Measurement of Coherent Vibrational Dynamics with X-ray Transient Absorption Spectroscopy Simultaneously at the Carbon K- and Chlorine L$_{2,3}$- Edges

TL;DR

This study uses simultaneous X-ray transient absorption spectroscopy at carbon and chlorine edges to measure vibrational dynamics and core-excited potential energy surface slopes in CCl4, revealing atomic displacements and forces with high sensitivity.

Contribution

First combined experimental and theoretical measurement of core-excited potential energy surface slopes in CCl4 using X-ray transient absorption spectroscopy at multiple edges.

Findings

Measured slopes of core-excited potential energy surfaces for Cl and C in CCl4.

Identified bond elongation effects on X-ray absorption energies.

Demonstrated sensitivity of X-ray spectroscopy to tiny atomic displacements.

Abstract

X-ray Transient Absorption Spectroscopy near the carbon K-edge (1s, $\sim$ 285 eV) and chlorine L$_{2,3}$ edges (2p, $\sim$ 200 eV) is used to study the nuclear dynamics of CCl$_4$ vibrationally activated by impulsive stimulated Raman scattering with a few-cycle 800 nm pump pulse. The totally symmetric stretching mode leads to a strong response in the inner-shell spectra, with the concerted elongation (contraction) in bond lengths leading to a red (blue) shift in the X-ray absorption energies associated with core-to-antibonding excitations. The relative slopes of the potential energy surfaces associated with the relevant core-excited states along the symmetric stretching mode are experimentally measured and compared to results from restricted open-shell Kohn-Sham calculations. A combination of experiment and theory indicates that the slope of the core-excited potential energy surface vs totally symmetric bond elongation is $-11.1 \pm 0.8$ eV/{\AA} for the Cl 2p$\to7a_1^*$ excitation, $-9.0\pm0.6$ eV/{\AA} for the Cl 2p$\to8t_2^*$ excitation and $-5.2\pm 0.4$ eV/{\AA} for the C 1s$\to8t_2^*$ excitation, to 95% confidence. The much larger slopes for the Cl 2p excitations compared to the C 1s state are attributed to greater contributions from Cl to the $7a_1^*$ or $8t_2^*$ antibonding orbitals to which the inner-shell electrons are being excited. No net displacement of the center of the vibrational wavefunction along the other vibrational modes is induced by the pump pulse, leading to absence of transient signal. The results highlight the ability of X-ray Transient Absorption Spectroscopy to reveal nuclear dynamics involving tiny ($<0.01$ {\AA}) atomic displacements and also provide direct measurement of forces on core-excited potential energy surfaces.