Dynamics of the 13Πg state of K2 on helium nanodroplets

Abstract

Fluorescence following optical excitation of the 1³Σ_u⁺ state of K₂ prepared on helium nanodroplets to the predissociative 1³Π_g state yields molecular emission from both the (B)1¹Π_u and (A)1¹Σ_u⁺ K₂ states as well as atomic emission from the expected 4²P_3/2,1/2 → 4²S_1/2 dissociation channel. A ∽12 cm⁻¹ red shift is observed in the molecular emission excitation spectrum compared to the atomic emission excitation spectrum. Time-correlated photon counting measurements demonstrate the rise time for both atomic and molecular products to be <80 ps, independent of vibrational level excited. This lifetime is interpreted as the total depopulation time for the optically excited 1³Π_g state, 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. Symmetry restrictions for the isolated K₂ imply that the intersystem crossing from the 1³Π_g state to the (B)1¹Π_u and (A)1 ¹Σ_u⁺ states must be induced by interaction with the helium nanodroplet.