Edward S. Chang

Energy levels of atomic aluminum with hyperfine structure

A new energy level table for Al i has been constructed to include hyperfine structure from observations within the last decade. Improvement in accuracy over older tables is about an order of magnitude. The analysis of high‐l Rydberg levels utilizing the polarization formula results in a new value for the ionization potential which is 0.110 cm−1 or five standard deviations above the old value.

Modified effective-range theory for electron scattering from molecules

An expression for the total cross section of electron-molecule scattering is derived in the modified effective-range theory. An excellent fit with the low-energy data in H2 of a recent time-of-flight experiment is found. In addition, the results are shown to be consistent with the diffusion cross section deduced from electron swarm data, pointing to a value of 1.27 a0 for the scattering length and a value of 5.4 a03 for the isotropic polarisability of H2. Finally a prediction is made for the total cross section in N2 at low energies.

The nf complexes in molecular nitrogen

The authors have investigated the vacuum ultraviolet absorption of N2 at 77K. The bands at 810, 815 and 825 AA are identified as the (0, 0) bands of the 7f, 6f and 5f complexes, each with O, Q and S branches. Theoretical analysis of the data confirms a theoretical value for the quadrupole moment of 2.1 e a02, and predicts a value of 19 a03 for the polarisability of the N2+ 2 Sigma g+ ( nu =0) core.

Solar emission lines revisited: extended study of magnesium

Edlens polarization formula is found to fit the high-l solar infrared emission lines attributed to Mg I to the accuracy of the high-precision observations. For several previously unidentified weak lines, assignments are made, a few to Mg. I. With some revision of laboratory energy levels, a more precise ionization limit for Mg I is determined to be 61 671.056 (10)cm-1. The fine structure of the two strongest lines at 811 and 818 cm-1 is found to consist of 4 components spaced about 0.001cm-1 apart with nearly equal intensities. Their behavior in the penumbra magnetic field is well described by the Paschen–Back effect. Their line profile, polarization, and directional properties are discussed.

Extended analysis of the 5g→4f emissions in H2

An analysis starting from Hund’s case d has been used to extend the work of Herzberg and Jungen on the 5g→4f emissions in H2. A simple analytical expression for the line intensities is presented that agrees with their calculations is about 1%. All of the experimentally observed lines have been accounted for by including higher vibrational levels in our calculations.

Non-penetrating Rydberg states of silicon from solar data

A general theory of non-penetrating Rydberg states in atoms is presented. It is applied to Si I and comparison is made of the calculated transitions with the remaining unidentified solar emission lines around 800 cm-1. Identification is completed for the groups 7i-6h, 7h-6g and 7g-6f.

Electric field strenghts from the solar 12 micron lines

The 12 μm and other similar lines in the ATMOS solar spectra are computer-fitted to obtain the emission and absorption components and their relative line center displacement. It is found that the emission peaks agree very well with laboratory wavenumbers and so the displacement are attributed to the absorption components. Interpreted as quadratic Stark shifts due to a quasi-static electric field, the displacements yield electric field strengths on the order of 100 V/cm, corresponding to an electron density of 10 12 cm -3

Properties of ions from spectroscopic data

From the spectroscopic data of non-penetrating Rydberg states of C I, N I, O I and Ne I the authors have extracted the polarisability and the quadrupole moments of the ionic core. The values so obtained from different Rydberg nl levels are mutually consistent to within 1%. Comparison with the values from ab initio calculation shows agreement to a few per cent. Further, some empirical relations between certain Slater integrals are shown to be the direct consequence of our treatment.

Accurate Determination of Electron Densities in Active and Quiescent Prominences: the Mid-Infrared Advantage

We have analyzed all lines in the MIR (8 to 20 micron) spectra of a quiescent and two time-frames of an active prominence. In the quiescent prominence, in addition to those lines found by Zirker (1985), we have identified a higher excitation hydrogen line and two helium recombination lines. Accounting for instrumental broadening, we can further separate out the Doppler and the Stark contributions to the line width. The former yields maximum temperatures of 6200 K, 34000 K and 12000 K and the latter electric field strengths of 7, 17, and 10 V cm-1 for the above prominences, respectively. We show that these electric fields when divided by 2.2 are equal to the normal electric field in Holtsmarks quasi-static Stark broadening theory. Hence, we obtain electron densities of N3=2.4(0.3), 9.1(1.2), and 5.5(0.6) in units of 1010 cm-3 respectively. Using the same assumptions as made by Zirker, namely, (1) the strongest line (7-6) is optically thin, (2) the population of the lower level (n=6) is determined by direct radiative recombination and photo-ionization, (3) the equality of proton and electron densities, and (4) the thickness of the prominence is at least 108 cm, we derive a new inequality, Ne ≤ 1.83 × 108 T0.75 e-2195/T. Substituting our maximum temperatures into the right-hand side, we find upper bound Ne values of 9, 43, and 30 in the same units as above. These upper bound values are comfortably higher than our measurement, unlike those of Zirkers derived from the same set of assumptions. We have also observed the helium recombination spectrum which has been postulated by Tandberg-Hanssen as one of three possible ways of equilibrating the triplet/singlet ratio. Surprisingly, it is present in the quiescent as well as in the active prominence. We show that no meaningful values can be found for the turbulent velocities by combining the helium with the hydrogen line widths.

Non-penetrating states of atomic oxygen

Atomic Rydberg transitions have been observed in the 1-5μm emission spectrum of an oxygen discharge. Proper analysis of these lines requires reinterpretation of previous 3d-nf measurements by explicit inclusion of the theoretical F-level fine structure in the experimental line profiles. The revised triplet-quintet differences in the nF levels are now seen to vary smoothly with n, analogous to the polarization energy in an Edlen plot. The new levels, 5g, 6g, 7g, and 7h also form a straight line according to the polarization formula, thereby confirming the ionization limit to a higher accuracy.