| Our study concerns collision-induced energy flow in various S1 vibrational states of para-difluorobenzene (pDFB) in gas phase in a bulb at room temperature. The primary goal is to determine absolute rate constants for vibrational energy transfer (VET) from selected vibrational states into a field of neighboring vibrational states of pDFB target during a single collision with inert gases. This rate constant depends on the vibrational state identity, total internal energy content, and the collision partner. We examine these dependencies. In particular, VET is investigated from a wide range of vibrational levels up to 4600cm-1 where states are highly dense (∼10 4 states per cm-1) and mixed, generating a vibrational state continuum, and where dynamics is dominated by fast intramolecular vibrational redistribution (IVR). In the course of our experiments, we also probe two other accompanying and competing phenomena: electronic state quenching and IVR.; This work extends VET measurements into high vibrational levels which are distinct from earlier studied levels, and for which the standard VET experimental technique fails. An alternative 'chemical timing' VET method has been developed by which the problem of extensive IVR is circumvented. This method involves controlled and rapid fluorescence quenching by O2 under high pressures.; Absolute VET rate constants of highly vibrationally excited large molecules are essential for scaling the energy transfer probability per collision P(E,E ') into energy transfer rate R(E,E'). Even though our technique falls way short of truly hot molecules with {dollar}10 4 cm-1 vibrational energy (by ∼10 times), our results nevertheless offer insight into the problem of effective VET size of such molecules. It is often assumed that VET rate constant is well-represented by the elastic Lennard-Jones cross-section. We directly probe validity of this assumption by looking at the trend of VET rate constant with vibrational energy. We find that trend continues to increase gradually, slowly and nonuniformly for some partners, while it levels off for others. Knowledge of such trends and of how they extend into a region of very high state densities and chemically significant amounts of energy, is a prerequisite for understanding how large molecules manage to surmount energy barriers and undergo unimolecular reactions. |
