Abstract
Helium nanodroplet isolation is used to produce van der Waals-bound quartet state alkali trimers (Na3 and K3) selectively over the corresponding chemically bound doublet trimers. Frequency-resolved excitation and emission spectroscopy reveals the presence of nonadiabatic spin–flip processes in the electronically excited states. A total of four quartet to quartet electronic transitions are observed: the 2 4E′,1 4E←1 4A2′ transitions of Na3 and the 1 4A1″,2 4E′←1 4A2′ transitions of K3. Time-resolved spectroscopy reveals that intersystem crossing times in Na3 decrease from 1.4 ns after excitation of the 0–0 band to approximately 400 ps for the higher vibronic levels (3,5/2). Analysis of the resonant quartet fluorescence reveals that the excited electronic state cools vibrationally on a time scale that is comparable to, but slower than, the intersystem crossing time.
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