Observation of dynamic Stark resonances in strong-field excitation

We investigated AC Stark-shifted resonances in Ar with ultrashort near-infrared light of two different durations (30 fs and 6 fs). With the 30 fs pulses, we clearly observed a periodic enhancement of the excitation yield in the intensity regions where the absorption of 13 and 14 photons occurs. When few-cycle 6 fs pulses are used, these enhancements become ambiguous due to the bandwidth of the pulse. We have a good agreement compared to simulations of the process using the TDSE and demonstrate evidence that the enhancements are due to AC Stark-shift resonances efficiently populating the 5g and 6h states.