Helium nanodroplet isolation spectroscopy of perylene and its complexes with oxygen.

Helium nanodroplet isolation ~HENDI! is an established and unique way to carry out spectroscopic investigations of a broad range of molecular species. The very low temperature of and the very weak interaction with the helium matrix allow for the observation of significantly simplified spectra. As the superfluid environment is similar to the gas phase, frequently rotational structure can be observed. In the present work, we have taken advantage of the fact that bolometric detection allows for the observation of species that do not fluoresce or do so too weakly to be observed by conventional optical spectroscopic detection systems. By using a beam depletion bolometric detection method, we have been able to observe the S1←S0 excitation spectra of complexes of perylene with one or more molecules of oxygen. Because of the quenching properties of molecular oxygen, this experiment would not have been possible using fluorescence detection. To our knowledge, no gas phase measurements of the perylene-oxygen complex S1←S0 excitation spectra have been reported to date in the literature. Since the work we report here is a preliminary study, as we present the results, we will explain why we believe that HENDI spectroscopy can provide new insights into the very important process of fluorescence quenching. The HENDI spectrometer used in these experiments has already been described in detail elsewhere and we will only briefly describe the features that are particular to the experiments reported here. Perylene is evaporated in an oven at a temperature of 135 °C which corresponds to a vapor pressure of about 0.1 mbar. The droplets cross a 2 cm long heated ~190 °C! pick-up cell connected to the oven in order to be doped by the molecules of interest. The laser source is a frequency-doubled Ti:Al2O3 laser pulsed at a repetition rate of 2.7 kHz with a bandwidth of the order of 0.5 cm. In order to increase the interaction time between the droplet beam and the laser, we use a multipass arrangement which makes the laser interact with the droplets beam at least 50 times. Upon electronic excitation of the embedded molecules