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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.

[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.