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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.

Understanding the relaxation pathways of photoexcited molecules is essentialto gain atomistic level insight into photochemistry. Herein, we perform atime-resolved study of ultrafast molecular symmetry breaking via geometricrelaxation (Jahn-Teller distortion) on the methane cation. Attosecond transientabsorption spectroscopy with soft X-rays at the carbon K-edge reveals that thedistortion occurs within $10\pm 2$ femtoseconds after few-femtosecondstrong-field ionization of methane. The distortion activates coherentoscillations in the scissoring vibrational mode of the symmetry broken cation,which are detected in the X-ray signal. These oscillations are damped within$58\pm13$ femtoseconds, as vibrational coherence is lost with the energyredistributing into lower-frequency vibrational modes. This study completelyreconstructs the molecular relaxation dynamics of this prototypical example andopens new avenues for exploring complex systems.

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

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.

X-ray Transient Absorption Spectroscopy (XTAS) and theoretical calculationsare used to study CCl$_4^+$ prepared by 800 nm strong-field ionization. XTASsimultaneously probes atoms at the carbon K-edge (280-300 eV) and chlorineL-edge (195-220 eV). Comparison of experiment to X-ray spectra computed byorbital-optimized density functional theory (OO-DFT) indicates that afterionization, CCl$_4^+$ undergoes symmetry breaking driven by Jahn-Tellerdistortion away from the initial tetrahedral structure (T$_d$) in 6$\pm$2 fs.The resultant symmetry-broken covalently bonded form subsequently separates toa noncovalently bound complex between CCl$_3^+$ and Cl over 90$\pm$10 fs, whichis again predicted by theory. Finally, after more than 800 fs, L-edge signalsfor atomic Cl are observed, indicating dissociation to free CCl$_3^+$ and Cl.The results for Jahn-Teller distortion to the symmetry-broken form of CCl$_4^+$and formation of the Cl -- CCl$_3^+$ complex characterize previously unobservednew species along the route to dissociation.

The measurement and control of light field oscillations enable the study of ultrafast phenomena on sub-cycle time scales. Electro-optic sampling (EOS) is a powerful field characterization approach, in terms of both sensitivity and dynamic range, but it has not reached beyond infrared frequencies. Here, we show the synthesis of a sub-cycle infrared-visible pulse and subsequent complete electric field characterization using EOS. The sampled bandwidth spans from 700 nm to 2700 nm (428 to 110 THz). Tailored electric-field waveforms are generated with a two-channel field synthesizer in the infrared-visible range, with a full-width at half-maximum duration as short as 3.8 fs at a central wavelength of 1.7 µm (176 THz). EOS detection of the complete bandwidth of these waveforms extends it into the visible spectral range. To demonstrate the power of our approach, we use the sub-cycle transients to inject carriers in a thin quartz sample for nonlinear photoconductive field sampling with sub-femtosecond resolution.

Today's high-power laser systems are capable of reaching photon intensities up to 1022 W cm-2, generating plasmas when interacting with material. The high intensity and ultrashort laser pulse duration (fs) make direct observation of plasma dynamics a challenging task. In the field of laser-plasma physics and especially for the acceleration of ions, the spatio-temporal intensity distribution is one of the most critical aspects. We describe a novel method based on a single-shot (i.e. single laser pulse) chirped probing scheme, taking nine sequential frames at frame rates up to THz. This technique, to which we refer as temporally resolved intensity contouring (TRIC) enables single-shot measurement of laser-plasma dynamics. Using TRIC, we demonstrate the reconstruction of the complete spatio-temporal intensity distribution of a high-power laser pulse in the focal plane at full pulse energy with sub-picosecond resolution.

The shape of a wave carries all information about the spatial and temporal structure of its source, given that the medium and its properties are known. Most modern imaging methods seek to utilize this nature of waves originating from Huygens' principle. We discuss the retrieval of the complete kinetic energy distribution from the acoustic trace that is recorded when a short ion bunch deposits its energy in water. This novel method, which we refer to as Ion-Bunch Energy Acoustic Tracing (I-BEAT), is a refinement of the ionoacoustic approach. With its capability of completely monitoring a single, focused proton bunch with prompt readout and high repetition rate, I-BEAT is a promising approach to meet future requirements of experiments and applications in the field of laser-based ion acceleration. We demonstrate its functionality at two laser-driven ion sources for quantitative online determination of the kinetic energy distribution in the focus of single proton bunches.

The shape of a wave carries all information about the spatial and temporalstructure of its source, given that the medium and its properties are known.Most modern imaging methods seek to utilize this nature of waves originatingfrom Huygens' principle. We discuss the retrieval of the complete kineticenergy distribution from the acoustic trace that is recorded when a short ionbunch deposits its energy in water. This novel method, which we refer to asIon-Bunch Energy Acoustic Tracing (I-BEAT), is a generalization of theionoacoustic approach. Featuring compactness, simple operation,indestructibility and high dynamic ranges in energy and intensity, I-BEAT is apromising approach to meet the needs of petawatt-class laser-based ionaccelerators. With its capability of completely monitoring a single, focusedproton bunch with prompt readout it, is expected to have particular impact forexperiments and applications using ultrashort ion bunches in high flux regimes.We demonstrate its functionality using it with two laser-driven ion sources forquantitative determination of the kinetic energy distribution of single,focused proton bunches.

Direct measurements of the electric field of light enable new observations of light–matter interactions. In the near‐infrared and visible spectral ranges, this typically relies on techniques that exploit nonlinearities in gases or solids, which limits their sensitivity. Here, a method for the detection of broadband near‐infrared fields spanning more than one octave from 110 to 220 THz based on linear absorption in a semiconductor is demonstrated. This technique, which avoids complex vacuum setups and works under ambient conditions, employs linear photoconductive sampling (LPS) in gallium phosphide. Simulations reveal that the response function of LPS is concerned with the intensity envelope of the gate field, in contrast to electro‐optic sampling, relaxing the stringent temporal requirements on the gate pulse. [ABSTRACT FROM AUTHOR]

Access to subtle ultrafast effects of light-matter interaction often requires highly sensitive field detection schemes. Electro-optic sampling, being an exemplary technique in this regard, lacks high sensitivity in an imaging geometry. We demonstrate a straightforward method to significantly improve the contrast of electric field images in spatially resolved electro-optic sampling. A thin-film polarizer is shown to be an effective tool in enhancing the sensitivity of the electro-optic imaging system, enabling an adjustment of the spectral response. We show a further increase of the signal-to-noise ratio through the direct control of the carrier envelope phase of the imaged field.