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

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 fewcycle 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.Recently, attosecond transient absorption (ATA) spectroscopy has demonstrated great success in accessing ultrafast dynamics of bound and autoionizing systems in rare gas atoms [1-10] and diatomic molecules [11][12][13]. In this technique, dipole allowed states (bright states) are coherently populated by a broadband attosecond pulse, forming a wavepacket. The coherence between each individual excited state and the initial ground state forms a polarization dipole. Upon the arrival of a delayed short near infrared (NIR) pulse, these dipoles are subject to a complex amplitude change that occurs within a few femtoseconds, leading to novel absorption features in the transmitted spectrum of the attosecond pulse. Various dynamical aspects of the initially launched wavepacket are imprinted in the delay-dependent absorption spectra. Thus far, ATA studies have generally been carried out at photon energies ranging from 20 to 100 eV due to more efficient high harmonic generation above 20 eV. Here, we report ATA experiments on Ar atoms at photon energies around 15 eV, enabling a detailed study of electronic wavepacket dynamics for Rydberg states approaching the first ionization threshold of Ar.There are two major effects of the NIR pulse in ATA experiments. First, it shifts the energy of the individual states, generally known as the ac Stark shift. Secondly, it transfers population between different bright states by a two photon process. How these two effects manifest themselves in an ATA measurement is the essence of understanding the observables in such an experiment. The ac Stark shift corresponds to the energy difference between a dressed energy level and its field-free counterpart. In the time domain, it originates from the extra phase modulation, introduced by the NIR field, on the states of interest. The ac Stark shift manifests itself as reshaping of the absorption line profile in an ATA experiment [14,15]. On the other hand, population transfer between different b...

Phase-resolved two-dimensional spectroscopy of electronic wave packets by laser-induced XUV free induction decay

“…This indicates that it originates from laser-induced population transfer between Rydberg states and the unpopulated 4s states of argon (11.7229 eV and 11.8228 eV). Since the population at a given energy originates from several Rydberg states, we observe oscillations in the signal, corresponding to interferences between ladder type transitions [10]. The oscillation frequencies are dictated by the energy differences between the Rydberg states involved, and are thus the same as in the Λ-type coupling modulating the Rydberg emission (0.164 eV and 0.174 eV measured in the high-resolution scan).…”

“…The wavepacket relaxes by coherently emitting XUV radiation (called XUV Free Induction Decay, XFID) that interferes with the incident light. A delayed IR laser field is used to follow or control the relaxation dynamics, such that these experiments can be seen as Transient Reshaping of the Absorption spectrum of the XUV light (TRAX) [9,10].…”

Self-heterodyned detection of dressed state coherences in helium by noncollinear extreme ultraviolet wave mixing with attosecond pulses

“…Nevertheless, attosecond transient absorption (ATA) with collinear XUV pump and optical or near infrared (NIR) probe pulses can be described as a self-heterodyned nonlinear spectroscopy [12]. These measurements provide evidence of four-wave mixing interactions in the form of quantum beat oscillations [22][23][24][25][26], but they lack the selectivity of higher-order wave-mixing techniques to disentangle complex spectra [27]. Capitalizing on the phasematching conditions inherent in wave-mixing processes, recent experiments employed a noncollinear beam geometry between an XUV attosecond pulse train and either one [28,29] or two [30][31][32] moderately intense NIR pulses to measure transient spectra of spatially isolated XUV wave-mixing signals.…”

Core-level attosecond transient absorption spectroscopy of laser-dressed solid films of Si and Zr

“…Theoretically, it is described as the manipulation of 38 the electron-wave-packet dynamics of the unoccupied and 39 occupied electronic states of the material [4]. 40 For atomic and molecular gases such as argon [5], neon 41 [6], helium [7,8], N 2 [9], Br 2 [10], and hydrocarbons [11], probe [3,[12][13][14] were the subject of fewer studies. Besides 47 the change in absorption, Stark shift in He [15] and Ar [5], 48 Rabi oscillations [16] in Ne, line broadening [5], and quantum 49 beats [17,18] were recognized in the recorded XUV or photo-50 electron spectra.…”