Product angular distributions in the ultraviolet photodissociation of N2O

Abstract

The angular distribution of products from the ultraviolet photodissociation of nitrous oxide yielding O( 1 D) and N 2 (X 1 + g ) was investigated using classical trajectory calculations. The calculations modeled absorption only to the 2 1 A electronic state but used surface-hopping techniques to model nonadiabatic transitions to the ground electronic state late in the dissociation. Observed values of the anisotropy parameter β, which decrease as the product N 2 rotational quantum number j increases, could be well reproduced. The relatively low observed β values arise principally from nonaxial recoil due to the very strong bending forces present in the excited state. In the main part of the product rotational distribution near 203 nm, an unusual dynamical effect produces the decrease in β with increasing j; nonaxial recoil effects remain approximately constant while higher j product molecules arise from parent molecules that had their transition dipole moments aligned more closely along the molecular axis. In both low and high j tails of the rotational distribution, the variations in β with j are caused by changes in the extent of nonaxial recoil. In the high-j tail, additional torque present on the ground state potential energy surface following nonadiabatic transitions causes both the additional rotational excitation and the lower β values.

The ultraviolet spectrum of OCS from first principles: Electronic transitions, vibrational structure and temperature dependence

“…We have studied the photodissociation of N 2 O in a series of papers and essentially all experimental results have been well reproduced. [41][42][43][44][45][46][47][48][49] A consistent and nearly complete picture has thus been obtained. This paper is part of a similar investigation of the UV photodissociation of OCS.…”

Ultraviolet photodissociation of OCS: Product energy and angular distributions

“…In Ref. 17 it was shown for N 2 O that the coupling term with respect to motion in R is much smaller and the same is assumed for OCS; because of the considerably smaller excess energy and the higher reduced mass the velocityṘ is smaller for OCS dissociation. The CO vibration remains almost unexcited along the trajectory (see below) and therefore the coupling with respect to r also can be neglected.…”

“…This restriction provided a trustworthy description of the photoexcitation of N 2 O. 13,17,28 Subsequent calculations, however, showed that for OCS higher states need to be included in the CASSCF averaging. The reason is probably intensity borrowing from the bright 4 1 A state.…”

“…17 This parameter describes the distribution of product velocity vectors about the polarization vector of the dissociating light according to 42 I(θ ) ∝ 1 + βP 2 (cos θ ), where P 2 (x) is a Legendre polynomial and θ is the angle between the product velocity and electric field vectors. The transition dipole moment for this A ← A transition must lie in the triatomic plane.…”

“…Following our theoretical work on the UV photodissociation of N 2 O, [13][14][15][16][17] which has the same number of 16 valence electrons, we recently began to study the complex photodissociation dynamics of OCS. 18,19 We calculated global three-dimensional PESs for the four lowest singlet states (letters in parentheses denote abbreviations used below)-1 1 A (X), 2 1 A (A), 1 1 A (B), and 2 1 A (C),-and the four lowest triplet states-1 3 A (a), 2 3 A (d), 1 3 A (b), and 2 3 A (c)-employing the multi-configuration reference internally contracted configuration interaction (MRCI) theory.…”