# Experimental Determination of Three-body Forces: Far Infrared Vibration-Rotation-Tunneling Spectroscopy (FIR-VRTS) of $Ar_{2}HCl$

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 Title: Experimental Determination of Three-body Forces: Far Infrared Vibration-Rotation-Tunneling Spectroscopy (FIR-VRTS) of $Ar_{2}HCl$ Creators: Elrod, M. J.; Steyert, D. W.; Loeser, J. G.; Saykally, R. J.; Cooper, A. R.; Hutson, Jeremy M. Issue Date: 1993 Publisher: Ohio State University Abstract: Although, most molecular dynamics simulations assume the validity of the pairwise-additive approximation in the representation of intermolecular forces, this hypothesis remains untested. Although various theoretical methods have predicted the existence of many-body forces, there exists very little direct experimental evidence of the form and size of these $effects.^{1}$ We have undertaken extensive high resolution FIR studies of the low-lying intermolecular vibrations of the $Ar_{2}HCl$ trimer in an attempt to experimentally determine the relative importance of three-body effects in this $cluster.^{2}$ In order to determine the effects of three-body forces, it is necessary to perform dynamical calculations on a number of potential surfaces in order to compare with experimental results. In particular, since the pairwise-additive potential surface is already well known (sum of Ar-Ar and Ar-HCl pair interactions), the deviation of experimental values from these calculated values will directly indicate the existence of three-body forces. Preliminary results seem to definitively show a significant three-body effect manifested in the measured spectroscopic observables. Interestingly, model three-body calculations seem to indicate that effects due to an exchange-induced quadrupole moment on the two argon atoms (which in term interacts with the permanent moments of the HCl monomer) are likely to be responsible for observed experimental deviation from calculated pairwise-additive values. Similar work is in progress for the $Ar_{2}DCl$ cluster. Description: $^{1}$ G.C. Maitland, M. Rigby, E.B. Smith, and W.A. Wakeham, Intermolecular Forces (Clarendon, Oxford, 1981). $^{2}$ M.J. Elrod, D.W. Steyert, and R.J. Saykally, J. Chem. Phys. 94,58(1991); ibid 95,3182 (1991). Author Institution: Department of Chemistry, University of California.; Department of Chemistry, University of Durham. URI: http://hdl.handle.net/1811/18625 Other Identifiers: 1993-TD-6