Dissociative photoionization of isoprene: experiments and calculations

Vacuum ultraviolet (VUV) dissociative photoionization of isoprene in the energy region 8.5–18 eV was investigated with photoionization mass spectroscopy (PIMS) using synchrotron radiation (SR). The ionization energy (IE) of isoprene as well as the appearance energies (AEs) of its fragment ions C₅H₇⁺, C₅H₅⁺, C₄H₅⁺, C₃H₆⁺, C₃H₅⁺, C₃H₄⁺, C₃H₃⁺ and C₂H₃⁺ were determined with photoionization efficiency (PIE) curves. The dissociation energies of some possible dissociation channels to produce those fragment ions were also determined experimentally. The total energies of C₅H₈ and its main fragments were calculated using the Gaussian 03 program and the Gaussian‐2 method. The IE of C₅H₈, the AEs for its fragment ions, and the dissociation energies to produce them were predicted using the high‐accuracy energy model. According to our results, the experimental dissociation energies were in reasonable agreement with the calculated values of the proposed photodissociation channels of C₅H₈. Copyright © 2009 John Wiley & Sons, Ltd.     

Complete valence double photoionization of SF6

Single photon double ionization of SF(6) has been investigated at the photon energies 38.71, 40.814, and 48.372 eV by using a recently developed time-of-flight photoelectron-photoelectron coincidence spectroscopy technique which gives complete two-dimensional e(-)-e(-) spectra. The first complete single photon double ionization electron spectrum of SF(6) up to a binding energy of approximately 48 eV is presented and accurately interpreted with the aid of Green's function ADC(2) calculations. Spectra which reflect either mainly direct or mainly indirect (via interatomic coulombic decay of F 2s holes) double ionization of SF(6) are extracted from the coincidence map and discussed. A previous, very low value for the onset of double ionization of SF(6) is found to energetically coincide with a peak structure related to secondary inelastic scattering events.     

Dissociative excitation of benzene by electron impact

In the wavelength region 1850–9000 Å radiation from H, C and CH fragments is observed as a result of the dissociative excitation of benzene by electron impact (0–1000 eV). Emission cross sections and threshold energies have been determined for the Balmer series of the hydrogen atom and the A²Δ - X²Π emission of the CH fragment.     

Dissociative photoionization of 1,3-butadiene: experimental and theoretical insights

The vacuum-ultraviolet photoionization and dissociative photoionization of 1,3-butadiene in a region ～8.5-17 eV have been investigated with time-of-flight photoionization mass spectrometry using tunable synchrotron radiation. The adiabatic ionization energy of 1,3-butadiene and appearance energies for its fragment ions, C(4)H(5)(+), C(4)H(4)(+), C(4)H(3)(+), C(3)H(3)(+), C(2)H(4)(+), C(2)H(3)(+), and C(2)H(2)(+), are determined to be 9.09, 11.72, 13.11, 15.20, 11.50, 12.44, 15.15, and 15.14 eV, respectively, by measurements of photoionization efficiency spectra. Ab initio molecular orbital calculations have been performed to investigate the reaction mechanism of dissociative photoionization of 1,3-butadiene. On the basis of experimental and theoretical results, seven dissociative photoionization channels are proposed: C(4)H(5)(+) + H, C(4)H(4)(+) + H(2), C(4)H(3)(+) + H(2) + H, C(3)H(3)(+) + CH(3), C(2)H(4)(+) + C(2)H(2), C(2)H(3)(+) + C(2)H(2) + H, and C(2)H(2)(+) + C(2)H(2) + H(2). Channel C(3)H(3)(+) + CH(3) is found to be the dominant one, followed by C(4)H(5)(+) + H and C(2)H(4)(+) + C(2)H(2). The majority of these channels occur via isomerization prior to dissociation. Transition structures and intermediates for those isomerization processes were also determined.     

Dissociative excitation of water by metastable argon

The reaction Ar(²P_2,0) + H₂O → Ar + H + OH(A²Σ⁺)was studied in crossed molecular beams by observing the luminescence from OH(A²Σ⁺). No significant dependence of the spectrum on collision energy was found over the 22–52 meV region. Spectral simulation was used to obtain the OH(A) vibrational distribution and rotational temperature, assuming a Boltzmann rotational distribution. Since predissociation is known to strongly affect the rovibrational distribution, the individual rotational state lifetimes were included in the simulation program and were used to obtain the average vibrational state lifetimes. Excellent agreement with experiment was obtained for vibrational population ratios N₀/N₁/N₂ of 1.00/ 0.40/0.013 and a rotational temperature of 4000 K. Correction for the different average vibrational lifetimes gave formation rate ratios P₀/P₁/P₂ of 1.00/0.49/0.25. The differences between these results and those from flowing afterglow studies on the same system are discussed. Three reaction mechanisms are considered, and the vibrational prior distributions are calculated from a simple density-of-states model. Only fair agreement with experiment is obtained. The best agreement for the mechanisms giving OH(A) in two 2-body dissociation steps is obtained by assuming 1.0 eV of internal energy remains in the second step. The OH(A) vibrational population distribution of the present work is similar to that found in the photolysis of H₂O at 122 nm, where there is 1.10 eV of excess internal energy.     

Dissociative excitation of water by electron impact

Absolute emission cross sections and threshold energies have been measured for radiation (1850–9000Å) from excited fragments (OH, O and H) produced by electron impact (0–1000 eV) on water vapour. The results are compared with previous experiments and the discrepancies are discussed. The measurements indicate that hydroxyl radicals excited in the A² ∑⁺ state originate from excitation of both singlet and triplet states of the water molecule. Excited atomic fragments arise partly from predissociation of Rydberg states of the water molecule converging to the third ionization potential.     

Dissociative photoionization of methyl chloride studied with threshold photoelectron-photoion coincidence velocity imaging

Utilizing threshold photoelectron-photoion coincidence (TPEPICO) velocity imaging, dissociation of state-selected CH(3)Cl(+) ions was investigated in the excitation energy range of 11.0-18.5 eV. TPEPICO time-of-flight mass spectra and three-dimensional time-sliced velocity images of CH(3)(+) dissociated from CH(3)Cl(+)(A(2)A(1) and B(2)E) ions were recorded. CH(3)(+) was kept as the most dominant fragment ion in the present energy range, while the branching ratio of CH(2)Cl(+) fragment was very low. For dissociation of CH(3)Cl(+)(A(2)A(1)) ions, a series of homocentric rings was clearly observed in the CH(3)(+) image, which was assigned as the excitation of umbrella vibration of CH(3)(+) ions. Moreover, a dependence of anisotropic parameters on the vibrational states of CH(3)(+)(1(1)A') provided a direct experimental evidence of a shallow potential well along the C-Cl bond rupture. For CH(3)Cl(+)(B(2)E) ions, total kinetic energy released distribution for CH(3)(+) fragmentation showed a near Maxwell-Boltzmann profile, indicating that the Cl-loss pathway from the B(2)E state was statistical predissociation. With the aid of calculated Cl-loss potential energy curves of CH(3)Cl(+), CH(3)(+) formation from CH(3)Cl(+)(A(2)A(1)) ions was a rapid direct fragmentation, while CH(3)Cl(+)(B(2)E) ions statistically dissociated to CH(3)(+) + Cl via internal conversion to the high vibrational states of X(2)E.     

Coherent excitation and isotope selective photoionization of cadmium

Coherence between the magnetic sub-levels of the 5s5p ³ P ₁ level of cadmium is induced by excitation. A second excitation step and photo-ionization are used to detect the coherence and perturbations by the nuclear angular momentum. The ionization probability of even isotopes is suppressed with respect to the odd isotopes by a factor 59.     

Dissociative photoionization mechanism of 1,8-dihydroxyanthraquinone: an experimental and theoretical study

The photoionization and dissociative photoionization mechanism of 1,8-dihydroxyanthraquinone (1,8-DHAQ) have been investigated by infrared laser desorption/tunable synchrotron vacuum ultraviolet photoionization mass spectrometry (IR LD/VUV PIMS) technique and theoretical calculations. Consecutive losses of two carbon monoxides and elimination of hydroxyl group are found to be the major fragmentation channels in low photon energy range. Photoionization efficiency (PIE) spectrum of 1,8-DHAQ was measured in the photon energy range of 8.2-15.0 eV. Adiabatic ionization energy (IE) of 1,8-DAHQ (M) and appearance energies (AEs) of the major fragments (M-CO) (+), (M-C 2O 2) (+), and (M-OH) (+) are determined to be 8.54 +/- 0.05, 10.8 +/- 0.1, 11.0 +/- 0.1, and 13.1 +/- 0.1 eV, respectively, which are in fair agreement with calculated results. The B3LYP method with the 6-31+G(d) basis set was used to study fragmentation of 1,8-DHAQ. Theoretical calculations indicate that five lowest-energy isomers of 1,8-DHAQ cations can coexist by virtue of bond rotation and intramolecular proton transfer. A number of decarbonylation and dehydroxylation processes of 1,8-DHAQ cations are well established.     

Dissociative excitation of some aliphatic hydrocarbons by electron impact

Cross sections and threshold energies are compared for radiation from fragments produced by electron impact on methane, ethylene, ethane and acetylene. Some previous measurements have been repeated. The emission cross sections for corresponding Balmer radiation are within 10% equal for these hydrocarbons. Also the thresholds for Balmer radiation lie close together. These results can be explained in a model where H fragments arise from Rydberg states excited by promotion of an inner valence electron to a non-bonding orbital. In this model a comparison between dissociative ionization yielding H⁺ and dissociative excitation yielding H⁺ is made. For radiation from molecular fragments it is shown that the CH(A² Δ-X²Π) emission cross sections are particularly high in the case of acetylene. The electron impact data appear to be consistent with photoabsorption data.     

Triple ionization of CO2 by valence and inner shell photoionization

Spectra of triply ionized CO(2) have been obtained from photoionization of the molecule using soft x-ray synchrotron light and an efficient multi-electron coincidence technique. Although all states of the CO(2) (+++) trication are unstable, the ionization energy for formation of molecular ions at a geometry similar to that of the neutral molecule is determined as 74 ± 0.5 eV.     

Time-dependent mass spectra and breakdown graphs. 10. Dissociative photoionization of anisole

Time-resolved photoionization mass spectrometry in the millisecond range has been employed to study the reaction C₆H₅OCH⁺₃ → C₆H⁺₆ + CH₂O in anisole. Photoionization efficiency (PIE) curves gave a long-time limiting appearance energy value, AE = 10.85 ± 0.05 eV at 298 K. Experimental PIE curves and breakdown graphs at t = 6 μs and 2 ms were compared to those predicted by the statistical theory (RRKM/QET) and by previous photoelectron—photoion coincidence spectrometry results. A sensitivity analysis yielded the following activation parameters: critical energy of activation, E₀ = 59.6 ± 0.6 kcal/mol, and entropy of activation, ΔS³(1000 K) = 7.25 ± 2.2 eu.     

Multiple photoionization from 3p excitation of Kr and 4p excitation of Xe

The photoionization cross sections for multiply charged ions produced by 3p excitation of Kr and 4p excitation of Xe have been obtained by means of a time-of-flight mass spectrometer and synchrotron radiation. It is found that the main formation of doubly to quadruply charged ions in both Kr and Xe is caused from the each initialp-hole state through a Coster-Kronig transition followed by Auger or double Auger processes. The formation of singly charged ions in these excitation energy regions is caused by direct photoionization from outermost shell electrons in both Kr and Xe. Triply charged ions are prominently produced among the multiply charged ions. The quadruple photoionization cross sections show clearly the structures due to the Rydberg series, 3p ^?1 nl of Kr and 4p ^?1 nl of Xe. Their main structures were assigned to the 3p ^?1 nd series in Kr and the 4p ^?1 nd series in Xe.     

Dissociative double photoionization of benzene molecules in the 26-33 eV energy range

A time-of-flight mass spectrometer with a position sensitive ion detector was used to study the dissociative double ionization of benzene by UV synchrotron radiation. The threshold energy for the main dissociative processes, leading to CH(3)(+) + C(5)H(3)(+), C(2)H(3)(+) + C(4)H(3)(+) and C(2)H(2)(+) + C(4)H(4)(+) ion pairs were characterized by exploiting a photoelectron-photoion-photoion-coincidence technique, giving 27.8 ± 0.1, 29.5 ± 0.1, and 30.2 ± 0.1 eV, respectively. The first reaction also proceeds via the formation of a metastable C(6)H(6)(2+) dication. The translational kinetic energy of the ionic products was evaluated by measuring the position of ions arriving to the detector. Theoretical calculations of the energy and structure of dissociation product ions were performed to provide further information on the dynamics of the charge separation reactions following the photoionization event.     

Dissociative excitation and fragmentation of S8 by electron impact

The vacuum-ultraviolet emission spectrum from 136 nm to 168 nm following the dissociative excitation of a predominantly S(8) target by electron impact at 100 eV incident energy was measured. The relative cross sections for the dominant multiplets at 138.9, 142.9, 147.9, and 166.7 nm are presented. Excitation functions are shown for electron-impact energies from below threshold to 360 eV for the two most prominent emissions at 142.5 nm and 147.4 nm. Five thresholds are clearly apparent in both excitation functions. For the four highest energy channels, the energy separation between the adjacent thresholds is approximately constant and the cross sections reduce regularly as the threshold energies increase. We suggest possible fragmentation pathways of the dissociating S(8) molecule that reproduce the energies of our observed thresholds.     

Dissociative photoionization and thermochemistry of dihalomethane compounds studied by threshold photoelectron photoion coincidence spectroscopy

The dissociative photoionization studies have been performed for a set of dihalomethane CH(2)XY (X,Y = Cl, Br, and I) molecules employing the threshold photoelectron photoion coincidence (TPEPICO) technique. Accurate dissociation onsets for the first and second dissociation limits have been recorded in the 10-13 eV energy range, and ionization potentials have been measured for these compounds. By using our experimental dissociation onsets and the known heat of formation of CH(2)Cl(2) molecule, it has been possible to derive the 0 and 298 K heats of formation of all six neutral dihalomethanes as well as their ionic fragments, CH(2)Cl(+), CH(2)Br(+), and CH(2)I(+), to a precision better than 3 kJ/mol. These new measurements serve to fill the lack of reliable experimental thermochemical information on these molecules, correct the old literature values by up to 19 kJ/mol, and reduce their uncertainties. From our thermochemical results it has also been possible to derive a consistent set of bond dissociation energies for the dihalomethanes.     

Calculation of excitation and photoionization spectra by quasi-degenerate perturbation theory

The ability of the QDPT CI method, in which the quasi-degenerate perturbation theory is applied within the configuration interaction (CI ) approach, in dealing with the calculation of excitation and photoionization spectra is shown through an overview of comparisons between the QDPT CI values and the full CI ones in the same basis set. A direct comparison with the experimental data is given for the core photoelectron spectrum of Ne and the valence photoelectron spectra of Ar and HCl. The quality of the results obtained is very satisfactory, both as regards the energies and the wave functions, indicating the validity and the flexibility of the method that can confidently be applied also in cases where strong correlation effects are present. © 1994 John Wiley & Sons, Inc.     

Dissociative double photoionization of singly deuterated benzene molecules in the 26-33 eV energy range

This work provides new experimental and theoretical results about the formation and dissociation of benzene dication. The experiment has been carried out by using a vacuum ultraviolet radiation from a synchrotron source together with a time-of-flight spectrometer and a position sensitive ion detector. Isotopically labeled benzene molecules with a single deuterium atom have been used in order to study the symmetric dissociation of the benzene dication, not well evident in previous experiments. A threshold of 30.1 ± 0.1 eV has been observed for this dissociation reaction. Moreover, the lifetime of the dissociation of the benzene metastable dication producing CH(3)(+) and C(5)H(3)(+) has been obtained as a function of the photon energy, by the use of a Monte Carlo trajectory analysis of the coincidence distributions. The determined lifetime is independent of the photon energy and has an average value of 0.75 ± 0.22 μs. Theoretical calculations of the energy and structure of dissociation product ions have been also performed to provide crucial information about the dynamics of the charge separation reactions following the photoionization event.     

Vertical excitation energies to valence states of DMS and DMSO

Coupled cluster linear response formalism has been used to compute the vertical excitation energies and oscillator strengths of the lowest valence singlet states of dimethyl sulfide (DMS) and dimethyl sulphoxide (DMSO). Unless for one exception, the results are in very good agreement with experiment, but a new assignment of one DMSO transition is proposed. Contribution to the Serafin Fraga Memorial Issue.