Energy Partitioning and Spin-orbit Effects in the Photodissociation of Higher Chloroalkanes as Isolated Molecules and Clusters

Abstract

I. S.

Vinklárek1,2, J. Suchan3, J.

Rakovský1, K. Moriová1,2, V.

Poterya1, P. Slavíček3 and M.

Fárník1 1J. Heyrovský Institute of Physical Chemistry, Czech Academy of Sciences, Dolejskova 2155/3, 182 23 Prague 8 - Czechia 2Charles University, Faculty of Mathematics and Physics, Ke Karlovu 3, 121 16 Praha 2 - Czechia 3University of Chemistry and Technology, 166 28 Prague 6 - Czechia We apply velocity map imaging (VMI) technique 1,2 and molecule dynamics (MD) simulations based on novel molecule dynamics continuous-wave sampling method3 in our current research of the photolysis of four selected isolated alkyl chlorides (CxHyCl), i.e., methyl-, n-propyl-, i-propyl-, n-pentyl-chloride.

The halogenated hydrocarbons are benchmark systems for studying intersurface dynamics of polyatomic molecules and the excess energy partitioning into internal modes of molecules and clusters within the fast ~fs photodissociation. In our experiment, the alkyl chlorides are dissociated by optically active repulsive excited singlet 1Q1 and triplet 3Q0 states, which are coupled by significant spin-orbital interaction and diabatically correlated with the Cl-fragment in ground and spin-orbit excited state, respectively.

The molecule fragmentation is triggered by 193 nm excitation followed by resonantly enhanced ionization probe pulse to detect the ground or spin-orbit excited Cl-fragments. The energy-resolved distributions show that Cl-fragment kinetic energy is almost independent on the paternal molecule and that around 50% of the available energy after photodissociation is stored in inner modes of hydrocarbon fragment compared to only 10% in the case of CH3Cl.

This agrees well with both MD simulations and classical spectator model, which assumes low rigidity of investigated molecules. The angle-resolved distributions of Cl fragments combined with quantum yield ratio of Cl*/Cl fragments indicate according to model described in 2 enhanced spin-orbital coupling in A-band region correlated with hydrocarbon group prolongation.

This leads to strong absorption to 3Q0 state (15%) and over 60% probability of intersystem crossing between 1Q1 and 3Q0 states in both directions. These outcomes have impact on our future experiments with alkyl chlorides incorporated in water cluster environment mimicking the atmospheric aerosols.

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A 124, 38 (2020). [2] D. Townsend, S.K.

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A 108 39 (2004). [3] J. Suchan et al., Faraday discussions 212 (2018).