Valence Photoionization of Thymine: Ionization Energies, Vibrational Structure, and Fragmentation Pathways from the Slow to the Ultrafast

Abstract

The photoionization of thymine has been studied by using vacuum ultraviolet radiationa nd imaging photoelectronp hotoion coincidences pectroscopy after aerosol flash vaporization andb ulk evaporation.T he two evaporation techniquesh ave been evaluated by comparison of the photoelectrons pectra and breakdown diagrams. The adiabatic ionization energies for the first four electronic states were determined to be 8.922 AE 0.008,9 .851 AE 0.008, 10.30 AE 0.02, and 10.82 AE 0.01 eV.V ibrational features have been assigned for the first three electronic states with the help of Franck-Condon factor calculations based on densityf unctional theory and wave function theory vibrational analysis within the harmonic approximation. The breakdown diagram of thymine, as supported by composite methoda bi nitio cal-culations,s uggestst hat the main fragment ions are formed in sequential HNCO-, CO-, and H-loss dissociation steps from the thymine parenti on, with the first step corresponding to ar etro-Diels-Alder reaction. The dissociation rate constants were extractedf rom the photoion time-of-flight distributions and used together with the breakdown curvest oc onstruct as tatistical model to determine 0K appearance energies of 11.15 AE 0.16 and 11.95 AE 0.09 eV for the m/z 83 and 55 fragment ions, respectively.T hese results have allowedu st o revise previously proposed fragmentation mechanismsa nd to proposeamodel for the final, nonstatistical H-losss tep in the breakdown diagram, yielding the m/z 54 fragment ion at an appearance energy of 13.24 eV.

New analytical tools for advanced mechanistic studies in catalysis: photoionization and photoelectron photoion coincidence spectroscopy

“…73 Furthermore, FC factors can be routinely calculated in the double harmonic approximation, and can reliably aid the interpretation and assignment of even crowded spectra. 74 No comparable approaches exist for the interpretation of PI spectra, which explains why iPEPICO provides superior isomerselectivity for reactive intermediates in pyrolysis studies, 75,76 flame sampling, 77,78 kinetic experiments, 79 and, more recently, also in catalysis. 14,15,61,62 Figure 7a shows ms-TPE spectra of the three xylylene isomers together with FC simulations in excellent agreement.…”

“…Excellent agreement with excited electronic state calculations and FC simulations are also common. 72,74 The velocity map electron images add a further layer of experimental information. Electrons are only imaged according to their inplane momenta in the raw images, but the 3D momentum distribution can be reconstructed to obtain the kinetic energy distribution as well as the angular distribution as a function of energy.…”

Conformers, electronic states, and diabolical conical intersections in the valence photoelectron spectroscopy of halocyclohexanes

“…This approach has proven to be a powerful, isomer-selective analytical tool in the analysis of reactive mixtures. 16,17 It has revealed the role of the electronic state in fragmentation processes of small halogen-substituted molecular ions, 18,19 unveiled complex dissociation mechanisms with parallel and sequential steps, 20,21 and delivered accurate thermochemistry and energetics of elusive species. [22][23][24] Furthermore, the two diastereomers, trans and cis, of the 1-methylallyl radical in pyrolysis 25 and combustion 26 environments could also be identified, similar to the fingerprints of the axial and equatorial conformers of fluorocyclohexane, C 6 H 11 F, with only a few tens of meV ionization energy difference.…”

Ultrafast Photo‐ion Probing of the Ring‐Opening Process in Trans‐Stilbene Oxide

“…In general, one can identify the same ions along the mass/charge axis of Figure 2a as what was observed in the UV/UV experiment (Figure 1). Figure 2b shows lineouts taken from data seen in Figure 2a for fixed masses, giving the mass‐selected threshold photoelectron spectra (ms‐TPES) [30] . The appearance energies of photo‐ions are not affected by the threshold method, however the shape of the presented curves generally differs from non‐threshold spectra.…”

“…The sample was introduced into the spectrometer through heated piping connected to a sample reservoir, with the reservoir held at 53 °C and the piping at a higher temperature to avoid unwanted sample deposition and blockages. The ion arrival events in this measurement were selected by coincident electrons with low kinetic energy, creating a threshold ion spectrum [30] . Whilst only a sub‐section of the results of this experiment are presented here, full VUV experimental results covering a larger range of photon energies will be published at a later date.…”