R. M. Schectman

New cascade analysis techniques for determining spontaneous atomic transition probabilities

Abstract Several new analysis techniques which account for the effects of cascades in the measurement of atomic transition probabilities have been developed at the University of Toledo, and will be described here. These techniques involve the incorporation of information from the direct measurement of the decay curves of cascading transitions into the analysis of the decay curve of the main level of interest. The traditional curve fitting techniques, as well as the new analysis techniques, are investigated by the use of computer simulated data containing various numbers of known exponentials. A diagrammatic mnemonic which trivially generates the theoretical decay curves for cascade schemes of arbitrary complexity will be described. The traditional curve fitting techniques are extended to include constraints imposed by the coefficients in the theoretical decay curve, which can be measured in terms of relative intensities of the cascading transitions. The population differential equation is converted to an integral equation involving only experimentally measurable quantities and the desired transition probability. Integrals over some arbitrary time interval of the decay curves can be photometrically measured, and given a common normalization through a wavelength relative efficiency calibration of the detection system. Integrated decay curves of all transitions, either directly into or out of the level of interest, can be summed in a manner which determines its transition probability. By varying the choice of time interval it can be verified that all contributing transitions have been correctly included. A variation of this technique allows the construction of the decay curve of an unmeasured cascade, provided the transition probability of the level into which it cascades is known. This variation can be used to investigate radiationless transitions and transitions outside the range of available detectors. Further, if there is additional a priori information concerning the shape of the unseen cascade decay curve, both its litetime and that level into which it cascades can be determined.

Accurate oscillator strengths for interstellar ultraviolet lines of Cl I

Analyses on the abundance of interstellar chlorine rely on accurate oscillator strengths for ultraviolet transitions. Beam-foil spectroscopy was used to obtain f-values for the astrophysically important lines of Cl I at 1088, 1097, and 1347 A. In addition, the line at 1363 A was studied. Our f-values for 1088, 1097 A represent the first laboratory measurements for these lines; the values are f(1088)=0.081 +/- 0.007 (1 sigma) and f(1097) = 0.0088 +/- 0.0013 (1 sigma). These results resolve the issue regarding the relative strengths for 1088, 1097 A in favor of those suggested by astronomical measurements. For the other lines, our results of f(1347) = 0.153 +/- 0.011 (1 sigma) and f(1363) = 0.055 +/- 0.004 (1 sigma) are the most precisely measured values available. The f-values are somewhat greater than previous experimental and theoretical determinations.

Selected Lifetime and Oscillator Strength Measurements in Si II

We have remeasured the lifetimes of the 3s24s and 3s25s2S1/2 levels in Si II using beam foil spectroscopic techniques. Measured values for the lifetimes and oscillator strengths derived from them are presented and compared with previous measurements and theoretical calculations. Agreement with recent theoretical calculations is now quite good: for 3s24s it is excellent and for 3s25s it is satisfactory, although the theoretical uncertainties in that calculation are still somewhat larger than desired.

Accurate oscillator strengths for ultraviolet lines of Ar I - Implications for interstellar material

Analysis of absorption from interstellar Ar I in lightly reddened lines of sight provides information on the warm and hot components of the interstellar medium near the sun. The details of the analysis are limited by the quality of the atomic data. Accurate oscillator strengths for the Ar I lines at 1048 and 1067 A and the astrophysical implications are presented. From lifetimes measured with beam-foil spectroscopy, an f-value for 1048 A of 0.257 +/- 0.013 is obtained. Through the use of a semiempirical formalism for treating singlet-triplet mixing, an oscillator strength of 0.064 +/- 0.003 is derived for 1067 A. Because of the accuracy of the results, the conclusions of York and colleagues from spectra taken with the Copernicus satellite are strengthened. In particular, for interstellar gas in the solar neighborhood, argon has a solar abundance, and the warm, neutral material is not pervasive.

Lifetime Measurements in Sn II

Lifetime measurements are reported for levels arising from the 5s25d and 5s24f configurations in Sn II. Measured decay curves were jointly analyzed using the Arbitrarily Normalized Decay Curve (ANDC) method to remove the effects of cascade repopulation from the determination of the lifetimes of the 5s25d 2D3/2 level. The branching ratio of the decay of this level to the ground term fine-structure levels 2P1/2 and 2P3/2 was carefully measured, and we have obtained an accurate value for the absorption oscillator strength of the resonance transition to this level at 1400.52 A. The results are discussed in the context of interpreting vacuum ultraviolet absorption spectra observed with the Goddard High Resolution Spectrograph on board the Hubble Space Telescope.

OSCILLATOR STRENGTHS OF SELECTED RESONANCE TRANSITIONS IN NEUTRAL SULFUR

As part of our continuing program of measuring f-values which are of astrophysical interest, meanlives and branching ratios for the 4s {sup 3}S{sup o} level was determined using beam foil techniques with the Toledo Heavy Ion Accelerator, THIA. The value 1.875{plus_minus}0.094 ns was obtained for the meanlife from multiexponential fitting to decay curves, a procedure which is reliable when all significant cascades are from levels with much longer meanlives. Oscillator strengths for the transitions 3p{sup 4} {sup 3}P{sub 0,1,2} - 4s {sup 3}S{sup o}{sub 1} were determined from this value together with the measured branching ratios. The preliminary result for the multiplet oscillator strength, 0.088{plus_minus}0.015, is slightly below other experimental measurements but is of improved accuracy. The mean of previous experimental measurements and theoretical calculations for this multiplet are in good agreement with our result. The 3p{sup 4} {sup 3}P{sub 2}- 4s {sup 3}S{sup o}{sub 1} transition of this multiplet at 1807.34{Angstrom}, is of particular interest to the study of interstellar matter since it involves a ground state transition. An investigation of the 3p{sup 4} {sup 3}P{sub 0,1,2} - 4s{double_prime} {sup 3}P{sup o}{sub 2,1,0} transitions in neutral sulfur is in progress and its results will also be reported.

Orientation and alignment of atoms by beam-foil excitation

Abstract We have measured the beam energy and foil tilt angle dependence of the orientation and alignment of the 3p 1 P state of 4 He I after beam-foil excitation. The results for carbon and aluminum foils are compared.

The Toledo heavy ion accelerator

Abstract The recently installed 330 kV electrostatic positive ion accelerator at the University of Toledo is described. Experiments have been performed using ions ranging from H + to Hg 2+ and exotic molecules such as HeH + . Most of these experiments involve the beam-foil studies of the lifetimes of excited atomic states and the apparatus used for these experiments is also described. Another beamline is available for ion-implantation. The Toledo heavy ion accelerator facility welcomes outside users.

Absolute transition probabilities for the 2p 5 3s–2p 5 3p transition array in Ne i

A continuation of a previous study of the 1s–2p transition array in Ne i is reported. Phenomenological intermediate-coupling wave functions that predict relative line strengths in good agreement with those measured here, as well as predicting energy levels and g values that agree with available data, are obtained and are compared with those generated by the ab initio calculations of Cowan. Absolute transition probabilities for the (1s2–2p3) and (1s4–2p3) transitions of 0.0040±0.0002 and 0.053±0.002 ns−1, respectively, corresponding to a mean life for the 2p3 level of 17.5±0.7 ns, were measured by pulsed electron excitation of low-pressure Ne gas with cascade-repopulation effects explicitly accounted for by direct measurement.

Oscillator strengths for ultraviolet transitions in Cl II and Cl III

Oscillator strengths for transitions in Cl II and III are derived from lifetimes and branching fractions measured with beam-foil techniques. The focus is on the multiplets at 1071 A in Cl II and 1011 A in Cl III whose lines are seen in spectra acquired with the Far Ultraviolet Spectroscopic Explorer. These data represent the first complete set of experimental f-values for the lines in the multiplets. Our results for Cl II λ1071 agree very well with the most recent theoretical effort and with Mortons newest recommendations. For Cl III, however, our f-values are significantly larger than those given by Morton; instead, they are more consistent with recent large-scale theoretical calculations. Extensive tests provide confirmation that LS coupling rules apply to the transitions for these multiplets.