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

Vibrations of the low energy states of toluene (X̃ (1)A1 and à (1)B2) and the toluene cation (X̃ (2)B1)

We commence by presenting an overview of the assignment of the vibrational frequencies of the toluene molecule in its ground (S0) state. The assignment given is in terms of a recently proposed nomenclature, which allows the ring-localized vibrations to be compared straightforwardly across different monosubstituted benzenes. The frequencies and assignments are based not only on a range of previous work, but also on calculated wavenumbers for both the fully hydrogenated (toluene-h8) and the deuterated-methyl group isotopologue (α3-toluene-d3), obtained from density functional theory (DFT), including artificial-isotope shifts. For the S1 state, one-colour resonance-enhanced multiphoton ionization (REMPI) spectroscopy was employed, with the vibrational assignments also being based on previous work and time-dependent density functional theory (TDDFT) calculated values; but also making use of the activity observed in two-colour zero kinetic energy (ZEKE) spectroscopy. The ZEKE experiments were carried out employing a (1 + 1(')) ionization scheme, using various vibrational levels of the S1 state with an energy <630 cm(-1) as intermediates; as such we only discuss in detail the assignment of the REMPI spectra at wavenumbers <700 cm(-1), referring to the assignment of the ZEKE spectra concurrently. Comparison of the ZEKE spectra for the two toluene isotopologues, as well as with previously reported dispersed-fluorescence spectra, and with the results of DFT calculations, provide insight both into the assignment of the vibrations in the S1 and D0(+) states, as well as the couplings between these vibrations. In particular, insight into the nature of a complicated Fermi resonance feature at ∼460 cm(-1) in the S1 state is obtained, and Fermi resonances in the cation are identified. Finally, we compare activity observed in both REMPI and ZEKE spectroscopy for both toluene isotopologues with that for fluorobenzene and chlorobenzene.

Probing ultrafast internal conversion of o-xylene via femtosecond time-resolved photoelectron imaging

The dynamics of excited states in o-xylene molecules has been studied by femtosecond time-resolved photoelectron imaging coupled with time-resolved mass spectroscopy. The ultrafast internal conversion from the S(2) state to the vibrationally hot S(1) state on timescale of 60 fs is observed on real time. The secondarily populated high vibronic S(1) state deactivates further to the S(0) state on timescale of 9.85 ps. Interestingly, the lifetime of the low vibronic S(1) state is much longer, extrapolated to ~12.7 ns. The great differences of lifetime of different vibronic S(1) state are due to their different radiationless dynamics.

Observation of a simple vibrational wavepacket in a polyatomic molecule via time-resolved photoelectron velocity-map imaging: A prototype for time-resolved IVR studies

We have prepared a coherent superposition of the two components of a Fermi resonance in the S1 state of toluene at approximately 460 cm(-1) with a approximately 1 ps laser pulse and monitored time-resolved photoelectron velocity-map images. The photoelectron intensities oscillate with time in a manner that depends on their kinetic energy, even though full vibrational resolution in the cation is not achieved. Analysis of the time-dependent photoelectron spectra enables information on the composition of the S1 wavepacket to be deduced. Such an experiment, in which a whole set of partially dispersed cation vibrational states are detected simultaneously, suggests an efficient method of studying intramolecular vibrational energy redistribution processes in excited states.

Photoelectron spectroscopy of S1 toluene: I. Photoionization propensities of selected vibrational levels in S1 toluene

Laser photoelectron spectra have been obtained following the preparation of eight vibrational states in S(1) toluene. For four of the vibrational states (up to approximately 550 cm(-1) excess energy) excitation and ionization with nanosecond laser pulses give rise to photoelectron spectra with well-resolved vibrational peaks. For the other states (>750 cm(-1) excess energy) the photoelectron spectra show a loss of structure when nanosecond pulses are used, as a result of intramolecular dynamics [see Whiteside et al., J. Chem. Phys. 123, 204317 (2005), following paper]. A number of vibrational peaks in the photoelectron spectra are assigned, and we find that the common series of ion vibrational peaks observed following the ionization of p-fluorotoluene in various S(1) vibrational states is not reproduced in toluene.