Studies in State-to-State Energy Transfer

The measurement and understanding of the rates at which energy is transferred in intermolecular collisions are problems of long-standing interest in chemical physics. Such processes play a crucial role in many environments for example, in atmospheric chemistry, especially at high altitudes, in gas lasers, in combustion and in the chemistry of deep space, especially in the characterisation of dense interstellar clouds. Gas-phase collisional energy transfer is an area in which the Reporters group has been active for many years. In the past few years, it has been one of several groups world-wide to apply time-resolved double resonance techniques to examine the details of energy transfer at the state-to-state level e.g. in HCN, C2H2 and NO. In Birmingham, these experiments have employed a tunable infrared IR laser to promote simple molecules to selected rovibrational states by direct infrared absorption and a tunable ultraviolet UV laser to study the subsequent fate of this sub-set of excited molecules as they return to thermodynamic equilibrium with the thermal bath. In general, double resonance methods can be applied to study both rotational and vibrational energy transfer. Since these two processes usually occur with quite different collisional efficiencies, equilibration over rotational levels within the excited vibrational state is usually complete before significant vibrational relaxation occurs. Consequently, by appropriate choice of the pressure of the sample and the delays between the pulses from the IR pump and UV probe lasers, the rates of both rotational and vibrational relaxation can be measured.