The study of ionization by intense laser fields is an important component of understanding light/matter interactions in highly nonlinear regimes. Typical intensities are between 10 12 and $10 14 W/cm 2 , generated in this case from 120 fs pulses from an 800 nm Ti:Sapphire laser system. Study of this highly nonlinear, so-called above threshold ionization (ATI) of atoms has led to a single active electron model. In this model, the laser interacts only with a single valence electron, which can absorb upwards of 30 photons from the field during ionization. Ionization yield is highly enhanced via resonance with Stark shifted atomic states, leading to peaks in the photoelectron energy spectrum known as Freeman resonances.
This work extends the study of ATI photoelectron spectroscopy from noble gases to diatomic molecules, and finds no clear deviation from the single active electron picture. Photoelectron spectra from N 2 (IP = 15.58 eV), O 2 (IP = 12.06 eV), and CO (IP = 14.01 eV) were collected and analyzed. These spectra are remarkably similar to noble gas spectra and appear perfectly consistent with dynamics dominated by a single electron interaction. Clear Freeman resonances are observed for all species. In fact, two Rydberg series are observed for N 2 and O 2 , perhaps originating from ionization through states associated with excited states of the ion.
We propose that the single active electron picture is an accurate shorthand for these, and perhaps all non-fragmenting, ionization events. This suggests the universality of single electron behavior. Moreover, our work demonstrates the difficulty of examining atomic or molecular structure using ATI photoelectron spectroscopy — the characteristic Freeman resonances are more indicative of valence electron behavior rather than atomic structure.