Attosecond transient absorption of argon atoms in the vacuum ultraviolet region: line energy shifts versus coherent population transfer

Wei Cao,Stephen R Leone,Erika R Warrick,Daniel M Neumark

doi:10.1088/1367-2630/18/1/013041

Abstract

Using attosecond transient absorption, the dipole response of an argon atom in the vacuum ultraviolet (VUV) region is studied when an external electromagnetic field is present. An isolated attosecond VUV pulse populates Rydberg states lying 15 eV above the argon ground state. A synchronized few-cycle near infrared (NIR) pulse modifies the oscillating dipoles of argon impulsively, leading to alterations in the VUV absorption spectra. As the NIR pulse is delayed with respect to the VUV pulse, multiple features in the absorption profile emerge simultaneously including line broadening, sideband structure, sub-cycle fast modulations, and 5–10 fs slow modulations. These features indicate the coexistence of two general processes of the light–matter interaction: the energy shift of individual atomic levels and coherent population transfer between atomic eigenstates, revealing coherent superpositions. An intuitive formula is derived to treat both effects in a unifying framework, allowing one to identify and quantify the two processes in a single absorption spectrogram.

Highlights

A replica of the NIR pulse centered around 780 nm is picked off by a beamsplitter and combined with the attosecond pulse using an annular mirror before the target cell
There are four distinct effects attributable to the NIR pulse, each of which is labelled in figure 1. (a) It broadens/shifts individual absorption lines at small positive delays. (b) Horizontal sideband structures between two adjacent absorption resonances develop at large positive delays. (c) Fast modulations with a period of 1.3 fs appear on the two 4d states lying around 15 eV. (d) Slow modulations with a period of 5–10 fs appear on the states approaching the ionization limits (∼15.75 eV)
To understand the role of the laser-imposed phase in the absorption spectra, we evaluate the optical density for an oscillating dipole with the form: d(t ) = e-t/G sin [w0t + j (t > t)], where w0 is the energy difference between the ground and excited state, G is the lifetime of the excited state, and j (t > t) specifies the laser-imposed phase when the IR pulse arrives at time t

Summary

Introduction

Both Stark shift and population transfer processes are found to play important roles in this energy range, giving rise to abundant transient absorption features such as line broadening, sideband structure, sub-cycle fast modulations, and 5–10 femtosecond slow modulations. The second term of equation (7) results from the coherent nature of the wavepacket prepared by the broadband VUV pump pulse; a precise control of the delay will allow effective control of the inter-level population transfer.

Results

Conclusion