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Published on: July 27, 2018
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Dynamics and yields for CHBrCl2 photodissociation from 215-265 nm
Wyatt G Merrill1, F Fleming Crim1, Amanda S Case1
1Department of Chemistry, University of Wisconsin - Madison, Madison, Wisconsin 53706, USA. fcrim@chem.wisc.edu ascase@chem.wisc.edu.
Physical Chemistry Chemical Physics : PCCP
|November 26, 2016
Summary
This study examines the photodissociation of CHBrCl2, revealing how different wavelengths influence bromine atom (Br) and excited bromine atom (Br*) production. The findings highlight the role of excited states and potential crossings in determining product yields and fragment energy partitioning.
Area of Science:
- Photochemistry
- Chemical Physics
- Molecular Dynamics
Background:
- Understanding photodissociation dynamics is crucial for atmospheric chemistry and reaction mechanisms.
- Halomethanes are important model systems for studying bond cleavage and energy transfer in molecules.
Purpose of the Study:
- To investigate the Ã-band photodissociation of CHBrCl2 across a range of UV wavelengths.
- To quantify the relative yields of Br and Br* production and analyze photofragment energy partitioning.
- To elucidate the roles of specific excited electronic states and potential energy surface crossings in the dissociation pathways.
Main Methods:
- Resonance enhanced multiphoton ionization (REMPI) coupled with time-of-flight mass spectrometry (TOF-MS) for product detection and yield quantification.
- Velocity-map imaging (VMI) to determine energy partitioning and anisotropy of photofragments.
- Impulsive modeling and comparison with related halomethane systems.
Main Results:
- Selective detection of Br/Br* production channels following C-Br bond cleavage.
- Energy partitioning analysis revealed wavelength-dependent behavior and significant internal excitation of the CHCl2 fragment.
- Anisotropy data indicated the importance of the 3Q0(A') excited state and its avoided crossing with the 1Q1(A') state, influencing Br yield.
- Spin-orbit excitation of Br largely occurred at the expense of CHCl2 internal energy.
Conclusions:
- The photodissociation pathways of CHBrCl2 are complex, involving competition between different excited states and influenced by potential energy surface crossings.
- Adiabatic passage through the avoided crossing between 3Q0(A') and 1Q1(A') states is a key factor in Br yield at longer wavelengths.
- Dissociation leads to highly rotationally excited CHCl2 fragments, consistent with impulsive models and C-s symmetry.
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