Observation of relativistic E x e vibronic coupling in Rb3 and K3 quartet states on helium droplets.

Spectroscopy of alkali-metal complexes on helium droplets (He(N), N=1000-10 000) provides the unique opportunity to observe high-spin species, which exhibit prominent spin-orbit (SO) effects. In this work we present laser-induced-fluorescence and magnetic-circular-dichroism (MCD) spectra of the (2)(4)E(')<--(1)(4)A(2) (') band of Rb(3) and K(3) on He(N). For Rb(3) we find a progression of four SO split bands, weakly perturbed by linear vibronic [Jahn-Teller (JT)] coupling. The K(3) transition was previously observed and interpreted in terms of linear and quadratic JT effects in the (2)(4)E(') state [J. H. Reho et al., J. Chem. Phys. 115, 10265 (2001)]. The structure of the MCD spectrum, however, clearly reveals the importance of SO coupling also for the K(3) transition and suggests a different assignment with weak linear JT and nonsignificant quadratic JT coupling. The observed strong C-type MCD spectra arise from different populations of the ground-state Zeeman sublevels. Thus a quantitative analysis allows a determination of the spin temperature, and since alkali-metal complexes are situated on the surface of the helium droplet this corresponds to a determination of the surface temperature. Our results from the trimer spectra are consistent with the value of approximately 0.4 K, which is found in the interior of a droplet and which we already confirmed for the surface from the spectra of triplet-state alkali-metal dimers in a previous work.