Observation Of A Freeman Resonance In The Femtosecond Ionization Of Methyl Iodide

We perform velocity map imaging in the electrons generated from the ionization of CH3I molecules at 800 nm, in the intensity range 3.1 x 10(13) W/cm(2)<= I <= 6 x 10(13) W/cm(2). The photoelectron spectra reveal a Freeman resonance at a peak intensity of approximate to 3.5 x 10(13) W/cm(2). The photoelectron kinetic energy dependence on the laser intensity, as well as the observation of the 8-th photon channel closing offers absolute intensity calibration in our experiment, which allows for the accurate determination of the ponderomotive potential induced by the laser field and ultimately the determination of the Rydberg state involved in the Freeman resonance.