Statehyphen;tohyphen;state rotational energy transfer measurements in methane (CHD3) by infrared double resonance with a tunable diode laser

摘要

An infrared doublehyphen;resonance laser spectroscopic technique is used to study statehyphen;resolved rotational energy transfer (RET), vibrationndash;vibration (Vndash;V) transfer, and symmetryhyphen;exchanging collisions in asymmetrically deuterated methane (CHD3). The molecules are prepared in selected rovibrational states of the lcub;v3,v6rcub;=1 dyad using coincidences between CO2laser lines and dyadlarr;ground state transitions. Measurements of both the total rate of depopulation by collisions and the rates of transfer into specific rovibrational (v,J,K) levels are carried out using timehyphen;resolved tunable diode laser absorption spectroscopy. Total excitedhyphen;state depopulation and groundhyphen;state recovery rates range from 0.5 to 1.0 times the Lennardhyphen;Jones collision rate, consistent with relaxation due to shorthyphen;range forces. Vndash;V (ngr;6rarr;ngr;3) processes contribute about 10percnt; of the total relaxation rate, and symmetryhyphen;changing (Aharr;E) collisions occur at a rate another order of magnitude smaller, viz. (0.17plusmn;0.02) mgr;sminus;1thinsp;Torrminus;1, corresponding to an effective cross section of 0.64 Aring;2, around 10minus;2sgr;LJ. The symmetryhyphen;exchanging collision efficiency for CHD3as well as for other systems reported elsewhere (CD3Cl,CH3F) can be quantitatively estimated using a simple Fouml;rster resonant exchange mechanism. The statehyphen;tohyphen;state RET rates are modeled using a kinetic master equation. A strong propensity rule, Dgr;K=plusmn;3x(integer), similar to that found for highly dipolar symmetric tops such as ammonia, applies to CHD3as well. We conclude that the flow of energy and angular momentum in molecular relaxation is dominated by the internal level structure of the molecule, rather than by specific details of the intermolecular potential.