Tuning out vibrational levels in molecular electron energy-loss spectra

Abstract

The phenomenon whereby features associated with certain vibrational levels in molecular states of mixed electronic character disappear under specific scattering conditions in electron energy-loss spectra is investigated. In particular, using a combination of experimental measurements and coupled-channel calculations, anomalous vibrational intensities in the mixed valence-Rydberg 1 u ← X 1 + g transition of N 2 are explained. A single parameter, i.e., the ratio of the generalized electronic transition moments to the diabatic valence and Rydberg components of the mixed states, dependent on the experimental scattering conditions, is found to be essentially capable of describing all observed relative vibrational intensities, including the near disappearance of the b 1 u (v = 5) feature for momentum-transfer-squared values K 2 ≈ 0.3 a.u. This result highlights the interesting possibility of experimental control of molecular quantum-interference effects in electron energy-loss spectra, something that is not possible in optical spectra.

THE HIGH-RESOLUTION EXTREME-ULTRAVIOLET SPECTRUM OF N 2 BY ELECTRON IMPACT

“…This was done to aid the assignment of the complex and, in many cases, blended spectrum. A very similar model was utilized in previous studies of N 2 electron-impact-induced fluorescence in this laboratory (Liu et al 2008), electron energyloss spectroscopy (Heays et al 2012), and the atmospheres of the Earth (Liu et al 2009) and Titan (Stevens et al 2011;Lavvas et al 2011).…”

“…It is necessary to define numerical potentialenergy curves describing each excited electronic state (and its rotational excitation) as well as all interactions responsible for the perturbation of these states. These various model parameters have been optimized with reference to a large database of experimental energy levels and are given in a detailed form elsewhere (Lewis et al 2005a(Lewis et al , 2005b(Lewis et al , 2008aHaverd et al 2005;Heays et al 2012). The model potential-energy curves of all electric-dipole accessible states included in the model are plotted in Figure 1 in Potential energy (eV) (Heays 2011), and the assignments made here reflect the dominant component only.…”

“…The approximation of electron excitation as optical absorption is only appropriate if there is a negligible amount of momentum transferred during each scattering event (Inokuti 1971), a condition which is not well satisfied by the 20 and 100 eV electron beams used in the current experiment. The dependence of the N 2 excitation cross section on collision energy and the electronic-vibrational state being excited are well documented (Ajello et al 1989;James et al 1990;Heays et al 2012;Malone et al 2012). The optical absorption cross sections provide a useful guide when determining the relative importance of overlapping emission bands but were ultimately adjusted empirically for each excited vibrational-electronic level in order to provide a quantitative fit to the experiment.…”

“…For some levels, predissociation rates have been deduced from observations of line broadening in absorption spectra or excited state lifetimes in time-resolved measurements (Lewis et al 2005b(Lewis et al , 2008aHeays et al 2012, and references therein). Then, ad hoc modifications have been made to the model predissociation rates.…”

“…For this study, the observed emission is compared with the output of a sophisticated quantum-mechanical model, which simulates N 2 excitation, predissociation, and fluorescence. This model solves the coupled-Schroedinger equation (CSE), has been in development for some time (Lewis et al 2005a(Lewis et al , 2005b(Lewis et al , 2008aHaverd et al 2005;Heays 2011), and has been successfully applied in previous electron-excitation (Heays et al 2012) and UV fluorescence (Wu et al 2012) experiments. The branching between emissive and predissociative decay critically controls which excited vibrational and rotational levels fluoresce, and leads to a temperature dependence of the observed band intensities.…”