Dynamics of the 1 (3)Pi g state of K-2 helium nanodroplets

Fluorescence following optical excitation of the 1 (3)Sigma (+)(u) state K- 2 of prepared on helium nanodroplets to the predissociative 1 (3)Pi (g) sta te yields molecular emission from both the (B)1 (1)Pi (u) and (A)1 (1)Sigma (+)(u) states as well as atomic emission from the expected 4 P-2(3/2), (1/ 2) --> 4 S-2(1/2) dissociation channel. A similar to 12 cm(-1) red shift is observed in the molecular emission excitation spectrum compared to the ato mic emission excitation spectrum. Time-correlated photon counting measureme nts demonstrate the rise time for both atomic and molecular products to be <80 ps, independent of vibrational level excited. This lifetime is interpre ted as the total depopulation time for the optically excited 1 (3)<Pi>(g) s tate, which is dominated by intersystem crossing at low vibrational energy and by predissociation at the highest vibrational level. It is deduced that the timescale for intersystem crossing must be of the order of 10 ps. Symm etry restrictions for the isolated K-2 imply that the intersystem crossing from the 1 (3)Pi (g) state to the (B)1 (1)Pi (u) and (A)1 (1)Sigma (+)(u) s tates must be induced by interaction with the helium nanodroplet.