Ubiquitous Nature of Multimode Vibrational Resonances in Positron-Molecule Annihilation

Abstract

Positron annihilation on many molecules occurs via positron capture into vibrational Feshbach resonances, with annihilation rates often further enhanced by energy transfer to vibrational excitations weakly coupled to the positron continuum. Data presented here uncover another scenario in which the positron couples directly to a quasicontinuum of multimode vibrational states. A model that assumes excitation and escape from a statistically complete ensemble of multimode vibrations is presented that reproduces key features of the data. DOI: 10.1103/PhysRevLett.108.093201 PACS numbers: 34.80.Uv, 34.50.Às, 34.80.Lx Positron annihilation on atoms and molecules is important in many areas of science and technology including positron emission tomography in medicine, astrophysics, and studies of material properties. For many molecules, the annihilation rates as a function of incident positron energy " exhibit vibrational Feshbach resonances (VFRs) (e.g., populated via excitation of infrared-active modes) [1]. These resonances occur at specific values of incident positron energy given bywhere ! is the vibrational mode energy and " b is the positron-molecule binding energy. The observation of these VFRs indicates that the molecule can form a bound state with a positron. The magnitudes of these VFRs are often further enhanced by intramolecular vibrational energy redistribution (IVR). A theory has been developed to explain quantitatively VFR-enhanced annihilation rates in selected small molecules [2]. However the details of the IVR enhancements remain sketchy [1]. Described here are new data for positron annihilation resolved as a function of positron energy. The initial motivation was to understand the annihilation spectra in the small molecules CCl 4 and CBr 4 that are known to have very large annihilation rates for thermal positrons at 293 K. For these targets, simple theoretical estimates for VFRs and IVR-enhanced VFRs appeared to be incapable of explaining the magnitudes of the observed rates. In particular, the positron binding energy in CBr 4 is expected to be greater than all of the mode energies. This makes the VFR energies in Eq. (1) negative, thus shutting off the resonant process.Based upon the study described here, we are led to conclude that broad quasicontinuous spectra of multimode vibrations (e.g., combinations and overtones) are responsible for the observed behavior. Further, this mechanism appears to be universal in the sense that it is present, at least at a significant level, in the spectra of most molecules studied to date. Qualitative agreement is obtained between the experimental data and the predictions of a model that assumes positron capture into (and possible detachment from) a statistically complete ensemble of strongly mixed multiquantum vibrational states. We term this mechanism statistical multimode resonant annihilation (SMRA). The more general implications of these results are discussed.The experimental procedures are described in [1]. An energy tunable, trap-based positron beam is used t...

Positron binding to alcohol molecules

“…We have become aware of a generic multimode process that contributes a broad component to the spectra that diminishes with increasing positron energy. This phenomenon is currently under investigation and will be discussed elsewhere [10].…”

“…While IVR enhancement might account for some of this intensity, the observed behavior is thought to result primarily from a new, generic resonant annihilation mechanism, which has been observed more clearly in studies of other molecules. To understand this new mechanism is the focus of ongoing research and will be discussed elsewhere [10].…”

“…Depending on the spectrum and energy range within the spectrum, these errors range from 5 to 40% of the measured magnitudes. The largest uncertainties occur at higher energies where the magnitude of Z eff is due to direct annihilation and a recently discovered, generic form of multimode annihilation that will be discussed elsewhere [10].…”

Interplay between permanent dipole moments and polarizability in positron-molecule binding

“…In some molecules, there is an apparent shift in the inferred value of b for lower frequency modes, such that these modes appear to indicate a smaller shift by an amount 15 meV. This appears to be due to an underlying component of annihilation on multimode vibrations that rises relatively steeply at lower values of positron energy [18]. When this is accounted for, the observed energy shifts for all modes in a given molecule appear to give the same binding energy.…”

Mode coupling and multiquantum vibrational excitations in Feshbach-resonant positron annihilation in molecules

“…However, VPT2 does not treat four-and higher-quantum transitions, but these are unlikely to play an important role in the processes under investigation. (The exception is the contribution of statistical multimode resonant annihilation, which is known to provide a smooth background to the Z eff signal [36]; see Sec. IV.)…”