|Title||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|
|Year of Publication||1995|
|Authors||Farrell, Jr., JT|
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 = 2←0 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.