Knowledge Bank

Saturation dip spectroscopy has been used to measure rovibronic transitions in the $b~1B1\leftarrow a~1A1$ 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 $IH=1$ and para-CH$2$ with $IH=0$. 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 $I.J$ coupling, typically of the order of kHz. However, a small number of rotational levels in the $v=0$ level of the lower $a~$ state are known to be perturbed by accidentally near degenerate, 1058-1072 (1993)} $X~3B1$-state levels via spin-orbit coupling. Spectra involving such levels in ortho-CH$2$ exhibit resolvable triplet, $I.S$, 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.