Angular and energy distribution of fragment ions in dissociative double photoionization of acetylene molecules at 39 eV M. Alagia, C. Callegari, P. Candori, S. Falcinelli, F. Pirani et al. Citation: J. Chem. Phys. 136, 204302 (2012); doi: 10.1063/1.4720350 View online: http://dx.doi.org/10.1063/1.4720350 View Table of Contents: http://jcp.aip.org/resource/1/JCPSA6/v136/i20 Published by the American Institute of Physics.
Additional information on J. Chem. Phys. Journal Homepage: http://jcp.aip.org/ Journal Information: http://jcp.aip.org/about/about_the_journal Top downloads: http://jcp.aip.org/features/most_downloaded Information for Authors: http://jcp.aip.org/authors
THE JOURNAL OF CHEMICAL PHYSICS 136, 204302 (2012)
Angular and energy distribution of fragment ions in dissociative double photoionization of acetylene molecules at 39 eV M. Alagia,1 C. Callegari,2 P. Candori,3 S. Falcinelli,3,a) F. Pirani,4 R. Richter,2 S. Stranges,1,5 and F. Vecchiocattivi3 1
IOM CNR Laboratorio TASC, I-34012 Trieste, Italy Sincrotrone Trieste, Area Science Park, 34149 Basovizza, Trieste, Italy 3 Dipartimento di Ingegneria Civile ed Ambientale, 06125 Perugia, Italy 4 Dipartimento di Chimica dell’Università di Perugia, 06123 Perugia, Italy 5 Dipartimento di Chimica, Università di Roma “La Sapienza”, 00185 Roma, Italy 2
(Received 1 March 2012; accepted 8 May 2012; published online 22 May 2012) The two-body dissociation reactions of the dication, C2 H2+ 2 , produced by 39.0 eV double photoionization of acetylene molecules, have been studied by coupling photoelectron-photoion-photoion coincidence and ion imaging techniques. The results provide the kinetic energy and angular distributions of product ions. The analysis of the results indicates that the dissociation leading to C2 H+ +H+ products occurs through a metastable dication with a lifetime of 108 ± 22 ns, and a kinetic energy release (KER) distribution exhibiting a maximum at ∼4.3 eV with a full width + at half maximum (FWHM) of about 60%. The reaction leading to CH+ 2 + C occurs in a time shorter than the typical rotational period of the acetylene molecules (of the order of 10−12 s). The KER distribution of product ions for this reaction, exhibits a maximum at ∼4.5 eV with a FWHM of about 28%. The symmetric dissociation, leading to CH+ + CH+ , exhibits a KER distribution with a maximum at ∼5.2 eV with a FWHM of 44%. For the first two reactions the angular distributions of ion products also indicate that the double photoionization of acetylene occurs when the neutral molecule is mainly oriented perpendicularly to the light polarization vector. © 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4720350] INTRODUCTION
The ionization of acetylene molecules is an important process for many different fields, for instance, plasma devices,1 flames,2 planetary atmospheric chemistry,3 and semiconductor industry.4 Among the possible ions originating from the ionization of acetylene, the C2 H2+ 2 dication received, in recent years, much attention and different techniques have been used in order to obtain dynamical information about its formation and its dynamical evolution. The double photoionization of acetylene has been studied by Thissen et al.5 in 1993, by photoelectron-ion-ion coincidence technique using both synchrotron radiation and some laboratory light sources. The study reported the threshold and the photon energy dependence of three two-body dissociation reactions of C2 H2+ 2 dication: + + C2 H2+ 2 → H + C2 H ,
(1)
+ + C2 H2+ 2 → C + CH2 ,
(2)
+ + C2 H2+ 2 → CH + CH ,
(3)
and also of other three-body and four-body dissociation reactions. Reaction (1) has been found by these authors to be the a) Author to whom correspondence should be addressed. Electronic mail:
[email protected]. Tel.: +39 075 5853856. Fax: +39 075 5853864.
0021-9606/2012/136(20)/204302/6/$30.00
most probable one in the whole energy range that they have investigated (35–65 eV). Moreover, its threshold appeared to be near 34 eV and it was evident that such a reaction occurs also through the formation of a metastable dication, with a lifetime of about 80 ns.5 By theoretical calculations these authors have also shown that reaction (2) occurs through the formation of the vinylidene dication, H2 CC2+ , intermediate.5 This last reaction has been also studied by the use of other experimental methods,6–11 since it represents a very interesting example of an elementary and fast isomerization. In the present paper we report an investigation of the two body dissociative reactions of C2 H2+ 2 dication formed in the double photoionization of acetylene by linearly polarized 39.0 eV photons, detecting angular and energy distributions of final ion products with respect the light polarization vector. This technique has been already successfully applied by our group to the dissociation reactions of N2 O2+ and CO2+ 2 dications.12–14 However, in the present case, since the double ionization cross section appears to be rather small,5 the ion signal is weak and therefore we have studied the reactions (1)–(3) at 39.0 eV only, that is at an energy where all the three channels are open. Nevertheless, the present results provide rather interesting information that clarifies some important aspects about the dynamics of these reactions. The photoelectron-photoion-photoion coincidence results that we report here allow an accurate determination of the lifetime of the metastable component of reaction (1) and of the kinetic energy released into the ion products in the three reactions. Moreover, from the analysis of the angular distribution
136, 204302-1
© 2012 American Institute of Physics
204302-2
Alagia et al.
J. Chem. Phys. 136, 204302 (2012)
of product ions, it is possible to draw some interesting conclusions about the dynamics of the double photoionization of acetylene.
EXPERIMENTAL PROCEDURE
The experiment has been carried out at the synchrotron light laboratory ELETTRA (Trieste, Italy) by the use of the ARPES end station of the Gasphase Beamline. Details about the beamline and the end station have been already reported elsewhere,15 while the apparatus used for the present experiment has also been described previously.12–14 Therefore, only some features relevant for the present investigation are sketched here. The monochromatic synchrotron light beam crosses an effusive molecular beam of C2 H2 and the product ions are then detected in coincidence with photoelectrons. The molecular acetylene beam, the light direction, and the ion and electron detection axes are mutually perpendicular. The monochromator uses a 400 L/mm spherical grating in first order diffraction. The entrance and exit slits of the monochromator have been adjusted in order to give a photon energy resolution of about 1.5 meV. In order to avoid spurious effects, due to ionization by photons of higher energy, a magnesium film filter was placed in the synchrotron radiation beam. The light beam is linearly polarized15 and the direction of the polarization vector is aligned parallel to the molecular beam axis. The ion extraction and the detection system have been assembled following the design described by Lavollèe.16 Such a time-of-flight spectrometer, with an ion position sensitive detector, is especially designed to measure the spatial momentum vectors of the ionic dissociation products. The electron detector, located just below the interaction volume, consists of a stack of three micro-channel-plates followed by a copper anode. The ion detector also consists of a stack of three microchannel plates located at the end of the drift tube. However, the ion signals are read by an array of anodes arranged in 32 rows and 32 columns. Such an arrangement allows the detection of the arrival position of each ion on the detector plate. In the experiment the photoelectron signals are used as start pulses, and then ions are counted as a function of their arrival time and their position on the detector. The delay times are measured by an array of 32 + 32 time-to-digital converters connected with the multi-anode. Therefore, the obtained images are resolved in time, allowing an easy transformation to angle and energy distributions in the center of mass frame. All experiment components are controlled by a computer that also records experimental data. The incident photon flux and the gas pressure have been monitored and stored in separate acquisition channels. Acetylene, from a commercial cylinder at room temperature, was supplied to a needle effusive beam source. The gas had a ∼99.0% nominal purity and was been used after a ∼193 K cold trap purification, that essentially removes acetone impurities. The performance of the cold trap has been verified by recording mass spectra. An adjustable leak valve along the input gas pipe line was used in order to control the
FIG. 1. The photoion-photoion coincidence plot. Coincidence events are represented by points, as a function of the arrival time to the detector, t1 , for the first ion and t2 , for the second one. In the upper part the rough data are reported, while in the lower part the same data are reported with the a contour line around the coincidence spots for the three dissociation processes and around the typical tail indicating a metastable precursor is evident.
gas flow, which was monitored by checking the pressure in the main vacuum chamber.
RESULTS AND DATA ANALYSIS
The coincidence plot obtained for 39.0 eV photons is reported in Fig. 1. The points in the figure represent the coincidence events as a function of the arrival time of the first ion, t1 , and of the second ion, t2 . In the lower part of Fig. 1, the typical spot for H+ /C2 H+ coincidences can be easily recognized, together with a tail that characterizes the presence of a dication metastable state dissociating in H+ and C2 H+ . Also in the figure are evident C+ /CH2 + coincidences, together with the region that contains, all overlapping CH+ /CH+ coincidences, + C2 H2+ 2 dications and also CH ions produced by the single ionization of acetylene which contribute to the same mass to charge ratio. We have analyzed the plot in order to obtain the lifetime of the metastable dication during the H+ + C2 H+ dissociation.17, 18 To do that we have plotted the number of coincidence points along the track of the tail, as a function of the (t2 −t1 ) difference, and have fitted the data by a Monte Carlo simulation of the ion trajectories in the mass spectrometer.18 In the upper panel of Fig. 2 the experimental data are compared with the best fit simulation, while in the lower panel
204302-3
Alagia et al.
FIG. 2. Analysis of the “tail” in the coincidence plot of FIG. 1, in order to derive the lifetime of the dissociation reaction leading to H+ +C2 H+ . The upper panel shows the intensity of coincidences along the tail track, as a function of the t2 −t1 difference (open circles), with the fit (continuous line) by a Monte Carlo trajectory calculation. The lower panel shows the mean square deviation, χ 2 , between the simulation and the experimental results, as a function of the lifetime used in the simulation.
of the same figure the mean square values, χ 2 , of the difference between the experimental data and the simulation, are plotted as a function of the dication lifetime: the minimum deviation is obtained for a lifetime of 108 ± 22 ns. The dissociation of the C2 H2+ 2 dications, leading to the formation of C+ + CH2 + product ions, does not show a tail in the coincidence plot, indicating that the reaction occurs in a time shorter than the characteristic time of our apparatus, that is ∼50 ns for the conditions of the present experiment.14 No lifetime can be extracted for the dissociation leading to CH+ + CH+ because the limitation of the experimental method when applied to product ions with same mass to charge ratio due to the overlap with many events given by the arrival of stable + C2 H2+ 2 dications and also CH ions produced by single ionization. However, the ion images of product ions in the three dissociation reactions have been analyzed providing the total kinetic energy distributions, as reported in Fig. 3. The ion image analysis provides also the angular distribution of product ions with respect the polarization vector direction. It is well known19, 20 that such angular distributions can provide valuable information about the dissociation dynamics
J. Chem. Phys. 136, 204302 (2012)
FIG. 3. Kinetic energy distribution of product ions for the three dissociation reactions.
and are usually represented by I(θ )sinθ β σtot 2 1 + (3cos θ − 1) , I (θ )sin(θ ) = 4π 2
(4)
where I(θ ) and σ tot are the differential and the total cross section of the process, respectively, while θ is the angle between the velocity vector of the fragment ion and the light polarization vector. The β parameter, also called anisotropy parameter, ranges from −1, for the emission of product ions along a direction perpendicular to the polarization vector, up to a value of 2 for a parallel direction. An isotropic distribution of fragment ions is characterized by a value β = 0. The center of mass distributions for the H+ /C2 H+ and C+ /CH2 + ion products are reported in Figs. 4 and 5, respectively, where they are also analyzed in terms of the β parameter. It has been found that for H+ /C2 H+ the best fit is provided by β = −0.19, while for C+ /CH2 + ion products by β = −0.47. DISCUSSION
The first three low lying states of the C2 H2+ 2 dication can be easily rationalized considering the removal of two electrons from the HOMO π orbitals of acetylene molecule in the neutral ground state: when one removes two unpaired electrons from each one of the two π bonding orbitals, one
204302-4
Alagia et al.
FIG. 4. Angular distribution, I(θ ), of C2 H+ + H+ product ions in the center of mass frame. In the upper panel, the experimental distribution is compared with limiting situations (β = −1, 0, 2), while, in the lower panel, is compared with the best fit β = −0.19 value.
obtains the X3 g − state of the dication; on the other hand, when two electrons are removed from the same π bonding orbital, the a1 g state is obtained; finally removing two paired electrons, each one from one of the two π bonding orbitals, the b1 g + state is formed.5 This sequence of states has been seen in the C2 H2+ 2 dication spectrum measured and discussed by Kinugawa et al.21 In a comparison between the experimental thresholds and theoretical calculations it has been shown that reaction (1) is the most probable from the X3 g − ground state,22–25 while reaction (3) takes place through a transition state with a trans bent C2h structure.22–25 Reaction (2), that involves as a first step the isomerization to the vinylidene dication, H2 CC2+ , is considered to proceed on both the triplet and singlet state surfaces.22–25 The kinetic energy release (KER) into product ions for each dissociation reaction, together with the threshold energy and the asymptotic energy of these products, is rather important in order to assess the energetics of the reaction. The KER distributions extracted from our measurement for the three processes are reported in Fig. 3 and it should be noted that the maxima of the three distributions are in reasonable agreement with the value provided by Thissen et al.,5 obtained by photo electron-ion-ion coincidence spectroscopy. It is interesting to note that for the process (1), which proceeds also through metastable states, we find a wider KER distribution,
J. Chem. Phys. 136, 204302 (2012)
FIG. 5. Angular distribution, I(θ ), of CH2 + + C+ product ions in the center of mass frame. In the upper panel, the experimental distribution is compared with limiting situations (β = −1, 0, 2), while, in the lower panel, is compared with the best fit β = −0.47 value.
when compared with that of the other two processes. Probably the longer lifetime of the dication before dissociation allows the energy to redistribute. Also the lifetime of 108 ± 22 ns for process (1) is in reasonable agreement with the value of 80 ns reported by Thissen et al.,5 while in a more recent experiment, by electron impact, Feil et al.25 found, for the same reaction, two lifetime components, one of 0.3 μs and another one of 7.3 μs. We did not observe any evidence, in our experimental conditions, for the long lifetime components. However, the observed lifetime has to be considered as a weighted average related to the several possible metastable states, since our experiment probes only the time window accessible with our set up (from ∼50 ns to few μs). In order to better understand the information content of the angular distribution of product ions that we observed for reactions (1) and (2), it is necessary to stress that this angular distribution derives from the combination of three main factors: the stereo-specificity of the photoionization, the lifetime of the dissociation, and the rotational state of the molecule. The molecules cross the light beam with their velocity vector mainly parallel to the polarization vector and, when they
204302-5
Alagia et al.
interact with the light, they rotate with their rotational axis (or the rotational angular momentum) randomly oriented. Therefore, the usual formula that correlates the β parameter with the σ , where σ || and σ ⊥ are respectively the σ || /σ ⊥ ratio, σ⊥|| = 1+β 2−β parallel and the perpendicular cross section for the process,14 has to be considered with some cautions, the observed β parameter being the result of the convolution over some effects contributing together. In other words, what is measured in this kind of experiment is the outgoing direction of product ions and this is obviously related to the σ || /σ ⊥ . However, the rotational period of the neutral molecule and of its dication, and the lifetime of the dication before dissociation, have both the tendency to redistribute the products in a wider cone. In the limiting case of a sufficiently long lifetime, the angular distribution of products becomes isotropic, even though the ionization event is anisotropic. Looking at the distribution in Fig. 4, which refers to reaction (1), we note that the experimental data are quite close to the behavior expected for an isotropic (β = 0) distribution, although they are all systematically lower than the β = 0 distribution. This indicates that the lifetime of the metastable state in this reaction is long enough, in comparison with the rotational period, to give an isotropic distribution, but there must be an additional faster component producing a negative value of the effective β. Such effect is larger in the case of the distribution of Fig. 5, related to reaction (2). Actually it is well known that this reaction is very fast,6–11 and occurs in a time shorter than the usual rotation period of the precursor dication, which is of the order of picoseconds. The results indicate that rotation and lifetime have the tendency to smooth the angular distribution and therefore the values of β given here have to be considered, being negative, as an upper limit to the intrinsic β value. However, it appears clear that the photoionization has the propensity to occur with the molecular axis oriented perpendicular to the light polarization vector. We recall that, in the case of acetylene molecules, the molecular axis is also the cylindrical symmetry axis of the electron density distribution. Since the molecules are rotating, we can also assert that the ionization mainly occurs when their rotational angular momentum vector is parallel to the light polarization vector, while, when the rotational angular momentum vector has a different orientation, the ionization probability is lower. It is interesting to note that for the other two systems that we have studied12–14 so far with this technique, CO2 and N2 O, also linear molecules, the ionization occurs with the molecular axis oriented mainly parallel to the light polarization vector. CONCLUSIONS
The reactions (1)–(3) have been studied by combining photoelectron-photoion-photoion coincidence and ionimaging techniques, at a photon energy of 39.0 eV, obtaining different results. For the first reaction, the KER of the C2 H+ +H+ product ions has been recorded showing a rather broad distribution with the maximum at about 4.3 eV and a full width at half maximum (FWHM) of about 60%. Moreover, it has been observed that this reaction proceeds via a
J. Chem. Phys. 136, 204302 (2012)
metastable dication formation with a lifetime of 108 ± 22 ns. The angular distribution of products, with respect the polarization direction of the light, has been found to be slightly narrower than the one expected for an isotropic distribution, indicating the presence of a fast component together with the metastable state dissociation. The ionization has a higher probability when the neutral molecule is oriented perpendicularly to the polarization direction. For the reaction (2), the one occurring through the formation of a vinylidene dication, H2 CC2+ , a lifetime has been observed, which is shorter than the typical rotational period of the acetylene molecule (of the order of ∼10−12 s). Moreover it has been observed that the H2 C+ + C+ dissociation products are collected in a direction mainly perpendicular to the light polarization direction. In this reaction the KER distribution has been found narrower when compared with the others, with a maximum at about 4.5 and a FWHM of about 28%. The reaction (3) has also been observed, but it was not possible to analyze the data in detail because of the limitation of the experimental method, as already discussed above. Nevertheless, it has been found that the KER distribution exhibits a maximum at about 5.2 eV, with a FWHM of about 44%. Unfortunately, because of the low ionization cross section5 and the limited time of access to the ELETTRA synchrotron beam line, only the photon energy of 39.0 eV has been investigated in detail. However, we are rather confident that a systematic investigation in a broader range of energies can be very productive to obtain detailed information on the dynamics of the double photoionization processes in acetylene, just above the threshold. Such results could allow us to better understand the dissociation dynamics. To do that we can also take into account the several results about two step formation of the dication,6, 9 as it occurs, for instance, in Auger spectroscopy.7, 8, 10 Actually, in those cases the involved states are the same only in the exit channel and the microscopic reaction path is expected, in principle, to be different from that occurring in the present single step double photoionization. ACKNOWLEDGMENTS
Financial contributions from MIUR (Ministero dell’Istruzione, dell’Università e della Ricerca) are gratefully acknowledged. We acknowledge partial travel support by “Sincrotrone Trieste S.C.P.A.” We wish to thank the “Fondazione Cassa di Risparmio di Perugia” for financial support. The authors are also grateful to the IOM-CNR TASC Laboratory for financial support at the Gasphase beamline. P.C. gratefully acknowledges financial support by Regione Umbria, project POR UMBRIA FSE 2007-2013 Asse II “Occupabilità”, Obiettivo specifico “e”- Asse IV “Capitale Umano”, Obiettivo specifico “I”. 1 R.
K. Janev, Atomic and Molecular Processes in Fusion Edge Plasmas (Plenum, New York, 1995). 2 I. A. Larionova, B. S. Fialkov, K. Y. Kalinich, A. B. Fialkov, and B. S. Ospanov, Combust., Explos., Shock Waves 29, 341 (1993). 3 S. Petrie and D. K. Bohme, in Modern Mass Spectrometry (SpringerVerlag, Berlin, 2003), Vol. 225, p. 37. 4 F. J. Gordillo-Vazquez and J. M. Albella, Plasma Sources Sci. Technol. 11, 498 (2002).
204302-6 5 R.
Alagia et al.
Thissen, J. Delwiche, J. M. Robbe, D. Duflot, J. P. Flament, and J. H. D. Eland, J. Chem. Phys. 99, 6590 (1993). 6 A. Hishikawa, A. Matsuda, M. Fushitani and E. J. Takahashi, Phys. Rev. Lett. 99, 258302 (2007). 7 T. Osipov, T. N. Rescigno, T. Weber, S. Miyabe, T. Jahnke, A. S. Alnaser, M. P. Hertlein, O. Jagutzki, L. P. H. Schmidt, M. Schöffler, L. Foucar, S. Schössler, T. Havermeier, M. Odenweller, S. Voss, B. Feinberg, A. L. Landers, M. H. Prior, R. Dörner, C. L. Cocke, and A. Belkacem, J. Phys. B: At., Mol. Opt. Phys. 41, 091001 (2008). 8 R. Flammini, E. Fainelli, F. Maracci and L. Avaldi, Phys. Rev. A 77, 044701 (2008). 9 Y. H. Jiang, A. Rudenko, O. Herrwerth, L. Foucar, M. Kurka, K. U. Kühnel, M. Lezius, M. F. Kling, J. van Tilborg, A. Belkacem, K. Ueda, S. Düsterer, R. Treusch, C. D. Schröter, R. Moshammer, and J. Ullrich, Phys. Rev. Lett. 105, 263002 (2010). 10 J. Laksman, D. Céolin, M. Gisselbrecht, S. E. Canton, and S. L. Sorensen, J. Chem. Phys. 131, 244305 (2009). 11 R. Flammini, M. Satta, E. Fainelli, and L. Avaldi, Phys. Chem. Chem. Phys. 13, 19607 (2011); Phys. Rev. A 83, 014501 (2011). 12 M. Alagia, P. Candori, S. Falcinelli, M. Lavollée, F. Pirani, R. Richter, S. Stranges, and F. Vecchiocattivi, Chem. Phys. Lett. 432, 398 (2006). 13 M. Alagia, P. Candori, S. Falcinelli, M. Lavollée, F. Pirani, R. Richter, S. Stranges, and F. Vecchiocattivi, J. Chem. Phys. 126, 201101 (2007).
J. Chem. Phys. 136, 204302 (2012) 14 M.
Alagia, P. Candori, S. Falcinelli, M. Lavolleé, F. Pirani, R. Richter, S. Stranges, and F. Vecchiocattivi, Phys. Chem. Chem. Phys. 12, 5389 (2010). 15 R. R. Blyth, R. Delaunay, M. Zitnik, J. Krempasky, R. Krempaska, J. Slezak, K. C. Prince, R. Richter, M. Vondracek, R. Camilloni, L. Avaldi, M. Coreno, G. Stefani, C. Furlani, M. de Simone, S. Stranges, and M. Y. Adam, J. Electron Spectrosc. Relat. Phenom. 101, 959 (1999). 16 M. Lavollée, Rev. Sci. Instrum. 70, 2968 (1999). 17 M. Alagia, P. Candori, S. Falcinelli, K. C. Mundim, M. S. P. Mundim, F. Pirani, R. Richter, S. Stranges, and F. Vecchiocattivi, Chem. Phys. 398, 134 (2011). 18 M. Alagia, P. Candori, S. Falcinelli, M. S. P. Mundim, F. Pirani, R. Richter, M. Rosi, S. Stranges, and F. Vecchiocattivi, J. Chem. Phys. 135, 144304 (2011). 19 R. N. Zare, Mol. Photochem. 4, 1 (1972). 20 J. L. Dehmer and D. Dill, Phys. Rev. A 18, 164 (1978). 21 T. Kinugawa, P. Lablanquie, F. Penent, J. Paladoux, and J. H. D. Eland, J. Electron Spectrosc. Relat. Phenom. 141, 143 (2004). 22 D. Duflot, J.-M. Robbe, and J.-P. Flament, J. Chem. Phys. 102, 355 (1995). 23 X. Li and H. B. Schlegel, J. Phys. Chem. A 108, 468 (2004). 24 T. S. Zyubina, Y. A. Dyakov, S. H. Lin, A. D. Bandrauk, and A. M. Mebel, J. Chem. Phys. 123, 134320 (2005). 25 S. Feil, K. Gluch, A. Bacher, S. Matt-Leubner, D. K. Böhme, P. Scheier, and T. D. Märk, J. Chem. Phys. 124, 214307 (2006).
