Hot-band spectroscopy of ground-state levels of perylene following predissociation of van der Waals complexes

Abstract

Experiments involving laser-induced hot bands have interrogated the energies and populations of ground-state vibrational levels of perylene. In the bare molecule, many ground-state levels persist into the 10-7-106-s regime without significant relaxation. Even in the perylene/Arl van der Waals complex, a ground-state quantum of 353 cm-l persists for >120 ns. On the other hand, residual collisions in the expanding free jet cause significant relaxation of energy deposited in low-energy (G) modes of perylene. Hot bands of free perylene are also generated following vibrational predissociation of van der Waals complexes. The argon, methane, and nitrogen complexes all show efficient hot-band population and show distribution of energy into low-energy, out-of-plane modes of the product aromatic molecule at higher energies. The COz complex dissociated at 11270 cm-1 in SI and the ethylene complex at 11320 cm-l. Corresponding ground-state binding energies are S1235 and 1 1 160 cm-l. In these two cases, hot-band excitation spectroscopy at x2-cm-l resolution provided the means to detect the onset of hot bands of nascent free perylene, as the pump laser scanned through a congested region of the spectrum.

Experimental and Theoretical Determination of Dissociation Energies of Dispersion-Dominated Aromatic Molecular Complexes

“…These hot fragments are probed by a second laser which is tuned to a hot band and fired ≈ 100 ns after the excitation laser. 18 [19][20][21][22][23] To discern between stable and metastable M + ·S ions, Neusser and co-workers have developed a method to determine the appearance energies of M + ions, using a reflectron time-of-flight mass spectrometer (RETOF). 24 The metastable ions that decay in the drift-region of the RETOF have a different kinetic energy than the stable ions.…”

“…17 For larger systems or for complexes with large dissociation energies, the spectra of both M* and M*•S become more complicated, which often leads to overlapping emission spectra. To overcome this limitation, a two-laser scheme has been introduced by Wittmeyer and Topp 18 in which the first laser vibronically excites the investigated complex. If enough energy for VP in the excited state is deposited into the complex, the formation of vibrationally excited M ‡ ground state fragments is expected.…”

“…These hot fragments are probed by a second laser which is tuned to a hot band and fired ≈100 ns after the excitation laser. 18…”

“…(b) A two-laser variant of the method described in ref. ; detection of the M* dissociation product is performed on a “hot” level that is not populated in the cold supersonic jet environment.…”

“…If enough energy for VP in the excited state is deposited into the complex, the formation of vibrationally excited M ‡ ground state fragments is expected. These hot fragments are probed by a second laser which is tuned to a hot band and fired ≈100 ns after the excitation laser …”