Fragmentation of Valence and Core–Shell (Cl 2p) Excited C2Cl4

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Fragmentation of Valence and Core−Shell (Cl 2p) Excited C2Cl4 Molecule A. C. F. Santos,*,†,‡ M. A. MacDonald,‡ A. B. Rocha,§ N. Appathurai,‡ M. M. Sant’Anna,† W. Holetz,† R. Wehlitz,∥ and L. Zuin‡ †

Instituto de Física, Universidade Federal do Rio de Janeiro, 21941-972 Rio de Janeiro, RJ, Brazil Canadian Light Source, Inc., Saskatoon, SK S7N 2V3, Canada § Instituto de Química, Universidade Federal do Rio de Janeiro, 21941-909 Rio de Janeiro, RJ, Brazil ∥ Physics Department, University of WisconsinMadison, Madison, Wisconsin 53706, United States ‡

ABSTRACT: The dynamics of the photofragmentation pathways of tetrachloroethylene with photon energies from 15 up to 250 eV encompassing the Cl 2p edge is presented. In order to distinguish the fragmentation channels, the ionic fragments were separated according to their mass-to-charge ratio, measured in coincidence with the photoelectrons, and collected as a function of the incident photon energy. Distinct minima or maxima are found in the partial ion yield in the region between 40 and 50 eV. These features are believed to be associated with the Cooper minimum which results from a molecular orbital with a strong atomic 3p subshell character. In the shallow core region, some fragmentation patterns are considered in terms of fast fragmentation of the C2Cl4 molecule, despite the heavy mass of its fragments. In the present case, the fast fragmentation is favored by the very strong antibonding character of the LUMO, understandable in the frame of the core equivalent model for halogen-containing molecules. In addition, ab initio calculations were performed to obtain states at the Cl 2p edge. Singlet and triplet states at the Cl 2p edge of the C2Cl4 molecule, corresponding to the Cl(2p → 9b1u*) and Cl(2p → 8b2u*) transitions, were calculated in order to form a basis set of molecular states from which the spin−orbit splitting can be inferred. Multiconfigurational self-consistent field (MCSCF) calculation followed by multireference configuration interaction (MRCI) was the method chosen to establish a set of singlet and triplet states at the 2p excitation edge in addition to the ground state.

I. INTRODUCTION Direct valence ionization, core-level excitation, and ionization of molecules are attractive means for the study of the dissociation dynamics. In the case of core-level studies, the choice of the photon energy makes it possible to excite specific resonances neighboring a selected inner-shell edge, affording to a selective fragmentation.1−3 More specifically, the study on the competition between Auger decay and the so-called ultrafast dissociation4−6 in neutral highly excited molecular species is of particular interest, giving rise to a transfer of an electron from an inner shell to an unoccupied valence orbital or to the ejection of one or more electrons into the continuum. Core excitation and ionization of molecules using soft X-rays give rise to core−hole creation with lifetimes of the order of a few femtoseconds, about the time it takes for the nuclei to shift substantially their positions away from their equilibrium © XXXX American Chemical Society

configuration. Thus, it makes it possible to generate an interesting scenario where atomic and molecular decay regimes can be compared.7 In the case of the chlorocarbons, electronic relaxation in a molecular framework may take place by means of a nonradiative Auger decay, while the internuclear distance C−Cl increases. For molecules that relax through Auger decay at larger internuclear C−Cl distances, neutral dissociation into Cl* may take place followed by an atomic decay of the coreexcited Cl atom. As for the electronic relaxation in a molecular framework, the decay outcome is a parent molecular ion electronically and vibrationally excited which can later Received: March 20, 2017 Revised: May 12, 2017 Published: May 17, 2017 A

DOI: 10.1021/acs.jpca.7b02632 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

excited C2Cl4 molecule, (ii) to study the stability of the multiply charged ions, (iii) to ascertain if one can benefit from the initial excitation of a specific resonance in the C2Cl4molecule to induce a specific bond breaking, (iv) to shed some light onthe nature of the Cooper minimum in molecules, (v) to determine the final charge state of the core excited/ionized C2Cl4 molecule after Auger decay, and (vi) to study the possibility of ultrafast dissociation in polyatomic systems with heavy fragments. The results have been obtained by an ion time-of-flight experiment and are discussed in terms of fragmentation patterns following the excitation process and depending on the photon energy. Single and double ionization and ultrafast fragmentation leading to different and characteristic patterns in the mass spectra are discussed in detail.

deexcitate via ionic dissociation. In the atomic decay mode the main decay outcomes are excited molecular fragments and Cl*. Auger emissions giving rise to Cl+ ions can be characterized via Auger electron spectroscopy.5 For polyatomic molecules, the use of a pseudodiatomic model5 in which each fragment is reduced to a point mass is usual. In the case of larger molecules, such as dichloroethenes (C2H2Cl2), the dissociation is expected to be slower than in Cl2 if it takes place strictly along C−Cl bond coordinates. Nevertheless, signatures of ultrafast fragmentation were detected for Cl 2p core-excited dichloroethenes.4 The dynamics of the ultrafast fragmentation, usually investigated by photoelectron spectroscopy, can be likewise studied in fragmentation experiments. It is depicted by the presence in the mass spectra of certain fragments, which are generated after the electronic decay of the neutral core-excited moiety. The branching ratios of these fragments exhibit individual photon energy dependences associated with the neutral antibonding states. For instance, Alcantara et al.1 studied the translational kinetic energy release of the fragments from the core-excited CH2Cl2 molecule. They observed that the kinetic energy release of the CH2+ ion presents a minimum at the Cl (2P3/2→ 10a1*) resonance. Lu et al.8 observed that the CH2+ ion yield presents a maximum at the Cl (2P3/2 → 10a1*) resonance, which they interpreted as due to a fast dissociation through a highly repulsive potential curve. Alcantara et al.1 presented calculations of the potential curves in order to give support to this assertion. From their calculations, it is evident that the curve is repulsive and can promote fast dissociation. Such a process consists of dissociation of the nuclear framework before Auger decay, which takes place in the atom. They concluded that the process of ultrafast dissociation CH2Cl + + Cl+ is quite likely via the inner-shell curve. Another aim of this work is to contribute to the topic of stratospheric chlorine, which has been an extensively studied subject since the 1970s,9 when it was suggested that atomic chlorine released from chlorofluorcarbons (CFCs) in the stratosphere would precede the catalytic damage of the ozone layer.10 Since then, stratospheric ozone models have been developed, and the protagonist role of chlorine compounds has been evaluated by several studies. The significance of the original hypothesis on the role of man-made chlorine in stratospheric ozone was granted by the 1995 Nobel Prize in Chemistry presented to Molina, Rowland, and Crutzen. As a halogenated molecule of atmospheric interest with anthropogenic sources, C2Cl4 has been subject of various studies on the environmental consequences of anthropogenic emissions.11−13 More specifically, C2Cl4 is broadly employed as a dry-cleaning solvent and a degreasing agent. Moreover, its presence has been studied as a tracer for urban and industrial activities. C2Cl4 is emitted worldwide in alarmingly large quantities into the atmosphere, being further photodecomposed to highly poisonous species, for instance, as phosgene and chloroacetyl chlorides. Some papers have been devoted to the fragmentation of the C2Cl4 molecule, both in the valence and in the inner-shell regions.11−13 Mass analysis of ion fragmentation patterns of the C2Cl4 molecule also revealed a multiphoton ladder switching excitation mechanism.11 This work describes the valence and shallow core ionization and the selective excitation of the resonances around the Cl 2p edge of the C2Cl4 molecule. Besides the points discussed above, the subjects of interest addressed in this work are (i) to shed some light on the fragmentation dynamics of the shallow core

II. EXPERIMENT Monochromatic synchrotron radiation from the varied line space plane grating monochromator (VLS-PGM) beamline (energy range from 5.5 to 250 eV, resolution E/ΔE > 10 000) of the Canadian Light Source Inc. synchrotron radiation laboratory (Saskatoon, Canada) with entrance slits of 5.0 μm × 5.0 μm is focused onto an effusive gas jet.14 The liquid sample (99.8% purity) was freeze−pump−thawed two times to remove gases. The base pressure of the chamber was in the 10−8 Torr range. The incident photon-energy calibration was accomplished by scanning the monochromator through the Ar 3s3p5 → np (n = 10) resonance and monitoring the total photoion production. Throughout the experiment the pressure was maintained around 10−6 Torr. In order to avoid contributions from undesirable high order harmonics, Sn (in the 15−24 eV energy range) and Al filters (only at 40 eV) were used. Photoelectrons and photofragments were removed from the interaction region by an intense electrostatic field (1138 V/cm) which drives the electrons straight into a microchannel (MCP) detector without energy analysis and the photoions into a Wiley and McLaren type15 time-of-flight mass spectrometer (TOF) for the mass-to-charge ratio analysis.16 The photoelectron− photoion coincidence (PEPICO) technique has been used to obtain the TOF spectra, allowing the detection of electrons correlated with the photofragments. The measurements have been performedwith the TOF drift tube perpendicular to the axis of polarization of the synchrotron light. The spectrometer system hasno bias for photoelectrons with energies up to 200 eV kinetic energy. The high voltage employed on the front of the MCP detector, in chevron configuration, ensures the uniformity of the photoions efficiencies with respect to the ion mass. We have also determined the electron and ion collection efficiencies. By measuring the ratio of double-to-single ionization of argon at 160 eV photon energy and comparing it to the absolute cross section ratio from Holland et al.,17 we find that the efficiency of detecting at least one electron from a double ionization event is 3.2 times the efficiency of detecting an electron from a single ionization event. In addition, although it is not significant for the present results, we have also measured the ion collection efficiency, f i, by measuring the ratio between the coincidence rate and the total ion rate. In short, the electron−ion coincidence rate Cei is given by the product of the number of single ionization events per second, I+, by e1 f i, i.e., Cei = I+e1 f i. On the other hand, the total electron counting rate Re is given by Re = I+e1. Thus, f i = Cei/Re for photon energies below the double ionization threshold. In the present B

DOI: 10.1021/acs.jpca.7b02632 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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Figure 1. Time-of-flight mass spectra of the C2Cl4 at selected photon energies.

case, we find f i = 0.040 ± 0.002 at 15 eV, being essentially independent of the nature of the recoiling ions.

(2b1g)2 (12.58 eV) (8b1u)2 (12.58 eV) (9ag)2 (12.58 eV), where the energies represent calculated vertical ionization energies for those states. The highest occupied molecular orbital possesses a π (CC) character and is to some extent delocalized throughout the chlorine atoms.12 Figure 1 shows the time-offlight (TOF) mass spectra of the C2Cl4 molecule at selected photon energies. With the purpose of methodically studying the fragmentation dynamics, it is useful to examine the regions below and above the chlorine 2p ionization threshold. The 50− 200 eV photon energy range is sufficiently energetic to open all fragmentation channels associated with the direct single and multiple ionization/excitation of outer-shell electrons. However, this EUV/soft X-ray photon energy range is not sufficient to produce the ionization of the core electrons. Hence, the mass spectra in the 50−200 eV range should allow us a suitable comparison concerning the fragmentation pattern following core electron excitation or ionization. It is worthwhile to note some large-scale highlights shown in Figure 1: the mass spectra portray rich structures; increasing the photon energy up to 40 eV leads naturally to a higher degree of fragmentation. The branching ratios show that apart from the peak intensity of the species, the fragmentation pattern is not drastically altered for photon energies in the 50−200 eV range; singly charged fragments prevail in mass spectra, as expected. Some of the TOF peaks appear as multiplets due to the two Cl isotopes (35Cl with 75% and 37Cl with 25%). The significant presence of the parent ion, which results mainly from the single ionization of low-lying valence orbitals, even at high photon energies, is evident. The C2+cation was observed at all photon energies, and its relative intensity remains constant in the 50− 200 eV photon energy range. The photoion branching ratios in the photon energy range from 15 to 250 eV, corrected by the electron detection and ion efficiencies, are presented in Figure 2 and Table 1. The threshold energies for the observed fragments are11 C2Cl3+ (12.1 eV), C2Cl2+ (14.8 eV), C2Cl+ (18.0 eV), CCl2+ (18.7 eV), CCl+ (24.4 eV), C2+ (25.8 eV), Cl+ (25.9 eV), C+ (27.6 eV).

III. THEORETICAL CALCULATIONS Ab initio calculations were done to obtain the states at the Cl 2p edge. The inclusion of spin−orbit coupling is essential to describe such states. The approach was the same as previously reported for the CH2Cl2 molecule.1 Briefly, singlet and triplet states at the Cl 2p edge of the C2Cl4 molecule, corresponding to the Cl (2p → 9b1u*) and Cl (2p → 8b2u*) transitions, were calculated in order to form a basis set of molecular states from which the spin−orbit splitting can be inferred. Multiconfigurational self-consistent field (MCSCF) calculation followed by multireference configuration interaction (MRCI) was the method chosen to establish a set of singlet and triplet states at the 2p excitation edge in addition to the ground state. The active space was composed of the Cl 2p and two valence orbitals, namely, 9b1u* and 8b2u*. Orbitals have been localized before MCSCF procedure in order to make a conversion of the calculation easier. So, orbitals did not keep the symmetry, but the electronic states of the molecule do keep the full symmetry of the D2h group. The resulting states were used to diagonalize the full Breit−Pauli Hamiltonian, resulting in the spin−orbit manifold. Finally, the transition moment involving the ground state and the states of the spin−orbit manifold are calculated in order to determine the relative transition intensities. All calculations were done with the Molpro package18 with Dunning’s aug-cc-pVTZ-DK basis set for all atoms. Scalar relativistic effects are taken into account by the Douglas− Kroll−Hess Hamiltonian up to third order in all steps of the calculation. IV. DATA ANALYSIS 1. Ionic Fragmentation Following Valence-Shell Ionization. The electronic configuration of the ground state of C2Cl4 (D2h point group) is represented by19 (3b3u)2 (9.51 eV) (7b3g)2 (11.37 eV) (2au)2 (12.19 eV) (7b2u)2 (12.19 eV) C

DOI: 10.1021/acs.jpca.7b02632 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

At photon energies below the Cl 2p edge, 2p electrons can be excited to an unoccupied orbital, and the inner-shell vacancy is filled by a resonant Auger decay,which usually lead to singly ionized states, in which the molecule is often unstable and breaks into smaller atomic and molecular fragments. Above the Cl 2p edge, normal Auger process predominates, producing multiply charged species that are even more unstable. The TEY spectrum displays contributions from the underlying continuum due to the direct ionization of the valence levels. The multiplet structures with energies 201.4 eV (feature A), 202.7 eV (feature B), 203.5 eV (feature C), 203.9 eV (feature D), and 204.3 eV (feature E) in the TEY spectrum are due to the spin− orbital splitting of the 2p3/2 and 2p1/2 levels of chlorine possessing a σ* character, in a diatomic-like picture (see Table 2). The different peak widths point to different lifetimes or Franck−Condon factors of the corresponding excited states. Peak A is assigned to Cl (2p3/2 → 9b1u*) transition. Peaks B and C are attributed to transitions of the type Cl (2p1/2 → 9b1u*). The arising of two peaks is probably related to the molecular-field splitting. Peaks D and E are attributed to transition 2p3/2 → 8b2u*. Peak F is assigned to the 2p1/2 → 8b2u* transition and peak G to 2p1/2 → 8b2u* + continuum transitions. Following transitions of the excited core electron to the lowest Rydberg energy levels, the spectator Auger decay dictates the relaxation dynamics. In addition, for transitions into higher Rydberg levels, the shakeup process contributes significantly in resonant Auger decay. Table 2 presents the energy positions and interpretations of the features shown in Figure 3. Table 3 shows the calculated dipole transition moments and transition energies at 2p edge of chlorine Figure 3 also shows our ab initio calculations. Although agreement of theoretical and experimental spectra is not perfect, it is quite clear from calculations that peaks from A to C are due to the spin−orbit manifold of the Cl (2p → 9b1u*) transition, i.e., Cl (2p3/2 → 9b1u*) and Cl (2p1/2 → 9b1u*), confirming the σ* character mentioned above. The remaining peaks are due to Cl (2p3/2 or 2p1/2 → 8b2u*) transitions. In the last case, the intensity is very low for all peaks. The reason is that there are transitions to the continuum, not considered in our calculations, which contribute to the intensity. Additionally, intensities of all transitions may be affected by vibronic coupling mechanism, also not considered in the calculations. The first three resonances A, B, and C in Figure 3 are separated by an energy value (1.3 and 1.6 eV, respectively) roughly equal to the spin−orbit splitting of the Cl 2p (1.64 eV).21 The resonance A is considerably broader than the resonances band C (fwhm of ≅1.5 eV for peak A and of 0.69 eV for peak B and 0.46 eV for peak C), indicating different lifetimes or Franck−Condon factors of the corresponding excited states. In contrast to the outer-shell electrons, which are delocalized throughout the entire molecule, the Cl 2p electrons are retained near to the nucleus of the Cl atoms. At the resonances, the excitation occurs in a well-localized electronic state to a repulsive virtual molecular state. The Z + 1 approximation allows us to describe the system by the core equivalent system, in which the core excited chlorine atom is replaced by the argon atom, C2Cl3Ar. Thus, the Cl 2p excited states are suggested to be very repulsive along the C−Ar coordinate, since no bonding character is expected between argon and carbon atoms. Thus, a fast fragmentation of the molecule is predicted, where the fragmentation takes place within the Cl 2p hole lifetime (femtoseconds). This ultrafast fragmentation leads to the creation of a core excited Cl* atom, which relaxes mostly via

Figure 2. Partial ion yield of the C2Cl4 fragments as a function of the photon energy. For the sake of clarity error bars are not shown. Typical values are [0.6% (in the valence) < C2Cl4+ < 5% (near the Cl 2p edge], [0.4% (near the Cl 2p edge) < Cl+ < 2% (in the valence)]. The CCl2+ yield may contain some C2Cl42+ ions. Figure 4 shows the region between 200 and 210 eV in an expanded scale.

Below 25 eV, the C2Cl4+ parent ion is the most intense signal (65%−20%), followed by the C2Cl3+ (30%−15%) ion. On the other hand, the intensity of the Cl+ ion (adding the two isotopes 35Cl+ and 37Cl+) increases from 0 at 25 eV to 40% just before the Cl 2p threshold, being the most intense structure in the mass spectra above 30 eV. Distinct minima, shoulder (for the C2+ and C2Cl+ and less well-defined for the C2Cl2+ and CCl+ fragments), and a maximum (for the Cl2+) are found in the partial ion yield shown in Figure 2. These structures are credited to the Cooper minimum,20 which arises from molecular orbitals with a radial node due to their strong atomic 3p subshell character. As far as the authors are aware, these features have not been observed in PEPICO data for any molecule. The main reason for this lack of observance is due to the fact that most fragmentation studies are devoted to near-edge studies. 2. Ionic Fragmentation Following Cl 2p Edge Excitation and Ionization. With the purpose of studying the fragmentation processes following shallow inner-shell excitation and ionization, the total electron yield (TEY) spectra were measured covering the Cl 2p edge (Figure 3). The TEY spectrum, although is much richer in features due to the high photon energy resolution, closely bears a resemblance to the corresponding spectrum of other chlorine-containing molecules.1−3 D

DOI: 10.1021/acs.jpca.7b02632 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A Table 1. Ion Yield of the C2Cl4 Molecules as Function of the Photon Energy

a

energy (eV)

C2Cl4+

C2Cl3+

C2Cl2+

CCl2+

C2Cl+

CCl+

15.0 20.0 24.0 28.0 30.0 32.0 35.0 40.0 60.0 80.0 100.0 120.0 160.0 195.0 200.0 200.1 200.2 200.3 200.4 200.5 200.6 200.7 200.8 200.9 201.0 201.1 201.2 201.3 201.4 201.5 201.6 201.7 201.8 201.9 202.0 202.1 202.2 202.3 202.4 202.5 202.6 202.7 202.8 202.9 203.0 203.1 203.2 203.3 203.4 203.5 203.6 203.7 203.8 203.9 204.0 204.1 204.2 204.3 204.4 204.5 204.6

65.4 65.4 27.2 21.1 19.0 16.3 12.8 8.9 7.4 7.3 7.6 6.2 5.5 7.2 6.0 5.5 5.3 5.3 4.9 4.5 4.3 3.2 3.3 2.7 2.3 2.4 1.8 1.9 1.7 1.7 1.7 1.7 1.9 2.0 2.1 2.1 2.3 2.3 1.9 1.6 1.5 1.4 1.8 1.7 1.9 1.9 2.1 2.2 2.3 2.3 2.2 1.9 2.4 2.1 2.0 1.5 1.3 1.0 1.3 1.3 2.0

31.9 31.9 21.2 17.4 15.1 13.7 11.3 7.8 5.6 5.1 5.0 4.3 3.9 4.0 4.3 3.8 4.1 3.6 4.5 3.3 2.7 2.8 2.7 2.1 2.2 2.1 2.0 1.8 1.6 1.8 1.8 1.9 1.9 1.9 2.0 2.2 2.2 1.8 1.9 1.7 1.4 1.6 1.7 1.7 2.0 1.7 2.0 2.0 2.1 1.9 1.9 1.9 1.8 1.8 1.4 1.1 1.2 0.9 1.0 1.3 1.5

0.4 0.4 26.1 19.6 17.1 16.1 14.5 14.3 9.8 8.7 8.1 7.5 7.6 8.8 9.2 8.6 8.6 9.3 9.1 9.0 9.5 9.2 10.2 10.2 10.3 10.4 10.4 10.3 10.1 10.1 9.8 10.1 9.6 9.5 9.2 9.3 9.3 8.7 7.9 8.0 7.8 8.1 8.0 8.6 8.5 8.6 8.6 8.5 8.4 8.3 8.0 7.3 6.8 6.8 5.8 5.2 4.8 3.8 4.6 4.9 5.3

0.6 0.6 2.9 7.1 13.1 16.1 18.3 15.6 11.8 11.5 11.3 10.3 11.6 11.1 12.4 12.4 12.5 12.1 11.2 10.3 10.6 10.3 9.0 8.5 8.0 7.4 7.3 6.9 7.1 6.8 7.2 7.1 7.5 7.1 7.9 7.4 8.0 8.4 8.4 8.2 7.9 7.5 7.5 7.5 7.5 7.9 7.8 7.9 8.0 8.4 8.2 8.5 9.2 8.7 9.4 9.8 9.6 8.4 9.8 10.4 10.7

0.3 0.3 8.4 13.4 10.2 8.2 6.7 5.5 6.6 5.9 5.8 5.7 5.5 5.8 6.2 6.3 5.9 5.7 5.8 6.7 6.8 7.3 7.0 8.1 7.7 7.8 8.3 7.8 8.4 8.3 8.4 8.3 8.5 8.6 8.7 8.5 8.2 8.2 8.1 8.7 9.0 8.8 8.6 8.2 8.4 8.5 8.4 8.0 8.1 8.3 8.4 7.9 8.0 8.0 8.1 7.7 8.2 7.5 7.9 7.6 7.3

0.5 0.5 11.5 14.8 15.6 15.8 17.1 19.9 19.7 18.7 17.9 17.8 17.9 16.7 17.8 19.0 18.3 19.4 19.8 18.0 19.1 19.3 18.9 18.9 19.4 19.9 19.5 19.8 19.9 19.6 19.6 19.8 19.1 19.8 19.5 19.1 19.3 19.9 20.4 19.9 20.0 20.0 20.3 20.1 20.0 19.8 19.4 19.9 19.7 19.5 19.5 20.0 20.2 20.0 19.8 20.8 19.9 19.3 20.0 19.9 20.1

E

Cl+

4.4 5.9 8.1 12.3 20.3 30.6 34.4 36.0 38.4 38.6 35.3 35.0 35.1 35.9 35.2 35.9 38.4 36.7 38.3 38.8 39.8 40.3 40.1 41.0 41.6 41.1 41.5 40.9 41.0 41.4 40.8 40.4 40.8 40.4 40.2 41.1 41.1 42.3 41.6 41.5 41.7 41.0 41.2 41.2 41.0 41.0 41.4 40.8 41.4 40.5 41.3 42.1 42.2 42.8 46.9 43.3 42.7 41.5

C2+

0.5 1.9 3.1 3.2 1.9 3.2 3.1 3.2 3.6 3.4 3.3 3.2 3.3 3.6 3.8 3.1 3.6 4.0 3.6 3.8 3.8 4.2 4.3 4.2 4.1 4.1 4.3 4.4 4.3 4.2 4.4 4.3 4.2 4.0 4.2 4.0 4.4 4.0 4.3 4.2 4.3 4.4 4.3 4.4 4.5 4.4 4.0 4.6 4.2 4.4 4.4 4.6 4.8 5.0 4.8 5.0 5.0 4.8

Cl2+

C+

0.15 0.20 0.30 0.30 0.39 0.40 0.38 0.39 0.39 0.42 0.39 0.34 0.34 0.36 0.31 0.30 0.27 0.30 0.29 0.28 0.31 0.30 0.30 0.35 0.34 0.35 0.35 0.32 0.33 0.33 0.27 0.29 0.33 0.27 0.33 0.32 0.32 0.31 0.32 0.32 0.35 0.34 0.31 0.34 0.27 0.25 0.27 0.30 0.29 0.23 0.29

0.1 0.1 0.1 0.1 0.4 0.8 1.3 1.9 2.9 3.3 3.5 4.1 4.0 4.3 4.1 4.2 3.8 3.7 3.8 4.2 4.4 4.3 4.6 4.3 4.2 4.2 4.3 4.4 4.5 4.5 4.7 4.3 4.4 4.3 4.2 4.9 4.6 4.6 4.6 5.0 4.9 5.0 4.9 4.8 4.7 4.6 4.5 4.7 4.5 4.5 4.8 5.1 5.2 5.1 5.5 5.7 5.7 5.8 5.6 5.7 5.4

DOI: 10.1021/acs.jpca.7b02632 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A Table 1. continued energy (eV)

C2Cl4+

C2Cl3+

C2Cl2+

CCl2+

C2Cl+

CCl+

Cl+

C2+

Cl2+

C+

204.7 204.8 204.9 205.0 205.1 205.2 205.3 205.4 205.5 205.6 205.7 205.8 205.9 206.0 206.1 206.2 206.3 206.4 206.5 206.6 206.7 206.8 206.9 207.0 207.1 207.2 207.3 207.4 207.5 207.6 207.7 207.8 207.9 208.0 208.1 208.2 208.3 208.4 208.5 208.6 208.7 208.8 208.9 209.0 209.1 209.2 209.3 209.4 209.5 209.6 209.7 209.8 209.9 210.0 220.0 230.0 240.0

2.5 3.2 3.6 3.5 3.6 2.8 3.3 3.0 2.7 2.0 1.7 1.3 1.0 0.9 1.1 1.1 1.3 1.4 1.1 1.0 0.9 0.8 0.8 0.8 0.9 0.8 0.7 0.6 0.5 0.5 0.4 0.4 0.3 0.2 0.2 0.2 0.2 0.2 0.1 0.1 0.2 0.1 0.1 0.1 0.2 0.2 0.1 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.2

2.0 2.6 3.0 2.7 2.6 2.4 2.4 1.9 2.1 2.1 1.5 1.3 1.1 1.1 1.3 1.2 1.4 1.4 1.2 1.2 1.1 1.2 1.2 1.1 1.2 1.1 1.1 1.1 1.2 1.2 1.2 1.2 1.1 1.1 1.1 1.0 1.0 1.0 1.0 1.0 1.0 0.7 0.9 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.9 1.0 1.0 0.6 0.5 0.5

5.7 5.8 6.5 6.5 6.1 6.2 5.8 5.8 5.1 5.3 4.7 4.5 4.2 4.1 4.2 4.7 4.7 5.0 4.9 4.8 4.7 5.3 5.3 4.9 5.2 5.4 5.3 5.8 6.3 6.8 7.7 8.4 8.8 8.8 9.1 9.1 9.3 9.5 9.4 9.5 9.4 6.9 9.5 9.4 9.4 9.5 9.5 9.7 9.8 9.9 10.1 10.0 10.0 10.2 6.4 5.4 5.2

10.3 11.2 12.0 11.6 11.6 11.4 11.1 11.1 10.7 11.5 11.2 11.0 11.3 10.8 11.4 11.2 11.7 11.4 11.0 10.6 10.8 10.5 9.7 9.7 9.0 8.8 8.7 8.3 9.1 9.3 9.7 9.6 9.8 9.9 10.0 9.8 9.8 9.8 9.8 9.7 9.6 8.3 9.5 9.4 9.4 9.4 9.4 9.3 9.4 9.4 9.5 9.3 9.3 9.4 6.4 5.5 5.1

7.4 6.7 6.6 6.6 7.0 6.2 6.6 6.3 6.6 6.1 6.7 6.6 6.4 6.3 6.0 5.4 4.9 5.0 4.9 4.7 4.6 4.4 4.5 4.4 4.0 4.3 4.1 4.1 4.3 4.1 4.0 4.2 4.1 4.0 4.0 4.2 4.1 4.2 4.2 4.2 4.1 3.7 4.2 4.2 4.2 4.2 4.1 4.1 4.2 4.2 4.2 4.3 4.2 4.2 4.5 3.7 3.7

19.7 19.1 18.7 19.3 19.6 19.6 19.3 19.5 19.1 19.8 19.6 19.4 19.5 19.9 19.7 19.1 18.9 18.5 19.2 18.4 19.0 18.3 18.3 18.6 18.3 18.4 18.7 18.4 18.7 19.4 20.0 20.4 21.0 21.0 20.9 21.2 21.4 21.4 21.2 21.3 21.4 20.4 21.3 21.3 21.3 21.2 21.4 21.6 21.6 21.6 21.6 21.8 21.8 21.8 20.1 18.2 17.8

40.9 40.1 38.3 39.2 38.5 40.2 40.3 40.6 42.1 41.9 42.3 43.6 44.1 44.0 43.8 44.3 44.1 44.8 44.6 46.2 45.7 46.4 47.2 46.9 47.7 48.0 48.2 48.4 47.1 46.9 46.0 45.2 44.5 44.5 44.7 44.5 44.2 44.1 44.2 44.2 44.2 49.8 44.6 44.6 44.4 44.5 44.6 44.3 44.2 44.1 43.9 43.9 44.0 43.7 50.8 53.6 54.4

4.8 4.8 4.5 3.9 4.2 4.6 4.8 4.6 4.8 4.7 5.4 5.4 5.2 5.6 5.3 5.3 5.7 5.7 5.5 5.5 5.6 5.4 5.3 5.6 5.5 5.3 5.2 5.3 4.9 4.8 4.5 4.2 4.1 4.2 3.9 3.9 3.9 3.8 3.9 3.9 3.9 3.8 3.8 3.9 3.9 3.9 3.8 3.7 3.8 3.7 3.7 3.8 3.7 3.6 4.0 4.6 4.5

0.29 0.31 0.31 0.44 0.39 0.42 0.35 0.35 0.30 0.28 0.30 0.24 0.29 0.28 0.27 0.29 0.24 0.23 0.27 0.31 0.27 0.29 0.29 0.28 0.28 0.28 0.26 0.26 0.30 0.24 0.24 0.22 0.23 0.20 0.19 0.21 0.20 0.20 0.21 0.20 0.23 0.22 0.21 0.21 0.22 0.21 0.21 0.21 0.23 0.23 0.21 0.21 0.22 0.22 0.30 0.36 0.37

5.3 5.0 5.0 4.5 4.9 4.8 4.9 5.4 5.2 5.2 5.5 5.7 5.6 5.7 5.7 6.3 6.0 5.5 6.1 6.0 6.1 6.2 6.2 6.3 6.8 6.6 6.5 6.6 6.2 5.7 5.2 5.1 5.0 4.9 4.8 4.7 4.8 4.7 4.8 4.7 4.8 5.1 4.8 4.8 4.9 4.8 4.7 4.7 4.6 4.6 4.5 4.6 4.5 4.5 5.3 6.3 6.5

a

Typical values are [0.6% (in the valence) ≪ C2Cl4+ ≪ 5% (near the Cl 2p edge], [0.4% (near the Cl 2p edge) ≪ Cl+ ≪ 2% (in the valence)]. The CCl2+ yield may contain some C2Cl42+ ions.

F

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should notice that as contrary to the CH2Cl2 case,1 the energy dependence of the Cl+ and Cl2+ yields are very dissimilar, implying that in general both fragments come from distinct fragmentation processes. The branching ratios, corrected by the electron detection and ion efficiencies, are shown in Figure 4 as a function of the

Figure 3. Black line: total electron yield spectra of C2Cl4 around the Cl L2,3 edge taken with 0.1 eV steps. Red line: theoretical calculations, convoluted to mimic the experimental data. Because of the limitations of the theoretical methods in taking into account all the significant effects in the experimental spectrum, there is no one to one correspondence between the experimental and theoretical spectra.

Table 2. Attribution of the Resonances Appearing in Figure 3a feature label

energy (eV)

origin

A B C D E F G H I

201.4 202.7 203.5 203.9 204.3 205.3 205.9 207 208.8

2p3/2 → 9b1u* 2p1/2 → 9b1u* 2p1/2 → 9b1u* 2p3/2 → 8b2u* 2p3/2 → 8b2u* 2p1/2 → 8b2u* 2p1/2 → 8b2u* + continuum not calculated not calculated

a

The energies are from experiment, while the origin of the features are obtained from theoretical calculations.

Figure 4. Partial ion yields of the C2Cl4 parent ion and its fragments as a function of the photon energy around the Cl 2p edge. The red lines represent the total electron yield for the sake of comparison.

Table 3. Calculated Squared Dipole Transition Moments and Transition Energies at 2p Edge of Chlorinea energy (eV) 202.2142 202.6598 203.6071 203.704 203.7057 204.3122 207.6965 207.705 207.7361 207.9303 208.0843

DM2 0.00141 3.90555 × 10−4 0 0.00104 0.00118 0.00107 2.401 × 10−8 1.05884 × 10−4 9.61 × 10−8 1.11022 × 10−5 2.46277 × 10−6

energy (eV) 208.0883 208.1002 208.1609 208.6685 208.6773 208.7154 209.0754 209.8484 209.8564 209.8647 209.9442

photon energy. Surrounding the chlorine 2p edge, the intensity of the CCl+ ion presents a peculiar photon energy dependence and is the most intense fragment after the Cl+. At 201.4 eV the yield of the Cl+ ion exhibits a maximum and a minimum in the production of Cl2+. This enhancement can be interpreted as an ultrafast fragmentation, taking place through a strongly repulsive potential curve. It implies that neutral dissociation of the core excited molecule comes from the strongly antibonding nature of the excited orbital to which the core electron was promoted, followed by electronic relaxation of the core excited ion. The increase in the fragmentation produces an increase in the neutral dissociation of C2Cl4 to Cl* plus other fragments, where Cl* is in a core excited state autoionizing to Cl+. In addition, the Cl+ also shows a maximum near the ionization potential (Rydberg levels) where the orbitals are more diffuse and the spectator Auger decay is more probable to take place. The present mass resolution and excellent signal-to-noise ratio has enabled us to observe a very weak signal (less than 0.1%, not shown in Figure 4) of some doubly and triply charged fragments (C2+ and Cl3+), showing the contribution of

DM2 1.29258 5.07601 0.00121 3.08498 1.81899 8.51375 4.22065 4.26836 1.04469 4.18634 0.00163

−4

× 10 × 10−6 × × × × × × ×

10−5 10−4 10−5 10−5 10−6 10−6 10−6

a

Energies do not perfectly match experimental values due to limitations of the calculation method, especially concerning the size of basis set and the inclusion of correlation effects. Nevertheless, the increasing energy order of the states has helped in the attribution presented in Table 2.

autoionization to bring in a major number of Cl+ ions and also an enhancement in the relative intensity of Cl2+ ions. One G

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experimental setup was designed to minimize mass and energy discrimination effects. As for the subjects of interest, listed in the Introduction, some conclusions can be drawn: (i) A state-selective fragmentation is observed along the Cl 2p pre-edge resonances. The fragmentation of the C2Cl4 molecule near the Cl 2p edge exhibits common features with other chlorine-containing molecules, which provides key insights for understanding the fragmentation of highly chlorinated molecules relevant tochemical processes in the stratosphere. (ii) With reference to the stability of the multiply charged ions, we have not observed any stable multiply charged molecular ions in the mass spectra. This is due to the Coulomb repulsion between positivelycharged, subsequentlyproduced fragments. (iii) It was observed that we can benefit from the initial excitation of a specific resonance in the C2Cl4 molecule to induce a specific bond breaking. (iv) In the valence region, distinct minima or maxima are found in the partial ion yield. These features are believed to be associated with the Cooper minimum which results from a molecular orbital with a strong atomic 3p subshell character. (v) Concerning the determination the final charge state of the core excited/ionized C2Cl4 molecule after Auger decay, we observe low probabilities of seeing C2+ and Cl3+. As for as objective (vi), the present work focuses on a specific case where a Cl 2p electron is promoted into a highly repulsive antibonding orbital. In that situation the dissociative character of the excited state promotes a very fast motion of the nuclei, despite their high masses, leading to the atomic autoionization following ultrafast dissociation. Thus, the evidence for fast fragmentation in the C2Cl4 system is favored by the very strong antibonding nature of the 9b1u* excited state, explicable in the frame of the core equivalent model for halogen-containing molecules. This idea of ultrafast dissociation of the C2Cl4 molecule around the Cl 2p edge is corroborated by Auger electron spectroscopy.22

the electron correlation for direct valence ionization. The molecular dications such as the parent molecular ion C2Cl42+could not be separated in the mass spectra (from the CCl2+ fragment) and point out the significant role of the Coulomb repulsion that overcomes the chemical bonding. As discussed in the Introduction, the ultrafast fragmentation process can also be observed in time-of-flight mass spectra22 and is distinguished by the manifestation of specific fragments, generated after electronic decay of the neutral core excited fragment. The branching ratios of these moieties show specific photon energy dependences, corresponding to the neutral antibonding states. From Figure 4 one can see that there is an increase in the production of Cl+ and a corresponding decrease in Cl2+. Some resonant photoelectron spectra have also been measured22 at photon energies corresponding to the pre-edge features and to the continuum. At the energy of the first transition, the photoelectron spectrum shows well-resolved features on top of a broad, unresolved band. On the other hand, spectra recorded at higher energies can be described only as broad, unresolved bands. These resolved features are usually assigned to atomic chlorine autoionization and the broad continuum underneath to molecular autoionization. The remark of the narrow structures in the resonant photoelectron spectra22 can be ascribed to the fact that the resonant spectrum recorded on the first excitation presents only transitions involving the Cl 2p3/2 hole. On the other hand, the spectra recorded at higher energies involve both Cl 2pl/2 and Cl 2p3/2 holes are more complicated and present less well-resolved structures. A strong change in the equilibrium geometry is expected to be induced by the population of the σ*-like molecular orbital, and the excitation of vibrational series associated with the C−Cl bond might be expected. An effect usually associated with the double ionization yield enhancement is the decrease of the branching ratio of the parent monocation, C2Cl4+, after the Cl 2p ionization edge. As the molecular monocation cannot be produced by normal Auger process, its relative intensity decreases relatively to the other moieties, which can be due to a fragmentation following the double ionization. The strong variation of the branching ratios in the spectra between 200 and 210 eV reflects the roles of the resonant and nonresonant Auger processes. The central effect in the mass spectra recorded at the resonances is the strong relative increases of the Cl+ and C+ fragment intensities. One attributes these state selective effects to the electronic relaxation processes which follow the production of a highly excited neutral state. A core excited neutral can relax either through participator or spectator Auger decay. In both cases, the population of the Cl 2p hole takes place by means of ionization of outer-shell electrons. Thus, one expects final ionic states with a hole in an outer-shell orbital localized on the Cl atom. This can be due to the increased overlap between the inner-shell and valence electronic wave functions. An increase of the branching ratio of Cl+ fragments can thus be anticipated after chlorine core electron excitation or ionization.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (A.C.F.S.). ORCID

A. C. F. Santos: 0000-0001-7402-6594 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported in part by FAPERJ, Capes, and CNPq.

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REFERENCES

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V. CONCLUSIONS In this article, we have presented time-of-flight mass spectrometry experiments in the valence and near the Cl 2p edge to shed some light on the photofragmentation dynamics of the C2Cl4 molecule in the region of the electromagnetic spectrum corresponding to the energetic solar radiation, which is present in the upper levels of the Earth’s atmosphere. The H

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