Dynamics at Conical Intersections

Abstract

The nonadiabatic coupling of electronic and vibrational degrees of freedom is the defining feature of electronically excited states of polyatomic molecules. Once considered a theoretical curiosity, conical intersections (CIs) are now generally accepted as being the dominant source of coupled charge and vibrational energy flow in molecular excited states. Passage through CIs leads to the conversion of electronic to vibrational energy, which drives the ensuing photochemistry, isomerization being a canonical example. It has often been remarked that the CI may be thought of as a transition state in the excited state. As such, we expect that both the direction and the velocity of approach to the CI will matter. We explore this suggestion by looking for dynamical aspects of passage through CIs and for analogies with well-known concepts from ground-state reaction dynamics. Great progress has been made in the development of both experimental techniques and ab initio dynamics simulations, to a degree that direct comparisons may now be made. Here we compare time-resolved photoelectron spectroscopy results with on-the-fly ab initio multiple spawning calculations of the experimental observables, thereby validating each. We adopt a phenomenological approach and specifically concentrate on the excited-state dynamics of the C=C bond in unsaturated hydrocarbons. In particular, we make use of selective chemical substitution (such as replacing an H atom by a methyl group) so as to alter the inertia of certain vibrations relative to others, thus systematically varying (mass-weighted) directions and velocities of approach to a CI. Chemical substituents, however, may affect both the nuclear and electronic components of the total wave function. The former, which we call an inertial effect, influences the direction and velocity of approach. The latter, which we call a potential effect, modifies the electronic structure and therefore the energetic location and topography of the potential energy surfaces involved. Using a series of examples, we discuss both types of effects. We argue that there is a need for dynamical pictures and simple models of nonadiabatic dynamics at CIs and hope that the phenomenology presented here will help inspire such developments.

Revealing electronic state-switching at conical intersections in alkyl iodides by ultrafast XUV transient absorption spectroscopy

“…The dynamics of chemical reactions are traditionally viewed within the Born–Oppenheimer approximation, which assumes separation of nuclear and electronic degrees of freedom in the system. While often applicable to reactions in ground electronic states, this approximation is frequently insufficient to describe excited state dynamics following photoexcitation where extensive couplings between electron and nuclear motions arise 1 – 4 . At crossings between the potential energy surfaces of electronic states where conical intersections are formed, the presence of strong couplings allow a molecule to abruptly transfer from one surface to another, thereby switching its electronic character.…”

Comparison of moving and fixed basis sets for nonadiabatic quantum dynamics at conical intersections

“…Intersections and near-intersections of potential energy surfaces (PESs) can occur in polyatomic molecules 1,2 and lead to ultrafast nonradiative decay of electronically excited states. [3][4][5][6] Often the nonradiative electronic transitions may be accompanied with remarkable nuclear dynamics, [7][8][9][10][11][12] the isomerization of molecular photoswitches being a typical example. [13][14][15][16] A prototypical situation of intersections between PESs in polyatomic molecules is the so-called conical intersection (CoIn) of two PESs.…”

Exploring the Excited-State Dynamics of Hydrocarbon Radicals, Biradicals, and Carbenes Using Time-Resolved Photoelectron Spectroscopy and Field-Induced Surface Hopping Simulations

“…To answer these questions, we require an experimental method that is sensitive to the character of the electronic states involved in the dynamics and at the same time suitable to detect molecules at low concentrations. Time-resolved-photoelectron spectroscopy (TRPES) of valence electrons has been the method of choice to investigate the excited-state dynamics of many closed-shell molecules and anions [8][9][10][11][12] , as it offers several advantages:…”