Rate Constants For Rotational Excitation Research Articles

Coupled-States Statistical Investigation of Vibrational and Rotational Relaxation of OH(2Π) by Collisions with Atomic Hydrogen

We report state-to-state cross sections and thermal rate constants for vibrational and rotational relaxation of OH(2pi) by collision with H atoms. The cross sections are calculated by the coupled-states (CS) statistical method including the full open-shell character of the OH + H system. Four potential energy surfaces (PESs) ((1,3)A' and (1,3)A'') describe the interaction of OH(X2pi) with H atoms. Of these, three are repulsive, and one (1A') correlates with the deep H2O well. Consequently, rotationally and ro-vibrationally inelastic scattering of OH in collisions with H can occur by scattering on the repulsive PESs, in a manner similar to the inelastic scattering of OH by noble gas atoms, or by collisions which enter the H2O well and then reemerge. At 300 K, we predict large (approximately 1 x 10(-10) cm3 molecule(-1) s(-1)) vibrational relaxation rates out of both v = 2 and v = 1, comparable to earlier experimental observations. This anomalously fast relaxation results from capture into the H2O complex. There exists a significant propensity toward formation of OH in the pi(A') lambda-doublet level. We also report state-resolved cross sections and rate constants for rotational excitation within the OH v = 0 manifold. Collisional excitation from the F1 to the F2 spin-orbit manifold leads to an inverted lambda-doublet population.

Improved low-temperature rate constants for rotational excitation of CO by H$_\mathsf{2}$

Cross sections for the rotational (de)excitation of CO by ground state para- and ortho-H2 are obtained using quantum scattering calculations for collision energies between 1 and 520 cm-1. A new CO-H2 potential energy surface is employed and its quality is assessed by comparison with explicitly correlated CCSD(T)-R12 calculations. Rate constants for rotational levels of CO up to 5 and temperatures in the range 5-70 K are deduced. The new potential is found to have a strong influence on the resonance structure of the cross sections at very low collision energies. As a result, the present rates at 10 K differ by up to 50% with those obtained by Flower (2001) on a previous, less accurate, potential energy surface.

Cross sections and rate constants for rotational excitation of NH 3 colliding with H 2( j = 0) and H 2( j = 1)

Rate constants for rotational excitation of NH 3 colliding with H 2 in the temperature range 50–300 K have been calculated using the semiclassical coupled states method.

Rate constants for rotational excitation of ammonia colliding with hydrogen

Ab initio calculations of the interaction energy between H 2 and NH 3 are carried out using self-consistent field (SCF) and many-body perturbation theory (MBPT). The surface is represented by a mixture of analytical and spline fits. It is used to calculate 110 rate constants for the rotational excitation of ortho- and para-NH 3 in the temperature range 50–300 K.

Cross sections for rotational excitation of ammonia colliding with helium and hydrogen

Extensive calculations of cross sections and rate constants for rotational excitation of ammonia colliding with He and H 2 on ab initio potential energy surfaces are carried out using a semiclassical collision model.

Rate constants for rotational excitation of ortho- and para-NH 3 colliding with 4He on an Ab initio potential energy surface

Ab initio self-consistent-field (SCF) calculations have been carried out at 180 configurations (θ, ϕ′) and distances ( R′) for the 4He-N

Accuracy of the IOS approximation for highly inelastic RT collisional energy transfer. CO-Ar

Rate constants for rotational excitation of CO by collisions with Ar atoms have been computed within the infinite order sudden (IOS) approximation and compared with values from exact classical trajectories. For CO-Ar, as for COHe, accurate values for R(O → j) - at least for small and intermediate j - may be obtained by taking the geometric average of rates computed assuming the IOS energy is the initial energy for upward O → j transitions and the initial energy for downward j → O transitions. however, especially for higher j, it appears preferable to interpret the IOS energy as the final kinetic energy. The best overall results are probably obtained by averaging rates computed assuming the IOS energy is the final energy for upward transitions and the final enery for downward transitions.

Rotational excitation of diatomic molecules in collisions with atoms

The rate constants for rotational excitation, the coefficient of RT diffusion for the Ar-N2 system, and the rotational relaxation time in N2 are calculated using the quasiclassical approximation.

Accuracy of the IOS approximation for highly inelastic R-T collisional energy transfer

Rate constants for rotational excitation of CO by collisions with He atoms computed within the infinite order sudden (IOS) approximation are compared with accurate quantum (coupled-states) and classical trajectory values. Taking the IOS energy as the initial kinetic energy for upward. 0 → J, transitions is found to overestimate the rates, especially for higher J (larger inelasticity). Taking the IOS energy as the initial energy for downward, J → 0, transitions underestimates the rates by a comparable amount. The geometric average of IOS rates computed in these two ways is found to provide accurate values.

Rotational excitation of hydrogen molecules by collisions with hydrogen atoms

Rate constants for rotational excitation of hydrogen molecules by collisions with hydrogen atoms have been obtained from quantum-mechanical calculations for kinetic temperatures between 100 and 5000 K. These calculations involve the rigid-rotator approximation, but other possible sources of error should be small. The calculations indicate that the early values of Nishimura are larger than accurate rigid-rotator values by about a factor of 20 or more.

Rate constants for rotational excitation in NH3–He collisions

The semiclassical method (classical translational motion and quantum mechanical rotational motion) is used to calculate cross sections and rate constants for (J?3, K) → (J?4, K) rotational excitations in NH3–He collisions on Green’s uniform electron gas (Gordon–Kim) potential surface. The assumptions involved in the use of the rigid rotor approximation for the inversion motion are examined in some detail. Agreement between semiclassical and accurate close-coupling cross sections is good, comparable to the agreement between the coupled-states and the close-coupling cross sections. A ’’semiclassical coupled-states’’ approximation was found to give reliable cross sections except at low energies. Microwave double resonance ΔI/I values were calculated from the rate constants and compared with experiment. Good agreement was found for the Δk=3 transitions, but poor agreement for the Δk=0 transitions. Possible explanations for the discrepancies are discussed.

Rotational excitation of molecular ions in interstellar clouds

Rate constants for rotational excitation of N2H(+) by low-energy collisions with He have been obtained from accurate quantum-mechanical calculations. Rates for excitation of HCO(+) and rates for excitation by collisions with H2 are expected to be qualitatively similar. The excitation rate for molecular ions is found to be only slightly faster than that for similar neutral species, and not much faster as had been previously suggested. The implications of this for interpreting microwave observations of interstellar N2H(+) and HCO(+) are discussed.

Rotational excitation of HD by collisions with He

It is shown that the interaction potential recently derived for H 2-He is also consistent with sound absorption and NMR spin-lattice relaxation measurements on HD. Based on this potential, collision-induced pressure broadening coefficients and also rate constants for rotational excitation are predicted. The latter may be important for understanding cooling processes in the interstellar medium.

Rotational Excitation of HCN by Collisions

Rate constants for rotational excitation of HCN by collisions with low-energy He atoms (kinetic temperatures below 100 K) have been computed from first principles. The accuracy of individual rates is conservatively estimated to be I 30 percent above 30 K and within a factor of 2 below 20 K. These rates are also believed to be representative of excitation by collisions with H2. Subject headings: interstellar matter - molecules, interstellar - transition probabilities