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Nitrogen and oxygen core excitations in solid NaNO2 studied by X-ray absorption and resonant photoemission
Chemical Physics 249 Ž1999. 249–258 www.elsevier.nlrlocaterchemphys
Nitrogen and oxygen core excitations in solid NaNO 2 studied by X-ray absorption and resonant photoemission A.S. Vinogradov a,) , A.B. Preobrajenski a,b, S.L. Molodtsov a,c , S.A. Krasnikov a , d R. Szargan b, A. Knop-Gericke d , M. Havecker ¨ a
b
Institute of Physics, St. Petersburg State UniÕersity, 198904, St. Petersburg, Russian Federation W. Ostwald-Institut fur ¨ Physikalische und Theoretische Chemie, UniÕersitat ¨ Leipzig, Linnestr. ´ 2, D-04103 Leipzig, Germany c Institut fur und Mikrostrukturphysik, Technische UniÕersitat ¨ Oberflachen¨ ¨ Dresden, D-01062 Dresden, Germany d Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany Received 2 March 1999
Abstract The origin and properties of the nitrogen and oxygen K excitations in solid NaNO 2 were investigated combining high-resolution X-ray absorption and resonant photoemission spectroscopy. The absorption spectra obtained were treated on the basis of comparison between the spectra of solid NaNO 2 and gas-phase NO 2 and CH 3 NO 2 molecules, as a result of which main absorption bands were assumed to be associated with the core electron transitions to unoccupied p and s molecular orbitals of the planar NO 2 Na quasimolecule – the structural unit of the crystal. Examining series of the photoemission spectra taken at the N K and O K edges, it was found that the p and s excitations decay in different ways: three decay channels Žspectator, participator and normal Auger processes. were observed for the p excitations whereas only the normal and spectator Auger decay channels were seen for the s excitations. This fact was qualitatively explained in terms of differences in the spatial localization and lifetime of the corresponding core excitations. The results of the resonant photoemission study give a direct evidence for the quasimolecular NOy 2 origin of the lowest unoccupied electronic states of the p type in solid NaNO 2 . q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction The dominant shape resonances in X-ray absorption spectra of the simple low-Z molecules N2 , CO, NO, HCN, etc., are known to be caused by the multiple scattering of low-energy X-ray photoelectrons from nearest-neighbor atoms w1x. Therefore, these resonances are strongly localized within these small atomic groups and may be described by unoc-
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Corresponding author. Fax: q7-812-428-7240
cupied molecular orbitals ŽMOs. of the latter. Development of these core excitations with increasing size of a polyatomic system is of fundamental interest because their spectral characteristics embody valuable information on the local atomic and electronic structure of the system. In this respect the polyatomic molecules, complexes and solids, containing the first-row atoms in clearly separated units like COy, CNy, NOy 2 , etc., are very promising candidates for experimental studies of the resonances and their relation to the properties of empty electronic states in complicated systems. Among them is an
0301-0104r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 0 1 0 4 Ž 9 9 . 0 0 2 8 7 - 6
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ionic-molecular crystal of sodium nitrite NaNO 2 inw2x. corporating planar molecular anions NOy 2 Recently X-ray absorption spectra in vicinity of the nitrogen, oxygen, and sodium K edges have been investigated for the polycrystalline samples of NaNO 2 with moderate energy resolution using bremsstrahlung of a X-ray tube w3–6x. Near-edge features like those in the case of simple molecules, namely, intense and rather narrow low-energy peak and higher-energy broad absorption band having additional structure w3x, were monitored in the N K and O K absorption spectra. Comparative analysis of the obtained spectra with the corresponding spectra of the NO molecule has shown that the near-edge N K and O K excitations of the sodium nitrite crystal have a quasimolecular origin and can be regarded as a result of the core electron transitions to the empty p w x and s MOs of the molecular NOy 2 anion 3 . On the basis of a quasiatomic consideration, it was also found that these excitations can be successfully described by the p and s components of the atomic 1s–2p atomic resonance splitted by molecular anion field w4x. A relation of the latter to electronic states of the conduction band of NaNO 2 was then discussed in the Ref. w5x. The main feature of the electronic structure obtained for the NaNO 2 crystal is the rather narrow Ž; 1 eV. first sub-band near the bottom of the conduction band, which is separated from the second sub-band by a considerably large energy gap Ž; 2.5 eV.. These unusual properties of the first sub-band can be understood by its quasimolecular origin: its electronic states are assumed to be strongly localized within the molecular NOy 2 anion. In contrast to that, electronic states of the second sub-band are extended over the whole crystal. In addition, an indirect evidence for the strong localization of the core-to-p-state excitations has been presumably derived from a comparative analysis of the N K absorption spectra of NaNO 2 measured under the same experimental conditions by means of direct photoabsorption and total electron yield techniques w6x. In the present study, high-resolution absorption and resonant photoemission spectra of a NaNO 2 ionic-molecular crystal were measured using synchrotron radiation in order to obtain new information on localization, lifetime and decay processes of core excitations and their relation to the unoccupied electronic states of the polyatomic system.
2. Experimental The experiments were carried out at the SX 700-I beamline of the Berliner Elektronenspeicherring fur ¨ Synchrotronstrahlung ŽBESSY. w7x. The monochromator was operated with energy resolution of 0.3 eV at the N K edge and 0.6 eV at the O K edge. The absorption spectra Žstep width between experimental points 0.1–0.2 eV, integration time per point 1 s. were recorded by detecting the total electron yield. The photon energy was calibrated with an accuracy of "0.5 eV by means of measurements of N K and O K core level photoemission ŽPE. of NaNO 2 with first- and second-order light and energies of the corresponding 1s electron excitations into the empty p state known from Ref. w3x. The NaNO 2 samples were films prepared from thoroughly dehydrated powder by thermal evaporation in situ from a tantalum Knudsen cell onto polished copper substrates in a vacuum of 2 = 10y7 mbar. Thickness of the sam˚ with a quartz ples was estimated to be about 200 A crystal monitor. No sample charging effects were observed during the measurements. A VG CLAM 100 electron-energy analyzer was used for the photoemission experiments. The analyzer resolution Žfull width at half maximum. was set to 0.5 eV corresponding to an analyzer pass energy of 50 eV. No absolute electron energy calibration was done for the PE spectra Žstep width 0.2 eV, integration time per point 1 s, time for collecting the spectrum 20–50 min.. The latter were analyzed by aligning the Na 2p peaks to equal binding energies. During measurements the base pressure in the analyzer chamber was about 1 = 10y1 0 mbar. The photon flux of incident radiation was monitored from a total electron yield of a clean Cu Ž110. surface.
3. Results and discussion Fig. 1 shows N K and O K photoabsorption spectra of NaNO 2 recorded with the total electron yield. These spectra normalized to incident photon flux are in good agreement with those reported earlier w3,5x and show no additional structures except some asymmetry on the high-energy side of peak A. A weak low-energy shoulder Žs. at the base of absorption band B arises from slow decomposition of NOy 2 ions
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holes and excited electrons. This experimental result indicates that a common set of the unoccupied electronic states of NaNO 2 is responsible for the nitrogen and oxygen K excitation spectra. Comparing the N K absorption spectra for solid NaNO 2 , gas-phase molecules NO 2 w8x, and CH 3 NO 2 w9x ŽFig. 2., the origin of these electronic states in sodium nitrite can be easy understood. It should be noted that the spectra compared were measured in various works under different experimental conditions Ždifferent detection techniques of X-ray absorption and different energy resolution.: NO 2 Želectron energy losses, 0.14 eV w8x.; CH 3 NO 2 Ždirect X-ray transmission, 0.4 eV w9x.; NaNO 2 Žtotal electron yield, 0.3 eV.. Therefore, the comparison of energy positions of absorption structures only seems to be correct. Despite the apparent correlation of main bands A and C, there are considerable distinctions between the spectra under consideration: Ži. intense band AX in the NO 2 spectrum Žlow-intensity narrow peaks a, b, and c are Rydberg transitions. that is monitored neither in the
Fig. 1. N K and O K absorption spectra for NaNO 2 . The spectra were aligned in energy using measured energy separation Ž129.0 eV. between the nitrogen and oxygen 1s core levels. Numbered vertical arrows show photon energies used to excite the PE spectra shown in Figs. 4 and 5.
to NOy 3 ions induced by X-ray radiation. In a course of the experiment the sample quality was monitored by acquisition of a signal in the energy region of this feature. As soon as a more appreciable deterioration was observed, the light spot was moved into another position on the sample. The spectra were aligned in energy using measured by photoemission energy separation Ž129.0 eV. between the O 1s and N 1s core levels. Similarity of the fine structure and clear correlation in energy positions of absorption bands A, B, C, and CX for both spectra are evident in Fig. 1. The observed rather small deviations in energy locations of these bands Ž0.5–0.7 eV. are determined by the accuracy of the spectral calibration and somewhat different final-state interaction between N K and O K
Fig. 2. N K absorption spectra of solid sodium nitrite, gas-phase nitrogen dioxide w8x and nitromethane w9x.
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CH 3 NO 2 nor in the NaNO 2 spectra and Žii. a lack in the NO 2 spectrum of any obvious analogue of band B well-defined in the spectra of CH 3 NO 2 and NaNO 2 . First of all, these distinctions result from different chemical state of the NO 2 group in these polyatomic systems. The ground state of NO 2 molecule ŽC 2v symmetry group. is described by means of MOs as follows w8,10x: 1b 22 ŽO1s.1a21ŽO1s.2a21 ŽN1s.3a21 2b 22 4a21 5a21 3b 22 1b 12 1a22 4b 22 6a11 , 2A 1. First absorption band AX in the NO 2 spectrum is attributed to dipole-allowed transitions of N K and O K electrons to the half-filled 6a 1 MO, whereas bands A and C are assigned to the lowest unoccupied 2b 1 MO Žantibonding, p type. and to the energetically higher lying antibonding 5b 2 s MO, respectively w8x. Transitions of the core electrons to the antibonding 7a 1 s MO oriented in the molecular plane along the two-fold symmetry axis C 2 w8x may cause weak band B in the region of a, b, and c Rydberg states of NO 2 . From the above consideration the absence of band AX in the spectra of CH 3 NO 2 and NaNO 2 can be understood taking into account that the electron transfer from the sodium or carbon Žmethyl group. atom along the C 2 axis to the NO 2 group fills up the 6a 1 s MO and prohibits core electron transitions to this MO. The interaction of NO 2 with the neighboring atoms reveals also Ži. an energy shift of peak A Ž2b1 p MO., in particularly, for NaNO 2 ; Žii. considerable changes of intensity and energy position of band B Ž7a 1 s MO.; and Žiii. an occurrence of additional absorption band CX . As it is obvious in Fig. 2, these spectral changes depend clearly on the character of interaction between NO 2 and neighboring atoms: the carbon–nitrogen bonding ŽC–NO 2 . in CH 3 NO 2 w11x and the sodium–oxygen bonding ŽNO 2 –Na. in NaNO 2 w12x. An important point is that the NO 2 Na planar group is a structural unit of the NaNO 2 crystal characterized by a bodycentered orthorhombic structure w2x. Finally, it should be noted that the analysis of the corresponding O K absorption spectra ŽFig. 3. leads to similar conclusions. Different coupling of the NO 2 group with the non-equivalent neighboring atoms in CH 3 NO 2 and NaNO 2 leads to a low-energy shift of the p resonance in going from CH 3 NO 2 to NaNO 2 . The interaction along the C 2 symmetry axis causes first of all modifications of the a 1 s MOs oriented along this
Fig. 3. O K absorption spectra of solid sodium nitrite, gas-phase nitrogen dioxide w8x and nitromethane w9x.
axis resulting in occupation of the 6a 1 s MO, change of atomic orbital composition of the 7a 1 s MO and formation of the additional 8a 1 s MO. Thus, on the basis of the extended quasimolecular approach ŽNO 2 quasimolecule interacting with neighboring atoms., one can fully identify the resonance structure of the N K and O K absorption spectra of the NaNO 2 crystal attributing A, B, C, and CX bands to dipole-allowed electronic transitions to the empty antibonding MOs 2b1 p ŽN–O., 7a 1 s ŽN–O, O–Na., 5b 2 s ŽN–O., and 8a 1 s ŽO–Na. of the planar NO 2 Na quasimolecule having C 2v group symmetry w2,12x, respectively. The lowest 2b1 p MO is essentially localized within the NOy 2 molecular anion since the p resonance is not sensitive to the substitution of the sodium cation for potassium one w3x and its lineshape does not change considerably in going from the NO 2 and CH 3 NO 2 molecules to the NaNO 2 crystal ŽFigs. 2 and 3.. Therefore, we assume that the high-energy asymmetry of this resonance observed in both ab-
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PE spectra of NaNO 2 excited by photons of various energies in vicinity of the N K and O K absorption edges Žas marked in the absorption spectra, Fig. 1., are shown in Figs. 4 and 5. Photoemission intensities are normalized to the incident photon flux. As seen in the figures, this normalization gives similar intensities of the Na 2p peak. The spectra are plotted using the binding energy scale Žrelative to the Fermi level.. Note that in this plot PE structures stay at the same positions while Auger peaks are linearly shifted as a function of photon energy. The bottom curves in both figures correspond to photon energies below the N K and O K core excitations, respectively. Structures a, b, c, d, e, f, and Na 2p in these spectra are well consistent in their relative energy positions with those in a spectrum of NaNO 2 obtained previously
Fig. 4. Valence-band PE spectra for NaNO 2 at the N K absorption edge. Black triangles indicate positions of the second-order-light N 1s core-level PE signals. Vertical bars mark satellite structures associated with the normal and spectator Auger decays of the p and s excitations.
sorption spectra of NaNO 2 ŽFig. 1. is due to vibrational structure of the molecular NOy 2 anion. High intensity of the resonances in the absorption spectra of the first-row-atom compounds is caused by considerable contributions of the 2p atomic states to the final MOs. An energy splitting by a molecular field of the nitrogen 2pp and 2ps atomic states Žbands A and C, respectively. D EŽ p–s . is directly related to the interatomic distance w3–5x. A decrease of D EŽ p–s . in going from NO 2 to CH 3 NO 2 and NaNO 2 ŽNO 2 : 12.9 eV w8x; CH 3 NO 2 : 10.6 eV w9x; NaNO 2 : 10.1 eV. can easily be understood, therefore, comparing the interatomic distances dŽN–O. in ˚ w10x, d CH 3 NO 2 s these compounds Ž d NO 2 s 1.193 A ˚ ˚ w x w x. 1.23 A 11 , d NaNO 2 s 1.24 A 2 .
Fig. 5. Valence-band PE spectra for NaNO 2 at the O K absorption edge. Black triangles indicate positions of the second-order-light O 1s core-level PE signals. Vertical bars mark satellite structures associated with the normal and spectator Auger decays of the p and s excitations.
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with Mg K a radiation Ž hn s 1253.6 eV. w13x. Bands a–d are superpositions of the valence MO PE signals Ž6a 1 , 1a 2 , 4b 2 , 1b1 , 3b 2 , and 5a 1 . originating mainly from the oxygen and nitrogen 2p atomic states. Band e stems from the 4a 1 MO of mixed 2p–2s composition, while band f reflects the valence MOs 2b 2 and 3a 1 having predominantly oxygen and nitrogen 2s atomic character w14x. For the following it is important to note that upper a and b bands Ž6a 1 , 1a 2 , and 4b 2 MOs. are mainly determined by the O 2p states, whereas c and d ones Ž1b 1 , 3b 2 , and 5a 1 MOs. are dominated by the nitrogen 2p orbitals. Examining the series of PE spectra Žcurves 2, 3, and 4 in Fig. 4 and curves 2, 3, 4, 5, and 6 in Fig. 5. it is clear that valence-band PE signals a, b, c, d Žand possibly e. are significantly enhanced and intense satellite structures sAX , sA, nAX , and nA Ždenoted taking into account their origin, see below. appear when the photon energy is scanned across the p resonance. In the case of the core-to-s-state excitations Žcurve 7 in Fig. 4 and curves 7 and 8 in Fig. 5. there are only satellite structures sAX , sA, nAX , and nA and no resonant enhancement of the valence MO PE signals. The discussed above resonance effects, which are most clearly observed at 401.5 eV and 531.4 eV photon energies, should be evidently ascribed to the electron emission due to the nitrogen Žoxygen. K core-hole decay following the N K ŽO K. 2b1 p excitations. Since nitrogen and oxygen are
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first-row atoms, their K holes decay predominantly Ž) 99%. via radiationless Auger-like channels and, thus, these decay processes are responsible for the observed resonant effects. First we consider satellite structures nA, nAX , sA, and sAX . Comparing these structures in the on-resonance PE spectra and the KVV Auger spectra obtained far above the N K and O K edges ŽFig. 6. it is obvious that nA and nAX signals in the PE spectra are essentially the main peak ŽnA. and its high-kineticenergy shoulder or peak ŽnAX . of the normal NKVV and OKVV Auger spectra for NaNO 2 , as their kinetic energies coincide very closely. They are defined by 2p-double-hole final states that are most probable in the case of normal Auger decay of the K hole of a first-row atom. Registration of the normal Auger decay signals in the resonant PE spectra implies that the excited core electron can leave the parent atom and, hence, cannot take part in the following core-hole de-excitation. If the excited electron remains spatially localized within the parent atom, the core hole may also decay via conventional Auger process modified by a screening effect of this electron involved in the decay process as a spectator w15–18x. In the framework of a strict spectator model this resonant spectator Auger spectrum is a replica of the normal Auger spectrum shifted to higher kinetic energies. The spectator Auger process is supposed to be responsible for bands sA and sAX in the electron
Fig. 6. Normal NKVV and OKVV Auger spectra acquired at high photon energies and on-resonance PE spectra measured at the N K and O K edges for solid NaNO 2 . The PE spectrum at the photon energy of 536.5 eV ŽFig. 5, curve 7. is taken for the OKy1 s excitation because there is a large overlap of the OKVV and Na 2p intensities for the on-resonance PE spectrum ŽFig. 5, curve 8..
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spectra under consideration ŽFigs. 4–6.. This assignment of sAX structures in the on-resonance PE spectra taken at the O K edge should not be, however, regarded as a final one, because the sA–sAX energy spacing Ž; 3.5 eV. is considerably smaller than the energy spacing Ž; 4.8 eV. between nA and nAX normal-Auger signals. The spectator NKVV and OKVV Auger spectra were found to be shifted by ; 6 eV from the normal Auger signals. It is worth noting that no detectable differences between the high-energy shifts of the spectator Auger spectra relative to the normal Auger signals were observed for the p and s excitations. The above shifts caused by screening effects of the p or s spectator electron in the initial and final states of the Auger transition are in good agreement with those for the spectator Auger spectra of the other first-row atom, fluorine, in various alkali fluorides Ž6.7–2.9 eV. w18x. From the PE spectra in Figs. 4 and 5 it is evident that the spectator Auger decay is more probable than the normal one for the nitrogen and oxygen Ky1 p excitations, while both decay channels have similar probabilities for the N Ky1 s excitations and the normal Auger decay process dominates the O Ky1 s excitations. Along with this, we can state that the spectator and normal Auger processes are the dominant decay channels of the s excitations in nitrogen and oxygen K absorption of solid NaNO 2 Žcurve 7 in Fig. 4 and curves 7 and 8 in Fig. 5.. In addition, it is seen from these curves that the total Žspectator and normal. Auger response has an extended Ž; 15 eV. tail on its high-kinetic-energy side. The similar behavior is observed for the Auger spectra measured at photon energies of 500 and 600 eV ŽFig. 6.. In the case of the p excitations, both Auger channels Žin particularly, the spectator one. result in a considerable sloping background increasing PE intensities of the valence bands. This background is simulated using the high-kinetic-energy tails of the normal NKVV and OKVV Auger signals Župper spectra in Fig. 6. and displayed by broken lines for the corresponding p-resonance PE spectra. In order to explain the observed enhancement of valence bands a–d in the decay of p excitations it is necessary to take into account the participator Auger process. In this process the core electron that was initially excited into the empty bound state takes part in the Auger transition leaving behind the same
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single hole final state as in the direct valence band photoemission w16,18x. Consequently the interference of the participator Auger decay and direct photoemission channels causes the resonant behavior of the valence-band PE intensities, when the photon energy is scanned across a core-to-bound-state excitation. This interference gives rise to Fano profiles for resonances in the absorption cross-section w19x. The interference effects are more pronounced for coreto-bound excitations with cross-sections, which are comparable in a magnitude with those of direct valence-band photoemission. No experimental data on absolute absorption cross-sections near the N K and O K edges in NaNO 2 are available. Nonetheless, by normalizing the total electron yield spectra ŽFig. 1. to NaNO 2 cross-sections evaluated on the basis of atomic calculations for nitrogen, oxygen and sodium atoms w20x we can see that the cross-sections for the p resonances are higher by a factor of ; 30 than the valence-band cross-sections near the N K and O K edges. Therefore, the interference effects are weak and cannot cause the above-mentioned high-energy asymmetry of the p resonances. The resonant enhancement is apparent for PE valence bands a and b, which are superimposed by only an insignificant Auger background. For these almost unresolved in the experiment bands coefficients of amplification Žwithout account for Auger background. amount to about 3 and 6 for the p excitations at the N K and O K thresholds Ž401.5 and 531.4 eV., respectively. Large unknown Auger background as well as overlap in part with the spectator Auger signal make difficult an estimation of values of the resonant enhancement for PE signals c, d, and e. Nevertheless, strong resonant behavior of all PE bands a, b, c, d, and e Žprobably, with the exception of the latter for the O Ky1 p excitation. caused by the participator Auger decay channel can be concluded from the obtained spectra for the p excitations. From the on-p-resonance PE spectra ŽFigs. 4 and 5. it is also obvious that the participator Auger decay has lower intensity in comparison with the spectator one. It should be noted that the participator de-excitation channel is generally believed to be very unlikely for core excitations of the first-row atoms in solids w16,18x. Hence, the present observation is apparently the first one except similar results obtained for condensed molecules w21,22x. It is generally ac-
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cepted that the participator decay involving valenceband electronic states can play a prominent role only if core electron is excited into localized intermediate states Že.g., into 3d or 4f collapsed orbitals in transition metals or rare-earths. strongly coupled to these valence-band states. In our case of molecular NOy 2 anion in solid NaNO 2 the upper valence MO are mainly formed by the nitrogen and oxygen 2p atomic states w14x. On the other hand, the dominant 2p atomic origin of the p excitations follows directly from the above analysis of the X-ray absorption spectra of NaNO 2 . Thus, we should anticipate for the p 2p excitations a strong coupling with the upper valence electrons, which is reflected in particular by the participator Auger-decay channel for the p 2p excitations in solid NaNO 2 . In the framework of the common two-step description of Auger processes assuming creation and decay of the inner-shell hole as consecutive and independent Žincoherent. events the lower intensity of the participator decay of the p 2p excitation as compared to that of the spectator decay can be easily explained. The participator or spectator Auger processes are described by Coulomb matrix elements between the Ky1 p state as the initial state and valence-band hole plus free electron or two valenceband holes, p electron plus free electron as the final state, respectively. These matrix elements are expected to have similar values, since the valence-band and excited p states are essentially the same 2p atomic states. Thus, intensity of the participator decay must be lower than that of the spectator decay, because there are several valence 2p electrons and only one excited p 2p electron. Within the above approach the participator decay channel of nitrogen or oxygen p 2p excitation will only enhance the PE signals of the valence MOs having appreciable contributions from the 2p atomic orbitals of the nitrogen or oxygen, respectively. As mentioned above, the upper valence MOs that are responsible for PE signals a and b are dominated by the oxygen 2p atomic orbitals w14x. Therefore we expect these PE bands to be resonantly enhanced primarily in the decay of the oxygen p 2p excitation. Our experimental observations ŽFigs. 4 and 5. correlate qualitatively with this assumption. It should be noted that the correct quantitative comparison between the resonant PE spectra taken for the nitrogen and oxygen p 2p excitations is
only possible using absolute Žor at least relative. absorption cross-sections for the nitrogen and oxygen K p 2p transitions those are presently not available for solid NaNO 2 . In spite of this, we underline the fact that PE valence bands a–d are similarly enhanced upon decay of the nitrogen and oxygen p 2p excitations. For both p resonances, c and d bands are more enhanced than a and b bands. At the same time, from comparative analysis of N K and O K emission spectra of NaNO 2 w14x it is known that PE bands a, b and c, d originate from the valence MOs having mainly oxygen and nitrogen 2p orbital character, respectively. Therefore, they are expected to show resonant enhancement only for the corresponding Žoxygen or nitrogen. core excitations. Thus, the observed resonant behavior upon decay of the p excitations is not clear assuming two-step model for the de-excitation process. It is possible that in our case of the p excitations in the N K and O K absorption of NaNO 2 the core-hole creation and its following decay should be treated as coherent events in a one-step process w23,24x. The probable reason for this is a strong coupling between the p 2p excited states and the upper valence MO associated with the same 2p atomic orbitals of nitrogen and oxygen. It is evident that extensive theoretical studies are necessary for detailed explanation of the obtained results. Nevertheless, certain of them can be qualitatively understood by analyzing spatial localization and lifetime of the core excitations. Lifetime of the core excitation, t , is determined by that of the core hole, t h , and that of the excited core electron in the final quasimolecular state, te : 1rt s 1rt h q 1rte . The lifetime of the core hole is an intrinsic atomic property, which is weakly affected by atomic surroundings in solids, whereas te is influenced by modifying quasimolecular region of localization of the excited core electron. As seen in Figs. 4 and 5, in the case of the p excitation we observe three decay channels: Ž1. spectator, Ž2. participator, and Ž3. normal Auger de-excitation. Since the two first channels, which are characterized by excited electron remaining spatially localized within the NOy 2 molecular anion, are dominant in the de-excitation process, it is evident that the lifetime of the p excitations in NaNO 2 is mainly defined by the lifetime of the N K or O K hole. On the other hand, non-zero intensity of
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the normal Auger signal means that the excited electron can still leave the parent quasimolecule NOy 2 within the hole lifetime by interacting with the atomic surroundings and by hopping to neighboring quasimolecules. The latter points to a certain degree of delocalization of the 1s– 1 p 2p state outside the parent anion NOy 2 . Thereby, the relative intensity of the normal Auger decay as compared to the total intensity of two other de-excitation processes can be used to characterize quantitatively a delocalization of the core excitation. The decay of the s excitations Žcurve 7 in Fig. 4 and curves 7 and 8 in Fig. 5. exhibits distinct differences in comparison with the decay of the p excitations: Ži. an absence of any noticeable resonant enhancement of the valence bands via the participator Auger channel and Žii. an observation of the normal and spectator Auger signals with similar intensities for the N Ky1 s excitation and with the dominance of the normal Auger process in the decay of the O Ky1 s excitation. These findings prove that the s excitations have lifetime, which is essentially shorter than that of the core hole and is primarily determined by the lifetime of the s electron within 1 the parent NOy 2 anion in the 1s– s state. It is also obvious that the excited s electron leaves the parent anion quicker in the case of the O Ky1 s excitation. These facts can be explained by a strong interaction of the s electron with the neighboring atoms in the plane of the NO 2 Na quasimolecule, which defines crystal planes in NaNO 2 , resulting in a larger hopping rate to other anions. Therefore, the s electron appears to be more delocalized within the whole crystal than the p electron with the orbital oriented normally to the quasimolecular plane. This effect is expected to be more appreciable for the oxygen excitation, because the interaction between the NO 2 group and the surroundings occurs through oxygensodium bonding. Finally we discuss the photoemission results in relation to the nature of unoccupied electronic states in solid NaNO 2 . First of all, the observed decay properties of the p excitations give evidence for the strong localization of the lowest unoccupied electronic states in the solid allowing one to consider them as quasimolecular p states of the NOy 2 anion. Furthermore, these vacant electronic states in solid NaNO 2 originate mainly from the nitrogen and oxy-
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gen 2p valence states. Therefore, the participator Auger channel is clearly observed in the decay of the N Ky1 p 2p and O Ky1 p 2p excitations. As to the s excitations, we see from the analysis of the de-excitation spectra that they are only partially localized at the quasimolecular anion. The absence of the participator transitions in the decay of the s excitations means that the unoccupied s electronic states differ considerably from the 2p atomic states. According to a correlation diagram for XH 2 molecules w10x, the a 1 s and b 2 s MOs of the quasimolecular NO 2 anion, that are formed by the nitrogen and oxygen 2p orbitals, have already some properties of 3s and 3p atomic orbitals, respectively. An increase of the node number in radial wavefunctions of the s MOs in comparison with the 2p atomic orbitals and the p MO causes the drastic decrease of the participator decay probability relative to that of the spectator decay. According to calculations w25x, the participator decay intensity for the Ky1 3p excited state of the first-row atom, Ne, is less than the spectator decay intensity by a factor of 450. Therefore, the origin and properties of empty electronic states in NaNO 2 that are responsible for the s excitations are defined by more complicated polyatomic groups involving anion itself and also neighbors, in particular, sodium atom.
4. Conclusions The high-resolution X-ray absorption and the resonant photoemission spectra were used to study the nature of the N K and O K excitations in solid NaNO 2 . The similarity of the fine structure for both absorption spectra and their clear correlation in energy positions of the absorption bands indicate a common set of unoccupied electronic states that are responsible for the nitrogen and oxygen core excitations. Based on the comparative analysis of the N K absorption spectra for solid NaNO 2 , gas-phase molecules NO 2 , and CH 3 NO 2 , the core excitations in the absorption spectra from solid NaNO 2 were found to be quasimolecular and assigned to core electron transitions to unoccupied MOs of the planar NO 2 Na quasimolecule – the structural unit of the crystal.
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The measured PE spectra show strong resonant effects at the p and s excitations. The spectator, participator, and normal Auger processes were found to be responsible for the decay of the p excitations, whereas only the normal and spectator Auger structures were observed for the s excitations. The decay properties of the p excitations give evidence for anion and, their strong localization at the NOy 2 consequently, for the quasimolecular origin of the lowest unoccupied electron states of the p type in NaNO 2 . On the basis of photoemission data the s excitations are shown to be only partially localized within the quasimolecular anion. The obtained results are considered as a direct proof of the validity of the quasimolecular treatment of the N K and O K absorption spectra of solid NaNO 2 . Acknowledgements We would like to acknowledge the assistance of the BESSY staff and to thank V.K. Adamchuk, A.A. Pavlychev and R. Schlogl ¨ for stimulating discussions. This work was supported in part by Russian Foundation for Basic Research under grant number 98-02-18177. References w1x J. Stohr, ¨ NEXAFS Spectroscopy, Springer, Berlin, 1992. w2x M.I. Kay, B.C. Frazer, Acta Crystallogr. 14 Ž1961. 56. w3x S.V. Nekipelov, V.N. Akimov, A.S. Vinogradov, Fiz. Tverd. Tela ŽLeningrad. 30 Ž1988. 3647, wSov. Phys. - Solid State 30 Ž1988. 2095x. w4x A.A. Pavlychev, A.S. Vinogradov, V.N. Akimov, S.V. Nekipelov, Phys. Scr. 41 Ž1990. 160.
w5x A.S. Vinogradov, V.N. Akimov, Opt. Spectrosk. 75 Ž1993. 816. w6x V.N. Akimov, A.S. Vinogradov, Opt. Spectrosk. 82 Ž1997. 360, wOpt. Spectrosc. 82 Ž1997. 326x. w7x H. Petersen, Nucl. Instrum. Methods A 246 Ž1986. 260. w8x W. Zhang, K.H. Sze, C.E. Brion, X.M. Tong, J.M. Li, Chem. Phys. 140 Ž1990. 265. w9x A.S. Vinogradov, V.N. Akimov, S.V. Nekipelov, A.A. Pavlychev, A.A. Boronoev, A.V. Zhadenov, Opt. Spectrosk. 72 Ž1992. 1094, wOpt. Spectrosc. 72 Ž1992. 599x. w10x G. Herzberg, Molecular Spectra and Molecular Structure, Vol. 3, Electronic Spectra and Electronic Structure of Polyatomic Molecules, Van Nostrand Reinhold, New York, 1966. w11x A.P. Cox, J. Mol. Struct. 97 Ž1983. 61. w12x F. Ramondo, Chem. Phys. Lett. 156 Ž1989. 346. w13x M. Kamada, K. Ichikawa, K. Tsutsumi, J. Phys. Soc. Japan 50 Ž1981. 170. w14x N. Kosuch, E. Tegeler, G. Wiech, A. Faessler, J. Electron. Spectrosc. Relat. Phenom. 13 Ž1978. 263. w15x W. Eberhardt, E.W. Plummer, C.T. Chen, W.K. Ford, Aust. J. Phys. 39 Ž1986. 853. w16x T. Tiedje, K.M. Colbow, D. Rogers, W. Eberhardt, Phys. Rev. Lett. 65 Ž1990. 1243. w17x M. Elango, A. Ausmees, A. Kikas, E. Nommiste, R. Ruus, ˜ A. Saar, J.F. van Acker, J.N. Andersen, R. Nyholm, I. Martinsson, Phys. Rev. B 47 Ž1993. 11736. w18x H. Aksela, E. Kukk, S. Aksela, A. Kikas, E. Nommiste, A. ˜ Ausmees, M. Elango, Phys. Rev. B 49 Ž1994. 3116. w19x U. Fano, Phys. Rev. 124 Ž1961. 1866. w20x J.E. Yeh, I. Lindau, Atomic Data and Nuclear Data Tables 32 Ž1985. 1. w21x D. Menzel, G. Rocker, H.-P. Steinruck, ¨ D. Coulman, P.A. Heimann, W. Huber, P. Ziebisch, D.R. Lloyd, J. Chem. Phys. 96 Ž1992. 1724. w22x P.A. Bruhwiler, A.J. Maxwell, P. Rudolf, C.D. Gutleben, B. ¨ Wastberg, N. Martensson, Phys. Rev. Lett. 71 Ž1993. 3721. ¨ ˚ ˚ w23x T. Aberg, Phys. Scr. 41 Ž1992. 71. w24x O. Karis, A. Nilsson, M. Weinelt, T. Wiell, C. Puglia, N. Wassdahl, N. Martensson, M. Samant, J. Stohr, ˚ ¨ Phys. Rev. Lett. 76 Ž1996. 1380. ˚ w25x H. Aksela, S. Aksela, J. Tulkki, T. Aberg, G.M. Bancroft, K.H. Tan, Phys. Rev. A 39 Ž1989. 3401.
Nitrogen and oxygen core excitations in solid NaNO 2 studied by X-ray absorption and resonant photoemission A.S. Vinogradov a,) , A.B. Preobrajenski a,b, S.L. Molodtsov a,c , S.A. Krasnikov a , d R. Szargan b, A. Knop-Gericke d , M. Havecker ¨ a
b
Institute of Physics, St. Petersburg State UniÕersity, 198904, St. Petersburg, Russian Federation W. Ostwald-Institut fur ¨ Physikalische und Theoretische Chemie, UniÕersitat ¨ Leipzig, Linnestr. ´ 2, D-04103 Leipzig, Germany c Institut fur und Mikrostrukturphysik, Technische UniÕersitat ¨ Oberflachen¨ ¨ Dresden, D-01062 Dresden, Germany d Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany Received 2 March 1999
Abstract The origin and properties of the nitrogen and oxygen K excitations in solid NaNO 2 were investigated combining high-resolution X-ray absorption and resonant photoemission spectroscopy. The absorption spectra obtained were treated on the basis of comparison between the spectra of solid NaNO 2 and gas-phase NO 2 and CH 3 NO 2 molecules, as a result of which main absorption bands were assumed to be associated with the core electron transitions to unoccupied p and s molecular orbitals of the planar NO 2 Na quasimolecule – the structural unit of the crystal. Examining series of the photoemission spectra taken at the N K and O K edges, it was found that the p and s excitations decay in different ways: three decay channels Žspectator, participator and normal Auger processes. were observed for the p excitations whereas only the normal and spectator Auger decay channels were seen for the s excitations. This fact was qualitatively explained in terms of differences in the spatial localization and lifetime of the corresponding core excitations. The results of the resonant photoemission study give a direct evidence for the quasimolecular NOy 2 origin of the lowest unoccupied electronic states of the p type in solid NaNO 2 . q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction The dominant shape resonances in X-ray absorption spectra of the simple low-Z molecules N2 , CO, NO, HCN, etc., are known to be caused by the multiple scattering of low-energy X-ray photoelectrons from nearest-neighbor atoms w1x. Therefore, these resonances are strongly localized within these small atomic groups and may be described by unoc-
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Corresponding author. Fax: q7-812-428-7240
cupied molecular orbitals ŽMOs. of the latter. Development of these core excitations with increasing size of a polyatomic system is of fundamental interest because their spectral characteristics embody valuable information on the local atomic and electronic structure of the system. In this respect the polyatomic molecules, complexes and solids, containing the first-row atoms in clearly separated units like COy, CNy, NOy 2 , etc., are very promising candidates for experimental studies of the resonances and their relation to the properties of empty electronic states in complicated systems. Among them is an
0301-0104r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 0 1 0 4 Ž 9 9 . 0 0 2 8 7 - 6
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ionic-molecular crystal of sodium nitrite NaNO 2 inw2x. corporating planar molecular anions NOy 2 Recently X-ray absorption spectra in vicinity of the nitrogen, oxygen, and sodium K edges have been investigated for the polycrystalline samples of NaNO 2 with moderate energy resolution using bremsstrahlung of a X-ray tube w3–6x. Near-edge features like those in the case of simple molecules, namely, intense and rather narrow low-energy peak and higher-energy broad absorption band having additional structure w3x, were monitored in the N K and O K absorption spectra. Comparative analysis of the obtained spectra with the corresponding spectra of the NO molecule has shown that the near-edge N K and O K excitations of the sodium nitrite crystal have a quasimolecular origin and can be regarded as a result of the core electron transitions to the empty p w x and s MOs of the molecular NOy 2 anion 3 . On the basis of a quasiatomic consideration, it was also found that these excitations can be successfully described by the p and s components of the atomic 1s–2p atomic resonance splitted by molecular anion field w4x. A relation of the latter to electronic states of the conduction band of NaNO 2 was then discussed in the Ref. w5x. The main feature of the electronic structure obtained for the NaNO 2 crystal is the rather narrow Ž; 1 eV. first sub-band near the bottom of the conduction band, which is separated from the second sub-band by a considerably large energy gap Ž; 2.5 eV.. These unusual properties of the first sub-band can be understood by its quasimolecular origin: its electronic states are assumed to be strongly localized within the molecular NOy 2 anion. In contrast to that, electronic states of the second sub-band are extended over the whole crystal. In addition, an indirect evidence for the strong localization of the core-to-p-state excitations has been presumably derived from a comparative analysis of the N K absorption spectra of NaNO 2 measured under the same experimental conditions by means of direct photoabsorption and total electron yield techniques w6x. In the present study, high-resolution absorption and resonant photoemission spectra of a NaNO 2 ionic-molecular crystal were measured using synchrotron radiation in order to obtain new information on localization, lifetime and decay processes of core excitations and their relation to the unoccupied electronic states of the polyatomic system.
2. Experimental The experiments were carried out at the SX 700-I beamline of the Berliner Elektronenspeicherring fur ¨ Synchrotronstrahlung ŽBESSY. w7x. The monochromator was operated with energy resolution of 0.3 eV at the N K edge and 0.6 eV at the O K edge. The absorption spectra Žstep width between experimental points 0.1–0.2 eV, integration time per point 1 s. were recorded by detecting the total electron yield. The photon energy was calibrated with an accuracy of "0.5 eV by means of measurements of N K and O K core level photoemission ŽPE. of NaNO 2 with first- and second-order light and energies of the corresponding 1s electron excitations into the empty p state known from Ref. w3x. The NaNO 2 samples were films prepared from thoroughly dehydrated powder by thermal evaporation in situ from a tantalum Knudsen cell onto polished copper substrates in a vacuum of 2 = 10y7 mbar. Thickness of the sam˚ with a quartz ples was estimated to be about 200 A crystal monitor. No sample charging effects were observed during the measurements. A VG CLAM 100 electron-energy analyzer was used for the photoemission experiments. The analyzer resolution Žfull width at half maximum. was set to 0.5 eV corresponding to an analyzer pass energy of 50 eV. No absolute electron energy calibration was done for the PE spectra Žstep width 0.2 eV, integration time per point 1 s, time for collecting the spectrum 20–50 min.. The latter were analyzed by aligning the Na 2p peaks to equal binding energies. During measurements the base pressure in the analyzer chamber was about 1 = 10y1 0 mbar. The photon flux of incident radiation was monitored from a total electron yield of a clean Cu Ž110. surface.
3. Results and discussion Fig. 1 shows N K and O K photoabsorption spectra of NaNO 2 recorded with the total electron yield. These spectra normalized to incident photon flux are in good agreement with those reported earlier w3,5x and show no additional structures except some asymmetry on the high-energy side of peak A. A weak low-energy shoulder Žs. at the base of absorption band B arises from slow decomposition of NOy 2 ions
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holes and excited electrons. This experimental result indicates that a common set of the unoccupied electronic states of NaNO 2 is responsible for the nitrogen and oxygen K excitation spectra. Comparing the N K absorption spectra for solid NaNO 2 , gas-phase molecules NO 2 w8x, and CH 3 NO 2 w9x ŽFig. 2., the origin of these electronic states in sodium nitrite can be easy understood. It should be noted that the spectra compared were measured in various works under different experimental conditions Ždifferent detection techniques of X-ray absorption and different energy resolution.: NO 2 Želectron energy losses, 0.14 eV w8x.; CH 3 NO 2 Ždirect X-ray transmission, 0.4 eV w9x.; NaNO 2 Žtotal electron yield, 0.3 eV.. Therefore, the comparison of energy positions of absorption structures only seems to be correct. Despite the apparent correlation of main bands A and C, there are considerable distinctions between the spectra under consideration: Ži. intense band AX in the NO 2 spectrum Žlow-intensity narrow peaks a, b, and c are Rydberg transitions. that is monitored neither in the
Fig. 1. N K and O K absorption spectra for NaNO 2 . The spectra were aligned in energy using measured energy separation Ž129.0 eV. between the nitrogen and oxygen 1s core levels. Numbered vertical arrows show photon energies used to excite the PE spectra shown in Figs. 4 and 5.
to NOy 3 ions induced by X-ray radiation. In a course of the experiment the sample quality was monitored by acquisition of a signal in the energy region of this feature. As soon as a more appreciable deterioration was observed, the light spot was moved into another position on the sample. The spectra were aligned in energy using measured by photoemission energy separation Ž129.0 eV. between the O 1s and N 1s core levels. Similarity of the fine structure and clear correlation in energy positions of absorption bands A, B, C, and CX for both spectra are evident in Fig. 1. The observed rather small deviations in energy locations of these bands Ž0.5–0.7 eV. are determined by the accuracy of the spectral calibration and somewhat different final-state interaction between N K and O K
Fig. 2. N K absorption spectra of solid sodium nitrite, gas-phase nitrogen dioxide w8x and nitromethane w9x.
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CH 3 NO 2 nor in the NaNO 2 spectra and Žii. a lack in the NO 2 spectrum of any obvious analogue of band B well-defined in the spectra of CH 3 NO 2 and NaNO 2 . First of all, these distinctions result from different chemical state of the NO 2 group in these polyatomic systems. The ground state of NO 2 molecule ŽC 2v symmetry group. is described by means of MOs as follows w8,10x: 1b 22 ŽO1s.1a21ŽO1s.2a21 ŽN1s.3a21 2b 22 4a21 5a21 3b 22 1b 12 1a22 4b 22 6a11 , 2A 1. First absorption band AX in the NO 2 spectrum is attributed to dipole-allowed transitions of N K and O K electrons to the half-filled 6a 1 MO, whereas bands A and C are assigned to the lowest unoccupied 2b 1 MO Žantibonding, p type. and to the energetically higher lying antibonding 5b 2 s MO, respectively w8x. Transitions of the core electrons to the antibonding 7a 1 s MO oriented in the molecular plane along the two-fold symmetry axis C 2 w8x may cause weak band B in the region of a, b, and c Rydberg states of NO 2 . From the above consideration the absence of band AX in the spectra of CH 3 NO 2 and NaNO 2 can be understood taking into account that the electron transfer from the sodium or carbon Žmethyl group. atom along the C 2 axis to the NO 2 group fills up the 6a 1 s MO and prohibits core electron transitions to this MO. The interaction of NO 2 with the neighboring atoms reveals also Ži. an energy shift of peak A Ž2b1 p MO., in particularly, for NaNO 2 ; Žii. considerable changes of intensity and energy position of band B Ž7a 1 s MO.; and Žiii. an occurrence of additional absorption band CX . As it is obvious in Fig. 2, these spectral changes depend clearly on the character of interaction between NO 2 and neighboring atoms: the carbon–nitrogen bonding ŽC–NO 2 . in CH 3 NO 2 w11x and the sodium–oxygen bonding ŽNO 2 –Na. in NaNO 2 w12x. An important point is that the NO 2 Na planar group is a structural unit of the NaNO 2 crystal characterized by a bodycentered orthorhombic structure w2x. Finally, it should be noted that the analysis of the corresponding O K absorption spectra ŽFig. 3. leads to similar conclusions. Different coupling of the NO 2 group with the non-equivalent neighboring atoms in CH 3 NO 2 and NaNO 2 leads to a low-energy shift of the p resonance in going from CH 3 NO 2 to NaNO 2 . The interaction along the C 2 symmetry axis causes first of all modifications of the a 1 s MOs oriented along this
Fig. 3. O K absorption spectra of solid sodium nitrite, gas-phase nitrogen dioxide w8x and nitromethane w9x.
axis resulting in occupation of the 6a 1 s MO, change of atomic orbital composition of the 7a 1 s MO and formation of the additional 8a 1 s MO. Thus, on the basis of the extended quasimolecular approach ŽNO 2 quasimolecule interacting with neighboring atoms., one can fully identify the resonance structure of the N K and O K absorption spectra of the NaNO 2 crystal attributing A, B, C, and CX bands to dipole-allowed electronic transitions to the empty antibonding MOs 2b1 p ŽN–O., 7a 1 s ŽN–O, O–Na., 5b 2 s ŽN–O., and 8a 1 s ŽO–Na. of the planar NO 2 Na quasimolecule having C 2v group symmetry w2,12x, respectively. The lowest 2b1 p MO is essentially localized within the NOy 2 molecular anion since the p resonance is not sensitive to the substitution of the sodium cation for potassium one w3x and its lineshape does not change considerably in going from the NO 2 and CH 3 NO 2 molecules to the NaNO 2 crystal ŽFigs. 2 and 3.. Therefore, we assume that the high-energy asymmetry of this resonance observed in both ab-
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PE spectra of NaNO 2 excited by photons of various energies in vicinity of the N K and O K absorption edges Žas marked in the absorption spectra, Fig. 1., are shown in Figs. 4 and 5. Photoemission intensities are normalized to the incident photon flux. As seen in the figures, this normalization gives similar intensities of the Na 2p peak. The spectra are plotted using the binding energy scale Žrelative to the Fermi level.. Note that in this plot PE structures stay at the same positions while Auger peaks are linearly shifted as a function of photon energy. The bottom curves in both figures correspond to photon energies below the N K and O K core excitations, respectively. Structures a, b, c, d, e, f, and Na 2p in these spectra are well consistent in their relative energy positions with those in a spectrum of NaNO 2 obtained previously
Fig. 4. Valence-band PE spectra for NaNO 2 at the N K absorption edge. Black triangles indicate positions of the second-order-light N 1s core-level PE signals. Vertical bars mark satellite structures associated with the normal and spectator Auger decays of the p and s excitations.
sorption spectra of NaNO 2 ŽFig. 1. is due to vibrational structure of the molecular NOy 2 anion. High intensity of the resonances in the absorption spectra of the first-row-atom compounds is caused by considerable contributions of the 2p atomic states to the final MOs. An energy splitting by a molecular field of the nitrogen 2pp and 2ps atomic states Žbands A and C, respectively. D EŽ p–s . is directly related to the interatomic distance w3–5x. A decrease of D EŽ p–s . in going from NO 2 to CH 3 NO 2 and NaNO 2 ŽNO 2 : 12.9 eV w8x; CH 3 NO 2 : 10.6 eV w9x; NaNO 2 : 10.1 eV. can easily be understood, therefore, comparing the interatomic distances dŽN–O. in ˚ w10x, d CH 3 NO 2 s these compounds Ž d NO 2 s 1.193 A ˚ ˚ w x w x. 1.23 A 11 , d NaNO 2 s 1.24 A 2 .
Fig. 5. Valence-band PE spectra for NaNO 2 at the O K absorption edge. Black triangles indicate positions of the second-order-light O 1s core-level PE signals. Vertical bars mark satellite structures associated with the normal and spectator Auger decays of the p and s excitations.
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with Mg K a radiation Ž hn s 1253.6 eV. w13x. Bands a–d are superpositions of the valence MO PE signals Ž6a 1 , 1a 2 , 4b 2 , 1b1 , 3b 2 , and 5a 1 . originating mainly from the oxygen and nitrogen 2p atomic states. Band e stems from the 4a 1 MO of mixed 2p–2s composition, while band f reflects the valence MOs 2b 2 and 3a 1 having predominantly oxygen and nitrogen 2s atomic character w14x. For the following it is important to note that upper a and b bands Ž6a 1 , 1a 2 , and 4b 2 MOs. are mainly determined by the O 2p states, whereas c and d ones Ž1b 1 , 3b 2 , and 5a 1 MOs. are dominated by the nitrogen 2p orbitals. Examining the series of PE spectra Žcurves 2, 3, and 4 in Fig. 4 and curves 2, 3, 4, 5, and 6 in Fig. 5. it is clear that valence-band PE signals a, b, c, d Žand possibly e. are significantly enhanced and intense satellite structures sAX , sA, nAX , and nA Ždenoted taking into account their origin, see below. appear when the photon energy is scanned across the p resonance. In the case of the core-to-s-state excitations Žcurve 7 in Fig. 4 and curves 7 and 8 in Fig. 5. there are only satellite structures sAX , sA, nAX , and nA and no resonant enhancement of the valence MO PE signals. The discussed above resonance effects, which are most clearly observed at 401.5 eV and 531.4 eV photon energies, should be evidently ascribed to the electron emission due to the nitrogen Žoxygen. K core-hole decay following the N K ŽO K. 2b1 p excitations. Since nitrogen and oxygen are
™
first-row atoms, their K holes decay predominantly Ž) 99%. via radiationless Auger-like channels and, thus, these decay processes are responsible for the observed resonant effects. First we consider satellite structures nA, nAX , sA, and sAX . Comparing these structures in the on-resonance PE spectra and the KVV Auger spectra obtained far above the N K and O K edges ŽFig. 6. it is obvious that nA and nAX signals in the PE spectra are essentially the main peak ŽnA. and its high-kineticenergy shoulder or peak ŽnAX . of the normal NKVV and OKVV Auger spectra for NaNO 2 , as their kinetic energies coincide very closely. They are defined by 2p-double-hole final states that are most probable in the case of normal Auger decay of the K hole of a first-row atom. Registration of the normal Auger decay signals in the resonant PE spectra implies that the excited core electron can leave the parent atom and, hence, cannot take part in the following core-hole de-excitation. If the excited electron remains spatially localized within the parent atom, the core hole may also decay via conventional Auger process modified by a screening effect of this electron involved in the decay process as a spectator w15–18x. In the framework of a strict spectator model this resonant spectator Auger spectrum is a replica of the normal Auger spectrum shifted to higher kinetic energies. The spectator Auger process is supposed to be responsible for bands sA and sAX in the electron
Fig. 6. Normal NKVV and OKVV Auger spectra acquired at high photon energies and on-resonance PE spectra measured at the N K and O K edges for solid NaNO 2 . The PE spectrum at the photon energy of 536.5 eV ŽFig. 5, curve 7. is taken for the OKy1 s excitation because there is a large overlap of the OKVV and Na 2p intensities for the on-resonance PE spectrum ŽFig. 5, curve 8..
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spectra under consideration ŽFigs. 4–6.. This assignment of sAX structures in the on-resonance PE spectra taken at the O K edge should not be, however, regarded as a final one, because the sA–sAX energy spacing Ž; 3.5 eV. is considerably smaller than the energy spacing Ž; 4.8 eV. between nA and nAX normal-Auger signals. The spectator NKVV and OKVV Auger spectra were found to be shifted by ; 6 eV from the normal Auger signals. It is worth noting that no detectable differences between the high-energy shifts of the spectator Auger spectra relative to the normal Auger signals were observed for the p and s excitations. The above shifts caused by screening effects of the p or s spectator electron in the initial and final states of the Auger transition are in good agreement with those for the spectator Auger spectra of the other first-row atom, fluorine, in various alkali fluorides Ž6.7–2.9 eV. w18x. From the PE spectra in Figs. 4 and 5 it is evident that the spectator Auger decay is more probable than the normal one for the nitrogen and oxygen Ky1 p excitations, while both decay channels have similar probabilities for the N Ky1 s excitations and the normal Auger decay process dominates the O Ky1 s excitations. Along with this, we can state that the spectator and normal Auger processes are the dominant decay channels of the s excitations in nitrogen and oxygen K absorption of solid NaNO 2 Žcurve 7 in Fig. 4 and curves 7 and 8 in Fig. 5.. In addition, it is seen from these curves that the total Žspectator and normal. Auger response has an extended Ž; 15 eV. tail on its high-kinetic-energy side. The similar behavior is observed for the Auger spectra measured at photon energies of 500 and 600 eV ŽFig. 6.. In the case of the p excitations, both Auger channels Žin particularly, the spectator one. result in a considerable sloping background increasing PE intensities of the valence bands. This background is simulated using the high-kinetic-energy tails of the normal NKVV and OKVV Auger signals Župper spectra in Fig. 6. and displayed by broken lines for the corresponding p-resonance PE spectra. In order to explain the observed enhancement of valence bands a–d in the decay of p excitations it is necessary to take into account the participator Auger process. In this process the core electron that was initially excited into the empty bound state takes part in the Auger transition leaving behind the same
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single hole final state as in the direct valence band photoemission w16,18x. Consequently the interference of the participator Auger decay and direct photoemission channels causes the resonant behavior of the valence-band PE intensities, when the photon energy is scanned across a core-to-bound-state excitation. This interference gives rise to Fano profiles for resonances in the absorption cross-section w19x. The interference effects are more pronounced for coreto-bound excitations with cross-sections, which are comparable in a magnitude with those of direct valence-band photoemission. No experimental data on absolute absorption cross-sections near the N K and O K edges in NaNO 2 are available. Nonetheless, by normalizing the total electron yield spectra ŽFig. 1. to NaNO 2 cross-sections evaluated on the basis of atomic calculations for nitrogen, oxygen and sodium atoms w20x we can see that the cross-sections for the p resonances are higher by a factor of ; 30 than the valence-band cross-sections near the N K and O K edges. Therefore, the interference effects are weak and cannot cause the above-mentioned high-energy asymmetry of the p resonances. The resonant enhancement is apparent for PE valence bands a and b, which are superimposed by only an insignificant Auger background. For these almost unresolved in the experiment bands coefficients of amplification Žwithout account for Auger background. amount to about 3 and 6 for the p excitations at the N K and O K thresholds Ž401.5 and 531.4 eV., respectively. Large unknown Auger background as well as overlap in part with the spectator Auger signal make difficult an estimation of values of the resonant enhancement for PE signals c, d, and e. Nevertheless, strong resonant behavior of all PE bands a, b, c, d, and e Žprobably, with the exception of the latter for the O Ky1 p excitation. caused by the participator Auger decay channel can be concluded from the obtained spectra for the p excitations. From the on-p-resonance PE spectra ŽFigs. 4 and 5. it is also obvious that the participator Auger decay has lower intensity in comparison with the spectator one. It should be noted that the participator de-excitation channel is generally believed to be very unlikely for core excitations of the first-row atoms in solids w16,18x. Hence, the present observation is apparently the first one except similar results obtained for condensed molecules w21,22x. It is generally ac-
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cepted that the participator decay involving valenceband electronic states can play a prominent role only if core electron is excited into localized intermediate states Že.g., into 3d or 4f collapsed orbitals in transition metals or rare-earths. strongly coupled to these valence-band states. In our case of molecular NOy 2 anion in solid NaNO 2 the upper valence MO are mainly formed by the nitrogen and oxygen 2p atomic states w14x. On the other hand, the dominant 2p atomic origin of the p excitations follows directly from the above analysis of the X-ray absorption spectra of NaNO 2 . Thus, we should anticipate for the p 2p excitations a strong coupling with the upper valence electrons, which is reflected in particular by the participator Auger-decay channel for the p 2p excitations in solid NaNO 2 . In the framework of the common two-step description of Auger processes assuming creation and decay of the inner-shell hole as consecutive and independent Žincoherent. events the lower intensity of the participator decay of the p 2p excitation as compared to that of the spectator decay can be easily explained. The participator or spectator Auger processes are described by Coulomb matrix elements between the Ky1 p state as the initial state and valence-band hole plus free electron or two valenceband holes, p electron plus free electron as the final state, respectively. These matrix elements are expected to have similar values, since the valence-band and excited p states are essentially the same 2p atomic states. Thus, intensity of the participator decay must be lower than that of the spectator decay, because there are several valence 2p electrons and only one excited p 2p electron. Within the above approach the participator decay channel of nitrogen or oxygen p 2p excitation will only enhance the PE signals of the valence MOs having appreciable contributions from the 2p atomic orbitals of the nitrogen or oxygen, respectively. As mentioned above, the upper valence MOs that are responsible for PE signals a and b are dominated by the oxygen 2p atomic orbitals w14x. Therefore we expect these PE bands to be resonantly enhanced primarily in the decay of the oxygen p 2p excitation. Our experimental observations ŽFigs. 4 and 5. correlate qualitatively with this assumption. It should be noted that the correct quantitative comparison between the resonant PE spectra taken for the nitrogen and oxygen p 2p excitations is
only possible using absolute Žor at least relative. absorption cross-sections for the nitrogen and oxygen K p 2p transitions those are presently not available for solid NaNO 2 . In spite of this, we underline the fact that PE valence bands a–d are similarly enhanced upon decay of the nitrogen and oxygen p 2p excitations. For both p resonances, c and d bands are more enhanced than a and b bands. At the same time, from comparative analysis of N K and O K emission spectra of NaNO 2 w14x it is known that PE bands a, b and c, d originate from the valence MOs having mainly oxygen and nitrogen 2p orbital character, respectively. Therefore, they are expected to show resonant enhancement only for the corresponding Žoxygen or nitrogen. core excitations. Thus, the observed resonant behavior upon decay of the p excitations is not clear assuming two-step model for the de-excitation process. It is possible that in our case of the p excitations in the N K and O K absorption of NaNO 2 the core-hole creation and its following decay should be treated as coherent events in a one-step process w23,24x. The probable reason for this is a strong coupling between the p 2p excited states and the upper valence MO associated with the same 2p atomic orbitals of nitrogen and oxygen. It is evident that extensive theoretical studies are necessary for detailed explanation of the obtained results. Nevertheless, certain of them can be qualitatively understood by analyzing spatial localization and lifetime of the core excitations. Lifetime of the core excitation, t , is determined by that of the core hole, t h , and that of the excited core electron in the final quasimolecular state, te : 1rt s 1rt h q 1rte . The lifetime of the core hole is an intrinsic atomic property, which is weakly affected by atomic surroundings in solids, whereas te is influenced by modifying quasimolecular region of localization of the excited core electron. As seen in Figs. 4 and 5, in the case of the p excitation we observe three decay channels: Ž1. spectator, Ž2. participator, and Ž3. normal Auger de-excitation. Since the two first channels, which are characterized by excited electron remaining spatially localized within the NOy 2 molecular anion, are dominant in the de-excitation process, it is evident that the lifetime of the p excitations in NaNO 2 is mainly defined by the lifetime of the N K or O K hole. On the other hand, non-zero intensity of
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the normal Auger signal means that the excited electron can still leave the parent quasimolecule NOy 2 within the hole lifetime by interacting with the atomic surroundings and by hopping to neighboring quasimolecules. The latter points to a certain degree of delocalization of the 1s– 1 p 2p state outside the parent anion NOy 2 . Thereby, the relative intensity of the normal Auger decay as compared to the total intensity of two other de-excitation processes can be used to characterize quantitatively a delocalization of the core excitation. The decay of the s excitations Žcurve 7 in Fig. 4 and curves 7 and 8 in Fig. 5. exhibits distinct differences in comparison with the decay of the p excitations: Ži. an absence of any noticeable resonant enhancement of the valence bands via the participator Auger channel and Žii. an observation of the normal and spectator Auger signals with similar intensities for the N Ky1 s excitation and with the dominance of the normal Auger process in the decay of the O Ky1 s excitation. These findings prove that the s excitations have lifetime, which is essentially shorter than that of the core hole and is primarily determined by the lifetime of the s electron within 1 the parent NOy 2 anion in the 1s– s state. It is also obvious that the excited s electron leaves the parent anion quicker in the case of the O Ky1 s excitation. These facts can be explained by a strong interaction of the s electron with the neighboring atoms in the plane of the NO 2 Na quasimolecule, which defines crystal planes in NaNO 2 , resulting in a larger hopping rate to other anions. Therefore, the s electron appears to be more delocalized within the whole crystal than the p electron with the orbital oriented normally to the quasimolecular plane. This effect is expected to be more appreciable for the oxygen excitation, because the interaction between the NO 2 group and the surroundings occurs through oxygensodium bonding. Finally we discuss the photoemission results in relation to the nature of unoccupied electronic states in solid NaNO 2 . First of all, the observed decay properties of the p excitations give evidence for the strong localization of the lowest unoccupied electronic states in the solid allowing one to consider them as quasimolecular p states of the NOy 2 anion. Furthermore, these vacant electronic states in solid NaNO 2 originate mainly from the nitrogen and oxy-
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gen 2p valence states. Therefore, the participator Auger channel is clearly observed in the decay of the N Ky1 p 2p and O Ky1 p 2p excitations. As to the s excitations, we see from the analysis of the de-excitation spectra that they are only partially localized at the quasimolecular anion. The absence of the participator transitions in the decay of the s excitations means that the unoccupied s electronic states differ considerably from the 2p atomic states. According to a correlation diagram for XH 2 molecules w10x, the a 1 s and b 2 s MOs of the quasimolecular NO 2 anion, that are formed by the nitrogen and oxygen 2p orbitals, have already some properties of 3s and 3p atomic orbitals, respectively. An increase of the node number in radial wavefunctions of the s MOs in comparison with the 2p atomic orbitals and the p MO causes the drastic decrease of the participator decay probability relative to that of the spectator decay. According to calculations w25x, the participator decay intensity for the Ky1 3p excited state of the first-row atom, Ne, is less than the spectator decay intensity by a factor of 450. Therefore, the origin and properties of empty electronic states in NaNO 2 that are responsible for the s excitations are defined by more complicated polyatomic groups involving anion itself and also neighbors, in particular, sodium atom.
4. Conclusions The high-resolution X-ray absorption and the resonant photoemission spectra were used to study the nature of the N K and O K excitations in solid NaNO 2 . The similarity of the fine structure for both absorption spectra and their clear correlation in energy positions of the absorption bands indicate a common set of unoccupied electronic states that are responsible for the nitrogen and oxygen core excitations. Based on the comparative analysis of the N K absorption spectra for solid NaNO 2 , gas-phase molecules NO 2 , and CH 3 NO 2 , the core excitations in the absorption spectra from solid NaNO 2 were found to be quasimolecular and assigned to core electron transitions to unoccupied MOs of the planar NO 2 Na quasimolecule – the structural unit of the crystal.
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The measured PE spectra show strong resonant effects at the p and s excitations. The spectator, participator, and normal Auger processes were found to be responsible for the decay of the p excitations, whereas only the normal and spectator Auger structures were observed for the s excitations. The decay properties of the p excitations give evidence for anion and, their strong localization at the NOy 2 consequently, for the quasimolecular origin of the lowest unoccupied electron states of the p type in NaNO 2 . On the basis of photoemission data the s excitations are shown to be only partially localized within the quasimolecular anion. The obtained results are considered as a direct proof of the validity of the quasimolecular treatment of the N K and O K absorption spectra of solid NaNO 2 . Acknowledgements We would like to acknowledge the assistance of the BESSY staff and to thank V.K. Adamchuk, A.A. Pavlychev and R. Schlogl ¨ for stimulating discussions. This work was supported in part by Russian Foundation for Basic Research under grant number 98-02-18177. References w1x J. Stohr, ¨ NEXAFS Spectroscopy, Springer, Berlin, 1992. w2x M.I. Kay, B.C. Frazer, Acta Crystallogr. 14 Ž1961. 56. w3x S.V. Nekipelov, V.N. Akimov, A.S. Vinogradov, Fiz. Tverd. Tela ŽLeningrad. 30 Ž1988. 3647, wSov. Phys. - Solid State 30 Ž1988. 2095x. w4x A.A. Pavlychev, A.S. Vinogradov, V.N. Akimov, S.V. Nekipelov, Phys. Scr. 41 Ž1990. 160.
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