Using picosecond time-resolved photoelectron imaging we have studied the intramolecular vibrational energy redistribution (IVR) dynamics that occur following the excitation of the 3 1 5 1 level which lies 2068 cm -1 above the S 1 origin in p-difluorobenzene. Our technique, which has superior time resolution to that of earlier studies but retains sufficient energy resolution to identify the behavior of individual vibrational states, enables us to determine six distinct beating periods in photoelectron intensity, only one of which has been observed previously. Analysis shows that the IVR dynamics are restricted among only a handful of vibrational levels, despite the relatively high excitation energy. This is deduced to be a consequence of the high symmetry and rigid structure of p-difluorobenzene. TOC graphic:

The vibrations of the ground state cation ( ) of-difluorobenzene (DFB) have been investigated using zero-electron-kinetic-energy (ZEKE) spectroscopy. A comprehensive set of ZEKE spectra were recorded via different vibrational levels of the S state (<0 + 1300 cm). The adiabatic ionization energy for DFB was measured as 73 869 ± 5 cm. Use of different intermediate levels allows different cationic vibrational activity to be obtained via the modification of the Franck-Condon factors for the ionization step, allowing the wavenumbers of different vibrational levels in the cation to be established. In addition, assignment of the vibrational structure in the ZEKE spectra allowed interrogation of the assignments of the S ← S transition put forward by Knight and Kable [J. Chem. Phys. , 7139 (1988)]. Assignment of the vibrational structure has been aided by quantum chemical calculations. In this way, it was possible to assign seventeen of the thirty vibrational modes of the ground stateDFB cation. Evidence for complex Fermi resonances in the S state, i.e., those that involve more than two vibrations, was established. One of these was investigated using picosecond time-resolved photoelectron spectroscopy. In addition, we discuss the appearance of several symmetry-forbidden bands in the ZEKE spectra, attributing their appearance to a Rydberg state variation of an intrachannel vibronic coupling mechanism.

In this Comment we submit the results of an experiment in which we use the technique of time-resolved photoelectron velocity map imaging to probe the intramolecular dynamics occurring following the preparation of the 3151 vibrational level in S1 p-difluorobenzene with a 1 ps laser pulse. The extracted photoelectron angular distributions are discussed in the context of earlier comparable measurements from our group [J. Chem. Phys. 111, 1438 (1999)], and we conclude that the specific interpretation of the earlier results was incorrect as a consequence of systematic errors that are removed in the present study.

No abstract

No abstract