Angle-Resolved Auger Spectroscopy as a Sensitive Access to Vibronic Coupling

In the angle-averaged excitation and decay spectra of molecules, vibronic coupling may induce the usually weak dipole-forbidden transitions by the excitation intensity borrowing mechanism. The present complementary theoretical and experimental study of the resonant Auger decay of core-to-Rydberg excited ${\mathrm{CH}}_4$ and Ne demonstrates that vibronic coupling plays a decisive role in the formation of the angle-resolved spectra by additionally involving the decay rate borrowing mechanism. Thereby, we propose that the angle-resolved Auger spectroscopy can in general provide very insightful information on the strength of the vibronic coupling.

Figure 1

Photoionization cross section (upper panel) and angular distribution parameter (lower panel) measured (open circles) and computed in different approximations in the vicinity of the C $1s\to n\ell$ excitations for the participator ${{\mathrm{CH}}_4}^{+}(X\hspace{0.17em}\hspace{0.17em}1t_2^{-1})$ final ionic state. The experimental cross section is set on absolute scale with the help of the final theoretical result (the black solid curve), whereas no normalization for the asymmetry parameter is required. Calculations performed without including nuclear motion (see the legend in the lower panel) illustrate individual ${\beta }_X^e(\omega )$ parameters for the direct (cyan dash-dot-dotted line) and resonant (green horizontal bars) ionization channels, and for their interference (the green dashed curve). The ${\sigma }_X(\omega )$ and ${\beta }_X^e(\omega )$ computed by including the symmetric stretching mode (the blue dash-dotted curve), as well as through the present simulation of the excitation intensity borrowing via vibronic coupling (VC Excitation, red short-dashed curve) and the decay rate borrowing via vibronic coupling (VC Decay, black solid curve), are also shown (see the upper legend and the text for details). Assignments of the vibronic resonances $n\ell {\nu }_1$ and $n\ell \text{vib}$ (“vib” stands for ${\nu }_2–{\nu }_4$) made according to Refs. [17, 18, 19] are indicated at the top.

Figure 2

Photoionization cross section (left panel) and angular distribution parameter (right panel) computed (solid line) and measured (open circles) in the vicinity of the Ne $1s\to 3p$ excitation for the participator ${\mathrm{Ne}}^{+}(2p^{-1})$ final ionic state (unresolved $2p_{1/2,3/2}^{-1}$ doublet). The experimental cross sections is set on the absolute scale with the help of the present theoretical data, whereas no normalization for the asymmetry parameter is required. The individual contributions computed for the direct and resonant ionization channels are also shown by broken curves (see the legend).