Dynamics of H2O2photodissociation: OH product state and momentum distribution characterized by subhyphen;Doppler and polarization spectroscopy

摘要

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 laserhyphen;induced fluorescence technique. The entire internal state distribution (vibration, rotation, spin, and Lgr; 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 theXthinsp;2Pgr;3/2,1/2ground state with 90percnt; of the available energy (248 kJ/mol) being released as OH recoil translation. The angular distribution is nearly a sin2thinsp;thgr; 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 invlsquo;=0 can be described by a Boltzmann distribution with a temperature parameter ofTrot=(1530plusmn;150) K. The two spin states are found to be populated nearly statistically, in contrast to the Lgr; 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 excitationndash;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)0le;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 bgr;20(20), bgr;00(22), and bgr;20(22). The bipolar moment bgr;20(20) corresponds to the conventionally defined anisotropy parameter bgr;=2bgr;20(20) =minus;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 H2O2being of1Asymmetry. The moment bgr;00(22) increaseswithJOHshowing a bias towardsvOHandJOHbeing parallel to one another. The moment bgr;20(22) is a measure of the mutual correlation of the fragment translational and rotational vectors and the transition dipole vector mgr; in the parent molecule. The positive value of this moment lsqb;bgr;20(22)=0.11rsqb; indicates that the expectation value of lang;J2yrang; should be very small when mgr; is parallel to thezaxis andvOHperpendicular to mgr; (xaxis). lang;J2xrang; originates in the torsional motion in the H2O2parent molecule, while lang;J2zrang; reflects the bending vibration of nearly planar H2O2where the H atoms are in thetransposition.