Vibrational Feshbach Resonances Mediated by Nondipole Positron-Molecule Interactions

Measurements of energy-resolved positron-molecule annihilation show the existence of positron binding and vibrational Feshbach resonances. The existing theory describes this phenomenon successfully for the case of infrared-active vibrational modes that allow dipole coupling between the incident positron and the vibrational motion. Presented here are measurements of positron-molecule annihilation made using a recently developed cryogenic positron beam capable of significantly improved energy resolution. The results provide evidence of resonances associated with infrared-inactive vibrational modes, indicating that positron-molecule bound states may be populated by nondipole interactions. The anticipated ingredients for a theoretical description of such interactions are discussed.

Figure 1

Annihilation spectra for (a) 1,2-trans-dichloroethylene (${\mathrm{C}}_2{\mathrm{H}}_2{\mathrm{Cl}}_2$) and (b) tetrachloroethylene (${\mathrm{C}}_2{\mathrm{Cl}}_4$): filled circle, measured data; solid curve, total $Z_{\mathrm{eff}}^{(\mathrm{tot})}$ from Eq. (3); $\cdots$, $Z_{\mathrm{eff}}^{(\mathrm{dir})}$; --, $Z_{\mathrm{eff}}^{(\mathrm{res})}$, described by Eq. (4) and including only IA modes, using ${\mathrm{\Gamma }}_{\nu }^e/{\mathrm{\Gamma }}_{\nu }=1$; and - -, $Z_{\mathrm{eff}}^{(\mathrm{MRA})}$, scaled by the factor $\eta$. Vertical bars show resonant energies ${ϵ}_{\nu }$; tall black bars and short red bars denote IA and infrared-inactive modes, respectively. The fit parameters from Eq. (3) are (a) ${ϵ}_b=15\mathrm{meV}$, $\eta =0.76$, and (b) ${ϵ}_b=57\mathrm{meV}$, $\eta =0.13$. Insets show unidentified resonances in detail, where $\cdots$ show $Z_{\mathrm{eff}}^{(\mathrm{tot})}$ with $Z_{\mathrm{eff}}^{(\mathrm{res})}$ obtained from Eq. (4) including the contribution of infrared-inactive C-C stretch modes with ${\mathrm{\Gamma }}_{\nu }^e/{\mathrm{\Gamma }}_{\nu }$ chosen to give the best fit of the data, yielding ${\mathrm{\Gamma }}_{\nu }^e/{\mathrm{\Gamma }}_{\nu }=0.39$ and 0.22 in (a) and (b), respectively.

Figure 2

Energies of multimode vibrational states within 5 meV of the C-C stretch up to mode order 5 for (a) 1,2-trans-dichloroethylene, and (b) tetrachloroethylene.