- Email: [email protected]
The high resolution Gas Phase Photoemission beamline, Elettra
Journal of Electron Spectroscopy and Related Phenomena 101–103 (1999) 959–964
The high resolution Gas Phase Photoemission beamline, Elettra R.R. Blyth a , R. Delaunay a , M. Zitnik a , J. Krempasky a , R. Krempaska a , J. Slezak a , a, a a b b c K.C. Prince *, R. Richter , M. Vondracek , R. Camilloni , L. Avaldi , M. Coreno , d e e f g G. Stefani , C. Furlani , M. de Simone , S. Stranges , M.-Y. Adam a Sincrotrone Trieste, I-34012 Trieste, Italy CNR-Istituto di Metodologie Avanzate Inorganiche, Montelibretti, I-00016 Rome, Italy c INFM, Gas Phase Beamline at Elettra, I-34012 Trieste, Italy d Dipartimento di Fisica e Unita’ INFM, Universita’ di Roma III, Via della Vasca Navale 84, I-00146 Rome, Italy e Dipartimento di Fisica, Universita’ di Roma III, Via della Vasca Navale 84, I-00146 Rome, Italy f Dipartimento di Chimica, Universita’ degli Studi di Roma, ‘ La Sapienza’, P.le A. Moro 5, I-00185 Rome, Italy g LURE, Centre Universitaire de Paris Sud, F-91045 Orsay, France b
Abstract The Gas Phase Photoemission beamline at Elettra has been commissioned with outstanding success. All photoabsorption spectra taken between 90 and 900 eV have shown resolution which is equal to or higher than any published spectra. The monochromator is a variable angle spherical grating instrument (plane mirror and grating between entrance and exit slits), with an undulator as the source. Some of the problems encountered in commissioning and their solutions are discussed. In particular the calibration is complicated by the fact that an infinite number of angle pairs of the mirror and grating exist for a particular photon energy, whereas only one angle pair gives the highest resolution. A second problem is that the resolution is so high that above about 80 eV, it is much smaller than the lifetime broadening of any known absorption resonance, making any determination of resolution difficult. The experimental chambers available for users are described together with some examples of spectra which have been taken. 1999 Elsevier Science B.V. All rights reserved. Keywords: Photoelectron spectroscopy; Monochromator; Beamline
1. Introduction This paper describes the characteristics of the Gas Phase Photoemission beamline at Elettra with a view to giving potential users the information they need to plan an experiment. The beamline is now open to users and has been designed for atomic and molecular physics and chemistry experiments. The design parameters included the possibility of working with gases, vapours, metastable species, and of perform*Corresponding author. E-mail address: [email protected] (K.C. Prince)
ing photoabsorption, photoemission, and coincidence measurements. The main requirements for the monochromator were high photon resolution over a wide energy range, stable spot size and high flux. The design goal for the energy range was set as 20–800 eV. The resolution was required to be significantly better than the natural widths of the main atomic and molecular excitations in this range, and so the goal for the resolving power was 10 000. A stable spot size means that changes in spot size, shape and position as a function of energy were to be minimised. This implied the use of a fixed exit slit
0368-2048 / 99 / $ – see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 98 )00381-8
960
R.R. Blyth et al. / Journal of Electron Spectroscopy and Related Phenomena 101 – 103 (1999) 959 – 964
and re-focusing system. The latter provides two additional advantages: firstly, the experimental station, which operates at pressures of 10 25 –10 26 mbar, is further away from the monochromator optical elements. Secondly a circular focus can be provided, which is important when aligning several spectrometers and / or different excitation sources on the same target volume. For the end-stations, the project foresaw the construction of two chambers available for users, and also to allow for the possibility that users bring their own chamber. The Multi-coincidence Chamber is optimised primarily to measure electron–electron coincidences with an array of analysers. The second end station, the Angle Resolved Photoemission Chamber, is designed for photoemission spectroscopies with vapours and a wide range of gases, including unstable and radical species which are usually corrosive. Both chambers are available to users, and versatile support systems have been built for users who bring their own chambers. The beam exits at an angle of 4 degrees with respect to the horizontal, at a height of 180 cm. Both chambers have an exit port to which a windowless gas cell can be attached. The cell can be filled with gas independently of the sample in the main chamber, and used for photoabsorption spectroscopies. At the back of the gas cell, a calibrated photodiode can be inserted. Thus under suitable conditions, the user can measure simultaneously his / her spectrum; an absorption spectrum which gives a very precise calibration and a check of the monochromator resolution; and an I0 spectrum for normalisation to the absolute flux.
2. Results
2.1. Light source The light source of the beamline is an undulator of period 12.5 cm in section 6.2 of the Elettra storage ring; details can be found on the Elettra home page [1]. The lowest photon energy produced with the ring running at 2 GeV is 16 eV, although we have not yet tested the monochromator at energies this low. The lower curve of Fig. 1 shows a typical absorption spectrum with fixed gap: both the ion yield and the
Fig. 1. Photoabsorption spectra of the He doubly excited states below the N52 threshold. Lower curve: ion yield spectrum, normalised to the flux. Centre curve, ‘Undulator Gap 63.5’: photodiode signal. Inset: high resolution scan of the peaks 121 to 221. Resolution: 2.2 meV.
photodiode spectra are shown. The photodiode curve shows weak absorption due to the gas in the gas cell and indicates that absorption is still in the linear regime. The overall shape is determined by the variation of the flux due to the undulator output, and the full width at half maximum is about 4% of the peak energy. In principle, the higher harmonics should be narrower, but effects such as finite acceptance of the photon beam, finite emittance and energy spread mean that the width is still about 4%. Most energy scanning spectroscopies are still done with fixed gap, although the gap of the undulator can be adjusted over a wide range manually or by software. Changing the gap is a relatively slow process (about 2 min), due to the time taken for software overheads. We are presently developing software to permit automated gap scanning. Table 1 shows the preliminary results of a series of measurements of flux at the gas cell after the experimental chamber.
2.2. Optical layout and monochromator The optical concept has been published previously [2] as well as the first results [3]. In brief, there are two mirrors before the entrance slit; a plane mirror and a set of five gratings (Table 2) between the
R.R. Blyth et al. / Journal of Electron Spectroscopy and Related Phenomena 101 – 103 (1999) 959 – 964
961
Table 1 Flux and resolution at selected energies Energy (eV)
Resolving power
Flux (photons / s / 100 mA)
45 65 86 245 401 540 680
.25 000 .28 000 – 12 200 .12 000 10 000 10 000
6.3310 10 2.2310 11 1.5310 11 1.5310 10 1.1310 10 2.0310 10 3.0310 9
entrance and exit slits; and two re-focusing mirrors afterwards. The first mirror, the plane Switching Mirror is used to steer the beam vertically so that it enters the entrance slit. This is important at the minimum slit width of 10 mm, but is a minor adjustment that is done once or twice a day. At large slit openings this is not normally necessary. The entrance slit is usually used at values between 10 and 30 mm, through which about 95% of the flux passes. The monochromator can be scanned in two different modes. In energy scan mode, both the mirror and grating of the monochromator are scanned simultaneously. This is the preferred mode of operation at low to medium energy resolving power, up to about 4000. The scanning of two mechanical systems, mirror and grating, doubles the contribution of mechanical errors to the degradation of the resolution. To avoid this, at high resolving power (circa 10 000) fixed mirror scanning is preferred. The mirror is set to the average value for the range of energy to be scanned, and only the grating is scanned. Typical spectra at high resolution are shown in the inset of Fig. 1, and in Fig. 2; see also Refs. [3–5]. This mode gives the highest resolving power, from 30 000 at 48 eV to 10 000 at 540 eV. It Table 2 Gratings and energy ranges Grating number
Lines / mm
Energy range (calculated)
1 2 3 4 5
400 800 1200 1200 1200
20–50 40–90 80–180 160–430 360–1000
Fig. 2. Photoabsorption spectra of the Ne, 2s 21 np states. Lower curve shows a broad scan; inset shows the peaks n516 to 27. Resolution: 1.6 meV.
has the disadvantage that the defocus contribution to the resolution changes slightly over the scan. However at high resolution, the required step size is usually small and so the scan width is limited. If scans are confined to about 2% of the average energy, the resolution is approximately constant as the defocusing term is sufficiently small that it does not affect resolution. All photoabsorption spectra taken between 90 and 900 eV have shown resolution which is higher than or at least equal to published spectra [3–5]. At the nitrogen 1s→p absorption resonance, the ratio of the third peak height to the first minimum is often taken as a criterion of resolution. We obtain a value of 0.6260.01, considerably lower than most published values [6–12]. For instance, Domke et al. [9] used an SX700 monochromator and obtained a value of approximately 1 in first order, although their spectrum taken with second order light and masking of most of the grating was better. Watanabe et al. [10] used a variable line space grating and their Fig. 6 seems to give a value of about 0.7. They obtained an estimated resolution of 40.9 meV for an intrinsic line width of 117 meV. We estimate our resolution to be 35 meV for a line width of 115 meV. At low energies, the resolution is also very high, being approximately 1.6 meV and 2.2 meV at 45 and 65 eV (measured using neon and helium photo-
962
R.R. Blyth et al. / Journal of Electron Spectroscopy and Related Phenomena 101 – 103 (1999) 959 – 964
absorption spectra, respectively). These results were obtained using first order light with no masking of the grating and the minimum slit settings, 10 mm. A higher resolution of 1.0 meV at 64 eV has been obtained on beamline 9.0.1 at the Advanced Light Source [13,14]. A few spectra taken with second order light indicate that our resolution can be improved further. The energy calibration has been carried out by measuring a series of absorption spectra at different mirror settings and selecting the value which maximises resolution. This provides a calibration point with a value of the photon energy, mirror and grating settings. A mathematical model has been developed [15] into which this data point is inserted, and then the functional describing the calibration curve is solved numerically. At least three data points per grating are necessary. Recalibration after maintenance (which may change certain mechanical constants of the system) is simpler as it is assumed that the new calibration curve is the same as the previous one but that the grating and mirror are displaced in angle with respect to the old value. This assumption works reasonably well and a grating can be recalibrated with only two calibration points. A number of problems were encountered during commissioning which are still being resolved. The resolution is so high that the mechanical scanning (which is sometimes in steps as small as 100 nm) is working at its limit. In particular stick-slip behaviour and small jumps are observed which are probably due to the surface finish of the components of the drive screw. We are planning to install a piezo drive later this year (1998) to resolve this problem. The original specification was for a range up to 800 eV and so the high energy grating was coated with nickel, to maximise reflectivity up to this energy. At the Ne K edge, the Ni L 2,3 absorption reduces the reflectivity to a few percent. In spite of this we were able to measure the best resolved Ne absorption spectra so far reported [4]. Presently we are having the grating re-coated with gold, and it will be installed in October 1998. The low energy grating (20–40 eV) suffered from excessive high order contributions and this is also being re-coated, this time with Al / MgF 2 , and it will also be installed in October. In addition, filters will be inserted to reduce higher orders at other energies.
2.3. Experimental stations 2.3.1. Multicoincidence chamber The 800 mm I.D. multicoincidence end-station is lined with a 2-mm-thick m-metal shield, additionally screened by three pairs of Helmholtz coils; the residual magnetic field is less than 10 mG. Two independently rotatable arrays of electrostatic electron energy analysers are housed in the chamber. Seven spectrometers are mounted on a frame that rotates in the plane of the polarisation vector of the incident radiation, while three other spectrometers are mounted on a smaller frame rotatable around the same axis. This latter array enables measurements out of the polarisation plane, to study, for example, non-dipole contributions to the photoionization cross-section. All 10 analysers are mounted at angular intervals of 308. The spectrometers are composed of two fourelement lenses (194 mm long) that focus the photoelectrons from the target region onto the entrance slits of the hemispherical deflector. The lens system [16] can be operated in two modes: a low resolution mode characterised by a DE /Ek 510 22 , where Ek is the kinetic energy of the photoelectrons, and an angular acceptance in the dispersion plane of 638. This mode is most suited for coincidence experiments. The second mode is a high resolution mode with DE /Ek 510 24 and an angular acceptance of 60.58 used for non-coincidence photoelectron and Auger electron spectroscopies. The lens stack can work with retarding ratios of 2–5 and 5–100 in the low- and high-resolution modes, respectively. The mean radius of the hemispherical deflector is 33 mm and the gap 9.9 mm. These values have been chosen in order to obtain a maximum time spread of 2 ns in the trajectories of 100 eV electrons in the dispersing element. An example of the performance of the analyser when operated in the high-resolution mode is shown in Fig. 3, where the spectrum of Ar 1 (3p 21 ) taken at 244.4 eV incident photon energy, the energy of the 2p 3 / 2 →4s resonance, is shown. The pass energy was 3 eV, corresponding to a retarding ratio of 76. The entrance and exit slits were 1 and 4 mm, respectively. The FWHM of the measured peaks was 65 meV giving a resolution of 50 meV after the deconvolution of the contribution from the incident light. This
R.R. Blyth et al. / Journal of Electron Spectroscopy and Related Phenomena 101 – 103 (1999) 959 – 964
963
Fig. 4. Auger electron–photoelectron coincidence spectrum (squares) of Kr at hn5135 eV, when a 3d 3 / 2 photoelectron is detected. Also shown is the non-coincidence M 4,5 N 23 N 23 Auger spectrum (dots), measured simultaneously. Dwell time: 1 h per point.
Fig. 3. Ar 1 (3p 21 ) photoelectron spectrum at 244.4 eV incident energy. A pseudo-Voigt function (full line) has been fitted to the data. Pass energy 3 eV; FWHM and analyser resolution 65 and 50 meV, respectively.
shows that despite the small size of the hemispheres good energy resolution can be achieved with a reasonable count rate. Fig. 4 displays the results of an Auger electron– photoelectron coincidence experiment to study the formation and decay of the Kr 1 (3d 21 ) states. This illustrates one of the main advantages of coincidence experiments: the possibility of distinguishing the components due to different initial holes in the Auger spectrum. In this case the M 4 N 23 N 23 ( 2 D) line 3,1 overlaps completely with the M 5 N 23 N 23 ( P) lines, so its contribution to the Auger spectrum cannot be evaluated directly without a coincidence experiment.
2.3.2. Angle resolved photoemission spectroscopy chamber The experimental chamber of the ARPES end station consists of a 500-mm I.D. cylindrical vessel lined with a double m-metal shield. Gaseous and volatile liquid samples can be introduced into the ionisation region through a hypodermic needle
mounted on an XYZ manipulator. Solids can be vaporised in a high temperature, anti-inductively wound oven, in which temperatures up to 11008C can be obtained and maintained for several hours. The oven and the ionisation volume are enclosed in a cooled jacket designed to minimize contamination. An efficient cryo and turbo pumping system and a quartz capillary mounted between the beamline and the chamber prevent contamination of the beamline. These design features allow the study of aggressive gases and vapours. Two VSW hemispherical photoelectron energy analysers (50 mm mean radius) can be mounted with a mutual angle of 908 on a turntable. The fourelement lens system of each analyser has an acceptance angle of approximately 638 and has been modified to suit high temperature experiments. Pass energies from 1 to 50 eV can be selected. A resolution of |50 meV (FWHM) has been obtained for 25 eV kinetic energy electrons, using 2 eV pass energy, and 180 meV resolution for 130 eV kinetic energy electrons with 10 eV pass energy. The modular design of the chamber and the large side flanges also allow users’ spectrometers to be mounted. Fig. 5 shows the photoelectron spectrum of SnBr 2 obtained using ARPES. An effusive beam of SnBr 2 molecules was produced by heating the solid sample to |3208C. The full valence PE spectrum, as well as the intense core Sn 4d 21 ionisation, are displayed.
964
R.R. Blyth et al. / Journal of Electron Spectroscopy and Related Phenomena 101 – 103 (1999) 959 – 964
ni Sostero of the Soft X-ray Optical Laboratory and Luca Romanzin and Gilio Sandrin for technical assistance. JS was supported by EC contract CIPACT94-0217, and MV by EC contract IC15 CT97700.
References
Fig. 5. SnBr 2 photoelectron spectrum recorded at the magic angle with hn550.5 eV and 5 eV pass energy. Photon flux: about 5310 12 photons / s, resolution|1000 (entrance / exit slit550 / 100 mm). The full valence spectral region and the Sn 4d 21 core ionisations are shown, as well as the Br 3d 21 core ionisations from second order light.
One of the interests of this investigation was to study electron correlation effects in valence photoionisation processes. These effects usually produce low intensity spectral features, which are difficult to detect in high temperature systems because of the low target density. The high photon flux reveals new features due to electron correlation in the spectrum in the region of the a 21 and Br 4s 21 valence ionisation, 1 in agreement with recent CI ab-initio calculations. A detailed account of these experimental and theoretical results will be given elsewhere [17].
3. Conclusions The Gas Phase Photoemission beamline at Elettra has produced outstanding results and is now ‘open for business’. The end stations are flexible and allow a wide variety of experiments to be done in atomic and molecular physics and chemistry.
Acknowledgements We thank our colleagues at Elettra for their support, and particularly Daniele Cocco and Giovan-
[1] www.elettra.trieste.it [2] P. Melpignano, S. Di Fonzo, A. Bianco, W. Jark, Rev. Sci. Instrum. 66 (1995) 2125. [3] K.C. Prince, R.R. Blyth, R. Delaunay, M. Zitnik, J. Krempasky, J. Slezak, R. Camilloni, L. Avaldi, M. Coreno, G. Stefani, C. Furlani, M. de Simone, S. Stranges, J. Sync. Rad. 5 (1998) 565. [4] M. Coreno, L. Avaldi, R. Camilloni, M. de Simone, J. Karvonen, R. Colle, S. Simonucci, Phys. Rev. A, submitted. [5] K.C. Prince, M. Vondracek, J. Karvonen, M. Coreno, R. Camilloni, L. Avaldi, M. de Simone, these proceedings. [6] C.T. Chen, Y. Ma, F. Sette, Phys. Rev. A 40 (1989) 6737. [7] C. Quaresima, C. Ottaviani, M. Matteucci, C. Crotti, A. Antonini, M. Capozi, S. Rinaldi, M. Luce, P. Perfetti, K.C. Prince, C. Astaldi, M. Zacchigna, L. Romanzin, A. Savoia, Nucl. Instr. Methods A 364 (1995) 374. [8] D. Cvetko, L. Floreano, R. Gotter, M. Malvezzi, L. Marassi, A. Morgante, G. Naletto, A. Santaniello, G. Stefani, F. Tommasini, G. Tondello, A. Verdini, Proc. SPIE 3150 (1997) 86. [9] M. Domke, T. Mandel, A. Puschmann, C. Xue, D.A. Shirley, G. Kaindl, H. Petersen, P. Kuske, Rev. Sci. Instrum. 63 (1992) 80. [10] M. Watanabe, A. Toyoshima, Y. Azuma, T. Hayaishi, Y. Yan, A. Yagishita, Proc. SPIE 3150 (1997) 277. [11] K.J. Randall, J. Feldhaus, W. Erlebach, A.M. Bradshaw, W. Eberhardt, Z. Xu, Y. Ma, P.D. Johnson, Rev. Sci. Instrum. 63 (1992) 1367. ¨ O.-P. Sairanen, A. Naves de Brito, [12] S. Aksela, A. Kivimaki, E. Nommiste, Rev. Sci. Instrum. 66 (1995) 1621. [13] K. Schulz, G. Kaindl, M. Domke, J.D. Bozek, P.A. Heimann, A.S. Schlacter, J.M. Rost, Phys. Rev. Lett. 77 (1996) 3086. [14] G. Kaindl, K. Schulz, P.A. Heimann, J.D. Bozek, A.S. Schlacter, Synchrotron Rad. News 8 (1995) 29. ´ A. Bianco, A. Abrami, R. [15] J. Krempasky, R. Krempaska, ` J. Karvonen, M. Coreno, M. de Simone, Puliese, F. Bille, Proc. SPIE 3150 (1998) 76. [16] D. Cvetko, L. Floreano, R. Gotter, M. Malvezzi, L. Marassi, A. Morgante, G. Naletto, A. Santaniello, G. Stefani, F. Tommasini, G. Tondello, A. Verdini, Proc. SPIE 3150 (1997) 86. [17] M. de Simone, R.C. Richter, M. Alagia, S. Stranges, P. Decleva, A. Lisini, M. Coreno, K.C. Prince, C. Furlani, in preparation.
The high resolution Gas Phase Photoemission beamline, Elettra R.R. Blyth a , R. Delaunay a , M. Zitnik a , J. Krempasky a , R. Krempaska a , J. Slezak a , a, a a b b c K.C. Prince *, R. Richter , M. Vondracek , R. Camilloni , L. Avaldi , M. Coreno , d e e f g G. Stefani , C. Furlani , M. de Simone , S. Stranges , M.-Y. Adam a Sincrotrone Trieste, I-34012 Trieste, Italy CNR-Istituto di Metodologie Avanzate Inorganiche, Montelibretti, I-00016 Rome, Italy c INFM, Gas Phase Beamline at Elettra, I-34012 Trieste, Italy d Dipartimento di Fisica e Unita’ INFM, Universita’ di Roma III, Via della Vasca Navale 84, I-00146 Rome, Italy e Dipartimento di Fisica, Universita’ di Roma III, Via della Vasca Navale 84, I-00146 Rome, Italy f Dipartimento di Chimica, Universita’ degli Studi di Roma, ‘ La Sapienza’, P.le A. Moro 5, I-00185 Rome, Italy g LURE, Centre Universitaire de Paris Sud, F-91045 Orsay, France b
Abstract The Gas Phase Photoemission beamline at Elettra has been commissioned with outstanding success. All photoabsorption spectra taken between 90 and 900 eV have shown resolution which is equal to or higher than any published spectra. The monochromator is a variable angle spherical grating instrument (plane mirror and grating between entrance and exit slits), with an undulator as the source. Some of the problems encountered in commissioning and their solutions are discussed. In particular the calibration is complicated by the fact that an infinite number of angle pairs of the mirror and grating exist for a particular photon energy, whereas only one angle pair gives the highest resolution. A second problem is that the resolution is so high that above about 80 eV, it is much smaller than the lifetime broadening of any known absorption resonance, making any determination of resolution difficult. The experimental chambers available for users are described together with some examples of spectra which have been taken. 1999 Elsevier Science B.V. All rights reserved. Keywords: Photoelectron spectroscopy; Monochromator; Beamline
1. Introduction This paper describes the characteristics of the Gas Phase Photoemission beamline at Elettra with a view to giving potential users the information they need to plan an experiment. The beamline is now open to users and has been designed for atomic and molecular physics and chemistry experiments. The design parameters included the possibility of working with gases, vapours, metastable species, and of perform*Corresponding author. E-mail address: [email protected] (K.C. Prince)
ing photoabsorption, photoemission, and coincidence measurements. The main requirements for the monochromator were high photon resolution over a wide energy range, stable spot size and high flux. The design goal for the energy range was set as 20–800 eV. The resolution was required to be significantly better than the natural widths of the main atomic and molecular excitations in this range, and so the goal for the resolving power was 10 000. A stable spot size means that changes in spot size, shape and position as a function of energy were to be minimised. This implied the use of a fixed exit slit
0368-2048 / 99 / $ – see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 98 )00381-8
960
R.R. Blyth et al. / Journal of Electron Spectroscopy and Related Phenomena 101 – 103 (1999) 959 – 964
and re-focusing system. The latter provides two additional advantages: firstly, the experimental station, which operates at pressures of 10 25 –10 26 mbar, is further away from the monochromator optical elements. Secondly a circular focus can be provided, which is important when aligning several spectrometers and / or different excitation sources on the same target volume. For the end-stations, the project foresaw the construction of two chambers available for users, and also to allow for the possibility that users bring their own chamber. The Multi-coincidence Chamber is optimised primarily to measure electron–electron coincidences with an array of analysers. The second end station, the Angle Resolved Photoemission Chamber, is designed for photoemission spectroscopies with vapours and a wide range of gases, including unstable and radical species which are usually corrosive. Both chambers are available to users, and versatile support systems have been built for users who bring their own chambers. The beam exits at an angle of 4 degrees with respect to the horizontal, at a height of 180 cm. Both chambers have an exit port to which a windowless gas cell can be attached. The cell can be filled with gas independently of the sample in the main chamber, and used for photoabsorption spectroscopies. At the back of the gas cell, a calibrated photodiode can be inserted. Thus under suitable conditions, the user can measure simultaneously his / her spectrum; an absorption spectrum which gives a very precise calibration and a check of the monochromator resolution; and an I0 spectrum for normalisation to the absolute flux.
2. Results
2.1. Light source The light source of the beamline is an undulator of period 12.5 cm in section 6.2 of the Elettra storage ring; details can be found on the Elettra home page [1]. The lowest photon energy produced with the ring running at 2 GeV is 16 eV, although we have not yet tested the monochromator at energies this low. The lower curve of Fig. 1 shows a typical absorption spectrum with fixed gap: both the ion yield and the
Fig. 1. Photoabsorption spectra of the He doubly excited states below the N52 threshold. Lower curve: ion yield spectrum, normalised to the flux. Centre curve, ‘Undulator Gap 63.5’: photodiode signal. Inset: high resolution scan of the peaks 121 to 221. Resolution: 2.2 meV.
photodiode spectra are shown. The photodiode curve shows weak absorption due to the gas in the gas cell and indicates that absorption is still in the linear regime. The overall shape is determined by the variation of the flux due to the undulator output, and the full width at half maximum is about 4% of the peak energy. In principle, the higher harmonics should be narrower, but effects such as finite acceptance of the photon beam, finite emittance and energy spread mean that the width is still about 4%. Most energy scanning spectroscopies are still done with fixed gap, although the gap of the undulator can be adjusted over a wide range manually or by software. Changing the gap is a relatively slow process (about 2 min), due to the time taken for software overheads. We are presently developing software to permit automated gap scanning. Table 1 shows the preliminary results of a series of measurements of flux at the gas cell after the experimental chamber.
2.2. Optical layout and monochromator The optical concept has been published previously [2] as well as the first results [3]. In brief, there are two mirrors before the entrance slit; a plane mirror and a set of five gratings (Table 2) between the
R.R. Blyth et al. / Journal of Electron Spectroscopy and Related Phenomena 101 – 103 (1999) 959 – 964
961
Table 1 Flux and resolution at selected energies Energy (eV)
Resolving power
Flux (photons / s / 100 mA)
45 65 86 245 401 540 680
.25 000 .28 000 – 12 200 .12 000 10 000 10 000
6.3310 10 2.2310 11 1.5310 11 1.5310 10 1.1310 10 2.0310 10 3.0310 9
entrance and exit slits; and two re-focusing mirrors afterwards. The first mirror, the plane Switching Mirror is used to steer the beam vertically so that it enters the entrance slit. This is important at the minimum slit width of 10 mm, but is a minor adjustment that is done once or twice a day. At large slit openings this is not normally necessary. The entrance slit is usually used at values between 10 and 30 mm, through which about 95% of the flux passes. The monochromator can be scanned in two different modes. In energy scan mode, both the mirror and grating of the monochromator are scanned simultaneously. This is the preferred mode of operation at low to medium energy resolving power, up to about 4000. The scanning of two mechanical systems, mirror and grating, doubles the contribution of mechanical errors to the degradation of the resolution. To avoid this, at high resolving power (circa 10 000) fixed mirror scanning is preferred. The mirror is set to the average value for the range of energy to be scanned, and only the grating is scanned. Typical spectra at high resolution are shown in the inset of Fig. 1, and in Fig. 2; see also Refs. [3–5]. This mode gives the highest resolving power, from 30 000 at 48 eV to 10 000 at 540 eV. It Table 2 Gratings and energy ranges Grating number
Lines / mm
Energy range (calculated)
1 2 3 4 5
400 800 1200 1200 1200
20–50 40–90 80–180 160–430 360–1000
Fig. 2. Photoabsorption spectra of the Ne, 2s 21 np states. Lower curve shows a broad scan; inset shows the peaks n516 to 27. Resolution: 1.6 meV.
has the disadvantage that the defocus contribution to the resolution changes slightly over the scan. However at high resolution, the required step size is usually small and so the scan width is limited. If scans are confined to about 2% of the average energy, the resolution is approximately constant as the defocusing term is sufficiently small that it does not affect resolution. All photoabsorption spectra taken between 90 and 900 eV have shown resolution which is higher than or at least equal to published spectra [3–5]. At the nitrogen 1s→p absorption resonance, the ratio of the third peak height to the first minimum is often taken as a criterion of resolution. We obtain a value of 0.6260.01, considerably lower than most published values [6–12]. For instance, Domke et al. [9] used an SX700 monochromator and obtained a value of approximately 1 in first order, although their spectrum taken with second order light and masking of most of the grating was better. Watanabe et al. [10] used a variable line space grating and their Fig. 6 seems to give a value of about 0.7. They obtained an estimated resolution of 40.9 meV for an intrinsic line width of 117 meV. We estimate our resolution to be 35 meV for a line width of 115 meV. At low energies, the resolution is also very high, being approximately 1.6 meV and 2.2 meV at 45 and 65 eV (measured using neon and helium photo-
962
R.R. Blyth et al. / Journal of Electron Spectroscopy and Related Phenomena 101 – 103 (1999) 959 – 964
absorption spectra, respectively). These results were obtained using first order light with no masking of the grating and the minimum slit settings, 10 mm. A higher resolution of 1.0 meV at 64 eV has been obtained on beamline 9.0.1 at the Advanced Light Source [13,14]. A few spectra taken with second order light indicate that our resolution can be improved further. The energy calibration has been carried out by measuring a series of absorption spectra at different mirror settings and selecting the value which maximises resolution. This provides a calibration point with a value of the photon energy, mirror and grating settings. A mathematical model has been developed [15] into which this data point is inserted, and then the functional describing the calibration curve is solved numerically. At least three data points per grating are necessary. Recalibration after maintenance (which may change certain mechanical constants of the system) is simpler as it is assumed that the new calibration curve is the same as the previous one but that the grating and mirror are displaced in angle with respect to the old value. This assumption works reasonably well and a grating can be recalibrated with only two calibration points. A number of problems were encountered during commissioning which are still being resolved. The resolution is so high that the mechanical scanning (which is sometimes in steps as small as 100 nm) is working at its limit. In particular stick-slip behaviour and small jumps are observed which are probably due to the surface finish of the components of the drive screw. We are planning to install a piezo drive later this year (1998) to resolve this problem. The original specification was for a range up to 800 eV and so the high energy grating was coated with nickel, to maximise reflectivity up to this energy. At the Ne K edge, the Ni L 2,3 absorption reduces the reflectivity to a few percent. In spite of this we were able to measure the best resolved Ne absorption spectra so far reported [4]. Presently we are having the grating re-coated with gold, and it will be installed in October 1998. The low energy grating (20–40 eV) suffered from excessive high order contributions and this is also being re-coated, this time with Al / MgF 2 , and it will also be installed in October. In addition, filters will be inserted to reduce higher orders at other energies.
2.3. Experimental stations 2.3.1. Multicoincidence chamber The 800 mm I.D. multicoincidence end-station is lined with a 2-mm-thick m-metal shield, additionally screened by three pairs of Helmholtz coils; the residual magnetic field is less than 10 mG. Two independently rotatable arrays of electrostatic electron energy analysers are housed in the chamber. Seven spectrometers are mounted on a frame that rotates in the plane of the polarisation vector of the incident radiation, while three other spectrometers are mounted on a smaller frame rotatable around the same axis. This latter array enables measurements out of the polarisation plane, to study, for example, non-dipole contributions to the photoionization cross-section. All 10 analysers are mounted at angular intervals of 308. The spectrometers are composed of two fourelement lenses (194 mm long) that focus the photoelectrons from the target region onto the entrance slits of the hemispherical deflector. The lens system [16] can be operated in two modes: a low resolution mode characterised by a DE /Ek 510 22 , where Ek is the kinetic energy of the photoelectrons, and an angular acceptance in the dispersion plane of 638. This mode is most suited for coincidence experiments. The second mode is a high resolution mode with DE /Ek 510 24 and an angular acceptance of 60.58 used for non-coincidence photoelectron and Auger electron spectroscopies. The lens stack can work with retarding ratios of 2–5 and 5–100 in the low- and high-resolution modes, respectively. The mean radius of the hemispherical deflector is 33 mm and the gap 9.9 mm. These values have been chosen in order to obtain a maximum time spread of 2 ns in the trajectories of 100 eV electrons in the dispersing element. An example of the performance of the analyser when operated in the high-resolution mode is shown in Fig. 3, where the spectrum of Ar 1 (3p 21 ) taken at 244.4 eV incident photon energy, the energy of the 2p 3 / 2 →4s resonance, is shown. The pass energy was 3 eV, corresponding to a retarding ratio of 76. The entrance and exit slits were 1 and 4 mm, respectively. The FWHM of the measured peaks was 65 meV giving a resolution of 50 meV after the deconvolution of the contribution from the incident light. This
R.R. Blyth et al. / Journal of Electron Spectroscopy and Related Phenomena 101 – 103 (1999) 959 – 964
963
Fig. 4. Auger electron–photoelectron coincidence spectrum (squares) of Kr at hn5135 eV, when a 3d 3 / 2 photoelectron is detected. Also shown is the non-coincidence M 4,5 N 23 N 23 Auger spectrum (dots), measured simultaneously. Dwell time: 1 h per point.
Fig. 3. Ar 1 (3p 21 ) photoelectron spectrum at 244.4 eV incident energy. A pseudo-Voigt function (full line) has been fitted to the data. Pass energy 3 eV; FWHM and analyser resolution 65 and 50 meV, respectively.
shows that despite the small size of the hemispheres good energy resolution can be achieved with a reasonable count rate. Fig. 4 displays the results of an Auger electron– photoelectron coincidence experiment to study the formation and decay of the Kr 1 (3d 21 ) states. This illustrates one of the main advantages of coincidence experiments: the possibility of distinguishing the components due to different initial holes in the Auger spectrum. In this case the M 4 N 23 N 23 ( 2 D) line 3,1 overlaps completely with the M 5 N 23 N 23 ( P) lines, so its contribution to the Auger spectrum cannot be evaluated directly without a coincidence experiment.
2.3.2. Angle resolved photoemission spectroscopy chamber The experimental chamber of the ARPES end station consists of a 500-mm I.D. cylindrical vessel lined with a double m-metal shield. Gaseous and volatile liquid samples can be introduced into the ionisation region through a hypodermic needle
mounted on an XYZ manipulator. Solids can be vaporised in a high temperature, anti-inductively wound oven, in which temperatures up to 11008C can be obtained and maintained for several hours. The oven and the ionisation volume are enclosed in a cooled jacket designed to minimize contamination. An efficient cryo and turbo pumping system and a quartz capillary mounted between the beamline and the chamber prevent contamination of the beamline. These design features allow the study of aggressive gases and vapours. Two VSW hemispherical photoelectron energy analysers (50 mm mean radius) can be mounted with a mutual angle of 908 on a turntable. The fourelement lens system of each analyser has an acceptance angle of approximately 638 and has been modified to suit high temperature experiments. Pass energies from 1 to 50 eV can be selected. A resolution of |50 meV (FWHM) has been obtained for 25 eV kinetic energy electrons, using 2 eV pass energy, and 180 meV resolution for 130 eV kinetic energy electrons with 10 eV pass energy. The modular design of the chamber and the large side flanges also allow users’ spectrometers to be mounted. Fig. 5 shows the photoelectron spectrum of SnBr 2 obtained using ARPES. An effusive beam of SnBr 2 molecules was produced by heating the solid sample to |3208C. The full valence PE spectrum, as well as the intense core Sn 4d 21 ionisation, are displayed.
964
R.R. Blyth et al. / Journal of Electron Spectroscopy and Related Phenomena 101 – 103 (1999) 959 – 964
ni Sostero of the Soft X-ray Optical Laboratory and Luca Romanzin and Gilio Sandrin for technical assistance. JS was supported by EC contract CIPACT94-0217, and MV by EC contract IC15 CT97700.
References
Fig. 5. SnBr 2 photoelectron spectrum recorded at the magic angle with hn550.5 eV and 5 eV pass energy. Photon flux: about 5310 12 photons / s, resolution|1000 (entrance / exit slit550 / 100 mm). The full valence spectral region and the Sn 4d 21 core ionisations are shown, as well as the Br 3d 21 core ionisations from second order light.
One of the interests of this investigation was to study electron correlation effects in valence photoionisation processes. These effects usually produce low intensity spectral features, which are difficult to detect in high temperature systems because of the low target density. The high photon flux reveals new features due to electron correlation in the spectrum in the region of the a 21 and Br 4s 21 valence ionisation, 1 in agreement with recent CI ab-initio calculations. A detailed account of these experimental and theoretical results will be given elsewhere [17].
3. Conclusions The Gas Phase Photoemission beamline at Elettra has produced outstanding results and is now ‘open for business’. The end stations are flexible and allow a wide variety of experiments to be done in atomic and molecular physics and chemistry.
Acknowledgements We thank our colleagues at Elettra for their support, and particularly Daniele Cocco and Giovan-
[1] www.elettra.trieste.it [2] P. Melpignano, S. Di Fonzo, A. Bianco, W. Jark, Rev. Sci. Instrum. 66 (1995) 2125. [3] K.C. Prince, R.R. Blyth, R. Delaunay, M. Zitnik, J. Krempasky, J. Slezak, R. Camilloni, L. Avaldi, M. Coreno, G. Stefani, C. Furlani, M. de Simone, S. Stranges, J. Sync. Rad. 5 (1998) 565. [4] M. Coreno, L. Avaldi, R. Camilloni, M. de Simone, J. Karvonen, R. Colle, S. Simonucci, Phys. Rev. A, submitted. [5] K.C. Prince, M. Vondracek, J. Karvonen, M. Coreno, R. Camilloni, L. Avaldi, M. de Simone, these proceedings. [6] C.T. Chen, Y. Ma, F. Sette, Phys. Rev. A 40 (1989) 6737. [7] C. Quaresima, C. Ottaviani, M. Matteucci, C. Crotti, A. Antonini, M. Capozi, S. Rinaldi, M. Luce, P. Perfetti, K.C. Prince, C. Astaldi, M. Zacchigna, L. Romanzin, A. Savoia, Nucl. Instr. Methods A 364 (1995) 374. [8] D. Cvetko, L. Floreano, R. Gotter, M. Malvezzi, L. Marassi, A. Morgante, G. Naletto, A. Santaniello, G. Stefani, F. Tommasini, G. Tondello, A. Verdini, Proc. SPIE 3150 (1997) 86. [9] M. Domke, T. Mandel, A. Puschmann, C. Xue, D.A. Shirley, G. Kaindl, H. Petersen, P. Kuske, Rev. Sci. Instrum. 63 (1992) 80. [10] M. Watanabe, A. Toyoshima, Y. Azuma, T. Hayaishi, Y. Yan, A. Yagishita, Proc. SPIE 3150 (1997) 277. [11] K.J. Randall, J. Feldhaus, W. Erlebach, A.M. Bradshaw, W. Eberhardt, Z. Xu, Y. Ma, P.D. Johnson, Rev. Sci. Instrum. 63 (1992) 1367. ¨ O.-P. Sairanen, A. Naves de Brito, [12] S. Aksela, A. Kivimaki, E. Nommiste, Rev. Sci. Instrum. 66 (1995) 1621. [13] K. Schulz, G. Kaindl, M. Domke, J.D. Bozek, P.A. Heimann, A.S. Schlacter, J.M. Rost, Phys. Rev. Lett. 77 (1996) 3086. [14] G. Kaindl, K. Schulz, P.A. Heimann, J.D. Bozek, A.S. Schlacter, Synchrotron Rad. News 8 (1995) 29. ´ A. Bianco, A. Abrami, R. [15] J. Krempasky, R. Krempaska, ` J. Karvonen, M. Coreno, M. de Simone, Puliese, F. Bille, Proc. SPIE 3150 (1998) 76. [16] D. Cvetko, L. Floreano, R. Gotter, M. Malvezzi, L. Marassi, A. Morgante, G. Naletto, A. Santaniello, G. Stefani, F. Tommasini, G. Tondello, A. Verdini, Proc. SPIE 3150 (1997) 86. [17] M. de Simone, R.C. Richter, M. Alagia, S. Stranges, P. Decleva, A. Lisini, M. Coreno, K.C. Prince, C. Furlani, in preparation.