IR excitation of ethylene molecules and clusters embedded in 4He droplets

Ethylene and ethylene clusters embedded in superfluid 4He droplets have been spectroscopically investigated with continuous wave line tunable CO2 and N2O lasers, utilizing the strong absorption of the ν7 mode of C2H4. The monomer and the different clusters of ethylene absorb in the same spectral region and show significant overlap of their spectra. By measuring the dependence of the signal on the pick-up pressure and fitting the resulting curves with a Poisson distribution and by further studying the spectral dependence on the droplet size, it was possible to assign two peaks. The origin of the 10 cm−1 broad gas phase dimer spectrum has been the subject of much discussion; we show that the broad peak is not due to homogeneous broadening, because the low temperature of the He droplet leads to a dimer full width at half maximum width of about 1 cm−1.