Vibrational excitation via shape resonances in electron scattering from the NO dimer

Vibrational electron-energy loss spectra of polycrystalline thin films (∼50 Å) of dimerized nitric oxide [i.e., (NO)2] have been recorded near 20 K at incident energies of 2,9, and 14 eV. The 2 and 9 eV spectra consist mainly of three vibrational peaks which are the first three members of two unresolved progressions ascribed to in-phase (ν1) and out-of-phase (ν5) vibrations of NO molecules within the dimer. The ‘‘background’’ intensity and the broadening of the elastic and energy-loss peaks are believed to result from unresolved multiple losses including vibrations within (NO)2 and phonon modes. At incident energies in the range 11–15 eV, a progression of seven vibrational members belonging to the ν1 and ν5 modes has been observed. This enhancement in the overtones of the fundamental frequencies is interpreted to arise from the formation of transient (NO)−2 states. The electron excitation functions recorded between 0–19 eV reveal the presence of two broad ‘‘humps’’ at 11.6±0.2 and 14.2±0.2 eV and a strong maximum near 2 eV. The latter is present in all decay channels including the v=1, 2, and 3 vibrational states of the ν1 and ν5 modes, the ‘‘background’’ losses and the elastic peak; but, the broad ‘‘humps’’ are exclusively related to vibrational excitation of the ν1 and ν5 modes. These two resonances appear to be derived from a splitting, via ‘‘through space’’ orbital interaction, of unfilled degenerate NO orbitals. In the 2 eV loss spectra the preponderance of a strong infrared mode (ν5) and the absence of anharmonicity in the vibrational peaks suggests that the vibrational excitation cross sections may be dominated by dipole scattering. No clear-cut evidence of compound states derived from the low-lying 3Σ−, 1Δ, and 1Σ+ NO− states was found in the dimer.