Ethylene and ethylene clusters embedded in superfluid He-4 droplets have be
en spectroscopically investigated with continuous wave line tunable CO2 and
N2O lasers, utilizing the strong absorption of the nu (7) mode of C2H4. Th
e monomer and the different clusters of ethylene absorb in the same spectra
l region and show significant overlap of their spectra. By measuring the de
pendence of the signal on the pick-up pressure and fitting the resulting cu
rves with a Poisson distribution and by further studying the spectral depen
dence 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 muc
h discussion; we show that the broad peak is not due to homogeneous broaden
ing, because the low temperature of the He droplet leads to a dimer full wi
dth at half maximum width of about 1 cm(-1). (C) 2001 American Institute of
Physics.