Gas-Phase Molecules

Mapping Wave Packet Bifurcation at a Conical Intersection in CH3I by Attosecond XUV Transient Absorption Spectroscopy

J. Chem. Phys. 154, 234301 (2021)

Kristina F. Chang,  Han Wang,  Sonia M. Poullain,  David Prendergast,  Daniel M. Neumark, and  Stephen R. Leone

Extreme ultraviolet (XUV) transient absorption spectroscopy has emerged as a sensitive tool for mapping the real-time structural and electronic evolution of molecules. Here, attosecond XUV transient absorption is used to track dynamics in the A-band of methyl iodide (CH3I). Gaseous CH3I molecules are excited to the A-band by a UV pump (277 nm, ∼20 fs) and probed by attosecond XUV pulses targeting iodine I(4d) core-to-valence transitions. Owing to the excellent temporal resolution of the technique, passage through a conical intersection is mapped through spectral signatures of nonadiabatic wave packet bifurcation observed to occur at 15 ± 4 fs following UV photoexcitation. The observed XUV signatures and time dynamics are in agreement with previous simulations [H. Wang, M. Odelius, and D. Prendergast, J. Chem. Phys. 151, 124106 (2019)]. Due to the short duration of the UV pump pulse, coherent vibrational motion in the CH3I ground state along the C–I stretch mode (538 ± 7 cm−1) launched by resonant impulsive stimulated Raman scattering and dynamics in multiphoton excited states of CH3I are also detected.