Keith B. MacAdam

Theoretical He I emissivities in the case B approximation

We calculate the He I case B recombination cascade spectrum using improved radiative and collisional data. We present new emissivities over a range of electron temperatures and densities. The differences between our results and the current standard are large enough to have a significant effect not only on the interpretation of observed spectra of a wide variety of objects, but also on determinations of the primordial helium abundance.

He I Emission in the Orion Nebula and Implications for Primordial Helium Abundance

We apply a recently developed theoretical model of helium emission to observations of both the Orion Nebula and a sample of extragalactic H II regions. In the Orion analysis, we eliminate some weak and blended lines and compare theory and observation for our reduced line list. With our best theoretical model we find an average difference between theoretical and observed intensities Ipred/Iobs ? 1 = 6.5%. We argue that both the red and blue ends of the spectrum may have been inadequately corrected for reddening. For the 22 highest quality lines, with 3499 ? ? ? ? 6678 ?, our best model predicts observations to an average of 3.8%. We also perform an analysis of the reported observational errors and conclude that they have been underestimated. In the extragalactic analysis, we demonstrate the likelihood of a large systematic error in the reported data and discuss possible causes. This systematic error is at least as large as the errors associated with nearly all attempts to calculate the primordial helium abundance from such observations. Our Orion analysis suggests that the problem does not lie in the theoretical models. We demonstrate a correlation between equivalent width and apparent helium abundance of lines from extragalactic sources that is most likely due to underlying stellar absorption. Finally, we present fits to collisionless case B He I emissivities as well as the relative contributions due to collisional excitations out of the metastable 2s 3S term.

Uncertainties in theoretical He i emissivities: H ii regions, primordial abundance and cosmological recombination

A number of recent works in astronomy and cosmology have relied upon theoretical He I emissivities, but we know of no effort to quantify the uncertainties in the atomic data. We analyse and assign uncertainties to all relevant atomic data, perform Monte Carlo analyses, and report standard deviations in the line emissivities. We consider two sets of errors, which we call ‘optimistic’ and ‘pessimistic’. We also consider three different conditions, corresponding to prototypical Galactic and extragalactic H II regions and the epoch of cosmological recombination. In the extragalactic H II case, the errors we obtain are comparable to or larger than the errors in some recent Y p calculations, including those derived from cosmic microwave background observations. We demonstrate a systematic effect on primordial abundance calculations; this effect cannot be reduced by observing a large number of objects. In the cosmological recombination case, the errors are comparable to many of the effects considered in recent calculations.

Millimetre-wave spectroscopy of Au I Rydberg states: S, P and D terms

Energy levels of laser-excited Au I Rydberg states n = 25–38 in n2S1/2, n2P1/2,3/2 and n2D3/2 series have been measured by means of millimetre-wave spectroscopy in the frequency range 80–300 GHz. In this n region, Rydberg progressions were smooth and regular within the ±1 MHz measurement accuracy, and quantum-defect Rydberg–Ritz coefficients for these series have been obtained in a single-channel analysis. The doublet fine-structure splittings of P terms for n = 14–36, combining old and new data, have been fitted by an expansion formula. An improved value of the first ionization potential for Au I is VIP = 74 409.11 ± 0.03 cm−1.

Charge transfer from coherent elliptic states

We present experimental data on charge transfer from coherent elliptic states of Rydberg atoms, which are oriented relative to the impact velocity vector of monoenergetic, singly charged ions. The data cover a broad range of scaled velocities around unity, selected angles of approach over the whole sphere, all eccentricities from zero to one and principal quantum numbers ranging from n = 20 to 35. The cross sections show good qualitative agreement with classical trajectory Monte Carlo calculations, but clear deviations are also observed.

Laser-microwave spectroscopy of Cu I atoms in S, P, D, F and G Rydberg states

Energy-level positions of neutral copper atoms in S, P, D, F and G Rydberg series for principal quantum number n = 23–38 have been determined with sub-MHz accuracy by means of laser-microwave spectroscopy in the 73–321 GHz frequency range. Quantum defects of L = 0–4 levels and fine-structure splittings of P, D, F and G levels have been found from a global analysis of the measured frequencies of 79 single- and two-photon resonance transitions. Hyperfine multiplets were observable in several cases below n = 30 whose splitting frequencies are consistent with n−3 scaling from low-lying levels. An improved value of the Cu I ionization potential 62 317.46 ± 0.03 cm−1 has been obtained.

Inhomogeneous‐field stripper for field ionization detection and analysis of fast beams of highly excited atoms

A new device is described for detection, dispersion, and analysis of keV beams of highly excited atoms. Field‐ionization events are electrostatically tagged by inhomogeneous electric fields in a cylindrical geometry. State identification is based on the relationship between ionizing fields and atomic‐state quantum numbers. Calibration is accomplished by injecting fast beams of selectively excited Na atoms into the stripper. Application of the detector to measurement of charge transfer from Rydberg atoms is demonstrated.

Microwave spectroscopy of singlet Mg I in L = 0–4 Rydberg states

Microwave resonances among high-lying spin-singlet Rydberg levels in neutral Mg have been measured with approximately 1 MHz accuracy for principal quantum numbers n = 25–40 and orbital angular momentum L = 0–4. P, D and F levels were excited by multistep laser excitation in an atomic beam, and one- or two-quantum microwave transitions to nearby levels having Δn = 0, 1 or 2 at frequencies 79–234 GHz were detected by pulsed field ionization. Quantum defects and Ritz expansion coefficients were obtained for S, P, D, F and G levels from a global analysis in which it was unnecessary, within experimental accuracy, to make any allowance for channel perturbation from the Mg I 3p2 1D2 doubly excited configuration. The present results allow prediction and identification of Rydberg transition frequencies in Mg I, for instance in radio recombination lines from the interstellar medium, with accuracy greater by two or three orders of magnitude than previously possible.

Microwave spectroscopy of Al I atoms in ℓ = 0 to 4 Rydberg states: comprehensive quantum-defect analysis

In this work, measured frequencies of neutral aluminium n2P1/2,3/2 ? (n + 1)2P1/2,3/2, n2P1/2,3/2 ? (n + 1)2S1/2 and n2P1/2,3/2 ? n2D3/2,5/2 transitions in the range of principal quantum number n = 22?36 and frequency 81 to 281 GHz are presented, comprising 32 new two-photon resonances and 30 new single-photon resonances. The P-state fine-structure splittings have been determined. A comprehensive quantum-defect fit of these results, together with our earlier results in D, F and G Rydberg states of Al and accurate data of other researchers, has been carried out. This analysis provides a consistent set of Rydberg?Ritz quantum-defect constants for Al I in S, P, D, F and G states, including resolved P- and D-state fine structure (but no hyperfine structure) by means of which levels and transitions can be accurately calculated for any n ? 7.

Quantum defects of the sodium atom in f, g and h states

Quantum-defect coefficients have been found to describe the energy levels of neutral Na for based on fitting all recent data for . Separate coefficients are found for each level of the fine-structure doublets. The results may be used to calculate all unperturbed Rydberg levels of Na, with an accuracy no worse than 0.2 MHz for n > 20. The quantum-defect predictions and an expansion in inverse odd powers of the effective quantum number match experimental d- and f-state doublet splittings to an even better accuracy.