Nonintrusive monitoring and quantitative analysis of strong laser-field-induced impulsive alignment

We report the observation of impulsive alignment of $\mathrm{C}{\mathrm{O}}_{2}$ molecules produced through their interaction with a nonresonant, strong laser pulse. The periodic alignment is monitored using a polarization technique generally employed in optical Kerr effect experiments; the birefringence produced by alignment of the molecular sample is measured with a weak pulse, time-delayed with respect to the alignment pulse. The technique provides a signal proportional to $⟨{\mathrm{cos}}^{2}\phantom{\rule{0.2em}{0ex}}\ensuremath{\theta}⟩\ensuremath{-}\frac{1}{3}$, where $\ensuremath{\theta}$ is the polar angle between the molecular axis and the strong-field polarization axis. Experimen…

Femtosecond time resolved coherent anti-Stokes Raman spectroscopy: Experiment and modelization of speed memory effects on H2-N2 mixtures in the collision regime

194317; With the aim of temperature diagnostic, femtosecond time-resolved CARS (coherent anti-Stokes Raman spectroscopy) is applied to probe H2 in H2-N2 mixtures. In a first part, a Lorentzian profile is used to model the femtosecond CARS response. A difference between the experimental broadening and the expected one is observed in the collision regime. The observed broadening increases strongly in an inhomogeneous way with respect to the perturber concentration. This is of considerable importance for temperature measurements. In a second part, we show that in the collision regime, this inhomogeneous broadening is due to the speed dependence of the collisional parameters and the memory effe…

Quantifying alignment with transient grating technique

Measurement of laser-induced alignment of molecules by cross defocusing

0146-9592; The field-free alignment of CO2 produced in response to the excitation of a molecule by a high-intensity femtosecond pump pulse is measured with a simple coronography-like technique. The technique is based on the defocusing of a time-delayed probe pulse produced by the spatial distribution of aligned molecules. In the intensity regime explored here, the technique is shown to give valuable information about dynamic alignment. With the help of simulations, the degree of alignment is extracted from the data. (C) 2005 Optical Society of America.

Wavelength dependence of multiphoton ionization of xenon

We have studied the multiphoton ionization of xenon atoms by $160\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$ pulses at intensities of $5\ifmmode\times\else\texttimes\fi{}{10}^{12}$ and $1.3\ifmmode\times\else\texttimes\fi{}{10}^{13}\phantom{\rule{0.3em}{0ex}}\mathrm{W}∕{\mathrm{cm}}^{2}$ and present photoelectron kinetic energy and angular distribution spectra measured with a photoelectron imaging spectrometer. A noncollinear optical parametric amplifier allows us to tune the wavelength of the laser pulse over a range between 500 and $700\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. Resonant and nonresonant processes as well as channel switching effects have been observed in this intensity and wavel…

Laser spatial profile effects in measurements of impulsive molecular alignment

0953-4075; We use a non-intrusive technique based on a polarization scheme to probe the periodic alignment of the CO2 and N2 molecules induced by a strong laser pulse linearly polarized. A weak probe field experiences the birefringence resulting from the alignment of the molecules. By comparing the probe depolarization signal with the numerical simulation of the time-dependent Schrödinger equation, it is possible to quantify the alignment of the molecular sample. The modelling takes into account the spatial profile of the pump intensity. It allows from the alignment signal to determine the onset of saturation due to ionization, despite the intrinsic saturation of the alignment arising for a…

Postpulse molecular alignment measured by a weak field polarization technique

We report a direct nonintrusive observation of alignment and planar delocalization of ${\mathrm{C}\mathrm{O}}_{2}$ after an intense linearly polarized femtosecond laser pulse excitation. The effects are measured by a polarization technique involving a perturbative probe that itself does not induce appreciable alignment. We show that this technique allows one to measure a signal proportional to $⟨{cos}^{2}\ensuremath{\theta}⟩\ensuremath{-}1/3$, with $\ensuremath{\theta}$ the angle between the molecular axis and the laser polarization. Simulations that support this analysis allow one to characterize the experimentally observed alignment and planar delocalization quantitatively.

Optical gratings induced by field-free alignment of molecules

We analyze the alignment of molecules generated by a pair of crossed ultra-short pump pulses of different polarizations by a technique based on the induced time-dependent gratings. Parallel polarizations yield an intensity grating, while perpendicular polarizations induce a polarization grating. We show that both configurations can be interpreted at moderate intensity as an alignment induced by a single polarized pump pulse. The advantage of the perpendicular polarizations is to give a signal of alignment that is free from the plasma contribution. Experiments on femtosecond transient gratings with aligned molecules were performed in CO2 at room temperature in a static cell and at 30 K in a …

Spectroscopie Raman Anti-Stokes Cohérente femtoseconde (DRASC – fs) : expériences et modélisation dans le cas du mélange H2 – N2 à basse pression

Dans l'objectif du diagnostic de la temperature dans les milieux en combustion, la Diffusion Raman Anti-Stokes Coherente resolue en temps (DRASC - fs) est utilisee pour sonder H 2 dans les melanges H 2 -N 2 a basse pression. Le dispositif DRASC mis en place est decrit en detail. Un nouveau modele de la reponse DRASC temporelle, prenant en compte tous les effets collisionnels specifiques a l'hydrogene (effets dits « de vitesse ») est presentee, ainsi que la comparaison avec l'experience, l'accord se revelant tres satisfaisant.

Suppression of plasma contribution in femtosecond degenerate four-wave mixing (fs-DFWM) at high intensity

Femtosecond degenerate four-wave mixing (fs-DFWM) experiments in CO2 exhibit a strong background due to plasma produced at high intensity (≥20 TW/cm2), when significant molecular alignment is likely to arise. This perturbing phenomenon renders the measurements of alignment very difficult. It is shown that the plasma contribution can be avoided by employing perpendicular polarizations for the two pump pulses. The effect is explained on the basis of the different diffraction angles between signals produced by molecular alignment and plasma. Copyright © 2007 John Wiley & Sons, Ltd.