On the Nature of Subharmonics of the Electron Emission from Ultracold Plasmas

TL;DR

This paper proposes a new interpretation for the subharmonics observed in electron emission from ultracold plasmas, attributing them to multiphoton ionization of Rydberg atoms rather than standing Langmuir waves, supported by numerical simulations.

Contribution

It introduces an alternative explanation for subharmonics in ultracold plasmas based on multiphoton ionization, overcoming limitations of previous models.

Findings

Numerical simulations show peaks in ionization efficiency at specific conditions.

The proposed mechanism is insensitive to boundary conditions and cloud shape.

The new interpretation aligns better with observed temperature dependence.

Abstract

One of the most interesting phenomena in the ultracold plasmas are multiple subharmonics of the electron emission observed after its irradiation by the monochromatic radiowaves. Unfortunately, the early interpretation of this phenomenon as the so-called Tonks-Dattner resonances (i.e., actually the standing Langmuir waves) encountered a number of serious obstacles, such as a lack of the adequate boundary conditions, an incorrect dependence on the electron temperature, and an insensitivity to the shape of the cloud. Here, we suggest an alternative interpretation based on the quasi-classical multiphoton ionization of the 'secondary' Rydberg atoms formed in the expanding and cooling plasma clouds. As follows from our numerical simulations, the efficiency of such ionization exhibits a series of well-expressed peaks. Moreover, this process is evidently irrelevant to the boundary conditions and global shape of the cloud. Therefore, this should be a viable alternative to the earlier idea of Tonks--Dattner resonances.