Timehyphen;resolved double resonance study ofJhyphen; andKhyphen;changing rotational collisional processes in CH3Cl

摘要

Timehyphen;resolved double resonance spectroscopy using infrared pump radiation and millimeterhyphen;wave and submillimeterhyphen;wave probe radiation (IRMMDR) has been used to study rotational energy transfer (RET) in CH3Cl. A collisional energy transfer model using only five parameters for RET plus those needed for vibrational processes is shown to accurately model 350 IRMMDR time responses for two different pump states and 43 probe transitions covering a wide range of rotational states. Previous studies in this laboratory have revealed thatJhyphen; andKhyphen;changing RET have vastly different characters in CH3F lsqb;J. Chem. Phys.92, 6480 (1990)rsqb;. BothJhyphen; andKhyphen;changing RET were accurately modeled with four parametersmdash;one for dipolendash;dipole collisions, two for the Dgr;Jscaling law, and one for the cumulative rate ofKhyphen;changing collisions. As was found for CH3F,Jhyphen;changing rotational collision rates in CH3Cl are modeled accurately by both the statistical power gap (SPG) law and the infinite order sudden approximation using a power law expression for the basis rates (IOShyphen;P). However, in contrast to CH3F, where all IRMMDR time responses forKhyphen;changing collisions have the same shape, many time responses of CH3Cl states populated byKhyphen;changing collisions contain an additional early time feature (ETF) that varies with pump and probe states. Nonetheless, a simple generalization of the previously reported model forKhyphen;changing collisions is shown to account for all of the additional features observed in CH3Cl. Rather than observing a fixed temperature forKhyphen;changing collisions as was the case for CH3F, the temperature is found to be a function of time for CH3Cl. Moreover, the two new parameters this adds to the RET model are related to known physical quantities. A qualitative argument ofKhyphen;changing collisions based on a classical picture is offered to explain the difference between the measuredJhyphen; andKhyphen;changing statehyphen;tohyphen;state rates in CH3Cl.