-enhanced nuclear spin conversion in CH3F

The nuclear spin conversion of a molecule is the modification of the total nuclear spin I of its equivalent atoms. This phenomenon is observed by measuring the relaxation rate of a gas sample initially prepared with a population of spin isomers far from the equilibrium given by nuclear spin statistics. New experimental data obtained at low pressure show a surface-induced enhancement of the nuclear spin conversion in ¹³CH₃F. Contrary to binary collisions in the gas phase, hitting the surface induces direct conversion. Several mechanisms are proposed.     

Laser Stark spectroscopy in the 10 μm region: The ν3 bands of CH3F

Laser Stark spectroscopy of the ν₃ band of CH₃F has been carried out using coincidences with the 9.4 μm band CO₂ laser lines. About 350 Stark resonances were measured for the ν₃ fundamental bands of ¹²CH₃F and ¹³CH₃F. About 30 of them were measured by using a Stark-Lamb dip technique to increase the resolution and the accuracy of the data. These Stark resonances, together with the recent results of infrared-microwave two-photon Lamb dip measurements, were analyzed to give the following vibration-rotation parameters and the dipole moments in the ν₃ state,

     

9.

A study on the GVFF of CHF₃, CH₂F₂, and CH₃F     

C. E. Blom  A. Müller 《Journal of Molecular Spectroscopy》1978,70(3):449-458

The complete GVFF of CHF₃, CH₂F₂, and CH₃F has been calculated from self-consistent-field ab initio energies, using a 4–31 G basis set. The larger part of the interaction force constants is close to those of the best available force fields from experimental data. Only one interaction term in CH₃F and the interaction force constants of the A₁ species in CH₂F₂ differ appreciably from the experimental ones. Using constraints from the ab initio studies we have improved the GVFF of CH₃F and CH₂F₂. It is shown that all comparable stretch-stretch interaction terms are of the same order of magnitude in the three molecules. The sign of all stretch/bend force constants are in accordance with those predicted by the hybrid orbital force field.     

10.

Level crossing in microwave spectroscopy of CH₃F     

P. Glorieux  J. Legrand  B. Macke  B. Segard 《Physics letters. A》1974,48(3):187-188

Stark tuned level crossing resonances have been detected in CH₃F through non-linear microwave absorption. In our experiments, the level crossing width is about ten times smaller than the Doppler width.     

11.

Saturable absorption of CO₂-laser radiation by CH₃F     

J.P. Pichamuthu  U.N. Sinha 《Optics Communications》1978,24(2):195-198

A theory of the saturable absorption of pulsed CO₂-laser radiation by CH₃F is described. It is valid for arbitrary pulse shapes and durations in the rate-equation regime. The theoretical predictions agree satisfactorily with available experimental data. The effect of vibrational exchange and the influence of the Boltzmann population factor on transmission are discussed.     

12.

Relation between electrooptical constants and polar tensors of CH₃F,CD₃F,CH₃Cl,CD₃Cl,CH₃Br,CD₃Br,CD₃I,and CD₃I     

G. Swarna Kumari  N. Rajeswara Rao 《Journal of Molecular Spectroscopy》1984,106(1):1-11

Infrared intensity analysis was done in two ways. In one approach, formulas connecting electrooptical parameters (eop's) and intensities were derived and, using the intensities, the former were obtained. In evaluating the eop's the intensity sum equations were used directly so that the sign ambiguity of $\text{δP}\text{δQ}$ is avoided. In the other method, polar tensors were obtained directly from intensities. The two methods have now been connected, and polar tensors have been obtained in terms of electrooptical parameters so that the former were also free from sign ambiguity of $\text{δP}\text{δQ}$. They compare well with those obtained directly from intensities. In similar connecting formulas by other authors [see, for example, P. L. Prasad, J. Chem. Phys., 69, 4403, (1978)], both polar tensors and eop's were affected by the sign ambiguity.     

13.

交叉分子束方法研究H+CH₄和H+CD₄反应:CH₃/CD₃的振动激发态产物通道     

陈文韬  俞盛锐  袁道福  谢婷  杨家岳  王思雯  罗畅  谭玉欣  缪月  张未卿  吴国荣  杨学明  王兴安 《化学物理学报》2017,30(6):609-613

We study the H+CH₄/CD₄→H₂/HD+CH₃/CD₃ reactions using the time sliced velocity map ion imaging technique. Ion images of the CH₃/CD₃ products were measured by the (2+1) resonance enhanced multi-photon ionization (REMPI) detection method. Besides the CH₃/CD₃ products in the ground state, ion images of the vibrationally excited CH₃/CD₃ products were also observed at two collision energies of 0.72 and 1.06 eV. It is shown that the angular distribution of the products CH₃/CD₃ in vibrationally excited states gradually vary from backward scattering to sideways scattering as the collision energy increases. Compared to the CH₃/CD₃ products in the ground state, the CH₃/CD₃ products in vibrationally excited states tend to be more sideways scattered, indicating that larger impact parameters play a more important role in the vibrationally excited product channels.     

14.

Line-mixing between rotational Stark components of CH₃F self-perturbed and perturbed by helium: Experimental results and IOS analysis     

J. Koubek  C. Boulet  A. Perrin  Š. Urban 《Journal of Molecular Spectroscopy》2011,266(1):12-20

Self- and He-broadening coefficients of microwave transitions of CH₃F have been measured with and without the presence of an external electric field. This provides values for the J, K → J + 1, K (K = 0 − J) transitions for J = 1 and J = 3 as well as for the various J, K, M → J + 1, K, M′ (|M| = 0 − K, |M - M′| = 0, 1) Stark components. The results and those of a previous experimental study for pure CH₃F, which show significant line-mixing effects, are analyzed with a model based on the Infinite Order Sudden approximation. It is shown that the latter leads to very satisfactory modeling of observed values even though no parameter was adjusted since previously and independently determined basic cross-sections are used. The quality of the present predictions is comparable with that obtained previously with a semi-classical approach. Furthermore, it is shown that the previously stated inaccuracy of the IOS model was due to an oversimplified use of this approach.     

15.

10 kW cavity operation of a submillimetre CH₃F laser     

L.E. Sharp  W.A. Pebbles  B.W. James  D.E. Evans 《Optics Communications》1975,14(2):215-218

Operation of a CH₃F laser at 496μ has been obtained in a hemispherical optical cavity pumped by up to 42 J of 9.55μ CO₂ TEA laser radiation. Highly modulated 1μ pulses delivering 6 mJ at peak powers greater than 10 kW have been recorded.     

16.

Short wavelength operation of the CH₃F Raman FIR laser     

Jerald R. Izatt  Bimal K. Deka 《International Journal of Infrared and Millimeter Waves》1984,5(11):1473-1482

Using the isotopic species¹³CH₃F in the methyl-fluoride Raman laser, we have observed kilowatt-level laser pulses which can be tuned over a series of intervals that are centered on pure-rotational transitions in thev ₃ ground state with initial-state J values between 35 and 43, inclusively. Taken together, these tuning intervals cover 35% of the spectrum between 142 and 174 m. Several straightforward improvements in the experimental setup, including the use of isotopically purer¹³CH₃F, should enhance the spectral coverage in this region.     

17.

Extensive high-resolution study of the crowded rovibrational CH₃F spectrum around 3000 cm⁻¹     

Georges Graner  Guy Guelachvili 《Journal of Molecular Spectroscopy》1981,89(1):19-41

The infrared spectrum of CH₃F has been remeasured from 2800 to 3160 cm^?1 with a Fourier spectrometer at a resolution of 0.005 cm^?1. By a systematic use of ground-state combination differences, we have assigned 5200 transitions, including several hundred “forbidden” transitions. They can be distributed into seven vibrational bands, which are mixtures of ν₄, ν₁, 2ν₅², 2ν₅⁰, ν₂ + ν₅, 2ν₂, and 3ν₃. More than 20 local resonances have been detected as well as 11 subbands with A₁A₂ splittings. We have attempted to fit all these data in a global model of the “limited-matrix” type including the 6 × 6 resonance due to ζ₂₅^ν, two Fermitype resonances linking ν₁ with 2ν₅⁰ and 2ν₂, respectively, a Coriolis resonance between ν₄ and 2ν₅², and other minor interactions. We conclude that the use of an “infinite-chain” program is unavoidable. We have also identified 80 transitions as belonging to the ν₄ band of ¹³CH₃F occurring in natural abundance.     

18.

FIR Stark spectroscopy on optically pumped CH₃OH laser     

M. Ingusio  A. Moretti  G. Moruzzi  F. Strumia 《International Journal of Infrared and Millimeter Waves》1981,2(5):943-986

An extensive theoretical and experimental analysis of the absorption and emission spectrum of a CH₃OH FIR-laser excited by a conventional CO₂ laser is presented. Particular interest is devoted to the Stark shifts of the pump and lasing lines and to the electric field dependence of the Fir-laser output of the various lines. The offsets with respect to the exciting radiation and the Stark shifts of the IR absorption (pump) lines are measured by means of the transferred Lamb dip technique. The theoretical behaviours of the Stark patterns are calculated for several choices of the quantum numbers and selection rules involved in the transitions. A large variety of experimental results are reported and compared to theory. Non-linear Stark shifts have been observed for the 37.5m FIR laser line and for the IR-pump transitions excited by the 9-P(38) and 10-R(38) CO₂ laser Lines. Line assignments are proposed and new FIR laser lines are reported.     

19.

New far-infrared laser lines produced by resonant Q-branch-pumping of CH₃F     

P. T. Lang  F. Sessler  U. Werling  K. F. Renk 《International Journal of Infrared and Millimeter Waves》1989,10(5):529-534

Conclusion We showed that by pumping the ₃-Q-branch of CH₃F using the 9P-branch radiation produced by a continuously tunable 20 atmospheres CO₂ laser all Q-branch transitions with quantum number 5J28 can be pumped. From the 23 laser lines with frequencies from 10 cm^–1 to 47 cm^–1, 13 were detected for the first time. The optimum operating pressure for the FIR laser was 11 Torr.     

20.

Emission spectra of the tunable Raman CH₃F optically pumped fir laser     

V. A. Batanov  A. O. Radkevich  A. L. Telyatnikov  A. Yu. Volkov 《International Journal of Infrared and Millimeter Waves》1990,11(1):31-47

The six-level model using a density matrix approach to simulate properties of a gas resorant media in optically pumped FIR lasers was developed. The model accounts for multiphoton as well as parametric processes. Advantages of the developed model are demonstrated by calculation and discussion of the gain and saturated emission spectra of CH₃F laser when transitions with different J rotational numbers of CH₃F are pumped.     

	12CH3F	13CH3F
$\text{ν}{}_0$	1048.610767 (62)	1027.493191 (69)	cm?1
$\text{B}$	25197.57 ± 0.03	24542.07 ± 0.43	MHz
$\text{A - A}{}_0$	?294.09 ± 0.60	?288.81 ± 0.37	MHz
$\text{D}{}_{\mathrm{J}}$	55.5 ± 1.2	56 ± 12	kHz
$\text{D}{}_{\mathrm{JK}}$	575 ± 63	464 ± 24	kHz
μ	1.9054 ± 0.0006	1.9039 ± 0.0006	$\text{D}$

A study on the GVFF of CHF3, CH2F2, and CH3F

The complete GVFF of CHF₃, CH₂F₂, and CH₃F has been calculated from self-consistent-field ab initio energies, using a 4–31 G basis set. The larger part of the interaction force constants is close to those of the best available force fields from experimental data. Only one interaction term in CH₃F and the interaction force constants of the A₁ species in CH₂F₂ differ appreciably from the experimental ones. Using constraints from the ab initio studies we have improved the GVFF of CH₃F and CH₂F₂. It is shown that all comparable stretch-stretch interaction terms are of the same order of magnitude in the three molecules. The sign of all stretch/bend force constants are in accordance with those predicted by the hybrid orbital force field.     

Level crossing in microwave spectroscopy of CH3F

Stark tuned level crossing resonances have been detected in CH₃F through non-linear microwave absorption. In our experiments, the level crossing width is about ten times smaller than the Doppler width.     

Saturable absorption of CO2-laser radiation by CH3F

A theory of the saturable absorption of pulsed CO₂-laser radiation by CH₃F is described. It is valid for arbitrary pulse shapes and durations in the rate-equation regime. The theoretical predictions agree satisfactorily with available experimental data. The effect of vibrational exchange and the influence of the Boltzmann population factor on transmission are discussed.     

Relation between electrooptical constants and polar tensors of CH3F,CD3F,CH3Cl,CD3Cl,CH3Br,CD3Br,CD3I,and CD3I

Infrared intensity analysis was done in two ways. In one approach, formulas connecting electrooptical parameters (eop's) and intensities were derived and, using the intensities, the former were obtained. In evaluating the eop's the intensity sum equations were used directly so that the sign ambiguity of $\text{δP}\text{δQ}$ is avoided. In the other method, polar tensors were obtained directly from intensities. The two methods have now been connected, and polar tensors have been obtained in terms of electrooptical parameters so that the former were also free from sign ambiguity of $\text{δP}\text{δQ}$. They compare well with those obtained directly from intensities. In similar connecting formulas by other authors [see, for example, P. L. Prasad, J. Chem. Phys., 69, 4403, (1978)], both polar tensors and eop's were affected by the sign ambiguity.     

交叉分子束方法研究H+CH4和H+CD4反应:CH3/CD3的振动激发态产物通道

We study the H+CH₄/CD₄→H₂/HD+CH₃/CD₃ reactions using the time sliced velocity map ion imaging technique. Ion images of the CH₃/CD₃ products were measured by the (2+1) resonance enhanced multi-photon ionization (REMPI) detection method. Besides the CH₃/CD₃ products in the ground state, ion images of the vibrationally excited CH₃/CD₃ products were also observed at two collision energies of 0.72 and 1.06 eV. It is shown that the angular distribution of the products CH₃/CD₃ in vibrationally excited states gradually vary from backward scattering to sideways scattering as the collision energy increases. Compared to the CH₃/CD₃ products in the ground state, the CH₃/CD₃ products in vibrationally excited states tend to be more sideways scattered, indicating that larger impact parameters play a more important role in the vibrationally excited product channels.     

Line-mixing between rotational Stark components of CH3F self-perturbed and perturbed by helium: Experimental results and IOS analysis

Self- and He-broadening coefficients of microwave transitions of CH₃F have been measured with and without the presence of an external electric field. This provides values for the J, K → J + 1, K (K = 0 − J) transitions for J = 1 and J = 3 as well as for the various J, K, M → J + 1, K, M′ (|M| = 0 − K, |M - M′| = 0, 1) Stark components. The results and those of a previous experimental study for pure CH₃F, which show significant line-mixing effects, are analyzed with a model based on the Infinite Order Sudden approximation. It is shown that the latter leads to very satisfactory modeling of observed values even though no parameter was adjusted since previously and independently determined basic cross-sections are used. The quality of the present predictions is comparable with that obtained previously with a semi-classical approach. Furthermore, it is shown that the previously stated inaccuracy of the IOS model was due to an oversimplified use of this approach.     

10 kW cavity operation of a submillimetre CH3F laser

Operation of a CH₃F laser at 496μ has been obtained in a hemispherical optical cavity pumped by up to 42 J of 9.55μ CO₂ TEA laser radiation. Highly modulated 1μ pulses delivering 6 mJ at peak powers greater than 10 kW have been recorded.     

Short wavelength operation of the CH3F Raman FIR laser

Using the isotopic species¹³CH₃F in the methyl-fluoride Raman laser, we have observed kilowatt-level laser pulses which can be tuned over a series of intervals that are centered on pure-rotational transitions in thev ₃ ground state with initial-state J values between 35 and 43, inclusively. Taken together, these tuning intervals cover 35% of the spectrum between 142 and 174 m. Several straightforward improvements in the experimental setup, including the use of isotopically purer¹³CH₃F, should enhance the spectral coverage in this region.     

Extensive high-resolution study of the crowded rovibrational CH3F spectrum around 3000 cm−1

The infrared spectrum of CH₃F has been remeasured from 2800 to 3160 cm^?1 with a Fourier spectrometer at a resolution of 0.005 cm^?1. By a systematic use of ground-state combination differences, we have assigned 5200 transitions, including several hundred “forbidden” transitions. They can be distributed into seven vibrational bands, which are mixtures of ν₄, ν₁, 2ν₅², 2ν₅⁰, ν₂ + ν₅, 2ν₂, and 3ν₃. More than 20 local resonances have been detected as well as 11 subbands with A₁A₂ splittings. We have attempted to fit all these data in a global model of the “limited-matrix” type including the 6 × 6 resonance due to ζ₂₅^ν, two Fermitype resonances linking ν₁ with 2ν₅⁰ and 2ν₂, respectively, a Coriolis resonance between ν₄ and 2ν₅², and other minor interactions. We conclude that the use of an “infinite-chain” program is unavoidable. We have also identified 80 transitions as belonging to the ν₄ band of ¹³CH₃F occurring in natural abundance.     

FIR Stark spectroscopy on optically pumped CH3OH laser

An extensive theoretical and experimental analysis of the absorption and emission spectrum of a CH₃OH FIR-laser excited by a conventional CO₂ laser is presented. Particular interest is devoted to the Stark shifts of the pump and lasing lines and to the electric field dependence of the Fir-laser output of the various lines. The offsets with respect to the exciting radiation and the Stark shifts of the IR absorption (pump) lines are measured by means of the transferred Lamb dip technique. The theoretical behaviours of the Stark patterns are calculated for several choices of the quantum numbers and selection rules involved in the transitions. A large variety of experimental results are reported and compared to theory. Non-linear Stark shifts have been observed for the 37.5m FIR laser line and for the IR-pump transitions excited by the 9-P(38) and 10-R(38) CO₂ laser Lines. Line assignments are proposed and new FIR laser lines are reported.     

New far-infrared laser lines produced by resonant Q-branch-pumping of CH3F

Conclusion We showed that by pumping the ₃-Q-branch of CH₃F using the 9P-branch radiation produced by a continuously tunable 20 atmospheres CO₂ laser all Q-branch transitions with quantum number 5J28 can be pumped. From the 23 laser lines with frequencies from 10 cm^–1 to 47 cm^–1, 13 were detected for the first time. The optimum operating pressure for the FIR laser was 11 Torr.     

Emission spectra of the tunable Raman CH3F optically pumped fir laser

The six-level model using a density matrix approach to simulate properties of a gas resorant media in optically pumped FIR lasers was developed. The model accounts for multiphoton as well as parametric processes. Advantages of the developed model are demonstrated by calculation and discussion of the gain and saturated emission spectra of CH₃F laser when transitions with different J rotational numbers of CH₃F are pumped.