Above-Threshold Ionization of H-atom in Classical and Quantum Plasma Environments

Introduction

Above-Threshold Ionization (ATI) is a fundamental strong-field process, in which photoelectrons exhibit a characteristic multi-peak energy spectrum. While well-studied for isolated atoms, this process is less understood in a screened plasma environment. This work utilizes simulations to investigate how a quantum plasma affects the ATI spectrum, comparing the results with those of a classical plasma.

Methods

The one-dimensional time-dependent Schrödinger equation (TDSE) is solved numerically for a hydrogen atom interacting with an intense laser field, defined by a Gaussian pulse envelope. The TDSE is propagated using a stable, unitary Crank–Nicolson algorithm. To compare the two regimes, classical screening is modeled with a Debye–Hückel (Yukawa) potential, while the quantum environment is modeled using an Exponential Cosine Screened Coulomb (ECSC) potential to capture its characteristic oscillatory nature.

Results

The simulations show a clear contrast between the classical and quantum models. In the classical model, stronger screening causes a uniform blue shift of the ATI peaks to higher energies, corresponding to a reduced electron binding energy. The quantum model reproduces this shift but also introduces new features. The oscillatory component of the ECSC potential alters the spectrum's structure, modulating the relative peak heights and creating new spectral peaks indicating absorption of more photons, features that are entirely absent in the classical Debye model. For low laser intensities, the peak heights are reduced due to plasma screening; however, at higher intensities, plasma screening is insufficient to reduce peak heights.

Conclusion

The systematic blue-shift of the spectral peaks with decreasing Debye lengths provides a direct measure of the binding energy reduction caused by plasma screening. Furthermore, the complex modulations and additional spectral features introduced by the oscillatory component of the potential may serve as a unique way of controlling ATI spectra, distinguishing it from simple monotonic screening.