Femtosecond XUV–IR induced photodynamics in the methyl iodide cation

Abstract

The time-resolved photodynamics of the methyl iodide cation (CH3I+) are investigated by means of femtosecond XUV-IR pump-probe spectroscopy. A time-delay-compensated XUV monochromator is employed to isolate a specific harmonic, 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 dynamics on the first excited (A) over tilde (2)A(1) state potential energy surface. Photoelectrons and photofragment ions-CH3+ and I+- are detected by velocity map imaging. The experimental results are complemented with high level ab initio calculations for the potential energy curves of the electronic states of CH3I+ as well as with full dimension on-the-fly trajectory calculations on the first electronically excited state (A) over tilde (2)A(1), considering the presence of the IR pulse. The CH3+ and I+ pump-probe transients reflect the role of the IR pulse in controlling the photodynamics of CH3I+ in the (A) over tilde (2)A(1) state, mainly through the coupling to the ground state (X) over tilde E-2(3/2,1/2) and to the excited (B) over tilde E-2 state manifold. Oscillatory features are observed and attributed to a vibrational wave packet prepared in the (A) over tilde (2)A(1) state. The IR probe pulse induces a coupling between electronic states leading to a slow depletion of CH3+ fragments after the cation is transferred to the ground (X) over tilde E-2(3/2,1/2) states and an enhancement of I+ fragments by absorption of IR photons yielding dissociative photoionization.