Positron Annihilation Resonances in Molecules: Going Beyond Fundamental Modes

Slow positrons ($< 0.5$ eV) bind to molecules through Feshbach resonant excitation of dipole-active fundamental vibrational modes, and this leads to annihilation rates that are much faster than any direct annihilation process. For modes with strong infrared activity, intramolecular vibrational redistribution (IVR) leads to a further enhancement of the annihilation by spreading the coupling over a large number of near degenerate combination and overtone modes. Recently, distinct enhanced annihilation resonances have been observed that involve vibrational modes beyond the fundamentals. In this talk, evidence for these resonances will be presented for ring and chain alkane molecules, an effect that appears to be generic. The energy spectra of the new resonances will be compared to infrared absorption spectra and shown to correlate to a region populated by combinations and/or overtones of the fundamental vibrations. Although the infrared amplitude is much weaker than for the fundamentals, the magnitude of these annihilation resonances show that many modes must be participating, and thus IVR appears to be strong even for these non-fundamental modes. For molecules with binding energies larger than those of the highest lying fundamental modes, such resonances are expected to dominate the annihilation spectrum. An example of this effect is seen in polycyclic aromatic hydrocarbon (PAH) molecules, a potentially important component of annihilation in the interstellar medium. A room temperature (FWHM $\sim 36$ meV), as well as a cryogenically cold (FWHM $\sim 20$ meV), positron beam is used in these studies. Implications of the energy distributions of these beams in determining the shape and magnitude of the observed resonances will also be discussed.