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High Resolution Infrared Spectroscopy of van der Waals and Hydrogen Bonded Clusters: Pairwaise and Nonpairwise Additive Intermolecular Forces in ArnHF/DF and (HF)n complexes

TitleHigh Resolution Infrared Spectroscopy of van der Waals and Hydrogen Bonded Clusters: Pairwaise and Nonpairwise Additive Intermolecular Forces in ArnHF/DF and (HF)n complexes
Publication TypeThesis
Year of Publication1995
AuthorsFarrell, Jr., JT
Abstract

This dissertation describes studies of intermolecular and intramolecular dynamics in weakly bound van der Waals and hydrogen bonded complexes. The complexes are synthesized in the low temperature (T=5–15 K) environment of a slit supersonic expansion. The slit geometry provides i) a high number density, ii) an approximately 5-fold increase in peak absorption vs pinhole expansions through suppression of Doppler broadening along the laser axis, and iii) a long absorption pathlength amenable to direct absorption techniques. Together with the advantages of sensitive dual beam detection of high resolution (< 0.0005 cm-1) tunable IR laser light, this technique provides a very sensitive method for studying the spectroscopy and dynamics of weakly bound molecular species at a state-to-state level.

The underlying goal of these studies is to provide information about the potential energy surfaces which govern weak intermolecular interactions. Several investigations are reported for the prototypical Ar-HF system. The spectroscopic characterization of intermolecular bending and stretching states of ArHF excited in combination with the νHF = 20 stretch provides information on the angular and radial anisotropy of the intermolecular potential in an excited intramolecular vibrational level. In addition, the near-IR spectra of ArnDF (n=1–3) provide a glimpse into the dynamics of a DF rotor sequentially "solvated" by a well defined number of argon atoms. The observation of intermolecular bending states of AR2HF and Ar2DF allow a detailed assessment of the importance of nonpairwise additive, or many body, terms to the intermolecular potential.

The effects of intramolecular excitation on intermolecular dynamics are investigated for the HF-HF and N2-HF systems. The vibrational frequency shifts and predissociation lifetimes for HF-HF and N2-HF in the νHF=2 vibrational level provide detailed information on the coupling between the intermolecular and intramolecular degrees of freedom. This is probed at a more subtle level for the HF-HF system, by characterizing how the vibrational frequencies and lifetimes change upon isotopic substitution (to form the deuterated isotopomers HF-DF, DF-HF, and DF-DF) in the νHF,DF manifold. The infrared spectra of (DF)3 affords a unique opportunity for investigating the role of intramolecular vibrational relaxation in a photoinitiatec unimolecular ring opening.