ANOMALIES IN THE REACTIONS OF HE+ WITH SF6 EMBEDDED IN LARGE HE-4 CLUSTERS

The embedding of single SF6 molecules in large liquid He-4 clusters (N almost-equal-to 3 X 10(3) atoms) and their ion molecule reactions with He+ within the clusters are studied in a molecular beam apparatus equipped with a sensitive mass spectrometer. After passing the He cluster beam (stagnation conditions: P0 = 80 bar, T0 = 24 K) through a scattering chamber filled with SF6, the electron impact mass spectra reveal essentially only the ion fragments SF5+ and SF6+ in addition to the usual He cluster ion fragments. This contrasts with the well-known mass spectrum of a free 300 K beam of SF6 molecules which shows all the expected ion fragments SF(n)+ (n = 0-5) and F+ but no SF6+. The anomalous suppression of smaller ion fragments and the electron impact energy threshold for SF5+ and SF6+ indicate that the primary excitation is creation of a positive hole in the liquid He. This hole migrates within the He cluster until it arrives at the embedded SF6 molecule, leading to the reaction of He+ with SF6. The results are shown to be consistent with the presence of the SF6 molecules located inside and not at the surface of the He clusters. The unexpected appearance of SF6+ is attributed to an extremely rapid energy transfer rate (10(16) K/s) of the nascent SF6+ to the surrounding He atoms, leading to its stabilization.