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In this paper we analyse the influence of laser power intensity fluctuations in multiphoton ionization processes. In particular, we have carried out the analysis for the specific case of four-photon ionization of benzene molecules, which we have recently investigated both experimentally and theoretically. We have calculated the total ion yield for the above process by taking into account the experimentally observed energy fluctuations of our pulsed laser system and compared it to the value obtained in the hypothesis of perfect pulse-to-pulse reproducibility. Finally, we also report the results obtained by using different Gaussian distributions for the laser energies, thus testing the sensitivity of the ion yield to the laser intensity spread in more general conditions.

We report on studies of the vibrational-translational relaxation time of CHClF 2 (freon-22) molecules, carried out by using an interferometric technique. The relaxation time has been analysed under experimental conditions of particular interest for the process of isotopic selective dissociation of freon-22, namely, at relatively high laser fluxes and gas pressures. We have also characterized the dependence of the relaxation time on the exciting CO 2 laser wavelength, and on the pressure of a buffer gas. Finally, the results are compared with our previous findings on freon-12 (CF 2 Cl 2 ) and SF 6 .

In this paper we describe the analysis of non-linearities in the vibrational relaxation rates of SF 6 molecules in a regime of strong laser excitation. Our studies were carried out using an interferometric technique which is particularly suitable to the investigation of the time dependence of VT relaxation times in molecular systems. At very high excitation energies a dramatic decrease in the VT relaxation time was found. This behaviour is related to the beginning of multiphoton dissociation processes of SF 6 molecules.

We report experimental studies of the frequency dependence of vibrational-translational (VT) relaxation times of pure SF 6 , and SF 6 with a buffer gas, strongly excited by a tunable TEA CO 2 laser. A notable decrease of the VT relaxation time has been observed at laser frequencies which are strongly red-shifted and broadened if compared to the linear IR absorption spectrum of SF 6 molecules. In particular, our results indicate that in specific experimental conditions the relaxation process is characterized by two different rates the first of which is related to the occurrence of partial dissociation of the highly excited molecules.

In this paper we briefly recall the Floquet theory and show how to apply it to the problem of laser-molecule interaction. In particular, we use the Floquet theory to obtain analytical expressions for the energy shifts in two- and three-level systems induced by the radiation source. This allows the evaluation of the influence of the electric strength on energy shifts in molecular systems.

An explanation of the cause of discharge constriction in an electrically excited XeCl* laser system is presented. Our analysis, carried out for a specific system, is more generally applicable to other rare-gas halide lasers and represents a contribution to the theory of discharge stability in high-pressure, strongly electron-attaching gas mixture. The conclusions obtained in various theories of the growth of instabilities in high-pressure laser discharges, thus far appeared in the literature, are briefly resumed and compared with our experimental and theoretical findings. Finally, prospects for obtaining a better discharge stability in rare-gas halide systems are discussed.

We report experimental studies, carried out by using an interferometric technique, of the frequency and flux dependence of vibrational–translational relaxation times of pure SF6, and SF6 with a buffer gas strongly excited by a tunable TEA CO2. laser A notable decrease in the relaxation time has been observed at laser frequencies that are red-shifted and broadened compared with the linear infrared spectrum of SF6 molecules. In particular, our results indicate that under specific experimental conditions the relaxation process is characterized by two different rates, the first of which is related to the occurrence of partial dissociation of the highly excited molecules. A simple, phenomenological model that assesses the influence of dissociation on the nonlinear relaxation time is also discussed.

We report experimental studies of the frequency dependence of the vibrational-translational relaxation time of polyatomic molecules strongly excited by resonant laser radiation. In particular, we have examined the behavior of SF 6 and CF 2 Cl 2 , two molecules of great practical and theoretical interest. Our analysis has highlighted a strong dependence of the relaxation time on the exciting laser wavelength. The laser ‘spectra’ of the relaxation times are very similar to the optical absorption spectra for both molecules, thus suggesting the possibility of using the study of the relaxation times to obtain useful spectroscopic information of polyatomic molecules.

In the present paper we describe the effect of the location of the ion source on the mass spectra obtained by multiphoton ionisation processes. Mass spectra measured with two different locations and sets of electrical parameters of the ion source are reported for the benzene molecule. Both sets of experimental results are compared with the mass spectra calculated using a Monte Carlo program specifically developed for our apparatus. The comparison clearly highlights the importance of the contribution of ionisation dissociation processes from impact with spurious secondary electrons which may arise with specific ion source geometries.

Taking advantage of a new non-linear technique, the authors have analysed the pulse-to-pulse fluctuations of the mode size and pulse length of the picosecond pulses delivered by a mode-locked neodymium-doped yttrium aluminium garnet laser. The technique can be exploited to process, in real time, laser pulses independently of their pulse length.

We present a very simple theoretical model aimed at the analysis of non-linear relaxation processes in molecular gases in the presence of partial molecular dissociation induced by vibrational–vibrational exchange between highly excited molecules. The model has a phenomenological character, since it analyzes the behavior of a system of anharmonic diatomic molecules, which is a very rough approximation of a polyatomic molecule such as SF6. Nonetheless, it provides an interesting key for the interpretation of a number of peculiar features characterizing our experimental observation, with which a comparison is made. In particular, the model takes realistic account of the influence of dissociation processes on the relaxation time.

We report interferometric studies of nonlinear relaxation processes in vibrationally highly excited CF 2 Cl 2 molecules, present in natural abundance ratio in the three isotropic varieties. The influence of laser frequency and flux, and of the presence of a buffer gas on the time characterizing the relaxation has been established. Our results provide a strong experimental confirmation of the influence that dissociation processes have on the relaxation dynamics. This influence was already clearly established in the case of SF 6 , with which a comparison is made.

In this work we report the first observation of intrinsic optical transistor action in a homeotropically aligned nematic liquid crystal film. The transistor action is produced by the sample intrinsic optical nonlinearities, so that no external feed-back mechanism (e.g. a Fabry-Perot resonator) is necessary. We also present a theoretical analysis of the problem in two different approximations: a linear, time-dependent approach, and a stationary non linear analysis. The theoretical results show to be in good agreement with our measurements.

We report for the first time the observation of ionized atomic hydrogen in the resonant four-photon ionization of benzene in the visible. A number of experimental tests carried out rules out the possibility that hydrogen ions are caused by electron impact process with photoelectrons produced by stray laser-light striking the vacuum chamber walls. Thus, our experimental results clearly seem to indicate that the ionized atomic hydrogen is the result of the multiphoton ionization, and consequent photodissociation, of the benzene molecules.

A new technique is successfully used to analyze in real time the pulse-to-pulse fluctuations of mode size and time duration in a picosecond Nd:YAG laser. In particular we show that the pulse length (30 psec) of our active–passive mode-locked Nd:YAG laser is stable to within 10% when the cavity is perfectly tuned and the saturable absorber is fresh. This technique is experimentally shown to be effective and reliable for real-time analysis of the stability of ultrashort laser pulses under a broad range of experimental conditions.

We report the experimental characterization of the charged species produced in excimer laser ablation of a superconducting intermetallic compound (YNi2B2C). By using energy-selective time-of-flight mass spectrometry, we have obtained direct measurements of both mass spectra and kinetic energy distributions of ions. The investigation has been carried out in the laser fluence range 1–5 J cm-2, which is typical of laser ablation thin film deposition. High kinetic energies of the charged component (up to 0.4 keV) have been observed even at moderate laser fluences.

The V–T relaxation from Freon‐22 molecules has been studied by using an interferometric technique. By raising the excitation energy from 500 cm−1 up to 3000 cm−1 (about three photons) a decrease of the effective relaxation time followed by a leveling off, has been observed. In agreement with this behavior, the average energy 〈ΔE〉 transferred per collision was found to depend linearly on 〈E〉 for extremal values of the energy absorbed, and stronger than linearly for intermediate energy values. This behavior parallels that observed in SF6 and other polyatomic molecules, and confirms the role played by the sparse and quasicontinuum regions of the vibrational spectrum.

Use of diffraction-free beams to generate low-order harmonies in xenon jets has been investigated. Nearly diffractionless beams have been generated from a Ti: sapphire, 100 fs, laser system by focussing ring-type radiation. which has been produced in two different ways: either, by inserting an annular slit into the incident Gaussian beam, or by placing a doublet of axicons in a vertex-to-vertex configuration before the focussing lens. The results obtained with the first scheme are discussed and interpreted in terms of a simple perturbative model of the harmonic generation (HG) process. The first measurements performed with the doublet of axicons are also reported and discussed. (C) 2002 Elsevier Science Ltd. All rights reserved.

A lidar system with the characteristic of simultaneously monitoring several atmospheric parameters such as atmospheric aerosols load, water vapor content, temperature and density, atmospheric transmissivity, and atmospheric pollutants concentration, has been realized. This system, based on two OPA lasers pumped by a Nd:YAG laser operating at maximum repetition rate of 100 Hz, is primary dedicated to tropospheric measurements. The system is continuously tunable over the spectral range 410 nm - 2500 nm, allowing DIAL measurements in a region where most of atmospheric pollutants display absorption lines. Preliminary measurements have been carried out. We report the simultaneous temporal evolution of the aerosol backscattering coefficient profiles at 355 nm and 829 nm in a rural area. A method to select optimal couples of wavelength for DIAL measurements in the near/mid IR spectral region has been developed. A sensitivity analysis has been carried out in order to find the minimum detectable concentration, that has been demonstrated of the order of few tenths up to few part per million in volume.

We report in this paper an experimental study of the spatial profile of harmonic beams (up to the 23rd order) generated in argon with 100 fs titanium-sapphire laser pulses at high repetition rate. In particular, we have investigated high-order harmonic radiation features as a function of the gas target position with respect to the pump laser focus. We have, thus, obtained the experimental conditions that simultaneously optimize generation efficiency and divergence angles of the harmonic beams. As a general trend, both the laser intensity and the harmonic order affect the harmonic beam divergence and spatial profile, depending on whether the harmonics lie in the plateau or in the cutoff region. Finally, we discuss our results in the light of recent theoretical analyses of the process. (C) 1998 Elsevier Science B.V.

In this paper we investigate the influence of magnetization changes on the amplitude of resonant magnetoelastic waves in amorphous ribbons of metallic glass. The magnetization changes were produced by external, polarizing magnetic fields both parallel and transverse to the longitudinal acoustic waves along the sample. According to the different experimental conditions, both enhancement and reduction of the magnetoelastic wave amplitude were observed. These effects are explained on the basis of theoretical and experimental results, and their possible applications are discussed.

The beam divergence of high-order harmonics generated in Helium by an ultra-short Ti:sapphire laser (7 fs and 30 fs) is experimentally characterized by means of a flat-field, high-resolution spectrometer. The harmonic beam divergence is also analysed as a function of the gas-jet position relative to the laser beam waist. Results, which are partly different from previous measurements performed at longer laser pulse duration, are discussed.