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Lines between 11.3 and 17.2 A of lithium-like, helium-like, and hydrogen-like fluorine have been observed in spectra of laser-produced plasmas. These lines include nine members of the Lyman series of F IX; eight members of the principal series of F VIII; and satellite lines arising from doubly excited configurations of F VII and F VIII. Similar satellite lines of the abundant solar elements have been identified in soft X-ray spectra of solar flares. A wavelength list of fluorine lines is given, and physical conditions in the plasma are discussed.

Measurements of cross sections are presented for excitation of Balmer alpha and Lyman alpha emissions in collisions of 1.98- to 24.6-keV hydrogen atoms with N2 and O2. Estimates of corrections required for cascade contributions and lack of excited state equilibrium are also given. At 1.87 keV, Lyman alpha and Balmer alpha cross sections reach their largest value, indicating maxima in these cross sections at or below 2 keV. Except for differences in absolute values for Lyman alpha cross sections, the results agree well with the work of others where comparison can be made.

A new approach to the technique of determining absolute intensity of radiation in the XUV by a rare-gas photoelectron counter is described. This method is applied satisfactorily in the region of photon energy above 40 eV where the method using rare-gas ionization chambers is hampered by technical difficulties. Two examples of application of this method, the determination of the spectral response of a monochromator by using synchrotron radiation in a region between 23 eV and 54 eV, and determination of intensities for the Lyman series of He II emitted by a stationary high density plasma, are presented.

Single shot photographic spectra of hydrogen‐like aluminum Lyman series lines are obtained from a low inductance vacuum spark. Corresponding pinhole pictures yield information on the source size. The observed line profiles are found to increase in width along the series and the higher series members Lγ and Lδ can be matched with convolved Stark and Gaussian profiles. The Gaussian profile includes the effects of source size, thermal Doppler broadening, and crystal resolution. The space and time‐averaged electron density determined from the profile analysis is 2.5×1021–1×1022 cm−3.

Optical oscillator strengths for the seven most intense dipole-allowed transitions in the 100-eV incident energy electron scattering spectrum of atomic oxygen have been measured from forward scattering spectra. An apparatus was used in which a microwave discharge source, producing atomic oxygen seeded in a helium beam, was coupled to an electron spectrometer. The O ¹D population in the target beam was less than 1.5% of O ³P, and transitions from the O ³P ground electronic state only were observed. The performance of the apparatus was checked by measurement of optical oscillator strengths for helium and atomic hydrogen. Agreement with the National Bureau of Standards (NBS) values for a single spectrum of better than 10% was obtained for three He transitions and 13% for the atomic hydrogen n = 3 transitions when the He 1¹S - 2¹P and H n = 1 - n = 2 Lyman series transitions were normalized to the NBS values. The atomic oxygen 2p4 ³P - 2p³3s ³S° (1304 A) transition was normalized to an average experimental value of f = 0.048. Optical oscillator strengths obtained for the other transitions were 2p4 ³P - 2p³4s ³S° (1040 A), 0.010±0.002; 2p4 ³P - 2p³3d ³D° (1026 A), 0.019±0.001; 2p4 ³P - 2p³3s′ ³D° (989 A), 0.061±0.006; 2p4 ³P - 2p³4d ³D° (972 A), 0.016±0.006; 2p4 ³P - 2p³3s″ ³P° (878 A), 0.086±0.006; 2p4 ³P -; 2s2p5 ³P° (792 A), 0.070±0.004. (All values have ±5% uncertainty in addition to the experimental scatter as indicated.) The electric dipole-forbidden transition 2p4 ³P - 2p³3p ³P was also observed as a weak process.

A theta pinch has been employed to investigate satellites to the H-like and He-like ion resonance lines of boron and carbon. Identification of the spectra shows an overlap of the Ar IX, Ar XI, and Ar XII lines with the predicted satellite wavelengths, and it is concluded that many of the previously reported satellite lines are due to argon impurities.

Optical determination of molecular hydrogen Lyman band oscillator strength from individual rotational lines

IT was reported in a previous communication l that, in the course of experiments by Dr. J. S. Badami and me on the spectrum of arsenic by the Paschen hollow cathode discharge in helium, the Lyman series of hydrogen was obtained with remarkable strength, the important feature of the series being the curious intensity anomaly exhibited by the members. Instead of the intensity gradually falling off, a sudden enhancement was observed in that of the 10th and the llth members (σ108,771 and 108,916). Experiments under varying conditions indicated definitely that this intensity anomaly occurs only in the presence of arsenic. The suggestion was made that the energy of excitation of this resonance spectrum is to be sought for in a transfer, by collisions of the second kind, from the excited arsenic atoms. Since sending this report, values of the characteristic energy levels of the neutral and ionised atom of arsenic 2 have become available, and a consideration of these has confirmed the above explanation.

The oscillator strength of the Lyman-α transition in the hydrogen atom has been determined by the line-absorption method. Hydrogen atoms were produced in a fast-flow system by a microwave discharge in a H2–He mixture. The hydrogen atoms absorbed the Lyman-α radiation at 1216 A emitted by a hydrogen-neon lamp, and absolute atom concentration was determined by titration with NO2. On the basis of the photometric calibration curve determined in this way, oscillator strengths were calculated as a function of assumed emission-line profiles. The best fit to the experimental data yielded an oscillator strength of 1.09±0.27.

TAKING for granted the dynamical legitimacy of the crossed-orbit model as originally proposed by Bohr (Zeits. fur Physik, ix., 1922, p. 1) for normal helium, I find for its negatived total energy, with the usual Coulomb law of force, and treating the two orbits as “circular” in the literal sense of the word, where F is the complete elliptic integral of the first kind, i the inclination of the planes of the two one-quantic orbits, in Bohr's case 120°, and the remaining letters are the usual symbols of the universal constants. In accordance with symmetry, the electrons are assumed simultaneously to pass the nodes, that is, the opposite ends of the common diameter. (Details of deduction of (1) are given in a paper to be published shortly.) If one of the electrons be removed to “infinity”, the energy of the remaining ionised atom He+ is 4Nch, where N = N: (1 + m/M). The small difference N–N being irrelevant for our purpose, the ionisation work thus becomes, by (1), W = Nch(3–7F/4), or, the equivalent wave-number (of the flash emitted at the return of the removed electron)

The laser power densities for the Lyman and Werner bands are calculated with a complete rotational analysis. For the Lyman band, P(3) and P(1) are favored and are stronger than Werner Q(1) and P(3) transitions even though the latter are favored in excitation. The transitions from Lyman levels to the lower ground state vibrational levels, where most of the strong Werner transitions terminate, reduce the net inversion density for the Werner band.

The processes allowing the escape of ionizing photons from galaxies into the intergalactic medium are poorly known. To understand how Lyman continuum (LyC) photons escape galaxies, we constrain the HI covering fractions and column densities using ultraviolet HI and metal absorption lines of 18 star-forming galaxies which have Lyman series observations. Nine of these galaxies are confirmed LyC emitters. We fit the stellar continuum, dust attenuation, metal, and HI properties to consistently determine the UV attenuation, as well as the column densities and covering factors of neutral hydrogen and metals. We use synthetic interstellar absorption lines to explore the systematics of our measurements. Then we apply our method to the observed UV spectra of low-redshift and z-2 galaxies. The observed HI lines are found to be saturated in all galaxies. An indirect approach using OI column densities and the observed O/H abundances yields HI column densities of 18.6 to 20 cm-2. These columns are too high to allow the escape of ionizing photons. We find that the known LyC leakers have HI covering fractions less than unity. Ionizing photons escape through optically thin holes/channels in a clumpy interstellar medium. Our simulations confirm that the HI covering fractions are accurately recovered. The SiII and HI covering fractions scale linearly, in agreement with observations from stacked Lyman break galaxy spectra at z-3. Thus, with an empirical correction, the SiII absorption lines can also be used to determine the HI coverage. Finally, we show that a consistent fitting of dust attenuation, continuum and absorption lines is required to properly infer the covering fraction of neutral gas and subsequently to infer the escape fraction of ionizing radiation. These measurements can estimate the LyC escape fraction, as we demonstrate in a companion paper., Accepted for publication in Astronomy & Astrophysics