STRUCTURE OF COLD, MIXED PARAHYDROGEN-DEUTERIUM CLUSTERS

Author Institution: Department of Chemistry, University of Southern California, Los Angeles, CA 90089Vibrational spectra of para-H$_2$ molecules in clusters consisting of para-H$_2$ or para-H$_2$/D$_2$ seeded in He have been obtained using coherent anti-Stokes Raman spectroscopy. The Q$_1$(0) line of para-H$_2$, obtained upon expansion of neat para-H$_2$/D$_2$, exhibits a high frequency vibrational shift of about 2.6 \wn with change of D$_2$ concentration from 0 to 98\%. This shift is assigned to a decrease of the para-H$_2$ vibron band width in mixed clusters, similar to that previously observed in bulk H$_2/D$_2$ solid. 1\% para-H$_2$/D$_2$ clusters in He are liquid and show a much smaller shift of about 0.5 \wn and indicate phase separation into a predominantly D$_2$ core and H$_2$ shell. The onset of phase separation in para-H$_2$/D$_2$ mixtures is predicted at approximately 3 K providing further evidence of super-cooled liquid hydrogen clusters previously studied