Threshold Photoelectron Photoion Coincidence Spectroscopy Research Articles

The use of threshold photoelectron–photoion coincidence spectroscopy to probe the spectroscopic and dynamic properties of the valence states of CCl3F+, CCl3H+ and CCl3Br+

Using tunable vacuum-ultraviolet radiation from a synchrotron source in the range 10–25 eV, threshold photoelectron–photoion coincidence (TPEPICO) spectroscopy has been used to determine the decay pathways of the valence electronic states of CCl3X+ (X=F, H, Br). TPEPICO spectra are recorded continuously as a function of photon energy, allowing threshold photoelectron spectra and yields of the fragment ions to be obtained. At fixed photon energies, spectra are also recorded with improved time resolution, allowing total mean translational kinetic energy releases, 〈KE〉t , into some dissociation channels to be determined. By comparing 〈KE〉t values for single bond-fission processes (i.e. cleavage of a C–Cl or C–X bond) with those predicted for the limiting extremes of a statistical and an impulsive dissociation, information on the nature of the photodissociation dynamics can be inferred. Excited states of all three parent cations show evidence for isolated-state behaviour, and the 〈KE〉t values suggest a relationship between the part of the molecule where ionisation occurs and the bond that breaks to form daughter ion plus neutral atom products; impulsive values of 〈KE〉t are more likely to be obtained when the breaking bond lies close to the part of the molecule where ionisation occurs, statistical values when ionisation occurs further away from the breaking bond. At higher photon energies, smaller fragment ions are formed following cleavage of more than one bond. With CCl3F and CCl3Br, the appearance energies of the daughter ions are close to the thermochemical energy for production of that ion with isolated neutral atoms, suggesting strongly that these ions form by bond-fission processes only. With CCl3H, at certain energies some fragment ions can only form with molecular neutral fragments (e.g. CCl2++HCl), involving bond-breaking and bond-making processes. It is suggested that this phenomenon is related to the small size of the hydrogen atom, and hence less steric hindrance in a tightly constrained transition state along the reaction coordinate.

Fragmentation of the valence electronic states of NF 3+ studied by threshold photoelectron–photoion coincidence spectroscopy

The valence threshold photoelectron spectrum of NF 3 is reported for the first time in the literature, and threshold photoelectron–photoion coincidence (TPEPICO) spectroscopy has measured, state-selectively, the decay dynamics of the valence states of NF 3 + in the range 13–23 eV. Vacuum–UV radiation from the Daresbury synchrotron source dispersed by a 1 m Seya-Namioka monochromator photoionises the parent molecules. 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 coincidence ion yields of the fragment ions and the breakdown diagram to be obtained. A comparison of the integrated threshold photoelectron and the total ion signals as a function of energy suggests that, in the range 16–19 eV, autoionisation via Rydberg states of NF 3 makes a significant contribution to the production of threshold electrons. The 50% crossover energy for production of NF 2 + from NF 3 + is determined to be 14.10±0.05 eV. The first onsets for NF 2 + and NF + production are 13.95±0.05 and 17.6±0.1 eV, respectively. The majority of the Franck–Condon region of the X ̃ 2 A 1 ground state of NF 3 + is stable with respect to dissociation to NF 2 +, whereas the unresolved A ̃ 2 E / B ̃ 2 A 2 states and most of the C ̃ 2 E state dissociate exclusively to NF 2 +. The D ̃ 2 A 1 and E ̃ 2 E states dissociate to NF +. Translational kinetic energy releases have been measured in NF 2 + and NF + at the energies of the Franck–Condon maxima of the valence states of NF 3 +. The results are compared with models assuming statistical and impulsive dissociation. The Ã/B̃ states of NF 3 + dissociate directly from the excited-state potential energy surface to NF 2 +, whereas the higher-lying C̃ state probably dissociates off the ground-state surface following rapid internal conversion. It is not possible to correlate unambiguously the formation of NF + with either F 2 or 2F, although on energetic grounds the latter products are more likely. Assuming that the neutral products are 2F, no information is obtained whether the two N–F bonds break simultaneously or sequentially.

Ethene loss kinetics of methyl butanoate ions studied by threshold photoelectron–photoion coincidence:: The enol ion of methyl acetate heat of formation

Threshold photoelectron–photoion coincidence spectroscopy has been used to investigate the unimolecular chemistry of gas-phase methyl butanoate ions (CH 3CH 2CH 2COOCH 3 ·+). This ester ion isomerizes to a lower energy distonic ion (CH 2CH 2CH 2COHOCH 3 ·+) prior to dissociating by the loss of C 2H 4. The asymmetric time of flight distributions, which arise from the slow rate of dissociation at low ion energies, provide information about the ion dissociation rates. By modeling these rates with assumed k( E) functions, the thermal energy distribution for room temperature and molecular beam samples, and the analyzer function for threshold electrons, it was possible to extract the dissociative photoionization limit for methyl butanoate which at 0 K is 10.275 ± 0.010 eV as well as the dissociation barrier of the distonic ion of 0.735 ± 0.010 eV. By combining these with an estimated heat of formation of methyl butanoate, we derive a 0 K heat of formation of the distonic ion CH 2CH 2CH 2COHOCH 3 ·+ of 119.0 ± 2.0 kcal/mol. The product ion is the enol of methyl acetate, CH 2COHOCH 3 ·+, which has a derived heat of formation at 0 K of 122.0 ± 2.0 kcal/mol. This energy agrees well with a derived energy obtained recently from the rate analysis of energy selected methyl acetate ions. Also measured was the adiabatic ionization energy of the methyl butanoate, 9.95 ± 0.05 eV.

Theoretical and experimental studies of unimolecular dissociation of phosphorus tribromide ions

The heat of formation of PBr 2 +, 842.9±1.5 kJ/mol at 0 K, has been determined by the dissociative photoionization of the phosphorus tribromide by threshold photoelectron photoion coincidence spectroscopy (TPEPICO) technique. Also measured was the adiabatic ionization energy of the phosphorus tribromide, 9.35±0.02 eV, and the new value of the 0 K heat of formation of PBr 3 ·+ ion was determined, 787±2 kJ/mol. The performance of G2 and density functional theory calculations with four functionals, B3PW91, B3P86, B3LYP, and BLYP in characterizing the ionization energy of PBr 3 and the appearance energy of the fragment PBr 2 + ion was examined.

Threshold photoelectron spectroscopy of PX3 (X=Cl, Br), and fragmentation of the valence electronic states of PX3+ studied by TPEPICO spectroscopy

The valence threshold photoelectron spectra (TPES) of phosphorous trichloride (PCl3) and phosphorous tribromide (PBr3) are presented for the first time. Threshold photoelectron–photoion coincidence (TPEPICO) spectroscopy has also been used to measure, state selectively, the decay pathways of the valence states of PX3+ (X=Cl, Br) in the range 10–25 eV. Vacuum-UV radiation from the Daresbury synchrotron source dispersed by a 1 m Seya-Namioka monochromator photoionises the parent molecules, electrons and ions being detected by threshold electron analysis and time-of-flight mass spectrometry, respectively. TPEPICO spectra are recorded continuously as a function of photon energy, allowing coincidence ion yields of the fragment ions to be obtained. The ground and first excited states of both molecular parent ions are stable with respect to dissociation to PX2+, whereas the B2E, C2E, D2A1 and E2E states dissociate exclusively to PX2+. The first experimental observation of the F2A1 state of PBr3+ is presented, which we deduce to have a very small cross-section to photoionisation. Fragmentation of the F2A1 states of both parent ions leads to exclusive production of PX+. Kinetic-energy (KE) releases into PX2++X have been measured at the Franck–Condon maxima of the B2E, C2E, D2A1 and E2E states of PCl3+ and the B2E, C2E and E2E states of PBr3+. For the first time, the method used to calculate the KE releases allows for the full range of daughter ion isotopes to be accommodated.

Isomerization and dissociation in competition: the two-component dissociation rates of methyl acetate ions

Threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy has been used to investigate the unimolecular chemistry of metastable methyl acetate ions, CH 3COOCH 3 .+. The rate of molecular ion fragmentation with the loss of CH 3O . and CH 2OH radicals as a function of ion internal energy was obtained from the coincidence data and used in conjunction with Rice-Ramsperger-Kassel-Markus and ab initio molecular orbital calculations to model the dissociation/isomerization mechanism of the methyl acetate ion (A). The data were found to be consistent with the mechanism involving a hydrogen-bridged complex CH 3CO···H···OCH 2 .+(E) as the direct precursor of the observed fragments CH 3CO + and CH 2OH .. The two-component decay rates were modeled with a three-well-two-product potential energy surface including the distonic ion CH 3C(OH)OCH 2 .+(B) and enol isomer CH 2C(OH)OCH 3 .+(C), which are formed from the methyl acetate ion by two consecutive [1,4]-hydrogen shifts. The 0 K heats of formation of isomers B and C as well as transition states TSAB, TSBC, and TSBE (relative to isomer A) were calculated from Rice-Ramsperger-Kassel-Markus (RRKM) theory.

The Dimethylamine Dimer Ion Is an Ion−Radical Complex: A Combined TPEPICO, Variational RRKM, and ab Initio MO Study of the Fragmentation of Ionized Dimers of Dimethylamine

Threshold photoelectron photoion coincidence spectroscopy has been used to study the fragmentation mechanism of dimethylamine dimer ions. The fragmentation rate constant data obtained from the experiment were modeled with variational RRKM theory using potential energies and vibrational frequencies from ab initio molecular orbital calculations at the MP2/6-31G* level. The results are consistent with the fragmentation of the dimethylamine dimer ion into protonated dimethylamine and the •CH2N(H)CH3 radical. This was supported by ab initio calculations in which the dimer ion was found to consist of a N−H−C hydrogen-bonded complex between the above two products. The RRKM fit to the experimental k(E) vs E data for the ion gave a dimethylamine dimer ion ΔfH°0 of 653 ± 11 kJ mol-1 and thus a dimer ion binding energy of 147 ± 16 kJ mol-1. In agreement with recent experimental results, the neutral dimethylamine dimer was calculated to be an N−H−N hydrogen-bonded dimer with a 0 K binding energy of 13 ± 3 kJ mol-1. The resulting neutral dimer adiabatic ionization energy is 6.8 ± 0.2 eV.

The unimolecular chemistry of ionized 1- and 2-butanethiol

Threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy was employed to study the unimolecular chemistry of metastable ionized 1- and 2-butanethiol. The ions were both found to form two products, m/z 56 (C 4H +· 8) and m/z 60 (C 2H 4S +·) corresponding to the losses of H 2S and C 2H 6 respectively. On the basis of their breakdown diagrams, it was evident that these two ions do not interconvert prior to fragmentation. From energetic arguments, the C 4H +· 8 ion formed by the two thiolswas found to be either 2-butene +· or isobutene +·. This is in contrast to the analogous ionic 1-butanol, which loses only H 2O to form methylcyclopropane ions, and 2-butanol, which loses CH 3, CH 4, C 2H 5 and C 2H 6. RRKM theory was used to model the rates for each fragment channel to obtain barrier heights for their formation and aid in identifying the nature of the C 4H +· 8 ion formed by the two thiols. The ionization energy of 1-butanethiol was determined to be 9.14 ± 0.02 eV and that of 2-butanethiol to be 9.03 ± 0.02 eV.

The heat of formation of CISO +

The heat of formation of CISO +, 802 ± 1 kJ mol −1 at 0 K, has been obtained by the dissociative photoionization of supersonically cooled thionyl chloride, Cl 2SO. Threshold photoelectron photoion coincidence spectroscopy was used to obtain the appearance energy from the cross-over point in the breakdown diagram for the dissociation. Also measured was the adiabatic ionization energy of thionyl chloride, 10.85 ± 0.05 eV. The experimentally measured kinetic energy release was found to agree with that calculated by statistical theory confirming that there is no reverse energy barrier to the Cl loss reaction. The interpretation was aided by ab initio MO calculations.

Dissociation of the COS+ ion by photoionisation: experiment and ab initio calculations The Franck-Condon energy gap an the vibronic state Franck-Condon energy regions

Abstract The predissociation or dissociation of selected vibronic levels of the X 2 Π, A 2 Π, and B 2 Σ + states of the COS+ ions has been studied experimentally. The product state distribution has been probed by photoabsorption of the synchrotron radiation in the 13.5–16.5 eV range and threshold photoelectron-photoion coincidence spectroscopy. Ab initio calculations have been performed to model the potential energy surfaces of the X 2 Π, A 2 Π, and B 2 Σ + electronic states and also of the lowest energy 4Σ− state. This latter is coupled by spin-orbit to X 2 Π and is confirmed to be responsible of the predissociation in the A − X Franck-Condon gap. In the A − X Franck-Condon energy gap the predissociation mechanism by 4Σ− is not statistical and favours the production of vibrationally excited CO (v″) with maximum probability for the highest energy accessible levels. It involves also a highly effective transfer into rotational motion. For the A 2 Π state, there is internal conversion to X 2 Π followed by predissociation to the lowest limit and, for the levels above the second limit, the predissociation by 4Σ− is in competition with direct dissociation of X 2 Π to the second limit. The B 2 Σ + state also undergoes internal conversion to X 2 Π followed by dissociation to the second limit and predissociation by 4Σ− to the first limit.

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.

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.

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.

A threshold photoelectron-photoion coincidence spectrometric study of dimethyl ether (CH 3OCH 3)

The technique of threshold photoelectron-photoion coincidence spectroscopy has been used to study the dissociation of state-selected dimethyl ether (CH 3OCH 3) cations. The crossover region (10.5–11.5 eV) of the breakdown curve for the lowest energy dissociation (CH 3OCH 3 + → CH 3OCH 2 + + H) has been recorded for the first time using threshold photoelectron-photoion coincidence with variable ion residence time. No shift in the crossover point is detected for a change in residence time of 0.66–5.57 μs, indicating that the H loss dissociation is a fast ( k ≥ 10 7 s −1) process. A calculated fit to the 5.57μs data produces a threshold of 11.115±0.010 eV at a temperature of 0 K. Information on higher energy fragmentations is obtained from the breakdown curve. The shape of the m/z 29 ion curve indicates that m/z 29 ions are being formed by at least two processes. At low energy, HCO + is the product of a secondary dissociation from the unstable H 3CO + ( m/z 31) fragment ion. This dissociation is joined at higher energies by the secondary reaction of the H 2COH + ( m/z 31) and CH 3OCH 2 + ( m/z 45) fragments to the HCO + ion.

Unimolecular dissociation of energy-selected t-butylbenzene ions and effect of thermal energy on data analysis

The technique of threshold photoelectron-photoion coincidence (PEPICO) spectroscopy has been used to determine unimolecular rate constants for CH 3 loss from the molecular ion of t-butylbenzene. It is shown that in such large molecules the actual distribution of the thermal internal energy must be taken into account in order to extract accurate k(E) values near the threshold for decomposition. A simple method is presented for the inclusion of thermal energy in the calculations for the extraction of the rate constants. The k(E) function derived is compared to one obtained from field-ionization kinetics measurements and to RRKM/QET calculations.