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

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.

Real-Time Probing of Electron Dynamics Using Attosecond Time-Resolved Spectroscopy

“…Theoretical efforts have been crucial to understanding the intense few-cycle pulse interaction with atoms and the resulting quantum pathways that underlie the spectral features measured by an attosecond XUV pulse (7,86,93,94). The near-infrared pulses in these experiments also cause a phase shift in the induced dipole moment, resulting in changes to the lineshape of the transmitted XUV spectrum from Lorentzian to Fano (9,95). Time-dependent measurements with pulse sequence XUV first, time-delayed near-infrared pulse second have been used to characterize the lifetimes of autoionizing states in rare gas atoms, where the population is depleted in the initially excited state by coupling to the ionization continuum or in other states by the near-infrared pulse as a function of the pump-probe delay.…”

“…A metal such as copper serves as the condensed-phase example: An electron traveling in the conduction band at the Fermi velocity (2) can traverse a distance equal to its lattice constant in ∼230 as. Correlation-driven electrons and holes in polyatomic molecules or widely spaced electronic superposition states are predicted to evolve on suboptical-cycle timescales (3)(4)(5)(6)(7)(8)(9). Generating attosecond pulses that can take snapshots of these processes is a prerequisite for understanding electron dynamics that precede, and perhaps even dictate, longer-time nuclear vibrational dynamics.…”

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

“…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.…”

Dressing effects in the attosecond transient absorption spectra of doubly excited states in helium

“…In this context, attosecond transient absorption spectroscopy (ATAS) [2][3][4] has affirmed itself as a valid technique to monitor and control electronic wave packets [2,5,6]. For example, it has been used to demonstrate hole alignment in neon [7], to track the fieldfree ultrafast dynamics of coherent ensembles of valence-hole excited states in krypton [2,8], of singly excited bound states in neon [6,9] and helium [10][11][12][13][14], and of doubly excited states in helium [5]. Compared to photofragment detection [15,16], ATAS provides easier access to higher energy resolution.…”