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We have studied the clustering of para-H2 (pH2) in cryogenic free jet expansions by quantitative Raman spectroscopy. The weak Raman spectra of the (pH2)N clusters have been recorded near the vibrational Q(0) line of the pH2 monomer at 4161.169 cm-1 [1,2]. The high resolution in space (10 micron), time(10 ns) and cluster size makes possible to track the cluster growth kinetics along the expansion axis. A 50 micron nozzle and a wide range of source conditions have been employed: P0= 0.3 -- 2 bar and T0= 27.5 -- 100 K. Each expansion can be characterized by an isentropic path in the pressure-temperature (PT) diagram. Expansions following the same isentrope yield similar clustering patterns (though delayed in space/time), despite their differences in source conditions. Regions of maximum cluster size can be established in the PT diagram. The effect of the third colliding body has been studied in jets of pH2/He mixtures, and based on those new experimental results, a simple model for the dimer formation and destruction, involving three-body collisions, is proposed. As a result, the three-body dimerization rate $K_D$ has been determined between 0.8 and 10 K. [1] G. Tejeda, J. M. Fernandez, S. Montero, D. Blume and J. P. Toennies, "Raman spectroscopy of small para-H2 clusters formed in cryogenic free jets" Phys. Rev. Lett. 92, 223401 (2004). [2] R. J. Hinde, "A six-dimensional H2-H2 potential energy surface for bound state spectroscopy" J. Chem. Phys. 128, 154308 (2008). |
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