- Email: [email protected]
A new multidetection spectrometer for studies of angle-resolved electron distributions
Journal of Electron Spectroscopy and Related Pherwmenu, 52 (1990) ‘787-796
Elsevier Science Publishers B.V., Amsterdam -Printed
787
in The Netherlands
A NEW MULTIDETECTION SPECTROMETERFORSTUDIESOF ANGLE-RESOLVED ELECTRONDISTRIBUTIONS
DROY’, D.TREMBlAY’, B.TREMBLAY’, AND D.DUBt2 1LPAM, DBpartement de physique, Pavillon Vachon, Universit6 Laval, Quebec, Qc, Gl K 7P4, Canada
2D6partement des sciences, Universit6 du Quebec ZI Hull, Hull, Qc, J8X 3X7, Canada SUMMARY A novel multidetection electron spectrometer is presented, designed for the parallel measurements of angular distribution in gas-phase electron scattering experiments. It is called MAPDESS. The same design is suitable for angle-resolved photoelectron spectroscopy. The main feature is its geometry given by the analyzer: a truncated spherical mirror symmetric about the scattering center and whose ring-shaped exit is coupled to 19 channel electron multipliers, thereby allowing parallel multi-angle detection. The angular resolution is k 3”, with an overall energy resolution of about 25 meV in some measurements. As examples, various angle dependence measurements are presented, such as electron energy loss spectra for the low-lying states of helium, Feshbach resonances in the differential elastic and inelastic cross sections of rares gases, resonant vibrational excitation in nitrogen, along with spectra of ejected electrons following the autoionization in argon.
1.
INTRODUCTION
The interests for angular measurements have been demonstrated in many fields of collision physics and in spectroscopy, for the study of surfaces and gas phase atoms and molecules as well. Since repeated measurements at several angles are time consuming and sometimes less reliable because of drifting parameters, there is a great demand for new spectrometers capable of simultaneous multi-angle detection. New electron spectrometers indeed have been designed during the last decade for that purpose (see Ref. 1 and references therein). We present a novel electrostatic electron spectrometer first designed for the parallel measurement of angular distributions in gas phase electron scattering experiments. Accordingly it was called MAPDESS for The same multi-angle parallel detection electron scattering spectrometer. design is suitable for angle-resolved photoelectron spectroscopy and could be adapted for applications of some electron spectroscopies to surface analysis.
036%2043/90/$03.50
0 1990 Elsevier Science Publishers B.V.
2.
DESCRIPTIONOFTHE SPECTROMETER
2.1 General descriotion Figure 1 presents a schematic view of the MAPDESS. The unusual configuration of this spectrometer is determined by the special symmetry of the truncated spherical mirror analyzer surrounding the collision center. Starting from the symmetry axis, the right part is in the plane of the The left part is symmetrical about the axis and incoming electron beam. The various parts shown are represents one of the 19 scattering planes. the electron gun (EG), the monochromator (M), the monochromator exit slit (MO), the three electron lenses (LI, Ls, Ls), the electron collimator (LCA), the collision center (C), the annular lens (AL), the analyzer input aperture (Al), the analyzer output aperture (AO), the exit aperture collimator (C3), one of the 19 channel electron multipliers (CEM), and the gas inlet (GI).
Fig. 1. Schematic view of the MAPDESS. Starting from the symmetry axis, the right part is in the plane of the incoming beam. The left part is symmetrical about the axis and represents one of the 19 scattering planes.
. . 2.2 The electron ootical svstem fo r the ororectrle electron be= The electron gun includes an extractor electrode, a three-electrode lens, pairs of deflection plates, and a collimator for proper limitation of the angular divergence at the entrance of the monochromator (2). The latter is a standard 127” cylindrical deflector (CDA) with a main radius Rm of 30 mm. The width ASm of the slits is 0.3 mm and the maximum acceptance half-angle am is 4.2”. In an energy disperser as the CDA (2), the base energy resolution AEe (width of the transmission function AEa/E,-(2MAS/R, + C,c?)ID, where
Eo is the
pass
energy,
at the base) is given by
PI Ro,
the
characteristic
length,
M,
the
magnification, Ca, the second-order angular aberration, and D, the energy dispersion. The effective (half-maximum) energy resolution AE/Eo is given by
AVEo=(MAS/Ro+C,a2/4)lD.
PI
For the 127O-CDA, one has f&l, Ca=l.33, and D=l. This yields for the monochromator (AEslEo)m = 0.027 and (AEIEo)m = 0.012. These values could be determined from the nomographs calculated by Dub6 and Roy (3) as well. Between the exit of the monochromator and the collision center, the electron beam must be transported through the analyzer (see Fig. 1) on a In order to yield high transmission and all the desirable fairly long path. flexibility, the lens system has been designed with three focusing elements ( LI, L2, and Ls in Fig. 1) and two series of deflectors (dr and d2) for LI is a two-electrode lens and is in two perpendicular planes. deviation used to bend the trajectories toward the other lenses. L2 and L3 are both More details on the characteristics, three-part asymmetric voltage lenses. the performance, and the modes of operation of this lens system are given in Ref. 1.
Fig. 2. Panoramic view of the analyzer scattering zone. Slits C2 are the scattering angle (0) selectors followed by the O-collimator (0-C); FCS is the Faraday-cup shield; the lines with the arrows indicate the view planes shown in Fig. 1.
. . 2.3 The collrsron zone The collision zone is shown in Fig. 2. The gas beam is introduced with a hypodermic needle (1$=0.41 mm i.d.) without being skimmered or Thus the highest gas density is found differentially pumped along its path. at the very exit of the outlet and falls .slowly to zero around the center. Some scattering events may then occur in the diffuse gas around the center.
790
The direct-to-diffuse gas ratio of the contributions to the scattering intensity was found to be higher than 7-10 in typical measurements at a near 90” scattering angle . The ratio at extreme angles (near 18” and 162”) is reduced to 2-3 (1). Figure 2 shows that, before entering the analyzer, the scattered electrons have to pass through a slit (e.g. C2) defining a specific scattering direction 6. The angle between two neighboring slits is 8”, and each is 2.2” To each slit corresponds a discrete CEM beyond the exit of the wide. analyzer, as shown in Fig. 1. The slits are followed by an additional diaphragm system, the 6 collimator. This arrangent determines the angular acceptance of the electron detectors by insuring that the electrons scattered off-center by the background gas and crossing a slit at 6 cannot reach neighboring detectors placed at 8 + 8”. The 6 collimator has a rather complicated design in order to reduce the probability that electrons scattered from the walls of this baffle reach the exit. The electrons scattered off the center C are focused on the analyzer entrance slit (Al) at the desired energy by means of an annular zoom lens. The unscattered electrons are collected in a deep Faraday cup enclosed in a cylinder (FCS in Fig. 2) at the same potentialas the surrounding elements. The cup consists in a small tube ($=2.16 mm i.d.). 2.4 The analvzer As shown in Fig. 1, the analyzer itself is a truncated spherical mirror with an entrance angle Q0 of 40” and an exit angle $f of 101.6”, for an effective deflection of 61.6”. This geometry allows energy analysis and detection all around the collision center. It is based on a previous study [4] which generalized the properties of the spherical mirror, considering an electron source anywhere in the inner sphere and starting from the basic geometry proposed by Sar-El [5]. The diameter of inner and outer spheres are 2Ro=130 mm and 2R1= 260 mm, respectively. This gives direct energy reading from the potential difference AV applied to the spheres (4), since Eo is given by EoeAV/2(1-Ro /R1). The width of the slits are 0.43 mm (ASa) at the entrance (Al) and 0.37 mm (MASa) at the exit (AO), with a magnification of 0.86. The exit aperture is at the position Rf = 103 mm. The half-angle of the accepted beam (aa ) is 1.4“ in the radial analyzer plane. With Ca=3.82 and D12.2, one calculates (AEaIEo)a = 0.0062 and (AEIEo)a = 0.0028 from Eqs. [l] and [2]. The detection of the energy selected electrons is achieved through 19 CEM’s, one associated with each of 19 apertures around the collision zone, beginning at the scattering angle 18”, and then one every 8” up to 162”. 2.5 The electronics The power supply system for the MAPDESS was essentially inspired by the design proposed by Katz and al. (6). The CEM’s are home made. Operated simultaneously, they are coupled with 19 independent AMPTEK
791
101 amplifier discriminators. The data acquisition is performed of a home made interface board which involves twenty 32-bit All is controlled by a eight 1Pbit DAC, and two 12-bit ADC. 512-K computer. The software used for the control, display, is home made, exploiting mathematical data processing interactive Macintosh philosophy. 3.
by means counters, Macintosh and basic the highly
PERFORMANCESIN ANGLE-RESOLVEDMEASUREMENTS
3.1 -and res&&n The energy resolution of the MAPDESS was determined experimentally (1). For the analyzer, the obtained value is (AE/Eo)a= 0.0023. This better than predicted resolution is attributed to the filtering effect of the exit diaphragm (C3 in Fig. l), added for a better elimination of the stray electrons. The energy resolution of the monochromator was 21 meV, giving an overall resolution of 25 meV in electron energy loss spectra (with a pass energy of 7 eV in the analyzer). This determination showed that the current monochromator limits the performance of the MAPDESS and a new one is under construction. The angular resolution of this instrument was investigated through the measurement of the strong dip appearing in the elastic differential cross section of argon, at an incident electron energy of about 50 eV. By comparison with the accurate measurements carried out by Srivastava et al. (7), with an angular resolution of f 2”, it was determined that the angular resolution of the MAPDESS is f 3”. For this operation the detection
19.8
20.0
20.2
20.4
20.6
20.8
21.0
21.2
21.4
Energy Loss (eV) Fig. 3. Electron energy loss spectra in helium measured at various scattering angles at an incident electron energy of 29.6 eV. The peaks correspond to the low-lying n=2 states 3S, ‘S, 3P, ’ P. The spectra are normalized with respect to the 3P peak.
792
efficiencies of the discrete detectors were calibrated using the known cross section for helium at the same energy (8). 3.2 moles of measurements Figures 3 to 7 show examples of measurements involving angle dependence For the in various electron scattering (or electron ejection) processes. sake of clarity, most of these measurements are not presented with the 19 recorded curves and have been normalized since the involved cross sections often exhibit dramatic variations of orders of magnitude. Electron energy loss spectra in helium are presented in Fig. 3, for the energy range 19.6 to This shows the relative differential sections for the excitation of 21.4 eV. the four n-2 low-lying states 1~2s (Z1 S) and ls2p (3*1P) for impact energy of 29.6 eV. These observations are in good agreement with the measurements reported by Hall et al. (9) and Trajmar (10). Given the isotropic character of the 23P cross section at this energy (9), this peak was used for the normalization; the corresponding cross section is about 2X 10“ g cm2/sr.
Ar
0.8
0.6
Electron
energy
(eV)
Fig.4. The 3p54s2 (2P) resonances in argon, observed in the differential elastic cross section at various scattering angles. Each curve is normalized. For the normalization, the average between the ends of each curve is fixed at one.
793
Figure 4 shows the well known Feshbach resonance appearing just It is above 11 eV in the elastic differential cross section of argon. From these assigned to the states 3p54s2 (2P J), with &lf1/2. measurements, it was determined that the line width of this resonance is 2.3f0.2 meV (11).
He (1~2~) 3-P
57.5
59.0
58.5
59.0
Energy Fig. 5. The 2P and 2D resonances associated to the first doubly excited states in helium, observed in the inelastic channel of the state ls2p (3P). For the normalization of each curve, the average between the ends is fixed at one. The energy scale is the incident electron energy in eV.
794
In helium such resonances are associated to the first doubly excited states around 58 eV. Their assignments are 2s22p (2P) and 2s2p2 (2D), located at about 57.2 and 58.3 eV, respectively (12). The decay of these short-lived states can be observed in the inelastic channels corresponding to the excitation cross sections of the n=2 low-lying (singly excited) Never before, to our knowledge, a measurement was reported for states. this observation in the weak 3P channel, as shown in Fig. 5. The 2P resonance exhibits dramatic variations and an unusual strength at large angles.
24.5
25.0
25.5
26.0
26.5
Energy
27.0
27.5
28.0
28.5
(eV)
Fig. 6. Ejected electron spectra following the autoionization by electron impact in argon, measured at angles of 16”, 50”, 62”,122”, and 162O, with a constant residual energy of the projectile electrons of 10 eV. No normalization has been carried out. The energy scale is actually the incident electron energy minus the residual energy.
The MAPDESS can also be used to study ejected electron spectra following the autoionization process by electron impact. Figure 8 shows the case of argon, in the region of the states 3s3p4 nl, corresponding to the range of about 24.5 to 29 eV. Spectra are presented for five angles: 18O, 50”, 82’, 122”, and 162”, at a constant residual energy of 10 eV. Each feature appears double, due to the spin-orbit splitting of the ground state of Ar+ involved in the process. One can recognize the electrons ejected from the 4s level around 25 eV, from the 4p level around 26.5 eV, and from
795
the 3d level around 27.5 eV (13). Subsequent features are related to higher members of the same series, overlapping with features due to doubly excited states. Though actually present, the angular variations are weak in this case. The last example features resonances in nitrogen observed in a vibrational channel. Figure 7 shows the differential cross section for the excitation of the v=O to v-l vibration of the ground state. The two peaks correspond to the, decay of the two first vibrational levels of the resonant state 2Zg+ of N2- located at 11.48 eV (14). Since a (r wave is involved in the process, one expects no angular variation for the strength of the peaks. Still, variations are observed, particularly at large angles. 1
.. a. . .
11.25
18O
11.50
Energy
11.75
12.00
(eV)
Fig. 7. Differential cross section of v=O to v=l electron impact excitation of the ground state of N2, measured at three different scattering angles.
The authors their assistance. FCAR du Quebec.
1 2 3 4 5 6 7
are grateful to Dr. A. Adnot and to the technical staff for This work was supported by NSERC-Canada and Fonds
D. Tremblay, D. Dub& and D. Roy, Rev. Sci. Instrum.. 60 (1989) 879-885. D. Roy and J. D. Carrette. in : H. lbach (Ed.), Electron Spectroscopy for Surface Analysis, Vol. 4 of Topics in Current Physics, Springer, Berlin, 1977, pp. 13-58. D. Dub6 and D. Roy, Nucl. Instrum. Methods, 201 (1982) 291-301. D. Tremblay and D. Roy, Nucl. Instrum. Methods, 220 (1984) 270-287. H. 2. Sar-El, Nucl. Instrum. Methods, 42 (1966) 71-76. J. E. Katz, P. W. Davies, J. E. Crowell, and G. A. Somorjai, Rev. Sci. Instrum. , 53 (1986) 785-789. S. K. Srivastava. H. Tanaka, A. Chutjian, and S. Trajmar, Phys. Rev. A, 23 (1981) 2156-2166.
796
8 9 10 11
12 13 14
D. F. Register, S. Trajmar, and S. K. Srfvastava, Phys. Rev. A, 21 (1980) 1134-l 151. R.I. Hall, G. Joyez, J. Mazeau, J. Reinhardt, and C. Bchermann, J. Phys., 34 (1973) 827-843. S. Trajmar, Phys. Rev. A, 8 (1973) 191-203. D. Dub& D. Tremblay, and D. Roy, in: A. Salin (Ed.), Third European Conference on Atomic and Molecular Physics. Book of Abstracts, Bordeaux, 3-7 April 1989, European Physical Society, Bordeaux, 1989, p. 154. D. Roy, Phys. Rev. Lett., 38 (1977) 1062-1065. D. Roy, A. Poulin, M.J. Hubin-Franskin, and J. Delwiche, J. Electron Spectrosc. Relat. Phenom., 19 (1980) 101-114. H.G.M. Heideman, C.E. Kuyatt, and G.E. Chamberlain, J. Chem. Phys., 44 (1966) 355361.
Elsevier Science Publishers B.V., Amsterdam -Printed
787
in The Netherlands
A NEW MULTIDETECTION SPECTROMETERFORSTUDIESOF ANGLE-RESOLVED ELECTRONDISTRIBUTIONS
DROY’, D.TREMBlAY’, B.TREMBLAY’, AND D.DUBt2 1LPAM, DBpartement de physique, Pavillon Vachon, Universit6 Laval, Quebec, Qc, Gl K 7P4, Canada
2D6partement des sciences, Universit6 du Quebec ZI Hull, Hull, Qc, J8X 3X7, Canada SUMMARY A novel multidetection electron spectrometer is presented, designed for the parallel measurements of angular distribution in gas-phase electron scattering experiments. It is called MAPDESS. The same design is suitable for angle-resolved photoelectron spectroscopy. The main feature is its geometry given by the analyzer: a truncated spherical mirror symmetric about the scattering center and whose ring-shaped exit is coupled to 19 channel electron multipliers, thereby allowing parallel multi-angle detection. The angular resolution is k 3”, with an overall energy resolution of about 25 meV in some measurements. As examples, various angle dependence measurements are presented, such as electron energy loss spectra for the low-lying states of helium, Feshbach resonances in the differential elastic and inelastic cross sections of rares gases, resonant vibrational excitation in nitrogen, along with spectra of ejected electrons following the autoionization in argon.
1.
INTRODUCTION
The interests for angular measurements have been demonstrated in many fields of collision physics and in spectroscopy, for the study of surfaces and gas phase atoms and molecules as well. Since repeated measurements at several angles are time consuming and sometimes less reliable because of drifting parameters, there is a great demand for new spectrometers capable of simultaneous multi-angle detection. New electron spectrometers indeed have been designed during the last decade for that purpose (see Ref. 1 and references therein). We present a novel electrostatic electron spectrometer first designed for the parallel measurement of angular distributions in gas phase electron scattering experiments. Accordingly it was called MAPDESS for The same multi-angle parallel detection electron scattering spectrometer. design is suitable for angle-resolved photoelectron spectroscopy and could be adapted for applications of some electron spectroscopies to surface analysis.
036%2043/90/$03.50
0 1990 Elsevier Science Publishers B.V.
2.
DESCRIPTIONOFTHE SPECTROMETER
2.1 General descriotion Figure 1 presents a schematic view of the MAPDESS. The unusual configuration of this spectrometer is determined by the special symmetry of the truncated spherical mirror analyzer surrounding the collision center. Starting from the symmetry axis, the right part is in the plane of the The left part is symmetrical about the axis and incoming electron beam. The various parts shown are represents one of the 19 scattering planes. the electron gun (EG), the monochromator (M), the monochromator exit slit (MO), the three electron lenses (LI, Ls, Ls), the electron collimator (LCA), the collision center (C), the annular lens (AL), the analyzer input aperture (Al), the analyzer output aperture (AO), the exit aperture collimator (C3), one of the 19 channel electron multipliers (CEM), and the gas inlet (GI).
Fig. 1. Schematic view of the MAPDESS. Starting from the symmetry axis, the right part is in the plane of the incoming beam. The left part is symmetrical about the axis and represents one of the 19 scattering planes.
. . 2.2 The electron ootical svstem fo r the ororectrle electron be= The electron gun includes an extractor electrode, a three-electrode lens, pairs of deflection plates, and a collimator for proper limitation of the angular divergence at the entrance of the monochromator (2). The latter is a standard 127” cylindrical deflector (CDA) with a main radius Rm of 30 mm. The width ASm of the slits is 0.3 mm and the maximum acceptance half-angle am is 4.2”. In an energy disperser as the CDA (2), the base energy resolution AEe (width of the transmission function AEa/E,-(2MAS/R, + C,c?)ID, where
Eo is the
pass
energy,
at the base) is given by
PI Ro,
the
characteristic
length,
M,
the
magnification, Ca, the second-order angular aberration, and D, the energy dispersion. The effective (half-maximum) energy resolution AE/Eo is given by
AVEo=(MAS/Ro+C,a2/4)lD.
PI
For the 127O-CDA, one has f&l, Ca=l.33, and D=l. This yields for the monochromator (AEslEo)m = 0.027 and (AEIEo)m = 0.012. These values could be determined from the nomographs calculated by Dub6 and Roy (3) as well. Between the exit of the monochromator and the collision center, the electron beam must be transported through the analyzer (see Fig. 1) on a In order to yield high transmission and all the desirable fairly long path. flexibility, the lens system has been designed with three focusing elements ( LI, L2, and Ls in Fig. 1) and two series of deflectors (dr and d2) for LI is a two-electrode lens and is in two perpendicular planes. deviation used to bend the trajectories toward the other lenses. L2 and L3 are both More details on the characteristics, three-part asymmetric voltage lenses. the performance, and the modes of operation of this lens system are given in Ref. 1.
Fig. 2. Panoramic view of the analyzer scattering zone. Slits C2 are the scattering angle (0) selectors followed by the O-collimator (0-C); FCS is the Faraday-cup shield; the lines with the arrows indicate the view planes shown in Fig. 1.
. . 2.3 The collrsron zone The collision zone is shown in Fig. 2. The gas beam is introduced with a hypodermic needle (1$=0.41 mm i.d.) without being skimmered or Thus the highest gas density is found differentially pumped along its path. at the very exit of the outlet and falls .slowly to zero around the center. Some scattering events may then occur in the diffuse gas around the center.
790
The direct-to-diffuse gas ratio of the contributions to the scattering intensity was found to be higher than 7-10 in typical measurements at a near 90” scattering angle . The ratio at extreme angles (near 18” and 162”) is reduced to 2-3 (1). Figure 2 shows that, before entering the analyzer, the scattered electrons have to pass through a slit (e.g. C2) defining a specific scattering direction 6. The angle between two neighboring slits is 8”, and each is 2.2” To each slit corresponds a discrete CEM beyond the exit of the wide. analyzer, as shown in Fig. 1. The slits are followed by an additional diaphragm system, the 6 collimator. This arrangent determines the angular acceptance of the electron detectors by insuring that the electrons scattered off-center by the background gas and crossing a slit at 6 cannot reach neighboring detectors placed at 8 + 8”. The 6 collimator has a rather complicated design in order to reduce the probability that electrons scattered from the walls of this baffle reach the exit. The electrons scattered off the center C are focused on the analyzer entrance slit (Al) at the desired energy by means of an annular zoom lens. The unscattered electrons are collected in a deep Faraday cup enclosed in a cylinder (FCS in Fig. 2) at the same potentialas the surrounding elements. The cup consists in a small tube ($=2.16 mm i.d.). 2.4 The analvzer As shown in Fig. 1, the analyzer itself is a truncated spherical mirror with an entrance angle Q0 of 40” and an exit angle $f of 101.6”, for an effective deflection of 61.6”. This geometry allows energy analysis and detection all around the collision center. It is based on a previous study [4] which generalized the properties of the spherical mirror, considering an electron source anywhere in the inner sphere and starting from the basic geometry proposed by Sar-El [5]. The diameter of inner and outer spheres are 2Ro=130 mm and 2R1= 260 mm, respectively. This gives direct energy reading from the potential difference AV applied to the spheres (4), since Eo is given by EoeAV/2(1-Ro /R1). The width of the slits are 0.43 mm (ASa) at the entrance (Al) and 0.37 mm (MASa) at the exit (AO), with a magnification of 0.86. The exit aperture is at the position Rf = 103 mm. The half-angle of the accepted beam (aa ) is 1.4“ in the radial analyzer plane. With Ca=3.82 and D12.2, one calculates (AEaIEo)a = 0.0062 and (AEIEo)a = 0.0028 from Eqs. [l] and [2]. The detection of the energy selected electrons is achieved through 19 CEM’s, one associated with each of 19 apertures around the collision zone, beginning at the scattering angle 18”, and then one every 8” up to 162”. 2.5 The electronics The power supply system for the MAPDESS was essentially inspired by the design proposed by Katz and al. (6). The CEM’s are home made. Operated simultaneously, they are coupled with 19 independent AMPTEK
791
101 amplifier discriminators. The data acquisition is performed of a home made interface board which involves twenty 32-bit All is controlled by a eight 1Pbit DAC, and two 12-bit ADC. 512-K computer. The software used for the control, display, is home made, exploiting mathematical data processing interactive Macintosh philosophy. 3.
by means counters, Macintosh and basic the highly
PERFORMANCESIN ANGLE-RESOLVEDMEASUREMENTS
3.1 -and res&&n The energy resolution of the MAPDESS was determined experimentally (1). For the analyzer, the obtained value is (AE/Eo)a= 0.0023. This better than predicted resolution is attributed to the filtering effect of the exit diaphragm (C3 in Fig. l), added for a better elimination of the stray electrons. The energy resolution of the monochromator was 21 meV, giving an overall resolution of 25 meV in electron energy loss spectra (with a pass energy of 7 eV in the analyzer). This determination showed that the current monochromator limits the performance of the MAPDESS and a new one is under construction. The angular resolution of this instrument was investigated through the measurement of the strong dip appearing in the elastic differential cross section of argon, at an incident electron energy of about 50 eV. By comparison with the accurate measurements carried out by Srivastava et al. (7), with an angular resolution of f 2”, it was determined that the angular resolution of the MAPDESS is f 3”. For this operation the detection
19.8
20.0
20.2
20.4
20.6
20.8
21.0
21.2
21.4
Energy Loss (eV) Fig. 3. Electron energy loss spectra in helium measured at various scattering angles at an incident electron energy of 29.6 eV. The peaks correspond to the low-lying n=2 states 3S, ‘S, 3P, ’ P. The spectra are normalized with respect to the 3P peak.
792
efficiencies of the discrete detectors were calibrated using the known cross section for helium at the same energy (8). 3.2 moles of measurements Figures 3 to 7 show examples of measurements involving angle dependence For the in various electron scattering (or electron ejection) processes. sake of clarity, most of these measurements are not presented with the 19 recorded curves and have been normalized since the involved cross sections often exhibit dramatic variations of orders of magnitude. Electron energy loss spectra in helium are presented in Fig. 3, for the energy range 19.6 to This shows the relative differential sections for the excitation of 21.4 eV. the four n-2 low-lying states 1~2s (Z1 S) and ls2p (3*1P) for impact energy of 29.6 eV. These observations are in good agreement with the measurements reported by Hall et al. (9) and Trajmar (10). Given the isotropic character of the 23P cross section at this energy (9), this peak was used for the normalization; the corresponding cross section is about 2X 10“ g cm2/sr.
Ar
0.8
0.6
Electron
energy
(eV)
Fig.4. The 3p54s2 (2P) resonances in argon, observed in the differential elastic cross section at various scattering angles. Each curve is normalized. For the normalization, the average between the ends of each curve is fixed at one.
793
Figure 4 shows the well known Feshbach resonance appearing just It is above 11 eV in the elastic differential cross section of argon. From these assigned to the states 3p54s2 (2P J), with &lf1/2. measurements, it was determined that the line width of this resonance is 2.3f0.2 meV (11).
He (1~2~) 3-P
57.5
59.0
58.5
59.0
Energy Fig. 5. The 2P and 2D resonances associated to the first doubly excited states in helium, observed in the inelastic channel of the state ls2p (3P). For the normalization of each curve, the average between the ends is fixed at one. The energy scale is the incident electron energy in eV.
794
In helium such resonances are associated to the first doubly excited states around 58 eV. Their assignments are 2s22p (2P) and 2s2p2 (2D), located at about 57.2 and 58.3 eV, respectively (12). The decay of these short-lived states can be observed in the inelastic channels corresponding to the excitation cross sections of the n=2 low-lying (singly excited) Never before, to our knowledge, a measurement was reported for states. this observation in the weak 3P channel, as shown in Fig. 5. The 2P resonance exhibits dramatic variations and an unusual strength at large angles.
24.5
25.0
25.5
26.0
26.5
Energy
27.0
27.5
28.0
28.5
(eV)
Fig. 6. Ejected electron spectra following the autoionization by electron impact in argon, measured at angles of 16”, 50”, 62”,122”, and 162O, with a constant residual energy of the projectile electrons of 10 eV. No normalization has been carried out. The energy scale is actually the incident electron energy minus the residual energy.
The MAPDESS can also be used to study ejected electron spectra following the autoionization process by electron impact. Figure 8 shows the case of argon, in the region of the states 3s3p4 nl, corresponding to the range of about 24.5 to 29 eV. Spectra are presented for five angles: 18O, 50”, 82’, 122”, and 162”, at a constant residual energy of 10 eV. Each feature appears double, due to the spin-orbit splitting of the ground state of Ar+ involved in the process. One can recognize the electrons ejected from the 4s level around 25 eV, from the 4p level around 26.5 eV, and from
795
the 3d level around 27.5 eV (13). Subsequent features are related to higher members of the same series, overlapping with features due to doubly excited states. Though actually present, the angular variations are weak in this case. The last example features resonances in nitrogen observed in a vibrational channel. Figure 7 shows the differential cross section for the excitation of the v=O to v-l vibration of the ground state. The two peaks correspond to the, decay of the two first vibrational levels of the resonant state 2Zg+ of N2- located at 11.48 eV (14). Since a (r wave is involved in the process, one expects no angular variation for the strength of the peaks. Still, variations are observed, particularly at large angles. 1
.. a. . .
11.25
18O
11.50
Energy
11.75
12.00
(eV)
Fig. 7. Differential cross section of v=O to v=l electron impact excitation of the ground state of N2, measured at three different scattering angles.
The authors their assistance. FCAR du Quebec.
1 2 3 4 5 6 7
are grateful to Dr. A. Adnot and to the technical staff for This work was supported by NSERC-Canada and Fonds
D. Tremblay, D. Dub& and D. Roy, Rev. Sci. Instrum.. 60 (1989) 879-885. D. Roy and J. D. Carrette. in : H. lbach (Ed.), Electron Spectroscopy for Surface Analysis, Vol. 4 of Topics in Current Physics, Springer, Berlin, 1977, pp. 13-58. D. Dub6 and D. Roy, Nucl. Instrum. Methods, 201 (1982) 291-301. D. Tremblay and D. Roy, Nucl. Instrum. Methods, 220 (1984) 270-287. H. 2. Sar-El, Nucl. Instrum. Methods, 42 (1966) 71-76. J. E. Katz, P. W. Davies, J. E. Crowell, and G. A. Somorjai, Rev. Sci. Instrum. , 53 (1986) 785-789. S. K. Srivastava. H. Tanaka, A. Chutjian, and S. Trajmar, Phys. Rev. A, 23 (1981) 2156-2166.
796
8 9 10 11
12 13 14
D. F. Register, S. Trajmar, and S. K. Srfvastava, Phys. Rev. A, 21 (1980) 1134-l 151. R.I. Hall, G. Joyez, J. Mazeau, J. Reinhardt, and C. Bchermann, J. Phys., 34 (1973) 827-843. S. Trajmar, Phys. Rev. A, 8 (1973) 191-203. D. Dub& D. Tremblay, and D. Roy, in: A. Salin (Ed.), Third European Conference on Atomic and Molecular Physics. Book of Abstracts, Bordeaux, 3-7 April 1989, European Physical Society, Bordeaux, 1989, p. 154. D. Roy, Phys. Rev. Lett., 38 (1977) 1062-1065. D. Roy, A. Poulin, M.J. Hubin-Franskin, and J. Delwiche, J. Electron Spectrosc. Relat. Phenom., 19 (1980) 101-114. H.G.M. Heideman, C.E. Kuyatt, and G.E. Chamberlain, J. Chem. Phys., 44 (1966) 355361.