Time-resolved x-ray spectroscopy of optical-field-ionized plasmas

The time-dependent soft X-ray emission of helium and nitrogen plasmas generated by optical-field ionization is reported. The experiments were carried out by focusing pulses of the high-power Ti:sapphire laser of the Lund Institute of Technology (lambda = 796 nm, pulse duration 150 fs, pulse energy 150 mJ) to a 50-mu m diameter spot close to a nozzle, using He and N-2 as target gases. The emission on He+, N4+, and N3+ resonance lines was recorded by means of a flat-field grating spectrometer coupled to an X-ray streak camera. A pronounced difference in the temporal shape of the emission of the Lyman-alpha line of hydrogen-like helium and of the 2p-3d resonance lines of lithium-like and beryllium-like nitrogen was observed. The helium line exhibited an initial spike followed by a slow revival of the emission, whereas the nitrogen lines showed a slow decay after a fast initial rise. These observations are explained with the help of simulations

Time-resolved Studies of Light-propagation In Paper

A method for time-resolved recording of light scattering in thin, highly scattering media is described. Subpicosecond pulses from a high-power Ti:sapphire laser are used, and single-shot recordings of the scattered light are made with a fast streak camera. The method is applied to the study of light scattering in paper, and a 1-ps resolution is demonstrated. The dependence of the light scattering on the basis of weight and density of the paper has been studied. A white-light continuum generated from the high-power pulses by the use of self phase modulation in water is used to study the wavelength dependence of the scattering process. A model for the propagation of light in paper has been developed and used in Monte Carlo simulations. The experimental results are used for testing this model, and absorption and scattering parameters are determined from that comparison

Two-color Time-resolved Spectroscopy of Helium Using High-order Harmonics

The radiative lifetime of the 1s2p(1)P state of helium is measured in a two-colour ionization experiment with a 5% accuracy. The state is excited by the 13th harmonic of a tunable 80 ps laser and ionized by a synchronous ultraviolet laser, with a variable time delay. This experiment demonstrates that the high harmonics generated in a jet of rare gas exposed to an intense laser field provide a tunable short-pulse xuv source ideally suited for pump/probe type of studies and, in particular, short lifetime measurements

Application des harmoniques générées dans un jet de gaz : mesure de sections efficaces d´ionisation des états excités de l´hélium

Les sections efficaces d'ionisation des états excités de l'hélium 1s2p 1P et 1s3p 1P ont été mesurées au voisinage du seuil d'ionisation en observant la saturation de l'ionisation. Les états excités de l'hélium sont préparés par l'absorption résonnante d'une harmonique d'ordre élevée produite par un laser picoseconde accordable. Ces états sont ensuite ionisés par un faisceau sonde en absorbant un photon. La modification de la fréquence du faisceau sonde, du proche infrarouge à l'ultraviolet, nous a permis de déterminer la dépendance de la section efficace d'ionisation en fonction de l'énergie de l'électron arraché. Les résultats expérimentaux confirment quantitativement les travaux théoriques effectués auparavant

Lifetime and predissociation yield of N-14(2) b (1)Pi(u)(v=1) revisited: Effects of rotation

The lifetime and predissociation yield o

Ionization and fragmentation of C-60 via multiphoton-multiplasmon excitation

We study the intensity dependence of ionization and fragmentation of buckminsterfullerene (C-60) in strong laser fields. Our data provide strong evidence that at intensities less than or similar to 10(14) W/cm(2) these processes occur predominantly via multiphoton excitation of the 20 eV plasmon resonance of C-60 At least two plasmons have to be created to initiate fragmentation or multiple ionization

High-order Harmonic-generation In Rare-gases With An Intense Short-pulse Laser

We present experimental studies of high-order harmonic generation in the rare gases performed with a short-pulse titanium sapphire laser operating at 794 nm in the 10(14)-10(15) W/cm2 range. The harmonic yields generated in neon and in argon are studied for all orders as a function of the laser intensity. They vary first rather steeply, in the cutoff region, then much more slowly in the plateau region, and, finally, they saturate when the medium gets ionized. The dependence of the high-order harmonic cutoff with the laser intensity in neon and argon is found to be lower than that predicted in single-atom theories. We observe high-order harmonics in argon and xenon (up to the 65th and 45th, respectively) at 10(15) W/cm2, which we attribute to harmonic generation from ions. We also show how the harmonic and fundamental spectra get blueshifted when the medium becomes ionized