Speaker
Description
The development of light sources in the extreme ultraviolet (XUV) based on high harmonic generation has opened new avenues for the investigation of time-resolved photodynamics in cationic excited electronic states of polyatomic molecules. Here, an XUV pump-infrared (IR) probe scheme with femtosecond time resolution is employed to study the dynamics of dissociative ionization in methyl iodide [1].
A time-delay-compensated XUV monochromator is employed to isolate the 9th harmonic of the fundamental 800 nm (13.95 eV, 88.89 nm), which is used as a pump pulse to prepare the cation in several electronic states. A time-delayed IR probe pulse is used to probe the dissociative ionization dynamics on the first excited à $^2$A$_1$ state. Photoelectrons and photofragment ions are detected by velocity map imaging. The experimental results are complemented with high-level ab initio calculations of potential energy curves (PECs) of the electronic states of CH$_3$I$^+$ as well as full-dimension on-the-fly trajectory calculations on the Ã$^2$A$_1$ state, considering the presence of the IR pulse. CH$_3^+$ and I$^+$ transients reflect the role of the IR pulse in probing the photodynamics of CH$_3$I$^+$ in the Ã$^2$A$_1$ state, mainly through the coupling to the ground state X$^2$E$_{3/2,1/2}$ and to the excited B$^2$E state manifold. Oscillatory features are observed and attributed to a vibrational wave packet prepared in the à $^2$A$_1$ state. The IR probe pulse induces a coupling between electronic states leading to a slow depletion of CH$_3^+$ fragments after the cation is transferred to the ground X$^2$E$_{3/2,1/2}$ states and an enhancement of I$^+$ fragments by absorption of IR photons yielding dissociative photoionization.
Complementary experiments have been carried out at the synchrotron SOLEIL using double imaging photoelectron photoion coincidence (i$^2$ PEPICO) spectroscopy to study the valence-shell dissociative photoionization of methyl iodide. The measured threshold photoelectron spectrum for CH$_3^+$ reveals that the ν$_5$ scissors vibrational mode promotes a transfer of population from the initially populated first excited state (Ã$^2$A$_1$) into the ground cationic state, leading to the formation of CH$_3^+$. Additional high-level ab initio calculations of PECs reveal the presence of an elusive conical intersection mediating this internal conversion.
References
[1] Murillo-Sánchez et al 2021 New J. Phys. 23, 073023
[2] González-Vázquez et al 2023, in preparation
