Ab initio multiple cloning simulations of pyrrole photodissociation: TKER spectra and velocity map imaging.

Dmitry V Makhov,Dmitrii V Shalashilin,Kenichiro Saita,Todd J Martinez

doi:10.1039/c4cp04571h

Abstract

We report a detailed computational simulation of the photodissociation of pyrrole using the ab initio Multiple Cloning (AIMC) method implemented within MOLPRO. The efficiency of the AIMC implementation, employing train basis sets, linear approximation for matrix elements, and Ehrenfest configuration cloning, allows us to accumulate significant statistics. We calculate and analyze the total kinetic energy release (TKER) spectrum and Velocity Map Imaging (VMI) of pyrrole and compare the results directly with experimental measurements. Both the TKER spectrum and the structure of the velocity map image (VMI) are well reproduced. Previously, it has been assumed that the isotropic component of the VMI arises from long time statistical dissociation. Instead, our simulations suggest that ultrafast dynamics contributes significantly to both low and high energy portions of the TKER spectrum.

Highlights

We report a detailed computational simulation of the photodissociation of pyrrole using the ab initio Multiple Cloning (AIMC) method implemented within MOLPRO
Our AIMC calculation reproduces very well both the total kinetic energy release (TKER) spectrum and the strong anisotropy of the Velocity Map Imaging (VMI) observed in recent experiments,1a validating the mechanistic information inferred from the simulations
Apart from trivial differences in the notations, AIMC12 differs from ab initio MCE8,9 in the introduction of cloning, time displaced basis sets or trains, and a more accurate estimation [14] of the matrix element of the potential energy

Summary

Introduction

Never been calculated theoretically for these molecules. This is a difficult task because TKER spectra reflect important details of quantum dynamics in multidimensional systems, where realistic calculations beyond simple reduced dimensionality models are challenging. There Ehrenfest trajectories, which are guided by potential energy surface average of those of individual electronic states, may move outside of the dynamically important region This is remedied in the AIMC method by a procedure called cloning, which is an adaptation of spawning procedure of AIMS. The ab initio MCE method (without cloning) has been used previously for a more complete analysis of the dissociation of pyrrole.8 This showed that the 2B1 state of the pyrrolyl radical is only weakly populated, confirming the original assignment of the HRAPTS data by experimentalists. We noticed that after partial departure of the H-atom, the two electronic states 2A2 and 2B1 are still coupled via an intersection between the potential energy surfaces of the pyrrolyl radical, which is nearly isolated This intersection is reached by the pyrrolyl vibrational motion, and as a result the population of the higher 2B1 state resulting from rapid nonadiabatic dynamics of the N–H bond fission leaks to the lower 2A2 state via longer time dynamics at this final intersection. Our AIMC calculation reproduces very well both the TKER spectrum and the strong anisotropy of the VMI observed in recent experiments,1a validating the mechanistic information inferred from the simulations

II.1 Working equations

II.2 Matrix element approximations

Computational details

Results

Summary