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 ÃA12 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 ÃA12 , 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 ÃA12 state, mainly through the coupling to the ground state X̃E3/2,1/22 and to the excited B̃E2 state manifold. Oscillatory features are observed and attributed to a vibrational wave packet prepared in the ÃA12 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 X̃E3/2,1/22 states and an enhancement of I+ fragments by absorption of IR photons yielding dissociative photoionization.