SUB-DOPPLER SPECTROSCOPY OF MIXED STATE LEVELS IN CH$_2$
Loading...
Date
2010
Journal Title
Journal ISSN
Volume Title
Publisher
Ohio State University
Abstract
Saturation dip spectroscopy has been used to measure rovibronic transitions in the $\tilde{b}\, ^1B_1 \leftarrow \tilde{a}\, ^1A_1$ band system of CH$_2$ at sub-Doppler resolution. The radical was made by 308 nm excimer laser photolysis of a slowly flowing, low-pressure, sample of ketene (CH$_2$CO), optionally with added inert buffer gas. Typical observed linewidths in the pure precursor are approximately 8 MHz (FWHM), due to a combination of collisional lifetime and pump-probe beam crossing angle. Due to the non-zero $^1$H proton nuclear spin, CH$_2$ exists as two distinct variants, ortho-CH$_2$ with $I_H = 1$ and para-CH$_2$ with $I_H = 0$. In ortho-CH$_2$, each rotational level consists of a triplet of hyperfine components corresponding to levels with $F= J, J \pm 1$. Most singlet CH$_2$ transitions show unresolved hyperfine structure in our experiment, since the largest splitting is due to $\bf{I.J}$ coupling, typically of the order of kHz. However, a small number of rotational levels in the $v=0$ level of the lower $\tilde{a}$ state are known to be perturbed by accidentally near degenerate, 1058-1072 (1993)} $\tilde{X}\,^3B_1$-state levels via spin-orbit coupling. Spectra involving such levels in ortho-CH$_2$ exhibit resolvable triplet, $\bf{I.S}$, hyperfine splittings, with the splittings providing a direct measure of triplet state character of the level. We have measured hyperfine splittings for a number of pairs of perturbed levels confirming and refining previous estimates of the singlet-triplet mixing coefficients. Measurements of the pressure-dependent saturation recovery rates with different collision partners can give new insights into dephasing, velocity-changing and inelastic collisions relevant to pressure broadening and intersystem crossing mechanisms. Acknowledgments: Work at Brookhaven National Laboratory was carried out under Contract No. DE-AC02-98CH10886 with the U.S. Department of Energy and supported by its Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences.
Description
Author Institution: Chemistry Department, Brookhaven National Laboratory, Upton, New York, 11973