Positron-Molecule Annihilation Resonances Beyond The Fundamental Vibrational Modes. *

Low energy positrons ($< 0.5$ eV) bind to molecules through Feshbach resonant excitation of dipole- or quadrupole-active fundamental vibrational modes, and this leads to greatly enhanced annihilation rates. Recently, new annihilation resonances were observed involving vibrational modes beyond the fundamentals. In this talk, evidence for these resonances will be presented for several n-alkane and cycloalkane molecules. These new resonances are compared to infrared absorption spectra and they appear to be due to combinations and/or overtones of the fundamental vibrations. For molecules with binding energies larger than that of the highest lying fundamental mode, such multimode processes are expected to dominate the annihilation spectrum. An example is the 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 cooled (FWHM $\sim 20$ meV), positron beam is used in these studies. Implications of the energy distributions of these beams in determining the shape of the observed resonances and remaining open questions will be discussed.