Attosecond Time-Domain Measurement of Core-Level-Exciton Decay in Magnesium Oxide.

Excitation of ionic solids with extreme ultraviolet pulses creates localized core-level excitons, which in some cases couple strongly to the lattice. Here, core-level-exciton states of magnesium oxide are studied in the time domain at the Mg L_{2,3} edge with attosecond transient reflectivity spectroscopy. Attosecond pulses trigger the excitation of these short-lived quasiparticles, whose decay is perturbed by time-delayed near-infrared pulses. Combined with a few-state theoretical model, this reveals that the infrared pulse shifts the energy of bright (dipole-allowed) core-level-exciton states as well as induces features arising from dark core-level excitons. We report coherence lifetimes for the two lowest core-level excitons of 2.3±0.2 and 1.6±0.5 fs and show that these are primarily a consequence of strong exciton-phonon coupling, disclosing the drastic influence of structural effects in this ultrafast relaxation process.

Toward two-dimensional photon echo spectroscopy with 200 nm laser pulses.

Knowledge of elementary relaxation processes in small molecules and proteins motivates the extension of two-dimensional photon echo (2DPE) spectroscopy further into the UV wavelength range. Here, we describe our development of a four-wave mixing spectrometer employing 200 nm laser pulses. Filamentation of laser beams in both air and argon yields 200 nm pulses with 60 fs durations. These 200 nm pulses are used to probe dynamics initiated at 267 nm in transient grating and 2DPE experiments conducted on adenosine. This study demonstrates that these femtosecond spectroscopies may indeed be carried out at the shortest wavelengths feasible in aqueous solutions.

Femtosecond symmetry breaking and coherent relaxation of methane cations via x-ray spectroscopy.

Understanding the relaxation pathways of photoexcited molecules is essential to gain atomistic-level insight into photochemistry. We performed a time-resolved study of ultrafast molecular symmetry breaking through geometric relaxation (Jahn-Teller distortion) on the methane cation. Attosecond transient absorption spectroscopy with soft x-rays at the carbon K-edge revealed that the distortion occurred within 10 ± 2 femtoseconds after few-femtosecond strong-field ionization of methane. The distortion activated coherent oscillations in the asymmetric scissoring vibrational mode of the symmetry-broken cation, which were detected in the x-ray signal. These oscillations were damped within 58 ± 13 femtoseconds because vibrational coherence was lost with the energy redistributing into lower-frequency vibrational modes. This study completely reconstructs the molecular relaxation dynamics of this prototypical example and opens avenues for exploring complex systems.

Correction: Jahn-Teller distortion and dissociation of CCl4 + by transient X-ray spectroscopy simultaneously at the carbon K- and chlorine L-edge.

[This corrects the article DOI: 10.1039/D2SC02402K.].

Jahn-Teller distortion and dissociation of CCl4 + by transient X-ray spectroscopy simultaneously at the carbon K- and chlorine L-edge.

X-ray Transient Absorption Spectroscopy (XTAS) and theoretical calculations are used to study CCl4 + prepared by 800 nm strong-field ionization. XTAS simultaneously probes atoms at the carbon K-edge (280-300 eV) and chlorine L-edge (195-220 eV). Comparison of experiment to X-ray spectra computed by orbital-optimized density functional theory (OO-DFT) indicates that after ionization, CCl4 + undergoes symmetry breaking driven by Jahn-Teller distortion away from the initial tetrahedral structure (Td) in 6 ± 2 fs. The resultant symmetry-broken covalently bonded form subsequently separates to a noncovalently bound complex between CCl3 + and Cl over 90 ± 10 fs, which is again predicted by theory. Finally, after more than 800 fs, L-edge signals for atomic Cl are observed, indicating dissociation to free CCl3 + and Cl. The results for Jahn-Teller distortion to the symmetry-broken form of CCl4 + and formation of the Cl-CCl+ 3 complex characterize previously unobserved new species along the route to dissociation.

Efficient table-top dual-wavelength beamline for ultrafast transient absorption spectroscopy in the soft X-ray region.

We present a table-top beamline providing a soft X-ray supercontinuum extending up to 370 eV from high-order harmonic generation with sub-13 fs 1300 nm driving pulses and simultaneous production of sub-5 fs pulses centered at 800 nm. Optimization of high harmonic generation in a long and dense gas medium yields a photon flux of  ~ 1.4 × 106 photons/s/1% bandwidth at 300 eV. The temporal resolution of X-ray transient absorption experiments with this beamline is measured to be 11 fs for 800 nm excitation. This dual-wavelength approach, combined with high flux and high spectral and temporal resolution soft X-ray absorption spectroscopy, is a new route to the study of ultrafast electronic dynamics in carbon-containing molecules and materials at the carbon K-edge.