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Dipole forbidden inner-shell excitation and decay of the N2 (1s)−1(2pπ) 3Π state studied by electron impact experiments
Journal of Electron Spectroscopy and Related Phenomena 161 (2007) 17–21
Dipole forbidden inner-shell excitation and decay of the N2 (1s)−1(2p) 3 state studied by electron impact experiments V. Feyer a,b,∗ , P. Bolognesi a , M. Coreno a,c , K.C. Prince c,d , L. Avaldi a,c a
CNR-IMIP, Area della Ricerca di Roma 1, CP10 I-00016 Monterotondo Scalo, Italy b Institute of Electron Physics, 21 Universitetska St., 88017 Uzhgorod, Ukraine c CNR-INFM-TASC, Gasphase Beamline at Elettra, Area Science Park, I-34012 Basovizza, Trieste, Italy d Sincrotrone Trieste, Area Science Park, I-34012 Basovizza, Trieste, Italy Available online 9 February 2007
Abstract The dipole forbidden inner-shell excitation of the N2 molecule to the (1s)−1 (2p) 3 state and its decay have been studied in an electron impact experiment at variable incident energy. The contribution of the dipole forbidden transitions to the N2 autoionization spectrum has been isolated via two different techniques. One is based on a subtraction procedure relying on non-coincidence measurements, while the other consists of an experiment where the electrons ejected in the decay process are detected in coincidence with scattered electrons which suffered an energy loss equal to the excitation energy of the (1s)−1 (2p) 3 state. © 2007 Elsevier B.V. All rights reserved. PACS: 34.80 Gs; 33.80 Eh Keywords: Nitrogen; Electron energy loss spectroscopy; Resonant Auger spectra; Electric-dipole forbidden transitions
1. Introduction For a long time electron energy loss experiments were the best suited tool to investigate the excitation of inner-shell states [1,2] until the advent of third generation synchrotron radiation sources. At these sources beamlines characterized by high resolution and high intensity in the soft X-ray region [3], have been built and photoabsorption measurements overtook electron impact experiments. However, one of the peculiarities of electron impact experiments is their ability to excite dipole forbidden transitions, and for this there is no comparable synchrotron radiation technique. Shaw et al. [4,5] pioneered the investigation of electric-dipole-forbidden transitions in atoms and small molecules. This was achieved by combining a high energy resolution spectrometer and a low incident electron energy (Ei ). At high Ei and small scattering angles the dipole selection rules dominate electron impact processes, but at lower energies the Born approximation breaks down and spin-forbidden processes due to exchange interaction become possible. ∗
Corresponding author at: CNR-IMIP, Area della Ricerca di Roma 1, CP10 I-00016 Monterotondo Scalo, Italy. E-mail address: [email protected] (V. Feyer). 0368-2048/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2007.02.006
In this work we have studied the excitation and decay of the N2 inner-shell dipole forbidden (1s)−1 (2p) 3 transition. As in the previous case of CO [6,7] the excitation of the transition was studied via electron energy loss experiments at several incident energies and large scattering angle. The decay of the inner-shell excited state was investigated by the measurement of the autoionization spectrum. The autoionization or “resonant Auger” spectra (using the nomenclature of synchrotron experiments), were measured at several incident energies. This provided an overview of the features due to the decay of the inner-shell excited states. Then the coincidence detection of the autoionizing electrons with scattered electrons which suffered an energy loss equal to the excitation of one of the 1s → (1s)−1 (2p) 1,3 transitions allowed us to separate the two spectra. This technique, introduced by Ungier and Thomas in 1983 [8], has been used to study the decay of inner-shell excited states of CO and N2 , but never applied to dipole forbidden states due its intrinsic low efficiency. We have taken advantage of the high efficiency of the multicoincidence endstation of the Gasphase beamline at Elettra [9] and the special geometry in which high momentum transfer collisions enhance dipole forbidden transitions, and thus we have isolated the decay of triplet excited state.
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V. Feyer et al. / Journal of Electron Spectroscopy and Related Phenomena 161 (2007) 17–21
2. Experiment The measurements were performed using the multicoincidence end-station of the Gasphase beamline of the Elettra storage ring. For these experiments the end-station was equipped with an electron gun (EKF 300, Omicron GmbH). The design and operating principles of the experimental apparatus are described in detail in Ref. [9] and will not be repeated here. The chamber hosts ten independent electrostatic analysers, arranged in two groups of three and seven analysers, respectively, and placed at 30◦ from each other. They are mounted on two independent turntables in a plane perpendicular to the incident beam. Thus, the scattering angle is fixed at 90◦ . The resonant Auger spectra were measured simultaneously by the seven analyzers of the bigger frame. The spectrum produced by each analyzer was first calibrated in energy and then corrected by the transmission function of the analyzer. Then the seven spectra were summed to improve statistics. To collect the energy loss spectra the three analyzers of the other frame were used. The energy resolution and the angular acceptance in the dispersion plane of the spectrometers were E1,2 = 0.45 and 0.85 eV for the non-coincidence and coincidence spectra, respectively, and ϑ1,2 = ±3◦ . The energy of the resonant Auger spectra was calibrated using the value of the first diagrammatic Auger line, labeled B1 by Moddeman et al. [10] located at 366.5 eV. The energy of the incident electron beam varied from 500 to 1000 eV. In the coincidence experiment the pulses of the inelastic scattered electrons are used to trigger the time circuit and the autoionizing electrons act as stops. The coincidence electronics is made by three independent time-to-digital, TDC, converters. In the experiment each TDC unit is operated in the common
start mode with the signals of each one of the three analysers of the small turntable used as starts and the signals from the other seven as stops. In this way 21 coincidence pairs are collected simultaneously. All the experimental settings and the data acquisition are controlled via a PC equipped with LabView software. The same software monitors the stability of the experiment during the long acquisition times of the coincidence scans via the measurement of the non-coincidence count rates of the 10 analysers at fixed time intervals. The coincidence signals of the 21 scattered-ejected pairs were added up, after a careful energy calibration of the non-coincidence autoionization spectra independently collected by the seven analyzers. This provision enhances the overall efficiency of the set-up and allows the investigation of a process with a small cross section, like the decay of the inner-shell triplet excited state. Under typical experimental conditions of about 1 × 10−5 mbar of gas pressure and 1.8 A current, acquisition times of about 6–10 h/point were needed in order to achieve the present statistics in the coincidence measurements. 3. Results and discussion The energy loss spectra of N2 in the region of excitation to the (1s)−1 (2p) 1,3 states, recorded at θ S = 90◦ , for 500 ≤ Ei ≤ 1000 eV are shown in Fig. 1. The structure observed in all the spectra is clearly composed of two features. The higherenergy feature corresponds to the (1s)−1 (2p) 1 state of N2 . The one on the low energy side, with relative intensity which increases as the incident energy decreases, is assigned to the dipole forbidden 1s → (2p) 3 transition [4]. Two Gaussian functions have been fitted to the features in the energy loss spectrum. The relative intensities, the position of the centroids and
Fig. 1. Energy-loss spectra in the region of N2 (1s−1 )(2p) 1,3 excited states at different incident electron energies and fixed scattering angle of 90◦ .
V. Feyer et al. / Journal of Electron Spectroscopy and Related Phenomena 161 (2007) 17–21
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Fig. 2. The ratio between triplet and singlet intensity in the energy loss spectrum vs. incident energy. Present data, measured at 90◦ : dots; data by Shaw et al. [4], measured at 0◦ : triangles.
widths of the Gaussian functions are used as fitting parameters. The singlet–triplet splitting obtained in the fit is 0.90 ± 0.07 eV, in good agreement with the values of previous measurements of 0.82 ± 0.07 eV by Shaw et al. [4] and the calculations by Rescigno and Oriel [11] of 0.9 eV. Moreover, the peak of the triplet excited state appears to be narrower by about 320 meV than the peak assigned to the singlet excited state. This is consistent with the high resolution results of Shaw et al. [4], which show that a smaller number of vibrational states are excited in the case of the triplet. The ratio of the triplet to singlet intensity strongly depends on the incident energy as well as on the momentum transfer in the collision. This is clearly seen in Fig. 2 where the ratios measured in the present work and by Shaw et al. [4] are reported. In both cases the ratio increases as the incident energy decreases. This is because the exchange interaction becomes more efficient as the incident energy decreases. At the same incident energy the ratios measured in this work are different from those of Shaw et al. [4], due to the different momentum transfer. Indeed in the present work the scattered electrons are detected at 90◦ , while in the case of Shaw et al. [4] the scattered electrons were measured at 0◦ . The N2 Auger spectra at different Ei are shown in Fig. 3. The spectrum measured at Ei = 1000 eV is in very good agreement with the one previously reported in the literature by electron impact [10]. According to these authors the decay of the innershell excited states should produce the features in the energy region above 367 eV, while the diagrammatic Auger spectrum starts with the narrow feature at 366.5 eV. However, recent work by photon excitation [12] showed that the resonant Auger spectrum extends also into the region below 367 eV. In the region above 367 eV at Ei = 1000 eV (see Fig. 3a) three main structures are observed. The first narrow feature with a small shoulder on the high energy side corresponds to the so called participator transitions, where the excited electron is involved in the decay process which results with a hole in the outer orbital. According to the high resolution experiments by Piancastelli et al. [13] the main peak is due to the N2 + 1−1 u final state, while the shoulder is due to the 3−1 final state. The two broad structures observed g at about 375 and 370 eV correspond to spectator transitions,
Fig. 3. (a–c) The Auger and autoionization spectra of N2 at different incident energy Ei .
where the excited electron does not participate in the process, and which end with a 2h-1p N2 + final state. The main effects on decreasing the incident energy (Fig. 3b–c) are the appearance of a new feature on the low energy side of the participator peak and a variation of the relative intensity of the features assigned to spectator transitions. These effects have been attributed to the decay of the triplet excited state, whose excitation probability increases as the incident energy decreases. The absence of such features in the spectrum taken at 1000 eV, while the energy loss at the same incident energy displays a non-negligible excitation of the triplet state can be explained as follows. The energy loss shown in Fig. 1 was measured at 90◦ scattering angle, while the autoionization spectrum shown in Fig. 3 is the sum of all the excitation events occurring when a beam of 1000 eV interacts with N2 molecules. The large majority of energy loss events occur with a low momentum transfer, i.e. at small scattering angle. Thus, the process is dominated by dipole selection rules, as clearly shown in Fig. 1 of Shaw et al. [4], where at 1300 eV and 0◦ scattering they observe no trace of dipole forbidden transitions. Thus, it is not surprising that no features due to the decay of the triplet excited state are observed in our spectrum at 1000 eV. In order to isolate the triplet contribution to the resonant Auger spectra two approaches can be used. In the first one, the resonant Auger spectrum measured at 1000 eV incident energy (Fig. 4a) can be subtracted from the one taken at 600 eV after a proper normalization. This is done upon the hypothesis that the
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Fig. 4. (a) The Auger and autoionization spectrum of N2 at 1000 eV incident energy after background subtraction used as a reference spectrum for the deexcitation of the (1s−1 )(2p) 1 excited state; (b) the autoionization spectrum of N2 (1s−1 )(2p) 3 excited state at 600 eV incident energy obtained via the subtraction procedure described in the text; the full and dashed lines represent the fit to the participator part of spectrum (380–387 eV) with three Gaussian functions; (c) the autoionization spectrum of N2 (1s−1 )(2p) 3 excited state measured in coincidence with scattered electrons which suffered an energy loss equal to the 1s → 2p 3 excitation at 600 eV incident energy.
relative intensities, position and width of the peaks due to the 1 resonant Auger spectra are independent of Ei . In the second one the resonant Auger spectrum is measured in coincidence with the inelastically scattered electrons which have suffered an energy loss equal to excitation energy of one of the N2 (1s)−1 (2p) 1,3 states. In this way the spectra due to the singlet and triplet excitations are directly separated. The resonant Auger spectrum resulting from the subtraction procedure is shown in Fig. 4b, while the one obtained in the coincidence experiment is shown in Fig. 4c. The subtraction procedure relies on a non-coincidence experiment and therefore the measurements are faster and allow better energy resolution and statistics. However, this procedure suffers from some drawbacks. First of all the background of secondary electrons under the normal Auger and autoionization features has to be removed, because its shape depends on the incident energy. This may result in small misalignment of the narrower features in the spectra and produces sharp and unrealistic features in the “subtracted
spectrum”, as can be seen in Fig. 4b at about 366.3, 360.6 and 358.9 eV. Moreover, at certain incident energies the energy loss features overlap with the spectrum of the ejected electrons. The coincidence experiments do not suffer from these drawbacks, but, of course, the low count rate makes them time consuming and limits the achievable energy resolution. The autoionization spectrum of the (1s)−1 (2p) 3 state is characterized by several features. They can be divided into two main groups, one centred at about 384 eV corresponding to participator transitions, and the one from 376 to 365 eV corresponding to spectator transitions. According to previous work on the resonant Auger spectrum of the singlet excited state by 2 + photon excitation [12–15] three final states, the N2 + 3−1 g X g , 2 −1 2 −1 + 1u A u and 2u B u states, respectively, are expected to contribute to the participator spectrum, with the 1−1 u one dominating. This has been explained by the fact that the potential energy curves of the intermediate and final states for this transition are parallel. In both Fig. 4b and c we observe that the feature attributed to participator decay is about 0.9 eV below the one observed for the decay of the singlet excited state. This is consistent with difference in the excitation energy of the singlet and triplet excited states. Both spectra in Fig. 4b and c confirm that also in the triplet decay the most likely final state is the A one. However, it is interesting to observe that a fit to the structure between 380 and 387 eV in the coincidence spectrum (Fig. 4c) provides clear evidence of the population of all three final states, while in the subtracted spectrum either the B state is not present or it is hidden in the background. The population of the same state in the case of the singlet state excited by photon absorption has been observed only recently in a high resolution and sensitivity experiment by Piancastelli et al. [15]. The slightly shorter internuclear distance [4] of the triplet excited state may explain the larger intensity of the B state in the decay spectrum of the triplet state. All the features below 380 eV can be assigned to spectator transitions. The features in the region between 379 and 374 eV may be assigned according to the high resolution PES 2 2 work by Baltzer et al. [16] to the C 2 + u , 2 g and D g ion −1 −1 −1 states with the 3g 1g and 3g 1u 1g dominant configurations. The shape of the spectrum in this energy region, with a small feature at about 378.8 eV and a sharp rise at 376.8 eV, can be explained using the decay spectrum of the singlet inner-shell excited state measured with vibrational resolution in both the intermediate and final states [15]. These experimental results supported by theoretical calculations show that the small feature results from the population of the lowest vibrational states of the D2 g final state, which, due to its equilibrium distance of ˚ larger than that of the (1s)−1 (2p) 1 state, can be pop0.3 A ulated only by high vibrational states of the inner-shell excited 2 state. Then the decay into both the C2 + u and D g final states contributes to the main feature in this energy region. Due to the ˚ [4]) of the internuclear distances of small difference (0.007 A the singlet and triplet inner-shell excited states, similar findings are expected also in the decay spectrum of the triplet state and therefore also in our spectrum. Moreover, the integration over the vibrational states of the intermediate and final state should emphasize the main feature.
V. Feyer et al. / Journal of Electron Spectroscopy and Related Phenomena 161 (2007) 17–21
At lower kinetic energy the broad feature centred at about 2 370 eV may be attributed to the F2 + g , G and H u states. The PES spectrum is dominated by the F state that is 2 eV wide and displays no vibrational structure. Therefore, it must have a repulsive potential curve in the Franck-Condon region. According to this we assign the broad feature centred at 370 eV to the F state. Finally, despite the scatter of the data a further feature can be observed in Fig. 4c centred at about 363.3 eV. No further information is available on N2 + inner valence states at binding energy above 30 eV, so the observed feature is unassigned for now. 4. Conclusions The de-excitation spectrum of the N2 inner-shell (1s)−1 (2p) 3 excited state has been isolated for the first time via two different techniques. The use of the scattered electron-autoionization electron coincidence technique, despite the long measurement time required, appeared to be the most suited tool for this purpose. The assignment of the features of the spectrum has been done using the information from the de-excitation spectrum of the 1 inner-shell excited state obtained in photoabsorption experiments and the spectroscopic data of N2 photoionization by Baltzer et al. [16]. This work confirms the richness of the deexcitation spectrum of the inner-shell excited state that extends well below the energy of the normal Auger spectrum of N2 . This of course implies that some of the assignments of the features in the reference Auger spectrum by electron impact [10] have to be reconsidered. In contrast to the CO case [6,7] where noticeable differences have been observed between the de-excitation spectra of the C 1s inner-shell excited singlet and triplet states, here the measured triplet de-excitation spectrum is very similar to that of the singlet state and it is enough to shift the data of the singlet spectrum by the difference in excitation energy to reproduce almost all the spectrum of the triplet state. This is due to the fact that the properties (internuclear distance, vibrational constants and natural width [4]) of the two intermediate states are very similar. The other property of the decay of an inner-shell excited triplet state is the possibility to populate quartet states of the final ion [6,7], which are not accessed in any other way. In order to identify such a contribution in the measured spectra detailed theoretical predictions of the energy position as well as
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of the relative intensities of the doublet final states used in the present assignment of the spectrum are needed. A comparison of the predicted and measured intensities will allow identification of anomalous intensities that may be attributed to the population of quartet states. Acknowledgments Work partially supported by the INTAS Research Project ‘Dynamics of correlated particles in the continuum’, No. 051-4706, the MIUR FIRB project ‘Probing the microscopic dynamics of chemical reactivity’. V. Feyer thanks the ICTP for a TRIL scholarship. References [1] G.C. King, F.H. Read, in: B. Crasemann (Ed.), Atomic Inner-Shell Physics, Plenum Press, New York, 1985, p. 317. [2] A.P. Hitchcock, D.C. Mancini, J. Electron Spectrosc. Relat. Phenom. 67 (1994) 1. [3] S. Svensson, J. Phys. B: At. Mol. Opt. Phys. 38 (2005) S 821. [4] D.A. Shaw, G.C. King, F.H. Read, D. Cvejanovic, J. Phys. B15 (1982) 1785. [5] D.A. Shaw, G.C. King, D. Cvejanovic, F.H. Read, J. Phys. B17 (1984) 2091. [6] V. Feyer, P. Bolognesi, M. Coreno, K.C. Prince, L. Avaldi, Rad. Phys. Chem. 76 (2007) 450. [7] V. Feyer, P. Bolognesi, M. Coreno, K.C. Prince, L. Avaldi, B. Jansik, V. Carravetta, J. Phys. B: At. Mol. Opt. Phys. 40 (2007) F35. [8] L. Ungier, T.D. Thomas, Chem. Phys. Lett. 96 (1982) 247. [9] R.R. Blyth, et al., J. Electron Spectrosc. Relat. Phenom. 101–103 (1999) 959–996. [10] W.E. Moddeman, T.A. Carlson, M.O. Krause, B.P. Pullen, W.E. Bull, G.K. Schweitzer, J. Chem. Phys. 55 (1971) 2317. [11] T.N. Rescigno, A.E. Oriel, in: F.A. Gianturco, G. Stefani (Eds.), Lecture Notes in Chemistry, vol. 35, Springer-Verlag, Berlin, 1984, p. 215. [12] A. Kivim¨aki, M. Neeb, B. Kempgen, H.M. K¨oppe, A.M. Bradshaw, Phys. Rev. A54 (1996) 2137. [13] M.N. Piancastelli, A. Kivim¨aki, B. Kempgens, M. Neeb, K. Maier, U. Hergenhahn, A. R¨udel, A.M. Bradshaw, J. Electron Spectrosc. Relat. Phenom. 98–99 (1999) 111. [14] M. Neeb, J.-E. Rubensson, M. Biermann, W. Eberhardt, J. Electron Spectrosc. Relat. Phenom. 67 (1994) 261. [15] M.N. Piancastelli, et al., J. Phys. B: At. Mol. Opt. Phys. 33 (2000) 1819. [16] P. Baltzer, M. Larsson, L. Karlsson, B. Wannberg, M. Carlsson G¨othe, Phys. Rev. A46 (1992) 5545.
Dipole forbidden inner-shell excitation and decay of the N2 (1s)−1(2p) 3 state studied by electron impact experiments V. Feyer a,b,∗ , P. Bolognesi a , M. Coreno a,c , K.C. Prince c,d , L. Avaldi a,c a
CNR-IMIP, Area della Ricerca di Roma 1, CP10 I-00016 Monterotondo Scalo, Italy b Institute of Electron Physics, 21 Universitetska St., 88017 Uzhgorod, Ukraine c CNR-INFM-TASC, Gasphase Beamline at Elettra, Area Science Park, I-34012 Basovizza, Trieste, Italy d Sincrotrone Trieste, Area Science Park, I-34012 Basovizza, Trieste, Italy Available online 9 February 2007
Abstract The dipole forbidden inner-shell excitation of the N2 molecule to the (1s)−1 (2p) 3 state and its decay have been studied in an electron impact experiment at variable incident energy. The contribution of the dipole forbidden transitions to the N2 autoionization spectrum has been isolated via two different techniques. One is based on a subtraction procedure relying on non-coincidence measurements, while the other consists of an experiment where the electrons ejected in the decay process are detected in coincidence with scattered electrons which suffered an energy loss equal to the excitation energy of the (1s)−1 (2p) 3 state. © 2007 Elsevier B.V. All rights reserved. PACS: 34.80 Gs; 33.80 Eh Keywords: Nitrogen; Electron energy loss spectroscopy; Resonant Auger spectra; Electric-dipole forbidden transitions
1. Introduction For a long time electron energy loss experiments were the best suited tool to investigate the excitation of inner-shell states [1,2] until the advent of third generation synchrotron radiation sources. At these sources beamlines characterized by high resolution and high intensity in the soft X-ray region [3], have been built and photoabsorption measurements overtook electron impact experiments. However, one of the peculiarities of electron impact experiments is their ability to excite dipole forbidden transitions, and for this there is no comparable synchrotron radiation technique. Shaw et al. [4,5] pioneered the investigation of electric-dipole-forbidden transitions in atoms and small molecules. This was achieved by combining a high energy resolution spectrometer and a low incident electron energy (Ei ). At high Ei and small scattering angles the dipole selection rules dominate electron impact processes, but at lower energies the Born approximation breaks down and spin-forbidden processes due to exchange interaction become possible. ∗
Corresponding author at: CNR-IMIP, Area della Ricerca di Roma 1, CP10 I-00016 Monterotondo Scalo, Italy. E-mail address: [email protected] (V. Feyer). 0368-2048/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2007.02.006
In this work we have studied the excitation and decay of the N2 inner-shell dipole forbidden (1s)−1 (2p) 3 transition. As in the previous case of CO [6,7] the excitation of the transition was studied via electron energy loss experiments at several incident energies and large scattering angle. The decay of the inner-shell excited state was investigated by the measurement of the autoionization spectrum. The autoionization or “resonant Auger” spectra (using the nomenclature of synchrotron experiments), were measured at several incident energies. This provided an overview of the features due to the decay of the inner-shell excited states. Then the coincidence detection of the autoionizing electrons with scattered electrons which suffered an energy loss equal to the excitation of one of the 1s → (1s)−1 (2p) 1,3 transitions allowed us to separate the two spectra. This technique, introduced by Ungier and Thomas in 1983 [8], has been used to study the decay of inner-shell excited states of CO and N2 , but never applied to dipole forbidden states due its intrinsic low efficiency. We have taken advantage of the high efficiency of the multicoincidence endstation of the Gasphase beamline at Elettra [9] and the special geometry in which high momentum transfer collisions enhance dipole forbidden transitions, and thus we have isolated the decay of triplet excited state.
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V. Feyer et al. / Journal of Electron Spectroscopy and Related Phenomena 161 (2007) 17–21
2. Experiment The measurements were performed using the multicoincidence end-station of the Gasphase beamline of the Elettra storage ring. For these experiments the end-station was equipped with an electron gun (EKF 300, Omicron GmbH). The design and operating principles of the experimental apparatus are described in detail in Ref. [9] and will not be repeated here. The chamber hosts ten independent electrostatic analysers, arranged in two groups of three and seven analysers, respectively, and placed at 30◦ from each other. They are mounted on two independent turntables in a plane perpendicular to the incident beam. Thus, the scattering angle is fixed at 90◦ . The resonant Auger spectra were measured simultaneously by the seven analyzers of the bigger frame. The spectrum produced by each analyzer was first calibrated in energy and then corrected by the transmission function of the analyzer. Then the seven spectra were summed to improve statistics. To collect the energy loss spectra the three analyzers of the other frame were used. The energy resolution and the angular acceptance in the dispersion plane of the spectrometers were E1,2 = 0.45 and 0.85 eV for the non-coincidence and coincidence spectra, respectively, and ϑ1,2 = ±3◦ . The energy of the resonant Auger spectra was calibrated using the value of the first diagrammatic Auger line, labeled B1 by Moddeman et al. [10] located at 366.5 eV. The energy of the incident electron beam varied from 500 to 1000 eV. In the coincidence experiment the pulses of the inelastic scattered electrons are used to trigger the time circuit and the autoionizing electrons act as stops. The coincidence electronics is made by three independent time-to-digital, TDC, converters. In the experiment each TDC unit is operated in the common
start mode with the signals of each one of the three analysers of the small turntable used as starts and the signals from the other seven as stops. In this way 21 coincidence pairs are collected simultaneously. All the experimental settings and the data acquisition are controlled via a PC equipped with LabView software. The same software monitors the stability of the experiment during the long acquisition times of the coincidence scans via the measurement of the non-coincidence count rates of the 10 analysers at fixed time intervals. The coincidence signals of the 21 scattered-ejected pairs were added up, after a careful energy calibration of the non-coincidence autoionization spectra independently collected by the seven analyzers. This provision enhances the overall efficiency of the set-up and allows the investigation of a process with a small cross section, like the decay of the inner-shell triplet excited state. Under typical experimental conditions of about 1 × 10−5 mbar of gas pressure and 1.8 A current, acquisition times of about 6–10 h/point were needed in order to achieve the present statistics in the coincidence measurements. 3. Results and discussion The energy loss spectra of N2 in the region of excitation to the (1s)−1 (2p) 1,3 states, recorded at θ S = 90◦ , for 500 ≤ Ei ≤ 1000 eV are shown in Fig. 1. The structure observed in all the spectra is clearly composed of two features. The higherenergy feature corresponds to the (1s)−1 (2p) 1 state of N2 . The one on the low energy side, with relative intensity which increases as the incident energy decreases, is assigned to the dipole forbidden 1s → (2p) 3 transition [4]. Two Gaussian functions have been fitted to the features in the energy loss spectrum. The relative intensities, the position of the centroids and
Fig. 1. Energy-loss spectra in the region of N2 (1s−1 )(2p) 1,3 excited states at different incident electron energies and fixed scattering angle of 90◦ .
V. Feyer et al. / Journal of Electron Spectroscopy and Related Phenomena 161 (2007) 17–21
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Fig. 2. The ratio between triplet and singlet intensity in the energy loss spectrum vs. incident energy. Present data, measured at 90◦ : dots; data by Shaw et al. [4], measured at 0◦ : triangles.
widths of the Gaussian functions are used as fitting parameters. The singlet–triplet splitting obtained in the fit is 0.90 ± 0.07 eV, in good agreement with the values of previous measurements of 0.82 ± 0.07 eV by Shaw et al. [4] and the calculations by Rescigno and Oriel [11] of 0.9 eV. Moreover, the peak of the triplet excited state appears to be narrower by about 320 meV than the peak assigned to the singlet excited state. This is consistent with the high resolution results of Shaw et al. [4], which show that a smaller number of vibrational states are excited in the case of the triplet. The ratio of the triplet to singlet intensity strongly depends on the incident energy as well as on the momentum transfer in the collision. This is clearly seen in Fig. 2 where the ratios measured in the present work and by Shaw et al. [4] are reported. In both cases the ratio increases as the incident energy decreases. This is because the exchange interaction becomes more efficient as the incident energy decreases. At the same incident energy the ratios measured in this work are different from those of Shaw et al. [4], due to the different momentum transfer. Indeed in the present work the scattered electrons are detected at 90◦ , while in the case of Shaw et al. [4] the scattered electrons were measured at 0◦ . The N2 Auger spectra at different Ei are shown in Fig. 3. The spectrum measured at Ei = 1000 eV is in very good agreement with the one previously reported in the literature by electron impact [10]. According to these authors the decay of the innershell excited states should produce the features in the energy region above 367 eV, while the diagrammatic Auger spectrum starts with the narrow feature at 366.5 eV. However, recent work by photon excitation [12] showed that the resonant Auger spectrum extends also into the region below 367 eV. In the region above 367 eV at Ei = 1000 eV (see Fig. 3a) three main structures are observed. The first narrow feature with a small shoulder on the high energy side corresponds to the so called participator transitions, where the excited electron is involved in the decay process which results with a hole in the outer orbital. According to the high resolution experiments by Piancastelli et al. [13] the main peak is due to the N2 + 1−1 u final state, while the shoulder is due to the 3−1 final state. The two broad structures observed g at about 375 and 370 eV correspond to spectator transitions,
Fig. 3. (a–c) The Auger and autoionization spectra of N2 at different incident energy Ei .
where the excited electron does not participate in the process, and which end with a 2h-1p N2 + final state. The main effects on decreasing the incident energy (Fig. 3b–c) are the appearance of a new feature on the low energy side of the participator peak and a variation of the relative intensity of the features assigned to spectator transitions. These effects have been attributed to the decay of the triplet excited state, whose excitation probability increases as the incident energy decreases. The absence of such features in the spectrum taken at 1000 eV, while the energy loss at the same incident energy displays a non-negligible excitation of the triplet state can be explained as follows. The energy loss shown in Fig. 1 was measured at 90◦ scattering angle, while the autoionization spectrum shown in Fig. 3 is the sum of all the excitation events occurring when a beam of 1000 eV interacts with N2 molecules. The large majority of energy loss events occur with a low momentum transfer, i.e. at small scattering angle. Thus, the process is dominated by dipole selection rules, as clearly shown in Fig. 1 of Shaw et al. [4], where at 1300 eV and 0◦ scattering they observe no trace of dipole forbidden transitions. Thus, it is not surprising that no features due to the decay of the triplet excited state are observed in our spectrum at 1000 eV. In order to isolate the triplet contribution to the resonant Auger spectra two approaches can be used. In the first one, the resonant Auger spectrum measured at 1000 eV incident energy (Fig. 4a) can be subtracted from the one taken at 600 eV after a proper normalization. This is done upon the hypothesis that the
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V. Feyer et al. / Journal of Electron Spectroscopy and Related Phenomena 161 (2007) 17–21
Fig. 4. (a) The Auger and autoionization spectrum of N2 at 1000 eV incident energy after background subtraction used as a reference spectrum for the deexcitation of the (1s−1 )(2p) 1 excited state; (b) the autoionization spectrum of N2 (1s−1 )(2p) 3 excited state at 600 eV incident energy obtained via the subtraction procedure described in the text; the full and dashed lines represent the fit to the participator part of spectrum (380–387 eV) with three Gaussian functions; (c) the autoionization spectrum of N2 (1s−1 )(2p) 3 excited state measured in coincidence with scattered electrons which suffered an energy loss equal to the 1s → 2p 3 excitation at 600 eV incident energy.
relative intensities, position and width of the peaks due to the 1 resonant Auger spectra are independent of Ei . In the second one the resonant Auger spectrum is measured in coincidence with the inelastically scattered electrons which have suffered an energy loss equal to excitation energy of one of the N2 (1s)−1 (2p) 1,3 states. In this way the spectra due to the singlet and triplet excitations are directly separated. The resonant Auger spectrum resulting from the subtraction procedure is shown in Fig. 4b, while the one obtained in the coincidence experiment is shown in Fig. 4c. The subtraction procedure relies on a non-coincidence experiment and therefore the measurements are faster and allow better energy resolution and statistics. However, this procedure suffers from some drawbacks. First of all the background of secondary electrons under the normal Auger and autoionization features has to be removed, because its shape depends on the incident energy. This may result in small misalignment of the narrower features in the spectra and produces sharp and unrealistic features in the “subtracted
spectrum”, as can be seen in Fig. 4b at about 366.3, 360.6 and 358.9 eV. Moreover, at certain incident energies the energy loss features overlap with the spectrum of the ejected electrons. The coincidence experiments do not suffer from these drawbacks, but, of course, the low count rate makes them time consuming and limits the achievable energy resolution. The autoionization spectrum of the (1s)−1 (2p) 3 state is characterized by several features. They can be divided into two main groups, one centred at about 384 eV corresponding to participator transitions, and the one from 376 to 365 eV corresponding to spectator transitions. According to previous work on the resonant Auger spectrum of the singlet excited state by 2 + photon excitation [12–15] three final states, the N2 + 3−1 g X g , 2 −1 2 −1 + 1u A u and 2u B u states, respectively, are expected to contribute to the participator spectrum, with the 1−1 u one dominating. This has been explained by the fact that the potential energy curves of the intermediate and final states for this transition are parallel. In both Fig. 4b and c we observe that the feature attributed to participator decay is about 0.9 eV below the one observed for the decay of the singlet excited state. This is consistent with difference in the excitation energy of the singlet and triplet excited states. Both spectra in Fig. 4b and c confirm that also in the triplet decay the most likely final state is the A one. However, it is interesting to observe that a fit to the structure between 380 and 387 eV in the coincidence spectrum (Fig. 4c) provides clear evidence of the population of all three final states, while in the subtracted spectrum either the B state is not present or it is hidden in the background. The population of the same state in the case of the singlet state excited by photon absorption has been observed only recently in a high resolution and sensitivity experiment by Piancastelli et al. [15]. The slightly shorter internuclear distance [4] of the triplet excited state may explain the larger intensity of the B state in the decay spectrum of the triplet state. All the features below 380 eV can be assigned to spectator transitions. The features in the region between 379 and 374 eV may be assigned according to the high resolution PES 2 2 work by Baltzer et al. [16] to the C 2 + u , 2 g and D g ion −1 −1 −1 states with the 3g 1g and 3g 1u 1g dominant configurations. The shape of the spectrum in this energy region, with a small feature at about 378.8 eV and a sharp rise at 376.8 eV, can be explained using the decay spectrum of the singlet inner-shell excited state measured with vibrational resolution in both the intermediate and final states [15]. These experimental results supported by theoretical calculations show that the small feature results from the population of the lowest vibrational states of the D2 g final state, which, due to its equilibrium distance of ˚ larger than that of the (1s)−1 (2p) 1 state, can be pop0.3 A ulated only by high vibrational states of the inner-shell excited 2 state. Then the decay into both the C2 + u and D g final states contributes to the main feature in this energy region. Due to the ˚ [4]) of the internuclear distances of small difference (0.007 A the singlet and triplet inner-shell excited states, similar findings are expected also in the decay spectrum of the triplet state and therefore also in our spectrum. Moreover, the integration over the vibrational states of the intermediate and final state should emphasize the main feature.
V. Feyer et al. / Journal of Electron Spectroscopy and Related Phenomena 161 (2007) 17–21
At lower kinetic energy the broad feature centred at about 2 370 eV may be attributed to the F2 + g , G and H u states. The PES spectrum is dominated by the F state that is 2 eV wide and displays no vibrational structure. Therefore, it must have a repulsive potential curve in the Franck-Condon region. According to this we assign the broad feature centred at 370 eV to the F state. Finally, despite the scatter of the data a further feature can be observed in Fig. 4c centred at about 363.3 eV. No further information is available on N2 + inner valence states at binding energy above 30 eV, so the observed feature is unassigned for now. 4. Conclusions The de-excitation spectrum of the N2 inner-shell (1s)−1 (2p) 3 excited state has been isolated for the first time via two different techniques. The use of the scattered electron-autoionization electron coincidence technique, despite the long measurement time required, appeared to be the most suited tool for this purpose. The assignment of the features of the spectrum has been done using the information from the de-excitation spectrum of the 1 inner-shell excited state obtained in photoabsorption experiments and the spectroscopic data of N2 photoionization by Baltzer et al. [16]. This work confirms the richness of the deexcitation spectrum of the inner-shell excited state that extends well below the energy of the normal Auger spectrum of N2 . This of course implies that some of the assignments of the features in the reference Auger spectrum by electron impact [10] have to be reconsidered. In contrast to the CO case [6,7] where noticeable differences have been observed between the de-excitation spectra of the C 1s inner-shell excited singlet and triplet states, here the measured triplet de-excitation spectrum is very similar to that of the singlet state and it is enough to shift the data of the singlet spectrum by the difference in excitation energy to reproduce almost all the spectrum of the triplet state. This is due to the fact that the properties (internuclear distance, vibrational constants and natural width [4]) of the two intermediate states are very similar. The other property of the decay of an inner-shell excited triplet state is the possibility to populate quartet states of the final ion [6,7], which are not accessed in any other way. In order to identify such a contribution in the measured spectra detailed theoretical predictions of the energy position as well as
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of the relative intensities of the doublet final states used in the present assignment of the spectrum are needed. A comparison of the predicted and measured intensities will allow identification of anomalous intensities that may be attributed to the population of quartet states. Acknowledgments Work partially supported by the INTAS Research Project ‘Dynamics of correlated particles in the continuum’, No. 051-4706, the MIUR FIRB project ‘Probing the microscopic dynamics of chemical reactivity’. V. Feyer thanks the ICTP for a TRIL scholarship. References [1] G.C. King, F.H. Read, in: B. Crasemann (Ed.), Atomic Inner-Shell Physics, Plenum Press, New York, 1985, p. 317. [2] A.P. Hitchcock, D.C. Mancini, J. Electron Spectrosc. Relat. Phenom. 67 (1994) 1. [3] S. Svensson, J. Phys. B: At. Mol. Opt. Phys. 38 (2005) S 821. [4] D.A. Shaw, G.C. King, F.H. Read, D. Cvejanovic, J. Phys. B15 (1982) 1785. [5] D.A. Shaw, G.C. King, D. Cvejanovic, F.H. Read, J. Phys. B17 (1984) 2091. [6] V. Feyer, P. Bolognesi, M. Coreno, K.C. Prince, L. Avaldi, Rad. Phys. Chem. 76 (2007) 450. [7] V. Feyer, P. Bolognesi, M. Coreno, K.C. Prince, L. Avaldi, B. Jansik, V. Carravetta, J. Phys. B: At. Mol. Opt. Phys. 40 (2007) F35. [8] L. Ungier, T.D. Thomas, Chem. Phys. Lett. 96 (1982) 247. [9] R.R. Blyth, et al., J. Electron Spectrosc. Relat. Phenom. 101–103 (1999) 959–996. [10] W.E. Moddeman, T.A. Carlson, M.O. Krause, B.P. Pullen, W.E. Bull, G.K. Schweitzer, J. Chem. Phys. 55 (1971) 2317. [11] T.N. Rescigno, A.E. Oriel, in: F.A. Gianturco, G. Stefani (Eds.), Lecture Notes in Chemistry, vol. 35, Springer-Verlag, Berlin, 1984, p. 215. [12] A. Kivim¨aki, M. Neeb, B. Kempgen, H.M. K¨oppe, A.M. Bradshaw, Phys. Rev. A54 (1996) 2137. [13] M.N. Piancastelli, A. Kivim¨aki, B. Kempgens, M. Neeb, K. Maier, U. Hergenhahn, A. R¨udel, A.M. Bradshaw, J. Electron Spectrosc. Relat. Phenom. 98–99 (1999) 111. [14] M. Neeb, J.-E. Rubensson, M. Biermann, W. Eberhardt, J. Electron Spectrosc. Relat. Phenom. 67 (1994) 261. [15] M.N. Piancastelli, et al., J. Phys. B: At. Mol. Opt. Phys. 33 (2000) 1819. [16] P. Baltzer, M. Larsson, L. Karlsson, B. Wannberg, M. Carlsson G¨othe, Phys. Rev. A46 (1992) 5545.