Wavelength Dependence of Atomic Excitation for Ar Subject to Intense Midinfrared Laser Pulses

We report experimental and theoretical investigations of wavelength dependence of Rydberg state excitation(RSE)process of Ar subject to intense laser fields.By simultaneously measuring ionization and RSE yields of Ar atoms subject to strong laser fields at a series of wavelengths,we obtain the wavelength scaling law of the ratio of Ar^(*)over Ar^(+)with respect to the laser intensity,and this result can be well reproduced by a nonadiabatic model,but not by the classical-trajectory Monte Carlo model.Our results indicate that the nonadiabatic corrections of the photoelectron tunneling exit and tunneling probability play a significant role at shorter wavelengths.Analysis shows that the wavelength dependence phenomenon is due to the interplay of the nonadiabatic effect,wave-packet diffusion and Coulomb focusing effect of the liberated electron.

Extreme Ultraviolet Frequency Comb with More than 100μW Average Power below 100nm

Extreme ultraviolet(XUV)frequency comb is a powerful tool in precision measurement.It also brings many new opportunities to the field of strong field physics since high harmonic generation related phenomena can be studied with high repetition rate.We demonstrate the generation of an XUV frequency comb with the aid of intra-cavity high harmonic generation process.The setup is driven by a high power infrared frequency comb,and an average power of 4.5 kW is reached in the femtosecond enhancement cavity.With Xe gas as the working media,harmonics up to the 19th order are observed.Power measurement indicates that as much as 115.9μW(1.3 mW)are generated at^94 nm(~148 nm).The shortest wavelength we can reach is^55 nm.The coherence of the generated light is tested with an optical-heterodyne-based measurement of the third harmonic.The resulted line width is^3 Hz.In addition,with this system,we also observe a strong suppression of below threshold harmonics from O2 compared to that from Xe.These results suggest that the current system is ready for precision spectroscopic measurements with few-electron atomic and molecular systems in XUV region as well as the study of strong field physics with an unprecedented 100 MHz repetition rate.