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Selective vibrational excitation in the resonant Auger decay following core-to-π∗ transitions in N2O
Journal of Electron Spectroscopy and Related Phenomena 181 (2010) 129–134
Contents lists available at ScienceDirect
Journal of Electron Spectroscopy and Related Phenomena journal homepage: www.elsevier.com/locate/elspec
Selective vibrational excitation in the resonant Auger decay following core-to-∗ transitions in N2 O O. Travnikova a,b,∗ , D. Céolin c , Z. Bao b , K.J. Børve d , T. Tanaka e , M. Hoshino e , H. Kato e , H. Tanaka e , J.R. Harries f , Y. Tamenori f , G. Prümper g , T. Lischke g , X.-J. Liu g , M.N. Piancastelli b , K. Ueda g a
Synchrotron SOLEIL, L’Orme des Merisiers, St. Aubin BP 48, F-91192 Gif-Sur-Yvette, France Department of Physics and Astronomy, Box 516, 75120 Uppsala, Sweden Department of Synchrotron Radiation Research, Institute of Physics, University of Lund, Box 118, SE-221 00 Lund, Sweden d Department of Chemistry, University of Bergen, N-5007 Bergen, Norway e Department of Physics, Sophia University, Tokyo 102-8554, Japan f Japan Synchrotron Radiation Research Institute, Sayo-gun, Hyogo 679-5198, Japan g Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan b c
a r t i c l e
i n f o
Article history: Available online 15 May 2010 Keywords: Resonant Auger Renner–Teller effect Core excitation Electron spectroscopy N2 O
a b s t r a c t In N2 O a detailed study of the vibrational distribution of the X˜ state reached after decay of core-to∗ excitation of N terminal, N central and O 1s core levels is reported. We observe a change in the relative intensity of bending versus stretching modes while scanning the photon energy across all three resonances. While this effect is known to be due to the Renner–Teller splitting in the core-excited states, we could derive that the antisymmetric stretching is excited mainly in the decay of the N terminal 1s-to∗ excitation. An explanation for such selectivity is provided in terms of interplay of vibrational structure on potential energy surfaces of different electronic states involved in the process. © 2010 Elsevier B.V. All rights reserved.
1. Introduction When a core electron in a molecule is excited, the core hole can relax by electron emission. The core-hole decay takes place within the time scale of few or few tens of femtoseconds. This time scale is of the same order as the period of molecular vibrations and therefore the nuclear motion of the molecule may affect the electronic decay dynamics [1]. An interesting aspect which can be probed by resonant Auger spectroscopy is the nuclear motion taking place in the core-excited state and its interplay with the Auger decay on the same time scale. Within a semiclassical picture, as soon as a wave packet is created on the potential energy surface of the core-excited state by the resonant excitation, this wave packet starts to propagate, exploring details of this surface and, at the same time, decaying to the various final states. The resonant Auger line profile measured will directly reflect the nuclear motion taking place in the core-excited state modulated by its relative shape and position with respect to the potential energy surface of the final state reached by Auger decay [2]. The geometry of the intermediate state can differ from that one of the ground state. As an example, core-excited states of linear
∗ Corresponding author at: Synchrotron SOLEIL, L’Orme des Merisiers, St. Aubin BP 48, F-91192 Gif-Sur-Yvette, France. Tel.: +33 1 69 35 81 19. E-mail address: [email protected] (O. Travnikova). 0368-2048/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2010.05.006
molecules can be bent and those of planar molecules can be pyramidal. Such geometry change reflects in the properties of the Auger decay. Good examples of this so-called “dynamical Auger emission” have been reported for BF3 and BCl3 [3–5]. In more complex cases, a core-excited linear molecule can undergo Renner–Teller splitting [6–8] to stabilize the intermediate state, and this Renner–Teller effect has visible consequences in the vibrational distribution of the final states reached after resonant Auger decay. In triatomic molecules with a doubly degenerate core-to-∗ excited state, the twofold degeneracy is removed by the vibronic coupling with bending vibrations through the Renner–Teller effect and the core-to-∗ excited state is split into a in-plane and a out-of-plane excited states with bent and linear equilibrium geometries, respectively. Typical examples are OCS, CO2 , CS2 and N2 O molecules which are linear in the ground state. Many spectroscopic studies were dedicated to the study of such nuclear dynamics induced by core excitation [2,9–18]. In the case of CO2 and N2 O, the Renner–Teller splitting was found to be large, while it is of medium magnitude in OCS and very small in CS2 [9,14]. In N2 O a conformational change from linear (C∞v point group) to bent (Cs point group) takes place when a N 1s or O 1s core electron is promoted into the ∗ (3) unoccupied molecular orbital. Upon bending the degeneracy of the ∗ orbitals in the C∞v symmetry is lifted, giving rise to in-plane and out-of-plane orbitals in the Cs symmetry, corresponding to bent and linear structures, respectively, the bent being lowered in energy by bending of
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Fig. 1. Total ion yield spectra recorded in the vicinity of the Nt 1s and Nc 1s → 3 (right part) and O 1s → 3 (left part) transitions. The arrows mark the photon energies at which RAS spectra were recorded.
the molecule. Due to the linear conformation of the molecule in the ground state, and using the Franck–Condon principle for the core-to-∗ transitions, the bending mode will be strongly excited when the photon energy is tuned below the resonance maximum (negative detuning), while only stretching modes can be effectively excited when the photon energy is tuned above the resonance maximum (positive detuning) [2]. There are three normal vibrational modes associated with triatomic linear molecules: two stretching (1 and 3 ) and a bending (2 ) motion which is doubly degenerate in the ground state. In the case of N2 O, 1 and 3 are called quasi-symmetric and quasiantisymmetric stretching modes which are associated mainly with the N–N and N–O stretching vibrations, respectively, and 2 corresponds to N–N–O bending vibrations. Resonant Auger spectroscopy is a powerful method to identify the influence of the Renner–Teller splitting on the dynamics of core excitation–deexcitation, in particular in what concerns which vibrational modes in the final states reached after electron emission are affected by the reduced symmetry of the intermediate state. In N2 O the vibrational structure of the X˜ state reached after participator decay following the excitation of the N terminal (Nt ) 1s to the ∗ virtual orbital has been studied in some detail [2]. In particular, it has been demonstrated that as the photon energy is varied across the resonance the vibrational structure of the final state changes dramatically. The change has been attributed to the fact that for excitations in the low-energy side of the resonance the bent intermediate state is more populated, with consequent increase in the bending vibrational progression in the final state, while for excitations in the high-energy side the stretching mode(s) become prominent in the final state. However, the resolution in [2] was not sufficient to perform a complete analysis of the vibrational distribution of the final state, in particular in what concerns the relative importance of the symmetric and antisymmetric stretching modes. We present here a detailed analysis of the vibrational structure of the X˜ state in N2 O, reached after decay following the excitation of all three core levels, N terminal (Nt ), N central (Nc ) and O 1s, to the ∗ orbital. We indeed observe a change in the relative intensity of bending versus stretching modes while scanning the photon energy through the resonances. Furthermore, we were able to distinguish between the vibrational progressions due to the two different stretching modes, and therefore we can assess that the antisymmetric stretching mode is excited mainly in the decay of the high-energy side of the Nt 1s → ∗ resonance. An explanation for such selective excitation is provided in terms of interplay of vibrational structure on potential energy surfaces of the ground, intermediate and final states. 2. Experiment The experiments were performed on the C2-branch of the soft X-ray beamline 27SU at the SPring-8 synchrotron radiation facility
in Japan [19]. The radiation source is a figure 8 undulator and provides linearly polarized light: the polarization vector E is horizontal for the first-order harmonic light and vertical for the 0.5-order harmonic light [20]. The measurements were performed with the horizontal linearly polarized light. The monochromator installed on this branch is of Hettrick type [21]. The monochromator resolution was set to be around 50 meV at the N 1s edge. For the O 1s edge, the monochromator resolution was set to be around 50 meV for the excitation on the low and high photon energy sides of the O 1s → ∗ resonance; and to around 100 meV off-resonance and on top of the resonance. The high-resolution electron spectroscopy system employed consists of an SES-2002 hemispherical electron energy analyser, a gas cell and a differentially pumped main chamber [22]. The lens axis of the analyser is in the horizontal direction, and the entrance slit of the analyser is set parallel to the photon beam direction. The analyser resolution was set to be about 78 meV at the N K-edge and about 47 meV at the O K-edge. The experimental total ion yield and resonant Auger decay spectra are presented in Figs. 1 and 2. 3. Calculations The geometries of the ground, X˜ final cationic, both linear and bent Nt , Nc and O 1s → ∗ core-excited states were calculated with Dalton, a molecular electronic structure program [23], using Restricted Active Space RAS(16,12) wave function and ANO-1 (7s5p3d2f) basis sets for N and O atoms with an additional polarizing g function with exponent 1.43 on nitrogen and exponent 1.85 on oxygen. For all states, the doubly occupied core orbitals were inactive, i.e. not correlated. For ground states of the neutral and singly-ionized molecule were described at the Complete Active Space (CAS) level of theory, correlating 16 and 15 electrons for X and X, respectively. Within CS symmetry, the active space consists of 9 A’ and 3 A” orbitals. The core-excited states were described in the RAS formalism, with 18 electrons distributed over the strictly singly-occupied core orbital (making up the RAS1 space) and a RAS2 space equal to the CAS space described for the ground states. Core-excited states were optimized with relaxed core orbital. Vibrational analysis calculations were also carried out for all the above-mentioned states. The results of the calculations are summarized in Table 1. The minima of the potential energy surfaces were estimated in normal coordinates relative to the ground state for 1 and 3 vibrational modes. The calculated 1s → ∗ transition energies are in a good agreement with experiment. The measured values of the peak maxima for the Nt , Nc and O 1s → ∗ resonances (401.1, 404.7 and 534.6, respectively [24]) fall between the theoretically predicted values of the linear and bent core-excited states of the Renner–Teller pair for all three edges. The calculated N–N–O angles of the bent core-excited states (115.6◦ (Nt –Nc –O∗ ), 118.6◦ (Nt –N∗c –O), 133.6◦ (N∗t –Nc –O)) are in a good agreement with the previ-
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Fig. 2. Decay spectra in the binding energy region of the X˜ final state recorded on top (T), high (H) and low (L) energy sides of the resonance in the photon energy region of the terminal nitrogen Nt 1s → ∗ , the central nitrogen Nc 1s → ∗ and the oxygen O 1s → ∗ transitions. The off-resonance spectra are shown for sake of comparison. Table 1 Results of Dalton calculations for equilibrium geometries of ground, X˜ final and core-excited states of N2 O. re [Nt –Nc ] (Å)
re [Nc –O] (Å)
1.2119 1.2207 1.2087
1.1989 1.3521 1.3159
a (bent) 1s−1 ∗1 N∗t Nc O Nt N∗c O Nt Nc O∗
1.1962 1.2492 1.1895
1.2066 1.3409 1.3963
X˜ N2 O+ Ground state
1.1406 a 1.1319
1.2098 1.1886
a (linear) 1s N∗t Nc O Nt N∗c O Nt Nc O∗
a b
−1
Angle (◦ )
Rel. E (eV)
(meV)
∗1
133.623 118.623 115.579
401.918 404.815 535.143
243 148 57 179 87 38 240 118 20
400.252 403.075 532.206
222 164 94 147 109 59 193 101 63
11.271 0b
218 128 52 61 282 160 75 75
Exp. N2 O ground state: re [Nt –Nc ] = 1.1273 Å , re [Nc –O] = 1.1851 Å ; N2 O+ X˜ state: re [Nt –Nc ] = 1.154 Å , re [Nc –O] = 1.185 Å [25], rel.E = 12.89 eV [26]. Calculated total energy −183.970821 a.u.
ous theoretical calculations (112–115◦ , 114◦ , 136◦ , respectively [24]). 4. Experimental data treatment The resonant Auger decay spectra following the Nt , Nc and O 1s → ∗ excitation in the binding energy region of the X˜ state (12.7–13.7 eV) were fitted with 1,2,3 vibrational progressions using SPANCF macro package [27] developed for IGOR Pro software. For the spectra recorded at the low-energy side of the 1s−1 ∗1
resonance the contribution of the bending vibrational modes is important due to the preferential core excitation of the bent intermediate state of the Renner–Teller pair. They are not resolved in the resonant Auger decay spectra due to their relatively small vibrational spacings and significant overlap, which is further complicated by the presence of the combinational bands. These bending vibrations cannot be fitted with a reliable accuracy. On the other hand, the contribution of the unresolved bending vibrations in the resonant Auger decay spectra recorded at the high-energy side of the 1s−1 ∗1 resonance are minimized and the stretching vibrational
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modes are highly excited and clearly observed because bending modes are not excited in the transition to the linear higher-energy 1s−1 ∗1 core-excited state of the Renner–Teller pair. Therefore, the spectra recorded at the high-energy side of the 1s−1 ∗1 resonance at all three Nt,c and O sites were fitted in order to analyze the influence of the 1 and 3 stretching vibrational modes to the profiles of the X˜ final state reached after autoionization of the 1s−1 ∗1 core-excited states in N2 O. The RAS spectra for the Nc and O 1s edge were fitted simultaneously, where the vibrational spacings of individual vibrational progressions were fixed to be the same for the 2 fitted spectra. The vibrational spacing of the 1 progression obtained from this fit was used in the fit of the RAS spectrum recorded at the high-energy side of the Nt 1s−1 ∗1 resonance. The fits bear rather qualitative information because unresolved bending vibrations cannot be very well included but they might influence the shape of the spectra and distribution of relative intensities for individual vibrational progressions. Moreover, inclusion of a large number of peaks leads to long running-procedure times and often results in crash of the fitting procedure. However, this analysis is sufficiently good to distinguish between 1 and 3 stretching vibrations, the vibrational spacings of which differ by 1.5 times (140 and 214 meV, respectively [28]), and it allows to determine their relative importance. The results of the fits are presented in Fig. 3.
5. Results and discussion The resonant Auger decay spectra following the excitation of all three core levels, N terminal (Nt ), N central (Nc ) and O 1s, to the ∗ orbital were recorded along the resonance at low-energy side, on top of the resonance and at high-energy side with the excitation energies marked by arrows in Fig. 1. The corresponding RAS spectra in the binding energy region of the X˜ state are shown in Fig. 2. The X˜ state is resonantly enhanced mostly after excitation from the Nt 1s and the O 1s core levels to the ∗ . This behaviour reflects the contribution of the atomic orbitals of the three atoms to the valence electronic density: the X˜ state (2−1 ) is mostly localized on the terminal nitrogen and the oxygen atoms [10]. According to the Franck–Condon principle, the bending mode is not excited in the transition to the linear (1s−1 ∗1 ) state but is strongly excited in the bent (1s−1 ∗1 ) state which is a lower-energy state in the Renner–Teller pair [24,2]. Therefore, for negative detunings (low-energy side of the 1s → ∗ resonance), a bent state is mainly populated and unresolved bending vibrations are observed in the final state of the consecutive resonant Auger decay. For positive detunings (high-energy side of the 1s → ∗ resonance), the probability would be higher to reach a linear core-excited intermediate state and hence mostly stretching vibrational progressions are dominant in the final state (see Fig. 2). From Figs. 2 and 3 the vibrational progression of about 210–220 meV attributed to the 3 antisymmetric stretching mode can be clearly seen in the resonant Auger decay spectra recorded only after Nt 1s → ∗ excitation, while the RAS spectra recorded at the Nc and O 1s sites show resolved vibrations with a smaller spacing of about 130–140 meV corresponding to the 1 symmetric stretching mode. To explain the observed differences in the population of vibrational modes in the X˜ final state reached after electronic relaxation of the 1s−1 ∗1 core-excited states, let us compare calculated minima of the potential energy surfaces for ground, core-hole and final states in normal coordinates for 1 and 3 stretching modes. Only higher-energy linear core-excited states of the Renner–Teller pair are considered in this simplified analysis as those are more likely to
Fig. 3. Fit results for the X˜ final state reached after resonant Auger decay of the Nt , Nc and O 1s−1 ∗1 core-excited states. Open circles correspond to the experimental data measured at the high-energy side of the resonance at the three edges; black curves total fit; filled blues curves - 1 quasi-symmetric stretching vibrational progression; filled red curves - 3 quasi-antisymmetric stretching vibrational progression; filled green curves - 2 bending vibrations.
be reached for the excitation energies corresponding to the highenergy side of the resonance. The potential energy surfaces for ground and X˜ final state are slightly shifted by −0.3 a.u. along the normal coordinate for the 1 stretching mode and are nearly parallel for the 3 stretching mode (the shift in normal coordinate is 0.05 a.u.). The contribution to the vibrational excitations in the final state is very weak through the direct channel and can be neglected. On the other hand, under Auger resonant Raman conditions the wave packet will have time to propagate on the shifted core-excited state potential and will be projected on the unshifted final state potential. It will accordingly transfer the vibrational modes excited in the intermediate coreexcited states to the final state [1,29]. Nc : The minimum of the potential energy surfaces of the linear Nc 1s → ∗ core-excited state appear to be shifted along the normal coordinate relative to the ground state for the 1 quasi-
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Fig. 4. A schematic representation of the resonant Auger decay process. (a) Potential surfaces of ground and final states are the same. (b) Potential surfaces of intermediate core-exited and final states are the same. (c) Potential surfaces of ground, intermediate core-excited and final states are the same.
symmetric stretching mode by −1.2 a.u. and it nearly coincides for the 3 vibrational mode with the shift in normal coordinate of −0.07 a.u. Thus, the promotion of the 1s electron to the ∗ orbital at the Nc atomic site leads to the considerable excitation of the 1 mode in the intermediate core-excited and consequently in the cationic X˜ final states; while the 3 quasi-antisymmetric stretching would not be excited in neither Nc 1s → ∗ core-excited nor in the resonant Auger X˜ final states (see Fig. 4a and c). O: The potential energy surfaces for the O 1s−1 ∗1 intermediate state have similar shapes as the Nc 1s−1 ∗1 ones. The minimum of the potential energy surface of the linear O 1s → ∗ core-excited state is shifted along the normal coordinate relative to the ground state for the 1 quasi-symmetric stretching mode by −1.0 a.u., and it nearly coincides for the 3 vibrational mode with the shift in normal coordinate of −0.08 a.u. Thus, the 3 quasi-antisymmetric stretching would not be excited in neither O 1s → ∗ core-excited nor in the resonant Auger X˜ final states (see Fig. 4a and c). Nt : In contrast to Nc and O 1s−1 ∗1 potential energy surfaces, for the Nt 1s → ∗ core-excited state the minima of the potential energy surfaces are shifted along the normal coordinate by −0.4 and −0.3 a.u. for 1 and 3 stretching vibrational modes, respectively. Hence the minima of the Nt 1s−1 ∗1 and X˜ final state are nearly parallel for the 1 quasi-symmetric stretching leading to the vibrational collapse for this mode [1,29] (Fig. 4b), while quasiantisymmetric stretching 3 would be dominant in the resonant Auger decay spectrum (Fig. 4a).
6. Conclusions The Renner–Teller splitting in the core-excited states of linear molecules is known to have a profound influence on the vibrational distribution of the final states reached after resonant Auger decay. We present here a detailed analysis of the vibrational substructure of the X˜ state in N2 O reached after the decay of N terminal, N central and O 1s-to-∗ core excitations. The change in relative intensity of the bending versus the stretching modes as a function of the photon energy scanning across the resonances is well known and explained in terms of the degeneracy removal in the intermediate states and subsequent presence of a bent state at lower photon energy and a linear state at higher photon energy. We report here this observation following the decay of all three core-to-∗ reso-
nances. Furthermore, due to the high resolution available, we are able to perform an extensive analysis of the vibrational distribution of the X˜ state by a fitting procedure which has allowed us to derive the vibrational spacings for both the stretching modes and a qualitative picture of the relative weight of all vibrational modes for the three decay processes. We observe that the antisymmetric stretching mode is excited mainly following the decay of the N terminal 1s-to-∗ resonance. Such selective excitation is explained in terms of interplay of vibrational structure on the potential energy surfaces of the ground, intermediate and final states. Acknowledgments This experiment was carried out with the approval of the SPring8 program advisory committee and supported in part by grants-inaid for Scientific Research from the Japan Society for the Promotion of Science and by the Matsuo Foundation. We are grateful to the staff at SPring-8 for their help. Financial support from Triangle de la Physique in France and from the Swedish Research Council (VR) is gratefully acknowledged. The authors would like to thank Dr. V. Kimberg for fruitful discussions. References [1] F. Gel’mukhanov, H. Ågren, Phys. Rep. 312 (1999) 87. [2] C. Miron, M. Simon, P. Morin, S. Nanbu, N. Kosugi, S.L. Sorensen, A. Naves de Brito, M.N. Piancastelli, O. Björneholm, R. Feifel, J. Chem. Phys. 115 (2001) 864. [3] M. Simon, C. Miron, N. Leclercq, P. Morin, K. Ueda, Y. Sato, S. Tanaka, Y. Kayanuma, Phys. Rev. Lett. 79 (1997) 3857. [4] C. Miron, R. Feifel, O. Björneholm, S. Svensson, A.N. de Brito, S.L. Sorensen, M.N. Piancastelli, M. Simon, P. Morin, Chem. Phys. Lett. 359 (2002) 48. [5] K. Ueda, S. Tanaka, Y. Shimizu, Y. Muramatsu, H. Chiba, T. Hayaishi, M. Kitajima, H. Tanaka, Phys. Rev. Lett. 85 (2000) 3129. [6] S.A. Tashkun, P. Jensen, J. Mol. Spectrosc. 165 (1994) 173. [7] G. Herzberg, E. Teller, Z. Phys. Chem. B 21 (1933) 410. [8] R. Renner, Z. Phys. 92 (1934) 172. [9] J. Adachi, N. Kosugi, E. Shigemasa, A. Yagishita, J. Chem. Phys. 107 (1997) 4919. [10] M.N. Piancastelli, D. Céolin, O. Travnikova, Z. Bao, M. Hoshino, T. Tanaka, H. Kato, H. Tanaka, J.R. Harries, Y. Tamenori, J. Phys. B: At. Mol. Opt. Phys. 40 (2007) 3357. [11] E. Kukk, J.D. Bozek, N. Berrah, Phys. Rev. A62 (2000) 032708. [12] K. Nobusada, J. Phys. B: At. Mol. Opt. Phys. 35 (2002) 3055. [13] J. Adachi, N. Kosugi, A. Yagishita, J. Phys. B: At. Mol. Opt. Phys. 38 (2005) R127. [14] T. Tanaka, M. Hoshino, C. Makochekanwa, M. Kitajima, G. Prümper, X. Liu, T. Lischke, K. Nakagawa, H. Kato, Y. Tamenori, Chem. Phys. Lett. 428 (2006) 34. [15] T. Tanaka, K. Ueda, R. Feifel, L. Karlsson, H. Tanaka, M. Hoshino, M. Kitajima, M. Ehara, R. Fukuda, R. Tamaki, Chem. Phys. Lett. 435 (2007) 182.
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Contents lists available at ScienceDirect
Journal of Electron Spectroscopy and Related Phenomena journal homepage: www.elsevier.com/locate/elspec
Selective vibrational excitation in the resonant Auger decay following core-to-∗ transitions in N2 O O. Travnikova a,b,∗ , D. Céolin c , Z. Bao b , K.J. Børve d , T. Tanaka e , M. Hoshino e , H. Kato e , H. Tanaka e , J.R. Harries f , Y. Tamenori f , G. Prümper g , T. Lischke g , X.-J. Liu g , M.N. Piancastelli b , K. Ueda g a
Synchrotron SOLEIL, L’Orme des Merisiers, St. Aubin BP 48, F-91192 Gif-Sur-Yvette, France Department of Physics and Astronomy, Box 516, 75120 Uppsala, Sweden Department of Synchrotron Radiation Research, Institute of Physics, University of Lund, Box 118, SE-221 00 Lund, Sweden d Department of Chemistry, University of Bergen, N-5007 Bergen, Norway e Department of Physics, Sophia University, Tokyo 102-8554, Japan f Japan Synchrotron Radiation Research Institute, Sayo-gun, Hyogo 679-5198, Japan g Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan b c
a r t i c l e
i n f o
Article history: Available online 15 May 2010 Keywords: Resonant Auger Renner–Teller effect Core excitation Electron spectroscopy N2 O
a b s t r a c t In N2 O a detailed study of the vibrational distribution of the X˜ state reached after decay of core-to∗ excitation of N terminal, N central and O 1s core levels is reported. We observe a change in the relative intensity of bending versus stretching modes while scanning the photon energy across all three resonances. While this effect is known to be due to the Renner–Teller splitting in the core-excited states, we could derive that the antisymmetric stretching is excited mainly in the decay of the N terminal 1s-to∗ excitation. An explanation for such selectivity is provided in terms of interplay of vibrational structure on potential energy surfaces of different electronic states involved in the process. © 2010 Elsevier B.V. All rights reserved.
1. Introduction When a core electron in a molecule is excited, the core hole can relax by electron emission. The core-hole decay takes place within the time scale of few or few tens of femtoseconds. This time scale is of the same order as the period of molecular vibrations and therefore the nuclear motion of the molecule may affect the electronic decay dynamics [1]. An interesting aspect which can be probed by resonant Auger spectroscopy is the nuclear motion taking place in the core-excited state and its interplay with the Auger decay on the same time scale. Within a semiclassical picture, as soon as a wave packet is created on the potential energy surface of the core-excited state by the resonant excitation, this wave packet starts to propagate, exploring details of this surface and, at the same time, decaying to the various final states. The resonant Auger line profile measured will directly reflect the nuclear motion taking place in the core-excited state modulated by its relative shape and position with respect to the potential energy surface of the final state reached by Auger decay [2]. The geometry of the intermediate state can differ from that one of the ground state. As an example, core-excited states of linear
∗ Corresponding author at: Synchrotron SOLEIL, L’Orme des Merisiers, St. Aubin BP 48, F-91192 Gif-Sur-Yvette, France. Tel.: +33 1 69 35 81 19. E-mail address: [email protected] (O. Travnikova). 0368-2048/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2010.05.006
molecules can be bent and those of planar molecules can be pyramidal. Such geometry change reflects in the properties of the Auger decay. Good examples of this so-called “dynamical Auger emission” have been reported for BF3 and BCl3 [3–5]. In more complex cases, a core-excited linear molecule can undergo Renner–Teller splitting [6–8] to stabilize the intermediate state, and this Renner–Teller effect has visible consequences in the vibrational distribution of the final states reached after resonant Auger decay. In triatomic molecules with a doubly degenerate core-to-∗ excited state, the twofold degeneracy is removed by the vibronic coupling with bending vibrations through the Renner–Teller effect and the core-to-∗ excited state is split into a in-plane and a out-of-plane excited states with bent and linear equilibrium geometries, respectively. Typical examples are OCS, CO2 , CS2 and N2 O molecules which are linear in the ground state. Many spectroscopic studies were dedicated to the study of such nuclear dynamics induced by core excitation [2,9–18]. In the case of CO2 and N2 O, the Renner–Teller splitting was found to be large, while it is of medium magnitude in OCS and very small in CS2 [9,14]. In N2 O a conformational change from linear (C∞v point group) to bent (Cs point group) takes place when a N 1s or O 1s core electron is promoted into the ∗ (3) unoccupied molecular orbital. Upon bending the degeneracy of the ∗ orbitals in the C∞v symmetry is lifted, giving rise to in-plane and out-of-plane orbitals in the Cs symmetry, corresponding to bent and linear structures, respectively, the bent being lowered in energy by bending of
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Fig. 1. Total ion yield spectra recorded in the vicinity of the Nt 1s and Nc 1s → 3 (right part) and O 1s → 3 (left part) transitions. The arrows mark the photon energies at which RAS spectra were recorded.
the molecule. Due to the linear conformation of the molecule in the ground state, and using the Franck–Condon principle for the core-to-∗ transitions, the bending mode will be strongly excited when the photon energy is tuned below the resonance maximum (negative detuning), while only stretching modes can be effectively excited when the photon energy is tuned above the resonance maximum (positive detuning) [2]. There are three normal vibrational modes associated with triatomic linear molecules: two stretching (1 and 3 ) and a bending (2 ) motion which is doubly degenerate in the ground state. In the case of N2 O, 1 and 3 are called quasi-symmetric and quasiantisymmetric stretching modes which are associated mainly with the N–N and N–O stretching vibrations, respectively, and 2 corresponds to N–N–O bending vibrations. Resonant Auger spectroscopy is a powerful method to identify the influence of the Renner–Teller splitting on the dynamics of core excitation–deexcitation, in particular in what concerns which vibrational modes in the final states reached after electron emission are affected by the reduced symmetry of the intermediate state. In N2 O the vibrational structure of the X˜ state reached after participator decay following the excitation of the N terminal (Nt ) 1s to the ∗ virtual orbital has been studied in some detail [2]. In particular, it has been demonstrated that as the photon energy is varied across the resonance the vibrational structure of the final state changes dramatically. The change has been attributed to the fact that for excitations in the low-energy side of the resonance the bent intermediate state is more populated, with consequent increase in the bending vibrational progression in the final state, while for excitations in the high-energy side the stretching mode(s) become prominent in the final state. However, the resolution in [2] was not sufficient to perform a complete analysis of the vibrational distribution of the final state, in particular in what concerns the relative importance of the symmetric and antisymmetric stretching modes. We present here a detailed analysis of the vibrational structure of the X˜ state in N2 O, reached after decay following the excitation of all three core levels, N terminal (Nt ), N central (Nc ) and O 1s, to the ∗ orbital. We indeed observe a change in the relative intensity of bending versus stretching modes while scanning the photon energy through the resonances. Furthermore, we were able to distinguish between the vibrational progressions due to the two different stretching modes, and therefore we can assess that the antisymmetric stretching mode is excited mainly in the decay of the high-energy side of the Nt 1s → ∗ resonance. An explanation for such selective excitation is provided in terms of interplay of vibrational structure on potential energy surfaces of the ground, intermediate and final states. 2. Experiment The experiments were performed on the C2-branch of the soft X-ray beamline 27SU at the SPring-8 synchrotron radiation facility
in Japan [19]. The radiation source is a figure 8 undulator and provides linearly polarized light: the polarization vector E is horizontal for the first-order harmonic light and vertical for the 0.5-order harmonic light [20]. The measurements were performed with the horizontal linearly polarized light. The monochromator installed on this branch is of Hettrick type [21]. The monochromator resolution was set to be around 50 meV at the N 1s edge. For the O 1s edge, the monochromator resolution was set to be around 50 meV for the excitation on the low and high photon energy sides of the O 1s → ∗ resonance; and to around 100 meV off-resonance and on top of the resonance. The high-resolution electron spectroscopy system employed consists of an SES-2002 hemispherical electron energy analyser, a gas cell and a differentially pumped main chamber [22]. The lens axis of the analyser is in the horizontal direction, and the entrance slit of the analyser is set parallel to the photon beam direction. The analyser resolution was set to be about 78 meV at the N K-edge and about 47 meV at the O K-edge. The experimental total ion yield and resonant Auger decay spectra are presented in Figs. 1 and 2. 3. Calculations The geometries of the ground, X˜ final cationic, both linear and bent Nt , Nc and O 1s → ∗ core-excited states were calculated with Dalton, a molecular electronic structure program [23], using Restricted Active Space RAS(16,12) wave function and ANO-1 (7s5p3d2f) basis sets for N and O atoms with an additional polarizing g function with exponent 1.43 on nitrogen and exponent 1.85 on oxygen. For all states, the doubly occupied core orbitals were inactive, i.e. not correlated. For ground states of the neutral and singly-ionized molecule were described at the Complete Active Space (CAS) level of theory, correlating 16 and 15 electrons for X and X, respectively. Within CS symmetry, the active space consists of 9 A’ and 3 A” orbitals. The core-excited states were described in the RAS formalism, with 18 electrons distributed over the strictly singly-occupied core orbital (making up the RAS1 space) and a RAS2 space equal to the CAS space described for the ground states. Core-excited states were optimized with relaxed core orbital. Vibrational analysis calculations were also carried out for all the above-mentioned states. The results of the calculations are summarized in Table 1. The minima of the potential energy surfaces were estimated in normal coordinates relative to the ground state for 1 and 3 vibrational modes. The calculated 1s → ∗ transition energies are in a good agreement with experiment. The measured values of the peak maxima for the Nt , Nc and O 1s → ∗ resonances (401.1, 404.7 and 534.6, respectively [24]) fall between the theoretically predicted values of the linear and bent core-excited states of the Renner–Teller pair for all three edges. The calculated N–N–O angles of the bent core-excited states (115.6◦ (Nt –Nc –O∗ ), 118.6◦ (Nt –N∗c –O), 133.6◦ (N∗t –Nc –O)) are in a good agreement with the previ-
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Fig. 2. Decay spectra in the binding energy region of the X˜ final state recorded on top (T), high (H) and low (L) energy sides of the resonance in the photon energy region of the terminal nitrogen Nt 1s → ∗ , the central nitrogen Nc 1s → ∗ and the oxygen O 1s → ∗ transitions. The off-resonance spectra are shown for sake of comparison. Table 1 Results of Dalton calculations for equilibrium geometries of ground, X˜ final and core-excited states of N2 O. re [Nt –Nc ] (Å)
re [Nc –O] (Å)
1.2119 1.2207 1.2087
1.1989 1.3521 1.3159
a (bent) 1s−1 ∗1 N∗t Nc O Nt N∗c O Nt Nc O∗
1.1962 1.2492 1.1895
1.2066 1.3409 1.3963
X˜ N2 O+ Ground state
1.1406 a 1.1319
1.2098 1.1886
a (linear) 1s N∗t Nc O Nt N∗c O Nt Nc O∗
a b
−1
Angle (◦ )
Rel. E (eV)
(meV)
∗1
133.623 118.623 115.579
401.918 404.815 535.143
243 148 57 179 87 38 240 118 20
400.252 403.075 532.206
222 164 94 147 109 59 193 101 63
11.271 0b
218 128 52 61 282 160 75 75
Exp. N2 O ground state: re [Nt –Nc ] = 1.1273 Å , re [Nc –O] = 1.1851 Å ; N2 O+ X˜ state: re [Nt –Nc ] = 1.154 Å , re [Nc –O] = 1.185 Å [25], rel.E = 12.89 eV [26]. Calculated total energy −183.970821 a.u.
ous theoretical calculations (112–115◦ , 114◦ , 136◦ , respectively [24]). 4. Experimental data treatment The resonant Auger decay spectra following the Nt , Nc and O 1s → ∗ excitation in the binding energy region of the X˜ state (12.7–13.7 eV) were fitted with 1,2,3 vibrational progressions using SPANCF macro package [27] developed for IGOR Pro software. For the spectra recorded at the low-energy side of the 1s−1 ∗1
resonance the contribution of the bending vibrational modes is important due to the preferential core excitation of the bent intermediate state of the Renner–Teller pair. They are not resolved in the resonant Auger decay spectra due to their relatively small vibrational spacings and significant overlap, which is further complicated by the presence of the combinational bands. These bending vibrations cannot be fitted with a reliable accuracy. On the other hand, the contribution of the unresolved bending vibrations in the resonant Auger decay spectra recorded at the high-energy side of the 1s−1 ∗1 resonance are minimized and the stretching vibrational
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modes are highly excited and clearly observed because bending modes are not excited in the transition to the linear higher-energy 1s−1 ∗1 core-excited state of the Renner–Teller pair. Therefore, the spectra recorded at the high-energy side of the 1s−1 ∗1 resonance at all three Nt,c and O sites were fitted in order to analyze the influence of the 1 and 3 stretching vibrational modes to the profiles of the X˜ final state reached after autoionization of the 1s−1 ∗1 core-excited states in N2 O. The RAS spectra for the Nc and O 1s edge were fitted simultaneously, where the vibrational spacings of individual vibrational progressions were fixed to be the same for the 2 fitted spectra. The vibrational spacing of the 1 progression obtained from this fit was used in the fit of the RAS spectrum recorded at the high-energy side of the Nt 1s−1 ∗1 resonance. The fits bear rather qualitative information because unresolved bending vibrations cannot be very well included but they might influence the shape of the spectra and distribution of relative intensities for individual vibrational progressions. Moreover, inclusion of a large number of peaks leads to long running-procedure times and often results in crash of the fitting procedure. However, this analysis is sufficiently good to distinguish between 1 and 3 stretching vibrations, the vibrational spacings of which differ by 1.5 times (140 and 214 meV, respectively [28]), and it allows to determine their relative importance. The results of the fits are presented in Fig. 3.
5. Results and discussion The resonant Auger decay spectra following the excitation of all three core levels, N terminal (Nt ), N central (Nc ) and O 1s, to the ∗ orbital were recorded along the resonance at low-energy side, on top of the resonance and at high-energy side with the excitation energies marked by arrows in Fig. 1. The corresponding RAS spectra in the binding energy region of the X˜ state are shown in Fig. 2. The X˜ state is resonantly enhanced mostly after excitation from the Nt 1s and the O 1s core levels to the ∗ . This behaviour reflects the contribution of the atomic orbitals of the three atoms to the valence electronic density: the X˜ state (2−1 ) is mostly localized on the terminal nitrogen and the oxygen atoms [10]. According to the Franck–Condon principle, the bending mode is not excited in the transition to the linear (1s−1 ∗1 ) state but is strongly excited in the bent (1s−1 ∗1 ) state which is a lower-energy state in the Renner–Teller pair [24,2]. Therefore, for negative detunings (low-energy side of the 1s → ∗ resonance), a bent state is mainly populated and unresolved bending vibrations are observed in the final state of the consecutive resonant Auger decay. For positive detunings (high-energy side of the 1s → ∗ resonance), the probability would be higher to reach a linear core-excited intermediate state and hence mostly stretching vibrational progressions are dominant in the final state (see Fig. 2). From Figs. 2 and 3 the vibrational progression of about 210–220 meV attributed to the 3 antisymmetric stretching mode can be clearly seen in the resonant Auger decay spectra recorded only after Nt 1s → ∗ excitation, while the RAS spectra recorded at the Nc and O 1s sites show resolved vibrations with a smaller spacing of about 130–140 meV corresponding to the 1 symmetric stretching mode. To explain the observed differences in the population of vibrational modes in the X˜ final state reached after electronic relaxation of the 1s−1 ∗1 core-excited states, let us compare calculated minima of the potential energy surfaces for ground, core-hole and final states in normal coordinates for 1 and 3 stretching modes. Only higher-energy linear core-excited states of the Renner–Teller pair are considered in this simplified analysis as those are more likely to
Fig. 3. Fit results for the X˜ final state reached after resonant Auger decay of the Nt , Nc and O 1s−1 ∗1 core-excited states. Open circles correspond to the experimental data measured at the high-energy side of the resonance at the three edges; black curves total fit; filled blues curves - 1 quasi-symmetric stretching vibrational progression; filled red curves - 3 quasi-antisymmetric stretching vibrational progression; filled green curves - 2 bending vibrations.
be reached for the excitation energies corresponding to the highenergy side of the resonance. The potential energy surfaces for ground and X˜ final state are slightly shifted by −0.3 a.u. along the normal coordinate for the 1 stretching mode and are nearly parallel for the 3 stretching mode (the shift in normal coordinate is 0.05 a.u.). The contribution to the vibrational excitations in the final state is very weak through the direct channel and can be neglected. On the other hand, under Auger resonant Raman conditions the wave packet will have time to propagate on the shifted core-excited state potential and will be projected on the unshifted final state potential. It will accordingly transfer the vibrational modes excited in the intermediate coreexcited states to the final state [1,29]. Nc : The minimum of the potential energy surfaces of the linear Nc 1s → ∗ core-excited state appear to be shifted along the normal coordinate relative to the ground state for the 1 quasi-
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Fig. 4. A schematic representation of the resonant Auger decay process. (a) Potential surfaces of ground and final states are the same. (b) Potential surfaces of intermediate core-exited and final states are the same. (c) Potential surfaces of ground, intermediate core-excited and final states are the same.
symmetric stretching mode by −1.2 a.u. and it nearly coincides for the 3 vibrational mode with the shift in normal coordinate of −0.07 a.u. Thus, the promotion of the 1s electron to the ∗ orbital at the Nc atomic site leads to the considerable excitation of the 1 mode in the intermediate core-excited and consequently in the cationic X˜ final states; while the 3 quasi-antisymmetric stretching would not be excited in neither Nc 1s → ∗ core-excited nor in the resonant Auger X˜ final states (see Fig. 4a and c). O: The potential energy surfaces for the O 1s−1 ∗1 intermediate state have similar shapes as the Nc 1s−1 ∗1 ones. The minimum of the potential energy surface of the linear O 1s → ∗ core-excited state is shifted along the normal coordinate relative to the ground state for the 1 quasi-symmetric stretching mode by −1.0 a.u., and it nearly coincides for the 3 vibrational mode with the shift in normal coordinate of −0.08 a.u. Thus, the 3 quasi-antisymmetric stretching would not be excited in neither O 1s → ∗ core-excited nor in the resonant Auger X˜ final states (see Fig. 4a and c). Nt : In contrast to Nc and O 1s−1 ∗1 potential energy surfaces, for the Nt 1s → ∗ core-excited state the minima of the potential energy surfaces are shifted along the normal coordinate by −0.4 and −0.3 a.u. for 1 and 3 stretching vibrational modes, respectively. Hence the minima of the Nt 1s−1 ∗1 and X˜ final state are nearly parallel for the 1 quasi-symmetric stretching leading to the vibrational collapse for this mode [1,29] (Fig. 4b), while quasiantisymmetric stretching 3 would be dominant in the resonant Auger decay spectrum (Fig. 4a).
6. Conclusions The Renner–Teller splitting in the core-excited states of linear molecules is known to have a profound influence on the vibrational distribution of the final states reached after resonant Auger decay. We present here a detailed analysis of the vibrational substructure of the X˜ state in N2 O reached after the decay of N terminal, N central and O 1s-to-∗ core excitations. The change in relative intensity of the bending versus the stretching modes as a function of the photon energy scanning across the resonances is well known and explained in terms of the degeneracy removal in the intermediate states and subsequent presence of a bent state at lower photon energy and a linear state at higher photon energy. We report here this observation following the decay of all three core-to-∗ reso-
nances. Furthermore, due to the high resolution available, we are able to perform an extensive analysis of the vibrational distribution of the X˜ state by a fitting procedure which has allowed us to derive the vibrational spacings for both the stretching modes and a qualitative picture of the relative weight of all vibrational modes for the three decay processes. We observe that the antisymmetric stretching mode is excited mainly following the decay of the N terminal 1s-to-∗ resonance. Such selective excitation is explained in terms of interplay of vibrational structure on the potential energy surfaces of the ground, intermediate and final states. Acknowledgments This experiment was carried out with the approval of the SPring8 program advisory committee and supported in part by grants-inaid for Scientific Research from the Japan Society for the Promotion of Science and by the Matsuo Foundation. We are grateful to the staff at SPring-8 for their help. Financial support from Triangle de la Physique in France and from the Swedish Research Council (VR) is gratefully acknowledged. The authors would like to thank Dr. V. Kimberg for fruitful discussions. References [1] F. Gel’mukhanov, H. Ågren, Phys. Rep. 312 (1999) 87. [2] C. Miron, M. Simon, P. Morin, S. Nanbu, N. Kosugi, S.L. Sorensen, A. Naves de Brito, M.N. Piancastelli, O. Björneholm, R. Feifel, J. Chem. Phys. 115 (2001) 864. [3] M. Simon, C. Miron, N. Leclercq, P. Morin, K. Ueda, Y. Sato, S. Tanaka, Y. Kayanuma, Phys. Rev. Lett. 79 (1997) 3857. [4] C. Miron, R. Feifel, O. Björneholm, S. Svensson, A.N. de Brito, S.L. Sorensen, M.N. Piancastelli, M. Simon, P. Morin, Chem. Phys. Lett. 359 (2002) 48. [5] K. Ueda, S. Tanaka, Y. Shimizu, Y. Muramatsu, H. Chiba, T. Hayaishi, M. Kitajima, H. Tanaka, Phys. Rev. Lett. 85 (2000) 3129. [6] S.A. Tashkun, P. Jensen, J. Mol. Spectrosc. 165 (1994) 173. [7] G. Herzberg, E. Teller, Z. Phys. Chem. B 21 (1933) 410. [8] R. Renner, Z. Phys. 92 (1934) 172. [9] J. Adachi, N. Kosugi, E. Shigemasa, A. Yagishita, J. Chem. Phys. 107 (1997) 4919. [10] M.N. Piancastelli, D. Céolin, O. Travnikova, Z. Bao, M. Hoshino, T. Tanaka, H. Kato, H. Tanaka, J.R. Harries, Y. Tamenori, J. Phys. B: At. Mol. Opt. Phys. 40 (2007) 3357. [11] E. Kukk, J.D. Bozek, N. Berrah, Phys. Rev. A62 (2000) 032708. [12] K. Nobusada, J. Phys. B: At. Mol. Opt. Phys. 35 (2002) 3055. [13] J. Adachi, N. Kosugi, A. Yagishita, J. Phys. B: At. Mol. Opt. Phys. 38 (2005) R127. [14] T. Tanaka, M. Hoshino, C. Makochekanwa, M. Kitajima, G. Prümper, X. Liu, T. Lischke, K. Nakagawa, H. Kato, Y. Tamenori, Chem. Phys. Lett. 428 (2006) 34. [15] T. Tanaka, K. Ueda, R. Feifel, L. Karlsson, H. Tanaka, M. Hoshino, M. Kitajima, M. Ehara, R. Fukuda, R. Tamaki, Chem. Phys. Lett. 435 (2007) 182.
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