Time-resolved four-wave-mixing spectroscopy for inner-valence transitions

Abstract

Non-collinear four-wave mixing (FWM) techniques at near-infrared (NIR), visible, and ultraviolet frequencies have been widely used to map vibrational and electronic couplings, typically in complex molecules. However, correlations between spatially localized inner-valence transitions among different sites of a molecule in the extreme ultraviolet (XUV) spectral range have not been observed yet. As an experimental step towards this goal we perform time-resolved FWM spectroscopy with femtosecond NIR and attosecond XUV pulses. The first two pulses (XUV-NIR) coincide in time and act as coherent excitation fields, while the third pulse (NIR) acts as a probe. As a first application we show how coupling dynamics between odd-and even-parity inner-valence excited states of neon can be revealed using a two-dimensional spectral representation. Experimentally obtained results are found to be in good agreement with ab initio time-dependent R-matrix calculations providing the full description of multi-electron interactions, as well as few-level model simulations. Future applications of this method also include site-specific probing of electronic processes in molecules.

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

“…Unlike traditional ATA measurements, this experiment utilizes two distinct NIR pulses that arrive at different delays. As in previously reported collinear experiments [22,26], these oscillations are indicative of wave-mixing emission in a self-heterodyned detection scheme and will be described fully in the Discussion section.…”

“…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.…”

RMT: R-matrix with time-dependence. Solving the semi-relativistic, time-dependent Schrödinger equation for general, multielectron atoms and molecules in intense, ultrashort, arbitrarily polarized laser pulses

“…r i |Φ p t (X N ) X lp,mp |X l p ,m p = = E(t) · d pt,p t δ lp,l p δ mp,m p , (A. 25) where the term E(t) · d pt,p t is the dot product between the x,y,z components of the electric field and the corresponding components of the permanent or transition dipole moments for the N -electron target states in the channels p and p . We can see that this potential is diagonal in {l, m}, but if at least two target states of different symmetries are included it will couple wavefunctions of different total symmetries.…”

R -matrix-with-time-dependence theory for ultrafast atomic processes in arbitrary light fields

“…It represents the latest embodiment of a time-dependent R-matrix formalism [61][62][63], whose flexibility and generality have been reflected in a wide variety of recent applications. These include multielectron correlation in doubly and core-excited states of Ne [64], strong-field rescattering in F − [65] and HHG from noble gas atoms in the near-infrared regime [66]. RMT theory has even been extended for the description of doubleelectron continua [67,68].…”