High-Resolution Measurements of Photoionization of ...

High-Resolution Measurements of Photoionization of Ions Using Synchrotron Radiation A. Aguilar∗† , A.M. Covington∗ , E.D. Emmons∗ , M.F. Gharaibeh∗ , I. Álvarez∗∗ , C. Cisneros∗∗ , G. Hinojosa∗∗ , I. Dominguez‡ , G. Ackerman† , J.D. Bozek† , S. Canton† , B. Rude† , M.M. Sant’Anna† , A.S. Schlachter† , F. Folkmann§ and R.A. Phaneuf∗ ∗

†

Department of Physics, MS 220, University of Nevada, Reno, NV 89557-0058, USA Advanced Light Source, Lawrence Berkeley National Laboratory, MS 7-100, Berkeley, CA 94720, USA ∗∗ Centro de Ciencias Físicas, UNAM, Apartado Postal 6-96, Cuernavaca 62131, México ‡ Centro Nacional de Metrología, Querétaro 76241, México § Institute of Physics and Astronomy, University of Aarhus, DK-8000 Aarhus C, Denmark

Abstract. Measurement of absolute cross sections for photoionization of ions has become feasible by merging a wellcollimated ion beam with a monochromatic beam of synchrotron radiation. An electron cyclotron resonance (ECR) ion source permits such measurements to be extended to multiply charged ions, and makes possible systematic studies along isoelectronic sequences. The evolution of atomic spectra along such sequences is commonly studied theoretically, but the predictive ability of the theoretical methods remains largely untested. Absolute cross-section measurements are presented for the first three ionic members of the isoelectronic sequence of nitrogen (O+ , F2+ and Ne3+ ).

INTRODUCTION

structures observed and for the determination of the metastable fractions present in the F2+ and Ne3+ ion beams. Specifically, a comparison with recently published experimental cross sections for photoionization of an admixture of metastable and ground-state O+ [7] is presented for F2+ and Ne3+ .

For two decades, extensive theoretical photoionization cross-section calculations have been performed along isolectronic sequences in connection with the Opacity Project [1] and the Iron Project [2]. These cross sections are widely used for modelling many astrophysical objects, as well as for the study of laboratory plasmas and their application to the development of fusion energy. Until recently, little experimental data existed to benchmark these calculations. Such data are now becoming available from ion-photon merged beams experiments at undulator beamlines of synchrotron radiation sources, using a technique pioneered by Peart et al [3] and recently reviewed by West [4]. At the Advanced Light Source (ALS), an all-permanent-magnet electron cyclotron resonance (ECR) ion source has been implemented for use with an ion-photon beam (IPB) endstation. This combination facilitates absolute photoionization cross section measurements on multiply charged ions and therefore allows systematic studies along isoelectronic sequences [5]. Moreover, the use of such a hot plasma source permits photoionization studies of long-lived metastable ionic states [6]. In this paper, data on photoionization of metastable ions of the nitrogen isoelectronic sequence are presented. Further analysis will be necessary for the complete identification of the

ION-PHOTON-BEAM APPARATUS The Ion-Photon-Beam (IPB) endstation at beamline 10.0.1 of the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory is shown in Figure 1. The O+ beam was produced with a hot-filament dischargetype ion source whereas for the F2+ and Ne3+ experiments, an all-permanent-magnet electron cyclotron resonance (ECR) ion source similar to that described by Trassl et al [8] was used. In all cases, ground state and metastable ions were accelerated by a potential of 6 kV and selected by a 60-degree analyzing magnet. A pair of 90-degree spherical bending-plates merged the ion beam with the counter-propagating photon beam. A highly monochromatic photon beam was provided by a 10-cm period undulator from which the radiation was dispersed by a spherical-grating monochromator. The photoions produced inside an electrically biased interaction region were separated from the primary ion beam and from

CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan © 2003 American Institute of Physics 0-7354-0149-7/03/$20.00

534

" " "

! " "

#

$ " %

# !

FIGURE 1.

! ! % %

Ion-Photon-Beam (IPB) endstation at beamline 10.0.1 of the Advanced Light Source (ALS)

FIGURE 2. Two-dimensional intensity profiles of the photon beam (solid curves) and ion beam (dashed curves). Top and bottom panels show the profiles obtained with rotating-wire beam-profile monitors upstream and downstream of the interaction region. The central panel shows the profiles obtained at the center of the interaction region by the slit scanner.

FIGURE 3. Interpolation of measured form factors to characterize the spatial overlap of the photon and ion beams in the interaction region.

gas in the ultra-high vacuum system. Two-dimensional intensity distributions of both beams were measured by rotating-wire beam profile monitors installed just upstream and downstream of the interaction region, and by a translating-slit scanner at the center of the region. Typical spatial profiles of the beams are shown in Figure 2.

photoions created elsewhere on the merged path by the dipole demerging magnet shown in Figure 1. The primary ion beam was collected in a Faraday cup and continuously measured by a precision current meter. The labelled photoions were directed through a set of 90-degree spherical bending plates to a negatively biased stainless steel plate from which secondary electrons were accelerated to a microchannel-plate detector operated in pulsecounting mode. The photon beam intensity was measured by a calibrated silicon photodiode, and was mechanically chopped to separate the photoion signal from background signal produced by collisions with residual

Absolute measurements Absolute photoionization cross section values can be obtained in terms of experimentally accessible parameters through the following equation:

535

FIGURE 4. Experimental cross sections for photoionization of admixtures of the metastable states 2 P, 2 D and ground-state 4 S of O+ , F2+ and Ne3+ . The vertical dashed lines indicate the ionization thresholds for the (2 P, 2 D) metastable states and the (4 S) ground states. The open circles on each of the panels indicate absolute measurements of the effective cross section, to which the relative photoion-yield spectra have been normalized.

RESULTS

Rqe2 vi ε R σ= + γ I I Ωδ ∆ F(z)dz

(1)

Effective absolute cross sections for photoionization of F2+ and Ne3+ were measured from the threshold energy of the 2 P metastable state to the 4 S ground-state threshold energy. In the corresponding energy range, experimental cross-section measurements have previously been reported for photoionization of an admixture of 15% 2 P, 42% 2 D metastable states and 43% 4 S ground state of O+ [7]. Figure 4 compares the absolute measurements for these first three ionic members of the nitrogen isoelectronic sequence in their corresponding energy regions. The vertical dashed lines indicate the photoionization threshold energies of the 2 P, 2 D metastable states and of the 4 S ground states, as summarized in Table 1. The open circles on each of the panels indicate absolute measurements of the effective cross section, to which the relative photoion-yield spectra have been normalized. The direct photoionization (often referred to as the non-resonant photoionization background) of the three components (2s2 2p3 4 S,2 D,2 P) has been predicted to

where R is the photoion count rate, q is the charge state of the parent ion, e the electronic charge, vi is the ion beam velocity, ε is the responsivity of the photodiode, I + is the ion beam current, Iγ is the photodiode current, Ω is the photoion collection efficiency, δ is the pulse transmission fraction of the photoion detection electronics, ∆ is the measured absolute photoion detection efficiency, R and the beam overlap integral F(z)dz defines the spatial overlap of the photon and ion beams along the common interaction path. The two-dimensional form factor was calculated for the three measured overlaps (zi ) by:

F(zi ) = R R

RR + I (x, y)I γ (x, y)dxdy RR

I + (x, y)dxdy

I γ (x, y)dxdy

.

(2)

Figure 3 shows the calculated form factors from the beam-profile measurements and their interpolation

536

TABLE 1. Ionization energies (eV) for the metastable states and ground states of O+ , F2+ and Ne3+

O+ F2+ Ne3+

2P

2D

4S

30.103 56.316 89.376

31.797 58.482 92.004

35.121 62.708 97.117

7. Covington A.M., Aguilar A., Covington I.R., Gharaibeh M.F., Shirley C.A., Phaneuf R.A., Álvarez I., Cisneros C., Hinojosa G., Bozek J.D., Dominguez I., Sant’Anna M.M., Schlachter A.S., Berrah N., Nahar S.N., McLaughlin B.M., Phys. Rev. Lett. 87, 243002 (2001). 8. Trassl R., Thompson W.R., Brötz F., Pawlowski M., McCullough R.W. and Salzborn E., Physica Scripta T80, 504 (1999). 9. Zeippen C.J., Le Dourneuf M., Vo Ky Lan, , J. Phys. B: Atom. Molec. Phys. 13, 3763 (1980). 10. Aguilar A., Covington A.M., Hinojosa G., Phaneuf R.A., Álvarez I., Cisneros C., Bozek J.D., Dominguez I., Sant’Anna M.M., Schlachter A.S., Nahar S.N., McLaughlin B.M., submitted to Astrophys. J. (2002)

have the same cross-section magnitude [9]. The observed structure superimposed on the direct photoionization cross section in this energy range is attributed to Rydberg series of autoionizing resonances which converge to the excited terms of the ground configuration O2+ , F3+ , Ne4+ (2s2 2p2 1 De ,1 Se ). In the case of O+ , eight different Rydberg series were identified, all having in common the excitation of a 2p electron to a ns or nd shell [7]. In this case the metastable fractions present in the O+ beam were determined independently using a beam attenuation technique [10]. However, the metastable fractions for F2+ and Ne3+ have not yet been estimated. Further analysis is also needed in these two experiments to unambiguously assign the resonant structures in the cross sections. Such experimental studies along isoelectronic and isonuclear sequences permit the identification of systematic trends of oscillator strengths and energy positions, such as those in the present case for light open-shell systems (ns2 npx ).

ACKNOWLEDGMENTS This collaborative research was supported in part by the DOE Division of Chemical Sciences, Geosciences and Biosciences, by the DOE Facilities Initiative and by CONACyT. A. Aguilar acknowledges a fellowship granted by DGAPA-UNAM México and by a fellowship in residence at the ALS.

REFERENCES 1. Seaton M.J, J. Phys. B. 20, 6363-6378 (1987). 2. Nahar S.N. and Pradhan A.K., Phys. Rev. A 49,1816 (1994). 3. Peart D., Stevenson J.G., and Dolder K., J. Phys. B:At. Mol. Opt. Phys. 6, 146(1973). 4. West J.B., J. Phys. B:At. Mol. Opt. Phys. 34, R45R91(2001). 5. Aguilar A., West J.B., Phaneuf R.A., Brooks R.L., Folkmann F., Kjeldsen H., Bozek J.D., Schlachter A.S., Cisneros C., submitted to Phys. Rev. A (2002). 6. Müller A., Phaneuf R.A., Aguilar A., Gharaibeh M.F., Schlachter A.S., Álvarez I., Cisneros C., Hinojosa G., and McLaughlin B.M., J. Phys. B 35, L137-L143 (2002).

537