Combining Time-of-Flight Methods and Velocity-Aligned Doppler Spectroscopy to Measure Wavelength-Dependent Product State Distributions in H₂Se Photolysis

Abstract

Results are presented on the ultraviolet photolysis of H2Se. An experimental approach is introduced that combines velocity-aligned Doppler spectroscopy (VADS) with time-of-flight (TOF) methods. Atomic hydrogen is detected, and by utilizing a tunable photolysis source, one is able to map out propensities for populating HSe vibrational states as a function of photolysis wavelength. Interest in observing such behavior in triatomic systems was suggested theoretically by Schinke and co-workers in the mid-1980s.

Using Anisotropy Measurements from A-Band Photodissociation to Interrogate the Excited States of H2Se

“…These cross-sections (in some arbitrary unit) are assigned to the exit channels with the spin–orbit, vibrational, and rotational states of SeH all being distinguished. As part of the results of this simulation, the rotational and vibrational populations of the SeH fragment have already been reported in a previous paper . The angular dependence of the photolysis process is modeled using the β anisotropy parameter (see Section C discussion).…”

“…In these experiments, the probe light propagates perpendicular to the photolysis k-vector and against the traveling direction of the H-atom fragment (the so-called T-configuration). , Delay-time-synchronized VADS experiments are performed. In this type of experiment, the probe laser frequency and the photolysis/probe delay time are adjusted synchronously so that H atoms with the desired Doppler shift are interrogated at the correct time in the ionization region.…”

“…14−18 In analogy with their lighter homologues, the diffuse bands of H 2 Se and H 2 Te are assigned to np → (n + 1)s Rydberg excitations possibly overlaid by weak valence shell promotions. 19 In a previous paper, 15 220 nm. In the current article, we also use this modified VADS approach to probe the angular dependence of the H 2 Se photolysis system in the same wavelength range (266−210 nm).…”

“…In contrast with the first two members, the photodissociation of the next two members of the 6A group hydrides, H 2 Se and H 2 Te, in their first diffusive bands have been studied less extensively. − In analogy with their lighter homologues, the diffuse bands of H 2 Se and H 2 Te are assigned to np → (n + 1)s Rydberg excitations possibly overlaid by weak valence shell promotions . In a previous paper, we used a variation on velocity-aligned Doppler spectroscopy (VADS) and reported on rotational and vibrational state distributions for the SeH radical subsequent to the photodissociation of H 2 Se in the wavelength range of the A-band. It was found that the SeH(X 2 Π 3/2 , v = 1) population has a maximum around 230–220 nm.…”

Laser detection of spin-polarized hydrogen from HCl and HBr photodissociation: Comparison of H- and halogen-atom polarizations

“…10,27,28 In contrast, the fluorescence of excited H-atom states from the photodissociation of H 2 have been studied extensively. [29][30][31][32][33][34] The main reason for the lack of studies on ground-state H-atom photofragments has been that spin-sensitive detection of H atoms has-until now-been achieved only by resolving the fine structure of the 2p ← 1s transition; [35][36][37] this requires that the Doppler spread of the H atoms be very small, which will occur either in collimated atomic beams or from samples having a translational temperature of less than about 80 K. This constraint does not allow H-atom spin polarization to be detected without some stringent form of velocity selection; one example is the detection of H and D atoms from HCl and DCl photodissociation, by Koplitz and co-workers, [37][38][39][40] using velocity-aligned Doppler spectroscopy ͑VADS͒, which allows selective excitation of the fine structure of H-atom photofragments traveling parallel to the probe beam. This spatial velocity selection is achieved by detecting only the H atoms that have traveled about 0.5 m from the photolysis region to the probe, after a long time delay.…”