Determination of structural parameters from advanced molecular ...

Rendiconti Lincei 19, 215 – 221 (2008) DOI: 10.1007/s12210-008-0015-7

Michele Alagia · Claudio Furlani† · Ferdinando Pirani · Michel Lavollée · Robert Richter · Stefano Stranges · Pietro Candori · Stefano Falcinelli · Franco Vecchiocattivi

Determination of Structural Parameters from Advanced Molecular Electronic Spectroscopy: The Double Ionization of Nitrous Oxide by Synchrotron Radiation

Received: 13 December 2007 / Accepted: 8 February 2008 – © Springer-Verlag 2008

Abstract The double photoionization of N2 O molecules, in the 30–50 eV energy range, has been studied by synchrotron radiation. In the whole energy range, dissociative ionization producing N + + NO+ or N2+ + O+ has been M. Alagia ISMN-CNR Sez. Roma1, P.le A. Moro 5, 00185, Roma, Italy Tel.: +390403758418, Fax: +390403758029, E-mail: [email protected] C. Furlani Professor Emerito dell’Università di Roma Tre (già Prof. ordinario di Chimica generate e inorganica) Dipartimento di Fisica “Edoardo Amaldi”, Università Roma Tre F. Pirani Dipartimento di Chimica Università di Perugia, 06123 Perugia, Italy Tel.: +390755855528, Fax: +390755855606, E-mail: [email protected] M. Lavollée CNRS, Université Paris-Sud, LIXAM UMR8624, Bâtiment 350, Orsay Cedex, F-91405, France Tel.: +33169154154, E-mail: michel.lavollé[email protected] R. Richter Sincrotrone Trieste, Area Science Park, 34012 Basovizza, Trieste, Italy Tel: +39 0403758642, E-mail: [email protected] S. Stranges Dipartimento di Chimica, Università di Roma “La Sapienza”, 00185 Roma, Italy Tel.: +390649913362, E-mail: [email protected] P. Candori Dipartimento di Ingegneria Civile ed Ambientale, Università di Perugia, 06125 Perugia, Italy Tel. +390755855508, E-mail: [email protected] S. Falcinelli Dipartimento di Ingegneria Civile ed Ambientale, Università di Perugia, 06125 Perugia, Italy Tel.: +390755853859, E-mail: [email protected] F. Vecchiocattivi (B) Dipartimento di Ingegneria Civile ed Ambientale, Università di Perugia, 06125 Perugia, Italy Tel.: +390755853862, Fax: +390755853864, E-mail: [email protected]

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observed. These two processes appear to occur also below the vertical threshold of 35.8 eV, where the double ionization should be indirect. In the range between 35.8 and 38.5 eV. The two processes occur instead by direct coulomb explosion of the N2 O2+ dication. Above 38.5 eV, the dissociation leading to NO+ + N+ is also promoted by the formation of a dication metastable state, which decays by fluorescence to the ground state and then dissociates. Keywords Photoionization, Synchrotron Radiation, Dications, Coulomb explosion, Pre-dissociation Subject codes C16008, P23037 1 Introduction Electronic molecular spectroscopy has progressed considerably in recent times as a source of valuable information on energy and structure of molecules, not yet available hitherto by traditional means of investigation. Two factors support the increased potentialities of molecular spectroscopy, namely: a) advances in instrumentation: new spectral sources, particularly synchrotron light, allowing investigation of larger spectral regions with unprecedented intensity and resolution, and extensive use of related techniques, mainly coincidence and mass spectroscopic techniques; b) more efficient and dependable aid from comparison with quantum mechanic computations, supported and justified by the very large number of successfully treated molecular structures. The double photoionization of N2 O has been studied in other laboratories. An early study (Price et al. 1988) has shown that the nitrous oxide dication is almost completely dissociated within ∼ 10−8 s of its formation, mainly through the NO+ + N+ and N2+ + O+ dissociative channels. Regarding the energetics of the N2 O2+ dication, its production and dissociation have been experimentally studied by double charge transfer (Price et al. 1988; Appell et al. 1974; Harris et al. 1991), by photoion-photoion coincidence (Price et al. 1988; Curtis and Eland 1985), by Auger spectroscopy (Bolognesi et al. 2006; Connor et al. 1976), by time-of-flight photoelectron-photoelectron coincidence (Eland 2003) and by ion kinetic energy spectrometry (Cooks et al. 1974). Some theoretical studies (Price et al. 1988; Bolognesi et al. 2006; Connor et al. 1976; Larkins 1987; Levasseur and Millié 1990) also contributed to the characterization of this molecular dication. A recent paper (Taylor et al. 2006) reported the formation of a metastable state of the N2 O2+ dication that was identified by the use of some theoretical calculations, such as the 3 state with a lifetime of a few hundred nanoseconds, which first decays by fluorescence to the ground X3 state, and then dissociates producing only NO+ + N+ .

The Double Ionization of Nitrous Oxide by Synchrotron Radiation

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In this article we discuss the structural features revealed by synchrotron photoelectron spectroscopic investigation of the relatively simple molecule N2 O, whose experimental investigation has been reported previously (Alagia et al. 2006, 2007), and compared with previous assignments from related techniques (Cooks et al. 1974; Taylor et al. 2006). 2 Instrumentation The experimental setup for synchrotron measurements is depicted schematically in Fig. 1. It consists essentially of an ion chamber fed from three orthogonal directions by an effusive beam of N2 O gas, by the synchrotron source of VUV photons, and equipped from the third direction with a time-of-flight tube containing two aligned MCP detectors, one for ions and one for electrons. A more detailed description of the apparatus is contained in previous papers (Taylor et al. 2006; Alagia et al. 2006, 2007; Lavollee 1999). Such instrumentation allows not only photoelectron spectra but also photoelectron and photoion coincidence measurements in a spectral range well above that of the conventional gas lamps used for the visible range. In our case, the selected energy range was in the region of ∼ 30–50 eV synchrotron radiation, MCP position sensitive ion detector t

y

x

time-of-flight tube VUV photons

ion optics N2O beam source

light polarization direction MCP electron detector Fig. 1 A schematic sketch of the experimental apparatus.

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i.e. the region of double molecular ionizations (two photoelectrons produced by a singly absorbed photon). The position sensitive detector also allows the energy and angle distribution of the product ions to be obtained, the reference axis being the linear polarization direction of the VUV light. 3 Quantum mechanical structure computations The structure and energetics of the nitrous oxide molecule have also been characterized by quantum mechanical computations, in the neutral, single and double ionized states. The ground state neutral molecule has a linear configuration, with an electronic structure similar to that of carbon dioxide. In the first and second ionization ground levels such a structure is maintained (Price et al. 1988; Bolognesi et al. 2006; Connor et al. 1976; Larkins, 1987; Levasseur and Millié 1990; Taylor et al. 2006). Recently, highly correlated MRCI calculations have been performed (Taylor et al. 2006) at a range of N2 O2+ geometries, from which both N–N and N–O bond stretching curves are generated. Substantial barriers along both coordinates have been observed for the ground state. However, it is difficult to assess the level of reliability of such theoretical results, since these potential energy data have not yet been used for a molecular dynamics check of the experimental findings. 4 Problem setting, results and assignments It is well known that N2 O exhibits two one-electron absorptions in the lower energy region at 12.89 and 16.38 eV (Biondini et al. 2005a; 2005b), assigned as the first ionization of the two inequivalent nitrogen atoms in the N2 O molecule, or rather to the two highest occupied molecular orbitals of the linear N–N–O moiety. In the higher energy range investigated here, ∼ 28–40 eV, a vertical transition occurs at ca. 35,8 eV leading to formation of some species of the pre-dissociative dication N2 O2+ . Structure assignment of the dication and of its decay products is not possible on the evidence of photoionization alone, but can be reached unambiguously on the grounds of correlated mass spectra of the decay products, taken in coincidence with the ionization process. Ionization in the ∼ 36–38.5 eV range gives a mixture of two final couples of products, (NO+ + N+ ) and (N2+ + O+ ) in a branching ratio of approximately 4:1, irrespective of the actual photon energy being lower or higher than the vertical threshold. In fact, below the vertical threshold at 35.8 eV, the excited dication must transform to a dissociative state before splitting, while above the vertical threshold the dication is sufficiently rich in energy to undergo directly a Coulomb explosion. Furthermore, above 38.5 eV a metastable form of the dication is formed (lifetime ca. < 10−6 s) decaying radiatively to the ground


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state and then dissociating only to NO+ + N+ . It is tempting to identify such metastable form as the 3 predicted by theory [11] as the lowest level in the electron-coupling scheme of the dication. A further characterization of the decay processes of N2 O2+ was attempted by us, making use of another function of the same experimental instrumentation already used in our previous experimental work on N2 O, in which the angular distribution of ionic fragments was measured using a specially designed position sensitive detector. The initial series of measurements gave us detailed evidence for the marked anisotropy of ion production and energy distribution, and determined the values of the beta parameter. We could not yet measure the single components of the three dimensional distribution, which will require full exploitation of the potentialities of our specially designed detectors, as well as a more powerful system of data analysis, such as the inverse Abel approach. In this article we will confine our analysis to the two-dimensional projections in the detector plane, which are a precious source of information on the dynamics of photoionization and of the subsequent destiny of the products. Furthermore, such preliminary analysis yields a full description of energy distribution among the fragments formed after impact. At this point it seems appropriate to resume discussion on reactivity and the fate of N2 O, on which a vast amount of literature has been presented until now, and where however several points are still unsettled or uncertain, and need further attention. The neutral molecule N2 O is a typical 16-electron example of a linear triatomic molecule of first row atoms with a 16-electron system, similar to CO2 , NO+ 2 etc., but characterized by non-equivalence of the external atoms. Neutral N2 O presents therefore, in an approximate M.O. picture, one N ≡ N bond, similar to N2 , but possibly slightly weakened by participation of the neighbouring ≡ N = O moiety, and on the N+ − O− or N = O bond, possibly strengthened by extended participation in the N ≡ N moiety. Both moieties present therefore fairly strong internal bonds (see bond distances of 1.13 in N2 and 1.19 in NO), the N ≡ N bond being only slightly weakened with respect to molecular N2 by partial involvement in the N–O bond system, and the N+ − O− bond being slightly strengthened with respect to molecular N2 by possible involvement in the N+ − O− bond system. The N+ − O− bond is slightly strengthened with respect to organic N-oxides by admixture with the N ≡ N bond system. There is therefore a tendency to partial equalization of both N–N and N–O bond strengths. The obvious consequence would be that N–O and N–N bonds tend to equalize bond strengths and therefore also their resistance to bond rupture in both sites, with some preference for N–O scission. This is in agreement with the experimental finding that thermal dissociation of N2 O occurs at fairly high temperatures of about 600 ◦ C and is reported to produce exclusively N2+1/2O2 (Beattie 1967). There are eliminate indications

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that the decomposition process is not as simple as it seems from the overall reaction, and the actual course of the apparently simple reaction is a complex one, possibly including formation of paramagnetic radical intermediates. Dissociation of N2 O occurs more smoothly by photodissociation (Alagia et al. 2006; Beattie 1967), or Auger dissociation (Appell 1974; Bolognesi et al 2006; Connor et al. 1976; Larkins 1987), or collision with other active molecules (Biondini et al. 2005b; Beattie 1967), and there is a general agreement on the existence of more than one decomposition channel and on the crucial role of the intermediate dication N2 O2+ . The structure of N2 O2+ is fairly well known, even if not by direct experimental measurement which is prevented by its very short lifetime (≈ 10−6 s). It can be investigated however, either theoretically or through experimental studies of its decay products. N2 O2+ is formed by photooxidation of N2 O and has an open-shell structure formed by loss of the two outermost valence electrons from the neutral configuration, giving [core] 4σ 2 5σ 2 6σ 2 1π 4 7σ 2 2π 2 with triplet coupling states 3 or 1 + or 1 − . Very close in energy there is also a 1π 4 7σ 1 2π 3 configuration, containing a triplet 3 P state. On the grounds of the above plausible configurations, we obtain a preliminary picture of the possible electron distribution in N2 O2+ . Physically, it seems likely that the above mentioned electronic configurations have a spatial electron distribution, i.e. a deformed wave function, concentrated on the two external atoms. In both cases therefore, the subsequent coulomb explosion would produce some form of the N+ –N–O+ chain with comparable bond breaking tendency at the N–N and N–O sites. The instability of the N2 O2+ ions would not direct further rupture preferentially along the N–N or N–O bond. Such a scheme would be therefore in keeping with parallel formation of the ionic pairs N+ +NO+ and N2+ +O+ . This situation is better in keeping with the results of the present work, suggesting that coulomb explosion leads almost contemporaneously to N+ +NO+ and O+ +N2+ in a ratio of approximately 4:1. This dissociation path is apparently the same both above and below the vertical threshold, i.e. irrespective of the actual dissociation mechanism leading to final fragments either directly or via a metastable intermediate. The branching ratio depends additionally, above vertical IP, on the contribution from the metastable dication 4σ 2 5σ 2 6σ 2 1π 4 7σ 1 2π 3 3 P state. As a continuation and confirmation of the present work we will in the future undertake a systematic study of the spatial distribution of ionized fragments from photochemistry of N2 O,which will produce additional details to support the fundamental scheme of absorption spectroscopy and subsequent fragmentation path of N2 O molecules. The role of molecular electronic spectroscopy is therefore not only to confirm partial results or previous assumptions, but rather to extend our knowledge to a more quantitatively defined level, and to introduce us therefore to a more


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deep and extended knowledge of structure, bonding and reactivity mechanisms of molecules. Acknowledgements. This work has been supported by the Italian Ministero dell’Università e della Ricerca (MUR) and by the “Sincrotrone Trieste S.C.p.A.”.

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