HIGH RESOLUTION SPECTROSCOPIC STUDIES OF SMALL MOLECULES

Abstract
The spectra of small molecules in the gas phase are often extremely complicated. Much of this complexity is gratuitous, and can be made to disappear when a suitable double resonance scheme is employed. Sometimes it is necessary to devise devious schemes for circumventing obstacles (selection rules, Franck-Condon factors, potential energy barriers) to important types of molecular structural information (e.g. dissociation energies) or classes of molecular energy levels (e.g. triplets, localized vibrational excitations suited for bond specific photochemistry). The traditional spectroscopic concepts and well-trodden path from spectrum to potential energy surface via molecular constants is ill suited for characterizing the large amplitude displacements of atoms from their equilibrium positions which occur in simple unimolecular dynamical processes. Cross-correlation of double resonance spectra may provide a direct view of specifiable intramolecular processes. Four spectroscopic examples will be discussed : (i) Stimulated Emission Pumping (SEP) spectroscopy of formaldehyde, an example of a well-behaved molecule in the small amplitude limit, for which a complete set of harmonic vibrational frequencies (ω₁) and anharmonicities (x_ij) is determined ; (ii) Perturbation Facilitated Optical-Optical Double Resonance (PFOODR) spectroscopy of Li₂, illustrating access to triplet states and the phenomenon of "accidental predissociation" ; (iii) determination of an upper bound to the HCC-H dissociation energy of acetylene by Zeeman Anti-Crossing (ZAC) spectroscopy, a cooperative predissociation scheme similar to the accidental predissociation in Li₂ whereby, despite potential energy barriers, one can ensure that the onset of dissociation will be detected not far above the thermochemical limit ; (iv) detection and characterization of a vinylidene vibrational level through the effect of acetylen-vinylidene isomerization on the cross-correlation of acetylene SEP spectra, illustrating a new, direct way of sampling intramolecular dynamics at such high internal energies that vibrational spectra are intrinsically unassignable.