On Resonance Enhancement of $E1-E2$ Nondipole Photoelectron Asymmetries in Low-Energy Ne $2p$-Photoionization

Abstract

Earlier, a significant enhancement of the nondipole parameters $\gamma_{2p}$, $\delta_{2p}$, and $\zeta_{2p} = \gamma_{2p} + 3\delta_{2p}$ in the photoelectron angular distribution for Ne $2p$ photoionization, owing to resonance interference between dipole ($E1$) and quadrupole ($E2$) transitions, was predicted. This enhancement manifests as narrow resonance spikes in the parameters due to the low-energy $2s \rightarrow 3p$ and $2s \rightarrow 4p$ dipole, as well as the $2s \rightarrow 3d$ quadrupole autoionizing resonances. Given the unique nature of this predicted enhancement, it requires further validation. Specifically, whether these narrow spikes in $\gamma_{2p}$, $\delta_{2p}$, and $\zeta_{2p}$ will or will not retain their values for experimental observation if one accounts for a typical finite frequency spread in the ionizing radiation. To address this, we revisit the previous study, now incorporating the effect of frequency spread in the ionizing radiation, assuming a spread as large as $5$ meV at the half-maximum of the radiation's intensity. We demonstrate in the present paper that while the frequency spread does affect the resonance enhancement of $\gamma_{2p}$, $\delta_{2p}$, and $\zeta_{2p}$, these parameters still retain quantitatively significant values to be observed experimentally. The corresponding calculations were performed using the random phase approximation with exchange, which accounts for interchannel coupling in both dipole and quadrupole photoionization amplitudes.