Ultrafast Internal Conversion in Highly Excited Toluene Monomers and Dimers^†

Abstract

The dynamics of the internal conversion in toluene molecules and dimers excited to the electronic S 2 state by 150 fs laser pulses at 202 nm has been studied in pump−probe experiments by detection of the ions and the coincident photoelectrons. The time-dependent ion signals reflect an ultrafast (∼50 fs) internal conversion from the excited S 2 state down to the lower electronic S 1 and S 0 states for the monomer as well as the dimer. The decay of the secondarily populated S 1 state proceeds within 4.3 ps for the toluene molecule and more than 100 ps for the dimer. The energy distribution of the photoelectron spectra for the monomer and the dimer ion demonstrate the significant geometry differences between the corresponding electronic states included in the processes of excitation, ionization and relaxation. The structure of the photoelectron spectra measured at simultaneous absorption of the pump and probe photons is tentatively assigned to vibrational modes of the ion states. The comparison of the electron spectra before and after the internal conversion out of the S 2 state directly reflects the growth of the vibrational energy in the secondarily populated S 1 states of the toluene monomer and dimer.

Time-resolved photoelectron imaging of S2 → S1 internal conversion in benzene and toluene

“…14 After S 2 −S 1 internal conversion, the molecule undergoes further internal conversion to S 0 and photochemical isomerization to various molecular forms, including Dewar benzene. 21,28 Similar experiments on toluene yielded lifetimes of 50 ± 10 fs in S 2 and 4.3 ± 0.2 ps in S 1 . [22][23][24][25][26][27] They observed that the photoexcited S 2 state changes to S 1 within 40 ± 10 fs and that vibrationally excited S 1 states created by internal conversion further decay within 6.7 ± 0.3 ps.…”

“…17,18 In the 1990s, Radloff et al [19][20][21] investigated the internal conversion from the S 2 state of benzene using timeresolved photoelectron spectroscopy (TRPES). 28 Lee et al obtained lifetimes of 43-54 fs and 9.4-88 ps, respectively, for S 2 and S 1 of benzene derivatives such as indene, styrene, and phenylacetylene using TRPES. 21,28 Similar experiments on toluene yielded lifetimes of 50 ± 10 fs in S 2 and 4.3 ± 0.2 ps in S 1 .…”

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

“…In the course of studies of the excited states of small aromatic molecules attention is often paid to issues such as intramolecular vibrational energy redistribution ͑IVR͒, [1][2][3][4][5][6][7][8] intersystem crossing and internal conversion, [9][10][11][12][13][14] torsional motion of side groups, 6,[15][16][17] and conformation. 18 Toluene in its first excited electronic state, S 1 , has been the subject of a number of extensive studies, 1,6 and is known to undergo intersystem crossing out of v =0, 10,13 and both internal conversion and IVR at higher levels of vibrational excitation.…”

Investigations of ultrafast dynamics in electronically excited alkylbenzenes

“…As the prototypical aromatic species benzene molecules have been the subject of a great number of investigations. Compared with benzene, the non-degeneracy of states in alkyl-benzenes leads to a higher density of the vibrational levels and to a reduction of symmetry forbidden interactions [1]. Hence, higher coupling rates for non-adiabatic transitions are expected.…”