Within this work the Even-Lavie valve was successfully employed as a new pulsed helium droplet source and was used for the electronic spectroscopy of various organic molecules doped into the droplets.
The detailed characterization of the pulsed helium droplet beam revealed a significantly different dependence of the droplet size on the stagnation conditions as compared to a continuous helium droplet beam. The combined investigation of Rayleigh-scattering from the droplets and laser induced fluorescence (LIF) from molecules doped into the droplets revealed a bimodal droplet size distribution. LIF from dopant molecules inside helium droplets could only be observed from the leading fraction of the droplet pulses. The size of these droplets can be varied between 10000 and 1000000 helium atoms per droplet. The other fraction of the droplet pulses carries very large droplets not useful for doping with molecules. Variations of the stagnation conditions mainly affect the relative abundance of the two fractions.
The optimum signal is almost constant from single shot up to 500 Hz operation and the density of the droplets is about 20 times higher than in the continuous droplet beam.
The electronic spectra of the anthracene-derivatives Anthracene, 9,10-Dichloroanthracene, 9-Chloroanthracene, 9-Cyanonanthracene, 9-Phenylanthracene, 9-Methylanthracene, 1-Methylanthracene and 2-Methylanthracene, the charge transfer molecules Phenylpyrrole and Fluorazene, and the Pyrromethene Dyes BDP, 8-PhPM, PM546, PM567, and PM650, doped into the pulsed helium droplet beam were recorded and compared to the corresponding spectra of the isolated molecules in a supersonic jet. Thereby, significant differences which could not be reduced to the lower temperature in the helium droplets, and which are consequently attributed to the interaction between the helium environment and the embedded molecules, could be observed.
The perturbations of the electronic transitions are reflected by substantial line broadening, the occurrence of a fine structure of electronic transitions (zero phonon lines accompanied by phonon wings), and in rare cases a multiplet splitting of zero phonon lines.
The broadening of molecular transitions is counterintuitive to what is expected for sub-Kelvin temperature and was attributed to a damping of the electronically excited state by the helium environment which occurs if the electronic excitation induces a significant nuclear rearrangement. Broad electronic spectra could also be observed due to the dominance of phonon wings over pure molecular transitions.
Both broadening effects reflect a strong perturbation of the electronic transitions of the dopant species by the helium droplet. This result deduced from a larger systematic investigation is of importance for planning photochemical experiments in helium droplets and contributes to the understanding of the solvation of molecules in superfluid helium droplets
