Near-ionization-threshold emission in atomic gases driven by intense sub-cycle pulses

TL;DR

This paper theoretically investigates the dipole radiation of a hydrogen atom driven by intense sub-cycle pulses, revealing a unique narrowband emission near the ionization threshold caused by nearly-bound electron oscillations.

Contribution

It demonstrates the occurrence of near-ionization-threshold emission driven by ultrashort pulses and shows this phenomenon is general across atomic structures using ab initio and strong field approximation methods.

Findings

Narrowband emission appears above ionization threshold.

Emission lasts longer than the driving pulse.

Phenomenon is independent of specific atomic structure.

Abstract

We study theoretically the dipole radiation of a hydrogen atom driven by an intense sub-cycle pulse. The time-dependent Schr\"odinger equation for the system is solved by ab initio calculation to obtain the dipole response. Remarkably, a narrowband emission lasting longer than the driving pulse appears at a frequency just above the ionization threshold. An additional calculation using the strong field approximation also recovers this emission, which suggests that it corresponds to the oscillation of nearly-bound electrons that behave similarly to Rydberg electrons. The predicted phenomenon is unique to ultrashort driving pulses but not specific to any particular atomic structure.