Y Liang

High harmonic generation in atomic and diatomic molecular gases using intense picosecond laser pulses-a comparison

High harmonic generation has been investigated in the atomic gases Ar and Xe as well as in the diatomic molecular gases H2, D2, N2 and O2 using a linearly polarized picosecond dye laser (594 nm) and a linearly polarized subpicosecond Ti:sapphire/Nd:glass laser (1053 nm). The harmonic conversion efficiencies are compared by choosing different gas pairs as a function of three parameters: (1) field-free ionization potential (2) mass and (3) static average polarizability. It is shown that the ionization potential and mass are not sensitive parameters for harmonic efficiency, but the polarizability is an important factor.

Non-sequential ionization of Xe and Kr in an intense femtosecond Ti:sapphire laser pulse

Using intense 200 fs Ti:sapphire laser pulses, multiphoton multiple ionization of Xe and Kr has been studied over a dynamic range of up to nine orders of magnitude. Huge enhancements in the probability of double ionization of both gases and triple ionization of Xe were observed and attributed to the occurrence of non-sequential (NS) ionization. It was found that the probability of NS ionization of any charge state is a linear combination of the tunnelling probability of the previous charge states.

Tunnel ionization of diatomic molecules by an intense CO2 laser

The non-resonant interaction of an intense nanosecond CO2 laser pulse with Hg2, N2, and H2 shows that the molecules are tunnel ionized as if they were atoms with the same ionization potentials as the molecules. Moreover, dissociation of the neutral molecule appears to be much less probable even though the dissociation energies are lower than the ionization potentials.

Population trapping in the CO molecule

Using very stable intense fs Ti:sapphire laser pulses, new structures in the ion versus intensity plot of are observed which can be attributed to electron trapping in excited electronic vibrational levels. This is further studied by a new and more sensitive technique based on high-order harmonic generation.

High-order harmonic generation in argon by elliptically polarized picosecond dye laser pulses

High-order harmonic generation is observed in an argon jet using elliptically polarized picosecond dye laser pulses. The experimental data appear to be in near agreement with the perturbation theory. However, there is a general trend of small deviation from the theory. We speculate that such a deviation could be due to the laser induced coupling of upper states in or near the continuum which relaxes the selection rules leading to an enhancement of harmonic emission.

Comparison of high harmonic conversion efficiency in atomic and diatomic molecular gases

High harmonic conversion efficiency has been experimentally compared in the atomic gas Ar and the diatomic molecular gases and . It is shown that, in the saturation region, if the atomic and diatomic molecular gases have similar polarizabilities, the one with the higher ionization potential has the higher harmonic efficiency. If the atomic and molecular gases have similar polarizabilities and ionization potentials, the atomic gas will have a higher harmonic efficiency than the molecular one.

Experimental investigation of the ellipticity dependence of high-harmonic generation and ionization in argon in the multiphoton regime

In the multiphoton regime, the correlation between high-harmonic generation and multiphoton ionization is investigated when the laser polarization changes from linear to elliptical. The ellipticity dependence of high harmonics and ion signal is measured experimentally. Both high harmonics and ion signal decrease with increased ellipticity at fixed laser intensity, but the decrease in ion signal is much slower compared to that of the harmonics. This phenomenon is due to the fact that high-harmonic generation is more strongly affected by the selection rules than ionization, when polarization changes.

Laser-induced inner shell vacancies in doubly ionized argon

The creation of laser-induced inner shell vacancies in doubly ionized Ar atoms is reported. First, theoretical calculations are performed for different laser systems and as much as 30% of doubly ionized Ar atoms in the [Ne]3s3p5 configuration with a 3s shell vacancy at optical and near-infrared frequencies are predicted. Analysis of the results shows that the mechanism for creation of the inner shell vacancy is a non-sequential double ionization process. Finally, the radiation spectrum of Ar in the field of an intense Ti:sapphire/Nd:glass laser system at 1053 nm has been measured, and the line corresponding to the fluorescence emission due to radiative filling of the created inner vacancy in the 3s shell is observed.

Investigation of photon emission spectra of hydrogen using intense subpicosecond Ti:sapphire/Nd:glass laser pulses

High-harmonic generation processes have been experimentally investigated in molecular hydrogen gas as both molecular dissociation and ionization occur in the gas. The high harmonics observed in our experiment can be mainly attributed to neutral hydrogen molecules. Besides the harmonic spectra, two strong Lyman lines from the dissociated hydrogen atoms and the emission from the neutral hydrogen molecules are observed. Several possible excitation and dissociation channels of hydrogen molecules are proposed. It is found that resonance effects are important for both high-harmonic generation and molecular dissociation. Theoretical calculations using the time-dependent Hartree-Fock (TDHF) method for H2 reproduce some of the resonance effects.

High harmonics generation in atoms and molecules using ultrashort laser pulses in the multiphoton regime

Summary form only given. We studied high harmonic generation in jets of atomic gases Ar and Xe as well as diatomic molecular gases H/sub 2/, D/sub 2/, N/sub 2/, and O/sub 2/ using a linearly polarized picosecond dye laser (594nm) and a linearly polarized subpicosecond Ti:sapphire/Nd:glass laser (1053nm). The harmonic conversion efficiencies are compared by choosing different gas pairs as a function of three parameters: (1) field-free ionization potential (2) mass and (3) static average polarizability.