Analyzing Angular Distributions for Two-Step Dissociation Mechanisms in Velocity Map Imaging

Increasingly, velocity map imaging
is becoming the method of choice
to study photoinduced molecular dissociation processes. This paper
introduces an algorithm to analyze the measured net speed, P(vnet), and angular, β­(vnet), distributions of the products from a two-step
dissociation mechanism, where the first step but not the second is
induced by absorption of linearly polarized laser light. Typically,
this might be the photodissociation of a C–X bond (X = halogen
or other atom) to produce an atom and a momentum-matched radical that
has enough internal energy to subsequently dissociate (without the
absorption of an additional photon). It is this second step, the dissociation
of the unstable radicals, that one wishes to study, but the measured
net velocity of the final products is the vector sum of the velocity
imparted to the radical in the primary photodissociation (which is
determined by taking data on the momentum-matched atomic cophotofragment)
and the additional velocity vector imparted in the subsequent dissociation
of the unstable radical. The algorithm allows one to determine, from
the forward-convolution fitting of the net velocity distribution,
the distribution of velocity vectors imparted in the second step of
the mechanism. One can thus deduce the secondary velocity distribution,
characterized by a speed distribution P(v1,2°) and an angular distribution I(θ2°), where θ2° is the
angle between the dissociating radical’s velocity vector and
the additional velocity vector imparted to the product detected from
the subsequent dissociation of the radical.