Dynamics of cw + pulsed optical-optical double resonance excitation of the $E^{2}\Sigma^{+}-A^{2}\Pi_{1/2}-X^{2}\Sigma^{+}$ transition in CaF

$^{1}$ P. F. Bernath and R. W. Field. J. Mol. Spec. 82, 339 (1980).Author Institution: Department of Chemistry, Massachusetts Institute of TechnologyWe present preliminary results of the OODR excitation of the $E^{2}\Sigma^{+}$ low-lying Rydberg state of CaF using one cw and one pulse-amplified single-mode dye laser. Dynamical information crucial to the study of higher-lying, as yet unobserved Rydberg states in CaF and related molecules will be discussed. Bernath has obtained and assigned rotationally resolved OODR spectra of the E-A-X excitation series using two single-mode cw dye $lasers.^{1}$ The use of a pulsed laser for the second excitation step is preferred for the study of higher energy states because of the case with which it can be frequency doubled and because the $Rydberg \leftarrow A$ transition moments are expected to be quite small. It is important to characterize this excitation scheme using a known transition which can be studied by both pulsed and cw methods. We compare the efficiencies of cw/pulsed with cw/cw fluorescence excitation of E-A-X. The density of molecules in our source and the transition moments for E-X and E-A are determined using these data and emission rate measurements. A first step towards the goal of characterizing the $n=5-10$ Rydberg states with double resonance is to obtain quantitative dynamical information on the $A^{2}\Pi_{1/2} - X^{2}\Sigma^{+}$ initial excitation step. This data is vital for the optimization of molecule source conditions and for determination of the best optical method (pulsed, cw or modulated cw) of exciting the intermediate state. Using an acousto-optically modulated, single-mode cw dye laser with a risetime of -10 ns, we observe an transient A-X fluorescence rate which is twice the steady-state value and which decays to the steady-state value with a lifetime of $\sim 160$ ns. The effects of varying background Ar pressure on both the transient and steady state emission rates were measured