Dynamical, spectroscopic and computational imaging of bond breaking in photodissociation: roaming and role of conical intersections

Recent experimental and theoretical advances in the study of dissociation of excited molecules are revealing unexpected mechanisms, when their outcomes are tackled by combining (i) space-time ion imaging of translational features, with (ii) spectroscopic probing of rotational and vibrational distributions; crucial is the assistance by (iii) quantum chemistry of structural investigations of rearrangements of chemical bonds, and by (iv) the simulations of molecular dynamics to follow the evolution of selective bond stretching and breaking. Here we present results of such an integrated approach to methyl formate, HCOOCH3, the simplest of esters; the main focus is on rotovibrationally excited CO (v=1) product and in general on the energy distribution in the fragments. Previous laser studies of dissociation at a sequence of various wavelengths discovered signatures of a roaming mechanism by late arrival in time-of-flight ion imaging of CO (v=0) products. Subsequent detailed investigations as a function of excitation energy provided the assessment of the threshold, which opens for the breakdown in further fragments H and CH3O, as spectroscopically characterized by ion imaging and FTIR respectively. Accompanying quantum mechanical electronic structure calculations and classical molecular dynamics simulations clarify the origin of these fragments through "roaming" pathways involving incipient radical intermediates at energies below the triple fragmentation threshold: a specific role is played by nonadiabatic transitions at a conical intersection between ground and excited states; alternative pathways focalize our attention to regions of the potential energy surfaces other than those in the neighbourhoods of saddle points along minimum energy paths: eventually this leads to find avenues in reaction kinetics beyond those of venerable transition state theories.