Product quantum-state-dependent anisotropies in photoinitiated unimolecular decomposition

Abstract

Angular distributions of state-selected NO and O products in the photoinitiated unimolecular decomposition of jet-cooled NO 2 have been measured by using both the photofragment ion imaging technique with velocity map imaging and ion time-of-flight translational spectroscopy. The recoil anisotropy parameter of the photofragments, ␤, depends strongly on the rotational angular momentum of the photoproduct. O( 3 P jϭ2,0 ) angular distributions are recorded at photolysis wavelengths 371.7, 354.7, and 338.9 nm. At these wavelengths, respectively, vibrational levels v ϭ0, vϭ0,1 and vϭ0 -2 of NO are generated. In addition, ␤ values for NO(vϭ2) in specific high rotational levels are determined at ϳ338 nm. The experimental observations are rationalized with a classical model that takes into account the transverse recoil component mandated by angular momentum conservation. The model is general and applicable in cases where fragment angular momentum is large, i.e., a classical treatment is justified. It is applied here both to the experimental NO 2 results, and results of quantum calculations of the vibrational predissociation of the Ne-ICl van der Waals complex. It is concluded that deviations from the limiting ␤ values should be prominent in fast, barrierless unimolecular decomposition, and in certain dissociation processes where a large fraction of the available energy is deposited in rotational excitation of the diatom. The application of the model to NO 2 dissociation suggests that the nuclear dynamics leading to dissociation involves a decrease in bending angle at short internuclear separations followed by a stretching motion. This interpretation is in accord with recent theoretical calculations.

Ultraviolet photodissociation of OCS: Product energy and angular distributions

“…In the main part of the rotational distribution, it decreases slowly with increasing j, and the behavior is essentially the same on the A and B surfaces. This slow change arises from the increased torque felt by molecules dissociating to higher j, and its basic physics is captured by the empirical models used by Demyanenko et al 46 and Brouard et al 3 In the upper part of the rotational distribution, representing only excitation to the A state followed by transitions to X, the deflection angle changes more steeply for the same reason.…”

UV photodissociation dynamics of CHI2Cl and its role as a photolytic precursor for a chlorinated Criegee intermediate

“…21,22 Angular distributions. ET-dependent anisotropy parameters derived from fitting the I and I* angular distributions are shown in Figure 2 and where χ is defined as the angle between the transition dipole moment, μ̂, and the Jacobi vector between the departing I atom and the center of mass of the CHICl radical, R. 79 CHI2Cl belongs to the Cs point group; the plane of symmetry is defined as the x-y plane, as depicted in Figure 6. The transition dipole moments lie within the x-y plane for excitation to A′ states and along the z-axis for excitation to A″ states.…”

Product angular distributions in the ultraviolet photodissociation of N2O

“…The components v y and v z of the final O atom velocity were obtained as described in Ref. 34, using the final orientation angle φ R of the R vector obtained through angular momentum constraints 35 propagated along the trajectory. No special procedures were needed to compute β from the surface hopping trajectories, except to check for possible artifacts as described below.…”

“…We conclude that the observed average values of β, substantially lower than the limiting value of 2 that would be expected for axial recoil from a linear initial configuration, appear largely because of nonaxial recoil due to the strong bending forces in the excited state. (We use the term "nonaxial recoil" to indicate final velocities that are not parallel to the initial direction of the breaking bond, rather than adopting the usage of Demyanenko et al 35 who apply it to velocities that are not parallel to the R vector late in the dissociation. )…”

“…With detailed trajectories in hand, we are in a position to examine the empirical model described by Demyanenko et al 35 in the context of NO 2 photodissociation and applied in modified form by Brouard et al to both OCS and N 2 O photodissociation. 36 The model, like our trajectory study, uses classical mechanics and assumes zero total angular momentum.…”