Attosecond Probing of Coherent Vibrational Dynamics in CBr$_4$

TL;DR

This study demonstrates how attosecond transient absorption spectroscopy can directly observe and analyze coherent vibrational wavepackets in CBr₄, revealing the influence of pump pulse intensity on vibrational dynamics with high temporal and energy resolution.

Contribution

The paper introduces a novel application of XUV ATAS combined with theoretical modeling to probe and quantify vibrational wavepacket dynamics in CBr₄, providing new insights into intensity-controlled vibrational excitation.

Findings

XUV ATAS resolves vibrational oscillations of a few meV and tens of femtoseconds.

Vibrational amplitude and population transfer depend on pump pulse intensity.

Theoretical calculations support the experimental observations and analysis.

Abstract

A coherent vibrational wavepacket is launched and manipulated in the symmetric stretch (a$_1$) mode of CBr$_4$, by impulsive stimulated Raman scattering from non-resonant 400 nm laser pump pulses with various peak intensities on the order of tens of 10$^{12}$ W/cm$^2$. Extreme ultraviolet (XUV) attosecond transient absorption spectroscopy (ATAS) records the wavepacket dynamics as temporal oscillations in XUV absorption energy at the bromine M$_{4,5}$ 3d$_{3/2,5/2}$ edges around 70 eV. The results are augmented by nuclear time-dependent Schr\"odinger equation simulations. Slopes of the (Br-3d$_{3/2,5/2}$)$^{-1}$10a$_1^*$ core-excited state potential energy surface (PES) along the a$_1$ mode are calculated to be -9.4 eV/{\AA} from restricted open-shell Kohn-Sham calculations. Using analytical relations derived for the small-displacement limit with the calculated slopes of the core-excited state PES, a deeper insight into the vibrational dynamics is obtained by retrieving the experimental excursion amplitude of the vibrational wavepacket and the amount of population transferred to the vibrational first-excited state, as a function of pump-pulse peak intensity. Experimentally, the results show that XUV ATAS is capable of easily resolving oscillations in the XUV absorption energy on the order of few to tens of meV and tens of femtosecond time precision, limited only by the averaging times in the experimental scans. This corresponds to oscillations of C-Br bond length on the order of 10$^{-4}$ to 10$^{-3}$ {\AA}. The results and the analytic relationships offer a clear physical picture, on multiple levels of understanding, for how the pump-pulse intensity controls the vibrational dynamics launched by non-resonant ISRS in the small-displacement limit.