An unusual π* shape resonance in the near-threshold photoionization of S1 para-difluorobenzene

Abstract

Previously reported dramatic changes in photoelectron angular distributions ͑PADs͒ as a function of photoelectron kinetic energy following the ionization of S 1 p-difluorobenzene are shown to be explained by a shape resonance in the b 2g symmetry continuum. The characteristics of this resonance are clearly demonstrated by a theoretical multiple-scattering treatment of the photoionization dynamics. New experimental data are presented which demonstrate an apparent insensitivity of the PADs to both vibrational motion and prepared molecular alignment, however, the calculations suggest that strong alignment effects may nevertheless be recognized in the detail of the comparison with experimental data. The apparent, but unexpected, indifference to vibrational excitation is rationalized by considering the nature of the resonance. The correlation of this shape resonance in the continuum with a virtual * antibonding orbital is considered. Because this orbital is characteristic of the benzene ring, the existence of similar resonances in related substituted benzenes is discussed.

Photoelectron spectroscopy of S1 toluene: II. Intramolecular dynamics of selected vibrational levels in S1 toluene studied by nanosecond and picosecond time-resolved photoelectron spectroscopies

“…33 We have investigated the possibility that the congestion observed in our photoelectron spectra arises from the photoionization of a vibrationally excited triplet state, T 1 * , by performing a nanosecond timeresolved two-color VMI experiment where the first photon is tuned to the S 1 ← S 0 origin and the wavelength of the second is 225 nm, judged to be short enough to efficiently ionize the triplet ͑i.e., to reach the Franck-Condon favored levels of the ion͒.…”

Time‐Resolved Photoelectron Spectroscopy of Nonadiabatic Dynamics in Polyatomic Molecules

“…Any potential fail- ure of this independent electron picture can always be experimentally tested directly through variation of the photoionization laser frequency: resonance structures should lead to variations in the form of the spectra with electron kinetic energy, although the effect of resonances is more likely to be prominent in PAD measurements, and indeed an observation of a shape resonance in para-difluorobenzene has been reported [153,154].…”

Photoelectron angular distributions: developments in applications to isolated molecular systems

“…Nonetheless, an investigation of near-threshold continuum resonances in ammonia has been undertaken, indicating that some sharp structure is present that causes a minor contamination to the observed PADs [69]. By contrast, a similar study in p-difluorobenzene indicated an unexpected shape resonance close to threshold which completely dominates the observed PADs [70,71]. Figure 5 shows photoelectron angular distributions obtained at six different kinetic energies following the same state-to-state ionization process in this molecule; clear differences are observed as a consequence of rapidly changing partial wave phases.…”