Photoelectron spectroscopy as a probe of intramolecular vibrational dynamics in electronically excited toluene

Green, Alistair Malcolm (2011) Photoelectron spectroscopy as a probe of intramolecular vibrational dynamics in electronically excited toluene. PhD thesis, University of Nottingham.

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Intramolecular vibrational energy redistribution (IVR) is a commonly-observed phenomenon whereby vibrational energy can be transferred between different parts of a polyatomic molecule. This process has profound implications for the understanding of chemical reactivity. In this work, IVR is studied in the S1 electronic state of toluene using time-resolved and frequency-resolved techniques. Both experiments are based upon laser photoelectron spectroscopy in the collision-free environment of a molecular beam.

The time-resolved experiments employ laser pulses of ~1 ps duration and ~15 cm^-1 bandwidth. In a pump-probe scheme, the molecule is first excited to a chosen superposition of vibrational states in the S1 manifold and then ionised by a second photon. The photoelectrons produced by the probe laser pulse are detected using velocity map imaging in order to obtain a vibrationally resolved photoelectron spectrum. Changes in the spectrum as a function of time give a direct view of the evolution of the vibrational state. The use of a two-colour ionisation scheme substantially improves the resolution compared with previous work.

High resolution zero kinetic energy (ZEKE) photoelectron spectra have also been obtained for the first time from excited vibrational levels in the S1 electronic state of toluene. These experiments employed laser pulses of ~5 ns duration and ~0.3 cm^-1 bandwidth, allowing the excitation of individual S1 vibrational levels rather than a superposition. The nanosecond and picosecond experiments therefore give complementary information.

A Fermi resonance at ~460 cm^-1 above the S1 origin in toluene is shown to be more complicated than previously thought, and provides the first demonstration of the use of time-resolved photoelectron spectroscopy to gain quantitative measurements of vibrational coupling matrix elements. Lifetimes of dissipative IVR have been determined following the preparation of high-frequency vibrations, and at intermediate energies several "doorway states" which mediate the IVR mechanism have been identified for the first time.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Reid, K.L.
Wright, T.G.
Subjects: Q Science > QD Chemistry > QD450 Physical and theoretical chemistry
Faculties/Schools: UK Campuses > Faculty of Science > School of Chemistry
Item ID: 12170
Depositing User: EP, Services
Date Deposited: 30 Apr 2012 09:29
Last Modified: 12 Oct 2017 11:54

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