Dynamics of H2O2 photodissociation: OH product state and momentum distribution characterized by sub-Doppler and polarization spectroscopy

Abstract

Hydrogen peroxide has been optically excited at a wavelength of 266 nm and the OH photofragment completely characterized by Doppler and polarization spectroscopy using the laser-induced fluorescence technique. The entire internal state distribution (vibration, rotation, spin, and Λ components), translational energy, angular distribution, rotational alignment, and vector correlations between rotational and translation motions of OH products is measured. The hydroxyl radicals are formed in the X 2Π3/2,1/2 ground state with 90% of the available energy (248 kJ/mol) being released as OH recoil translation. The angular distribution is nearly a sin2 θ distribution about the electric vector of the photolysis laser. The internal motion of OH is vibrationally cold (no vibrationally excited OH was found) while the rotational excitation in v″=0 can be described by a Boltzmann distribution with a temperature parameter of Trot=(1530±150) K. The two spin states are found to be populated nearly statistically, in contrast to the Λ components which show an increasing inversion with increasing OH rotation. The observed profiles of recoil Doppler broadened spectral lines are strongly dependent on the nature of the transition, the excitation–detection geometry, and the relative polarizations of the dissociating and analyzing laser light. However, the line intensities show only a minor dependence on geometry and polarization indicating a low alignment of OH photofragments (A(2)0≤0.1). For the first time the vector correlation between product rotational and translational motions was analyzed and evaluated in terms of the three bipolar moments β20(20), β00(22), and β20 (22). The bipolar moment β20(20) corresponds to the conventionally defined anisotropy parameter β=2β20(20) =−0.71. The angular distribution peaks in the direction perpendicular to the electric vector of the dissociating laser light, indicating the predominant electronic excited state in H2O2 being of 1A symmetry. The moment β00(22) increases with JOH showing a bias towards vOH and JOH being parallel to one another. The moment β20(22) is a measure of the mutual correlation of the fragment translational and rotational vectors and the transition dipole vector μ in the parent molecule. The positive value of this moment [β20(22)=0.11] indicates that the expectation value of 〈J2y〉 should be very small when μ is parallel to the z axis and vOH perpendicular to μ (x axis). 〈J2x〉 originates in the torsional motion in the H2O2 parent molecule, while 〈J2z〉 reflects the bending vibration of nearly planar H2O2 where the H atoms are in the trans position.