Absolute Rate Constants for Collisional Vibrational Relaxation in Dense Vibrational Regions of S1 p-Difluorobenzene

Test of a chemical timing method for measuring absolute vibrational relaxation rate constants for S1 p-difluorobenzene

A chemical timing (CT) method for measuring absolute rate constants for collisional vibrational relaxation has been tested for the 5(1) state of S(1) p-difluorobenzene (pDFB) where an alternative method exists to provide benchmark values. The CT method was originally developed to treat vibrational energy transfer (VET) in large molecules excited to high vibrational levels where the intramolecular vibrational redistribution (IVR) resulting from large vibrational state densities completely eliminates vibrational structure in the emission spectrum. Here we apply the same method to a low-lying state (5(1) with epsilon(vib) = 818 cm(-1)) located in the low-density region of the vibrational manifold where IVR plays no role. For high vibrational levels, the chemical timing method involves addition of high O(2) pressures (kTorr) to a low-pressure pDFB sample, introducing vibrational structure in the fluorescence spectrum. Response of this spectrum to vibrational relaxation by Ar is then examined. For levels such as 5(1), the fully structured fluorescence spectrum allows the rate constant for single-collision VET into the surrounding vibrational field to be measured directly without the presence of O(2). The measurements of 5(1) VET have been repeated with various O(2) pressures (kTorr) for comparison with the O(2)-free benchmark. In the presence of O(2), the rate constant for VET by Ar is (4.0 +/- 0.5) x 10(6) Torr(-1) s(-1) and independent of high O(2) pressure variations. The rate constant as found by the standard O(2)-free method is (3.6 +/- 0.4) x 10(6) Torr(-1) s(-1). This comparison suggests that the chemical timing method is capable of providing a reasonably accurate measure of the VET rate constant for high vibrational levels provided that details of the kinetics are known.

Vibrational energy relaxation of naphthalene in the S(1) state in various gases

Time-resolved fluorescence spectra of naphthalene in the S(1) state have been measured in various gases below 10(2) kPa. The band shape of the fluorescence changed in an earlier time region after the photoexcitation when an excess energy (3300 cm(-1)) above the 0-0 transition energy was given. The excitation energy dependence of the fluorescence band shape of an isolated naphthalene molecule was measured separately, and the time dependence of the fluorescence band shape in gases was found to be due to the vibrational energy relaxation in the S(1) state. We have succeeded in determining the transient excess vibrational energy by comparing the time-resolved fluorescence band shape with the excitation energy dependence of the fluorescence band shape. The excess vibrational energy decayed almost exponentially. From the slope of the decay rate against the buffer gas pressure, we have determined the collisional decay rate of the excess vibrational energy in various gases. The dependence of the vibrational energy relaxation rate on the buffer gas species was similar to the case of azulene. The comparisons with the results in the low temperature argon and the energy relaxation rate in the S(0) state in nitrogen were also discussed.