Threshold Photoelectron Photoion Coincidence Research Articles

High resolution threshold photoelectron and photoion spectroscopy of oxygen in the 12-50 eV photon range

High resolution threshold photoelectron and photoion spectra of the oxygen molecule have been measured in the photon energy range 12-50 eV using a penetrating field electrostatic analyser and a synchrotron radiation source. Besides the well known X 2 Pi g, a 4 Pi u, A 2 Pi u, b 4 Sigma g-, B 2 Sigma g-, c 4 Sigma u- O2+ states, several other series of vibrational structure are observed in the 21-24 eV range. One series is interpreted to result, not from an ion state, but from an autoionization process involving a neutral state and the nuclear continuum of an ion state. Above 26 eV, three relatively strong features are observed at approximately 27.3 eV, 32.5 eV and 39.5 eV, interspersed with several weaker ones. Dissociation processes from some O2+ states have been studied using a threshold photoelectron-photoion coincidence technique (TPEPICO). These observations identify the weak, broad feature at 24.96 eV as the nu =2 level of the c 4 Sigma u- state.

Kinetic energy analysis in time of flight mass spectrometry: application of time of flight methods to clusters and pyrolysis studies in supersonic expansions

A new experimental technique, based on high resolution time of flight analysis of ions in a molecular beam, is described with the use of several examples. Although the approach used here is based on threshold photoelectron photoion coincidence (TPEPICO), the technique can also be used with pulsed laser photoionization, albeit without the benefit of ion energy selection. The approach is based on the fact that the TOF distribution of parent ions formed from cold neutral molecules are narrow, while product ion TOF distributions are broad due to kinetic energy release. This distinction permits differentiating cluster ions formed by “simple” ionization of the corresponding neutral clusters from similar mass cluster ions formed by dissociative ionization. Results so far obtained indicate that most clusters ionize, even at threshold, via dissociative ionization. The technique is also suitable for obtaining the threshold photoelectron spectra (TPES) of mass selected cold species in the presence of a mixture of warm and cold species as might be encountered in a pyrolysis experiment.

Fragmentation of the C̃ 2T 2 and D̃ 2A 1 states of CCl +4 and SiCl +4 studied by threshold photoelectron—photoion coincidence spectroscopy

Threshold photoelectron—photoion coincidence (TPEPICO) spectroscopy has been used to study the decay pathways of the C̃ 2T 2 and D̃ 2A 1 excited electronic states of CCl + 4 and SiCl + 4 in the range 14–23 eV. Vacuum UV photons from a synchrotron radiation source ionise the parent neutral molecule, and the electrons and ions are detected by threshold electron analysis and time-of-flight mass spectrometry, respectively. TPEPICO spectra are recorded continuously as a function of photon energy, allowing ion yield curves to be obtained. The C̃ and D̃ states of CCl + 4 fragment exclusively to CCl + 2 and CCl + respectively, corresponding to non-statistical routes. Radiative decay from the C̃ state of SiCl + 4 is an important process and this state fragments indirectly to SiCl + 3 by fluorescence from the C̃ state, followed by dissociation from lower-lying repulsive states. The D̃ state of SiCl + 4 fragments exclusively to SiCl + 2. Kinetic energy releases into CCl + 2 + Cl 2 and SiCl + 3 + Cl have been measured at the Franck—Condon maximum of the C̃ 2T 2 states of the two parent ions.

Transition state structures and angular momentum effects in the dissociation dynamics of energy-selected C4H+8 ions

The photoionization and dissociation dynamics of energy-selected 1-butene ions have been investigated by the technique of threshold photoelectron photoion coincidence (TPEPICO) time of flight mass spectrometry. The absolute dissociation rates for the reactions leading to the loss of H, CH3, and CH4 have been measured for two samples prepared with very different internal energy and angular momentum distributions. First rotationally cold ions were prepared by photoionizing 1-butene molecules cooled in a seeded molecular beam. These rates were analyzed within the framework of RRKM theory with vibrator transition state structure for all three channels. Excellent agreement between theory and experiment was obtained when ab initio calculated transition state frequencies were used for the H loss and the CH3 loss transition states. A variational transition state theory (VTST) analysis shows that the CH3 loss transition state lies about 11 kJ/mol below the dissociation limit. Second, dissociation rates using an effusive source which contained a 298 K distribution of vibrational and rotational energy were measured. The vibrator-type transition state model, with inclusion only of the vibrational energy distribution, gives a good account of the total rates but significantly overestimates the H loss branching ratio. Excellent agreement is obtained, however, when the energies of the molecular ions and vibrator transition states are corrected for the rotational energy of each structure. K-rotor mixing with the vibrations does not change the calculated rates significantly. Finally, the analysis confirms a previous proposal [Faraday Discuss. Chem. Soc. 75, 57 (1983)] that an orbiting transition state (a la phase space theory) is not the rate limiting bottleneck at the energies used in this experiment.

Fragmentation of valence electronic states of CF +4 and SF +6 studied by threshold photoelectron-photoion coincidence spectroscopy

Threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy has been used to measure, state selectively, the decay pathways of all the valence states of CF + 4 and SF + 6 in the range 15–28 eV. Radiation in the vacuum UV from a synchrotron radiation source ionises the parent molecule, and the electrons and ions are detected by threshold electron analysis and time-of-flight mass spectroscopy, respectively. TPEPICO spectra are recorded continuously as a function of photon energy, allowing both threshold photoelectron spectra and yields of all the fragment ions to be obtained. Kinetic energy releases are measured at fixed photon energies with good time resolution. The results orf the X 2T 1, A 2T 2 and B 2E states of CF + 4 (all below the energy of He I radiation) confirm earlier measurements; results for the C 2T 2 and D 2A 1 states at 21.7 and 25.1 eV, respectively, are new. They confirm previous measurements that radiative decay from both states is an important process, and we have measured their state-selected fluorescence quantum yields. For SF + 6, fluorescence does not occur from any of the excited valence states, and we have measured the fragmentation channels and branching ratios for all of the valence states.

Photoelectron spectra of acetone and acetone dimer

In order to study the ionic states of acetone produced in a supersonic jet, we have carried out threshold photoelectron spectroscopy (TPES) with synchrotron radiation in the region 122.0–130.0 nm. Acetone has also been re-investigated by 58.4 nm He(I) photoelectron spectroscopy (PES). It has been found from a comparison of the PES and TPES spectra that there is an autoionizing state at 9.893 ± 0.015 eV. Furthermore, a threshold-photoelectron photoion coincidence (TPEPICO) spectrum due to the acetone dimer has been obtained for the first time in the region 124.0–135.0 nm with an interval of 0.2 nm. The dimer intensity in each spectrum was plotted against the excitation wavelength, giving rise to intensity distribution corresponding to a TPES spectrum of the non-dissociative acetone dimer. In contrast to the PES and TPES spectra of the acetone monomer, no vibrational structure appears in the dimer spectrum. From this spectrum the appearance ionization potential of the acetone dimer has been evaluated as being 9.210 ± 0.015 eV. Two bands appear in the dimer spectra, one of which may be assigned to a Rydberg state of the neutral dimer, and the other to the electronic ground state of the dimer cation.

The dissociation energies of the positive ions NH +3, NF +3, PH +3, PF +3 and PCl +3

We present new values for the appearance energies as well as breakdown diagrams for the fragmentation of the title compounds as measured with threshold photoelectron photoion coincidence spectroscopy in the energy range up to 20 eV.

The photoionization and dissociation dynamics of energy-selected acetylene dimers, trimers, and tetramers

The photoionization and dissociation characteristics of (C2H2)n (n=2, 3, and 4) have been studied by the threshold photoelectron–photoion coincidence technique applied to a molecular beam. For photon energies between 10.20 and 11.27 eV, only dissociative ionization pathways were observed for the clusters; no direct ionization of (C2H2)2, (C2H2)3, and (C2H2)4 to the stable ions C4H4+, C6H6+, and C8H8+ was observed. The shape of the time of flight peaks is consistent with statistical dissociation dynamics, and statistical analysis of the fragment peak shapes suggests that the dimer, trimer, and tetramer produce the fragments C4H2+/C4H3+, C4H4+, and C6H6+, respectively. Consistent with these observations, ab initio calculations are presented which indicate that the neutral T-shaped dimer and triangular trimer geometries are unstable on the ionic potential energy surface, and indeed have vertical ionization energies well above the dissociation limits of many of their possible product channels. The statistical modeling of the C4H4+ peak width is consistent with a process in which the trimer neutral produces a C4H4+–C2H2 complex, which then dissociates via monomer evaporation without exploring any of the covalent C6H6+ potential energy surface, i.e., benzene ion is not formed. The heat of formation of the observed C4H4+ daughter is estimated to be 1240±15 kJ/mol, indicating that it is cyclobutadiene, butatriene, or vinylacetylene ion. The peak width of C6H6+ is also analyzed, and suggests a process whereby the tetramer produces a C6H6+–C2H2 complex, which dissociates via monomer evaporation without exploring any of the covalent C8H8+ potential energy surface. The heat of formation of the C6H6+ daughter fragment is estimated as 1160±15 kJ/mol, which is consistent with the formation of benzvalene ion.

The Determination of the Transition State Structure from the J Dependence of the Dissociation Energy E0 (J): The Methane and Ethane Ion Dissociation

AbstractA threshold photoelectron photoion coincidence experiment has been setup which allows to measure breakdown curves of energy selected ions at different sample temperatures. These studies reveal the rotational energy of the transition state in unimolecular reactions, which in turn is used as a probe for the transition state structure. According to a model recently suggested this should allow the distinction of tight and loose transition states. The breakdown curves of methane and ethane ions in the region of the lowest reaction channel have been measured at 300 K and close to 0 K. In the case of the H loss from methane ions the shift between these two breakdown curves reveals that the dissociation energy Eo varies significantly with the rotational temperature as expected for a simple bond fission (loose transition state). In the case of H2 loss from ethane ions the shift between the breakdown curves at 300 and close to 0 K shows that E0 (J) does not vary significantly with the rotational temperature. This is to be expected for a concerted elimination reaction (tight transition state).

Unimolecular dissociation of energy-selected ethyl formate ions

The unimolecular dissociation of ethyl formate ions has been investigated using threshold photoelectron photoion coincidence mass spectrometry. The fragment ions appearing at low internal energies are C 2H 4CO + and C 2H 4 ·+, whose appearance potentials are higher than the ionization energy of the ethyl formate molecule by 0.18 eV and 0.28 eV respectively. Observed decay rates for the formation of C 2H 4CO + were found to be different to those of C 2H 4 ·+, although the direct fragmentation mechanism indicates that they should be identical. Furthermore, the rate constants predicted by RRKM/QET calculations are 10 4–10 5 times higher than the observed rate constants for both ions. The different decay rates probably result from different isomerization processes. One of the reaction channels is a stepwise McLafferty rearrangement involving a distonic intermediate, with charge retention on the olefin.

Energy partitioning in the dissociation reaction Ar3+→Ar2++Ar

Threshold-photoelectron photoion coincidence (TPEPICO) spectra of argon clusters ionized with synchrotron radiation have been observed, and the average kinetic energy of Ar2+, which originates from Ar3+, has been estimated. All the line shapes of the Ar2+ ion observed in the TPEPICO spectra have been appropriately simulated in terms of superposition of a sharp and a broad Gaussian function, where the sharp Gaussian function corresponds to ionization of the argon dimer Ar2. The observed line shapes agree very well with the simulated ones, implying that the excess energy in the Ar3 ionization is thermally or quasithermally partitioned to translational energy of the fragment species. From the average kinetic energies of Ar2+, it has been found that the percentage of excess energy in Ar3+, that is partitioned into the total translational energy of the fragment species, is dependent on the excitation energy. The percentage is slightly less than the value of 40% expected from statistical theory. It has also been found that complete dissociation of Ar3+ takes place in the excess energy range 0.3–1.1 eV.

On the dissociation dynamics of energy-selected dimethylether ions

Dissociation of the dimethylether ion has been studied using threshold photoelectron-photoion coincidence (TPEPICO) mass spectrometry. Our TPEPICO breakdown curves showed a significant occurrence of electronic autoionization at around 12.2 eV and over 16 eV. For the formation of [CH 3O] +, rectangular flight-time distributions owing to a large, sharp kinetic energy release were observed at high energies, and about 80% of internal excess energy of the molecular ion was found to be released as translational energy in this fragmentation. This remarkably high conversion was consistent with the spectator model of direct dissociation rather than with statistical theories (RRKM—QET). However, the relative abundance of [CH 3O +] remained less than 20% in the breakdown diagram although direct dissociation predicts its predoiminace. These results may be intepreted as a direct dissociation to CH 3O + on a repulsive state reached by transition from an initial state or as a statistical fragmentation to CH 2O + H strongly controlled by a tight transition exit on a high reverse activation barrier.

The metastable formation of di-ethylchloronium ions from ethylchloride dimer ions in a seeded molecular beam

Cluster ions of ethylchloride and their dissociation products have been produced in a supersonic expansion of ethylchloride seeded in Ar and energy selected by the threshold photoelectron photoion coincidence (TPEPICO) method. The peak widths of the ion time of flight distribution indicate that all of the clusters are produced by dissociative photoionization of higher order clusters. Thus, trimer ions dissociate to form dimer ions and an ethylchloride monomer. This dimer ion was found to be metastable with respect to the formation of the di-ethylchloronium ion and a chlorine atom. The measured dissociation rate as a function of the dimer ion internal energy was compared to the calculated rates based on the statistical RRKM/QET theory. Good agreement was found when the dimer adiabatic IP was assumed to be 10.2 eV. The Cl loss from the ethylchloride dimer ion is associated with a reverse activation energy of about 0.32 eV.

Metastable fragmentation of excited B 2H +6 ions

Threshold photoelectron photoion coincidence measurements and the peak shape analysis of an ion flight time distribution are reported showing metastable fragmentation of photoexcited B 2H 6 at 800 Å wavelength yielding the monomer ions BH + 3 (with rate constant k=10 μs −1), BH + 2 ( k=2 μs −1 and BH + ( k=0.6 μs −1).

Isotope effects in the dissociation of partially deuterated dimethyl ether, CH 3OCD +3 ions

The dissociation of partially deuterated dimethyl ether ions(CH 3OCD + 3) is studied from the ionization potential of 10.03–16eV photon energy. Ions are produced by photoionization with synchrotron radiation and selected in internal energy by threshold photoelectron—photoion coincidence. Up to 13eV the major fragments correspond to an H or D atom loss by a simple bond cleavage. However, the shift in the H and D loss onsets indicates that there is a small reverse activation energy as a result of the zero point energies in the CX deformation modes. A new value for Δ H° fo(CH 3OCH + 2) of 161.1 kcalmol −1 is derived. The CH 2OH + and CD 2OD + products appear at 11.75 eV, 0.95 eV above their thermochemical onset, and remain minor fragments in the whole energy range studied. No scrambling of the H and D atoms is observed. Modeling of the breakdown graph by RRKM theory indicates that the CX 2OX + (X  H or D) fragments are produced via a 1.35 eV barrier. We observe an isotope effect in both these dissociation channels which is well explained by zero point energy, and density of states differences in the framework of the RRKM calculations. The analysis shows that in the energy range investigated, tunneling is not an important process in the CH 2OH + formation. At 15 and 16 eV photon energies, the main fragments are XCO + and CX + 3 (X  H or D). Non-statistical scrambling of the hydrogen atoms in CX + 3 is observed.

Fragmentation of Energy-Selected Fluorobenzene Ion

Abstract Dissociation of the fluorobenzene ion has been studied using threshold photoelectron–photoion coincidence (TPEPICO) mass spectrometry. The detailed breakdown curves were measured over internal energies from 0 to 9 eV. Asymmetric flight time distributions due to metastable dissociation were observed for the formations of C6H5+ and C4H3F+, and their absolute decay rate constants were derived from their flight time distributions. The information-theoretical approach has been employed for the analysis of kinetic energy release distribution (KERD). The results of surprisal analysis demonstrated that experimental KERDs are in good agreement with the predictions based on the statistical theory of mass spectra (QET). The heat of formation of C4H3F·+ has been estimated to be 222±2 kcal mol−1.

Fragmentation of acetic acid ions with selected internal energies

The unimolecular dissociation of acetic acid ion in the photon energy range 10.5–17.0 eV was studied using threshold photoelectron photoion coincidence mass spectrometry. The detailed breakdown graph was obtained and the fragmentation pathways were elucidated. The breakdown graph calculated using statistical theories was found to be consistent with the experimental data up to a photon energy of about 12.5 eV. The average kinetic energy release observed is higher than that calculated on the basis of quasi-equilibrium theory for the formation of COOH + while it seems to be statistical for the formation of CH 3CO +. The origin of kinetic energy release accompanying the formation of these two ions is discussed. The structure of [COH 3] + ion ( m/z 31) is determined to be hydroxymethyl cation, CH 2OH +, which could be formed by a two-step rearrangement prior to dissociation.

Threshold photoelectron photoion coincidence study of the ethane loss from energy selected pentane ions cooled in a supersonic expansion

The loss of C2H6 from energy selected pentane ions has been investigated by the threshold protoelectron photoion coincidence method. Breakdown diagrams are reported at room temperature and for two different conditions in a molecular beam. From the observed shift in cross-over energies the 0 K onset for the C2H6 loss is determined to be 690 ± 20 meV. This result together with information about kinetic energy release and other thermochemical data supports the earlier conclusion that the corresponding ion in the C2H6 loss is the propene ion rather than the cyclopropane ion. It is shown that rotational and vibrational energies are both active in this dissociation. The rate constants of dissociation have been measured as a function of the ion internal energy and are shown to agree with the results of Rice-Rampsperger-Kassel-Marcus calculations. Evidence is given that isomerisation of pentane to methylbutane prior to dissociation does not play an important role.

TPEPICO studies near ionization threshold of argon and krypton clusters

Single photon ionization of argon and krypton clusters has been studied in the region between threshold and the ionization potential of the corresponding atom. Synchrotron radiation from the electron storage ring BESSY is used to ionize the clusters; threshold-photo-electron-photoion-coincidence (TPEPICO)-time-of-flight technique is used to detect ions correlated with the emission of zero-kinetic-energy-electrons. The spectra of the clusters in the range ofn=2 to 15 are discussed in view of the extensive fragmentation taking place in these systems. In order to characterize the properties of the clusters a method using scaling laws is applied. The principles and the deduction of Hagena's scaling parameter Γ* are briefly reviewed. Using Γ* an experimentally derived mean cluster size for molecular beams can be assigned. This allows one to clearly demonstrate the systematic variations of the measured spectra due to cluster fragmentation. As a general feature it is observed that, in the range studied, the peak in the measured ionization rate for a cluster ion (fragment) of a given size shifts to higher photon energies as the mean cluster size is increased.

Fragmentation dynamics of ammonia cluster ions after single photon ionisation

A reflecting time of flight mass spectrometer (RETOF) is used to study unimolecular and collision induced fragmentation of ammonia cluster ions. Synchrotron radiation from the BESSY electron storage ring is used in a range of photon energies from 9.08 up to 17.7 eV for single photon ionisation of neutral clusters in a supersonic beam. The threshold photoelectron photoion coincidence technique (TPEPICO) is used to define the energy initially deposited into the cluster ions. Metastable unimolecular decay (µs range) is studied using the RETOF's capacity for energy analysis. Under collision free conditions the by far most prominent metastable process is the evaporation of one neutral NH3 monomer from protonated clusters (NH3) n − 2NH 4 + . Abundance of homogeneous vs. protonated cluster ions and of metastable fragments are reported as a function of photon energy and cluster size up ton=10.