On the propensity rules for inelastic NH3–rare gas collisions

Abstract

Articles you may be interested inQuantum scattering studies of inelastic collisions of NH(A 3Π) with helium: Finestructure and Λdoublet propensities J. Chem. Phys. 95, 5036 (1991); 10.1063/1.461792Quantum theory and collisional propensity rules for rotationally inelastic collisions between polyatomic molecules (NH3 and CO2) and an uncorrugated surfaceThe observed and ab initio calculated propensity rules for collisions of NH 3 with rare gas atoms are found to be in reasonable agreement for NH 3 -Ar, whereas for NH 3 -He they show large discrepancies. In order to examine these discrepancies we have calculated state-to-state integral cross sections for collisions of NH 3 with He using the close coupling method. The NH 3 -He interaction potential has been obtained from SCF calculations, augmented by a multipole-expanded damped dispersion energy. Our calculations show that the discrepancies can be accounted for if the cross sections are corrected for the imperfect initial state preparation in the experiment. They also clarify why the discrepancies do not occur to the same extent for NH 3 -Ar. After comparing our new theoretical results with the experimental data we found that for one experimental cross section for NH 3 -He the earlier assignment must be corrected.

State-to-state resolved differential cross sections for rotationally inelastic scattering of ND3with He

“…Molecules prepared in the well-defined initial state ݆ േ ൌ 1 ଵ ି are excited by collisions with He into various final rotational states, most of which lie higher in energy, and velocity map images were obtained for various ݆Ԣ ᇱ േ levels using the well-known 30 (2+1) REMPI of ND 3 via the 2-photon ‫ܤ‬ ෨ ՚ ܺ ෨ transition. [39][40][41] One consequence of symmetry and nuclear spin constraints is that only transitions originating from the lower (+) inversion component of the ground state appear in bands in which the final ߥ ଶ is even, whereas only transitions from the upper (−) inversion 35 component appear in bands with final ߥ ଶ odd.…”

“…1,2 The discovery of ammonia in the 25 interstellar medium dates back to 1968, 3 when emissions were observed at a frequency corresponding to the inversion transitions of the rotational levels j k = 1 1 and 2 2 in the vibrational ground state of NH 3 . Since then, ammonia has been observed in various environments, such as comets, 4 the Galactic centre, 5 the T dwarf 30 class of brown dwarf stars, 6 and in diffuse and translucent molecular clouds. 7 Ammonia is also present in the Earth's troposphere, 8 and has been identified in the atmospheres of extrasolar planets.…”

“…In the current work, we tested both McBane's iterative 30 extraction method IMSIM 42 (IMage SIMulation, based on the repeated comparison of a simulated image with the experimental image), and the method of Monte Carlo simulation of the experiment, using a modification of the computer program of Eyles and Brouard. 43 Both codes provide density-to-flux 35 transformation of experimental images, so that DCSs can be extracted.…”

“…Current experiments were performed with a fixed wavelength 30 chosen to give maximum scattering signal, rather than scanning the wavelength over the full width of the Doppler profile of the REMPI transition (0.1 cm -1 for inelastic scattering of ND 3 from He). However, the laser bandwidth was ~0.3 cm -1 and the ~4.5 mJ energy per pulse of the laser power broadened the transitions 35 sufficiently that Doppler scanning was unnecessary.…”

“…Measurement quantum-state resolved DCSs obtained using beam (CMB) scattering, [22][23][24] combined with resonance enhanced multi-photon ionization (REMPI) detection and velocity 30 imaging (VMI). 25 At this level of resolution of the dynamics of a scattering process, rigorous comparisons can be drawn between experimental measurements and the outcomes of quantum mechanical (QM) scattering calculations.…”