Discerning Mode-Controlled Relaxation Pathways in Photoionized Methane via X-ray Transient Absorption Spectroscopy.

Deciphering ultrafast relaxation pathways following the excitation of specific vibrational modes can inform our understanding of mode-selective chemistry; however, direct spectroscopic signatures of the underlying nuclear dynamics remain elusive. Here, we show that X-ray transient absorption spectroscopy at the carbon K-edge can resolve how vibrational pre-excitation biases the ultrafast dynamics of CH4+ following strong-field ionization of methane. We utilize ab initio molecular dynamics combined with high-level multireference computations of core-excited states to track these femtosecond dynamics starting from methane excited in its asymmetric stretching (ν3 = 1) and asymmetric scissoring (ν4 = 1) modes. Our simulations show that core excitation to the singly occupied molecular orbital (SOMO) tracks bond angles, whereas core excitation to the lowest unoccupied molecular orbital (LUMO) reports on bond lengths. These spectroscopic signatures distinguish two major mode-selective relaxation pathways: asymmetric stretching excitation promotes the formation of a transient CH3+···H complex, whereas asymmetric scissoring enhances angular distortion along the Jahn-Teller distortion coordinate, increasing sampling of C2v CH4+ geometries. These results identify for each relaxation pathway an experimentally testable X-ray signature of vibrationally-steered ionization dynamics in methane.