Knowledge Bank

The \nub{3} band of CH$3$F isolated in solid $para$-H$2$ exhibits a clearly resolved series of lines due to clusters of CH$3$F with residual $ortho$-H$2$ molecules, CH$3$F-($ortho$-H$2$), with $N$ = 0 $\sim$ 12. \textbf{119}, 4731 (2003).} We have examined this spectrum in detail using a quantum cascade laser source. Solid hydrogen crystals were formed by direct vapor phase deposition on a BaF$2$ window at 2 K followed by annealing (4.6 K for 10 minutes) or super-annealing" (7 K for 10 seconds). The cw laser (Hamamatsu Corp.) was a room temperature DFB device with output powers up to ~30 mW and a tuning range of about 1042 - 1036 $\wn$ for device temperatures of 0 - $37^irc}$ C. This power level was easily strong enough to modify the sample by means of spectral hole-burning effects. It was therefore necessary to strongly attenuate the laser in order to record normal static" spectra. But we also took advantage of the power to make interesting ``dynamic" measurements in which a transition could be bleached away and forced to reappear in a new position. Line widths as narrow as ~0.0065 $\backslash wn$ were observed and line profiles were Lorentzian. The $N$ = 0 line at 1040.19 $\backslash wn$ was resolved into two closely-spaced ($\sim$0.008 $\backslash wn$) components which we believe correspond to the $K$ = 0 ($A$) and 1 ($E$) levels of CH$3$F, arising from partially free $a$-axis rotation in the crystal. \textbf{129}, 104502 (2008).} Numerous extra satellite lines were observed around each main feature ($N$ = 0, 1, 2, 3, 4) as well as some weak but sharp transitions almost mid-way between $N$ = 0 and 1.