The observation of large changes in the rotational constants of glyoxal in superfluid helium droplets upon electronic excitation

Abstract

The rotational fine structure of the vibrationless 000 band of the electronic S1←S0 transition of glyoxal in large superfluid He4 droplets (N̄4=2⋅103–2⋅104) has been interrogated with high resolution laser depletion spectroscopy. In the electronic ground singlet state S0 the rotational constants A and B̄=(B+C)/2 are less than for the free molecule by a factor of 2.87 and 2.16, respectively. In the electronic excited state S1 the rotational constant A is found to be 17% larger and the average rotational constants B̄ to be 25% smaller than in the S0 state. The unexpected large magnitudes and different signs in the observed changes are attributed to modifications in the shape of the outer electron density distribution of the molecule upon excitation. None of the previous models introduced to explain the changes in rotational constants is entirely satisfactory, presumably because of the much weaker interactions with the helium environment.