Optical Field Ionization Research Articles

Coherent ultrafast MI-FROG spectroscopy of optical field ionization in molecular H/sub 2/, N/sub 2/, and O/sub 2/

Quantitative phase-sensitive measurements of ultrafast optical-field ionization rates in molecular H/sub 2/, N/sub 2/, and O/sub 2/ are obtained using a temporally gated frequency-domain interferometric pulse measurement technique: multipulse interferometric frequency-resolved optical gating (MI-FROG). By measuring the pump-induced frequency change on a weak copropagating probe pulse, the optical field ionization dynamics can be completely time-resolved with sub-pulsewidth time resolution. A one-dimensional nonrelativistic electromagnetic fluid code model is used to compute the ionization dynamics and optical field propagation through the plasma. Using the Ammosov-Delone-Krainov (ADK) tunnel ionization rate model originally developed for atoms, the relatively simple model proposed here has been shown to compare favorably with the MI-FROG measured ionization rates in noble gases in the intermediate intensity regime (10/sup 14/ W/cm/sup 2/) (Siders et al, Phys. Rev. Lett.). We attempt to unify our studies in noble gases and molecules by performing experiments on N/sub 2/ and O/sub 2/, which have nearly identical ionization potentials to Ar and Xe, respectively. For the molecules studied here, we show that an ADK-like description of molecular ionization rates calculated from the model agree with the experimentally measured rates using the MI-FROG technique for H/sub 2/ and N/sub 2/. In the case of O/sub 2/, however, the experimentally measured ionization rate is approximately two orders of magnitude lower than that expected from the standard ADK formula. This is in agreement with the previously observed suppressed O/sub 2/ ionization rate in ion mass spectroscopy studies (Guo, 2000). We attribute the suppressed ionization rate in O/sub 2/ to a multielectron screening effect and show that a modified version of the ADK formula, taking into account the electron screening as proposed by Guo, well approximates the MI-FROG O/sub 2/ ionization rate data.

Inner-shell soft X-ray lasers in Ne-like ions driven by optical field ionization

A novel short-wavelength laser based upon monopole excitation of inner-shell electrons in Ne-like ions following optical field ionization with circularly polarized radiation is discussed. Calculations of the small-signal gain are presented for one such ion, Ne-like Ar, for which a small-signal gain of 13.7 cm −1 is predicted on the 2s→2p hole transition at 15.8 nm for an Ar pressure of 50 mbar. Extensions of this approach to other inner-shell transitions are suggested.

Soft-x-ray laser scheme in a plasma created by optical-field-induced ionization of nitrogen

An x-ray laser scheme based on the recombination of a fully stripped nitrogen plasma is presented. Plasma is assumed to be created by the optical-field ionization of a nitrogen gas jet of 10(19) cm-3 atomic density by an ultrashort (60 fs), high-intensity (3 x 10(19) W/cm2) Ti:sapphire laser. Results of two-dimensional particle-in-cell simulations, modeling laser-plasma interaction, parametric heating, and ponderomotive effects are presented. Hydrodynamic and kinetics calculations are performed and predict important local gain for H-like nitrogen transitions at 25 and 134 A, following fast collisional recombination for specific plasma conditions.

Single-shot chirped-pulse spectral interferometry used to measure the femtosecond ionization dynamics of air

A novel interferometry technique is presented by which, in one shot, one can measure phase changes with a resolution of tens of femtoseconds while extending the measurement over picoseconds or even longer. The method is based on spectral (frequency-domain) interferometry with a pair of linearly chirped pules as probes. With this technique we obtained single-shot measurements of the rapid phase changes induced by optical field ionization of air. This allowed us to calculate the time profile of the electron density created by an intense short laser pulse.

The effect of Coulomb explosion ion heating in optical-field ionised nitrogen X-ray laser schemes

This paper examines the effect of Coulomb ion heating in various nitrogen targets produced by optical-field ionisation from plane polarised high intensity laser radiation. Methods for controlling the explosion, such that plasma suitable for recombination X-ray lasers may be produced are devised. A numerical model of breakdown by high intensity pulses is used to calculate optical-field ionisation, above-threshold ionisation and inverse bremsstrahlung electron heating, and Coulomb ion heating mechanisms. A 1D Lagrangian hydrodynamics and atomic kinetics model is used as a postprocessor to simulate the recombination phase. Both optimised nitrogen and nitrogen doped with hydrogen mixtures are examined. Electron temperatures are found to be significantly lower in mixtures, yielding higher gains. Single and double (pre- and main) pulses are investigated with double pulses found to give good control of the ion temperature resulting from the Coulomb ion repulsion. An optimised nitrogen hydrogen mixture ionised by a double pulse produce a gain of 60 cm −1 at 247 Å with a low saturation intensity of ∼1×10 7 W cm −2. A two-dimensional pulse propagation model, which solves the paraxial wave equation and includes optical-field ionisation, refraction and diffraction effects, is used to study nitrogen hydrogen mixtures with double pulse configurations. A loosely focused pre-pulse of vacuum peak intensity 4×10 14 W cm −2 is applied to produce a wide and 4 mm long channel of singly ionised dissociated gas mixture. The plasma is further ionised to the He-like N state by a tighter main pulse of peak intensity 6×10 16 W cm −2, which is suitable for achieving the gain referred to above.

Demonstration of XUV amplification to the ground state in low-charged nitrogen ions

We report the first demonstration of the gain to the ground state in low-charged nitrogen ions by optical-field ionization. A small signal gain coefficient of 9.6 cm −1 and a gain–length product that it is in the range 3.8–3.9 for the NIII 3 s( 2 S)−2 p( 2 P) transition XUV laser at a wavelength of 45.2 nm were obtained by an only 25-mJ linearly-polarized 100-fs laser pulse.

Generation of Nonadiabatic Blueshift of High Harmonics in an Intense Femtosecond Laser Field

By applying intense 30-fs Ti:sapphire laser pulses to Ar gas well above the saturation intensity for optical-field ionization, a strong blueshift of high harmonics 2 times as large as the laser frequency was generated. A semiclassical calculation showed that the observed large blueshift resulted from a rapidly increasing electric field, existing much earlier in time than the peak of the laser pulse, i.e., a nonadiabatic effect.

Direct spectroscopic observation of multiple-charged-ion acceleration by an intense femtosecond-pulse laser

We have observed evidence of the emission of energetic He-and H-like ions of fluorine more than 1 MeV produced via the optical field ionization (OFI) from a solid target irradiated by an intense I=(2-4)x10(18) W/cm(2) (60 fs, lambda=800 nm), obliquely incident p-polarized pulse laser. The measured blue wing of He(alpha), He(beta), and Ly(alpha) lines of fluorine shows a feature of the Doppler-shifted spectrum due to the self-similar ion expansion dominated by superthermal electrons with the temperature T(h) approximately 100 keV. Using a collisional particle-in-cell simulation, which incorporates the nonlocal-thermodynamic-equilibrium ionization including OFI, we have obtained the plasma temperature, line shape, and maximal energy of accelerated ions, which agree well with those determined from the experimental spectra. The red wing of ion spectra gives the temperature of bulk plasma electrons.

Particle Modeling of Ionization and Three-Body Recombination in Fully Ionized Plasmas

In optical field ionized (OFI) plasmas, the electron energy distribution is highly deviated from equilibrium distribution; ionization and recombination in such plasmas should be treated using the particle model since the usual concept of a rate constant is not applicable in nonequilibrium. A particle model is proposed for the simulation of ionization and three-body recombination in fully ionized plasmas of Li3+. The nonequilibrium rate constant is defined and calculated using the cross section for reactive collision, which is derived from the equilibrium rate constant using the inverse Laplace transform. The use of the model, together with the Coulomb collision simulation, makes it possible to examine the relaxation of the electron energy distribution in nonequilibrium systems.

Terawatt Laser Chemistry.

Optical field ionization is potentially useful for molecular analysis. A molecule is efficiently ionized to theparent and multiply charged ions, and eventually atoms in highly charged states are produced. Ultrafast timeresolvedx-ray diffraction techniques are emerging as powerful ways to observe the dynamics and structures ofchemical as well as biological reactions. These two promising applications developed by a terawatt laser arereviewed in brief.

Highly charged carbon ions formed by femtosecond laser excitation ofC60:A step towards an x-ray laser

We study the interaction of ${\mathrm{C}}_{60}$ molecules with intense $(10--1000{\mathrm{T}\mathrm{W}/\mathrm{c}\mathrm{m}}^{2})$ femtosecond pulses of 790-nm wavelength. High charge states of carbon up to ${\mathrm{C}}^{4+}$ are produced in this intensity range, as determined by time-of-flight spectroscopy. These high charge states of carbon are produced at intensities more than an order of magnitude lower than would be required based on optical field ionization of isolated carbon atoms. From a line-shape analysis of the time-of-flight data, we derive the kinetic-energy distribution of the C ions. We find average energies of up to a few hundred eV, indicating that these ions are released through Coulomb explosion of the ${\mathrm{C}}_{60}$ molecules once the charge buildup due to ionization becomes sufficiently high. Ionic potentials of up to 200 V are derived from the kinetic-energy distributions. The ionic potential needed to retain electrons energetic enough to cause K-shell impact ionization is 392 V. This potential is not reached in case of ionization of ${\mathrm{C}}_{60}$ with 790-nm, 45-fs pulses since the disintegration of the cluster occurs on a time scale of a few femtoseconds, much shorter than the optical pulse. In this short time span, the heating of the cluster is not efficient enough for higher charge states to be ionized.

Ultrashort pulse laser ionization of ions in a plasma

Theoretical calculations are presented that show that optical field ionization of an ion in a plasma medium, induced by an intense ultrashort laser pulse, is sensitive to the plasma screening of the binding potential of the target ionic electrons by surrounding plasma electrons. Hence experimental measurements of ionization rate versus plasma electron density would provide a direct test of theoretical plasma models for hot, dense matter.

Optical field ionization effects on the generation of wakefields with short pulse lasers

In recent experiments involving high intensity short pulse lasers, large wakefields have been observed to occur even when the pulse is much longer than the length necessary to generate wakefields. It was observed in the experiments that the plasma wave starts near the position of the ionization front. The whole process of wake amplification from optical field ionization to Raman scattering is investigated by using a one-dimensional particle-in-cell simulation code which includes the effect of optical field ionization.

Chaotic behavior of a one-dimensional model atom in an intense field

In this paper we describe the rescattering process in optical field ionization through a one-dimensional model, which improves the well-known quasistatic model by adding the smoothed Coulomb potential in its second step. The above-threshold ionization spectra and high-order harmonic generation are calculated from this model. They are qualitatively in agreement with the quantum results and experiments. In particular, we find that this model is characterized by chaotic scattering. Modern nonlinear theory is used to analyze this dynamical system. It is found that singular self-similar fractal structure exists in the phase-dependence energy spectra, and the unstable manifolds constitute the chaotic scattering pattern. Our results also demonstrate a close connection of irregular trajectories with the energy spectra and high-order harmonic generation. We conclude that the chaotic behavior plays an important role in this one-dimensional model of optical field ionization.

Exact determination of spatially resolved ion concentrations in focused laser beams

The operation principle and the performance of a variable-volume selective TOF mass spectrometer are discussed in detail. Lateral confinement is achieved with an appropriate entrance aperture. Width and location of the axial confinement are established via the energy filtering capacity of a newly developed five-grid ion mirror. The minimum dimensions are in the μm range. The thus defined detection volume ΔV, can be scanned through a laser beam. Complete ionization of target gases such as SO 2 and Xe within ΔV, permits calibration of the overall detection sensitivity. This instrument can be utilized to measure spatially resolved ion concentrations, absolute ionization probabilities and the intensity dependence of ion yields which are not affected by volume expansion. As an example we investigate optical field ionization of water with a Ti:Sa laser at intensities up to 10 16 W/cm 2. Published by Elsevier Science B.V.

Eight- and nine-photon resonances in multiphoton ionization of xenon

We have measured and analyzed the clear emergence of ac-Stark-shifted multiphoton resonances with successive photon orders in xenon. The remarkable quality of our data illustrates the unambiguous evolution through parity-allowed resonances at the eight- and subsequent nine-photon levels. This marks a significant advance in showing that the transient resonance model is valid and strong optical-field ionization remains multiphoton in character at higher intensities. Furthermore, a simple Landau-Zener picture is sufficient to understand the basic principles.

Propagation effects in optical-field-induced gas mixture breakdown for recombination x-ray lasers

A two-dimensional pulse propagation model which solves the paraxial wave equation in cylindrical geometry, is used to study argon doped with hydrogen targets. The code includes optical-field ionization, refraction and diffraction effects, above-threshold ionization and inverse bremsstrahlung electron heating mechanisms. A range of target and pump laser parameters are examined and a 4 mm long, wide channel may be formed which is suitable for x-ray lasing. A one-dimensional Lagrangian hydrodynamics model is subsequently used as a postprocessor to simulate the recombination phase with both the plasma effects and atomic kinetics included. Calculated gain coefficients above at 232 Å in Na-like Ar are found throughout the channel but with a saturation intensity of the order of .

Chaotic Behavior in the Rescattering Process of Optical Field Ionization**The project supported by National Natural Science Foundation of China and by the Science Foundation of CAEP

In this paper we describe the rescattering process in optical field ionization through a new model which improves the well-known quasi-static model by adding Coulomb potential in its second procedure. We find that this new model shows chaotic scattering. Nonlinear theory is applied to analyze the system. We also find that there exist chaotic layers and self-similar fractal structure and those unstable manifolds which constitute the chaotic scattering pattern.

Optical-field induced gas mixture breakdown for recombination X-ray lasers

A spatially resolved analytic model is used to calculate the distribution of optical-field ionisation and heating including both above-threshold ionisation and inverse bremsstrahlung electron heating mechanisms. A 1D Lagrangian hydrodynamics model is subsequently used as a postprocessor to simulate the recombination phase with both plasma effects and atomic kinetics included. Calculation of gain coefficients for pure lasant and mixed species targets are presented, and important parameters examined. Both pure nitrogen (247 Å) and argon (232 Å) targets produce low gain, however hydrogen doping significantly enhances the gain particularly in argon up to values as high as 300 cm−1. Lithium (135 Å) and neon (98 Å) are shown to be ineffective even with doping, due to severe electron heating. Low saturation intensities (∼ 107 W cm−2) are found for argon and nitrogen limiting the output X-ray pulse energy available.

Second harmonic generation and its interaction with relativistic plasma waves driven by forward Raman instability in underdense plasmas

High conversion efficiency (0.1%) into second harmonic light generated in the interaction of a short-pulse intense laser with underdense plasma has been observed. In this experiment the plasma is created by optical field ionization of hydrogen or helium gas. Second harmonic spectra observed in the forward direction show Stokes and anti-Stokes satellites. This is due to the interaction of the second harmonic light with large-amplitude relativistic plasma waves. Second harmonic images taken at 30° from the propagation axis show that the radiation is generated over a length of a few times the Rayleigh length and that the origin of the second harmonic light is due to the radial electron density gradients created by the ionization process and the radial ponderomotive force.