Probing ultrafast C-Br bond fission in the UV photochemistry of bromoform with core-to-valence transient absorption spectroscopy

Benjamin W. Toulson, Mario Borgwardt, Han Wang, Florian Lackner, Adam S. Chatterley, C. D. Pemmaraju, Daniel M. Neumark, Stephen R. Leone, David Prendergast, Oliver Gessner

Research output: Contribution to journalArticleResearchpeer-review

Abstract

UV pump-extreme UV (XUV) probe femtosecond transient absorption spectroscopy is used to study the 268 nm induced photodissociation dynamics of bromoform (CHBr3). Core-to-valence transitions at the Br(3d) absorption edge (∼70 eV) provide an atomic scale perspective of the reaction, sensitive to changes in the local valence electronic structure, with ultrafast time resolution. The XUV spectra track how the singly occupied molecular orbitals of transient electronic states develop throughout the C-Br bond fission, eventually forming radical Br and CHBr2 products. Complementary ab initio calculations of XUV spectral fingerprints are performed for transient atomic arrangements obtained from sampling excited-state molecular dynamics simulations. C-Br fission along an approximately C S symmetrical reaction pathway leads to a continuous change of electronic orbital characters and atomic arrangements. Two timescales dominate changes in the transient absorption spectra, reflecting the different characteristic motions of the light C and H atoms and the heavy Br atoms. Within the first 40 fs, distortion from C 3 v symmetry to form a quasiplanar CHBr2 by the displacement of the (light) CH moiety causes significant changes to the valence electronic structure. Displacement of the (heavy) Br atoms is delayed and requires up to ∼300 fs to form separate Br + CHBr2 products. We demonstrate that transitions between the valence-excited (initial) and valence + core-excited (final) state electronic configurations produced by XUV absorption are sensitive to the localization of valence orbitals during bond fission. The change in valence electron-core hole interaction provides a physical explanation for spectral shifts during the process of bond cleavage.

Original languageEnglish
Article number054304
Number of pages14
JournalStructural Dynamics
Volume6
Issue number5
DOIs
Publication statusPublished - 1 Sep 2019

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Photochemical reactions
Absorption spectroscopy
photochemical reactions
fission
absorption spectroscopy
valence
Electronic states
Atoms
Electronic structure
Photodissociation
Molecular orbitals
Excited states
Molecular dynamics
Absorption spectra
electronics
electronic structure
atoms
orbitals
Pumps
Sampling

ASJC Scopus subject areas

  • Radiation
  • Instrumentation
  • Condensed Matter Physics
  • Spectroscopy

Fields of Expertise

  • Advanced Materials Science

Cite this

Probing ultrafast C-Br bond fission in the UV photochemistry of bromoform with core-to-valence transient absorption spectroscopy. / Toulson, Benjamin W.; Borgwardt, Mario; Wang, Han; Lackner, Florian; Chatterley, Adam S.; Pemmaraju, C. D.; Neumark, Daniel M.; Leone, Stephen R.; Prendergast, David; Gessner, Oliver.

In: Structural Dynamics, Vol. 6, No. 5, 054304, 01.09.2019.

Research output: Contribution to journalArticleResearchpeer-review

Toulson, BW, Borgwardt, M, Wang, H, Lackner, F, Chatterley, AS, Pemmaraju, CD, Neumark, DM, Leone, SR, Prendergast, D & Gessner, O 2019, 'Probing ultrafast C-Br bond fission in the UV photochemistry of bromoform with core-to-valence transient absorption spectroscopy' Structural Dynamics, vol. 6, no. 5, 054304. https://doi.org/10.1063/1.5113798
Toulson, Benjamin W. ; Borgwardt, Mario ; Wang, Han ; Lackner, Florian ; Chatterley, Adam S. ; Pemmaraju, C. D. ; Neumark, Daniel M. ; Leone, Stephen R. ; Prendergast, David ; Gessner, Oliver. / Probing ultrafast C-Br bond fission in the UV photochemistry of bromoform with core-to-valence transient absorption spectroscopy. In: Structural Dynamics. 2019 ; Vol. 6, No. 5.
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