CONTRIBUTED TALKS REVISTA MEXICANA DE FISICA S56 (2) 74–78 JUNIO 2010

Inner-shell photodetachment of boron negative ion

I. Dumitriua, R.C. Bilodeaua,b, C.W. Walterc, N.D. Gibsonc, T. Gorczycaa, O. Zatsarinnya, and N. BerrahaaDepartment of Physics, Western Michigan University,

Kalamazoo, 49008 MI.bLawrence Berkeley National Laboratory, Advanced Light Source Division,

94720 Berkeley, CA.cDepartment of Physics and Astronomy, Denison University,

Granville, 43023 OH.

Recibido el 14 de julio de 2008; aceptado el 23 de octubre de 2008

Measurements of the photodetachment cross section of B− (1s22s22p23P) have been performed with photon energies ranging from 187 to192 eV. The experimental method is based on collinearly merged beams technique and photoion spectroscopy. The absolute photodetachmentcross section exhibits three near-threshold shape resonances (due to the3S,3P, and3D final partial waves) which are accurately described bya shape resonance profile. K-shell photodetachment of B− has been also calculated using two R-matrix methods which show good overallqualitative agreement, but also unresolved quantitative discrepancies with the experimental data.

Keywords:Photodetachment; atomic ions; Auger effect.

Se han realizado medidas de la seccion transversal de la fotoseparacion del B− (1s22s22p23P) con energıas fotonicas en los lımites de 187a 192 eV. El metodo experimental esta basado en la tecnica de rayos combinados colinealmente y la espectroscopia de fotoion. La secciontransversal de fotoseparacion absoluta expone tres resonancias de forma cerca del umbral (debido al3S, 3P, y 3D ondas parciales finales)que es descrito con exactitud por un perfil de resonancia de forma. La fotoseparacion de K-capas de B− tambien ha sido calculada usandodos metodos de R-matrices que muestran un buen acuerdo cualitativo, pero tambien las discrepancias cuantitativas no resueltas con los datosexperimentales.

Descriptores:Fotoseparacion; iones atomicas; efecto Auger.

PACS: 32.80.Gc; 32.80.Hd

1. Introduction

Negative ions play an important role in various branches ofphysics, ranging from astrophysics, atmospheric and plasmaphysics to surface physics and accelerator physics [1]. It isknown that over 80% of naturally occurring elements formstable atomic negative ions [2]. For example, negative ionssuch as O− and O2− [2] influence radio communications intothe Earth’s ionosphere. The main source of opacity in the at-mosphere of the Sun at red and IR wavelengths is the absorp-tion by the H− ion [3].

Correlation effects are generally more enhanced in neg-ative ions than in atoms or positive ions [1]. This is a con-sequence of more efficient screening of the nucleus by theatomic electrons. The interplay of these attractive and repul-sive interactions in weakly bound system, such as negativeions, is of fundamental interest and allows us a better under-standing of correlated systems.

Negative ions differ fundamentally from neutral atomsor positive ions. In the atoms and positive ions the valenceelectron moves in the long-range Coulomb field of the nu-cleus. The resulting 1/r potential supports an infinite numberof bound states that converge to the ionization limit. In neg-ative ions, the outermost electron experiences a short-rangeinduced force that arises from the polarization of the atomiccore. The induced dipole potential is shallow and typicallysupports only a few bound states. The weakness of the bind-ing is reflected in the binding energies of negative ions, or

electron affinities of the parent atoms, which are usually ap-proximately an order of magnitude smaller that the ionizationenergies of atoms [2].

The stability of negative ions depends on the extent towhich the extra electron shares the attractive field of the nu-cleus with the other electrons. Essentially all information onthe structure and dynamics of negative ions is coming fromexperiments designed to ionize the ions in a controlled man-ner [2]. Negative ions are ionized in thephotodetachmentprocess when one or more electrons are ejected following theabsorption of a photon.

While many investigations of valence-shell electronsof negative ions have been performed using laser spec-troscopy [4], inner-shell photodetachment of negative ionsremains a largely unexplored territory. In order to investigateinner-shell excitation and detachment processes it is neces-sary to access the VUV or X-ray regions. These regionscan be accessed at synchrotron radiation facilities. Inner-shell photodetachment experiments were performed only inthe last seven years when high flux synchrotron radiation andsophisticated instruments became available.

2. The case of boron – process of interest

Ion implantation is one of the most important processingtechnologies in the integrated circuit industry. The produc-tion of a high negative ion beam current at very low energy

INNER-SHELL PHOTODETACHMENT OF BORON NEGATIVE ION 75

is one of the ion implantation challenges for shallow junctionformation. Implantation of small boron-containing clusters,such as Bn¯, SiBn¯, GeBn¯ and B10H14¯ is a potential solu-tion for this problem. Due to the higher cluster ion energy forthe lower partial energy of the boron atom, the space-chargelimit to the transport of the boron-containing cluster negativeion beams can be reduced. The boron-containing clusters canbe extracted from a Source of Negative Ions by Cesium Sput-tering and the ion beam current can be increased by 1 to 2orders of magnitude by simply adjusting the source parame-ters [5].

Boron, with a binding energy of 0.279723 (25) eV [6],forms the most weakly bound stable ion among the light ele-ments [7]. Three final partial waves result from K-shell pho-todetachment of B−(1s22s22p2 3P):

hν + B− = (1s22s2[2p2(3P )])

↓

{B∗(1s2s22p2[4P,2 P,2 D,2 S])εp}(3S,3 P,3 D)

↓

B+(1s22l2l′)εpεl (1)

Excitation of 1s electron for B− leads to quasidiscrete1s2s22p3 [3S, 3P, 3D] shape resonances. Also K-shell pho-todetachment of other light negative ions such as He− [8-10],Li−[11-14] and C−[15-17] showed triply excited resonances.

According to the process responsible for their decay, thenegative ion resonances are divided in two classes: Fesh-bach and shape resonances. Shape resonancesare situatedjust above the ground or an excited state of the parent atoms.In this case, the centrifugal force experienced by the detachedelectron (forl > 0) creates a barrier in the negative ion poten-tial. The primary decay mechanism of these states is tunnel-ing through the barrier into their atomic parent states makingtheir lifetimes fairly short, 10−13 – 10−14 s. As a conse-quence, the resonances are broad structures in the photode-tachment cross section [18-20].Feshbach resonancesare sit-uated below the excited state of the parent ion, and decay to alower lying state of the atom by simultaneously de-excitationof one electron and ejection of another. The lifetimes of thesestates are generally relatively long (10−11–10−12 s) result-ing in narrow structures in the photodetachment cross sec-tion [18-,20]. Feshbach resonances give rise to characteristicasymmetric structures in the photodetachment cross sectiondescribed by Fano profiles [21].

The photodetachment cross section for B− was obtainedby measuring the positive ion production in the photon en-ergy range from 187 to 195 eV. The data are presented inFig. 2 and are characterized by two structures, a first one atabout 189 eV and a second one at about 192 eV.

FIGURE 1. Ion-photon-beam endstation and schematic of beamline10.0.1.

FIGURE 2. Lower-resolution (390 meV) detection of B+ ions fol-lowing K-shell photodetachment of B−[see Eq. (1)] over a broadphoton energy range. The open red circles are the experimentaldata while the two solid red circles are the absolute cross sec-tion measurements used to calibrate the spectrum. The Theory I(nonorthogoanl basis, B-spline R-matrix results; long-dashed greencurve) and Theory II (orthogonal basis R-matrix results; short-dashed blue curve) convoluted (390 meV) results are also shown.The four vertical arrows show the positions of the four B∗ 1s2s22p2

(4P, 2D, 2P, 2S) thresholds from the Theory II calculations (bothTheory I and II results have been aligned such that the4P thresholdenergy is 188.63 eV) [30].

3. Experimental method

The experiment was performed at the Advanced Light Source(ALS) using the High-Resolution Atomic, Molecular andOptical Physics (HRAMO) undulator beamline 10.0.1 witha fixed ion beamline. The ion-photon-beam (IPB) endsta-tion [22] is based on the merged-beam technique and pho-toion spectroscopy using tunable synchrotron radiation.

An 8.5 keV negative ion beam was produced using a ce-sium sputter source (SNICS II from NEC) [23]. The mass-selected B− ions are merged collinearly with the counter-propagating photon beam as shown in the Fig. 1. An ion

Rev. Mex. Fıs. S56 (2) (2010) 74–78

76 I. DUMITRIU et al.

current of 100 nA was obtained in the interaction regionwhere inner-shell photodetachment and subsequent Augerdecay produce positive ions. The B+ ions (the signal) are de-flected by the de-merging magnet and detected with a chan-neltron as a function of photon energy.

In order to optimize the ion-photon beam overlap in theinteraction region we used two rotating wire beam profilemonitors and three translating-slit scanners. In order to mea-sure the background signal produced by the collision betweennegative ions and residual gas (∼3.5× 10−10 Torr) the pho-ton beam was modulated at a frequency of 6 Hz using acomputer-controlled shutter. The photodetachment signal isdetermined by subtracting the light-off signal from the light-on signal. The resulting signal is normalized to the incidentphoton flux and negative ion current. The incident photonflux was recorded by an absolutely calibrated silicon x-rayphotodiode and the ion current was monitored with a Fara-day cup placed after the de-merging magnet.

Many spectra of the photon energy range of interest wererecorded and summed to form the final spectrum. The pho-ton energy scale was calibrated against accurately known ab-sorption lines of Ar (4s line at 244.39 eV [24]) and SF6 (twolines at 183.4 and 184.57 eV [25]) using data from a previ-ous measurement. The total uncertainty for photon energycalibration, including Doppler correction was estimated to be50 meV.

4. Results and discussion

The experimentally obtained photodetachment cross sectionof B−(1s22s22p23P) as a function of photon energy from187to 195 eV, scaled to absolute cross section measurements, isshown in Fig. 2. The absolute cross section at 192.25 eV is3.04(70) Mb while at 194.26 eV is 1.88(43) Mb.

The K-shell photodetachment thresholds, 1s2s22p2 (4P)and 1s22s22p2(4P), turn on at about 188.6 and 191 eV, re-spectively. Above the first threshold at 188.6 eV the crosssection increases dramatically, due to the presence of shaperesonances. The cross section above threshold can be ap-proximated by the Wigner law in a limited region [26]. How-ever, the presence of a shape resonance near threshold isresponsible for significant deviation from the Wigner-lawbehavior, an effect also seen a number of valence studies,such as in Ca− [27], He− [28], and Os− [29], and alsoinner-shell studies in C− [15]. Following the selectionsrules, the excitation of 1s electron in B− leads to three fi-nal channels: 1s2s22p2(2S)εp(3P), 1s2s22p2(2D)εp(3P), and1s2s22p2(2D)εp(3D). So, we concluded that the first maxi-mum in Fig.2 at about 189 eV is due to the three 1s2s22p33S,3P, and3D shape resonances. The positions of the resonancesrelative to the threshold and their widths are varying due tothe geometrically different scattering potential seen by thedeparting electron. The experimental data have been fit us-ing the sum of three modified shape resonance profiles, first

suggested by Petersonet al. [30]:

σfit(E) =3∑

i=1

Ai

[(E − Ethr)

(Eresi − Ethr)

]l+ 12

× Γi/2π

(E − Eresi )2 + (Γi/2)2

(2)

TABLE I. B¯ 1s2s22p3 shape resonance parameters. Experimentalline positions include photon energy calibration uncertainty [30].

ThI ThII Exp. fit

Ethr eV 188.63a 188.63a 188.6(+0.1−0.2

)(3D

)Eres (eV) 188.73 188.73b 188.72±0.05

Γ (eV) 0.056 0.071 0.037±0.020

A (Mb-eV) 1.80 1.32 1.14±0.28(3S

)Eres (eV) 189.02 189.03b 189.03±0.05

Γ (eV) 0.178 0.165 0.071±0.022

A (Mb-eV) 4.00 3.10 1.92±0.48(3P

)Eres (eV) 189.22 189.22b 189.17±0.13

Γ (eV) 0.528 0.536 0.26(+0.26−0.10

)

A (Mb-eV) 1.82 1.60 1.92±0.52

aBoth theoretical results have been globally shifted in photon energy to give

this same threshold value.bThese two ThII resonances were artificially shifted by +0.1 eV relative to

threshold.

FIGURE 3. A partitioning, within the Theory II calculations, of thephotodetachment cross section in Eq. (1) into4P (solid red),2P(dashed black),2D (green crosses), and2S (dashed-dotted cyan)channel contributions. The2D, 2P, and2S channels have been mag-nified by a factor of 10 for clarity whereas the dominant4P channelhas been further broken into the incoherent contributions from eachof the three final3D, 3P, and3S symmetries, indicating a separateshape resonance within each partial wave[30].

Rev. Mex. Fıs. S56 (2) (2010) 74–78

INNER-SHELL PHOTODETACHMENT OF BORON NEGATIVE ION 77

FIGURE 4. Experimental cross sections for K-shell photodetach-ment of B− leading to B+ over the photon energy range of the firststructure shown in fig. 2. Here, the 1s2p22p3 (3D, 3S, 3P) shaperesonances are fitted using Eq. (2) [30].

Where Ethr is the 1s2s22p2 (4P) threshold energy, the or-bital angular momentum take the valuel=1 for the presentp- wave case, Eres

i are the energies of each of the three res-onances,Γi are the three corresponding natural (Lorentzian)widths, and Ai are the three separate amplitude factors.

The high resolution (63 meV) data in the near-thresholdregion scaled to absolute cross section measurements areshown in Fig. 4, where the red solid curve is the best fit curveconvoluted with a 63 meV Gaussian to simulate the spectralbandwidth and the green dashed lines represented the threedecomposed partial waves with spectral bandwidths effectsremoved. The best fit parameters along with estimated totaluncertainties are listed in Table I [31] with the theoretical val-ues. The resonance profiles are in a good agreement with themeasured cross section, except the widths and the total reso-nance strengths, which are generally overestimated by theory.

The structure observed in Fig. 2 at about 192 eV appeareddue to the threshold effects, the opening of the2P photode-tachment channel together with the weaker coupled2D and2S channels and it is composed of featureless continuum [31].

Two R-matrix methods were used to calculate the K-shell photodetachment cross section for boron negative ion.The first theory (Theory I) improves the target descrip-tion by using configuration-interaction expansions involvingnonorthogonal sets of term-dependent one-electron orbitalsand allows for relaxation effects in the 1s-vacancy states.The second theoretical method (Theory II) is based on thestandard, orthogonal-orbital R-matrix method [32] as imple-mented in the RmaX suite of codes [33] and is restricted tothe use of a single set of orbitals for describing each B−, B*,and/or B state. Thus a lot of extra configuration interaction(CI) is needed just to account for the strong relaxation ef-fects [31]. Both R-matrix results convoluted with a 390 meVGaussian to simulate the photon bandwitdth are also shownin Fig. 3.

In order to align photon energies between the experimen-tally and theoretically determined thresholds a global shiftin total photon energies by -0.13 eV for the Theory I and-0.36 eV for Theory II has been applied.

5. Conclusions

K-shell photodetachment of B−has been explored experi-mentally and theoretically. The absolute cross sections weremeasured and then compared with two R-matrix calculations.The three near-threshold shape resonances are accurately de-scribed by a shape resonance profile. The results for Theory Iwhich used a sufficiently large, separate basis of orbitals foreach state (initial B−, intermediate B*−and residual B or B*)are in very good agreement with experimentally data. TheTheory II which used the same orthogonal basis to describeeach state is giving a less accurate description of the K-shellphotodetachment of B−. Additional 4l and 5lorbitals need tobe considerate in order to make Theory II calculations moreaccurate, but this requires more demanding computation.

Acknowledgement

This research was supported by DOE, Office of Science,BES, Chemical, Geoscience and Biological Divisions.

1. J.C. Rienstra-Kiracofe, G.S. Tschumper, H.F. Schaefer, S.Nandi, and G.B. Ellison,Chem. Rev.102 (2002) 231; T. An-dersen, H.K. Haugen, H. Hotop,J. Phys. Chem. Ref. Data28(1999) 1511; T. Andersen,Phys. Reports394(2004) 157.

2. D.J. Pegg,Rep. Prog. Phys.67 (2004) 857.

3. R. Wildt, Astrophys. J.89 (1939) 295.

4. H. Kjeldsen, P. Andersen, F. Folkmann, B. Kristensen, and A.Andersen,J. Phys. B34L353.

(2001); A.M. Covingtonet al., J. Phys. B34L735.

(2001); J.R. Harrieset al., Phys. Rev. Lett.90 (2002) 133002;N.D. Gibsonet al., Phys. Rev. A67 (2003) 030703.

5. X. Wang,et al., Nucl. Instr. and Meth.251(2004) 278.

6. M. Scheer, R.C. Bilodeau, and H.K. Haugen,Phys. Rev. Lett.80 (1998) 2652.

7. T. Andersen, H.K. Haugen, and H. Hotop,Phys. Chem. Ref.28(1999) 1511.

8. N. Berrahet al., Phys. Rev. Lett.88 (2002) 093001.

9. R.C. Bilodeau, J.D. Bozek, A. Aguilar, G.D. Ackerman, G.Turri, and N. Berrah,Phys. Rev. Lett.93 (2004) 193001.

10. R.C. Bilodeauet al., Phys. Rev. Lett.95 (2005) 083001.

11. H. Kjeldsen, P. Andersen, F. Kristensen, and T.J. Andersen,J.Phys. B34 (2001) L353.

Rev. Mex. Fıs. S56 (2) (2010) 74–78

78 I. DUMITRIU et al.

12. N. Berrahet al., Phys. Rev. Lett87 (2001) 253002.

13. H.L. Zhou, S.T. Manson, L.Vo Ky, N. Feautrier, and A. Hibbert,Phys. Rev. Lett.87 (2001) 023001.

14. T.W. Gorczycaet al., Phys. Rev. A68 (2003) 050703(R).

15. N.D. Gibsonet al., Phys. Rev. A67 (2003) 030703(R).

16. C.W. Walteret al., Phys. Rev. A73 (2006) 062702.

17. G.Y. Kashenok and V.K. Ivanov,J. Phys. B39 (2006) 1379.

18. B.M. Smirnov, Negative Ions(McGraw-Hill Inc., New York1982).

19. S. Buckman and C.W. Clark,Rev. Mod. Phys66, 539 (1994).

20. V.A. Esaulov,Ann. Phys. F11 (1986) 493.

21. U. Fano,Phys. Rev.124(1961) 1866; U. Fano and J.W. Cooper,Phys. Rev. A137(1965) 1364.

22. A.M. Covingtonet al., Phys. Rev. A66 (2002) 062710.

23. R. Middleton, A Negative Ion Cookbook, (Dept. of Physics,Philadelphia, PA, 1990).

24. G.C. King, M. Tronc, F.H. Read, and R.C. Bradford,J. Phys. B10 (1977) 2479.

25. E. Hudsonet al., Phys. Rev. A47 (1993) 361.

26. T. Andersen,Phys. Reports394(2004) 157.

27. Y.K. Bae and J.R. Peterson,Phys. Rev. A32 (1985) 1917.

28. J.R. Peterson, Y.K. Bae, and D.L. Huestis,Phys. Rev. Lett.55(1985) 692.

29. R.C. Bilodeau and H.K. Haugen,Phys. Rev. Lett.85 (2000)534.

30. J.R. Peterson, Y.K. Bae, and D.L. Huestis,Phys. Rev Lett.55(1985) 692.

31. N. Berrahet al., Phys. Rev. A76 (2007) 032713.

32. P.G. Burke and K.A. Berrington,Atomic Molecular Process:An R-matrix Approach(IOP, Bristol, 1993).

33. K.A. Berrington, W.B. Eissner, and P.H. Norrington,Comput.Phys. Commun.92 (1995) 290.

Rev. Mex. Fıs. S56 (2) (2010) 74–78