State-to-state rotational energy-transfer measurements in the v2=1 state of ammonia by infrared–infrared double resonance

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Abstract
An infrared double‐resonance laser spectroscopic technique is used to study state‐resolved rotational (R–R, R–T) energy transfer in ammonia (14NH3) (self‐collisions and between ammonia and foreign gases). NH3 molecules are prepared in selected rovibrational states of the v2=1 level using coincidences between CO2 ‐laser lines and ν2 fundamental transitions. Measurements of both the total rate of depopulation by collisions, and the rates of transfer into specific final rovibrational states (v,J,K) have been carried out using time‐resolved tunable diode laser absorption spectroscopy. For NH3–NH3 collisions, measurements of total depopulation rates of selected JK states in v2=1 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 NH3–Ar 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 ‘‘dipolelike’’ 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 ΔJ=2 and 3, ΔK=0, and ΔJ=1–4, ΔK=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 the expected separate relaxation of different nuclear‐spin species, which can be labeled by their K‐quantum number. Rates of collision‐induced symmetry change (a↔s) in v2=1 are on the order of kas=4 μs−1 torr−1, smaller than kas in the ground state, but over an order of magnitude larger than that recently reported in the literature. 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.