Time-dependent photoionization of azulene: Competition between ionization and relaxation in highly excited states

Abstract

Pump-probe photoionization has been used to map the relaxation processes taking place from highly vibrationally excited levels of the S 2 state of azulene, populated directly or via internal conversion from the S 4 state. Photoelectron spectra obtained by 1+2' two-color time-resolved photoelectron imaging are invariant (apart from in intensity) to the pump-probe time delay and to pump wavelength. This reveals a photoionization process which is driven by an unstable electronic state (e.g. doubly excited state) lying below the ionization potential. This state is postulated to be populated by a probe transition from S 2 and to rapidly relax via an Auger like process onto highly vibrationally excited Rydberg states. This accounts for the time invariance of the photoelectron spectrum. The intensity of the photoelectron spectrum is proportional to the population in S 2 . An exponential energy gap law is used to describe the internal conversion rate from S 2 to S 0 . The vibronic coupling strength is found to be larger than 60±5 µeV.

Cause of Asphaltene Fluorescence Intensity Variation with Molecular Weight and Its Ramifications for Laser Ionization Mass Spectrometry

“…This means that, in the process illustrated in Scheme 1 (for azulene, but it may be generic for polyaromatic molecules 22 ), the IC step S 1 f S 0 becomes faster and faster with an increasing MW. Then, beyond a certain MW value, essentially all of the S 1 state will be consumed in S 1 f S 0 IC and the molecule will not be detectable in LIMS.…”

“…In an elegant, detailed study 22 published recently on the time-dependent photoionization of azulene and the competition between ionization and relaxation in highly excited states, it was shown that the phenomena observed in the study may be quite generic in polycyclic aromatic molecules. A detailed consideration of all of the results and their interpretations lies outside the scope of the present paper, and only the relevant salient features of this study will be discussed here, as follows:…”

“…Now, considering the data in 2-5 we can see that k(IC) 16 for the S 4 f S 2 transition (the analogue of the S n > 1 f S 1 transition in most polycyclic aromatic molecules) is independent of the vibrational energy and about 2-4 orders of magnitude larger than k(IC) for the S 2 f S 0 transition (the analogue of the S 1 f S 0 transition), which is a function of the excess vibrational energy in S 2 , ∼1.3 × 10 13 s -1 versus 10 8.4 -10 10.3 s -1 , 22 in line with the known general trend for k(IC) n > 1 f 1 and k(IC) 1 f 0 .…”

Femtosecond time-resolved electronic relaxation dynamics in tetrathiafulvalene

“…The Rydberg states, due to their peculiar strong propensity for ∆v = 0 ionization, are also expected to be distinct from the other contributions to the kinetic energy and angular distributions of the photoelectrons. 33,36 The excitation schemes for all wavelength combinations are summarized in Figure 1. One THG photon (267 nm) is enough to probe the wave packet dynamics onto D 0 or at the threshold of D 1 .…”

“…A decay rate increases exponentially with vibrational excitation in the initial state but not indefinitely. 36 A plateau is reached when the density of vibrational levels in the initial state is so large that the entropy of the initial state (M*) can no longer be neglected. 42 This corresponds to a wavepacket in which the vibronic coupling becomes more and more diluted in the uncoupled vibrational modes.…”