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

摘要

Ethylene and ethylene clusters embedded in superfluid ~4He droplets have been spectroscopically investigated with continuous wave line tunable CO_2 and N_2O lasers,utilizing the strong absroption of the V_7 mode of C_2H_4.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 futher 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 homoeneous broadening,because the low temperature of the He droplet leads to a dimer full widthat half maximum width of about 1 cm~(-1).