Non-adiabatic photodissociation dynamics of BrCl and BrNO
Date
2018-07-18
Authors
Loock, Hans-Peter
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Abstract
Photodissociation experiments on expansion cooled bromine chloride (BrCl) and nitrosyl
bromide (BrNO) were conducted using a pump / probe scheme. Resonance-enhanced- multiphoton-
ionization (REMPI) spectroscopy allowed for state selective ionization of the
photofragments. Using time-of-flight mass spectrometry (TOF-MS), it was possible to
make use of the E-μ-ν vector correlation in order to identify, and characterize, the parent
molecule's excited electronic states.
Photodissociation experiments on BrCl were carried out at six wavelengths between 500
nm and 389 nm. These experiments established the [special characters omitted]) fragment channel
as one of the diabatic dissociation limits of the [special characters omitted] state. Application of the theory
of diatomic dissociation as developed by Singer, Freed and Band (J. Chem. Phys., 79,
(1983), 6060) led to a new diabatic correlation diagram. This correlation diagram explained
not only our experimental results well, but was also coherent with earlier experimental and
theoretical investigations on other interhalogens.
The UV-Vis absorption spectrum of BrNO was recorded. At ten wavelengths between 740
nm and 225 nm photodissociation experiments were carried out. Guided by comparison
with the well-studied CINO dissociation dynamics, we propose the BrNO frontier orbitals
and assign the absorption bands to the various low lying electronic transitions. The
absorption from 740 nm to 370 nm is dominated by transitions to triplet states, which
borrow intensity from the higher energy singlet state, S5. A diabatic correlation diagram
was constructed to aid an understanding of the photodissociation dynamics. These
considerations combined with the recorded fragment quantum state distributions led to a
qualitative understanding of the topology of the excited state potential energy surfaces.
Non-adiabatic interactions following excitation to the [special characters omitted] state of BrNO were
investigated and could be understood with our correlation diagram. We observed a strong
correlation of the adiabalicity of the photodissociation process to the NO rotational levels.
This indicates that the interaction region of the [special characters omitted] state with the S3 and/or S4 state is
located early in the dissociation coordinate but late in the bending coordinate.
The lowest lying triplet state, T1 absorbs via two vibrational adiabats which correlate to
NO fragments in their v" = 0 and v" = 1 vibrational state, respectively. The NO rotational
distribution is bimodal indicating that the excited state is rather shallow.
Following excitation at 355 nm a narrow oscillation of the spatial distribution from a cos2θ
distribution to a sin2θ distribution was observed as a function of the NO rotational
excitation. Assuming that this effect is due to an interference between different
excitation/dissociation pathways, two tentative explanations based on the new correlation
diagram are offered.
Description
Keywords
Photoionization, Dissociation