Thomas J. McIlrath

Third-harmonic generation in argon, krypton, and xenon: Bandwidth limitations in the vicinity of Lyman-α

Using experimentally determined oscillator strengths and photoionization cross-sectional data, we compute the dispersion characteristics of Ar, Kr, and Xe up to their first ionization levels and determine the spectral regions in the VUV where these gases exhibit negative dispersion and so can be efficiently used for frequency tripling. We then investigate the bandwidths over which efficient tripling can be achieved in phase-matched gas mixtures. The bandwidth is limited by the rapidly varying dispersion in the vicinity of resonance transitions in the gases. In particular, we look at the case of frequency tripling 3647 A radiation to 1215.7 A (hydrogen Lyman-α) and show, that for fundamental wavelength bandwidths as narrow as 1 A, the rapid change in refractive index with wavelength can preclude phase matching over the entire bandwidth of the radiation.

Water vapor differential absorption lidar development and evaluation

A ground-based differential absorption lidar (DIAL) system is described which has been developed for vertical range-resolved measurements of water vapor. The laser transmitter consists of a ruby-pumped dye laser, which is operated on a water vapor absorption line at 724.372 nm. Part of the ruby laser output is transmitted simultaneously with the dye laser output to determine atmospheric scattering and attenuation characteristics. The dye and ruby laser backscattered light is collected by a 0.5-m diam telescope, optically separated in the receiver package, and independently detected using photomultiplier tubes. Measurements of vertical water vapor concentration profiles using the DIAL system at night are discussed, and comparisons are made between the water vapor DIAL measurements and data obtained from locally launched rawinsondes. Agreement between these measurements was found to be within the uncertainty of the rawinsonde data to an altitude of 3 km. Theoretical simulations of this measurement were found to give reasonably accurate predictions of the random error of the DIAL measurements. Confidence in these calculations will permit the design of aircraft and Shuttle DIAL systems and experiments using simulation results as the basis for defining lidar system performance requirements.

Nonlinear generation of Lyman‐alpha radiation

Tunable narrow‐band Lyman‐alpha radiation (∼1216 A) has been generated by frequency tripling in krypton. A near‐diffraction‐limited tunable 15‐nsec 10‐MW input at 3648 A was used to generate ∼3.6×1011 photons (60 W) per pulse at 1216 A. Measurements of the output as a function of gas pressure give the refractive index for krypton at 1216 A, demonstrating its negatively dispersive character.

Laser excitation and ionization of dense atomic vapors

It has been shown that a dense (>10(14) -cm(-3)) atomic vapor, irradiated by a saturating pulse of resonance radiation, will ionize on a time scale of <10(-6) sec. The ionization can be 95% complete and has been observed in Li, Na, Ca, Sr, and Ba. A large number of physical processes contribute to the ionization with different processes dominating at different stages in the ionization. These processes are discussed, and two models for calculating the ionization are described. The results of the various experiments and the applications to the spectroscopy of ions and to atomic physics studies in general are reviewed.

Laser techniques for extreme ultraviolet spectroscopy

In this paper we describe several techniques for using lasers to study core-excited energy levels in the spectral region between 10 eV and 100 eV. We are particularly interested in levels that are metastable against autoionization and, in some cases, against both autoionization and radiation.

Repetitively pulsed-plasma soft x-ray source.

A 10-Hz Nd:YAG laser system with 0.6-J, 25-nsec pulses was used to produce plasmas which emitted strongly in the soft x-ray region. Spectral, temporal, and spatial characteristics of these plasma emissions are presented together with an application of the source to soft x-ray lithography.

Fizeau wavemeter for pulsed laser wavelength measurement

A Fizeau wavelength meter optimized for use with pulsed laser sources has been developed and characterize which demonstrates a cw resolution better than 2 parts in 107 and a pulsed resolution better than 1 part in 106. The static optical design is based on a Fizeau wedge interferometer, which together with spatial filtering and collimating optics is used to produce a pattern of parallel fringes which is imaged on a linear photodiode array and analyzed by a minicomputer. We describe a series of cw and pulsed measurements of various narrowband laser sources and examine the particular difficulties involved in pulsed laser measurements with the wavemeter.

Fluorescence branching ratios from the A2Σ+(υ′ = 0) state of NO

Abstract The emission spectrum of NO from several rotational levels in the first excited state has been recorded. Branching ratios have been measured and Franck-Condon Factors calculated from the experimental data. Lifetimes of the lines used have been measured and Einstein A-values calculated.

Picosecond soft-x-ray pulse-length measurement by pump–probe absorption spectroscopy

We have demonstrated a system for subpicosecond, soft-x-ray-continuum, pump–probe absorption spectroscopy. By using a correlation measurement of visible and soft-x-ray photons we report the temporal profile of laser-generated x-ray pulses near 90 eV.

Resonant scattering of Lyman alpha radiation as a neutral density diagnostic for fusion plasmas

Abstract Resonance fluorescence of neutral hydrogen illuminated by Lyman alpha radiation (1216A) provides a technique for spatially and temporally resolved density measurements of neutral hydrogen isotopes in magnetically confined fusion plasmas or other similar environments. An analysis of the magnitude of expected fluorescence signals and integrated background radiation is developed and applied to typical fusion plasmas. Assuming that intense Lyman alpha sources, based on nonlinear laser frequency mixing techniques, are available, the feasibility of the method is demonstrated for neutral densities as low as a few times 10 7 cm -3 in DITE, PLT and TFTR. Suggestions are made for spectroscopic measurements on current and future Tokamak devices, to obtain additional data of interest for this application.