Coherent strong-field control of multiple states by a single chirped femtosecond laser pulse

TL;DR

This study demonstrates how chirped femtosecond laser pulses can selectively control the excitation of multiple sodium atom states through different ionization pathways, utilizing experimental measurements and theoretical modeling.

Contribution

It introduces a combined experimental and theoretical approach to control atomic states using chirped pulses, revealing the role of dynamic Stark-shifts and level crossings in state preparation.

Findings

Selective excitation of sodium states achieved by tuning chirp parameter

Identification of two main ionization pathways via REMPI processes

Observation of coherent superpositions through three-state level crossings

Abstract

We present a joint experimental and theoretical study on strong-field photo-ionization of sodium atoms using chirped femtosecond laser pulses. By tuning the chirp parameter, selectivity among the population in the highly excited states 5p, 6p, 7p and 5f, 6f is achieved. Different excitation pathways enabling control are identified by simultaneous ionization and measurement of photoelectron angular distributions employing the velocity map imaging technique. Free electron wave packets at an energy of around 1 eV are observed. These photoelectrons originate from two channels. The predominant 2+1+1 Resonance Enhanced Multi-Photon Ionization (REMPI) proceeds via the strongly driven two-photon transition $4s\leftarrow\leftarrow3s$, and subsequent ionization from the states 5p, 6p and 7p whereas the second pathway involves 3+1 REMPI via the states 5f and 6f. In addition, electron wave packets from two-photon ionization of the non-resonant transiently populated state 3p are observed close to the ionization threshold. A mainly qualitative five-state model for the predominant excitation channel is studied theoretically to provide insights into the physical mechanisms at play. Our analysis shows that by tuning the chirp parameter the dynamics is effectively controlled by dynamic Stark-shifts and level crossings. In particular, we show that under the experimental conditions the passage through an uncommon three-state "bow-tie" level crossing allows the preparation of coherent superposition states.