GERALD AUBÖCK, JOHANN NAGL, CARLO CALLEGARI AND WOLFGANG E. ERNST, Institute of Experimental Physics, TU Graz, Petersgasse 16, 8010 Graz, Austria/EU.
We present laser-induced-fluorescence (LIF) and magnetic-circular-dichroism (MCD) spectra of the (2) 4\!E' (1) 4\!A'2 band of \chemRb3 and \chemK3 on He nanodroplets. For \chemRb3 we find a progression of four bands, split by spin-orbit (SO) and weakly perturbed by linear vibronic Jahn-Teller (JT) coupling. For \chemK3 the transition had been previously observed, and interpreted in terms of a linear and quadratic Jahn-Teller coupling in the (2) 4\!E' state. An unambiguous assignment of spectra on He droplets is however difficult due to matrix broadening; MCD spectra are a valuable method to offset this unwanted effect. For \chemK3 they clearly reveal the importance of SO coupling and suggest a different assignment with weak linear, and no significant quadratic, JT coupling. Further, interpretation of our data in terms of free-molecule SO indicate that the trimers lie flat on the droplet surface. The observed strong \mathcalC-type MCD spectra arise from different populations of the ground-state Zeeman sublevels; a quantitative analysis allows a determination of the spin temperature, hence of the temperature at the droplet surface. Our simulations are consistent with a value of 0.4~K, which agrees with our previous findings from spectra of triplet-state alkali-metal dimers. This is the first observation of SO coupling in quartet states of trimers.