Investigating photofragment product-state distributions utilizing velocity-aligned Doppler spectroscopy
Experience the future of the Tulane University Digital Library.
We invite you to visit the new Digital Collections website.
Description
Results are presented for a novel approach to the measurement of product-state populations arising out of the photodissociation of small gas phase molecules. Photoionization at the endpoint of a time-of-flight (TOF) apparatus using two, alternative probe beam geometries allows for the interrogation of reaction photofragments in either Doppler space or TOF space. The probe beam orientation, colinear with the photofragment flight path, provides a high-resolution variation of velocity-aligned Doppler spectroscopy (VADS). The second orientation, orthogonal to the TOF axis, is a variation of the well established photofragment translational energy spectroscopy The variation on velocity-aligned Doppler spectroscopy uses 1+1$\sp\prime$ resonance-enhanced ionization through the Lyman-$\alpha$ transition to interrogate hydrogen atom photofragments having their velocity vector aligned anti-parallel to the probe beam's axis. The VADS experiment provides baseline-resolved spectra of the nuclear hyperfine states in the $\sp2$S$\sb{1/2}$ ground state of atomic hydrogen from the 193nm, 248nm and 266nm photodissociation of jet-cooled hydrogen halide parent molecules. In addition, the onset of Stark effects in the n = 2 resonance state of atomic hydrogen has been observed in the modest electric fields of the ion extraction region of the detector The probe beam can be set orthogonal to the average velocity vector of the photofragments. In this manner, the wavelength of the probe is set to the zero-Doppler resonance frequency for 1+1$\sp\prime$ ionization, and the time-of-flight is measured for the photofragment of interest. The TOF spectra for the 193nm, 248nm and 266nm photodissociation of H$\sb2$Se, transformed to center-of-mass translational energy distributions, show photolysis wavelength dependent vibrational distributions and 'cold' rotational distributions for the HSe fragment. Both one-and two-photon dissociation channels are observed, with all eight possible exit channels present for the 266nm dissociation. The current resolution, when using the 266nm YAG line as the photolysis source, is $\rm\Delta E/E\sim 0.7\%$, or $\sim$65cm$\sp{-1}$. For 266nm dissociation, the rotational structure in the v$\sb0$ (HSe) vibrational level is well resolved. An analysis of the energy disposal distributions for H$\sb2$Se photodissociation will allow assignment of bond dissociation energies and provide data for characterization of the potential energy surfaces involved in the reaction