Rotational relaxation time in CO free jets

The electron beam fluorescence technique was applied to measure the rotational energy relaxation cross-sections in carbon monoxide free jets. The data were obtained in the low temperature region from 20 to 200 K and compared with the results of ultrasound, thermal conductivity, free jet and line broadening experiments. The overall scatter of the experimental data runs to one order of magnitude, and their temperature dependence differs. This fact calls for further theoretical and experimental investigations of rotational relaxation rates in carbon monoxide.     

CO_2与高振动激发K_2碰撞中的全分辨转动态分布

在K_2+CO_2中,受激发射泵浦得到K_2(E=3 500和4 000cm~(-1))高位振动态,研究了高振动激发K_2与CO_2碰撞产生的CO_2全分辨转动态分布。利用高分辨瞬时激光诱导荧光(LIF)测量了CO_2(0000)J=2~74的转动和平移能量轮廓,利用双高斯函数拟合,分别确定各转动态的产生和倒空线宽,从而得到碰撞产生的Doppler展宽、平移温度和平移能。对于K_2不同的激发能E,能量转移的机制是相似的,为振动-转动/平移弛豫机制。但碰撞出现部分的平移温度均超出池温,而碰撞倒空部分的平移温度均略低于池温,平移能随E的增加而增大,E增加14%,平移能增加40%。CO_2(0000)转动态分布的半对数描绘给出了双指数分布,对于K_2E=3 500cm~(-1),低J态分布T_a=(523±60)K,高J态分布T_b=(1 890±210)K。Ta接近池温,说明低J态为近弹性碰撞,属单量子弛豫过程,而高J态为非弹性碰撞,属多量子驰豫过程。对于K_2E=4 000cm~(-1)同样有双指数行为,低J分布T_a=(620±65)K,高J分布T_b=(2 240±250)K。高振动态K_2(E)与CO_2碰撞,E=4 000cm~(-1)比E=3 500cm~(-1)的Ta和Tb均约高19%,说明转动分布对于K_2不同能量是敏感的,但弹性和非弹性分支比是基本相同的,弱碰撞约占82%,强碰撞约占18%。     

Shape and width of IR absorption lines of ammonia expanded in a supersonic jet

Line-width and-shape of absorption lines of the ₂ vibration of ammonia were measured for a series of transitions starting from vibrational groundstate levels with a different rotational energy. An increase in linewidth was found for larger rotational energies of the initial states. This effect has been attributed to the spatial rotational non-equilibrium in the supersonic jet expansion.For expansion of NH₃ seeded in He and Ar a very pronounced dip in the absorption profile has been observed, due to the formation of clusters.     

Infrared spectra of the Kr—CO and Xe—CO van der Waals complexes

The infrared spectra of the weakly bound complexes Kr—CO and Xe—CO have been studied in the region of the CO stretching vibration (4.7 μm) using a high-resolution tuneable diode laser probe. The complexes were observed in a long path (200 m) low temperature (76 K) gas cell (Kr—CO) and in a pulsed supersonic jet expansion (Kr—CO and Xe—CO). Previous long path cell measurements on these complexes at lower resolution analysed only the K = 0 and 1 stacks of rotational levels in the ground intermolecular vibrational state. The new data extend up to K = 3 (Xe—CO) or 4 (Kr—CO), and also include K = 0 and 1 stacks in the excited bending state, ν₂ = 1. The bending frequencies for Kr—CO and Xe—CO (in the ν_co = 1 upper state) were determined to be 13.156cm_?1 and 13.794cm^?1, respectively. Detailed molecular parameters were determined to describe the rotational energy levels of each complex using a simple empirical Hamiltonian. These results enable parameters to be compared for the entire series of rare gas—carbon monoxide complexes, from He—CO to Xe—CO. Also they will guide the future development and evaluation of accurate intermolecular potential energy surfaces for Kr—CO and Xe—CO.     

Coupled cluster ab initio potential energy surfaces for CO… He and CO… H2

Ab initio potential energy surfaces including the vibrational coordinate dependence are presented for CO… He and CO… H₂ using the coupled cluster method with Brueckner orbitals. The interaction energy is calculated using the supermolecule approach. The calculation of rate constants for the vibrational relaxation of CO(v = 1) by He and their comparison with the experimentally measured values over the temperature range 40–300 K is used to test the accuracy of the CO… He surface. Comparison with results from an earlier surface calculated by symmetry adapted perturbation theory shows that the two surfaces have similar scattering characteristics and reproduce the experimental measurements to a similar level of accuracy. The potential surface for the CO… H₂ system is presented as raw data in anticipation of future calculations.     

Diode laser study of IR multiphoton-induced depletion of rotational sublevels of the ground vibrational state of SF6 molecules cooled in a pulsed free jet

A pulsed frequency tunable diode laser was used to investigate the IR multiphoton-induced depletion of rotational sublevels of the ground vibrational state of SF₆ molecules cooled in a pulsed free jet at exciting energy densities between ≈10^-2 and 2.3 J cm^-2. The depletion of all rotational sublevels was effective at considerable (≈5–11 cm^-1) pumping frequency detunings from the linear absorption spectrum (LAS) of the molecule the width of which under the conditions of experiment (T_rot ≈ 18 K) was ≈2–3 cm^-1. The fraction of molecules excited by a pumping pulse from individual rotational sublevels was measured and its dependence on the exciting pulse frequency and energy density investigated. The effect of collisions on the depletion of the rotational sublevels was studied.     

Ultrasonic absorption in aqueous solutions of amino acids at neutral pH

Ultrasonic absorption coefficients in aqueous solutions of glycine, L-alanine, imidazole, L-phenylalanine, L-histidine and L-tryptophan at neutral pH were measured in the range from 0.8 to 220 MHz at 25 degrees C. A characteristic ultrasonic relaxation phenomenon was observed only in the solution of L-histidine with a relaxation frequency at around 2 MHz at neutral pH. It was proposed from the concentration independent relaxation frequency and the linear concentration dependence of the maximum absorption per wavelength that the relaxation mechanism was associated with a perturbation of the rotational isomeric equilibrium of the L-histidine molecule. The existence of two rotational isomeric forms of L-histidine in water was examined by semiempirical quantum chemical methods, in order to determine the free energy difference between the two states. The forward and backward rate constants were determined from the relaxation frequency and the energy change. Also, the standard volume change of the reaction was estimated from the concentration dependence of the maximum absorption per wavelength. It was speculated that L-histidine fulfills a specific function among amino acids because of the rotational motion in the molecule, in addition to its well-established acid-base properties.     

CO_2在与高振动激发NaH碰撞中的量子态分辨能量再分配

利用简并受激超拉曼泵浦激发NaH基态到高位振动态(ν″=14,J″=20)。研究了NaH(14,20)与CO_2(00°0)间的振转能量转移。利用吸收系数和瞬时Doppler线宽,得到不同池温下NaH(14,20)分子密度,测量CO_2(00°0,J)与NaH高振动态碰撞前后的瞬时泛频激光感应荧光谱线的相对强度,确定了CO_2(00°0,J=2~80)的初生态布居,它们呈现双指数转动分布。拟合实验数据得到两个转动温度T_(rot)=(650±80)和(1 531±150)K。较冷的分布约占CO_2(00°0)的79%,它是由弹性或弱非弹性碰撞产生的,因而CO_2只有很小的转动激发。另有21%的CO_2(00°0)较大地增加了转动能,故有较热的转动温度。对碰撞产生的CO_2(00°0,J)进行高分辨率瞬时泛频荧光谱线的轮廓测量,得到各转动态平移能的改变。对于CO_2(00°0,J=56~80),转移能从582cm~(-1)(对于J=60)增加到2 973cm~(-1)(对于J=80)。探测转动态布居数的改变,得到各转动态的产生速率系数k_(app)~J之和为(7.2±1.8)×10~(-10) cm~3·mol~(-1)·s~(-1),而平均倒空速率系数〈kdep〉=(6.9±1.7)×10~(-10)cm~3·mol~(-1)·s~(-1)。     

高振动激发LiCs与CO_2碰撞中的振动-转动能量转移

受激发射泵浦得到的LiCs高位振动态与CO_2碰撞,研究了LiCs不同能量对碰撞能量转移过程的影响.利用高分辨率瞬时激光感应荧光(LIF)测量得到CO_2(0000,J=2-74)原生转动态分布.转动布居数的半对数描绘给出了一个双指数结构,得到了一个T_a=660±73K[对LiCs(E=4300cm~(-1))]和550±61K[对LiCs(E=3700cm~(-1))]的低能分布,一个T_b=2380±261K[对LiCs(E=4300cm~(-1))]和1980±217K[对LiCs(E=3700cm~(-1))]的高能分布,低能分布属于弹性或弱非弹性碰撞,高能分布属于强非弹性碰撞.转动分布对LiCs能量是敏感的.但弹性与非弹性分支比基本是相同的.在一次碰撞的条件下,测量了各J态的出现和倒空速率系数,它们仅弱依赖于LiCs激发能.出现和倒空速率系数对不同LiCs能量基本一致,是振动-转动/平移弛豫轨道的重要证据.     

Magnetic trapping and Zeeman relaxation of NH (X3Sigma-)

NH radicals are magnetically trapped and their Zeeman relaxation and energy transport collision cross sections with helium are measured. Continuous buffer-gas loading of the trap is direct from a room-temperature molecular beam. The Zeeman relaxation (inelastic) cross section of magnetically trapped electronic, vibrational, and rotational ground state NH molecules in collisions with 3He is measured to be 3.8+/-1.1 x 10(-19) cm(2) at 710 mK. The NH-He energy transport cross section is also measured, indicating a ratio of diffusive to inelastic cross sections of gamma=7 x 10(4), in agreement with recent theory [R. V. Krems, H. R. Sadeghpour, A. Dalgarno, D. Zgid, J. K?os, and G. Cha?asiński, Phys. Rev. A 68, 051401 (2003)10.1103/PhysRevA.68.051401].     

The electron beam technique in carbon monoxide: Rotational transition probabilities

The electron beam fluorescence technique was developed to measure the rotational level population in the ground state of a carbon monoxide molecule. The data on rotational transition probabilities under ionization–excitation of CO (X¹Σ⁺) into the CO⁺ (B²Σ⁺) state by high energy electrons are presented, which noticeably differ from those predicted by the dipole excitation model. A flow in a free jet was used to obtain the experimental data in a low temperature rotationally equilibrium gas target. The probability matrix is represented in a compact form corresponding to the sudden approximation model.     

Rotational energy relaxation in CH4 and CH4-He,Ar collisions calculated from coherent and stimulated Raman spectroscopy data

Theoretical treatment of the evolution of the methane Q branch contour with pressure is given in the framework of a quantum model of rotational relaxation with allowance for the tetra-hedral symmetry of rotational levels. The results obtained are compared in detail with available experimental data. As a result, rotational energy relaxation cross-sections for methane and its mixtures with helium and argon have been determined.     

Double electron capture cross-sections by protons in collisions with He

Double electron capture (DEC) by protons from He was studied in collisions at energies in the keV regime, theoretically and experimentally. Theoretical cross-sections were calculated using the electron nuclear dynamic approach and a binomial distribution method in the energy range 1–100 keV. Experimental cross-sections were measured by means of the grow-rate method in the energy range of 4–10 keV. The cross-sections measured are consistent with those of the previous data at the high energies of the measured interval and show a different trend for the low energy. This behavior is consistent with those of other DEC cross-sections measured by protons from He-like targets. With the two collision models, it is proven that the reference data for this system are consistent only with the assumption of uncorrelated electrons and with independent target-projectile nuclei dynamics.     

Interaction potentials and rotational scattering for H2 and MuH with He

Elastic and inelastic differential cross-sections for the rotational scattering of p-H₂ and MuH by He at a single collision energy are calculated with potential surfaces obtained by varying the parameters in a model potential. The importance of the attractive and repulsive terms in the potential, and of the available kinetic energy in the different channels is demonstrated, and it is shown that the whole potential surface determines the scattering cross-sections. Inelastic and elastic scattering are compared, and the differences between the rotational scattering of homonuclear and heteronuclear homopolar molecules are ascribed to the molecular constants rather than to the numbers of open channels.     

Collisional depolarization of OH(A) studied by Zeeman quantum beat spectroscopy

The depolarization of the rotational angular momentum of electronically excited OH(2Σ) radicals through collisions with water molecules has been measured using Zeeman quantum beat spectroscopy. The new data have permitted the evaluation of OH(A) state-specific quenching and angular momentum depolarization cross-sections for superthermal OH(A) radicals with mean relative velocities centred around 3500 m s-1. The quenching cross-sections are compared both with values available in the literature, and with predictions based on a simple harpoon model, and are found to be in good qualitative accord with previous findings. For the lowest rotational levels studied, the depolarization cross-sections (which include contributions from both elastic and inelastic processes in OH(A)) are found to approach ～100 Å2, only slightly below the high-temperature cross-sections for rotational energy transfer determined elsewhere. The data suggest that under the present conditions rotational energy transfer is accompanied by significant depolarization. The cross-section for the translational moderation of superthermal OH(X) by water is also determined in the present study.     

Quantum calculations of rotational and NMR relaxation,depolarized Rayleigh and rotational Raman line shapes for H2(HD)−He mixtures

The two ab initio vibrotor potentials of H₂?He previously published by Meyer, Hariharan, and Kutzelnigg have been composed in one potential grid. After using this potential in close coupled scattering calculations we have subsequently obtained rotational relaxation cross sections for para- and ortho-H₂, NMR relaxation times for ortho-H₂, depolarized Rayleigh line broadening cross sections for para-, ortho-, and normal-H₂, and rotational Raman line width and shift cross sections for S₀(0), S₀(2) transitions, at temperatures between 20 and 450 K. The agreement with previous and more recent measurements has been found completely satisfying, with the exception of the rotational relaxation cross sections of para-H₂. Throughout the paper previous results derived from the multiple property fit of Shafer and Gordon have been included for comparison. The new ab initio potential is obviously better, except for the rotational relaxation cross sections. The interaction potential of HD?He transformed from the new H₂?He potential is also presented in this paper. It has been used recently in a very successful application of the Waldmann-Snider kinetic theory describing transport, relaxation, and reorientation phenomena in magnetic fields. Therefore, only the rotational relaxation cross sections, converged up to about 400 K, have been presented in this paper.     

Transport properties of H–N2 mixtures

Transport coefficients (shear viscosity, volume viscosity, thermal conductivity, and mass and thermal diffusion coefficients) of H–N₂ mixtures in the dilute-gas limit have been calculated from the intermolecular potential in the temperature range 300–2000K using the classical trajectory method. The intermediate results pertaining to H–N₂ binary interactions are reported, mainly in terms of cross-section ratios. Cross-sections evaluated with the Mason–Monchick approximation yield very good results for this system, the largest deviations, about 2.5%, being observed for the thermal diffusion coefficient. The accuracy here of this approximation can primarily be attributed to a light H atom and a weakly non-spherical potential resulting in a high rotational collision number. Furthermore, we investigate to which H–N₂ cross-sections and their ratios the values of the mixture transport coefficients are most sensitive. Our results indicate that, for some cross-section ratios, reliance on universal correlations at high temperatures, often used in flame codes, can induce sizeable errors in the thermal conductivity coefficient and especially in the thermal diffusion coefficients. We also observed that the volume viscosity is particularly sensitive to the value of the cross-section for internal energy relaxation in H–N₂ collisions.     

Specific heat of solid deuterium in the region of the proposed rotational glass phase

The specific heat of solid deuterium of 33% paradeuterium concentration has been measured for 1 K > T > 0.115K by a thermal relaxation technique. No evidence of a phase transition was found in the samples in the temperature range associated with the molecular rotational glass phase. No thermal remanence effects were observed for characteristic measurements times greater than 30 seconds. Integration of the specific heat indicates that rotational entropy of the solid below 0.35 Kelvin equals about 10% of the free rotator entropy, in agreement with ?P/?T)_V studies of solid H₂.     

The 3ν3 overtone bands of UF6 and UF6

The absorption specta of CO laser radiation by 3ν₃ overtone bands of ²³⁸UF₆ and ²³⁵UF₆ has been measured using photoacoustic spectroscopic techniques. For the two temperatures 250 K and 290 K, measured absorption coefficients and cross-sections of the 3ν₃ band of ²³⁵UF₆ are reported for the first time.     

Corona discharge as a temperature probe of atmospheric air microwave plasma jet

We developed and tested a new method for temperature measurements of near-LTE air plasmas at atmospheric pressure. This method is specifically suitable for plasmas at relatively low gas temperature (800–1700 K) with no appropriate radiation for direct spectroscopic temperature measurements. Corona discharge producing cold non-equilibrium plasma is employed as a source of excitation and is placed into the microwave plasma jet. The gas temperature of the microwave plasma jet is determined as the rotational temperature of N₂? produced in the corona discharge. The corona probe temperature measurement was tested by the use of a thermocouple. We found a fairly good agreement between the two methods after correcting the thermocouple measured temperatures for radiative losses. The corona probe method can be generally applied to determine the temperature of the near-LTE plasmas and contrary to the thermocouple it can be used for higher plasma temperatures and is not affected by radiative losses and problems of interaction with the microwave plasma and electromagnetic fields.