This chapter discusses some experimental manifestations of interference stabilization or population trapping in atoms and molecules during the filamentation of strong 800-nm femtosecond laser pulses propagating in air and other gases. Particular emphasis is given to nitrogen molecules whose fluorescence induced by the 800-nm pump is probed respectively by 400 nm, 1,338 nm and THz radiations. Fluorescence enhancement and reduction were observed under different probe conditions and at the revival times of the rotational wave packet of nitrogen. Population trapping in the Rydberg states of the molecule is central to the explanation of these observations. We conclude that population trapping through interference stabilization in the multiphoton regime is a universal phenomenon in atoms and molecules in intense laser fields. This includes the excitation of super-excited states of molecules.

We study the fluorescence emitted from filaments in air using a pump-probe scheme with a femtosecond Ti-sapphire laser. The fluorescence intensities from the first negative band (B-2 Sigma(+)(u) -> X-2 Sigma(+)(g)) and the second positive band (C-3 Pi(u) -> B-3 Pi(g)) show enhancement and change periodically as a function of the pump-probe time delay. We attribute this phenomenon to the universal yet probably forgotten phenomenon of population trapping of nitrogen molecules in highly excited states together with field-induced alignment of nitrogen molecules followed by revivals of the rotational wavepackets. Theoretical calculation of the alignment dynamics of nitrogen molecules is consistent with the experimental data.

The ionization of Ar and Xe in a dichroic pulsed laser field, comprising the fundamental and second harmonic of a Ti:sapphire laser, was studied. The ion yield was found to depend on the angle between the polarization vectors of the two fields. The dependence was explained by proposing a model that considered the ionization process to be resulting from the action of two independent mechanisms, namely quasi-static tunnelling and multiphoton ionization.

The nonsequential double ionization of four unsaturated organic molecules, C5NH5, C3H6, C2H4, and C2H2 interacting with strong Ti: Sapphire laser pulses are reported. Despite differences in the molecular geometry, the degree of ionization suppression and the fraction of molecules that undergo non-sequential double ionization were similar for all of these molecules.

The mass spectra of two isomers of butane, 1-butene and cis-2-butene, interacting with linearly polarized Ti:sapphire laser pulses, were compared. It was demonstrated that the difference between the two spectra are much more pronounced than the corresponding spectra prepared by 100 eV electron ionization ion source. In addition, the difference could be varied by changing the peak laser intensity. The observation suggests potential application of dissociative multiphoton ionization by ultrafast laser pulses as a source for high performance mass spectroscopy.

Multiphoton/tunnel ionization of benzene, interacting with strong Ti:sapphire laser pulses, to doubly charged ions was studied. In contrast to previously published results on multiphoton/tunnel ionization of atoms and molecules, it was demonstrated that double ionization of benzene occurs mainly through non-sequential processes.

Formation and wandering of filaments in air are studied both experimentally and numerically. Filament-center deflections are collected from 1100 shots of 190-fs and 800-nm pulses in the plane perpendicular to the propagation direction. To calculate the filament wandering in air we have developed a model of powerful femtosecond laser pulse filamentation in the Kolmogorov atmospheric turbulence and employed the Monte Carlo method to model the propagation of several hundred laser pulses. Statistical processing of experimental and numerical data shows that filament-center displacements in the transverse plane obey the Rayleigh-distribution law. Parameters of the Rayleigh distribution obtained for numerical and experimental data are close to each other.

As a result of focusing ultrafast Ti: Sapphire laser pulses in a gas, a plasma column is created. The spectrum of the radiation from the excited species in the column might be used as a new spectroscopic source free of plasma continuum. Furthermore, from the photoemission spectra, valuable information on the mechanism of interaction of strong laser pulses with atoms and molecules could be obtained.

High-power ultra-short laser pulses can propagate through a significant distance in air after self-focusing and filamentation.. Upon self-focusing, the pulse undergoes self-phase modulation turning into a broadband pulse spectrally ranging from the UV to IR, yet of strong spectral coherence, hence a white light laser pulse. Molecules in the filament will be ionized, fragmented and the resulting particles will fluoresce with well resolved molecular spectra without the interference of a plasma continuum even at atmospheric pressure. This talk gives a summary of some of our recent findings on the above mentioned phenomena as well as on ring formation, etc.

Dissociative ionization of the ${\mathrm{D}}_{2}$ molecule was studied using linearly polarized 400- and 800-nm laser pulses. At 800 nm, the ${\mathrm{D}}^{+}$ ions resulting from the dissociation of the molecule are observed to be parallel to the polarization of the laser field. At the shorter wavelength (400 nm), we observe that a considerable number of ${\mathrm{D}}^{+}$ ions are ejected perpendicular to the polarization of the laser field. This observation is interpreted in terms of the radiative coupling between the ground $1s{\ensuremath{\sigma}}_{g}$ and the high-lying $3d{\ensuremath{\sigma}}_{g}$ and $3p{\ensuremath{\pi}}_{u}$ electronic states of ${\mathrm{D}}_{2}^{+}.$ This interpretation was verified by comparing the experimentally measured and theoretically predicted kinetic energy of the ${\mathrm{D}}^{+}$ ions. Our results indicate that the radiative coupling between the ground and highly excited electronic states must be considered when intense laser fields with high frequencies are used.

We present an experiment demonstrating the generation of fast pulsed electric field from a plasma column created in air as a result of the interaction of N2 and O2 molecules with a 220 fs laser pulse. By first measuring the distribution of N2+ and then by measuring the net charge present at different positions in the focal region, we determined that this plasma column has an intrinsic dipole moment, which generates these pulses. The origin of this dipole moment was attributed to longitudinal separation of electrons due to ponderomotive acceleration of electrons created through multiphoton ionization of N2 and O2 molecules.

The fragmentation of benzene molecules in a linearly polarized Ti : Sapphire laser pulse was studied. It was found that the fragmentation of the molecules is the result of MPI of the inner shell electrons which results in a molecular ion in an excited state. Through radiationless transition the population is transferred to the highly excited levels of the ground electronic state. The molecular ion created in this manner is not stable and dissociates rapidly. The model was tested by comparing the measured laser intensity dependent abundance of C4H4+ fragments with the calculated ones and excellent agreement was obtained with the experimental results.

Measuring the photo-emission from a restricted section of a filament created in air as a result of propagation of a strong Ti:Sapphire laser pulse, the dependence of the propagation on the polarization (linear and circular) of the laser was studied. The results clearly indicate the dominant roles of two processes: self-focusing due to the nonlinear refractive index in the neutral gas and defocusing in the plasma as a result of multiphoton ionization. Multiple refocusing was observed in both polarizations.

We use a variational method to study the phenomenon of intense femtosecond pulse propagation in air. This method allows us to obtain a semianalytical solution to the problem in which a wide range of initial conditions can be studied. In addition, it provides a simple physical interpertation, where the problem is reduced to an analogous problem of a particle moving in a potential well. Different types of possible solutions are considered, with focus upon the main physical interpretations. The results recapture at least qualitatively some of the major experimental observations, and previous numerical simulations.

We present some results concerning the propagation of intense femtosecond Ti:sapphire laser pulses in matter and especially in air. We show that the supercontinuum created during the propagation preserves the spectral coherence of the initial pulse and both together form a chirped-white-light laser. Furthermore, we detected the photoemission of N-2 and N-2(+) to measure the intensity dependence of a focused laser pulse in air along the propagation distance and we observed a multiple refocusing. Finally, we underlined a clear charge separation in the plasma created a focused pulse in air giving rise to a short electric pulse.

Based on the possibility of multiphoton ionization of the inner valence electrons in polyatomic molecules, a model was proposed to predict the abundance of different fragments resulting from dissociative ionization of the polyatomic molecules interacting with short laser pulses. According to this model the fragmentation of these molecules occurs through multiphoton ionization of the inner valence electrons, which results in a molecular ion in an excited state. Through radiationless transition the population is transferred to the highly excited levels of the ground electronic state. The molecular ion created in this manner is not stable and dissociates rapidly. To test the applicability of the model, the fragmentation of a benzene molecule interacting with linearly and circularly polarized Ti:sapphire laser pulses was studied. The model was tested by comparing the measured laser-intensity-dependent abundance of C4H4+ fragment ions with the calculated ones and excellent agreement was obtained with the experimental results.

We present some results concerning the propagation of intense femtosecond Ti:sapphire laser pulses in matter and especially in air. We show that the supercontinuum created during the propagation preserves the spectral coherence of the initial prise and both together form a chirped-white light laser. Furthermore, we detected the photoemission of N-2 and N-2(+) to measure the intensity dependence of a focused laser pulse in air along the propagation distance and we observed a multiple refocusing. Finally, me underlined a clear charge separation in the plasma created a focused pulse in air giving rise to a short electric pulse.

Dissociative ionization of the D-2 molecule was studied using linearly polarized 400- and 800-nm laser pulses. At 800 nm, the D+ ions resulting from the dissociation of the molecule are observed to be parallel to the polarization of the laser field, At the shorter wavelength (400 nm), we observe that a considerable number of D+ ions are ejected perpendicular to the polarization of the laser field. This observation is interpreted in terms of the radiative coupling between the ground 1 s sigma(g), and the high-lying 3d sigma(g) and 3p pi(u) electronic states of D-2(+). This interpretation was verified by comparing the experimentally measured and theoretically predicted kinetic energy of the D+ ions. Our results indicate that the radiative coupling between the ground and highly excited electronic states must be considered when intense laser fields with high frequencies are used.

As a result of focusing ultrafast Ti:Sapphire laser pulses in a gas, a plasma column is created. Its density does not exceed some limiting value as a result of the defocusing by the electrons resulting from multiphoton ionization and the involvement of high order nonlinearities. The spectrum of the radiation from the excited species in the column has little contribution from the plasma continuum and the line broadening is less than that of the spectra radiated from a plasma generated by a long laser pulse. Thus, the spectroscopic source obtained using short laser pulses is an excellent source for spectroscopy.