Fano Lineshapes Revisited: Symmetric Photoionization Peaks from Pure Continuum Excitation

TL;DR

This paper investigates symmetric Fano lineshapes in photoionization spectra arising solely from continuum excitation, challenging traditional interpretations and emphasizing the role of phase-shifted continua in resonance shapes.

Contribution

It introduces a revised theoretical framework incorporating phase-shifted continua to explain symmetric peaks without discrete state excitation.

Findings

Symmetric peaks can occur without discrete state excitation.

Standard Fano theory predicts zero cross section in this scenario.

Resonance shape alone does not reveal excitation amplitude ratios.

Abstract

In a photoionization spectrum in which there is no excitation of the discrete states, but only the underlying continuum, we have observed resonances which appear as symmetric peaks, not the commonly expected window resonances. Furthermore, since the excitation to the unperturbed continuum vanishes, the cross section expected from Fano's configuration interaction theory is identically zero. This shortcoming is removed by the explicit introduction of the phase shifted continuum, which demonstrates that the shape of a resonance, by itself, provides no information about the relative excitation amplitudes to the discrete state and the continuum.