- Email: [email protected]
Double photoionization of N2O molecules in the 28–40 eV energy range
Chemical Physics Letters 432 (2006) 398–402 www.elsevier.com/locate/cplett
Double photoionization of N2O molecules in the 28–40 eV energy range Michele Alagia a,b, Pietro Candori c, Stefano Falcinelli c, Michel Lavolle´e d, Fernando Pirani e, Robert Richter f, Stefano Stranges b,g, Franco Vecchiocattivi
c,*
a ISMN-CNR Sez. Roma1, P.le A. Moro 5, 00185, Rome, Italy Laboratorio TASC-INFM, Area Science Park, 34012 Basovizza, Trieste, Italy c Dipartimento di Ingegneria Civile ed Ambientale, Universita` di Perugia, 06125 Perugia, Italy CNRS, Universite´ Paris-Sud, LIXAM UMR8624, Baˆtiment 350, Orsay Cedex, F-91405, France e Dipartimento di Chimica Universita` di Perugia, 06125 Perugia, Italy f Sincrotrone Trieste, Area Science Park, 34012 Basovizza, Trieste, Italy g Dipartimento di Chimica, Universita` di Roma ‘La Sapienza’, 00185 Roma, Italy b
d
Received 9 October 2006; in final form 20 October 2006 Available online 27 October 2006
Abstract The double photoionization of N2O molecules, in the 28–40 eV energy range, has been studied by synchrotron radiation. In the whole þ energy range, dissociative ionization producing N+ + NO+ or Nþ 2 þ O has been observed. Below 35.5 eV, these two processes involve the formation of some autoionizing states. In the range between 35.5 and 38.5 eV the two processes occur instead by a direct coulomb explosion of the N2O2+ 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. Ó 2006 Elsevier B.V. All rights reserved.
1. Introduction The nitrous oxide molecule, N2O, is a naturally occurring molecular species in the atmosphere of our planet. Although it represents a very small fraction of the total atmospheric gases, its presence is important because this molecule is two orders of magnitude more efficient as a green house gas than, for example carbon dioxide, CO2. This, together with a steady increase of its emissions by about 0.3% each year, make N2O an important gaseous species to be considered in atmospheric studies. An important molecular property for green house modeling is the lifetime of a species in the atmosphere, before it disappears by dissociation or reaction with other molecular species. Nitrous oxide absorbs ultraviolet light from the sun, and re-radiates this energy under various wavelengths, from the infrared to the visible. If the energy of the photons is *
Corresponding author. E-mail address: [email protected] (F. Vecchiocattivi).
0009-2614/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.10.100
sufficiently high, the molecule can be dissociated into fragments, such as NO and N, or stripped of one or even two of its electrons. Understanding the various fragmentation and ionization paths of the molecule are, therefore, of great importance for atmospheric and chemistry studies. Two recent papers from our laboratory [1,2] have reported experimental and theoretical studies about the single ionization of nitrous oxide by collisions with excited rare gas atoms (Penning ionization) and the results have been also compared with those by ultraviolet photon absorption. The ionization dynamics has been discussed in terms of structure and geometry of the atomic and molecular orbitals involved in the processes. The double photoionization of N2O has been already studied in other laboratories. An early study [3] has shown that the nitrous oxide dication is almost completely dissociated, within 108 s of its formation, mainly through þ the NO+ + N+ and Nþ dissociative channels. The 2 þO 2+ energetics of the N2O dication, its production and dissociation has been experimentally studied by double charge
M. Alagia et al. / Chemical Physics Letters 432 (2006) 398–402
transfer [3–5], by photoion–photoion coincidence [3,6], by Auger spectroscopy [7,8], by time-of-flight photoelectron– photoelectron coincidence [9] and by ion kinetic energy spectrometry [10]. Some theoretical studies [3,7,8,11,12] also contributed to the characterization of this molecular dication. In a recent paper, Taylor et al. [13] have characterized the formation of a metastable state of N2O2+ dication, that they identified, by the use also of some theoretical calculations, as the 3P state with life time of few hundred nanoseconds, which first decays by fluorescence to the ground X3R state, and then dissociates producing only NO+ + N+. 40 eV
N2O2+ 3Π 1Σ+ 1Δ
3Σ-
35
399
In recent years we have studied the double photoionization of HX (X = Cl, Br, and I) by synchrotron radiation and we have characterized the results by using some simple theoretical schemes [14–19]. With the present work we intend to extend such a study to the N2O triatomic molecule and we report here on the double ionization of N2O by vacuum ultraviolet photons of synchrotron radiation, in the 28–40 eV energy range. The energy levels of the double ionization states of N2O, involved in the presently investigated energy range, are shown in Fig. 1. The present results allow us to assess the threshold energies for the disþ sociative processes producing NO+ + N+ and Nþ 2 þO ions. It will be shown that below 35.5 eV, the two processes involve the formation of some autoionizing states. In the range between 35.5 and 38.5 eV the two processes occur instead by a direct coulomb explosion of the N2O2+ dication. However, above 38.5 eV the dissociation leading to NO+ + N+ is also promoted by the formation of a metastable state of the molecular dication which is expected to decay by fluorescence to the ground state and then dissociates [13]. 2. Experimental
N2+(2Σg+)+ O +(4S) 30 NO+(1Σ+)+ N +(3P)
Fig. 1. The energy levels relevant to the double ionization of N2O in the 28–40 eV range. The values are quoted with respect to the ground neutral state of the molecule. The molecular dication state energies are taken from Ref. [13].
The experiment has been performed at the ELETTRA Synchrotron Light Laboratory using the ARPES end station at the Gasphase Beamline. A detailed description of the beam line and the end station is given elsewhere [20]. Only the features relevant to the present experiment are schematically described here. In Fig. 2, a sketch of the experimental set up is shown. The energy selected synchrotron light beam crosses an effusive molecular beam of N2O and the product ions are then
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. 2. A schematic sketch of the experimental set up.
400
M. Alagia et al. / Chemical Physics Letters 432 (2006) 398–402
detected in coincidence with photoelectrons. The monochromator uses a 400 l/mm spherical grating in first diffraction order. The entrance and exit slits of the monochromator have been adjusted in order to have a resolution between 2.0 and 1.5 meV in the investigated 28– 40 eV photon energy range. The resolution and the energy scale calibration has been checked by measuring some known sharp resonances in the total ion yield of argon in this energy range. To avoid spurious ionization effects, due to photons of higher order energy, a magnesium film filter was used. The N2O molecular beam and the VUV light beam crosses at right angles, with the light polarization vector being parallel to the synchrotron ring plane and perpendicular to the time-of-flight direction of detected ions. The ion detection system has been built up according to the model described in Ref. [21]. Such a position sensitive detector is particularly designed in order to measure the spatial momentum components of the dissociation ionic products. However, for the experiment described here, the x–y dependence has been not used and therefore only total ion arrival times to the micro channel plate detector have been considered. Practically, mass spectra were recorded using photoelectrons as starting pulses, and then ions were counted as a function of the delay times. A computer controlled all the experiment components and also recorded experimental data. The incident photon flux and gas pressure have been monitored and stored in separate acquisition channels. Ion yields have been then corrected for pressure and photon flux changes while varying the photon energy. The nitrous oxide is supplied to the needle beam source from a commercial bottle filled with about 10 atm at room temperature. The gas has a 99.99% nominal purity and has been used without any further treatment. An adjustable leak along the input gas pipe line is used in order to control the gas flow, which is monitored by checking the background pressure in the main vacuum chamber.
Fig. 3. Mass spectrum and ion–ion time of flight correlation of ions produced by single and double photoionization of N2O at 39 eV. In this type of plot, which is typical of double photoionization experiments, the two time of flight values of a couple of ions produced in the same photoionization event define a point (see for instance Ref. [13]). The diagonal weak traces are false and spurious coincidences that have been neglected in the present analysis.
3. Results and discussion
Fig. 4. Extended plot of the portion of the ion–ion time of flight correlation diagram of Fig. 3 where is possible to distinguish the N+/NO+ + and Nþ coincidence spots and the trace for the metastable N2O2+ 2 /O dication formed at 39 eV photon energy.
The mass spectra detected in the 28–40 eV photon energy range indicate that single and double photo ionization of N2O, in such an energy range, produce mainly N+, + + O +, N þ 2 , NO and N2O ions. A mass spectrum, together with an ion–ion time of flight correlation diagram, are reported in Fig. 3, for a photon energy of 39.0 eV. All product ions are evident together with some background peaks. A blow up of the most relevant part of such a diagram is reported in Fig. 4, where it is possible to distinguish the ionic products of the double photoionization detected þ in coincidence, which are NO+ + N+ and Nþ 2 þ O . The time correlation diagram also shows some diagonal weak traces of false and spurious coincidences, which have been simply neglected in the present analysis. A typical trace due to the formation of a metastable N2O2+ dication is also evident in the diagram of Fig. 4
and such a metastable dication leads to the dissociation in NO+ + N+ final products. We have measured spectra and ion–ion time of flight correlation diagrams, like that one in Figs. 3 and 4, starting from 28 to 40 eV, with an energy step of 0.5 eV. We have found that the trace of the N2O2+ metastable dications is not present up to 38.5 eV, but it appears from 39.0 eV, in very good agreement with the threshold value of 38.5 eV given by Taylor et al. and confirming that this metastable dication is formed in a 3P state [13]. In order to study the threshold and the energy depenþ dence of the NO+ + N+ and Nþ dissociative chan2 þO nels, we have integrated the density of the two corresponding ion coincidence spots as a function of the photon energy, with a more dense energy grid. The results are plotted in Fig. 5. The curves in panels a and b indicate
M. Alagia et al. / Chemical Physics Letters 432 (2006) 398–402
that these two processes start to occur also below the expected threshold energy for the formation of the molecular dication N2O2+ (35.8 eV, following Ref. [13]) and this þ should imply that both NO+ + N+ and Nþ 2 þ O products are formed, below this threshold, through the involvement of some autoionizing states. Such a behavior, with similar characteristics, has been already observed in other cases [22]. Looking at the branching ratio plotted in panel c of Fig. 5, it is possible to see that the NO+ + N+ channel is always the most important one and the slope of the ratio changes around the threshold for the formation of the ground state N2O2+ dication. Following the paper by Taylor et al. [13], in the photon energy range investigated in the present experiment, the N2O2+ dication states that can be populated are X3R, 11D, 11R+, 13P and 11P. By comparing the relevant electronic configurations [13] with that of the neutral ground state molecule [2], it is possible to realize that all these states are formed by removing a couple of electrons from the outer shell p or r orbital of N2O: for the first three states the two electrons come both from a p-orbital, while for the remaining two states they come one from the p- and one from r-orbital. The photoelectron–photoelectron-
branching ratio
6
+
c
4
2
0 100
b
80
intensity (arb. units)
60
N2O + hν
40
+
+
N + NO
20 0 100
a 80 60
N2O + hν
+
+
N2 + O
40 20 0 28
30
32
34
36
38
coincidence spectrum reported by Taylor et al. [13] shows that all the dication states mentioned above are really produced by double photoionization in the energy range that we have here investigated. However, no stable N2O2+ ions have been observed in our experiment. This means that the lifetime of the produced molecular dications is never longer than 106 s, which is the typical average time-of-flight of ions in our detection device. An interesting aspect is the fact that the ground state N2O2+(3R) dication, once formed, dissociates in a rather þ short time in the two NO+ + N+ and Nþ 2 þ O channels, in a ratio of about 4:1. This is consistent with the assumption that the potential energy surface describing the molecular dication is populated by the vertical transition in a region from where the system can evolve towards the two exit channels with such a branching ratio. However, when the same potential energy surface is populated from above, that is from the radiating metastable 3P state, the system, before decaying, lives long enough to rearrange the bonds making possible only the NO+ + N+ exit channel, as we have observed. 4. Conclusions The double photoionization of N2O molecules in the 28– 40 eV photon energy range has been studied by the use of the synchrotron VUV light. The ions have been detected as a function of the photon energy, in coincidence with one of the two photoelectrons and recording the delay times. The analysis of the correlation time diagrams shows that, for energies below the vertical threshold for the N2O2+ dication formation (35.8 eV), autoionizing states are formed, followed by dissociative autoionization finally þ leading to N+ + NO+ and Nþ 2 þ O products. The dication formed above the threshold also dissociates through the same channels. However above 38.5 eV a metastable dication is formed, that decays radiatively to the ground state and then dissociates leading only to N+ + NO+ final products. A further investigation could provide additional information about the dynamics of this process by the use of the analysis of the spatial momentum components of the ionic products of the dissociation. The position sensitive detector that we have used has been designed also for such a measurement and therefore can be used for such an ‘ion-imaging’ detection. This experiment is presently in an advanced stage and we expect to obtain, in the near future, the angular and translational energy distribution of ion products with respect to the light polarization vector.
+
(N + NO ) + + (N2 + O )
401
40
photon energy (eV) + Nþ 2 /O
Fig. 5. In the panels a and b the photon energy dependence of the and NO+/N+ coincidences, from 28 to 40 eV are reported, while in the + panel c the NO+/N+ to Nþ 2 /O branching ratio is plotted.
Acknowledgements Financial contributions from the MUR (Ministero dell’Universita` e della Ricerca) is gratefully acknowledged. FV and FP gratefully acknowledge a travel support by the ‘Sincrotrone Trieste S.C.p.A.’.
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M. Alagia et al. / Chemical Physics Letters 432 (2006) 398–402
References [1] F. Biondini et al., J. Chem. Phys. 122 (2005) 164307. [2] F. Biondini et al., J. Chem. Phys. 122 (2005) 164308. [3] S.D. Price, J.H.D. Eland, P.G. Fournier, J. Fournier, P. Millie, J. Chem. Phys. 88 (1988) 1511. [4] J. Appell, J. Durup, F.C. Fehsenfeld, P. Fournier, J. Phys. B 7 (1974) 406. [5] F.M. Harris, C.J. Reid, J.A. Ballantine, D.E. Parry, J. Chem. Soc., Faraday Trans. 87 (1991) 1681. [6] D.M. Curtis, J.H.D. Eland, Int. J. Mass Spectrom. Ion Process. 63 (1985) 241. [7] P. Bolognesi, M. Coreno, L. Avaldi, L. Storchi, F. Tarantelli, J. Chem. Phys. 125 (2006) 054306. [8] J.A. Connor, I.H. Hillier, J. Kendrick, M. Barber, A. Barrie, J. Chem. Phys. 64 (1976) 3325. [9] J.H.D. Eland, Chem. Phys. 294 (2003) 171. [10] R.G. Cooks, D.T. Terwilliger, J.H. Beynon, J. Chem. Phys. 61 (1974) 1208.
[11] F.P. Larkins, J. Chem. Phys. 86 (1987) 3239. [12] N. Levasseur, P. Millie´, J. Chem. Phys. 92 (1990) 2974. [13] S. Taylor, J.H.D. Eland, M. Hochlaf, J. Chem. Phys. 124 (2006) 204319. [14] M. Alagia et al., J. Chem. Phys. 117 (2002) 1098. [15] M. Moix-Teixidor, F. Pirani, P. Candori, S. Falcinelli, F. Vecchiocattivi, Chem. Phys. Lett. 379 (2003) 139. [16] M. Alagia et al., J. Chem. Phys. 120 (2004) 6980. [17] M. Alagia et al., J. Chem. Phys. 120 (2004) 6985. [18] M. Alagia et al., J. Chem. Phys. 121 (2004) 10508. [19] M. Alagia et al., J. Chem. Phys. 124 (2006) 204318. [20] R.R. Blyth et al., J. Electron Spectr. Relat. Phenom. 101–103 (1999) 959. [21] M. Lavolle´e, Rev. Sci. Instrum. 70 (1999) 2968. [22] P. Bolognesi, D.B. Thompson, L. Avaldi, M.A. MacDonald, M.C.A. Lopes, D.R. Cooper, G.C. King, Phys. Rev. Lett. 82 (1999) 2075.
Double photoionization of N2O molecules in the 28–40 eV energy range Michele Alagia a,b, Pietro Candori c, Stefano Falcinelli c, Michel Lavolle´e d, Fernando Pirani e, Robert Richter f, Stefano Stranges b,g, Franco Vecchiocattivi
c,*
a ISMN-CNR Sez. Roma1, P.le A. Moro 5, 00185, Rome, Italy Laboratorio TASC-INFM, Area Science Park, 34012 Basovizza, Trieste, Italy c Dipartimento di Ingegneria Civile ed Ambientale, Universita` di Perugia, 06125 Perugia, Italy CNRS, Universite´ Paris-Sud, LIXAM UMR8624, Baˆtiment 350, Orsay Cedex, F-91405, France e Dipartimento di Chimica Universita` di Perugia, 06125 Perugia, Italy f Sincrotrone Trieste, Area Science Park, 34012 Basovizza, Trieste, Italy g Dipartimento di Chimica, Universita` di Roma ‘La Sapienza’, 00185 Roma, Italy b
d
Received 9 October 2006; in final form 20 October 2006 Available online 27 October 2006
Abstract The double photoionization of N2O molecules, in the 28–40 eV energy range, has been studied by synchrotron radiation. In the whole þ energy range, dissociative ionization producing N+ + NO+ or Nþ 2 þ O has been observed. Below 35.5 eV, these two processes involve the formation of some autoionizing states. In the range between 35.5 and 38.5 eV the two processes occur instead by a direct coulomb explosion of the N2O2+ 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. Ó 2006 Elsevier B.V. All rights reserved.
1. Introduction The nitrous oxide molecule, N2O, is a naturally occurring molecular species in the atmosphere of our planet. Although it represents a very small fraction of the total atmospheric gases, its presence is important because this molecule is two orders of magnitude more efficient as a green house gas than, for example carbon dioxide, CO2. This, together with a steady increase of its emissions by about 0.3% each year, make N2O an important gaseous species to be considered in atmospheric studies. An important molecular property for green house modeling is the lifetime of a species in the atmosphere, before it disappears by dissociation or reaction with other molecular species. Nitrous oxide absorbs ultraviolet light from the sun, and re-radiates this energy under various wavelengths, from the infrared to the visible. If the energy of the photons is *
Corresponding author. E-mail address: [email protected] (F. Vecchiocattivi).
0009-2614/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.10.100
sufficiently high, the molecule can be dissociated into fragments, such as NO and N, or stripped of one or even two of its electrons. Understanding the various fragmentation and ionization paths of the molecule are, therefore, of great importance for atmospheric and chemistry studies. Two recent papers from our laboratory [1,2] have reported experimental and theoretical studies about the single ionization of nitrous oxide by collisions with excited rare gas atoms (Penning ionization) and the results have been also compared with those by ultraviolet photon absorption. The ionization dynamics has been discussed in terms of structure and geometry of the atomic and molecular orbitals involved in the processes. The double photoionization of N2O has been already studied in other laboratories. An early study [3] has shown that the nitrous oxide dication is almost completely dissociated, within 108 s of its formation, mainly through þ the NO+ + N+ and Nþ dissociative channels. The 2 þO 2+ energetics of the N2O dication, its production and dissociation has been experimentally studied by double charge
M. Alagia et al. / Chemical Physics Letters 432 (2006) 398–402
transfer [3–5], by photoion–photoion coincidence [3,6], by Auger spectroscopy [7,8], by time-of-flight photoelectron– photoelectron coincidence [9] and by ion kinetic energy spectrometry [10]. Some theoretical studies [3,7,8,11,12] also contributed to the characterization of this molecular dication. In a recent paper, Taylor et al. [13] have characterized the formation of a metastable state of N2O2+ dication, that they identified, by the use also of some theoretical calculations, as the 3P state with life time of few hundred nanoseconds, which first decays by fluorescence to the ground X3R state, and then dissociates producing only NO+ + N+. 40 eV
N2O2+ 3Π 1Σ+ 1Δ
3Σ-
35
399
In recent years we have studied the double photoionization of HX (X = Cl, Br, and I) by synchrotron radiation and we have characterized the results by using some simple theoretical schemes [14–19]. With the present work we intend to extend such a study to the N2O triatomic molecule and we report here on the double ionization of N2O by vacuum ultraviolet photons of synchrotron radiation, in the 28–40 eV energy range. The energy levels of the double ionization states of N2O, involved in the presently investigated energy range, are shown in Fig. 1. The present results allow us to assess the threshold energies for the disþ sociative processes producing NO+ + N+ and Nþ 2 þO ions. It will be shown that below 35.5 eV, the two processes involve the formation of some autoionizing states. In the range between 35.5 and 38.5 eV the two processes occur instead by a direct coulomb explosion of the N2O2+ dication. However, above 38.5 eV the dissociation leading to NO+ + N+ is also promoted by the formation of a metastable state of the molecular dication which is expected to decay by fluorescence to the ground state and then dissociates [13]. 2. Experimental
N2+(2Σg+)+ O +(4S) 30 NO+(1Σ+)+ N +(3P)
Fig. 1. The energy levels relevant to the double ionization of N2O in the 28–40 eV range. The values are quoted with respect to the ground neutral state of the molecule. The molecular dication state energies are taken from Ref. [13].
The experiment has been performed at the ELETTRA Synchrotron Light Laboratory using the ARPES end station at the Gasphase Beamline. A detailed description of the beam line and the end station is given elsewhere [20]. Only the features relevant to the present experiment are schematically described here. In Fig. 2, a sketch of the experimental set up is shown. The energy selected synchrotron light beam crosses an effusive molecular beam of N2O and the product ions are then
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. 2. A schematic sketch of the experimental set up.
400
M. Alagia et al. / Chemical Physics Letters 432 (2006) 398–402
detected in coincidence with photoelectrons. The monochromator uses a 400 l/mm spherical grating in first diffraction order. The entrance and exit slits of the monochromator have been adjusted in order to have a resolution between 2.0 and 1.5 meV in the investigated 28– 40 eV photon energy range. The resolution and the energy scale calibration has been checked by measuring some known sharp resonances in the total ion yield of argon in this energy range. To avoid spurious ionization effects, due to photons of higher order energy, a magnesium film filter was used. The N2O molecular beam and the VUV light beam crosses at right angles, with the light polarization vector being parallel to the synchrotron ring plane and perpendicular to the time-of-flight direction of detected ions. The ion detection system has been built up according to the model described in Ref. [21]. Such a position sensitive detector is particularly designed in order to measure the spatial momentum components of the dissociation ionic products. However, for the experiment described here, the x–y dependence has been not used and therefore only total ion arrival times to the micro channel plate detector have been considered. Practically, mass spectra were recorded using photoelectrons as starting pulses, and then ions were counted as a function of the delay times. A computer controlled all the experiment components and also recorded experimental data. The incident photon flux and gas pressure have been monitored and stored in separate acquisition channels. Ion yields have been then corrected for pressure and photon flux changes while varying the photon energy. The nitrous oxide is supplied to the needle beam source from a commercial bottle filled with about 10 atm at room temperature. The gas has a 99.99% nominal purity and has been used without any further treatment. An adjustable leak along the input gas pipe line is used in order to control the gas flow, which is monitored by checking the background pressure in the main vacuum chamber.
Fig. 3. Mass spectrum and ion–ion time of flight correlation of ions produced by single and double photoionization of N2O at 39 eV. In this type of plot, which is typical of double photoionization experiments, the two time of flight values of a couple of ions produced in the same photoionization event define a point (see for instance Ref. [13]). The diagonal weak traces are false and spurious coincidences that have been neglected in the present analysis.
3. Results and discussion
Fig. 4. Extended plot of the portion of the ion–ion time of flight correlation diagram of Fig. 3 where is possible to distinguish the N+/NO+ + and Nþ coincidence spots and the trace for the metastable N2O2+ 2 /O dication formed at 39 eV photon energy.
The mass spectra detected in the 28–40 eV photon energy range indicate that single and double photo ionization of N2O, in such an energy range, produce mainly N+, + + O +, N þ 2 , NO and N2O ions. A mass spectrum, together with an ion–ion time of flight correlation diagram, are reported in Fig. 3, for a photon energy of 39.0 eV. All product ions are evident together with some background peaks. A blow up of the most relevant part of such a diagram is reported in Fig. 4, where it is possible to distinguish the ionic products of the double photoionization detected þ in coincidence, which are NO+ + N+ and Nþ 2 þ O . The time correlation diagram also shows some diagonal weak traces of false and spurious coincidences, which have been simply neglected in the present analysis. A typical trace due to the formation of a metastable N2O2+ dication is also evident in the diagram of Fig. 4
and such a metastable dication leads to the dissociation in NO+ + N+ final products. We have measured spectra and ion–ion time of flight correlation diagrams, like that one in Figs. 3 and 4, starting from 28 to 40 eV, with an energy step of 0.5 eV. We have found that the trace of the N2O2+ metastable dications is not present up to 38.5 eV, but it appears from 39.0 eV, in very good agreement with the threshold value of 38.5 eV given by Taylor et al. and confirming that this metastable dication is formed in a 3P state [13]. In order to study the threshold and the energy depenþ dence of the NO+ + N+ and Nþ dissociative chan2 þO nels, we have integrated the density of the two corresponding ion coincidence spots as a function of the photon energy, with a more dense energy grid. The results are plotted in Fig. 5. The curves in panels a and b indicate
M. Alagia et al. / Chemical Physics Letters 432 (2006) 398–402
that these two processes start to occur also below the expected threshold energy for the formation of the molecular dication N2O2+ (35.8 eV, following Ref. [13]) and this þ should imply that both NO+ + N+ and Nþ 2 þ O products are formed, below this threshold, through the involvement of some autoionizing states. Such a behavior, with similar characteristics, has been already observed in other cases [22]. Looking at the branching ratio plotted in panel c of Fig. 5, it is possible to see that the NO+ + N+ channel is always the most important one and the slope of the ratio changes around the threshold for the formation of the ground state N2O2+ dication. Following the paper by Taylor et al. [13], in the photon energy range investigated in the present experiment, the N2O2+ dication states that can be populated are X3R, 11D, 11R+, 13P and 11P. By comparing the relevant electronic configurations [13] with that of the neutral ground state molecule [2], it is possible to realize that all these states are formed by removing a couple of electrons from the outer shell p or r orbital of N2O: for the first three states the two electrons come both from a p-orbital, while for the remaining two states they come one from the p- and one from r-orbital. The photoelectron–photoelectron-
branching ratio
6
+
c
4
2
0 100
b
80
intensity (arb. units)
60
N2O + hν
40
+
+
N + NO
20 0 100
a 80 60
N2O + hν
+
+
N2 + O
40 20 0 28
30
32
34
36
38
coincidence spectrum reported by Taylor et al. [13] shows that all the dication states mentioned above are really produced by double photoionization in the energy range that we have here investigated. However, no stable N2O2+ ions have been observed in our experiment. This means that the lifetime of the produced molecular dications is never longer than 106 s, which is the typical average time-of-flight of ions in our detection device. An interesting aspect is the fact that the ground state N2O2+(3R) dication, once formed, dissociates in a rather þ short time in the two NO+ + N+ and Nþ 2 þ O channels, in a ratio of about 4:1. This is consistent with the assumption that the potential energy surface describing the molecular dication is populated by the vertical transition in a region from where the system can evolve towards the two exit channels with such a branching ratio. However, when the same potential energy surface is populated from above, that is from the radiating metastable 3P state, the system, before decaying, lives long enough to rearrange the bonds making possible only the NO+ + N+ exit channel, as we have observed. 4. Conclusions The double photoionization of N2O molecules in the 28– 40 eV photon energy range has been studied by the use of the synchrotron VUV light. The ions have been detected as a function of the photon energy, in coincidence with one of the two photoelectrons and recording the delay times. The analysis of the correlation time diagrams shows that, for energies below the vertical threshold for the N2O2+ dication formation (35.8 eV), autoionizing states are formed, followed by dissociative autoionization finally þ leading to N+ + NO+ and Nþ 2 þ O products. The dication formed above the threshold also dissociates through the same channels. However above 38.5 eV a metastable dication is formed, that decays radiatively to the ground state and then dissociates leading only to N+ + NO+ final products. A further investigation could provide additional information about the dynamics of this process by the use of the analysis of the spatial momentum components of the ionic products of the dissociation. The position sensitive detector that we have used has been designed also for such a measurement and therefore can be used for such an ‘ion-imaging’ detection. This experiment is presently in an advanced stage and we expect to obtain, in the near future, the angular and translational energy distribution of ion products with respect to the light polarization vector.
+
(N + NO ) + + (N2 + O )
401
40
photon energy (eV) + Nþ 2 /O
Fig. 5. In the panels a and b the photon energy dependence of the and NO+/N+ coincidences, from 28 to 40 eV are reported, while in the + panel c the NO+/N+ to Nþ 2 /O branching ratio is plotted.
Acknowledgements Financial contributions from the MUR (Ministero dell’Universita` e della Ricerca) is gratefully acknowledged. FV and FP gratefully acknowledge a travel support by the ‘Sincrotrone Trieste S.C.p.A.’.
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