Rydberg Excitation Of Triatomic Molecule CO₂ In Strong Femtosecond Laser Field

With the rapid development of ultrashort pulsed laser technology in the past 30 years, the interaction of strong laser field with atoms or molecules have become one of the hot research topics. In case of low laser intensity, the interaction of laser with atoms and molecules can deal with low-order perturbation theory. As the laser intensity increasing, the laser electric field reaches or even exceeds the Coulomb field of atom. Many interesting and novel physical phenomena are observed in strong laser field, such as: Multi-photon ionization, above threshold ionization, non-sequential double ionization, high-order harmonic generation, Coulomb explosion and so on. In recent years, neutral Rydberg excitation of atoms, which is a newly-found strong-field physical process, has attracted many of theoretical and experimental interests.In experiments, neutral Rydberg excitation had been observed in noble gas atoms as well as the atomic fragments from dissociation of some diatomic molecules in strong laser field. With the assistance of various theoretical models, a better understanding of the physical progress has been made. There still exist some problems which need further experimental and theoretical studies, for example, neutral Rydberg excitation in polyatomic molecule; the differences in Rydberg excitation between atom and polyatomic molecules; the effect of geometry and electronic structure of polyatomic in Rydberg excitation; the product pathway of neutral fragment. In addition, little information is available for understanding the similarities and differences between Rydberg excitation and other strong field physical processes.In this paper, we have measured the Rydberg excitation of triatomic molecules, CO₂, in femtosecond laser field using a pulsed electric field ionization method combined with time of flight mass spectrometer. The yields of Rydberg parent molecule and neutral fragments have been measured as a function of laser intensity,ellipticity and polarization direction, and have compared with those of direct ionization. The physical mechanism of Rydberg excitation of triatomic molecules, as well as connection between the Rydberg excitation and strong field ionization/dissociation and non-sequential double ionization, has been discussed. The main results of the study are summarized as followed:1) We have observed neutral Rydberg excitation of triatomic molecules in femtosecond laser field for the first time. By measuring the yield of the neutral parent molecule CO₂* as a function of laser intensity, ellipticity and polarization direction, and comparing them with that of Kr atom which has an ionization potential close to that of CO₂, we have analyzed the physical mechanism of Rydberg excitation of triatomic molecule, and studied the effects of geometry and electronic structure of molecule on the Rydberg excitation process.2) Neutral Rydberg excitations of the fragments C*, O* and CO* from CO₂ molecules have been observed. The fragment Rydbergs have been compared with fragment ions C+, O+ and CO+, for different laser intensity and ellipticity. The production pathway of neutral Rydberg fragment in femtosecond laser field, and the relationship between the neutral fragment and neutral parent molecule have been discussed.