Threshold Photoelectron Photoion Coincidence Research Articles

The photoionization dynamics of the three structural isomers of dichloroethene

Using tunable vacuum-UV radiation from a synchrotron, the threshold photoelectron spectrum, threshold photoelectron photoion coincidence spectrum and ion breakdown diagram of the 1,1, cis-1,2 and trans-1,2 isomers of C2H2Cl2 have been recorded in the range 9–23 eV. The energies of the peaks in the threshold photoelectron spectrum are in good agreement with outer-valence Greens function calculations. The major difference between the isomers, both predicted and observed experimentally is that the and states of are approximately degenerate for 1,1 and trans-1,2, but well separated for the cis-1,2 isomer. The ground and low-lying valence states of are bound, with higher-lying states dissociating to C2H2Cl+ or . The translational kinetic energy release into C2H2Cl+ + Cl is determined as a function of energy. Isolated-state behaviour for the low-lying electronic states of becomes more statistical as the energy increases.

Modeling ionic unimolecular dissociations from a temperature controlled TPEPICO study on 1-C 4H 9I ions

We report on the development of a new temperature controlled inlet system for the study of gas phase unimolecular reaction dynamics using threshold photoelectron photoion coincidence (TPEPICO) spectroscopy. Temperatures in the range of 220–400 K can be achieved, with a deviation of less than 5 K over the course of a 48 h experiment. Iodine loss from energy selected 1-butyl iodide (1-C 4H 9I) ions was studied at four temperatures, 220, 275, 298 and 400 K. The fractional ion abundances, in the form of breakdown diagrams are presented. Particular attention is paid to the slopes of the molecular and daughter ion abundances in crossover region of the breakdown curve since they are governed solely by the internal energy distribution, P( E), of the neutral precursor. P( E) is a function of both the temperature and vibrational frequencies of the neutral precursor. A detailed discussion regarding the transposition of the neutral thermal energy distribution to the ionic manifold is presented. From the four experimental measurements, the E 0 for the production of the 2-C 4H 9 + ion was determined to be 9.738 ± 0.015 eV.

Selected ion flow tube cation–molecule reaction studies and threshold photoelectron photoion coincidence spectroscopy of cyclic-C5F8

The product ion branching ratios and rate coefficients have been measured using a selected ion flow tube (SIFT) at 298 K for the bimolecular reactions of cyclic-C5F8 with several atomic and molecular cations. The majority of reactions occur at the collisional rate calculated by the modified average dipole orientation theory, with the exception of H2O+ for which the reaction efficiency is only 55%. Apart from H2O+ and N+, the similarity of the product ion branching ratios determined from threshold photoelectron photoion coincidence (TPEPICO) and ion-molecule data suggests that long-range electron transfer is the dominant mechanism for reactions involving ions with recombination energies between 12 and 17 eV. For N+, the product ion branching ratios are very different to those produced by photoionisation; this result may be explained if some of the N-atom products are formed electronically excited. The onset of an ionisation signal of c-C5F8 measured by TPEPICO spectroscopy occurs at 12.25 +/- 0.05 eV. This is much higher than the value of the first adiabatic ionisation energy determined from electron ionisation (11.24 +/- 0.10 eV), He (I) photoionisation (11.30 +/- 0.05 eV), and an independent high resolution threshold photoelectron spectrum (11.237 +/- 0.002 eV). The ground electronic state of c-C5F8+ has very weak intensity under threshold electron conditions. The TPEPICO spectrum of c-C5F8 recorded from 12-23 eV shows detection of the parent ion and the daughter ions C4F6+ and C5F7+, with their appearance energies increasing in this order. Ion yield curves and branching ratios have been determined. Using Gaussian 03, the enthalpy of formation of c-C5F8 at 298 K has been determined to be -1495 kJ mol(-1).

Dissociation Dynamics of Sequential Ionic Reactions: Heats of Formation of Tri-, Di-, and Monoethylphosphine

The sequential ethene (C2H4) loss channels of energy-selected ethylphosphine ions have been studied using threshold photoelectron photoion coincidence (TPEPICO) spectroscopy in which ion time-of-flight (TOF) distributions are recorded as a function of the photon energy. The ion TOF distributions and breakdown diagrams have been modeled in terms of the statistical RRKM theory for unimolecular reactions, providing 0 K dissociation onsets, E0, for the ethene loss channels. Three RRKM curves were used to model the five measurements, since two of the reactions differ only by the internal energy of the parent ion. This series of dissociations provides a detailed check of the calculation of the product energy distribution for sequential reactions. From the determined E0's, the heats of formation of several ethylphosphine neutrals and ions have been determined: Delta(f)H degrees 298K[P(C(2)H(5))3] = -152.7 +/- 2.8 kJ/mol, Delta(f)H degrees 298K[P(C(2)H(5))3+] = 571.6 +/- 4.0 kJ/mol, Delta(f)H degrees 298K[HP(C(2)H(5))2] = -89.6 +/- 2.1 kJ/mol, Delta(f)H degrees 298K[HP(C(2)H(5))2+] = 669.9 +/- 2.5 kJ/mol, Delta(f)H degrees 298K[H(2)PC(2)H(5)] = -36.5 +/- 1.5 kJ/mol, Delta(f)H degrees 298K[H(2)PC(2)H(5)+] = 784.0 +/- 1.9 kJ/mol. These values have been supported by G2 and G3 calculations using isodesmic reactions. Coupled cluster calculations have been used to show that the C2H4 loss channel, which involves a hydrogen transfer step, proceeds without a reverse energy barrier.

Photoion Photoelectron Coincidence Spectroscopy of Primary Amines RCH2NH2 (R = H, CH3, C2H5, C3H7, i-C3H7): Alkylamine and Alkyl Radical Heats of Formation by Isodesmic Reaction Networks

Alkylamines (RCH(2)NH(2), R = H, CH(3), C(2)H(5), C(3)H(7), i-C(3)H(7)) have been investigated by dissociative photoionization by threshold photoelectron photoion coincidence spectroscopy (TPEPICO). The 0 K dissociation limits (9.754 +/- 0.008, 9.721 +/- 0.008, 9.702 +/- 0.012, and 9.668 +/- 0.012 eV for R = CH(3), C(2)H(5), C(3)H(7), i-C(3)H(7), respectively) have been determined by preparing energy-selected ions and collecting the fractional abundances of parent and daughter ions. All alkylamine cations produce the methylenimmonium ion, CH(2)NH(2)+, and the corresponding alkyl free radical. Two isodesmic reaction networks have also been constructed. The first one consists of the alkylamine parent molecules, and the other of the alkyl radical photofragments. Reaction heats within the isodesmic networks have been calculated at the CBS-APNO and W1U levels of theory. The two networks are connected by the TPEPICO dissociation energies. The heats of formation of the amines and the alkyl free radicals are then obtained by a modified least-squares fit to minimize the discrepancy between the TPEPICO and the ab initio values. The analysis of the fit reveals that the previous experimental heats of formation are largely accurate, but certain revisions are suggested. Thus, Delta(f)Ho(298K)[CH(3)NH(2)(g)] = -21.8 +/- 1.5 kJ mol-1, Delta(f)Ho(298K)[C(2)H(5)NH(2)(g)] = -50.1 +/- 1.5 kJ mol(-1), Delta(f)Ho(298K)[C(3)H(7)NH(2)(g)] = -70.8 +/- 1.5 kJ mol(-1), Delta(f)Ho(298K)[C(3)H(7)*] = 101.3 +/- 1 kJ mol(-1), and Delta(f)Ho(298K)[i-C(3)H(7)*] = 88.5 +/- 1 kJ mol(-1). The TPEPICO and the ab initio results for butylamine do not agree within 1 kJ mol-1; therefore, no new heat of formation is proposed for butylamine. It is nevertheless indicated that the previous experimental heats of formation of methylamine, propylamine, butylamine, and isobutylamine may have been systematically underestimated. On the other hand, the error in the ethyl radical heat of formation is found to be overestimated and can be decreased to +/- 1 kJ mol(-1); thus, Delta(f)Ho(298K)[C(2)H(5).] = 120.7 +/- 1 kJ mol(-1). On the basis of the data analysis, the heat of formation of the methylenimmonium ion is confirmed to be Delta(f)Ho(298K)[CH(2)NH(2)+] = 750.3 +/- 1 kJ mol(-1).

Manganese−Chalcocarbonyl Bond Strengths from Threshold Photoelectron Photoion Coincidence Spectroscopy

The dissociation kinetics of CpMn(CO)2CX (X = S, Se) complexes have been investigated by threshold photoelectron photoion coincidence (TPEPICO) spectroscopy. Energy-selected CpMn(CO)2CX+ ions sequentially lose the two carbonyl and chalcocarbonyl ligands upon photoionization. The resulting asymmetric time-of-flight peak shapes and breakdown diagrams were simulated, and the following 0 K appearance energies were determined for the fragment ions: CpMn(CO)CS+, 8.79 ± 0.05 eV; CpMnCS+, 9.42 ± 0.05 eV; CpMn+, 11.53 ± 0.05 eV; CpMn(CO)CSe+, 8.70 ± 0.05 eV; CpMnCSe+, 9.39 ± 0.05 eV; and CpMn+, 12.05 ± 0.05 eV. These values, combined with the adiabatic ionization energies, yield Mn−CO bond energies in CpMn(CO)2CX+ (X = S, Se) and CpMn(CO)CX+ (1.20 ± 0.06, 0.63 ± 0.07 eV and 1.08 ± 0.06, 0.72 ± 0.07 eV, respectively) and Mn−CX bond energies in CpMnCX+ (X = S, Se, 2.12 ± 0.07 and 2.66 ± 0.07 eV, respectively).The 0 K heats of formation of the neutral and ionic species were determined using the heats of formation of...

Vacuum-UV and electron dissociative ionisation studies of SF 5Cl

A complementary study of the interaction of SF 5Cl in the gas phase with vacuum-UV photons and low-energy electrons from the onset of ionisation, ca. 12 eV, up to 20 eV is presented. The photon-induced experiments have used tunable vacuum-UV radiation from a synchrotron and threshold photoelectron photoion coincidence spectroscopy for product ion detection, the electron-induced experiments a trochoidal electron monochromator and a quadrupole mass spectrometer. The strengths and limitations of both experiments are contrasted, the main difference being the absence of state selectivity in the electron-induced study. The parent cation is not observed in either study, suggesting that its ground electronic state is repulsive following Franck–Condon vertical excitation. The fragment cations SF 5 +, SF 4Cl +, SF 4 + and SF 3 + have been observed in both studies, with reasonable agreement in their threshold appearance energies. Using a variant of threshold photoelectron photoion coincidence spectroscopy applicable when the ground state of the parent cation is repulsive, the first dissociative ionisation energy of SF 5Cl is determined to be 12.3 ± 0.2 eV, leading to a value for the adiabatic ionisation energy for the SF 5 radical of 9.92 ± 0.28 eV. The electron-induced experiment is sensitive to ion-pair production, and onsets for F + and Cl + production have been observed which are only possible energetically if the accompanying fragments are the anions SF 4Cl − and SF 5 −, respectively. A lower limit for the electron affinity of the SF 4Cl radical of 4.88 eV is determined, a value confirmed by ab initio calculations. The electron-induced experiment is very sensitive to gas impurities, and the effects of minute quantities of SF 4, FCl, Cl 2 and possibly SF 2 in the gas sample are observed.

A Photoelectron and TPEPICO Investigation of the Acetone Radical Cation

The valence shell photoelectron spectrum, threshold photoelectron spectrum, and threshold photoelectron photoion coincidence (TPEPICO) mass spectra of acetone have been measured using synchrotron radiation. New vibrational progressions have been observed and assigned in the X 2B2 state photoelectron bands of acetone-h6 and acetone-d6, and the influence of resonant autoionization on the threshold electron yield has been investigated. The dissociation thresholds for fragment ions up to 31 eV have been measured and compared to previous values. In addition, kinetic modeling of the threshold region for CH3* and CH4 loss leads to new values of 78 +/- 2 kJ mol(-1) and 75 +/- 2 kJ mol(-1), respectively, for the 0 K activation energies for these two processes. The result for the methyl loss channel is in reasonable agreement with, but slightly lower than, that of 83 +/- 1 kJ mol(-1) derived in a recent TPEPICO study by Fogleman et al. The modeling accounts for both low-energy dissociation channels at two different ion residence times in the mass spectrometer. Moreover, the effects of the ro-vibrational population distribution, the electron transmission efficiency, and the monochromator band-pass are included. The present activation energies yield a Delta(f)H298 for CH3CO+ of 655 +/- 3 kJ mol(-1), which is 4 kJ mol(-1) lower than that reported by Fogleman et al. The present Delta(f)H298 for CH3CO+ can be combined with the Delta(f)H298 for CH2CO (-47.5 +/- 1.6 kJ mol(-1)) and H+ (1530 kJ mol(-1)) to yield a 298 K proton affinity for ketene of 828 +/- 4 kJ mol(-1), in good agreement with the value (825 kJ mol(-1)) calculated at the G2 level of theory. The measured activation energy for CH4 loss leads to a Delta(f)H298 (CH2CO+*) of 873 +/- 3 kJ mol(-1).

Inner-valence states of N2+ and the dissociation dynamics studied by threshold photoelectron spectroscopy and configuration interaction calculation

The N2(+) states lying in the ionization region of 26-45 eV and the dissociation dynamics are investigated by high-resolution threshold photoelectron spectroscopy and threshold photoelectron-photoion coincidence spectroscopy. The threshold photoelectron spectrum exhibits several broad bands as well as sharp peaks. The band features are assigned to the N2(+) states associated with the removal of an inner-valence electron, by a comparison with a configuration interaction calculation. In contrast, most of the sharp peaks on the threshold photoelectron spectrum are allocated to ionic Rydberg states converging to N2(2+). Dissociation products formed from the inner-valence N2(+) states are determined by threshold photoelectron-photoion coincidence spectroscopy. The dissociation dynamics of the inner-valence ionic states is discussed with reference to the potential energy curves calculated.

Isomeric Effects in the Gas-Phase Reactions of Dichloroethene, C2H2Cl2, with a Series of Cations

A study of the reactions of a series of gas-phase cations (NH(4)(+), H(3)O(+), SF(3)(+), CF(3)(+), CF(+), SF(5)(+), SF(2)(+), SF(+), CF(2)(+), SF(4)(+), O(2)(+), Xe(+), N(2)O(+), CO(2)(+), Kr(+), CO(+), N(+), N(2)(+), Ar(+), F(+), and Ne(+)) with the three structural isomers of dichloroethene, i.e., 1,1-C(2)H(2)Cl(2), cis-1,2-C(2)H(2)Cl(2), and trans-1,2-C(2)H(2)Cl(2) is reported. The recombination energy (RE) of these ions spans the range of 4.7-21.6 eV. Reaction rate coefficients and product branching ratios have been measured at 298 K in a selected ion flow tube (SIFT). Collisional rate coefficients are calculated by modified average dipole orientation (MADO) theory and compared with experimental data. Thermochemistry and mass balance have been used to predict the most feasible neutral products. Threshold photoelectron-photoion coincidence spectra have also been obtained for the three isomers of C(2)H(2)Cl(2) with photon energies in the range of 10-23 eV. The fragment ion branching ratios have been compared with those of the flow tube study to determine the importance of long-range charge transfer. A strong influence of the isomeric structure of dichloroethene on the products of ion-molecule reactions has been observed for H(3)O(+), CF(3)(+), and CF(+). For 1,1-C(2)H(2)Cl(2) the reaction with H(3)O(+) proceeds at the collisional rate with the only ionic product being 1,1-C(2)H(2)Cl(2)H(+). However, the same reaction yields two more ionic products in the case of cis-1,2- and trans-1,2-C(2)H(2)Cl(2), but only proceeds with 14% and 18% efficiency, respectively. The CF(3)(+) reaction proceeds with 56-80% efficiency, the only ionic product for 1,1-C(2)H(2)Cl(2) being C(2)H(2)Cl(+) formed via Cl(-) abstraction, whereas the only ionic product for both 1,2-isomers is CHCl(2)(+) corresponding to a breaking of the C=C double bond. Less profound isomeric effects, but still resulting in different products for 1,1- and 1,2-C(2)H(2)Cl(2) isomers, have been found in the reactions of SF(+), CO(2)(+), CO(+), N(2)(+), and Ar(+). Although these five ions have REs above the ionization energy (IE) of any of the C(2)H(2)Cl(2) isomers, and hence the threshold for long-range charge transfer, the results suggest that the formation of a collision complex at short range between these ions and C(2)H(2)Cl(2) is responsible for the observed effects.

Dissociation of energy-selected c-C 3H 6S + studied with threshold photoelectron-photoion coincidence experiments and calculations

The dissociation of energy-selected c-C 3H 6S + was investigated in a region 10.2–10.9 eV with a threshold photoelectron-photoion coincidence technique. Branching ratios and average releases of kinetic energy for channel c-C 3H 6S + → H 2CS + + C 2H 4 were derived from coincidence mass spectra. The measured small releases of kinetic energy near the appearance onset agree with statistical calculations; a linearly extrapolated threshold at 10.39 ± 0.01 eV agrees with a predicted energy 10.35 eV with the G aussian-3 method. We discuss plausible mechanisms for c-C 3H 6S + dissociating to CH 3CS + + CH 3 based on G3B3 calculations to rationalize the absence of CH 3CS + signal in these experiments.

Dissociation dynamics and thermochemistry of chloroform and tetrachloroethane molecules studied by threshold photoelectron photoion coincidence

Threshold photoelectron photoion coincidence (TPEPICO) technique has been employed to determine accurate dissociative photoionization onsets for chloroform and 1,1,2,2-tetrachloroethane molecules. Our 0 K onsets for these reactions are CHCl 3 + hv → CHCl 2 + + Cl (11.500 ± 0.007 eV); C 2H 2Cl 4 + hv → C 2H 2Cl 3 + + Cl (11.565 ± 0.010 eV) and C 2H 2Cl 4 + hv → CHCl 2 + + CHCl 2 (11.669 ± 0.030 eV). The well established heat of formation of CHCl 3 was used as an anchor and the measured dissociation energies were used to determine the heats of formation, Δ f H 298 K o in kJ/mol, of the following species: CHCl 2 + (888.9 ± 1.3), CHCl 2 (87.1 ± 1.6), C 2H 2Cl 4 (−152.7 ± 3.5), and C 2H 2Cl 4 + (918.3 ± 5.9). Our results also made possible the determination of reliable 298 K bond enthalpies for: CHCl 2 Cl (311.1 ± 2.0 kJ/mol), CHCl 2 H (400.6 ± 2.0 kJ/mol), and CHCl 2 CHCl 2 (326.9 ± 4.1 kJ/mol).

Thermochemistry and Dissociative Photoionization of Si(CH3)4, BrSi(CH3)3, ISi(CH3)3, and Si2(CH3)6Studied by Threshold Photoelectron−Photoion Coincidence Spectroscopy

Threshold photoelectron-photoion coincidence spectroscopy (TPEPICO) has been used to study the dissociation kinetics and thermochemistry of Me(4)Si, Me(6)Si(2), and Me(3)SiX, (X = Br, I) molecules. Accurate 0 K dissociative photoionization onsets for these species have been measured from the breakdown diagram and the ion time-of-flight distribution, both of them analyzed and simulated in terms of the statistical RRKM theory and DFT calculations. The average enthalpy of formation of trimethylsilyl ion, Delta fH(o)298K(Me(3)Si(+)) = 617.3 +/- 2.3 kJ/mol, has been determined from the measured onsets for methyl loss (10.243 +/- 0.010 eV) from Me(4)Si, and Br and I loss from Me(3)SiBr (10.624 +/- 0.010 eV) and Me(3)SiI (9.773 +/- 0.015 eV), respectively. The methyl loss onsets for the trimethyl halo silanes lead to Delta fH(o)298K(Me(2)SiBr(+)) = 590.3 +/- 4.4 kJ/mol and Delta fH(o)298K(Me(5)Si(2)(+)) = 487.6 +/- 6.2 kJ/mol. The dissociative photoionization of Me(3)SiSiMe(3) proceeds by a very slow Si-Si bond breaking step, whose rate constants were measured as a function of the ion internal energy. Extrapolation of this rate constant to the dissociation limit leads to the 0 K dissociation onset (9.670 +/- 0.030 eV). This onset, along with the previously determined trimethylsilyl ion energy, leads to an enthalpy of formation of the trimethylsilyl radical, Delta fH(o)298K(Me(3)Si(*)) = 14.0 +/- 6.6 kJ/mol. In combination with other experimental values, we propose a more accurate average value for Delta fH(o)298K(Me(3)Si(*)) of 14.8 +/- 2.0 kJ/mol. Finally, the bond dissociation enthalpies (DeltaH(298K)) Si-H, Si-C, Si-X (X=Cl, Br, I) and Si-Si are derived and discussed in this study.

Threshold photoelectron photoion coincidence spectroscopy and selected ion flow tube reactions of CHF3: comparison of product branching ratios§

The threshold photoelectron and threshold photoelectron photoion coincidence spectra of CHF3 in the range 13.5–24.5 eV have been recorded. Ion yields and branching ratios have been determined for the three fragments , and CF+. The mean kinetic energy releases into fragment ions involving either C–H or C–F bond cleavage have been measured, and compared with statistical and impulsive models. behaves in a non-statistical manner characteristic of the small-molecule limit, with the ground electronic state and low-lying excited states of being largely repulsive along the C–H and C–F coordinates, respectively. The rate coefficients and product ion branching ratios have been measured at 298 K in a selected ion flow tube for the reactions of CHF3 with a large number of gas-phase cations whose recombination energies span the range 6.3 through 21.6 eV. A comparison between the branching ratios from the two experiments, together with an analysis of the threshold photoelectron spectrum of CHF3, shows that long-range charge transfer probably occurs for the Ar+ and F+ atomic ions whose recombination energies lie above ca. 15 eV. Below this energy, the mechanism involves a combination of short-range charge transfer and chemical reactions involving a transition state intermediate. §In honour of Professor John Simons’ 70th birthday Festschrift.

Threshold photoelectron photoion coincidence spectroscopy and selected ion flow tube cation-molecule reaction studies of cyclic-C4F8

Using tunable vacuum-UV radiation from a synchrotron, the threshold photoelectron and threshold photoelectron photoion coincidence (TPEPICO) spectra of cyclic-C4F8 in the range 11-25 eV have been recorded. The parent ion is observed very weakly at threshold, 11.60 eV, and is most likely to have cyclic geometry. Ion yield curves and branching ratios have been determined for five fragments. Above threshold, the first ion observed is C3F5+, at slightly higher energy C2F4+, then successively CF+, CF2+ and CF3+ are formed. The dominant ions are C3F5+ and C2F4+, with the data suggesting the presence of a barrier in the exit channel to production of C3F5+ whilst no barrier to production of C2F4+. In complementary experiments, the product branching ratios and rate coefficients have been measured in a selected ion flow tube (SIFT) at 298 K for the bimolecular reactions of cyclic-C4F8 with a large number of atomic and small molecular cations. Below the energy where charge transfer becomes energetically allowed, only one of the ions, CF2+, reacts. Above this energy, all but one of the remaining ions react. Experimental rate coefficients are consistently greater than the collisional values calculated from modified average dipole orientation theory. The inclusion of an additional ion-quadrupole interaction has allowed better agreement to be achieved. With the exception of N+, a comparison of the fragment ion branching ratios from the TPEPICO and SIFT data suggest that long-range charge transfer is the dominate mechanism for reactions of ions with recombination energy between 12.9 and 15.8 eV. For all other ions, either short-range charge transfer or a chemical reaction, involving cleavage and making of new bond(s), is the dominant mechanism.

Dissociative photoionization of mono-, di- and trimethylamine studied by a combined threshold photoelectron photoion coincidence spectroscopy and computational approach

Energy selected mono-, di- and trimethylamine ions were prepared by threshold photoelectron photoion coincidence spectroscopy (TPEPICO). Below 13 eV, the main dissociative photoionization path of these molecules is hydrogen atom loss. The ion time-of-flight (TOF) distributions and breakdown diagrams for H loss are analyzed in terms of the statistical RRKM theory, which includes tunneling. Experimental evidence, supported by quantum chemical calculations, indicates that the reverse barrier along the H loss potential energy curve for monomethylamine is 1.8 +/- 0.6 kJ mol(-1). Accurate dissociation onset energies are derived from the TOF simulation, and from this analysis we conclude that Delta(f)H degrees (298K)[CH(2)NH(2)(+)] = 750.4 +/- 1.3 kJ mol(-1) and Delta(f)H degrees (298K)[CH(2)NH(CH(3))(+)] = 710.9 +/- 2.8 kJ mol(-1). Quantum chemical calculations at the G3, G3B3, CBS-APNO and W1U levels are extensively used to support the experimental data. The comparison between experimental and ab initio isodesmic reaction heats also suggests that Delta(f)H degrees (298K)[N(CH(3))(3)] = -27.2 +/- 2 kJ mol(-1), and that the dimethylamine ionization energy is 8.32 +/- 0.03 eV, both of which are in slight disagreement with previous experimental values. Above 13 eV photon energy, additional dissociation channels appear besides the H atom loss, such as a sequential C(2)H(4) loss from trimethylamine for which a dissociation mechanism is proposed.

On the dissociation of the 2-pentanone ion studied by threshold photoelectron photoion coincidence spectroscopy

The photodissociation of 2-pentanone has been studied by threshold photoelectron photoion coincidence (TPEPICO) spectroscopy, in which ion time-of-flight (TOF) mass spectra are recorded as a function of the photon energy in the range of 9.6–12.2 eV. 2-Pentanone ions dissociate via four competitive channels: ethylene (C 2H 4) loss to produce the propen-2-ol ion, n-propyl ( C 3H 7) loss to produce the acetyl ion, and two parallel methyl ( CH 3) loss channels, producing the butanoyl ion at low energies and the but-3-en-2-ol ion at higher energies. The latter dissociates further via ethylene (C 2H 4) loss providing a second pathway to the acetyl ion. This final dissociation channel is observed experimentally by the appearance of an asymmetric ion peak in the time-of-flight (TOF) distribution at photon energies greater than 11.5 eV. The ion TOF distributions and breakdown diagram have been modeled in terms of the statistical RRKM theory for unimolecular reactions, yielding the 0 K dissociation onsets of 10.239 ± 0.015 eV for the butanoyl ion and 10.259 ± 0.019 eV for the propen-2-ol (acetone enolate) ion. By relating the measured onsets with other well established heats of formation, the 298 K heat of formation of the butanoyl and propen-2-ol ions were determined to be 586.9 ± 2.1 and 680.7 ± 1.8 kJ/mol, respectively. The acetone enolate ion is thus 37 kJ/mol more stable than the acetone ion, a value supported by G3B3, and CBS-QB3 calculations. The but-3-en-2-ol ion was found to lose ethylene to produce the acetyl ion without an energy barrier.

A Photoelectron Photoion Coincidence Study of the Vinyl Bromide and Tribromoethane Ion Dissociation Dynamics: Heats of Formation of C2H3+, C2H3Br, C2H3Br+, C2H3Br2+, and C2H3Br3

The threshold photoelectron photoion coincidence (TPEPICO) technique has been used to measure accurate dissociative photoionization onsets of vinyl bromide and 1,1,2-tribromoethane. The reactions investigated and their 0 K onsets are C2H3Br + hnu --> C2H3+ + Br (11.902 +/- 0.008 eV); C2H3Br3 + hnu --> C2H3Br2+ + Br (10.608 +/- 0.008 eV); and (C2H3Br3 + hnu --> C2H3Br+ + 2Br (12.301 +/- 0.035 eV). The vinyl ion heat of formation (Delta(f)H degrees 298K = 1116.1 +/- 3.0 kJ/mol) has been calculated using W1 theory and used as an anchor along with the measured dissociation energies to determine the heats of formation, Delta(f)H degrees 298K, in kJ/mol, of the following bromine-containing species: C2H3Br (74.1 +/- 3.1), C2H3Br+ (1021.9 +/- 3.1), C2H3Br2+ (967.1 +/- 4.0), and C2H3Br3 (53.5 +/- 4.3). These results represent accurate and consistent experimental determinations of heats of formation for these bromine-containing species, which serve to correct the discrepancies in the literature for C2H3Br and C2H3Br+ and provide the first experimental determination for the enthalpies of formation of C2H3Br2+ and C2H3Br3.

Dissociation processes of Kr2+ and Kr3+ studied by threshold photoelectron-photoion coincidence measurements

A time-of-flight (TOF) ion mass spectrum in coincidence with threshold photoelectrons was measured in the photon energy region between the first and second dissociation limits of Kr2(+) to examine the decay processes of the Kr2(+) II(1/2u) state. The measured TOF spectrum reveals that Kr+ fragment ions are produced through dissociation of the repulsive I(1/2g) state, which can be formed by the decay process of the II(1/2u) state accompanied with emission of photons. The potential-energy curve of the I(1/2g) state is deduced with detailed analysis of the observed TOF spectrum, in which the radiative lifetime of the II(1/2u) state was also derived to be 2.5 micros. Additionally, evidence of the dissociation process of Kr3(+) ions was obtained in the same photon energy region, where the dominant channel is Kr3(+) --> Kr2(+) + Kr.

Ionization energies of argon clusters: A combined experimental and theoretical study

We have measured appearance energies of Ar(n)+, n<or=30, by electron impact of gas phase clusters. Quantum-chemical calculations have been performed to determine the adiabatic and vertical ionization energies of argon clusters up to n=4 and 6, respectively. The experimental appearance energy of the dimer ion approaches, under suitable cluster source conditions, the adiabatic ionization energy. The agreement with values obtained by photoionization and threshold photoelectron-photoion coincidence (TPEPICO) spectra demonstrates that autoionizing Rydberg states are accessible by electron impact. Appearance energies of larger clusters, though, exceed the TPEPICO values by about 0.5 eV.