For investigating diatomic and triatomic molecules of astrophysical interest in the vacuum UV using the methods of laser spectroscopy we have set up several systems for generating coherent vacuum UV sources. Since we are interested in tunable and narrow banded (0.1 - 6. 01 cm ) sources it is advantageous to employ the methods of nonlinear optics.

Recently, experiments of the third harmonic generation in the Rb-Xe system. have been performed and a quantitative interpretation up to the highest conversion efficiencies of a few percent has been given. On the basis of these experiments we have investigated the optimum conditions for third harmonic generation in isotropic media. It has been possible to solve the third order problem analytically and it can be shown that the intensity conversion efficiency depends in the limit of large input intensities only on the wavelength dependent nonlinear susceptibilities of the nonlinear medium. For a Gaussian beam it can be shown that a Na-Xe system, for example, is capable of providing an energy conversion efficiency of up to 40% for the Nd:glass laser.

Resonance line CV and CVI plasma spectra in the 40 Å region indicate a reduced (Nn /wn) population density inversion of ∽4X between n=4 and n=3 excited levels. Inversion between n=5 and n=3 is also observed. The ions are created from a graphite target by a 5 J, 16 ns Nd glass laser pulse focused to a 0.5 mm spot, i.e., 2 x 1011 W/cm2 . These ions expand into a gaseous atmosphere (H, He, Ne, Ar, …) . The greatest degree of inversion is observed at ∽15 mm from the target surface at a pressure of 1-2 Torr in helium or hydrogen. At this distance both ions and neutrals are detected by space - and time-resolved spectroscopy, and an electron density of 5 x 1016 cm– is measured from Stark broadening.

The task of calculating line-shapes in ultra-dense fusion type plasmas is discussed. The application of line-broadening theory to derive a consistent formulation for ionized emitters including ion dynamics, Debye shifts and perturber-ion correlations is outlined. The difficulties in extending the standard theories are explored with special emphasis given to the modifications necessary to treat the cases which occur in the experimental situations. Preliminary results will be presented.

A technique is described in which a plasma is prepared using a tunable laser such that the population of only a single atomic bound state differs significantly from its equilibrium value. Subsequent observation of the relaxation of this single state in the absence of laser irradiation and with all other levels close to their equilibrium populations then yields unambiguous values for the collisional depopulation and re-population rates for the level, free from uncertainties present e.g. in laser-induced fluorescence experiments.

Results will be presented for the n = 2 state of hydrogen, but the technique is applicable to many first excited states in plasmas which need not be in either LTE or pure coronal equilibrium.

Measurements of the absolute cross section for photoionization from the 2p4 3P ground state of atomic oxygen will be described for wavelengths between the 4S° (91.2 nm) and 2D° (73.2 nm) limits. The cross section is determined from the absorption spectrum of shock heated mixtures of neon and oxygen at temperatures near 12000 K. The measured cross section will be compared to theoretical predictions and to earlier experimental work. Similar measurements of the photoionization cross section of H I which demonstrated the reliability of our windowless shock tube apparatus will be discussed briefly.

Since interstellar masers were first discovered in 1965, various attempts have been made to account for the variety of behaviour observed. In particular a number of theoretical models have been advanced to account for the pumping mechanisms but none have gained a wide acceptance.

Over forty molecules have now been identified in the interstellar medium, of these five exhibit maser action; they are detected by virtue of their small angular diameter and very large brightness temperatures > 1013°K. The molecules in question are H20, OH, SiO, CH3OH and CH, of these the Hydroxyl maser presents the widest range of behaviour and therefore if the physical mechanisms involved could be understood these masers could be used to probe the interstellar environment and obtain more information about this important region where new stars are formed

Observations of high density effects on spectral line shapes are reported for neutral, singly- and doubly-ionized emitters in a dense plasma with a small number of particles per Debye sphere. The source used in these experiments was a well-diagnosed linear pinch,run in argon, producing a plasma with electron density between 0.5 x 1018 and 3 x 1018 cm–3, and temperature between 22,000 K and 27,000 K.

Satellite-like features observed in the wings of H I Lyman α are attributed to the influence of unbound quasimolecular states. A number of the satellites are apparently due to states of H–Ar+ (similar to two such Lyman α satellites observed by Preston 1977). However, it seems likely that at least one of the satellite features observed should be attributed to the influence of Ar++ perturbers. Such quasimolecular features might be expected to become increasingly important at higher densities, and for more highly-ionized perturbing ions.

Since the last VUV conference in Hamburg, there has been speculation on the possible occurrence of collective effects in the 4d-f excitation channels of the lanthanide sequence. However, the data available so far has been restricted to solids, with the exceptions of Barium and Europium, due to the need for a high temperature absorption cell.

In the present paper we report the first observation of the 4d-f ‘Giant Resonance’ for the atomic vapours of Lanthanum, Samarium and Cerium using a 42 cm long induction furnace working up to 2700 K. The spectra were calibrated by superposing the emission lines from a Garton flash tube.

High spectral (0,05 Å) and spatial (⋍ 1000 km) resolution spectra of the Fe XII line 1349.4 Å reveal the existence of coronal fine structures in the quiet sun against the solar disk. These coronal bright elements have an average size of 2000-3000 km; their column density can be 3 x 1017 cm –2 . In the quiet sun, outward streaming velocities of 10-15 km sec –1 can be measured by means of the Doppler effect. The total kinetic and thermal energy of the outstreaming gas can be estimated to be larger than 1 x 10 5 ergs cm –2 sec –1, enough to account for the heating of the corona and the losses of the solar wind. At the outer limb (cos θ ⋍0.1) line profiles show a strong blue asymmetry, which could be caused by expanding material in a piston-driven shock, whereby the opaque, cool piston causes the asymmetry of the line profile.

Extreme ultraviolet observations of the chromospheric network obtained by the Harvard College Observatory experiment aboard Skylab on May 31st, 1973, are analyzed.

The data, which have a spatial resolution of 5“ x 5“, cover an area of 5‘ x 5‘ near the center of the sun, where a large coronal hole was present, and are recorded in several lines formed in a range of temperatures from 104 to 106 °K.

By studying the EUV spatial intensity distribution, the average intensity in cell and network, the contrast, and the emission measures have been derived.

The network distribution, highly asymmetrical and skewed to higher intensity, suggests an inhomogeneous network structure, probably associated with the presence of spicules, not resolved by the instrument.

A dynamical model of the transition zone is therefore developed, and the resulting EUV emission in spicules is computed and compared with available observations, and with previous network models.

High resolution solar spectra were recorded by the NRL normal incidence slit spectrograph on Skylab in the wavelength range from 1100 Å to 2000 Å. The spectral resolution is 0.06. Å and the spatial resolution at the Sun is 2“ x 60“. Several temperature and density-sensitive spectral lines occur in this wavelength region produced by ions formed in the transition zone at temperatures between ∽ 2 x 104 to ∽2 x 105 K. These lines are used to infer the electron density and plasma volumes in different solar regions, e.g., flares, surges, active regions, the quiet Sun and coronal holes. The densities in regions of activity are frequently found to be quite high (> 1011 cm–3), implying a filamentrary structure for the emitting plasma. In addition, from spectral line widths, regions of activity are frequently found to be highly turbulent. An attempt will be made to correlate non-thermal line widths with electron density enhancements.

Phase differences between intensity and doppler shift in each of the two bright emission lines of Sill at λ1816.93, λ1817.45 are determined for chromospheric oscillations with periods near 300 seconds and 95 seconds. Phase differences between the oscillations in the two lines are determined also.

For the 300 second oscillation, maximum intensity most often leads maximum blue shift by about 40 seconds and the oscillation in the fainter line at λ 1817.45 most often leads the oscillation in the stronger line at λ 1816.93 by about 35 seconds. For the 95 second oscillation, maximum intensity most often lags maximum blue shift by about 20 seconds and the oscillations in the fainter line most often lags the oscillations in the stronger line by about 12 seconds.

Intensities of resonance and subordinate lines in C III and O V are calculated for several non-equilibrium states expected to occur in the solar atmosphere. In particular, cooling (recombination) phases of solar flares, and regions of mass flow such as coronal holes and the network offer conditions where non-equilibrium phenomena can be significant. In such situations, the ionization state may not be in equilibrium with the electron temperature. Detailed comparison is made with the ultraviolet observations from the Harvard College Observatory experiment on Skylab which show temporal variations as well as clear non-equilibrium enhancement of the ratio of the 2s2p 3P - 2p23P transition 2 1 1 to the resonance line 2s21S - 2s2p 1P in both C III and O V.

A search for density sensitive line ratios was made in the 1400 Å to 300 Å region of the solar spectra. The emergent intensity of resonance and subordinate lines from ions of the beryllium and boron isoelectrcnic sequence were calculated for typical solar plasmas. For individual ions, line, ratios which are sensitive to electron densities changes between 107 to 1013cm –3were selected. These line ratios were then compared to observations made by the Harvard College Observatory spectrometer aboard SKYLAB. From all the potential candidates only few are adequately observed to estimate electron densities. In particular the Mg VIII lines at 430 and 436 Å are good indicators of the coronal densities. Comparisons of the densities of coronal holes, flares, and active regions are made.

The density sensitivity of lines in the Be I-like ion sequence has been discussed by several authors. The observed behaviour of the 977Å and 1175 Å lines of C III was studied using the Harvard instrument on Skylab, and limits were placed on the density variations across the quiet solar network and in active regions. Some limb data were also analyzed.

More recently, further observations of the intensities of the 1175 Å, 1909 Å and 1247 Å lines of C III at the solar limb have been published,. Similar observations have been made for the 1371 Å and 1218 Å lines of o v ,.

The present paper will discuss the interpretation of the above data in terms of Ne and other parameters in the solar atmosphere.

Observations of the solar equatorial electron scattered coronal radiance made during the Skylab mission show characteristics which dramatically differ with time. The results may be interpreted as indicating a general simplification of the coronal magnetic field, and, in comparison with harmonic analysis of observed solar surface magnetic fields, as indicating a rapid response of outer coronal structures to abrupt changes in the global solar surface field structure.

Spectra of plasmas produced by a CO2 laser have recently been obtained using a normal incidence slitless spectrograph and a high spectral resolution (0.028 mÅ) grazing incidence spectrograph. The slitless spectrograph forms images of the plasmas in spectral lines and is similar to the instrument flown by NRL on Skylab. The total wavelength coverage is from about 100 Å to about 600 Å. The shapes of the images depend markedly on the type of atomic transition. Time-averaged electron densities in the expanding plumes are calculated, and the expansion velocity is estimated from the profiles of lines recorded by the grazing incidence spectrograph. In addition, spectra of electron-ion beam plasmas between ∼200 Å and 2000 Å were obtained using a stigmatic normal incidence slit spectrograph. The distribution of plasma emisston between the anode and cathode, and the mass motions in the plasmas are discussed.

H and He-like Al Lines and their inner-shell satellites were obtained from Al and Polyethylene layered targets irradiated by a Nd-glass laser at a power flux of 2.1013 W/cm2 . Spatial resolution was achieved using a flat crystal spectrograph incorporating a wire shadow technique. Spatial profiles of Te and ne in the expanding plasma were deduced from line intensity ratios.

By varying the Al layer location in the target, the non-thermal origin of Al Kα radiation is investigated and the penetration depth of the heat wave front is determined. Comparison is made with classical transport mechanisms.

The L-shell spectra of heavy elements from germanium to molybdenum have been observed in plasmas generated by high power neodymium lasers at Queens University, Belfast and the Central Laser Facility (Rutherford Laboratory).

The spectra were recorded with convex crystal spectrometers with either a mica crystal (radius of curvature 3.81 cm) or a PET crystal (radius of curvature 15 cm). The spectral resolution is 1 in 1000. The spectra show lines from neon-like ions and associated satellites, fluorine-like ions, hydrogen and helium-like ion wavelength calibration lines and satellites from the latter. These will be discussed and some identifications given.

Emission spectra of laser-produced plasmas on targets of Cs, Ba and the lanthanides have been studied systematically in the region from 40 - 300Å. In the lighter of these elements the most characteristic feature is a strong emission maximum consisting of a mixture of continuum and discrete lines. The maximum is centered at 102 Å for cesium and moves to shorter wavelength with increasing Z. At praeseodymium a second maximum appears at longer wavelengths and with increasing Z both maxima broaden and eventually form a single continuum. The resonances are interpreted as excitation of 4d10 4fN9 to 4d 4fN+1 followed by re-emission modified by the presence of the exciting electron. The extensive continua which appear with the progressive filling of the f shell, are attributed to brehmstrahlung radiation.