The Rydberg Excitation Of Noble Gas And Diatomic Molecules In Strong Femtosecond Laser Field

With the development of modern science and technology, many newstrong field physics phenomena are produced by the interaction betweenultrafast laser with atoms and molecules, such as high harmonic generation,above threshold ionization and non-sequential double ionization. Theneutral Rydberg excitation in strong laser filed as a new strong fieldphysics phenomenon, which is consider to be not only an importantcomplement of the tunneling-plus-rescattering phenomena, but also anotherpossible means of neutral particle acceleration, has attracted a lot oftheoretical and experimental attention in recent years.Despite the fact that lots of studies about Rydberg excitation in strongfield has been published since2008, there are still some problems need tobe solved in further research. For example, the mechanism of atomRydberg excitation, the competition between Rydberg excitation and otherstrong field physics processes, the neutral Rydberg excitation of moleculesin strong laser field, the influence of various laser parameters(such as laserwavelength, intensity and ellipticity), and the comparison of Rydbergexcitation between atoms and molecules. In this dissertation, we study the Rydberg excitation of different atomsand molecules using the pulsed electronic field ionization methodcombined with time of flight mass spectrum. According to the dependenceof strong field ionization and Rydberg excitation probability on laserparameters(laser intensity and ellipticity), and the comparison betweenRydberg excitation and non-sequential double ionization of atoms, thecomparison of Rydberg excitation between atoms and molecules withidentical ionization potential, and the difference of Rydberg excitationbetween IR and UV laser field, we discuss the mechanism of Rydbergexcitation of atoms and molecules in IR and UV laser field, and analyze thecompetition between Rydberg excitation and other strong field physicsprocess. We obtained the following main results:1) We measured the yield of Rydberg excitation of inert gases (He, Arand Xe) in800nm femtosecond laser field, and the dependence of the yieldon the laser parameters, such as laser intensity and elliptiticy. As acomparison, the yield of non-sequential double ionization of these inertgases were also measured in this experiment. The research found that theRydberg excitation and non-sequential double ionization show similarphenomenon, for example, as the atomic number growing, both yieldincreases, and the dependence on laser ellipticity is weaker. Combined withthe existed reports and related theories, we analyze and discuss the intrinsic physical mechanism of Rydberg excitation in800nm strong laser field,namely frustrated tunneling ionization.2) We observed the Rydberg excitation of molecules in femtosecondlaser field experimentally for the first time. And a comparative study ofRydberg excitation yield was made between diatomic molecules N2and O2with its companion atoms Ar and Xe. Similar to strong field ionization, N2show similar Rydberg excitation probability with atom Ar, while comparedto Xe, the Rydberg excitation probability of O2was significantly inhibited,and the inhibition in Rydberg excitation is more obvious than that in strongfield ionization. Analysis shows that the different outgoing electron angulardistribution caused by orbital structures of different molecules is animportant factor for the Rydberg excitation probability.3) We studied the Rydberg excitation of NO both in IR and UVfemtosecond laser fields. For the first time, we observed the Rydbergexcitation process in UV femtosecond laser field in experiment. It is foundthat the dependence of Rydberg excitation probability on laser ellipticity in800nm and400nm is different, combined with the previous experimentaldata and theoretical model, we believe that the mechanism of Rydbergexcitation of NO in UV laser field is multi-photon resonance excitation.