Knowledge Bank

Vibrational 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.