Excitation Of Rydberg State Of Ar In Intense Laser Field

Femtosecond laser as an important mean to study ultrafast atomic and molecularprocesses is widely used in many fields such as physics, chemistry, biology, materialsetc. and it has received many remarkable achievements. Frustrated tunnelingionization （FTI） observed by a recent experiment is a Rydberg state excitationphenomenon in intense femtosecond laser field, while its mechanism anddependencies of the laser pulse parameters such as intensity, polarization, CEP etc. isnot clear enough, so it looks forward to our further research.In this paper, using pulsed electric field to achieve field ionization combined withtime-of-flight mass spectrometry and by adjusting the time delay between the pulsedelectric field and the laser pulse, we make the separation of the direct ionization ionand the Rydberg state field ionization ion in order to measure the Rydberg stateexcitation atom in strong femtosecond laser field directly. The experiment studies Aratom’s Rydberg state excitation process in strong femtosecond laser with800nmwavelength,90fs pulse width,10¹⁴-10¹⁵W/cm²pulse intensity and also study thedependency of the Rydberg state excitation with the laser intensity, polarization etc.and finally we analyze the the Rydberg state excitation mechanism of Ar atoms inintense laser field.The experimental research results are as follows1) By adjusting the delay time between the pulse electric field and laser, we detectthe Ar⁺signal while the Ar+is coming from the Ar^*Field Ionization and therebydetermine the Rydberg state producing, followed by calculated we find that thequantum state distribution of the Rydberg state is20<n <30.2) We measured the relationship of Rydberg state and the laser intensity, with thelaser intensity increasing, the signal strength of the Rydberg state first increases and then form a plateau with some oscillation, when the laser intensity increasesto a certain extent, the signal appeared an upward trend again.3) measuring the Rydberg states and the polarization relations and the result is thatat linearly polarized laser （=1） the signal is strongest and at the circularlypolarized laser （=±1）, under low intensity laser the FTI signal disappearedwhile under high intensity laser the FTI signal still exists.Through the analysis of experimental results combined with the existing literature,we believe that different intensity laser pulse cause different Rydberg states excitedmechanisms. Under low laser intensity it should be―re-scattering‖mechanismsleading to excitation of the Rydberg states, while under the high laser intensity a newmechanism of Rydberg state excitation join in which is not dependent on the laserellipticity and we speculate that it may be the multi-photon mechanism.Further we will study the dependencies of Rydberg states excitation with differentn values range and the parameters of laser pulses such as the CEP，pulse width and soon and different atomic and molecular Rydberg states excitation in the sameconditions. We hope to analyze the mechanism of the atomic and molecular Rydbergstate excitation more clearly.