- Email: [email protected]
An energy and intensity monitor for X-ray absorption near-edge structure measurements
ARTICLE IN PRESS Nuclear Instruments and Methods in Physics Research A 619 (2010) 154–156
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
An energy and intensity monitor for X-ray absorption near-edge structure measurements Martin D. de Jonge a,, David Paterson a, Ian McNulty b, Christoph Rau c, Jay A. Brandes d, Ellery Ingall e a
Australian Synchrotron, 800 Blackburn Road, Clayton, Victoria 3168, Australia Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439-4856, USA c Diamond Light Source Ltd, Didcot, Oxfordshire, United Kingdom d Skidaway Institute of Oceanography, 10 Ocean Science Circle, Savannah, GA 31411, USA e School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA 30332-0340, USA b
a r t i c l e in f o
a b s t r a c t
Available online 6 January 2010
An in-line X-ray beam energy and intensity monitor has been developed for use in focussed X-ray absorption near-edge spectroscopy (XANES) measurements. The monitor uses only the X-ray intensity that would otherwise bypass our zone-plate focussing optic and relies on a measurement of photoemission current. The monitor is inexpensive, easy to align, and provides valuable feedback about the X-ray energy. Operation of the monitor is demonstrated for measurements of phosphorus XANES. The precision of the energy determination is around 0.5 eV. & 2010 Published by Elsevier B.V.
Keywords: XANES X-ray energy monitor X-ray intensity monitor Phosphorus
We have developed an in-line energy monitor for scanning X-ray absorption near-edge spectroscopy (XANES) microscopy. The shape of an X-ray absorption near-edge structure (XANES) spectrum and the energy location of its features provide information about the oxidation state of an illuminated atom [1]. However, the energy separation of such features may be very small and so the requirements for the monochromation system are very demanding, and may exceed the system’s performance. Reliable spectral deconvolution of fine features requires that the measured spectra exhibit very good energy registration throughout the region of interest. For some elements this is a significant challenge, for example, phosphorus exhibits less than 2 eV shift in primary edge position from P(V) to P(III) [2]. It is therefore useful to have reliable source of feedback about the X-ray energy. Direct photon energy measurement techniques are often slow or cumbersome, requiring for example measurement of crystal rocking-curves, and may complicate an experimental arrangement. Energy dispersive detectors typically do not have the precision necessary for this task. Accurate energy calibration has been addressed in relation to high-resolution XANES spectroscopy by performing careful intensity-normalised calibrations of the monochromator operation [3]. There it is recommended that recalibration be performed at regular intervals and after major changes in the optical quality of the beamline, such as storagering refill or electron orbit shifts. However, in the absence of a
Corresponding author. Tel.: + 61 3 8540 4195; fax: + 61 3 8540 4200.
E-mail address: [email protected] (M.D. de Jonge). 0168-9002/$ - see front matter & 2010 Published by Elsevier B.V. doi:10.1016/j.nima.2010.01.001
reliable feedback mechanism, it can be difficult to assess the need for recharacterisation. XANES and X-ray absorption fine structure (XAFS) measurements of the highest accuracy require detailed investigations of a diverse range of systematic errors affecting both the attenuation and energy axes [4,5]. Absolute determination of absorption edge energy locations requires detailed and thorough independent calibration. In this article we describe a method for improving the relative precision of XANES spectroscopy measurements and which can be used for either the energy or intensity axis. At APS beamline 2-ID-B the incident photon energy is scanned by means of a multilayer spherical grating monochromator (MSGM) [6]. Unfortunately there can be some small mismatch of the delivered photon energy relative to the desired energy due to slow drifts in the MSGM angle. Due to the slowness of the effect this mismatch usually takes the form of a systematic energy offset—affecting an entire XANES scan—but there can also be occasional small discontinuities ( 0.5 eV) in the delivered photon energy occurring at some point during a scan. Fig. 1 presents a schematic of the equipment used for XANES measurements. Cross and Frenkel [7] have monitored the beam energy by inserting a thin scatterer into the beam—redirecting a fraction of the incident beam into a small side–branch calibration unit–, where they use it to obtain a transmission XAS from a standard specimen. This approach correctly samples the incident X-ray beam, however, is impractical at low X-ray energies due to the high absorption of the scatterer. Warwick et al. [8] are developing a system to provide constant energy feedback by monitoring the location of the zeroth-order reflection from their monochromator.
ARTICLE IN PRESS M.D. de Jonge et al. / Nuclear Instruments and Methods in Physics Research A 619 (2010) 154–156
155
SDD e-
monitor
monochromated x-ray beam
CS
ZP
OSA
S
nA e-
Fig. 1. Schematic of the apparatus used to measure XANES. The beam has been monochromated by reflection from a MSGM (not shown). A central stop (CS), zone plate (ZP), and order-sorting aperture (OSA) are used to form a small focus of X-rays at a specimen (S). A XANES spectrum is obtained by scanning incident X-ray energy while recording fluorescence using a silicon drift diode (SDD) detector. The incident energy is monitored by measuring the TEY current passing from the apatite (on the monitor) to the vacuum chamber. The monitor picks off only the portion of the incident X-ray beam that would otherwise bypass the focussing optic.
Fig. 2. Normalised counts recorded using the energy monitor device, as a function of X-ray energy. [Upper] prior to alignment and [lower] following alignment. The uncertainty in the X-ray energy – and hence the feature location – has been reduced significantly from around 4 eV to less than 1 eV.
We present a very simple and inexpensive device which enables the total electron yield (TEY) XANES spectrum of a calibration standard to be obtained in parallel with the XANES of an experimental specimen. The energy monitor consists of a small slit ( E190 mm wide, slightly larger than our 160-mm-diameter zone plate focussing optic) cut into a thin stainless steel shim. An ethanol slurry of the NIST standard reference material SRM 2910 calcium hydroxyapatite Ca10(PO4)6(OH)2 [9] was applied to the surface of the shim and allowed to dry. This material was chosen for its vacuum compatibility, its radiation hardness, and its ready availability. The shim was mounted in vacuum on an electricallyinsulated substrate about 1 m upstream of the zone-plate, about 12.5 m downstream of the MSGM. A Stanford Research current amplifier (model SR570) was used to measure the restoring current required to balance photoemission from the standard. When the X-ray energy is scanned the energy dependence of the cross-section gives rise to a strong and characteristic variation of the photoemission current. Here we demonstrate the operation of the energy monitor for measurements of phosphorus XANES. Over 100 XANES spectra were obtained during a recent investigation into phosphorus speciation. Fig. 2 shows the monitor photoemission current scaled for each scan so as to have unitary peak value. It is clear from the upper part of Fig. 2 that the delivered beam energy has varied by about 4 eV over the course of the measurements. The photoemission current at the primary peak ranged from 20 to 70 pA over the 100 spectra due to changes in slit settings. The reproducibility of the feature sizes following scaling indicates the linearity of the response of the energy monitor. The monochromator energy offset can be determined by identifying the location of any one feature in the spectrum. The most obvious candidate is to use the energy at which the maximum photoemission current is observed. We have done this by fitting the spectrum with a pseudo-physical model to determine the locations
of all features in the spectrum. We have fitted the spectra with a constant background, an arctangent step function, and four Gaussians, in accord with the observed structure in the spectrum [1]. The fitted curves are generally very consistent with the measured data. Following our analysis it is apparent that an alternate approach could instead scale and shift all spectra to agree with a single, arbitrarily-chosen reference spectrum. However, this approach requires the stability of the reference spectrum, which we were not prepared to assume at this early stage. Tracking of fitted peak locations and amplitudes over time is necessary to properly test such an approach. The lower portion of Fig. 2 shows the same spectra after alignment, effected by defining the fitted primary peak location to be 2152 eV [2]. The raw fitted locations of this peak ranged from 2148.4 to 2152.6 eV for the measurements shown in the upper half of Fig. 2. Several energy discontinuities are now apparent in the aligned spectra. These can be automatically detected from the results of the fit to the TEY XANES spectra, by interrogating either the chi-squared goodness-of-fit for the entire spectral region or only for a region of interest, such as in the neighbourhood of the white-line transition. Despite typical uncertainties for the fitted peak location below 0.1 eV, the precision of the current energy determination is around 0.5 eV. The current precision is limited by discontinuities in the delivered energy, and by the width of the primary XANES peak (1.5–2 eV). The width of this peak was quite constant even when the monochromaticity of the beam was adjusted, indicating that the width is determined perhaps by the electrochemical and surface properties of the standard. The device can clearly be used to monitor the intensity of the X-ray beam when the energy is fixed, for e.g., when performing a fluorescence map. However, it can also be used as an intensity monitor when the energy is scanned over a range to which the response of the monitor is flat—for example, the phosphorus
ARTICLE IN PRESS 156
M.D. de Jonge et al. / Nuclear Instruments and Methods in Physics Research A 619 (2010) 154–156
monitor can be used when measuring sulphur XANES. In both of these cases the photoemission current is dominated by the incident X-ray intensity, and so the device functions primarily as an intensity monitor. We plan to commission a sulphur XANES energy monitor using anhydrite CaSO4 which will enable us to monitor energy or intensity for either phosphorus or sulphur XANES measurements. Simultaneous energy and intensity monitoring can be obtained by employing two such devices with dissimilar standards with shim slits oriented perpendicular to one-another. The in-line standard can be used as an energy monitor for determination of the location of the primary XANES peak. The accuracy of this determination would benefit from complete characterisation of the TEY XANES for the selected standard compound. The standard can be used to align all spectra (in energy), correcting the effect of the monochromator drift that we have observed, and thereby enabling clear detection of small changes to the location of XANES features indicating different bonding configurations. We have demonstrated the application by aligning XANES from the one experiment, but note that the standard provides an excellent energy baseline for comparing the
results of different experiments and simplifies the tracking of relative energy offsets. The adoption of such a standard will enable consistent definition of the energy axis (for each element) within a facility and across facilities. Adoption of this relative standard will enable tracking of past conclusions as absolute energy determination is further refined. Use of the Advanced Photon Source was supported by the US Department of Energy, Basic Energy Sciences, Office of Energy Research, under Contract No. DE-AC02-06CH11357.
References ¨ J. Stohr, NEXAFS Spectroscopy, Springer, 2003. J.A. Brandes, E. Ingall, D. Paterson, Mar. Chem. 103 (2007) 250. ¨ et al., J. Electron Spectrosc. Relat. Phenom. 129 (2003) 1. A. Scholl, C.T. Chantler, et al., Phys. Rev. A 64 (2001). M.D. de Jonge, et al., Phys. Rev. A 71 (2005). I. McNulty, Y.P. Feng, S.P. Frigo, T.M. Mooney, SPIE Proc. 3150 (1997) 195. J.O. Cross, A.I. Frenkel, Rev. Sci. Instrum. 70 (1999) 38. T. Warwick, W. McKinney, E. Domning, A. Doran, H. Padmore, AIP Conf. Proc. 879 (2007) 69. [9] /https://srmors.nist.gov/view cert.cfm?srm=2910S. [1] [2] [3] [4] [5] [6] [7] [8]
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
An energy and intensity monitor for X-ray absorption near-edge structure measurements Martin D. de Jonge a,, David Paterson a, Ian McNulty b, Christoph Rau c, Jay A. Brandes d, Ellery Ingall e a
Australian Synchrotron, 800 Blackburn Road, Clayton, Victoria 3168, Australia Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439-4856, USA c Diamond Light Source Ltd, Didcot, Oxfordshire, United Kingdom d Skidaway Institute of Oceanography, 10 Ocean Science Circle, Savannah, GA 31411, USA e School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA 30332-0340, USA b
a r t i c l e in f o
a b s t r a c t
Available online 6 January 2010
An in-line X-ray beam energy and intensity monitor has been developed for use in focussed X-ray absorption near-edge spectroscopy (XANES) measurements. The monitor uses only the X-ray intensity that would otherwise bypass our zone-plate focussing optic and relies on a measurement of photoemission current. The monitor is inexpensive, easy to align, and provides valuable feedback about the X-ray energy. Operation of the monitor is demonstrated for measurements of phosphorus XANES. The precision of the energy determination is around 0.5 eV. & 2010 Published by Elsevier B.V.
Keywords: XANES X-ray energy monitor X-ray intensity monitor Phosphorus
We have developed an in-line energy monitor for scanning X-ray absorption near-edge spectroscopy (XANES) microscopy. The shape of an X-ray absorption near-edge structure (XANES) spectrum and the energy location of its features provide information about the oxidation state of an illuminated atom [1]. However, the energy separation of such features may be very small and so the requirements for the monochromation system are very demanding, and may exceed the system’s performance. Reliable spectral deconvolution of fine features requires that the measured spectra exhibit very good energy registration throughout the region of interest. For some elements this is a significant challenge, for example, phosphorus exhibits less than 2 eV shift in primary edge position from P(V) to P(III) [2]. It is therefore useful to have reliable source of feedback about the X-ray energy. Direct photon energy measurement techniques are often slow or cumbersome, requiring for example measurement of crystal rocking-curves, and may complicate an experimental arrangement. Energy dispersive detectors typically do not have the precision necessary for this task. Accurate energy calibration has been addressed in relation to high-resolution XANES spectroscopy by performing careful intensity-normalised calibrations of the monochromator operation [3]. There it is recommended that recalibration be performed at regular intervals and after major changes in the optical quality of the beamline, such as storagering refill or electron orbit shifts. However, in the absence of a
Corresponding author. Tel.: + 61 3 8540 4195; fax: + 61 3 8540 4200.
E-mail address: [email protected] (M.D. de Jonge). 0168-9002/$ - see front matter & 2010 Published by Elsevier B.V. doi:10.1016/j.nima.2010.01.001
reliable feedback mechanism, it can be difficult to assess the need for recharacterisation. XANES and X-ray absorption fine structure (XAFS) measurements of the highest accuracy require detailed investigations of a diverse range of systematic errors affecting both the attenuation and energy axes [4,5]. Absolute determination of absorption edge energy locations requires detailed and thorough independent calibration. In this article we describe a method for improving the relative precision of XANES spectroscopy measurements and which can be used for either the energy or intensity axis. At APS beamline 2-ID-B the incident photon energy is scanned by means of a multilayer spherical grating monochromator (MSGM) [6]. Unfortunately there can be some small mismatch of the delivered photon energy relative to the desired energy due to slow drifts in the MSGM angle. Due to the slowness of the effect this mismatch usually takes the form of a systematic energy offset—affecting an entire XANES scan—but there can also be occasional small discontinuities ( 0.5 eV) in the delivered photon energy occurring at some point during a scan. Fig. 1 presents a schematic of the equipment used for XANES measurements. Cross and Frenkel [7] have monitored the beam energy by inserting a thin scatterer into the beam—redirecting a fraction of the incident beam into a small side–branch calibration unit–, where they use it to obtain a transmission XAS from a standard specimen. This approach correctly samples the incident X-ray beam, however, is impractical at low X-ray energies due to the high absorption of the scatterer. Warwick et al. [8] are developing a system to provide constant energy feedback by monitoring the location of the zeroth-order reflection from their monochromator.
ARTICLE IN PRESS M.D. de Jonge et al. / Nuclear Instruments and Methods in Physics Research A 619 (2010) 154–156
155
SDD e-
monitor
monochromated x-ray beam
CS
ZP
OSA
S
nA e-
Fig. 1. Schematic of the apparatus used to measure XANES. The beam has been monochromated by reflection from a MSGM (not shown). A central stop (CS), zone plate (ZP), and order-sorting aperture (OSA) are used to form a small focus of X-rays at a specimen (S). A XANES spectrum is obtained by scanning incident X-ray energy while recording fluorescence using a silicon drift diode (SDD) detector. The incident energy is monitored by measuring the TEY current passing from the apatite (on the monitor) to the vacuum chamber. The monitor picks off only the portion of the incident X-ray beam that would otherwise bypass the focussing optic.
Fig. 2. Normalised counts recorded using the energy monitor device, as a function of X-ray energy. [Upper] prior to alignment and [lower] following alignment. The uncertainty in the X-ray energy – and hence the feature location – has been reduced significantly from around 4 eV to less than 1 eV.
We present a very simple and inexpensive device which enables the total electron yield (TEY) XANES spectrum of a calibration standard to be obtained in parallel with the XANES of an experimental specimen. The energy monitor consists of a small slit ( E190 mm wide, slightly larger than our 160-mm-diameter zone plate focussing optic) cut into a thin stainless steel shim. An ethanol slurry of the NIST standard reference material SRM 2910 calcium hydroxyapatite Ca10(PO4)6(OH)2 [9] was applied to the surface of the shim and allowed to dry. This material was chosen for its vacuum compatibility, its radiation hardness, and its ready availability. The shim was mounted in vacuum on an electricallyinsulated substrate about 1 m upstream of the zone-plate, about 12.5 m downstream of the MSGM. A Stanford Research current amplifier (model SR570) was used to measure the restoring current required to balance photoemission from the standard. When the X-ray energy is scanned the energy dependence of the cross-section gives rise to a strong and characteristic variation of the photoemission current. Here we demonstrate the operation of the energy monitor for measurements of phosphorus XANES. Over 100 XANES spectra were obtained during a recent investigation into phosphorus speciation. Fig. 2 shows the monitor photoemission current scaled for each scan so as to have unitary peak value. It is clear from the upper part of Fig. 2 that the delivered beam energy has varied by about 4 eV over the course of the measurements. The photoemission current at the primary peak ranged from 20 to 70 pA over the 100 spectra due to changes in slit settings. The reproducibility of the feature sizes following scaling indicates the linearity of the response of the energy monitor. The monochromator energy offset can be determined by identifying the location of any one feature in the spectrum. The most obvious candidate is to use the energy at which the maximum photoemission current is observed. We have done this by fitting the spectrum with a pseudo-physical model to determine the locations
of all features in the spectrum. We have fitted the spectra with a constant background, an arctangent step function, and four Gaussians, in accord with the observed structure in the spectrum [1]. The fitted curves are generally very consistent with the measured data. Following our analysis it is apparent that an alternate approach could instead scale and shift all spectra to agree with a single, arbitrarily-chosen reference spectrum. However, this approach requires the stability of the reference spectrum, which we were not prepared to assume at this early stage. Tracking of fitted peak locations and amplitudes over time is necessary to properly test such an approach. The lower portion of Fig. 2 shows the same spectra after alignment, effected by defining the fitted primary peak location to be 2152 eV [2]. The raw fitted locations of this peak ranged from 2148.4 to 2152.6 eV for the measurements shown in the upper half of Fig. 2. Several energy discontinuities are now apparent in the aligned spectra. These can be automatically detected from the results of the fit to the TEY XANES spectra, by interrogating either the chi-squared goodness-of-fit for the entire spectral region or only for a region of interest, such as in the neighbourhood of the white-line transition. Despite typical uncertainties for the fitted peak location below 0.1 eV, the precision of the current energy determination is around 0.5 eV. The current precision is limited by discontinuities in the delivered energy, and by the width of the primary XANES peak (1.5–2 eV). The width of this peak was quite constant even when the monochromaticity of the beam was adjusted, indicating that the width is determined perhaps by the electrochemical and surface properties of the standard. The device can clearly be used to monitor the intensity of the X-ray beam when the energy is fixed, for e.g., when performing a fluorescence map. However, it can also be used as an intensity monitor when the energy is scanned over a range to which the response of the monitor is flat—for example, the phosphorus
ARTICLE IN PRESS 156
M.D. de Jonge et al. / Nuclear Instruments and Methods in Physics Research A 619 (2010) 154–156
monitor can be used when measuring sulphur XANES. In both of these cases the photoemission current is dominated by the incident X-ray intensity, and so the device functions primarily as an intensity monitor. We plan to commission a sulphur XANES energy monitor using anhydrite CaSO4 which will enable us to monitor energy or intensity for either phosphorus or sulphur XANES measurements. Simultaneous energy and intensity monitoring can be obtained by employing two such devices with dissimilar standards with shim slits oriented perpendicular to one-another. The in-line standard can be used as an energy monitor for determination of the location of the primary XANES peak. The accuracy of this determination would benefit from complete characterisation of the TEY XANES for the selected standard compound. The standard can be used to align all spectra (in energy), correcting the effect of the monochromator drift that we have observed, and thereby enabling clear detection of small changes to the location of XANES features indicating different bonding configurations. We have demonstrated the application by aligning XANES from the one experiment, but note that the standard provides an excellent energy baseline for comparing the
results of different experiments and simplifies the tracking of relative energy offsets. The adoption of such a standard will enable consistent definition of the energy axis (for each element) within a facility and across facilities. Adoption of this relative standard will enable tracking of past conclusions as absolute energy determination is further refined. Use of the Advanced Photon Source was supported by the US Department of Energy, Basic Energy Sciences, Office of Energy Research, under Contract No. DE-AC02-06CH11357.
References ¨ J. Stohr, NEXAFS Spectroscopy, Springer, 2003. J.A. Brandes, E. Ingall, D. Paterson, Mar. Chem. 103 (2007) 250. ¨ et al., J. Electron Spectrosc. Relat. Phenom. 129 (2003) 1. A. Scholl, C.T. Chantler, et al., Phys. Rev. A 64 (2001). M.D. de Jonge, et al., Phys. Rev. A 71 (2005). I. McNulty, Y.P. Feng, S.P. Frigo, T.M. Mooney, SPIE Proc. 3150 (1997) 195. J.O. Cross, A.I. Frenkel, Rev. Sci. Instrum. 70 (1999) 38. T. Warwick, W. McKinney, E. Domning, A. Doran, H. Padmore, AIP Conf. Proc. 879 (2007) 69. [9] /https://srmors.nist.gov/view cert.cfm?srm=2910S. [1] [2] [3] [4] [5] [6] [7] [8]