Collisional energy transfer of highly vibrationally excited molecules: The role of long-range interaction and intramolecular vibronic coupling

In the experimental study of collisional energy transfer of highly vibrationally excited molecules (1, 2), several techniques have been developed and applied to monitoring the energy content of either the highly excited energy donor or the energy receiving bath molecules. The highly excited molecules are often prepared with electronic excitation followed by internal conversion (3) or with multiple resonance techniques such as Stimulated Emission Pumping (4). The probe techniques include thermal lensing (5), photoacoustic (6), Doppler width measurement, diode laser transient absorption spectroscopy (2), optical absorption (7, 8) and ionization (9), laser induced fluorescence (4), sensitized chemical reaction (10) and IR emission (11). The IR emission from vibrationally excited molecules can in principle be used to reveal the rovibrational energy content of both the energy donating and receiving molecules. This can be best achieved if the IR emission is detected with sufficient frequency and time resolution so that the energy content of the emitting molecules during a collisional relaxation process can be continuously monitored. The average energy and energy distribution of the highly excited molecules can then be directly extracted from the time-resolved IR emission spectra.