Experimental Photoionization Cross-Section Measurements in the Ground and Metastable State Threshold Region of Se
Absolute photoionization cross-section measurements are reported for Se in the photon energy range 18.0–31.0 eV, which spans the ionization thresholds of the ground state and the low-lying and metastable states. The measurements were performed using the Advanced Light Source synchrotron radiation facility. Strong photoexcitation-autoionization resonances due to 4 transitions are seen in the cross-section spectrum and identified with a quantum-defect analysis.
pacs:32.80.Fb, 32.80.Zb, 95.30.Dr, 95.30.Ky, 97.10.Cv, 98.38.Ly
Short title: Photoionization of Se
J. Phys. B: At. Mol. Opt. Phys: August 3, 2019
Photoionization is an important process in determining the ionization balance and hence the abundances of elements in astrophysical nebulae. In the last few years, it has become possible to detect neutron(n)-capture elements (atomic number ) in a large number of ionized nebulae sterling07 , sterling08 . These elements are produced by slow or rapid n-capture nucleosynthesis (the “s-process” and “r-process,” respectively). Measuring the abundances of these elements can reveal their dominant production sites in the Universe, as well as details of stellar structure, mixing and nucleosynthesis smith90 , wally97 , busso99 , trav04 , herwig05 , sneden08 , karakas09 . These astrophysical observations provide an impetus to determine the photoionization and recombination properties of n-capture elements.
Various n-capture elements have been detected in the spectra of planetary nebulae pequignot94 , sharpee07 , sterling08 , sterling09 , the photoionized ejecta of evolved low- and intermediate-mass stars (1–8 solar masses). Planetary nebula progenitor stars may experience s-process nucleosynthesis busso99 , stran06 , karakas09 , karakas10 , in which case their nebulae will exhibit enhanced abundances of trans-iron elements. The level of s-process enrichment for individual elements is strongly sensitive to the physical conditions and mixing processes in the stellar interior busso99 , herwig05 , karakas09 .
The principal difficulty in studying s-process enrichments in planetary nebulae is the large uncertainties (factors of 2 to 3) of n-capture element abundances derived from the observational data. There are two root causes for these uncertainties. First, due in part to their low cosmic abundances, only one or two ions of a given n-capture element can be detected in individual planetary nebulae. To derive elemental abundances, corrections must be applied for the abundances of unobserved ionization stages. These corrections can be large and uncertain when the unobserved ions constitute a significant fraction of an element’s overall abundance. Second, while robust ionization corrections can be derived from numerical simulations of nebulae ferland98 , kallman01 , this method relies on the availability of accurate atomic data for processes that affect the ionization equilibrium of each element. In photoionized nebulae, these atomic data include photoionization cross sections and rate coefficients for radiative and dielectronic recombination and charge exchange reactions. These data are unknown for the overwhelming majority of n-capture element ions. Uncertainties in the photoionization and recombination data of n-capture element ions can result in elemental abundance uncertainties of a factor of two or more sterling07 .
The present work is part of a larger study to determine the photoionization and recombination properties of n-capture element ions sterling_prep , motivated by the astrophysical detection of these species and the importance of measuring their elemental abundances accurately to test theories of nucleosynthesis and stellar structure. The ultimate goal of this effort is to produce atomic data suitable for incorporation into codes that numerically simulate the thermal and ionization structure of nebulae, enabling significantly more accurate abundance determinations of trans-iron elements in astrophysical nebulae than is presently achievable. Determining these data over the range of energies and temperatures encountered in astrophysical environments necessitates a predominantly theoretical approach. However, experimental measurements are needed to constrain and establish the veracity of such calculations, particularly in the case of complex systems such as low-charge states of trans-iron elements.
Se was chosen as the first element of our investigation because it has been detected in nearly twice as many planetary nebulae as any other trans-iron element sterling08 . Experimental photoionization studies of other astrophysically observed n-capture elements have already been conducted by other groups for select Kr lu06a , lu06b  and Xe ions bizau06 , emmons05 .
This paper presents experimental determinations of the absolute Se photoionization cross section near the ground-state ionization threshold, and is the first in a series of papers on the photoionization of low-charge Se ions (up to 5 times ionized). In Section 2, the experimental procedure for our photoionization cross-section measurements is described in detail. The results and analysis of the data are presented in Section 3, and in Section 4 we summarize our work.
High-resolution measurements of the Se photoionization cross section have been carried out at the Advanced Light Source (ALS) synchrotron radiation facility at Lawrence Berkeley National Laboratory in California. This experiment used the merged beams technique lyonBa+:jpb:86  with the Ion Photon Beamline (IPB) apparatus located at undulator beamline 10.0.1 of the ALS. A detailed description of the IPB apparatus is available in Covington et al. covingtonNe+:pra:02 . The IPB endstation has been used for photoionization cross-section measurements of a variety of singly- and multiply-charged ions mullerC2+:jpb:02 , schippersB+:03 , fred04 , scully05 , scully06 , muller07 .
Se ions were produced by gently heating solid selenium inside a resistive oven within an electron-cyclotron-resonance (ECR) ion source. These ions were accelerated to an energy of 6 keV, and a 60 analyzing magnet selected Se from the accelerated ion beam. The Se ions were collimated with two sets of vertical and horizontal slits and focused by three electrostatic einzel lenses. The resulting collimated ion beam had a typical diameter of a few millimeters, and a current ranging from 20 to 200 nA. The ions were merged onto the axis of the counter-propagating photon beam by a pair of 90 spherical bending plates. In the merged beam path, an electrical potential of 1.4 kV was placed on the “interaction region” to energy-label photoions produced in a well-defined volume, for the purpose of absolute photoionization cross-section measurements. The interaction region consists of an isolated stainless-steel mesh cylinder with entrance and exit apertures defining an effective length of 29.4 cm. Two-dimensional intensity distributions of the photon and ion beams were measured by commercial rotating-wire beam profile monitors installed on either side of the interaction region, and by three translating-slit scanners located within the cylinder. Downstream from the interaction region, the Se product ions were separated from the parent Se ion beam with a 45 dipole demerger magnet. This directed the Se ions to a Faraday cup, while the photoions were steered by a spherical 90 electrostatic deflector onto a negatively biased stainless steel plate. The secondary electrons produced by the Se collisions on this plate were recorded by a single-particle channeltron detector. The detection efficiency has been determined on several occasions by measuring a femtoampere ion current at the stainless steel plate and comparing with the count rate generated from the channeltron. These measurements have consistently shown 100% efficiency for this detection scheme.
Photons were produced by a 10-cm period undulator located in the 1.9 GeV electron storage ring of the ALS. A grazing-incidence spherical grating monochromator delivered a highly collimated photon beam of spatial width less than 1 mm and divergence less than 0.05. The photon energies were selected and scanned by rotating the grating and translating the exit slit of the monochromator, while simultaneously adjusting the undulator gap to maximize the beam intensity. The spectral resolution of the photon beam was controlled with the entrance and exit slits of the monochromator. The photon flux was typically 310 photons/sec, as measured by a silicon x-ray photodiode (IRD, SXUV-100) that was referenced to two identical photodiodes absolutely calibrated by the National Institute of Standards and Technology (NIST) and by the National Synchrotron Light Source (NSLS). To calibrate the photon energy, the well known doubly-excited states of He domke96  were measured in first, second and third order. These measurements indicated that the uncertainty in the photon energies of the reported cross sections can be conservatively estimated to be less than 10 meV. The photon beam was mechanically chopped to separate photoions from the background produced by collisions between the parent ion beam and residual gas inside the interaction region.
3 Results and discussion
The photoion yield for Se was measured from 18 eV to 31 eV at a photon energy resolution of 28 meV (Figures 1 and 2). The actual resolution was determined by fitting Lorentzian profiles to isolated features across the scanned energy region, and taking the average value of the fitted resonance widths. Absolute photoionization cross sections were measured at discrete photon energies with the same resolution. The photoionization spectrum was multiplied by a polynomial function to normalize the spectroscopic data to all absolute cross-section measurements, which are indicated in Figure 1 by solid circles with associated uncertainties.111All uncertainties quoted in this paper are 90% confidence level estimates. Absolute photoionization measurements with the IPB apparatus at the ALS typically are uncertain by 20% at a 90% confidence level covingtonNe+:pra:02 , aguilarO+:apjs:03 . For a detailed discussion of uncertainty estimates for photoionization measurements with the IPB apparatus, see covingtonNe+:pra:02 , aguilarO+:apjs:03 . Note that these uncertainties do not account for contamination from higher-order radiation from the undulator at low photon energies. A previous experiment on Xe emmons_thesis  using the same beamline estimated the contamination of higher-order radiation to be 2% near 40 eV. At lower photon energies, the contamination of higher-order radiation is expected to be larger, but not by more than a factor of 2–3 compared to the contamination at 40 eV. The total experimental uncertainties of the absolute measurements are estimated to be 30%, which accounts for the possible contamination of the photon beam by higher-order radiation. We note that the lowest-energy absolute cross section values at 18.5 and 21.1 eV may not be as accurate as the others due to the effects of higher-order radiation.
Given the similarities in electronic structure of Se and O — both have two electrons in their outermost shell that give rise to the same ground state and and metastable states — their photoionization spectra are expected to be similar. Indeed, a comparison between the measured Se photoionization cross section and that of O reported by Aguilar et al. aguilarO+:apjs:03  shows that to be the case. For O, the region of the spectrum encompassing the and thresholds consists of very strong resonances due to 2 electron excitations of the metastable ions, as well as weak resonances due to 2 transitions. At our resolution of 28 meV, only the 4 transitions are observed in the spectrum of Se.
Identifications for the observed Se structure (see Figure 2) are made based on the O resonance identifications and the use of the quantum-defect form of the Rydberg formula,
where is the charge of the nucleus, is the number of core electrons, is the principal quantum number, is the series limit and is the dimensionless quantum defect parameter that indicates the departure of the energy level from the hydrogenic value. Two series from the metastable state and one from have been identified. The 44() series converging to the series limit is shown in Figure 2 by open blue triangles for resonances originating from the state, and by filled blue triangles for resonances from . The 44() series converging to the series limit originating from the state is depicted by half-filled pink triangles in Figure 2. In addition, two other series from the ( = 5/2, 3/2) metastable states are indicated by inverted open and filled red triangles above the spectrum. These resonances correspond to the 44() series converging to the limit. In the measured energy range, only one Rydberg series is observed from the ground state, whose first autoionizing member is 44(). The series is indicated with inverted half-open purple triangles just above the ground state threshold.
Tables 1, 2 and 3 list the principal quantum numbers, resonance energies and quantum defects of the identified members of each series. The uncertainty in the quantum defect values of the first few resonances in each series is conservatively estimated to be 10%. These uncertainties are a function of the energy uncertainty as well as the relative precision to which individual resonances can be identified and their associated centroids resolved. High -value resonances are typically much more difficult to clearly identify as they have significantly lower intensities and are often obscured by strong low- resonances in adjacent series. High- resonances can also become unresolvable as series converge toward their respective limits. Due to these complications, uncertainties have not been estimated for high- resonances.
It is important to note that ECR ion sources are known to produce ions in the ground and metastable states in fractions that may differ from statistically-weighted values. Therefore, the reported cross-section measurements correspond to an unknown admixture of metastable and ground state fractions. In the case of O aguilarO+:apjs:03  the metastable fractions were determined using the beam attenuation method ( 43%, 42% and 15%), clearly differing from the statistically-weighted values ( 20%, 50% and 30%) but not as much as those reported using translational energy spectroscopy ( 60%, 19% and 21%) Enos_fractions:jpb:92 . Similarly, we do not expect statistically-weighted metastable fractions in our Se measurements.
|Initial Se state:|
|Rydberg Series||Rydberg Series|
|Energy (eV)||Energy (eV)|
|19.853333NIST tabulations NIST ||-||21.70311footnotemark: 1||-|
|Initial Se state:|
|Initial Se state:||Initial Se state:|
|Rydberg Series||Rydberg Series|
|Energy (eV)||Energy (eV)|
|21.100555NIST tabulations NIST ||-||21.16711footnotemark: 1||-|
|Initial Se state:|
|21.682777NIST tabulations NIST ||-|
The absolute photoionization cross section of Se has been measured in the energy region of the ground state ionization threshold. The cross section exhibits a wealth of resonances that form a clear pattern of Rydberg series. The strongest resonances are identified as 4 transitions belonging to the 44(D) and 44(S) series originating from the and metastable states. The sole series from the ground state is identified as 44(). The resonance positions and quantum defects are determined for the initial members of each of these series.
The photoionization cross sections from the present study (and for other low-charge Se ions, to be presented in forthcoming papers) will be used to calibrate a broader theoretical effort to determine the photoionization and recombination properties of astrophysically observed n-capture elements sterling_prep . The resulting atomic data determinations will enable the abundances of trans-iron species in astrophysical nebulae to be derived to a much higher degree of accuracy than is currently possible, which bears implications for the nucleosynthetic sites and chemical evolution of Se and other trans-iron elements. Our absolute cross sections for Se can be accessed via secure FTP at the IP address 18.104.22.168 (note that a username and password are required; these can be obtained by contacting A Aguilar at firstname.lastname@example.org), or by contacting the authors N C Sterling (email@example.com) and A Aguilar.
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