STATE-TO-STATE ROTATIONAL ENERGY-TRANSFER MEASUREMENTS IN METHANE (CHD3) BY INFRARED DOUBLE-RESONANCE WITH A TUNABLE DIODE-LASER

An infrared double-resonance laser spectroscopic technique is used to study state-resolved rotational energy transfer (RET), vibration-vibra tion (V-V) transfer, and symmetry-exchanging collisions in asymmetrica lly deuterated methane (CHD3). The molecules are prepared in selected rovibrational states of the {upsilon3, upsilon6} = 1 dyad using coinci dences between CO2 laser lines and dyad<--ground state transitions. Me asurements of both the total rate of depopulation by collisions and th e rates of transfer into specific rovibrational (upsilon,J,K) levels a re carried out using time-resolved tunable diode laser absorption spec troscopy. Total excited-state depopulation and ground-state recovery r ates range from 0.5 to 1.0 times the Lennard-Jones collision rate, con sistent with relaxation due to short-range forces. V-V (nu6-->nu3) pro cesses contribute about 10% of the total relaxation rate, and symmetry -changing (A<-- -->E) collisions occur at a rate another order of magn itude smaller, viz. (0.17+/-0.02) mus-1 Torr-1, corresponding to an ef fective cross section of 0.64 angstrom2, around 10(-2) sigma(LJ). The symmetry-exchanging collision efficiency for CHD3 as well as for other systems reported elsewhere (CD3Cl,CH3F) can be quantitatively estimat ed using a simple Forster resonant exchange mechanism. The state-to-st ate RET rates are modeled using a kinetic master equation. A strong pr opensity rule, DELTAK = +/-3x (integer), similar to that found for hig hly dipolar symmetric tops such as ammonia, applies to CHD3 as well. W e conclude that the flow of energy and angular momentum in molecular r elaxation is dominated by the internal level structure of the molecule , rather than by specific details of the intermolecular potential.