Experiments on small (H2)N clusters

4 pages, 4 figures, 1 table.-- PACS nrs.: 36.40.-c, 33.20.Fb, 05.30.Jp.Contributed to: ISSPIC'14 - 14th International Symposium on Small Particles and Inorganic Cluster (Valladolid, Sep 15-19, 2008).The nucleation process of small hydrogen clusters has been studied in detail with space, time, and number size resolution on the basis of supersonic expansions of the gas through a cryogenic nozzle operating in the thermal range 24 < T < 110 K. The diagnostic of the jet medium is based on high sensitivity (few photons/sec) Raman spectroscopy with very high spatial resolution (~ 2 μm), enabling for quantitative measurements with time resolution in the domain of nanoseconds. Temperatures down to 0.1 K have been attained in the jet at a collisional rate low enough to avoid sudden freezing of the gas. Clusters between two and eight molecular units are observed size-resolved. Features of the ortho-ortho and ortho-para hydrogen dimers, and the role of three-body collisions for nascent dimers and trimers are reported.This research has been supported by the Spanish Ministerio de Ciencia e Innovación, Projects PB97-1203, BFM2001-2276, FIS2004-02576, and FIS2007-61430.Peer reviewe