Conformational landscape of the SF6 dimer as revealed by various free jet conditions

International audienceSupersonic free jets are an ideal way to form and study van der Waalscomplexes [1], through the very fast cooling and rarefaction of the gasduring the expansion leading to the stabilization of these isolated species.Of course, the formation of these complexes depends dramatically on theexpansion conditions, i.e. the nozzle geometry, the nature of the gasmixture and the stagnation conditions (P0, T0). Urban and Takamipreviously studied one conformer (noted #1) of the SF6 dimer by highresolution absorption spectroscopy [2]. This spectral range has beenrevisited with a set-up coupling an external cavity quantum cascade laserand a pulsed supersonic expansion through a pin hole (D=0.35mm) or aslit nozzle (30mm x 50 mm). Different dilutions of the SF6 /He gasmixture ranging from 0.25 to 4% were used at a backing pressure P0 of6 bars. From this work, two new rotationally resolved structures havebeen identified in the n3 region (10.5 mm) they are respectively assignedto a new conformer of the SF6 dimer (#2), and to a (SF6)2-He complex(#3) [3].Interestingly, the expansion conditions that favor the two new structures(#2 and #3) and #1 correspond to different dynamics of formation #1 isintense at low concentrations and low temperature expansions while #2is favored by higher temperatures and concentrations (Fig. 1). Thisrelaxation behavior suggests a SF6-SF6 potential surface with a wideshallow basin corresponding to #2 separated by a low barrier from adeeper well associated to #1. Feature #3, is essentially present underlow temperature conditions, i.e., with a pin hole nozzle, as expected fora fragile He complex