Relaxation Cross Sections Research Articles

Surface temperature effects on the dynamics of H − low energy electron stimulated desorption from diamond films

In this work, we report on the influence of surface temperature on low energy H − electron stimulated desorption (ESD) occurring via dissociative electron attachment (DEA) from hydrogenated diamond films. By measuring the H − kinetic energy distribution (KED) induced by electron bombardment in the 7–18 eV range for surface temperatures ranging between 100 and 450 K we investigate the dynamics of the desorption process. It is determined that the H − ion yield continuously decreases with increasing temperature and that the most probable H − kinetic energy shifts to lower energies. It is proposed that the effect of temperature on the H − KED and consequently on the reduction in ion yield is due to an increase in the energy relaxation cross-section of the anion resonance and energy losses of the outgoing H − ion through interactions with the solid's multi-phonon background and collisions.

Bulk rotational relaxation time and cross-section in gas mixtures

In an earlier paper [M. Terzić, J. Jovanović-Kurepa, D.D. Markušev, J. Phys. B: At. Mol. Opt. Phys. 32 (1999) 1193] we have presented a method for evaluation of the bulk rotational relaxation times and cross-sections in gas mixtures from the measured saturation intensity, I S, vs. buffer gas pressure, p buff, at constant incident laser energy. According to this method, the bulk rotational parameters were determined from the slope, I S/ p buff (“the slope method”). In this work, we report an alternative method to determine the same parameters, which does not require the knowledge of the functional dependence of I S vs. p buff. The method, which we propose, allows the evaluation of bulk rotational relaxation parameters from measured I S at a selected p buff. Hence, we termed this method “the point method”. Both methods are tested with a calibrated photoacoustic technique for SF 6–CH 4 gas mixtures at constant SF 6 gas pressure (0.47 mbar) and variable CH 4 gas pressure (0–140 mbar). The gas mixtures were irradiated by a pulsed 10P(16) CO 2 laser line in fluence range 0.1–0.5 J cm −2. The accuracy of both methods is checked by quantitative comparison of the data with fitting laws. The result obtained shows that the proposed point method gives higher accuracy of the obtained parameters. The influence on the accuracy of the saturation intensity and of the cross-section of the absorbing molecule (SF 6) on rotational relaxation parameters are discussed also.

Vibrational and rotational energy transfer in collisions of vibrationally excited HF molecules with Ar atoms

This work presents converged vibrational close coupling-rotational coupled states calculations of cross sections and rate constants for rotational and vibrational transitions in collisions of vibrationally excited HF molecules with Ar atoms. Reduced channel basis sets assuming both a lower and an upper cutoff in vibrational quantum number are used for calculations at high internal energies of the diatomic molecule. The most recent potential energy surface is employed for the calculations and the correspondence of the results to the previous investigation of rovibrational dynamics in collisions of HF(v=1) with Ar is examined. It is shown that initial vibrational excitation stimulates vibrationally inelastic transitions to a great extent while the rotational energy transfer is essentially unaffected by initial v-number. The cross sections for vibrational relaxation of different vibrational levels of HF are shown to exhibit a strong dependence on initial rotational energy which is, however, of different magnitude for different vibrational states. The dependence of the vibrational relaxation of HF(v=1,3,6) on the translational energy of the atomic collision partner is different by an energy independent factor for different vibrational levels in the high energy limit and shows minima at low collision energies.

Low-Temperature Rotational Relaxation of N2 in Collisions with Ne

The rotational relaxation of nitrogen molecules in collisions with neon has been studied in supersonic expansions. N2 rotational temperatures have been determined from resonance-enhanced-multiphoton-ionization (REMPI) spectra performed in a series of supersonic molecular beams of N2 diluted in Ne. Terminal flow velocities and translational temperatures for the two expansion partners have been obtained from mass-selected time-of-flight measurements. From the measured data, the approximate isentropic behavior of the expansion could be verified and the thermal cross section for N2/Ne rotational relaxation at very low temperatures could be derived. The cross section increases from a value of ≈35 Å2 at 5 K, goes through a maximum of about 55 Å2 between 10 and 20 K, and then decreases smoothly to ≈25 Å2 for T = 60 K. The relaxation cross section for N2/Ne collisions is larger than that for N2/N2 collisions below roughly 20 K and smaller above this temperature.

Exchange-Coulomb potential energy surfaces and related physical properties for Ne-N2

An exchange-Coulomb (XC) potential energy model is developed for the Ne-N2 interaction. The construction of this new potential energy surface is based on recent results for the Heitler-London interaction energy, the long range dispersion energies, and the microwave spectra of the dimer. The adjustable parameters in the final XC1 potential energy surface have been determined by fitting the frequencies of three representative lines of the microwave spectrum for the two isotopomers 20Ne-14N2 and 20Ne-15N2 while simultaneously maintaining agreement with the experimental second virial coefficient data obtained with the initial (unadjusted XC0 potential. With no further adjustment of parameters, the final XC potential reproduces, within 0.005%, the frequencies of the 34 microwave transitions studied experimentally for four isotopomers of Ne-N2. Excellent agreement with experiment is obtained also for the binary diffusion, the interaction viscosity, and the mixture viscosity (for all compositions) coefficients for all temperatures. Agreement with experiment for the relaxation cross-sections associated with viscomagnetic effects, and with the pressure broadening of depolarized Rayleigh light scattering, is very good given the level of computation used for the calculations and the accuracy with which some of these cross-sections currently can be determined from experimental data. Comparisons are made with predictions calculated from the three best literature potential energy surfaces or Ne-N2. The final XC1 potential energy surface is overall the most reliable potential energy for this van der Waals complex to date. The flexibility still inherent in this potential can be exploited, if required, in future studies of the Ne-N2 system.

Rotational relaxation rates in CO-He mixtures

The rotational level populations of CO molecules were measured in CO(<10%) + He free jets by electron beam fluorescence (in a stationary jet) and resonantly enhanced multiphoton ionization (in a pulsed jet). The measured evolution of the non-equilibrium rotational energy was used to derive the rotational relaxation cross-sections in the temperature range from 6 K to 140 K. To compare and analyse on a common basis all available experimental data (ours and others) on rotational relaxation of CO in He, the infinite order sudden approximation was explored. The following quantities were investigated: integral rotationally inelastic cross-sections, state-to-state rate coefficients, rotational relaxation times, line broadening coefficients, and non-equilibrium rotational energy distributions in a free jet.

Millimeter-wave time-resolved studies of HCO +-H 2 inelastic collisions

A novel ion cell has been constructed for the purpose of studying rotationally inelastic collisions involving truly thermal molecular ions at low temperatures. With this ion cell, time-resolved double resonance (pump-probe) spectroscopic experiments have been performed to determine the cross sections for relaxation of the J=2 state of HCO + in collisions with normal-H 2 at temperatures around 40 and 77 K. The HCO + is pumped through the J=2←1 transition and probed via the J=3←2 transition. The cross sections at the lower temperature are slightly below those predicted by the simple Langevin theory, while those at the higher temperature are in good agreement with this theory.

Quantum-mechanical study of vibrational relaxation of HF in collisions with Ar atoms

Vibrational relaxation cross sections and rate constants of HF(v=1) by Ar are calculated on a recent semiempirical potential energy surface (PES) [J. Chem. Phys. 111, 2470 (1999)] using the quantum-mechanical coupled states approach. Accurate theoretical estimations of rate coefficients for vibrational relaxation of HF(v=1) at temperatures between 100 and 350 K are obtained. The vibrational relaxation is shown to be of a quasiresonant character and occur mostly to two nearest rotational levels of the ground vibrational state. The weak isotope effect after substitution of HF by DF is investigated and explained. The cross sections for vibrational relaxation of HF(v,j=0), where v=1,2,3,4, are calculated and shown to increase significantly as v increases. In the same calculations we observe a dramatic increase of multiple quantum vibrational transitions as the difference between the initial and final states falls in close resonance with the collision energy. A comparison of the cross sections obtained from the coupled states calculations with those performed with rotational infinite-order-sudden approximation proves a crucial role of molecular rotations for vibrational relaxation. Finally, we describe the close coupling coupled states calculations for relaxation and rotational excitation of HF(v=1, j=0) with a reduced number of open channels in the basis set and show that it is possible to obtain converged results for rotationally inelastic transitions between the various levels of v=1 neglecting all states below v=1, j=0.

Extension of the energy-corrected sudden model to anisotropic Raman lines: Application to pureN2

The Energy-Corrected Sudden (ECS) model is a well-established dynamical model to calculate rovibrational isotropic Raman linewidth and also to describe overlapping between lines through the off-diagonal elements of the relaxation matrix. The extension of this model to anisotropic Raman or infrared lines has been previously performed through the expansion of the tetradic (four indices) relaxation supermatrix elements in terms of dyadic (two indices) mechanical cross sections. This model required new parameters in the diagonal elements of the dyadic matrices that are the relaxation cross sections of rotational angular momentum and of its associated second-order tensor. In this paper, a new approach of the extension of the ECS model for anisotropic Raman and infrared branches is proposed, and developed on a strong physical basis. This new model only needs the original ECS parameters determined through a fit of the experimental isotropic Raman Q linewidths to generate anisotropic Raman and infrared linewidths as well as line mixing coefficients.

Vibrational relaxation of CO by collisions with 11He at ultracold temperatures

Quantum mechanical coupled channel scattering calculations are performed for the ro-vibrational relaxation of CO in collisions with ultracold He atoms. The van der Waals well in the interaction potential supports a number of shape resonances which significantly influence the relaxation cross sections at energies less than the well depth. Feshbach resonances are also found to occur near channel thresholds corresponding to the j=1 rotational level in the v=0 and v=1 vibrational levels. Their existence influences dramatically the limiting values of the elastic scattering cross sections and the rotational quenching rate coefficients from the j=1 level. We present complex scattering lengths for several low lying rotational levels of CO which characterize both elastic and inelastic collisions in the limit of zero temperature. Our results for the vibrational relaxation of CO (v=1) are in good agreement with available experimental and theoretical results.

Collision relaxation cross section of highly vibrationally excited molecules

Through quantum-beat spectroscopy collision relaxation of a high vibrational level of SO2 at 44 877.52 cm(-1) is characterized. This is a first measurement of collision relaxation for a single, highly excited vibrational level. The deduced relaxation cross section of this excited level by Ar is 216 A(2), 5 times the area of the hard sphere, and by an ambient temperature SO2 molecule is 969 A(2), almost 20 times the hard sphere. These cross sections indicate that relaxation collisions of highly vibrationally excited molecules have effective distances much longer than van der Waals radii and involve mechanisms qualitatively different from lower excitations.

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.

State-to-state rate coefficients for rotational relaxation of CO in Ar

The rotational level populations of CO molecules were measured in CO (&amp;lt;10%)+Ar free jets by electron beam fluorescence (in a stationary jet) and resonantly enhanced multiphoton ionization (in a pulsed jet). The measured evolution of the nonequilibrium rotational energy was used to derive the rotational relaxation cross sections in the temperature range from 7 to 150 K. Using the pressure-broadened linewidth data the state-to-state rotational relaxation rate coefficients were found using the most popular fitting law forms: The modified exponential gap, the statistical polynomial-exponential gap, and the energy corrected sudden approximation. The fitted rates were compared with the experimental and theoretical data presently available in the literature. They were checked also by consideration of the rotational kinetics in free jets and by comparison between the computed and experimental rotational level populations both in near and far regions of a flow.

Temperature dependence of the rotational relaxation cross-section in carbon monoxide

This Letter presents an investigation into the temperature dependence of the relaxation cross-section of the mean rotational energy, σ rot, in carbon monoxide. Our data obtained in a free jet at 20–200 K together with high-temperature measurements made by other authors point to non-monotonic behavior of the rotational relaxation cross-section as a function of temperature. A theoretical explanation for such a behavior of σ rot with temperature is offered.

Upper Stark State Collisional Relaxation Cross Sections for Single Rotational States of CH3F

A radiofrequency electric resonance cell has been used to modulate individual rotational states present in a state-selected molecular beam of symmetric top molecules prepared for use in a beam attenuation experiment. Tagging states in this way allows cross sections for collisionally induced ΔM transitions to be measured for single rotational states. A hexapole transmission curve attenuation method reported earlier suffered from poor rotational state resolution and the measurement of cross sections was very time consuming in comparison to the method reported here. Cross sections measured using the two different methods show remarkably good agreement.

Experimental study of rotational relaxation processes by pulsed photoacoustic technique

The IR pulsed photoacoustic method was used to investigate rotational relaxation processes in MPA. We have examined the bulk rotational relaxation times and the bulk rotational relaxation cross sections , related to R-R and R-T relaxation mechanisms. The saturation intensities for gas molecules have been measured in the gas mixtures with (buffer) molecules, at constant gas pressure (0.47 mbar) and variable gas pressure (1-140 mbar), using the 10P(16) laser line in the fluences range 0.1-0.7 J . It has been found that for proper investigation of the rotational relaxation processes, the buffer gas pressure range must be divided into two parts: the low pressure range (0-20 mbar) and the high pressure range (20-140 mbar), according to the behaviour of the saturation intensities versus buffer gas pressure. Consequently, two different sets of values for rotational relaxation parameters are obtained. The found step-function behaviour for shows that the rotational hole-filling effect during MPA only dominates at low buffer gas pressure ( mbar). At higher buffer gas pressures (between 20-140 mbar) collisional deactivation takes place. We also derive the enhanced absorption cross sections for absorbing gas molecules from the measured saturation intensities data (at zero buffer gas pressure) confirming the expectation that the enhanced absorbing cross sections are not only a function of laser fluence and buffer gas pressure, but are very strongly dependent on laser beam parameters, too.

THEORETICAL CALCULATION OF PRESSURE-BROADENED CROSS SECTION FOR PURE ROTATIONAL DIPOLE TRANSITION OF Hel IN Ar

The pressure-broadened cross section (kinetic energy of the collision range:30 cm-1—1000cm-1) and the avarage relaxation cross section (temperature of the interaction system:125K,175K,300K) for pure rotational dipole transition of HCl in Ar are calculated,by employing infinite or-der sudden approximation (IOSA) and Ar-HCl H6(2) analyti potential energy surface of Hutson.Comparing the IOSA results with Close-Coupling results and experiments,we find that IOSA can give satisfactory results for j=0←j=1 transition line of the molecules which have large rotational constant.Unfortunately,modfication must be performed for using IOSA in the exitation of transition lines.

REMPI spectroscopy of internal state populations in HBr+Ar free jets: Rotational relaxation of HBr

The (2+1) resonance enhanced multiphoton ionization spectra of HBr were obtained through the E 1 Σ + state for HBr seeded in Ar free jets. Analysis of the two-photon absorption line strength factors allowed determination of quantitative populations for the HBr rotational manifold. A study of the terminal HBr rotational state distributions was made for a wide variety of p 0 D values of the jets. These nonequilibrium population distributions were then modeled by numerical solutions of the set of coupled kinetic equations describing the rotationally inelastic collisions with the buffer. Using a power-law relaxation model, the j– j′ rotationally state specific relaxation cross-sections and rate coefficients were determined as a function of temperature over the range present in the post-continuum zone of the free jet flows (180 to 15 K).

Axialisation, cooling and quenching of $\mathsf{Ba^+}$ ions in a Penning trap

Collisions of ions, stored in a Penning trap, with neutral background molecules usually lead to rapid ion loss from the trap unless the ions are excited by the sum of the frequencies of the reduced cyclotron and magnetron motion. Then the ions are cooled by collisions and are driven to the trap centre leading to substantial increase of the storage time. Furthermore in a three level system including a long living metastable state collisions deexcite this metastable state and increase the population density in the ionic ground state. In a laser spectroscopic experiment we demonstrate the advantages of collisions on Ba+ ions stored in a Penning trap. The combined action of metastable state quenching, axialisation and cooling leads to a significantly enhanced laser induced fluorescence signal. The cross section for collisional relaxation of the ground state Zeeman levels has been determined and we find that it is of the same order of magnitude as quenching cross section for a metastable state. Cooling and increased signal strength allows us to observe extremely narrow resonances and motional sidebands in microwave induced Zeeman transitions in the Ba+ electronic ground level.

An intermolecular potential for nitrogen from a multi-property analysis

New measurements are reported for the velocity dependence of the integral cross-section for N2–N2 scattering showing the glory oscillations. These data are analysed along with recent high precision second-virial coefficient data and traditional transport property measurements to yield an improved N2–N2 potential energy surface by adjusting certain parameters in the fit to this surface by van der Avoird et al. The new potential gives improved agreement with the integral cross-section, virial and transport data. Transport and relaxation cross-sections sensitive to anisotropy are very close to those obtained previously using the van der Avoird et al. surface. New comparisons with rotational relaxation and NMR relaxation data are reported.