Relative time parameters of optogalvanic responses

New optogalvanic response relative time features, such as signal growth and decay speeds, are suggested. These criteria can characterize signal time response parameters, as well as determine which particular type of gas internal quantum process is dominant during a given stage of response development. The general theoretical concepts reported here are compared with experimental results which are based on resonant optogalvanic effects in neon discharges and on non-resonant optogalvanic signals in neon and argon prebreakdown discharges. These new time characteristics are conceptually pleasing and permit study of the influence of individual factors such as bias and incident laser irradiation on overall gas response.     

Energy distribution of photoelectrons generated from highly charged ions in a circularly polarized intense laser field

We observed the energy distribution of the photoelectrons generated from the highly charged ions in the tunneling regime by using a circularly polarized Ti:Sapphire laser (745 nm, 100 fs). The peaks for each successive charge state up to Ar³⁺, Kr⁴⁺, and Xe⁵⁺ were clearly resolved, and the peaks due to higher charge states were flattened in the high-energy region and deviated from those predicted by the quasistatic model. This deviation is explained by pondero-motive acceleration in the strong field gradient. In Xe, Xe⁸⁺ was generated at a peak intensity of 2.0 × 10¹⁶ W/cm².     

Near-UV sub-Doppler spectroscopy on a metastable Mg beam by a frequency-doubled diode laser

Near-UV radiation is generated by doubling the frequency of a semiconductor laser in a nonlinear crystal. The crystal is contained in a resonant cavity in order to improve the conversion efficiency. The cavity increases the efficiency by about three orders of magnitude. This radiation has been used to perform spectroscopy of metastable magnesium in an atomic beam. We observed the (3s3p)³ P–(3s3d)³ D transition multiplet, which is of interest for metrological applications. The isotopic shift between²⁴Mg and²⁶Mg was measured and new information on the hyperfine structure of²⁵Mg was obtained. This radiation source is promising also in order to improve the Mg frequency standard.     

Ionization via an ultrashort high-power KrF laser

Received: 10 January 1997     

Determination of rf electric field strengths in deuterium from satellite structure of Balmer lines

In a linear magnetoplasma withn _e5×10¹⁸ m^–3 microwaves with frequencyf=34.8 GHz and powerP100 kW were focussed with a quasi-optical horn-lens system. The profiles of the deuterium lines D, D, and D were recorded from the focus. The profiles show a satellite structure which is generated by the rf electric field. Beside the first order satellites which can be resolved from the main line, even second order satellites were detectable. Comparing the satellites' intensities with theory it is possible to determine the electric field strength in the focus. Due to a sensitive dependence of the satellites' intensities upon the polarization direction one can also determine an unknown polarization direction.     

Nonlinear behaviour of atomic fluorescence in mercury vapours following double-resonance laser excitation

When mercury atoms, present with a buffer gas (N₂ or Ar) in a quartz cell or in a graphite furnace are excited into the 7³ S ₁ level by means of two pulsed dye lasers tuned at 253.652 nm (6¹ S _o 6³ P ₁) and 435.835 nm (6³ P ₁ 7³ S ₁), a laser-like, collimated emission is observed along the axis of the cell at the green mercury line (7³ S ₁ 6³ P ₂), 546.074 nm). This radiation exhibits a pronounced non-linear dependence upon the number density of the mercury atoms in the cell. This behaviour is interpreted as being due to amplified spontaneous emission (ASE), which occurs as a result of the transient population inversion between the 7s ³ S ₁ and 6p ³ P ₂ ^o levels. The parameters governing the population inversion can be deduced by applying a rate equation analysis to the atomic system. A time-resolved observation of both ASE and spontaneous fluorescence signals confirms the totally different characteristics of the two emission processes.     

The impact of several atomic and molecular laser spectroscopic techniques for chemical analysis

Several laser-based methods, namely laser induced fluorescence, laser enhanced ionisation and thermal lensing spectrophotometry are discussed with respect to their capabilities of approaching the extremely high detection sensitivity which is nowadays required in many fields of application, notably in high purity materials, in biomedicine and in the nuclear industry. The discussion is restricted to atomisers operated at atmospheric pressure, i.e., combustion flames, plasmas and graphite furnaces. It is shown that the analytical limit of detection can be in the range of femtograms and that double-resonance excitation possesses significant advantages over single-resonance excitation, both in terms of signal-to-noise ratio and spectral selectivity. In addition, the combination of the fluorescence and ionisation techniques represents a remarkable diagnostic tool. In the nuclear field, the suitability of the technique of thermal lensing for the direct determination and chemical speciation of very low levels of uranium in water is discussed.     

Two-photon excitation of atomic oxygen using a Raman-shifted ArF-excimer laser

A novel method for exciting the two-photon 2p ³ 3p ³ P 2p ^{4 3} P atomic oxygen transition is reported. The transition energy is provided by the combined photon energies of the second and third Stokes orders of a deuterium-filled Raman shifter pumped by an ArF-excimer laser. The ArF-excimer-pumped Raman shifter performance is characterized and the approach is demonstrated in an atomic oxygen population created by molecular oxygen absorption and predissociation in room air.     

Time-resolved spectroscopy of the fluorescence quenching of a donor — acceptor pair by halothane

Donor (anthracene) sensitized acceptor (perylene) fluorescence is quenched more efficiently by halothane than is intrinsic perylene fluorescence. The underlying process of dynamic fluorescence quenching is investigated by time-resolved fluorescence spectroscopy.     

Continuous wave dye laser pumped by a HeNe laser

Continuous wave operation of a dye laser pumped at 633 nm by a HeNe laser is reported. A jet of a cooled solution of Oxazine 1 in ethanol was used as the laser medium in an asymmetric spherical cavity. With mirrors of high reflectivity a threshold as low as 10 mW could be achieved.     

Observation of X-ray spectra from nitrogen clusters irradiated with high-intensity ultrashort laser pulses

X-ray spectra from nitrogen clusters irradiated with ultrashort laser pulses are reported. The line spectra of H-like and He-like nitrogen ions have been observed by irradiation with 100 fs, 800 nm pulses at 7×10¹⁷ W/cm² irradiance. The generation of highly charged ions of N⁶⁺ and N⁷⁺ is explained by the optical field-induced ionization and the subsequent collisional ionizations in the clusters. The He-δline has anomalously high brightness compared to other He-like lines. It is ascribed to charge exchange, which preferentially populates the n=5 level of N⁵⁺. Received: 7 October 1999 / Published online: 23 February 2000     

Lithium ionization by a strong laser field

We study ab initio computations of the interaction of lithium with a strong laser field. Numerical solutions of the time-dependent fully correlated three-particle Schrodinger equation restricted to the one-dimensional soft-core approximation are presented. Our results show a clear transition from nonsequential to sequential double ionization for increasing intensities. Nonsequential double ionization is found to be sensitive to the spin configuration of the ionized pair. This asymmetry, also found in experiments of photoionization of Li with synchrotron radiation, shows evidence of the influence of the exclusion principle on the underlying rescattering mechanism.     

High-resolution diode-laser spectroscopy on a fast beam of metastable atoms for detecting very rare krypton isotopes

A fast krypton ion beam with an energy of 10 keV is transferred through a mass filter and neutralized in a Rb- (or Cs-) vapour charge exchange cell. The emerging beam of metastable Kr atoms of a selected Kr isotope is collinearly irradiated with a tunable cw GaAlAs diode laser at 811 nm. The spectrum of the 1s ₅–2p ₉ transition covers 10 GHz and consists of one line each for the five stable isotopes of Kr with even mass number and 15 hyperfine structure lines of⁸³Kr. The individual lines are recorded by detecting the fluorescence signal perpendicular to the beams. Photons are guided to the PM tube by diffuse reflection from a high reflectance thermoplastics light collector. Up to 20 photons per atom are emitted by cycling between the two states during the flight time of 1.2 µs through the detector. The observed absorption linewidth of 100 MHz is a combination of laser linewidth and energy uncertainty in the fast atomic beam. In order to achieve maximum isotope selectivity the beam energy has to be adjusted in such a way that the Doppler shifted lines of all isotopes form an optimum pattern for detecting the very rare isotopes⁸¹Kr and⁸⁵Kr in natural environmental samples.     

Gas density measurement of pulsed gas jets using XeF four-photon fluorescence induced by a KrF laser

The gas densities of two pulsed gas jets were measured together with spatial and temporal distributions by the XeF fluorescence induced by a KrF laser. The B-X and C-A transitions of XeF showed a biquadratic dependence on laser intensity when SF₆ was used as the F donor instead of F₂, and quadratic and cubic dependences on gas density, respectively.     

Above-threshold ionization of xenon by intense laser field

Above-threshold ionization of xenon atom is treated by the KFR theory withconsideration of screening effects in the initial state.Comparison is made with experiment us-ing circularly polarized laser, screening effects on the above-threshold ionization is also dis-cussed.Better agreement with experiment is found than the previous results using hydrogenicwavefunction.     

Continuous wave photonic crystal laser in ultraviolet range

It is shown that photonic band-gap (PBG) structures have great potential for the development of widely tunable continues wave and/or mode-locked ultrashort-pulse all-solid-stale lasers in UV and optical ranges. The basic idea is to decrease the laser threshold via inhibition of radiative decay of an upper laser level by embedding an active medium into spatial structure having PBG at the frequency of laser transition. This technique provides favorable conditions for coherent suppression of the excited state absorption crucial for short-wavelength solid-state lasing. It also resolves the laser gain dilemma, providing combination of high emission cross-section and large population inversion. Different designs of 2D and 3D photonic crystal laser are proposed.     

Multiphoton ionization of magnesium in a Ti-sapphire laser field

In this paper we report the theoretical results obtained for partial ionization yields and the above-threshold ionization (ATI) spectra of magnesium in a Ti: sapphire laser field (804 nm) in the range of short pulse duration (20-120 fs). Ionization yield, with linearly polarized light for a 120 fs laser pulse, is obtained as a function of the peak intensity motivated by recent experimental data [9]. For this, we have solved the time-dependent Schrödinger equation nonperturbatively on a basis of discretized states obtained with two different methods; one with the two-electron wavefunction relaxed at the boundaries, giving a quadratic discretized basis and the other with the two-electron wavefunction expanded in terms of Mg ⁺ -orbitals plus one free electron allowing the handling of multiple continua (open channels). Results, obtained with the two methods, are compared and advantages and disadvantages of the open-channel method are discussed.Received: 17 June 2003, Published online: 26 August 2003PACS: 32.80.Rm Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)G. Buic-Zloh: Permanent address: Institute for Space Sciences, P.O. Box MG-23, 76900 Bucharest-Mgurele, Romania.     

Molecular above-threshold ionization with a circularly polarized laser field

Molecular above-threshold ionization by a circularly polarized laser field is investigated. Our theoretical approach is based on the modified molecular strong-field approximation. Various gauge-dependent versions of this approximation are considered and homonuclear and heteronuclear molecular species characterized by different symmetries are used as targets. As in the case of a linearly polarized field, the imprint of the molecular multicenter structure can be observed in the above-threshold ionization spectra. It manifests itself as minima in the rate of the ionized electrons as a function of their energy. The locations of these minima are strongly influenced by the symmetry of the corresponding highest occupied molecular orbital as well as the internuclear separation. Analyzing the interference structures of the electron spectra one can obtain information about the molecular symmetry.