Switching the vibrational excitation of a polyatomic ion in multi-photon strong field ionization

The multiphoton ionization (MPI) of CH3I has been investigated by angular resolved photoelectron spectroscopy as a function of femtosecond laser excitation intensity. A sudden change in the electron kinetic energy is observed above a specific field strength. The multiphoton excitation at a fixed wavelength of 800 nm becomes vibronically resonant due to Stark shifting of intermediate Rydberg state levels. The present letter gives an experimental evidence for ultrafast optical control of the vibrational excitation in a polyatomic ion by adjusting the intensity of a femtosecond laser pulse.