STATE TO STATE ROTATIONAL ENERGY TRANSFER N THE $\nu_{2}$ = 1 STATE OF AMMONIA*
Loading...
Date
1992
Journal Title
Journal ISSN
Volume Title
Publisher
Ohio State University
Abstract
Time-resolved infrared double-resonance is used to study state-resolved rational energy transfer in ammonia $^{14}NH_{3}$) self-collisions and between ammonia and foreign gases. $NH_{3}$ molecules are prepared in selected rovibrational stales of the $v_{2}=1$ level using coincidences between $CO_{2}$ laser lines and $v_{2}$ fundamental transitions. Measurements of both the total depopulation rate and the rates of transfer into specific final rovibrational sates (v,J,K) have been carried out. For $NH_{3}-NH_{3}$ collisions, total depopulations rates and ground-state recovery rates are found to be three and eight times larger, respectively, than the Lennard-Jones collision rate, in accord with theoretical expectations for polar molecules. A kinetic master equation analysis of time-resolved level populations yields state to state rate constants and propensity rules for $NH_{3}-NH_{3}$ and $NH_{3} Ar$ collisions. Individual rotational energy transfer rates in $v_{2}= 1$ are slower than in the vibrational ground state. but still comparable to the Lennard-Jones collision frequency. Our experiments show that rotational energy transfer in $v_{2} = 1$ is not governed by simple ``dipole like” selection rules. They show fast rotational energy transfer, which can be related to long range interaction potentials, but at the same time considerable amounts of $\Delta$J= 2,3 and $\Delta$K = 3 transitions, which may be attributed to higher order terms in the multipole expansion of the intermolecular potential. No pronounced symmetry-state correlation and no preferred pathways were found except the preference for relaxation within a K-stack and expected separate relaxation of different nuclear spin species. Rates of collision induced symmetry change ($a<->s$) in this region are on the order of $k_{as}= 4 $ $\mu s^{-1}$ $torr,^{-1}$ smaller than $k_{as}$ in the ground state, but over an order of magnitude larger than that recently reported the literature for $v_{2} = 1$. Depopulation rates for other collision partners (Ar, $H_{2}$, $N_{2}$, and He) can be understood in terms of the intermolecular potentials. Comparisons are made between the relaxation rates measured in this work and infrared pressure-broadening coefficients reported in the literature.
Description
*Work supported by NASA Grants NAGW-1667 and NAGW-2387 and N.S.F. Grant CHE89-14953 to the G.R. Harrison Spectroscopy Laboratory.
Author Institution: Institut f\""{u}r Physikalische Chemie, Universit\'{a}t G\""{o}ttingen; Department of Chemistry, G.R. Harrison Spectroscopy Laboratory
Author Institution: Institut f\""{u}r Physikalische Chemie, Universit\'{a}t G\""{o}ttingen; Department of Chemistry, G.R. Harrison Spectroscopy Laboratory