- Email: [email protected]
Resonant Auger decay of 2p hole in argon induced by electron impact
Nuclear Instruments and Methods in Physics Research B 267 (2009) 260–262
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Resonant Auger decay of 2p hole in argon induced by electron impact }kési c M. Zˇitnik a,*, M. Kavcˇicˇ a, K. Bucˇar a, B. Paripás b, B. Palásthy b, K. To a b c
J. Stefan Institute, Jamova 39, P.O. Box 3000, SI 1000, Ljubljana, Slovenia Department of Physics, University of Miskolc, 3515 Miskolc-Egyetemváros, Hungary Institute of Nuclear Research of the Hungarian Academy of Sciences (ATOMKI), Debrecen, Hungary
a r t i c l e
i n f o
Available online 18 October 2008 PACS: 34.80.Dp Keywords: Coincidence spectroscopy (e, 2e) Study Auger emission
a b s t r a c t We present an experimental study of L-MM resonant Auger spectra of argon after electron impact excitation. The electron spectra were measured at ten different electron impact energies between 442.6 eV and 461.7 eV. During (e, 2e) measurement the energy of the second electron was kept fixed at 209.6 eV, corresponding to the energy of one of the strongest resonant Auger transitions from the [2p3/23d] state. Except for the monopole excitations, the recorded spectral structures are explained on the basis of photon impact data. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction The kinetic energy region of L-MM transitions of Ar extends from 200 eV to 214 eV and was studied extensively in the past by coincidence and non-coincidence techniques, using electron, photon and ion impact ([1] and references therein). Up to 207 eV one observes the ‘‘normal” (diagram) Auger lines; the multiplets 1 S0, 1D2 and 3P0,1,2 originate from the Auger decay Ar+[L2,3]Ar2+ [M2,3M2,3]. These lines appear as soon as the energy deposited in the atom exceeds 248.628 eV for L3 hole and 250.776 eV for L2 hole, respectively [2]. A series of singly excited atomic states [2p3/2,1/2]nl, where l is restricted to s and d in the dipole (photon impact) approximation, approach to the corresponding ionization thresholds from below. As a result of decay of these atomic states, the so-called resonant Auger electrons are emitted with energies lying mainly in the upper part of the L-MM kinetic energy region. After the decay, argon is usually left in one of the singly excited final ionic states [3p2]n0 l0 where n0 and l0 are not necessary equal to n and l, respectively (a spectator decay). Moreover, since the final states are the outer valence states, a mixture of single configuration states is normally required to describe them properly. With the advent of bright synchrotron sources in the last 90’s a considerable amount of high resolution spectroscopy work was done in the realm of photo-induced resonant Auger transitions [3] and of the final valence satellite states build on the double 3p hole in argon [4]. Fully isolated resonant Auger spectra were mea-
* Corresponding author. E-mail address: [email protected] (M. Zˇitnik). 0168-583X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2008.10.032
sured for [2p3/2]4s,3d,4d,5d and [2p1/2]4s resonances and results were compared to the calculated decay probabilities. However, apart from a very weak signal which was extracted from non-coincidence Auger spectra of high statistical quality and assigned to the resonant decay processes [5], no electron induced Auger resonant spectra have been reported so far. Since the resonant Auger lines appear only at a given electron energy loss, when the impact energy is above the threshold energy, in the non-coincidence experiment they are obscured by the normal Auger lines (or by lines coming from the cascade Auger or by double L-hole Auger decay) and the lines which originate from different resonances may overlap between themselves. Clearly, a coincidence (e, 2e) measurement is needed to measure the Auger spectrum emitted by the selected resonance. Since the signal is weak, a highly efficient collection of both electrons is required. This paper describes our recent results in this field. Another interesting aspect which is unavoidably present in photon or electron impact experiments, but is not in the focus of this paper, is the possibility of interferences. At any deposited energy in the range of the resonances, the same final states can be reached by direct ionization of the single 3p electron which is accompanied by the shake-up transition of another 3p electron [6]. Apart from the discrete-continuum type of interference, in an electron impact experiment appears a possibility of the state-to-state interference [7,8]. Namely, due to electron correlations two different resonances can decay into the same final ionic states and at certain energies of two detected electrons (the scattered and the resonant Auger electron) the two paths can be made indistinguishable by choosing a proper electron impact energy. For more exhaustive account of different possibilities please refer to [9].
M. Zˇitnik et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 260–262
2. Experiment
261
5x10
L 3 -M 23 M 23 Auger
Our experimental setup was described in details elsewhere [5] so that here we give only a very brief description. The instrument consists of two electrostatic spectrometers (see Fig. 1). Both spectrometers consist of a so called box type distorted field cylindrical mirror analysers (CMA). The energy spectrum of the first electron is measured by a double pass cylindrical box type analyzer, while the second electron, emitted in reaction: 2
! Ar ½3p nl þ eðE1 Þ þ eðE2 Þ;
[3p ]nl satellites
[3s] offset = 0.0 eV E0 - E1 = 252.1 eV
2.7
ð1Þ
is detected by the single pass box analyzer in coincidence with the first electron. E0 denotes the primary electron energy, while E1 (scattered projectile) and E2 (Auger electron) denote energies of the two outgoing electrons. The axis of the target gas beam is perpendicular to the common axis of the two spectrometers and also to the projectile electron beam. These three perpendicular lines cross each other exactly at the common focal point of the spectrometers in the near vicinity of the gas entrance into the chamber. Large accepted solid angles of spectrometers is provided by 5° wide entrance cones centered at an entrance angle close to the theoretical value of 43.5°. An overall entrance solid angle is around 0.015 4p srd. Since the projectile electron beam is perpendicular to the common spectrometer axis the (mean) scattering angle h is between 46.5° and 133.5°. The relative energy resolution, the half width at the half maximum (HWHM) of the single and double pass spectrometer is 0.45% and 0.25%, respectively. During the measurement the typical background pressure in the chamber was 107 mbar.
249.4 3.8
500
3. Results and discussion
4.6 247.4 4.9 247.1 5.9 246.2
2
[3p]
7.6 244.5
0
Noncoin. Auger
8.7
1
243.4
13.7
238.4
19.1
233.0
220
230
0 210
240
Electron energy 2E[eV]
A sequence of ten (e, 2e) spectra was recorded at different electron impact energies in the interval between 442.6 eV and 461.7 eV while E1 was kept fixed at 209.6 eV. This energy corresponds to one of the strongest resonant Auger transitions from [2p3/2]3d state [3]. The double pass CMA scanned a few tens of eV wide energy range around 200 eV (see Fig. 2). The energy scale of spectrometers was calibrated and kept stable by a comparison to the well known positions of Auger L-MM lines. The energy of the electron gun was derived from known ionization potential Ip = 29.3 eV of Ar[3s] state and the position E2[3s] of the corresponding spectral line in the coincidence spectrum:
ð2Þ
The measurement time was about one week for each spectrum. The intensity of each coincidence spectrum is normalized to the same number of counts in the [3s] peak which is assumed to be constant when the experimental conditions, i.e. beam current, target pressure and detection efficiency, are stable. A possible interference effects on the [3s] line [6] are considered to be smoothed out due to a broad spectrometer function. We note, that it is not convenient to normalize the spectra to the same number of single
Fig. 1. The cross section of our experimental setup.
3
248.3
200
E0 ¼ E1 þ E2 ½3s þ Ip ½3s:
4
single electron counts
þ
2
1000
Offset (e,2e) counts
Ar þ eðE0 Þ ! Ar½2pnl þ eðE1 Þ
6
E 1 = 209. 6 eV
Fig. 2. Coincidence spectra measured at 10 different values of E0 primary energies. The energy loss E0E1 falls into the region of argon L threshold. There is an offset in energy E2 to align the peaks pertaining to [3s] final state (a constant ionic state CIS spectrum). The energy of single pass detector was fixed to E1 = 209.6 eV. For comparison the non-coincidence Auger L-MM of double pass detector is given (full line curve).
counts in detector 1 and 2, since both non-coincidence signals vary with electron impact energy due to the contribution of the ‘‘moving” inelastic scattering yield (see below). There are three different kinds of structures in these spectra: The first, type I, are the peaks which are present only in coincident spectra. They pertain to electrons which leave behind the [3s], the [3p] or the satellite [3p2]nl states after being kicked-off the atom:
Ar þ eðE0 Þ ! Arþ þ eðE1 Þ þ eðE2 Þ:
ð3Þ
The excess energy above the ionization potentials is continuously distributed between the scattered projectile and ejected electron (E1 and E2). When one of the electrons is detected (let say at energy, E1), the energy of the other electron is also determined (E2), producing a peak in the coincidence spectrum. The coincidence spectral lines corresponding to a given outer shell hole move together with the electron impact energy E0 and are expected to display a fairly constant intensity in our experiment. The second, type II, are the resonant Auger lines which also move in energy in the same way as the satellite lines, but appear in the spectra only resonantly. In other words, they appear in the small range of E0, where the energy loss is in the vicinity of the resonance energy. The third, type III, are the normal (diagram) Auger lines which stick to the fixed spectral positions and start to appear when E0 is above the corresponding threshold. Assuming that the discrete-continuum interference effects can be neglected in our experiment the type I contribution was subtracted from experimental
M. Zˇitnik et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 260–262
262
B
A
C
Coincidence yield
j
i h g f
e d c 200
205
210
215
220
In Fig. 3, the sequence of our recent (e, 2e) spectra after the subtraction of the contribution of type I are compared with the photon impact data of de Gouw [3], taking into account a difference in energy resolution and the real value of energy loss. We note, that in our case these energies were not always matched with the resonance energies. In general, there is quite a good agreement of measured spectra with the model results based on photon data [3] and on our experimental energy loss values. The remaining differences are expected to come from neglected contribution of resonances based on [2p1/2] hole and from the non-dipolar character of our experiment, as shown by the electron energy loss spectrum recorded with the same set-up at 350 eV electron impact energy. Opposite to the photoexcitation case, the [2p3/2]4p line is clearly seen here (Fig. 4). Since the electron scattering angle is not small and the energy loss is not small either relative to E0, the angular momentum different from one can be transferred to the target [10]. At 206.3, 208.6 and 210.0 eV some extra signal is found which is attributed to the Auger decay of [2p3/2]4p state. At present, the post collision interaction (PCI) effect on the Type III Auger lines is taken into account only by a small energy shift – PCI lineshape distortion is not considered here [11].
E 2 [eV] Fig. 3. (e, 2e) Spectra after the type I contribution was subtracted. Full line is a model based on photoexcitation data of deGouw and includes the contribution of [2p3/2]4s,35d and [2p1/2]4s resonant Auger decays. Dotted line shows the contribution of [2p] (nonresonant) Auger decay. With letters A–C are the structures which are attributed to [2p3/2]4p resonant Auger decay according to [5]. The energy resolution is 2.2 eV and the 8 expt. spectra (from bottom) correspond to the energy losses, denoted in Fig. 4 from the left.
spectra. We are left with the sequence of resonant Auger spectra which evolves into the normal Auger spectrum when E0 E1 climbs above the corresponding threshold. 4s
4p
3d 4d 5d 4p
4s
Electron yield [arb. units]
expt., E0 = 350 eV noncovol. convol. E0 = 350 eV convol. E0 = 450 eV c
2000
d
[2p] 3/2 3d 4d 5d [2p] 1/2
We have measured for the first time a series of resonant Auger spectra emitted after the electron impact excitation of the [2p] hole in argon. The spectra are explained on the basis of photon impact data. A more precise modeling would require the inclusion of monopole excitations and PCI distortion effects. Due to the wide range of scattering angles seen by the spectrometers it is more practical to derive the ratio of monopole to dipole excitations from the fit to the electron energy loss spectra. From data presented in Fig. 4 and a similar set acquired at E0 550 eV, the ratio at 450 eV could be estimated. Having done both, the non-dipolar component of the Resonant Auger spectra could be generated and included into the modeling of the measured coincidence spectra.
Acknowledgements
h e
4. Conclusions
fg j
i
This work was supported by the TT Grant No. SLO-15/05 (BI/ HU-06-07-015), the grant ‘‘Bolyai” from the Hungarian Academy of Sciences, the Hungarian National Office for Research and Technology and the Hungarian Scientific Research Found OTKA (K72172).
References
1000
0 240
242
244
246
248
250
252
Energyloss [eV] Fig. 4. Electron energy loss spectrum recorded at E0 = 350 eV with the double pass spectrometer. Experimental FWHM resolution at that energy is 0.7 eV. The spectrum at E0 = 450 eV is estimated by taking into account experimental resolution 1.3 eV. The natural linewidth of 2p hole was fixed to 0.13 eV [2]. Straight lines denote central energy loss values at which the (e, 2e) spectra were taken.
[1] K. Bucˇar, M. Zˇitnik, Rad. Phys. Chem. 76 (2007) 487. [2] G.C. King, M. Tronc, F.H. Read, R.C. Bradford, J. Phys. B 10 (1977) 2479. [3] J.A. de Gouw, J. van Eck, A.C. Peters, J. van der Weg, H.G.M. Heideman, J. Phys. B 28 (1995) 2127. [4] A. Kikas, S.J. Osborne, A. Ausmees, S. Svensson, O.-P. Sairanen, S. Aksela, J. Electron Spectrosc. Relat. Phenom. 77 (1996) 241. [5] B. Paripás, B. Palásthy, Rad. Phys. Chem. 76 (2007) 565. [6] R. Camilloni, M. Zˇitnik, C. Comicioli, K.C. Prince, M. Zacchigna, C. Crotti, C. Ottaviani, C. Quaresima, P. Perfetti, G. Stefani, Phys. Rev. Lett. 77 (1996) 2646. [7] J.P. van den Brink, G. Nienhuis, J. van Eck, H.G.M. Heideman, J. Phys. B 22 (1989) 3501. [8] J.A. de Gouw, J. van Eck, A.C. Peters, J. van der Weg, H.G.M. Heideman, Phys. Rev. Lett. 71 (1993) 2875. [9] M. Zˇitnik, M. Kavcˇicˇ, K. Bucˇar, B. Paripás, B. Palásthy, MicroCad 2007, Section H, Physics and Education, University of Miskolc, 2007, p. 55. [10] C.D. Schröter, L. Avaldi, R. Camilloni, G. Stefani, M. Zˇitnik, M. Štuhec, J. Phys. B 32 (1999) 171. [11] B. Paripás, B. Palásthy, Nucl. Instr. and Meth. B 267 (2009) 275.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Resonant Auger decay of 2p hole in argon induced by electron impact }kési c M. Zˇitnik a,*, M. Kavcˇicˇ a, K. Bucˇar a, B. Paripás b, B. Palásthy b, K. To a b c
J. Stefan Institute, Jamova 39, P.O. Box 3000, SI 1000, Ljubljana, Slovenia Department of Physics, University of Miskolc, 3515 Miskolc-Egyetemváros, Hungary Institute of Nuclear Research of the Hungarian Academy of Sciences (ATOMKI), Debrecen, Hungary
a r t i c l e
i n f o
Available online 18 October 2008 PACS: 34.80.Dp Keywords: Coincidence spectroscopy (e, 2e) Study Auger emission
a b s t r a c t We present an experimental study of L-MM resonant Auger spectra of argon after electron impact excitation. The electron spectra were measured at ten different electron impact energies between 442.6 eV and 461.7 eV. During (e, 2e) measurement the energy of the second electron was kept fixed at 209.6 eV, corresponding to the energy of one of the strongest resonant Auger transitions from the [2p3/23d] state. Except for the monopole excitations, the recorded spectral structures are explained on the basis of photon impact data. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction The kinetic energy region of L-MM transitions of Ar extends from 200 eV to 214 eV and was studied extensively in the past by coincidence and non-coincidence techniques, using electron, photon and ion impact ([1] and references therein). Up to 207 eV one observes the ‘‘normal” (diagram) Auger lines; the multiplets 1 S0, 1D2 and 3P0,1,2 originate from the Auger decay Ar+[L2,3]Ar2+ [M2,3M2,3]. These lines appear as soon as the energy deposited in the atom exceeds 248.628 eV for L3 hole and 250.776 eV for L2 hole, respectively [2]. A series of singly excited atomic states [2p3/2,1/2]nl, where l is restricted to s and d in the dipole (photon impact) approximation, approach to the corresponding ionization thresholds from below. As a result of decay of these atomic states, the so-called resonant Auger electrons are emitted with energies lying mainly in the upper part of the L-MM kinetic energy region. After the decay, argon is usually left in one of the singly excited final ionic states [3p2]n0 l0 where n0 and l0 are not necessary equal to n and l, respectively (a spectator decay). Moreover, since the final states are the outer valence states, a mixture of single configuration states is normally required to describe them properly. With the advent of bright synchrotron sources in the last 90’s a considerable amount of high resolution spectroscopy work was done in the realm of photo-induced resonant Auger transitions [3] and of the final valence satellite states build on the double 3p hole in argon [4]. Fully isolated resonant Auger spectra were mea-
* Corresponding author. E-mail address: [email protected] (M. Zˇitnik). 0168-583X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2008.10.032
sured for [2p3/2]4s,3d,4d,5d and [2p1/2]4s resonances and results were compared to the calculated decay probabilities. However, apart from a very weak signal which was extracted from non-coincidence Auger spectra of high statistical quality and assigned to the resonant decay processes [5], no electron induced Auger resonant spectra have been reported so far. Since the resonant Auger lines appear only at a given electron energy loss, when the impact energy is above the threshold energy, in the non-coincidence experiment they are obscured by the normal Auger lines (or by lines coming from the cascade Auger or by double L-hole Auger decay) and the lines which originate from different resonances may overlap between themselves. Clearly, a coincidence (e, 2e) measurement is needed to measure the Auger spectrum emitted by the selected resonance. Since the signal is weak, a highly efficient collection of both electrons is required. This paper describes our recent results in this field. Another interesting aspect which is unavoidably present in photon or electron impact experiments, but is not in the focus of this paper, is the possibility of interferences. At any deposited energy in the range of the resonances, the same final states can be reached by direct ionization of the single 3p electron which is accompanied by the shake-up transition of another 3p electron [6]. Apart from the discrete-continuum type of interference, in an electron impact experiment appears a possibility of the state-to-state interference [7,8]. Namely, due to electron correlations two different resonances can decay into the same final ionic states and at certain energies of two detected electrons (the scattered and the resonant Auger electron) the two paths can be made indistinguishable by choosing a proper electron impact energy. For more exhaustive account of different possibilities please refer to [9].
M. Zˇitnik et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 260–262
2. Experiment
261
5x10
L 3 -M 23 M 23 Auger
Our experimental setup was described in details elsewhere [5] so that here we give only a very brief description. The instrument consists of two electrostatic spectrometers (see Fig. 1). Both spectrometers consist of a so called box type distorted field cylindrical mirror analysers (CMA). The energy spectrum of the first electron is measured by a double pass cylindrical box type analyzer, while the second electron, emitted in reaction: 2
! Ar ½3p nl þ eðE1 Þ þ eðE2 Þ;
[3p ]nl satellites
[3s] offset = 0.0 eV E0 - E1 = 252.1 eV
2.7
ð1Þ
is detected by the single pass box analyzer in coincidence with the first electron. E0 denotes the primary electron energy, while E1 (scattered projectile) and E2 (Auger electron) denote energies of the two outgoing electrons. The axis of the target gas beam is perpendicular to the common axis of the two spectrometers and also to the projectile electron beam. These three perpendicular lines cross each other exactly at the common focal point of the spectrometers in the near vicinity of the gas entrance into the chamber. Large accepted solid angles of spectrometers is provided by 5° wide entrance cones centered at an entrance angle close to the theoretical value of 43.5°. An overall entrance solid angle is around 0.015 4p srd. Since the projectile electron beam is perpendicular to the common spectrometer axis the (mean) scattering angle h is between 46.5° and 133.5°. The relative energy resolution, the half width at the half maximum (HWHM) of the single and double pass spectrometer is 0.45% and 0.25%, respectively. During the measurement the typical background pressure in the chamber was 107 mbar.
249.4 3.8
500
3. Results and discussion
4.6 247.4 4.9 247.1 5.9 246.2
2
[3p]
7.6 244.5
0
Noncoin. Auger
8.7
1
243.4
13.7
238.4
19.1
233.0
220
230
0 210
240
Electron energy 2E[eV]
A sequence of ten (e, 2e) spectra was recorded at different electron impact energies in the interval between 442.6 eV and 461.7 eV while E1 was kept fixed at 209.6 eV. This energy corresponds to one of the strongest resonant Auger transitions from [2p3/2]3d state [3]. The double pass CMA scanned a few tens of eV wide energy range around 200 eV (see Fig. 2). The energy scale of spectrometers was calibrated and kept stable by a comparison to the well known positions of Auger L-MM lines. The energy of the electron gun was derived from known ionization potential Ip = 29.3 eV of Ar[3s] state and the position E2[3s] of the corresponding spectral line in the coincidence spectrum:
ð2Þ
The measurement time was about one week for each spectrum. The intensity of each coincidence spectrum is normalized to the same number of counts in the [3s] peak which is assumed to be constant when the experimental conditions, i.e. beam current, target pressure and detection efficiency, are stable. A possible interference effects on the [3s] line [6] are considered to be smoothed out due to a broad spectrometer function. We note, that it is not convenient to normalize the spectra to the same number of single
Fig. 1. The cross section of our experimental setup.
3
248.3
200
E0 ¼ E1 þ E2 ½3s þ Ip ½3s:
4
single electron counts
þ
2
1000
Offset (e,2e) counts
Ar þ eðE0 Þ ! Ar½2pnl þ eðE1 Þ
6
E 1 = 209. 6 eV
Fig. 2. Coincidence spectra measured at 10 different values of E0 primary energies. The energy loss E0E1 falls into the region of argon L threshold. There is an offset in energy E2 to align the peaks pertaining to [3s] final state (a constant ionic state CIS spectrum). The energy of single pass detector was fixed to E1 = 209.6 eV. For comparison the non-coincidence Auger L-MM of double pass detector is given (full line curve).
counts in detector 1 and 2, since both non-coincidence signals vary with electron impact energy due to the contribution of the ‘‘moving” inelastic scattering yield (see below). There are three different kinds of structures in these spectra: The first, type I, are the peaks which are present only in coincident spectra. They pertain to electrons which leave behind the [3s], the [3p] or the satellite [3p2]nl states after being kicked-off the atom:
Ar þ eðE0 Þ ! Arþ þ eðE1 Þ þ eðE2 Þ:
ð3Þ
The excess energy above the ionization potentials is continuously distributed between the scattered projectile and ejected electron (E1 and E2). When one of the electrons is detected (let say at energy, E1), the energy of the other electron is also determined (E2), producing a peak in the coincidence spectrum. The coincidence spectral lines corresponding to a given outer shell hole move together with the electron impact energy E0 and are expected to display a fairly constant intensity in our experiment. The second, type II, are the resonant Auger lines which also move in energy in the same way as the satellite lines, but appear in the spectra only resonantly. In other words, they appear in the small range of E0, where the energy loss is in the vicinity of the resonance energy. The third, type III, are the normal (diagram) Auger lines which stick to the fixed spectral positions and start to appear when E0 is above the corresponding threshold. Assuming that the discrete-continuum interference effects can be neglected in our experiment the type I contribution was subtracted from experimental
M. Zˇitnik et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 260–262
262
B
A
C
Coincidence yield
j
i h g f
e d c 200
205
210
215
220
In Fig. 3, the sequence of our recent (e, 2e) spectra after the subtraction of the contribution of type I are compared with the photon impact data of de Gouw [3], taking into account a difference in energy resolution and the real value of energy loss. We note, that in our case these energies were not always matched with the resonance energies. In general, there is quite a good agreement of measured spectra with the model results based on photon data [3] and on our experimental energy loss values. The remaining differences are expected to come from neglected contribution of resonances based on [2p1/2] hole and from the non-dipolar character of our experiment, as shown by the electron energy loss spectrum recorded with the same set-up at 350 eV electron impact energy. Opposite to the photoexcitation case, the [2p3/2]4p line is clearly seen here (Fig. 4). Since the electron scattering angle is not small and the energy loss is not small either relative to E0, the angular momentum different from one can be transferred to the target [10]. At 206.3, 208.6 and 210.0 eV some extra signal is found which is attributed to the Auger decay of [2p3/2]4p state. At present, the post collision interaction (PCI) effect on the Type III Auger lines is taken into account only by a small energy shift – PCI lineshape distortion is not considered here [11].
E 2 [eV] Fig. 3. (e, 2e) Spectra after the type I contribution was subtracted. Full line is a model based on photoexcitation data of deGouw and includes the contribution of [2p3/2]4s,35d and [2p1/2]4s resonant Auger decays. Dotted line shows the contribution of [2p] (nonresonant) Auger decay. With letters A–C are the structures which are attributed to [2p3/2]4p resonant Auger decay according to [5]. The energy resolution is 2.2 eV and the 8 expt. spectra (from bottom) correspond to the energy losses, denoted in Fig. 4 from the left.
spectra. We are left with the sequence of resonant Auger spectra which evolves into the normal Auger spectrum when E0 E1 climbs above the corresponding threshold. 4s
4p
3d 4d 5d 4p
4s
Electron yield [arb. units]
expt., E0 = 350 eV noncovol. convol. E0 = 350 eV convol. E0 = 450 eV c
2000
d
[2p] 3/2 3d 4d 5d [2p] 1/2
We have measured for the first time a series of resonant Auger spectra emitted after the electron impact excitation of the [2p] hole in argon. The spectra are explained on the basis of photon impact data. A more precise modeling would require the inclusion of monopole excitations and PCI distortion effects. Due to the wide range of scattering angles seen by the spectrometers it is more practical to derive the ratio of monopole to dipole excitations from the fit to the electron energy loss spectra. From data presented in Fig. 4 and a similar set acquired at E0 550 eV, the ratio at 450 eV could be estimated. Having done both, the non-dipolar component of the Resonant Auger spectra could be generated and included into the modeling of the measured coincidence spectra.
Acknowledgements
h e
4. Conclusions
fg j
i
This work was supported by the TT Grant No. SLO-15/05 (BI/ HU-06-07-015), the grant ‘‘Bolyai” from the Hungarian Academy of Sciences, the Hungarian National Office for Research and Technology and the Hungarian Scientific Research Found OTKA (K72172).
References
1000
0 240
242
244
246
248
250
252
Energyloss [eV] Fig. 4. Electron energy loss spectrum recorded at E0 = 350 eV with the double pass spectrometer. Experimental FWHM resolution at that energy is 0.7 eV. The spectrum at E0 = 450 eV is estimated by taking into account experimental resolution 1.3 eV. The natural linewidth of 2p hole was fixed to 0.13 eV [2]. Straight lines denote central energy loss values at which the (e, 2e) spectra were taken.
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