Comparison of the spatial-phase retardation effects between Rayleigh and Taylor plane-wave multipole series approximations

TL;DR

This paper compares Rayleigh and Taylor plane-wave multipole series approximations for spatial-phase retardation effects, demonstrating Rayleigh's superior validity range and highlighting the importance of higher-order terms in strong-field laser interactions.

Contribution

It provides a detailed comparison showing Rayleigh expansion's greater accuracy over Taylor expansion in modeling retardation effects in strong-field physics.

Findings

Rayleigh expansion has a larger validity range than Taylor expansion.

Taylor's spherical Bessel functions deviate in the asymptotic region.

Higher-order terms significantly affect photoelectron spectra predictions.

Abstract

As experimentalists explore new opportunities in strong field radiation offered by current generation light sources, new theoretical tools become inevitable in dealing with the challenging non-linear dynamics that come into play as a result of the increasing laser intensities and the shorter wavelengths. While many theoretical studies employ the electric dipole approximation for convenience reasons, in the strong-field regime the validity of this approximation is questionable. We have made a detailed comparison of the expansion of the retardation term, $e^{i\mathbf{k} \cdot \mathbf{r}}$ in both Taylor and Rayleigh series multipole approximations with the angle between the radial vector and the direction of propagation chosen arbitrarily to be $45^{\circ}$. It is verified in this paper that the Rayleigh plane-wave expansion provides a larger validity range in comparison to the widely used Taylor expansion. We also take note that the Taylor approximated spherical Bessel functions reproduce the lower limits of the regular spherical Bessel functions but deviates strongly in the asymptotic region. We conclude that the use of the Rayleigh plane-wave expansion provides the most accurate contribution of any given order of the multipole expansion. The discrepancy in the dipole and non-dipole photoelectron energy spectra as predicted by these approximations using short-wavelength intense laser pulses interacting with hydrogen atom in its ground state show the importance of the higher-order terms absent in the Taylor expansion.