Knowledge Bank

Time-resolved infrared double-resonance is used to study state-resolved rational energy transfer in ammonia $\mathrm{<mrow\; data-mjx-texclass="ORD">14</mrow>}NH3$) self-collisions and between ammonia and foreign gases. $NH3$ molecules are prepared in selected rovibrational stales of the $v2=1$ level using coincidences between $CO2$ laser lines and $v2$ 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 $NH3-NH3$ 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 $NH3-NH3$ and $NH3Ar$ collisions. Individual rotational energy transfer rates in $v2=1$ 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 $v2=1$ 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 () in this region are on the order of $k_{as}= 4 $ $\mu s-1$ $torr,-1$ smaller than $kas$ in the ground state, but over an order of magnitude larger than that recently reported the literature for $v2=1$. Depopulation rates for other collision partners (Ar, $H2$, $N2$, 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.