N2- and CO-induced collisional broadening of forbidden ground-to-Rydberg-state transitions in sodium as measured by trilevel echoes

Abstract

We report the first study of molecular-perturber-induced broadening of forbidden ground-to-Rydberg-state transitions. The ${\mathrm{N}}_2$- and CO-induced-broadening cross sections for the $3S-\mathrm{nS} \mathrm{}\left(\mathrm{nD}\right)\mathrm{}$ transitions of sodium have been measured for values of $n$, the principal quantum number of the upper state, ranging from 4 to 41. These measurements, performed using the trilevel echo technique, were accomplished at sodium and perturber densities of ${10}^{-6\mathrm{}}$ and 1 torr, respectively. The broadening rates, observed in the present experiment, differ in some respects from those observed previously with noble-gas perturbers. These differences, which presumably arise because of the presence of accessible internal degrees of freedom in the ${\mathrm{N}}_2$ and CO perturbers (and in the case of CO also because of its permanent electric-dipole moment), are most clearly displayed in $3S-\mathrm{nS}$ transitions. Previous work has shown that with noble-gas perturbers the $3S-\mathrm{nS}$ broadening cross sections display a dramatic increase at low values of $n$; they then decline at intermediate values of $n$, and at high $n$ they approach a limiting value. In the present experiment we find that the initial increase in $3S-\mathrm{nS}$ broadening cross sections with $n$ is relatively small and more gradual. For high $n$, the broadening cross sections become approximately constant. As is the case with noble-gas perturbers, the broadening of $3S-\mathrm{nD}$ transitions is generally larger than the broadening of the corresponding $3S-\mathrm{nS}$ transitions. With CO as a perturber, an unexplained irreproducibility in the broadening cross sections of high-$n$ $3S-\mathrm{nD}$ transitions is observed. One explanation has to do with the CO-induced frequency shifts being unusually large and exceeding the spectral width of our excitation lasers.