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Partial electron yield spectrum of N2: doubly excited states at the K-shell threshold
7 April 2000
Chemical Physics Letters 320 Ž2000. 217–221 www.elsevier.nlrlocatercplett
Partial electron yield spectrum of N2 : doubly excited states at the K-shell threshold M. Neeb a
a,),1
a , A. Kivimaki , K. Maier a , ¨ a,2 , B. Kempgens a, H.M. Koppe ¨ A.M. Bradshaw a,3, N. Kosugi b
Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4–6, D-14195 Berlin, Germany b Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan Received 23 June 1999
Abstract The partial electron yield spectrum of N2 has been measured at a kinetic energy corresponding to the Auger decay of doubly excited core–hole states. The spectrum reveals previously unresolved double excitations just below and above the K-shell ionization threshold. Their excitation energies and dissociative nature agree with calculated potential energy curves of the doubly excited states. q 2000 Published by Elsevier Science B.V. All rights reserved.
Resonances below the core ionization threshold of small molecules are usually attributed to one-hole one-electron excitations, i.e. core-to-valence and core-to-Rydberg excitations. Recently, however, the electronic decay spectrum of N2 has revealed resonances below the N 1s threshold which do not stem from singly excited states w1x. These resonances were assigned to core-hole double excitations, where the excitation of the N 1s electron is accompanied by a simultaneous excitation of a valence electron. The existence of such states in the region of the core-toRydberg transitions has been predicted by Arneberg
) Corresponding author. Fax: q49-2461-61-4037; e-mail: [email protected] 1 Present address: Forschungszentrum Julich, Postfach 1913, ¨ D-52425 Julich, Germany. 2 Present address: Department of Physical Sciences, University of Oulu, PL 3000, 90401 Oulu, Finland. 3 Present address: Max-Planck-Institut fur ¨ Plasmaphysik, Boltzmannstr. 2, D-85748 Garching, Germany.
et al. w2x some 15 years earlier. Whereas double excitations in the continuum are easily observed in the normal X-ray absorption spectrum of N2 w3x, the doubly excited states below threshold are much more difficult to detect since they are obscured by the more intense core-to-Rydberg transitions. Indeed, prior to Ref. w1x doubly excited states had not been detected below threshold, although the core excitation spectrum of N2 had been studied in great detail by using photoabsorption w3,4x, electron-energy loss spectroscopy w5–7x, and fragment ion-yield spectroscopy w8–10x. Resonant Auger-decay spectroscopy proves to be the only sensitive probe for the detection of these core-excited states w1x. This technique has also been utilized successfully to expose other excitations that are not resolved in the absorption spectra, e.g., of adsorbed N2 , or of free O 2 and CO 2 w11–13x. Although it has been successfully used to identify absorption states obscured by other resonances it remains an indirect method. In general, it would be highly desirable to decompose degenerate
0009-2614r00r$ - see front matter q 2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 0 0 . 0 0 2 2 9 - 3
218
M. Neeb et al.r Chemical Physics Letters 320 (2000) 217–221
Rydberg transition. The next feature is composed of the 1sy1 3ps and 1sy1 3pp transitions. At still higher excitation energies the individual Rydberg transitions are not fully resolved. Symmetry-resolved ion yield absorption spectra have been used successfully to identify the symmetries of these core-excited states w8–10x. The broad double-peak structure above threshold, at ; 415 eV in Fig. 1, originates from double excitations. The maximum seen at the highest photon energies is due to the N 1s s ) shape resonance. The partial electron yield spectrum at the K-shell ionization edge of N2 is shown in Fig. 2. This spectrum shows the Auger-electron yield at 384 eV kinetic energy which was chosen because the feature at this energy has been shown to result from the resonant-Auger decay of doubly excited states w1x. Moreover, as the resonant Auger peak stays at a constant kinetic energy, due to fragmentation prior to the Auger decay, the partial electron yield spectrum can be readily measured. The spectrum was recorded with a kinetic energy resolution of ; 0.7 eV. It transpires that the partial yield spectrum is totally different from the total yield spectrum of Fig. 1. Below threshold a broad feature with a smooth shoulder at 408.5 eV is seen. The feature extends
™
Fig. 1. The secondary electron partial yield spectrum of N2 . The structures below the core ionization threshold at 409.9 eV are due to transitions into Rydberg orbitals. Double excitations in the 1s continuum and the s ) shape resonance are seen around 415 and 419 eV excitation energies, respectively.
features in the absorption spectrum and to determine their excitation energy, intensity, line width, and symmetry. In the present Letter we show that by a partial-electron yield spectrum the core-excited double excitations of N2 can be measured without contributions from the core-to-Rydberg transitions and the s ) shape resonance. The measurements were carried out on the soft X-ray undulator beamline X1B w14x at the National Synchrotron Light Source, Brookhaven. The partial electron yield spectra of N2 were measured with the magic-angle cylindrical mirror analyzer ŽCMA. w15x by scanning the photon energy and recording emitted electrons within a fixed kinetic energy window. The monochromator bandwidth was set to ; 100 meV for the spectra presented in this Letter. Fig. 1 shows the partial electron yield of secondary electrons with a kinetic energy of ; 1 eV which represents to a good approximation the total yield spectrum. The spectrum is quite similar to the absorption spectra measured by X-ray absorption w3x and electron energy loss spectroscopy w5–7x. The core electron ionization potential at 409.9 eV is marked by a vertical line in the figure. The intense features below threshold are due to the excitations of a core electron into various Rydberg orbitals. The lowest feature at 406 eV is due to the 1sy1 3ss
Fig. 2. The N2 partial electron yield spectrum using electrons at a kinetic energy of 384 eV. Transitions into Rydberg orbitals and the shape resonance do not appear in this spectrum. All resonances observed are due to double excitations below and above threshold.
M. Neeb et al.r Chemical Physics Letters 320 (2000) 217–221
from 408 eV up to 2 eV above the single hole ionization threshold with an intensity maximum at 410.5 eV. A partial yield measurement with a higher photon energy resolution revealed no further fine structure from this feature. The double peak structure in the continuum between 413 and 418 eV is similar to the peak structure in the total yield spectrum. Above 418 eV the absorption intensity drops down to nearly zero. As has been shown in Ref. w1x, neither the coreexcited Rydberg states nor the 1sy1 ionized state result in any Auger feature above 380 eV. Thus, the partial yield spectrum measured at 384 eV kinetic energy should not be influenced by the core-to-Rydberg transitions or by the behaviour of the N 1sy1 photoionization cross-section. Indeed, the ionization edge at 409.9 eV does not appear to cause any change in the partial yield spectrum and no structure can be seen at the shape resonance around 419 eV. Furthermore, only very small peaks are seen at positions corresponding to some of the Rydberg resonances. We conclude that all resonances in Fig. 2 arise from neutral doubly excited states. The double excitations between 406 and 412 eV have not been seen before in any absorption-type spectrum. Those between 413 and 418 eV are revealed for the first time without the contribution of the shape resonance. Note that the intensity of the continuum double excitations is reduced to almost zero above 418 eV. Direct valence photoemission from the 2 sgy1 final state, which moves across the chosen kinetic energy window of the CMA, may be responsible for the residual weak signal. Fig. 3 shows the calculated potential energy curves for the core-hole doubly excited states involving both molecular orbitals and Rydberg states. The potential energy scale is referenced to the energy of the ground vibrational level of the 1sy1 state at 409.9 eV Ždashed-dotted curve.. The calculations are based on multi-reference configuration interaction including single and double excitations ŽMR-SDCI.. The CI calculations were carried out using symmetry adapted SCF orbitals obtained with the GSCF3 w16x code. Ž4111111r4111. contracted Gaussian-type functions have been taken from Ž73r7. of Huzinaga et al. w17x. The diabatic potential curves Ždotted lines. of the doubly excited Rydberg states refer to the potential curves of the corresponding shake-up
219
Fig. 3. The calculated potential energy curves of N2 . The diabatic curves for 3s and 3p Rydberg doubly excited states Ždotted curves. converge to the shake-up ionized state. The adiabatic curves for all other states are of pure valence character Žbold curves..
core-hole state Ždashed-dotted line.. These states have been obtained by static exchange calculations similar to the approach described in Ref. w18x. The potential curve with a minimum at y8.8 eV in Fig. 3 represents the singly excited 1sy1 1p ) state which appears in the absorption spectrum at 401 eV. The vertical excitation energy of the first monopole shake up state Ž1ssuy1 1puy1 1pg1 Ž P .. was calculated at 9.02 eV above the core ionization threshold. The remaining curves are due to doubly excited states, two of S symmetry and four of P symmetry. The S states have main configurations 1ssuy1 3sgy1 1pg2 and 1ssgy1 1puy1 1pg1 3ssg1. The P states have main configurations 1ssgy1 1puy1 1pg2 and 1ssuy1 1puy1 1pg1 3ppu1. In the immediate neighbourhood of the 1s ionization threshold, one doubly excited state of S symmetry and one of P symmetry have been calculated within the Franck– ˚ The degenerate lowest Condon region at 1.1 A.
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M. Neeb et al.r Chemical Physics Letters 320 (2000) 217–221
unoccupied molecular orbital ŽLUMO. Žs 1pg . is filled in the S state by two electrons, one from the core orbital and one from the highest occupied molecular orbital ŽHOMO. Žs 3sg .. In the P state a 1s electron and an electron from the HOMOy 1 Žs 1pu . are simultaneously excited into the LUMO. Within a one-electron approximation the excitation energies of the two lowest doubly excited states can qualitatively be estimated to occur close to the core ionization threshold. Their excitation energies can be approximated empirically by adding the energy of the 1sy1 1pg1 state Ž400.9 eV. to the shake energies of the lowest 1sy1 3sgy1 1pg1 Ž1 P, 8.2 eV. w19x and second lowest 1sy1 1puy1 1pg1 shake up states Ž1 S, 9.4 eV. w19x, which sum up to 409.1 and 410.3 eV, respectively. The computed excitation energies of the predicted states are almost degenerate with the 1s ionization energy Ž409.9 eV.. The vertical transition energies of the two lowest double excitations are calculated at q0.46 and q0.6 eV with respect to the 1sy1 vibrational ground state. The calculations are in good agreement with the positions of the broad absorption feature at 410 eV in Fig. 2. Only a single peak is resolved in the experiment. Its width indicates a strong dissociative character of the doubly excited states which is due in turn to the twofold-excited antibonding LUMO. The antibonding character is confirmed by the shape of the potential curves of the molecular-type double excitations: the equilibrium distance of the doubly excited states is shifted to a higher value than that of ˚ . and they are shallow. the ground state Ž D re ) 0.3 A Hence, there is a high probability of excitation of vibrational states in the continuum and dissociation is therefore very likely to occur. Dissociation and atomic-like Auger decay following core excitation at the core-ionization threshold were indeed observed in the resonant Auger spectra of N2 w1x. The next highest doubly excited state Ž P . occurs at q3.29 eV above the 1s ionization threshold. The corresponding absorption feature is observed at 414 eV in Fig. 2. Another doubly excited state with total P symmetry is calculated at q6.45 eV. The transition energy overlaps with that of the double excitations with 1sy1 1puy1 1pg1 ŽRydberg.1 configurations. These are calculated at q5.12 and q6.22 eV. As expected, the potential curves of the two Rydberg states are similar to the potential curve of the core
ionised shake up state Ž1sy1 1puy1 1pg1 . which is located at q9.02 eV above threshold. The Rydberg double excitations are much less dissociative than the antibonding double excitations at lower excitation energies as can be seen by the potential curves in Fig. 3. As the peak at 384 eV originates from atomic-like Auger decay, the partial electron yield spectrum of Fig. 2 favours the detection of the dissociative molecular-like double excitations Žof type 1sy1 valy1 1pg2 . rather than the Rydberg-type double excitations. Therefore a quantitative estimation of the relative cross-sections of the core-excited molecular-type double excitations and the Rydbergtype double excitations is not possible. The low intensity at the transition energy of the lowest coreto-Rydberg single excitation at 406 and 407 eV is explained by a mixing of these Rydberg configurations with the double excitations. This contribution to the eigenvector composition of the core-to-Rydberg states will therefore give rise to absorption features exactly at the transition energies of these resonances. Thus the partial-yield spectrum turns out to be a general method not only to disentangle overlapping resonances, but also to separate the contributions from single and mixed electron configurations. Unfortunately, it is not possible to measure the intensity contribution of these doubly excited states to the photoabsorption spectrum, although we note from the difficulties previously experienced in assigning the spectra close to the threshold w3,9,10x that the effect is important. As we have already pointed out w1x, the possible presence of the doubly excited states below threshold means that curve resolving exercises based entirely on the assumption of singly excited Rydberg states may not be correct, however good the fit is. In summary, doubly excited states have been revealed both below and immediately above the core ionization threshold in N2 by measuring a partial electron yield spectrum at the kinetic energy corresponding to the Auger decay of the double excitations. In this partial yield spectrum the core-to-Rydberg transitions and the s ) shape resonance are absent from this absorption spectrum. The calculated potential energy curves suggest a strongly dissociative nature of the molecular-type double excitations. This agrees well with the observed width of the absorption feature in the partial yield spectrum.
M. Neeb et al.r Chemical Physics Letters 320 (2000) 217–221
References w1x M. Neeb, A. Kivimaki, A.M. ¨ B. Kempgens, H.M. Koppe, ¨ Bradshaw, Phys. Rev. Lett. 76 Ž1996. 2250. ˚ w2x R. Arneberg, H. Agren, J. Muller, R. Manne, Chem. Phys. ¨ Lett. 91 Ž1982. 362. w3x C.T. Chen, Y. Ma, F. Sette, Phys. Rev. A 45 Ž1992. 2915. w4x M. Nakamura, M. Sasanuma, S. Sato, M. Watanabe, H. Yamashita, Y. Iguchi, E. Ejiri, S. Nakai, S. Yamaguchi, T. Sagawa, Y. Nakai, T. Oshio, Phys. Rev. 178 Ž1969. 80. w5x G.R. Wight, C.E. Brion, M.J. Van der Wiel, J. Electron Spectrosc. Relat. Phenom. 1 Ž1972. 457. w6x A.P. Hitchcock, C.E. Brion, J. Electron Spectrosc. Relat. Phenom. 18 Ž1980. 1. w7x M. Tronc, G.C. King, F.H. Read, J. Phys. B 13 Ž1980. 999. w8x N. Saito, H. Suzuki, Phys. Rev. Lett. 61 Ž1988. 2740. w9x E. Shigemasa, K. Ueda, Y. Sato, T. Sasaki, A. Yagishita, Phys. Rev. A 45 Ž1992. 2915. w10x K. Lee, D.Y. Kim, C.-I. Ma, D.M. Hanson, J. Chem. Phys. 100 Ž1994. 8550.
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w11x A. Sandell, O. Bjorneholm, A. Nilsson, E.D.F. Zdansky, H. ¨ Tillborg, J.N. Andersen, N. Martensson, Phys. Rev. Lett. 70 ˚ Ž1993. 2000. w12x M. Neeb, J.-E. Rubensson, M. Biermann, W. Eberhardt, Phys. Rev. Lett. 71 Ž1993. 3091. w13x M.N. Piancastelli, A. Kivimaki, ¨ B. Kempgens, M. Neeb, K. Maier, A.M. Bradshaw, Chem. Phys. Lett. 274 Ž1997. 13. w14x K.J. Randall, J. Feldhaus, W. Erlebach, A.M. Bradshaw, W. Eberhardt, Z. Xu, Y. Ma, P.D. Johnson, Rev. Sci. Instrum. 63 Ž1992. 362. w15x J. Feldhaus, W. Erlebach, A.L.D. Kilcoyne, K.J. Randall, M. Schmidbauer, Rev. Sci. Instrum. 63 Ž1992. 1454. w16x N. Kosugi, Theor. Chim. Acta 72 Ž1987. 149. w17x S. Huzinaga, J. Andzelm, M. Klobukowski, E. RadzioAndzelm, Y. Sakai, H. Tatewaki, Gaussian Basis Sets for Molecular Calculations, Elsevier, Amsterdam, 1984. w18x A. Yagishita, E. Shigemasa, N. Kosugi, Phys. Rev. Lett. 72 Ž1994. 3961. w19x B. Kempgens, A. Kivimaki, A.M. ¨ M. Neeb, H.M. Koppe, ¨ Bradshaw, J. Phys. B 29 Ž1996. 5389.
Chemical Physics Letters 320 Ž2000. 217–221 www.elsevier.nlrlocatercplett
Partial electron yield spectrum of N2 : doubly excited states at the K-shell threshold M. Neeb a
a,),1
a , A. Kivimaki , K. Maier a , ¨ a,2 , B. Kempgens a, H.M. Koppe ¨ A.M. Bradshaw a,3, N. Kosugi b
Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4–6, D-14195 Berlin, Germany b Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan Received 23 June 1999
Abstract The partial electron yield spectrum of N2 has been measured at a kinetic energy corresponding to the Auger decay of doubly excited core–hole states. The spectrum reveals previously unresolved double excitations just below and above the K-shell ionization threshold. Their excitation energies and dissociative nature agree with calculated potential energy curves of the doubly excited states. q 2000 Published by Elsevier Science B.V. All rights reserved.
Resonances below the core ionization threshold of small molecules are usually attributed to one-hole one-electron excitations, i.e. core-to-valence and core-to-Rydberg excitations. Recently, however, the electronic decay spectrum of N2 has revealed resonances below the N 1s threshold which do not stem from singly excited states w1x. These resonances were assigned to core-hole double excitations, where the excitation of the N 1s electron is accompanied by a simultaneous excitation of a valence electron. The existence of such states in the region of the core-toRydberg transitions has been predicted by Arneberg
) Corresponding author. Fax: q49-2461-61-4037; e-mail: [email protected] 1 Present address: Forschungszentrum Julich, Postfach 1913, ¨ D-52425 Julich, Germany. 2 Present address: Department of Physical Sciences, University of Oulu, PL 3000, 90401 Oulu, Finland. 3 Present address: Max-Planck-Institut fur ¨ Plasmaphysik, Boltzmannstr. 2, D-85748 Garching, Germany.
et al. w2x some 15 years earlier. Whereas double excitations in the continuum are easily observed in the normal X-ray absorption spectrum of N2 w3x, the doubly excited states below threshold are much more difficult to detect since they are obscured by the more intense core-to-Rydberg transitions. Indeed, prior to Ref. w1x doubly excited states had not been detected below threshold, although the core excitation spectrum of N2 had been studied in great detail by using photoabsorption w3,4x, electron-energy loss spectroscopy w5–7x, and fragment ion-yield spectroscopy w8–10x. Resonant Auger-decay spectroscopy proves to be the only sensitive probe for the detection of these core-excited states w1x. This technique has also been utilized successfully to expose other excitations that are not resolved in the absorption spectra, e.g., of adsorbed N2 , or of free O 2 and CO 2 w11–13x. Although it has been successfully used to identify absorption states obscured by other resonances it remains an indirect method. In general, it would be highly desirable to decompose degenerate
0009-2614r00r$ - see front matter q 2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 0 0 . 0 0 2 2 9 - 3
218
M. Neeb et al.r Chemical Physics Letters 320 (2000) 217–221
Rydberg transition. The next feature is composed of the 1sy1 3ps and 1sy1 3pp transitions. At still higher excitation energies the individual Rydberg transitions are not fully resolved. Symmetry-resolved ion yield absorption spectra have been used successfully to identify the symmetries of these core-excited states w8–10x. The broad double-peak structure above threshold, at ; 415 eV in Fig. 1, originates from double excitations. The maximum seen at the highest photon energies is due to the N 1s s ) shape resonance. The partial electron yield spectrum at the K-shell ionization edge of N2 is shown in Fig. 2. This spectrum shows the Auger-electron yield at 384 eV kinetic energy which was chosen because the feature at this energy has been shown to result from the resonant-Auger decay of doubly excited states w1x. Moreover, as the resonant Auger peak stays at a constant kinetic energy, due to fragmentation prior to the Auger decay, the partial electron yield spectrum can be readily measured. The spectrum was recorded with a kinetic energy resolution of ; 0.7 eV. It transpires that the partial yield spectrum is totally different from the total yield spectrum of Fig. 1. Below threshold a broad feature with a smooth shoulder at 408.5 eV is seen. The feature extends
™
Fig. 1. The secondary electron partial yield spectrum of N2 . The structures below the core ionization threshold at 409.9 eV are due to transitions into Rydberg orbitals. Double excitations in the 1s continuum and the s ) shape resonance are seen around 415 and 419 eV excitation energies, respectively.
features in the absorption spectrum and to determine their excitation energy, intensity, line width, and symmetry. In the present Letter we show that by a partial-electron yield spectrum the core-excited double excitations of N2 can be measured without contributions from the core-to-Rydberg transitions and the s ) shape resonance. The measurements were carried out on the soft X-ray undulator beamline X1B w14x at the National Synchrotron Light Source, Brookhaven. The partial electron yield spectra of N2 were measured with the magic-angle cylindrical mirror analyzer ŽCMA. w15x by scanning the photon energy and recording emitted electrons within a fixed kinetic energy window. The monochromator bandwidth was set to ; 100 meV for the spectra presented in this Letter. Fig. 1 shows the partial electron yield of secondary electrons with a kinetic energy of ; 1 eV which represents to a good approximation the total yield spectrum. The spectrum is quite similar to the absorption spectra measured by X-ray absorption w3x and electron energy loss spectroscopy w5–7x. The core electron ionization potential at 409.9 eV is marked by a vertical line in the figure. The intense features below threshold are due to the excitations of a core electron into various Rydberg orbitals. The lowest feature at 406 eV is due to the 1sy1 3ss
Fig. 2. The N2 partial electron yield spectrum using electrons at a kinetic energy of 384 eV. Transitions into Rydberg orbitals and the shape resonance do not appear in this spectrum. All resonances observed are due to double excitations below and above threshold.
M. Neeb et al.r Chemical Physics Letters 320 (2000) 217–221
from 408 eV up to 2 eV above the single hole ionization threshold with an intensity maximum at 410.5 eV. A partial yield measurement with a higher photon energy resolution revealed no further fine structure from this feature. The double peak structure in the continuum between 413 and 418 eV is similar to the peak structure in the total yield spectrum. Above 418 eV the absorption intensity drops down to nearly zero. As has been shown in Ref. w1x, neither the coreexcited Rydberg states nor the 1sy1 ionized state result in any Auger feature above 380 eV. Thus, the partial yield spectrum measured at 384 eV kinetic energy should not be influenced by the core-to-Rydberg transitions or by the behaviour of the N 1sy1 photoionization cross-section. Indeed, the ionization edge at 409.9 eV does not appear to cause any change in the partial yield spectrum and no structure can be seen at the shape resonance around 419 eV. Furthermore, only very small peaks are seen at positions corresponding to some of the Rydberg resonances. We conclude that all resonances in Fig. 2 arise from neutral doubly excited states. The double excitations between 406 and 412 eV have not been seen before in any absorption-type spectrum. Those between 413 and 418 eV are revealed for the first time without the contribution of the shape resonance. Note that the intensity of the continuum double excitations is reduced to almost zero above 418 eV. Direct valence photoemission from the 2 sgy1 final state, which moves across the chosen kinetic energy window of the CMA, may be responsible for the residual weak signal. Fig. 3 shows the calculated potential energy curves for the core-hole doubly excited states involving both molecular orbitals and Rydberg states. The potential energy scale is referenced to the energy of the ground vibrational level of the 1sy1 state at 409.9 eV Ždashed-dotted curve.. The calculations are based on multi-reference configuration interaction including single and double excitations ŽMR-SDCI.. The CI calculations were carried out using symmetry adapted SCF orbitals obtained with the GSCF3 w16x code. Ž4111111r4111. contracted Gaussian-type functions have been taken from Ž73r7. of Huzinaga et al. w17x. The diabatic potential curves Ždotted lines. of the doubly excited Rydberg states refer to the potential curves of the corresponding shake-up
219
Fig. 3. The calculated potential energy curves of N2 . The diabatic curves for 3s and 3p Rydberg doubly excited states Ždotted curves. converge to the shake-up ionized state. The adiabatic curves for all other states are of pure valence character Žbold curves..
core-hole state Ždashed-dotted line.. These states have been obtained by static exchange calculations similar to the approach described in Ref. w18x. The potential curve with a minimum at y8.8 eV in Fig. 3 represents the singly excited 1sy1 1p ) state which appears in the absorption spectrum at 401 eV. The vertical excitation energy of the first monopole shake up state Ž1ssuy1 1puy1 1pg1 Ž P .. was calculated at 9.02 eV above the core ionization threshold. The remaining curves are due to doubly excited states, two of S symmetry and four of P symmetry. The S states have main configurations 1ssuy1 3sgy1 1pg2 and 1ssgy1 1puy1 1pg1 3ssg1. The P states have main configurations 1ssgy1 1puy1 1pg2 and 1ssuy1 1puy1 1pg1 3ppu1. In the immediate neighbourhood of the 1s ionization threshold, one doubly excited state of S symmetry and one of P symmetry have been calculated within the Franck– ˚ The degenerate lowest Condon region at 1.1 A.
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M. Neeb et al.r Chemical Physics Letters 320 (2000) 217–221
unoccupied molecular orbital ŽLUMO. Žs 1pg . is filled in the S state by two electrons, one from the core orbital and one from the highest occupied molecular orbital ŽHOMO. Žs 3sg .. In the P state a 1s electron and an electron from the HOMOy 1 Žs 1pu . are simultaneously excited into the LUMO. Within a one-electron approximation the excitation energies of the two lowest doubly excited states can qualitatively be estimated to occur close to the core ionization threshold. Their excitation energies can be approximated empirically by adding the energy of the 1sy1 1pg1 state Ž400.9 eV. to the shake energies of the lowest 1sy1 3sgy1 1pg1 Ž1 P, 8.2 eV. w19x and second lowest 1sy1 1puy1 1pg1 shake up states Ž1 S, 9.4 eV. w19x, which sum up to 409.1 and 410.3 eV, respectively. The computed excitation energies of the predicted states are almost degenerate with the 1s ionization energy Ž409.9 eV.. The vertical transition energies of the two lowest double excitations are calculated at q0.46 and q0.6 eV with respect to the 1sy1 vibrational ground state. The calculations are in good agreement with the positions of the broad absorption feature at 410 eV in Fig. 2. Only a single peak is resolved in the experiment. Its width indicates a strong dissociative character of the doubly excited states which is due in turn to the twofold-excited antibonding LUMO. The antibonding character is confirmed by the shape of the potential curves of the molecular-type double excitations: the equilibrium distance of the doubly excited states is shifted to a higher value than that of ˚ . and they are shallow. the ground state Ž D re ) 0.3 A Hence, there is a high probability of excitation of vibrational states in the continuum and dissociation is therefore very likely to occur. Dissociation and atomic-like Auger decay following core excitation at the core-ionization threshold were indeed observed in the resonant Auger spectra of N2 w1x. The next highest doubly excited state Ž P . occurs at q3.29 eV above the 1s ionization threshold. The corresponding absorption feature is observed at 414 eV in Fig. 2. Another doubly excited state with total P symmetry is calculated at q6.45 eV. The transition energy overlaps with that of the double excitations with 1sy1 1puy1 1pg1 ŽRydberg.1 configurations. These are calculated at q5.12 and q6.22 eV. As expected, the potential curves of the two Rydberg states are similar to the potential curve of the core
ionised shake up state Ž1sy1 1puy1 1pg1 . which is located at q9.02 eV above threshold. The Rydberg double excitations are much less dissociative than the antibonding double excitations at lower excitation energies as can be seen by the potential curves in Fig. 3. As the peak at 384 eV originates from atomic-like Auger decay, the partial electron yield spectrum of Fig. 2 favours the detection of the dissociative molecular-like double excitations Žof type 1sy1 valy1 1pg2 . rather than the Rydberg-type double excitations. Therefore a quantitative estimation of the relative cross-sections of the core-excited molecular-type double excitations and the Rydbergtype double excitations is not possible. The low intensity at the transition energy of the lowest coreto-Rydberg single excitation at 406 and 407 eV is explained by a mixing of these Rydberg configurations with the double excitations. This contribution to the eigenvector composition of the core-to-Rydberg states will therefore give rise to absorption features exactly at the transition energies of these resonances. Thus the partial-yield spectrum turns out to be a general method not only to disentangle overlapping resonances, but also to separate the contributions from single and mixed electron configurations. Unfortunately, it is not possible to measure the intensity contribution of these doubly excited states to the photoabsorption spectrum, although we note from the difficulties previously experienced in assigning the spectra close to the threshold w3,9,10x that the effect is important. As we have already pointed out w1x, the possible presence of the doubly excited states below threshold means that curve resolving exercises based entirely on the assumption of singly excited Rydberg states may not be correct, however good the fit is. In summary, doubly excited states have been revealed both below and immediately above the core ionization threshold in N2 by measuring a partial electron yield spectrum at the kinetic energy corresponding to the Auger decay of the double excitations. In this partial yield spectrum the core-to-Rydberg transitions and the s ) shape resonance are absent from this absorption spectrum. The calculated potential energy curves suggest a strongly dissociative nature of the molecular-type double excitations. This agrees well with the observed width of the absorption feature in the partial yield spectrum.
M. Neeb et al.r Chemical Physics Letters 320 (2000) 217–221
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