Attosecond transient absorption probing of electronic superpositions of bound states in neon: detection of quantum beats

Annelise R Beck,Shaohao Chen,Kenneth J Schafer,Mette B Gaarde,Daniel M Neumark,Stephen R Leone,Birgitta Bernhardt,Erika R Warrick,Mengxi Wu

doi:10.1088/1367-2630/16/11/113016

Abstract

Electronic wavepackets composed of multiple bound excited states of atomic neon lying between 19.6 and 21.5 eV are launched using an isolated attosecond pulse. Individual quantum beats of the wavepacket are detected by perturbing the induced polarization of the medium with a time-delayed few-femtosecond near-infrared (NIR) pulse via coupling the individual states to multiple neighboring levels. All of the initially excited states are monitored simultaneously in the attosecond transient absorption spectrum, revealing Lorentzian to Fano lineshape spectral changes as well as quantum beats. The most prominent beating of the several that were observed was in the spin–orbit split 3d absorption features, which has a 40 femtosecond period that corresponds to the spin–orbit splitting of 0.1 eV. The few-level models and multilevel calculations confirm that the observed magnitude of oscillation depends strongly on the spectral bandwidth and tuning of the NIR pulse and on the location of possible coupling states.

Highlights

Wavepackets, or coherent superpositions of multiple electronic, vibrational or rotational states of an atom or molecule, evolve on timescales determined by the energy separations between the levels that comprise the wavepacket
The broad and continuous spectrum of an isolated attosecond pulse is useful for launching electronic superpositions that consist of many states
The broad spectrum allows multiple absorption features to be monitored simultaneously while varying the time delay between the attosecond pulse and a second laser pulse that can couple the initially excited state to other states or to the continuum. This technique, known as attosecond transient absorption, was first used to measure the degree of coherence in an electronic superposition created by the strongfield ionization of krypton [9]

Summary

Introduction

Wavepackets, or coherent superpositions of multiple electronic, vibrational or rotational states of an atom or molecule, evolve on timescales determined by the energy separations between the levels that comprise the wavepacket. The broad spectrum allows multiple absorption features to be monitored simultaneously while varying the time delay between the attosecond pulse and a second laser pulse that can couple the initially excited state to other states or to the continuum. In the work presented here, an isolated attosecond pulse in the extreme ultraviolet (XUV) spectrum is used to create a coherent superposition in neon gas, inducing a time-dependent polarization in the sample.

Results

Conclusion