LiH state-to-state rotationally inelastic cross sections in collisions with HCl and DCl
A new technique is presented for the determination of state-to-state rotationally inelastic cross sections, which employs electric quadrupole state selection and laser fluorescence detection. Results for the scattering of /sup 7/LiH j=1 molecules by HCl and DCl scattering gas (E-bar/sub rel/= 0.74 eV) are reported. With knowledge of the number density and scattering path length l and the observed invariance of the experimental cross sections with decreased l, absolute integral state-to-state cross sections sigma/sub j=1..-->..j'/ have been obtained. For both /sup 7/LiH--HCl and /sup 7/LiH--DCl, the sigma/sub j=1..-->..j'/ values are large (10--200 A/sup 2/), the j'=2 level being most probable; the cross sections fall monotonically with increasing j' out to j'=6, the highest level probed. All the sigma/sub j=1..-->..j'/ values are approx.25% larger for /sup 7/LiH--DCl than for the corresponding transitions in the /sup 7/LiH--HCl system. Because both scattering partners have relatively large dipole moments, these systems are paradigms for rotational energy transfer induced by a dipole--dipole interaction. The Born approximation gives cross sections which are far too large and predicts insufficient flux into the higher j' states, even when higher-order electrostatic terms are included in the potential. The sudden approximation and the recently introduced adiabatically corrected sudden (ACS) approximation reproduce the experimental cross sections much better. The ACS method does significantly better and, in particular, successfully predicts the isotopic enhancement observed in the /sup 7/LiH--DCl system. The origin of this effect is shown to be due to energetic resonances: For dipole--dipole transitions of the type j/sub a/=1, j/sub b/..-->..j/sub a/ +- 1, j/sub b/-+1, the translational energy defect is less in DCl than HCl for those values of j/sub b/ which made the largest contributions to the 298 /sup 0/K Boltzmann rotational distribution of the hydrogen halides.
- Research Organization:
- Department of Chemistry, The Johns Hopkins University, Baltimore, Maryland 21218
- OSTI ID:
- 5927397
- Journal Information:
- J. Chem. Phys.; (United States), Journal Name: J. Chem. Phys.; (United States) Vol. 71:4; ISSN JCPSA
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
Molecular & Chemical Physics-- Collision Phenomena
74 ATOMIC AND MOLECULAR PHYSICS
ALKALI METAL COMPOUNDS
COLLISIONS
CROSS SECTIONS
DEUTERIUM COMPOUNDS
ENERGY LEVELS
ENERGY-LEVEL TRANSITIONS
EXCITED STATES
HYDRIDES
HYDROCHLORIC ACID
HYDROGEN COMPOUNDS
INELASTIC SCATTERING
INORGANIC ACIDS
LITHIUM COMPOUNDS
LITHIUM HYDRIDES
MOLECULE COLLISIONS
MOLECULE-MOLECULE COLLISIONS
ROTATIONAL STATES
SCATTERING