Dissociative Double Photoionization Research Articles

Two-body dissociation of isoxazole following double photoionization - an experimental PEPIPICO and theoretical DFT and MP2 study.

The dissociative double photoionization of isoxazole molecules has been investigated experimentally and theoretically. The experiment has been carried out in the 27.5-36 eV photon energy range using vacuum ultraviolet (VUV) synchrotron radiation excitation combined with ion time-of-flight (TOF) spectrometry and photoelectron-photoion-photoion coincidence (PEPIPICO) technique. Five well-resolved two-body dissociation channels have been identified in the isoxazole's coincidence maps, and their appearance energies have been determined. The coincidence yield curves of these dissociation channels have been obtained in the photon energy ranges from their appearance energies up to 36 eV. The double photoionization of isoxazole produces a C3H3NO2+ transient dication, which decomposes into fragments differing from previously reported photofragmentation products of isoxazole. We have found no evidence of pathways leading to the C3H2NO+, HCN+, C2H2O+, C3HN+, or C2H2+ fragments or their neutral counterparts that have been observed in previous neutral photodissociation and single photoionization studies. Instead, the dissociation of isoxazole after the ejection of two electrons is bond-selective and is governed by two reactions, HCO+ + H2CCN+ and H2CO+ + HCCN+, whose appearance energies are 28.6 (±0.3) and 29.4 (±0.3) eV, respectively. A third dissociation channel turns out to be a variant of the most intense channel (HCO+ + H2CCN+), where one of the fragment ions contains a heavy isotope. Two minor dissociation channels occurring at higher energies, CO+ + CH3CN+ and CN+ + H3CCO+, are also identified. The density functional and ab initio quantum chemical calculations have been performed to elucidate the dissociative charge-separating mechanisms and determine the energies of the observed photoproducts. The present work unravels hitherto unexplored photodissociation mechanisms of isoxazole and thus provides deeper insight into the photophysics of five-membered heterocyclic molecules containing two heteroatoms.

Time-resolved dissociative ionization and double photoionization of CO2.

CO2 single-photon double photoionization, Coulomb explosion, and dissociative ionization are studied with ultrafast extreme-ultraviolet pump and time-delayed near-infrared probe pulses. Kinetic energy release and momentum correlations for the two-body CO+ + O+ and three-body O+ + C+ + O fragmentation products are determined by 3D coincidence fragment imaging. The transient enhancement of the ratio of two-body vs three-body Coulomb explosion events and the time dependence of low and high kinetic energy release dissociation events are discussed in terms of dissociative ionization and Coulomb explosion dynamics.

Dissociative single and double photoionization of biphenyl (C12H10) by soft X-rays in planetary nebulae

ABSTRACT Biphenyl (C12H10), or phenylbenzene, is an important building block of polycyclic aromatic hydrocarbons (PAHs), whose infrared spectral features are present in a variety of galactic and extragalactic sources. In this work, we use synchrotron radiation coupled with time-of-flight spectrometry to study the photoionization and photodissociation processes of biphenyl upon its interaction with soft X-ray photons at energies around the inner-shell C1s resonance. These results are compared with our previous studies with benzene (C6H6) and naphthalene (C10H8), and discussed in the context of four planetary nebulae featuring PAH infrared emission: BD+30○3639, NGC 7027, NGC 5315, and NGC 40. We show that the mass spectrum of biphenyl before the C1s resonance energy is dominated by single photoionization processes leading to C6H$_{5}^+$, C6H$_{4}\, ^{+\cdot}$, and C12H$_{10}\, ^{+\cdot}$, while after the resonance dissociation following multiple photoionization processes is dominant. The release of neutral C6H6 and C6H$_{5}\, ^\cdot$ species accounts for one of the most relevant dissociation processes starting from the doubly ionized biphenyl, indicating that heterolytic charge separation of the two phenyl rings is also achieved. By using quantum chemical calculations, we show that the biphenylic structure is a high-lying isomer of the singly and doubly ionized C12H10 species, whose minimum energy geometries are related to the acenaphthene molecule, composed of a C2-bridged naphthalene. Furthermore, we estimate the lifetime of biphenyl for 275 and 310 eV in photon-dominated regions of planetary nebulae. We discuss distinct processes that may enhance its lifetime and those of other small-sized PAHs in such astrophysical environments.

Dissociation of cyclopropane in double ionization continuum.

Dissociative double photoionization of cyclopropane is studied in the inner-valence region using tunable synchrotron radiation. With the aid of ab initio quantum chemical calculations the energies of dication states and their favoured fragmentation pathways are determined. These are compared to the experimental appearance energies of two-body fragmentation processes and to the kinetic energy released upon dissociation. Photon energy dependent state-selective dissociation in the 25-35 eV range is found. Calculations of dissociation pathways suggest that cyclopropane ring-deformation is selectively triggered at certain photon energies. The calculations suggest that initial ring deformation essentially determines the population of different dication states that function as gateways for particular dissociation channels. The measurements show that stepwise ionization processes populate dissociative 3e'-2 states via ring-opening and Jahn-Teller active states at photon energies below the double-ionization threshold. For energies above the double-ionization threshold the kinematics indicate that double ionization takes place predominantly within the Franck-Condon region populating 3e'-1 1e''-1 states.

Angular and energy distributions of fragment ions in dissociative double photoionization of acetylene molecules in the 31.9-50.0 eV photon energy range

The two-body dissociation reactions of the dication C2H2+2, initiated via double ionization of acetylene molecules by photons in the energy range 31.9-50.0 eV, have been studied by coupling photoelectron-photoion-photoion coincidence and ion imaging techniques. The angular distributions and kinetic energy of product ions, measured in the 31.9-50.0 eV energy range, exhibit significant differences for the three leading dissociation reactions with respect to a previous investigation carried out at a fixed energy of 39.0 eV, providing thus new information on the dynamical evolution of the system. The analysis of the results indicates that such dissociation reactions occur with a different mechanism. In particular, the symmetric dissociation in two CH+ ions is characterized by different dynamics, and the anisotropy of the angular distribution of ionic products increases with photon energy in a more pronounced way than the other two reactions. Moreover, the kinetic energy distribution of the symmetric dissociation reaction exhibits several components that change with photon energy. The new experimental findings cast light on the microscopic evolution of the system and can provide a laboratory reference for new theoretical calculations on specific features of the multidimensional potential energy surface, namely, the structure, energy and symmetry of dication states, the electronic state of dissociation products, energy barriers and their dependence on the geometry of the intermediate state.

Kinetic Energy Release of the Singly and Doubly Charged Methylene Chloride Molecule: The Role of Fast Dissociation

The center of mass kinetic energy release distribution (KERD) spectra of selected ionic fragments, formed through dissociative single and double photoionization of CH2Cl2 at photon energies around the Cl 2p edge, were extracted from the shape and width of the experimentally obtained time-of-flight (TOF) distributions. The KERD spectra exhibit either smooth profiles or structures, depending on the moiety and photon energy. In general, the heavier the ionic fragments, the lower their average KERDs are. In contrast, the light H(+) fragments are observed with kinetic energies centered around 4.5-5.5 eV, depending on the photon energy. It was observed that the change in the photon energy involves a change in the KERDs, indicating different processes or transitions taking place in the breakup process. In the particular case of double ionization with the ejection of two charged fragments, the KERDs present have characteristics compatible with the Coulombic fragmentation model. Intending to interpret the experimental data, singlet and triplet states at Cl 2p edge of the CH2Cl2 molecule, corresponding to the Cl (2p → 10a1*) and Cl (2p → 4b1*) transitions, were calculated at multiconfigurational self-consistent field (MCSCF) level and multireference configuration interaction (MRCI). These states were selected to form the spin-orbit coupling matrix elements, which after diagonalization result in a spin-orbit manifold. Minimum energy pathways for dissociation of the molecule were additionally calculated aiming to give support to the presence of the ultrafast dissociation mechanism in the molecular breakup.

Angular and energy distribution of fragment ions in dissociative double photoionization of acetylene molecules at 39 eV

The two-body dissociation reactions of the dication, C(2)H(2)(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 C(2)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.

ChemInform Abstract: Dissociative Single and Double Photoionization of CH3F in the Region 20-110 eV Studied by Mass Spectrometry and the Photoion-Photoion Coincidence Method.

Dissociative single and double photoionization of methyl fluoride (CH3F) has been studied in the photon energy region of 20–110 eV by use of synchrotron radiation, a time‐of‐flight (TOF) mass spectrometer, and the photoion–photoion coincidence (PIPICO) method. The TOF mass spectrometer facilitates relatively efficient extraction of energetic ions, which is essential for quantitative measurements. Photoion branching ratios are reported for the observed 11 singly charged ions, H+, H+2, CH+n (n=0–3), F+, CHnF+ (n=0–3). Stable CH3F2+ is not observed in all of the energy region studied in a μs time scale. The double photoionization threshold is measured to be 35.0±0.5 eV. PIPICO spectra measured at higher photon energies show the following five types of dissociation processes:(1) CH3F2+→H++CHnF+ (n=0–2), (2) H+2+CHnF+ (n=0–1), (3) H++CH+n (n=0–2), (4) H+2+CH+, (5) H++F+, with ten total dissociation channels. These processes are characteristic in regard to bond breakage involved, whether a rearrangement reactio...

ChemInform Abstract: Dissociative Single, Double, and Triple Photoionization of OCS in the Region hν = 20-100 eV Studied by Mass Spectrometry and the Photoion- Photoion Coincidence Method.

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Dissociative double photoionization of CO2 molecules in the 36–49 eV energy range: angular and energy distribution of ion products

Dissociative double photoionization of CO(2), producing CO(+) and O(+) ions, has been studied in the 36-49 eV energy range using synchrotron radiation and ion-ion coincidence imaging detection. At low energy, the reaction appears to occur by an indirect mechanism through the formation of CO(+) and an autoionizing state of the oxygen atom. In this energy range the reaction leads to an isotropic distribution of products with respect to the polarization vector of the light. When the photon energy increases, the distribution of products becomes anisotropic, with the two ions preferentially emitted along the direction of the light polarization vector. This implies that the molecule photoionizes when oriented parallel to that direction and also that the CO(2)(2+) dication just formed dissociates in a time shorter than its typical rotational period. At low photon energy, the CO(+) and O(+) product ions separate predominantly with a total kinetic energy between 3 and 4 eV. This mechanism becomes gradually less important when the photon energy increases and, at 49 eV, a process where the two products separate with a kinetic energy between 5 and 6 eV is dominant.

Anisotropy of the angular distribution of fragment ions in dissociative double photoionization of N2O molecules in the 30–50eV energy range

The double photoionization of N2O molecules by linearly polarized light in the 30-50 eV energy range has been studied by coupling ion imaging technique and electron-ion-ion coincidence. For the two possible dissociative processes, leading to N++NO+ and O++N2+, angular distributions of ionic fragments have been measured, finding an evident anisotropy. This indicates that the molecules ionize when their axis is parallel to the light polarization vector and the fragments are separating in a time shorter than the dication rotational period. The analysis of results provides, in addition to the total kinetic energy of ionic fragments, crucial information about the double photoionization dynamics.

Dissociative double photoionization of N2 using synchrotron radiation: Appearance energy of the N2+ dication

Photoionization cross sections for the production of the doubly charged ion N2+ from N2 have been measured by means of synchrotron radiation in the photon energy range from 50 to 110 eV. The appearance energy for N2+ has been determined as 55.2+/-0.2 eV, i.e., about 1.3 eV higher than the spectroscopic dissociation limit leading to the charge asymmetric dissociation channel N2+(2P)+N(4S) at 53.9 eV. The onset of a second threshold at 59.9+/-0.2 eV is detected and the energy dependence of photoion intensities near the threshold regions is interpreted in terms of the Wannier theory. The production of the N2+ dication is discussed in terms of direct and indirect mechanisms for dissociative charge asymmetric photoionization and by comparison with the potential energy curves of the intermediate N(2)2+ dication. Experimental evidences for the opening of the Coulomb explosion channel N2++N+ at high photon energies are provided by measuring the kinetic energy release spectra of N2+ fragments at selected photon energies.

Spectroscopy and metastability of CO22+ molecular ions

The spectroscopy and metastability of the carbon dioxide doubly charged ion, the CO(2) (2+) dication, have been studied with photoionization experiments: time-of-flight photoelectron photoelectron coincidence (TOF-PEPECO), threshold photoelectrons coincidence (TPEsCO), and threshold photoelectrons and ion coincidence (TPEsCO ion coincidence) spectroscopies. Vibrational structure is observed in TOF-PEPECO and TPEsCO spectra of the ground and first two excited states. The vibrational structure is dominated by the symmetric stretch except in the TPEsCO spectrum of the ground state where an antisymmetric stretch progression is observed. All three vibrational frequencies are deduced for the ground state and symmetric stretch and bending frequencies are deduced for the first two excited states. Some vibrational structure of higher electronic states is also observed. The threshold for double ionization of carbon dioxide is reported as 37.340+/-0.010 eV. The fragmentation of energy selected CO(2) (2+) ions has been investigated with TPEsCO ion coincidence spectroscopy. A band of metastable states from approximately 38.7 to approximately 41 eV above the ground state of neutral CO(2) has been observed in the experimental time window of approximately 0.1-2.3 mus with a tendency towards shorter lifetimes at higher energies. It is proposed that the metastability is due to slow spin forbidden conversion from bound excited singlet states to unbound continuum states of the triplet ground state. Another result of this investigation is the observation of CO(+)+O(+) formation in indirect dissociative double photoionization below the threshold for formation of CO(2) (2+). The threshold for CO(+)+O(+) formation is found to be 35.56+/-0.10 eV or lower, which is more than 2 eV lower than previous measurements.

Dissociative single and double photoionization of CF4 and ionic fragmentation of CF4+ and CF42+ from 23 to 120 eV

Time-of-flight mass spectrometry and the photoion–photoion coincidence technique have been used to study dissociative single and double photoionization processes of CF4 in the energy range of 23–120 eV together with linearly polarized synchrotron radiation. Under the condition where the molecular and dissociative single and double photoionization processes take place concomitantly, we have explicitly determined the ion branching ratios for single photoionization. Together with previously reported data for the ion branching ratios of ion-pairs and doubly charged ions in double photoionization, we have derived the ratio of double to single photoionization (σ2+/σ+), the partial cross-sections for single (σ+) and double (σ2+) photoionization, the ionization yield and the partial cross-sections for the individual fragment ions produced from the singly and doubly charged parent ions, CF4+ and CF42+. Dissociation pathways of CF4 are discussed on the basis of the observed ion branching ratios and the derived partial photoionization cross-sections of fragment ions and ion-pairs.

Kinetic-energy release and intercharge distance of the sulfur dioxide dication (SO 22+)

The kinetic-energy release distributions (KERDs) of the fragment ion pairs (O + + SO + and O + + S +) produced in dissociative double photoionization of sulfur dioxide have been determined by analyzing the photoion-photoion coincidence spectra measured in the energy region of 37–130 eV by use of a time-of-flight mass spectrometer and synchrotron radiation. It should be noted that the KERDs obtained for the O + + S + channel represent lower limits because the kinetic energy of the neutral O atom is not involved. The kinetic-energy releases (KERs) in the O + + SO + dissociation channel do not vary but remain almost constant across this broad range of excitation energies. This means that low-lying electronic states of SO 2 2+ below 37 eV are mainly responsible for this channel. However, the average KERs in the O + + S + dissociation channel depend strongly on the excitation energies in the 40–60-eV region and weakly above 60 eV. This indicates that high-lying electronic states of SO 2 2+ as well as low-lying ones, contribute to this channel. From the observed KERDs in the O + + SO + and O + + S + channels of SO 2 2+, the range of the intercharge distances is estimated by assuming that the KERs are simply given by Coulomb repulsion. The estimated intercharge distances for the O + + S + channel are wide and deviate largely from the equilibrium internuclear distance, which is not possible by direct double ionization only and which indicates a significant role of indirect processes.

Angular distribution of ionic fragments in the dissociation of SO22+

The photoion–photoion coincidence technique has been used to measure the angular distributions of the O++S+ and O++SO+ fragments in the dissociative double photoionization of SO2 in the 37–120 eV range with linearly polarized synchrotron radiation. The present work shows that the obtained values of the asymmetry parameter β are close to zero (−0.2⩽β⩽0.2) over the whole energy range of photoexcitation, indicating that the fragmentation is almost isotropic in both cases of O++S+ and O++SO+ channels. It is found that the symmetry analysis which has been partially successful for explaining the behavior of β values observed for linear molecules or for molecules with cylindrical symmetry is unsatisfactory to account for the observed energy dependence of β values in the dissociation processes of SO22+. For more quantitative understanding of β values, the dynamical effects such as asymmetric bending or stretching motion of the SO2 molecule prior to the ionic fragmentation, rather than the static effects such as electron configuration of parent ions, may provide a satisfactory model for the observed β values.

Dissociative single and double photoionization with excitation between 37 and 69 eV in N2

Dissociative single and double photoionization of N2 as a function of the exciting-photon energy has been investigated using monochromatized synchrotron radiation in the exciting-photon energy range between 37 and 69 eV by dispersed VUV fluorescence spectroscopy (80 nmfl200 nm). Relative partial emission cross sections for N I and N II fluorescence have been recorded as a function of the exciting-photon energy. There is strong evidence that the precursors of dissociative single ionization with one of the fragments being excited are the N2+ 2g-1 2g+ state with its closely lying correlation states only. An approximately linear excess energy dependence of the cross sections for the molecular double photoionization into the N22+ D 1u+ state has been observed in the excitation spectrum of D 1u+ X 1g+ fluorescence in N22+ close to its threshold. Evidence for dissociative double photoionization with excitation has been found.

Anisotropic angular distribution of ionic fragments in the dissociation of CO22+

Abstract Angular distributions of fragment ion pairs (O++CO+ and C++O+) produced in dissociative double photoionization of carbon dioxide have been studied in the photon energy region 41–100 eV by use of a photoion–photoion coincidence (PIPICO) technique and a source of linearly polarized synchrotron radiation. The asymmetry parameter ranges from 0.89 at 41 eV to 0.01 at 90 eV for the O++CO+ dissociation channel and from 0.89 at 55 eV to −0.08 at 90 eV for the C++O+ channel. The observed anisotropy in dissociative double photoionization from the valence orbitals of CO2 is related to the symmetry involved in the transition.

Direct and indirect dissociative double photoionization of diatomic molecules

An enhanced photoelectron - ion - ion triple coincidence technique eliminates the effect of target molecule thermal velocity on measured ion pair kinetic energy releases. Kinetic energy release distributions at photon energies of 40.8 and 48.4 eV are reported for diatomic molecules and are compared with calculated distributions for direct vertical transitions to theoretically known dication states. Good agreement in the case of indicates that double photoionization of this molecule at 8 eV above threshold is mainly direct. Double photoionization of CO, and NO is purely indirect at the lowest photon energies and has both indirect and direct components at higher energies in this range. The indirect processes give a wide range of final product energies and often involve atomic autoionization. Selectivity in the direct double ionization processes is consistent with the application of symmetry-based propensity rules.

Single, double, and triple photoionization of molecules using synchrotron radiation

The recent advances concerning production and dissociation dynamics of multiply-charged molecules using synchrotron radiation are discussed with emphasis on dissociative double photoionization in the valence region. In the determination of the ratio of double to single photoionization, both molecular and dissociative processes are properly included. Kinetic-energy release and anisotropic fragmentation in dissociative double photoionization of carbon monoxide are also discussed.