Excited-state dynamics of the isolated ultracold ovalene molecule

Abstract

In this paper we report the results of an experimental study of the energetics and the dynamics of the first two electronically excited singlet states S1 and S2 of the ovalene molecule (C32H14) seeded in supersonic beams of Ar and of Kr. By an optimal choice of the stagnation pressure (100–300 Torr) of the Ar and Kr carrier gases expanded through a 200 m nozzle, efficient rotational–vibrational cooling of the large molecule was accomplished, while the spectrum was not obscured by the formation of van der Waals complexes. We have interrogated the energy-resolved fluorescence action spectrum, the time-resolved fluorescence, as well as the energy-resolved fluorescence of the bare, ultracold, isolated, large molecule. The first spin-allowed 1S0(1A1g)→1S1(1B3u−) transition with an origin at 21 449 cm−1, exhibits a sparse, well-resolved, vibrational level structure at low excess vibrational energies, EV, while the higher energy range of S1 reveals a congested level structure corresponding to overlapping resonances within a single electronic manifold. The time-resolved decay lifetimes of photoselected single vibronic levels of S1 are in the range 1.7–2.2 msec, exhibiting a very weak energy dependence on EV and being close to the pure radiative lifetimes of S1. The second S0(1A1g)→S2(1B2u) transition of ovalene reveals an irregular, closely-spaced structure, resulting from strong interstate coupling between the S2 and the S1 electronic states, whose electronic origins are separated by ∼1800 cm−1. The strength function for the scrambling of the S2 origin with the background S1 manifold was extracted from the spectroscopic data, providing information on the energetics and on the energy dependence of interstate coupling. The time-resolved decay lifetimes of the scrambled S2 molecular eigenstates are longer by about two orders of magnitude than the pure radiative lifetimes of S2, as estimated from the integrated oscillator strength, manifesting the dynamic consequences of the intermediate level structure. The energy-resolved fluorescence resulting from excitation into the S2 state exhibits emission in the S1→S0 energy region, which is in accord with the large dilution factor for the S2–S1 scrambling. The structure of the energy-resolved emission spectrum may provide some information regarding intrastate vibrational energy redistribution in this large molecule.