Advances in threshold photoelectron spectroscopy (TPES) and threshold photoelectron photoion coincidence (TPEPICO).

The history and evolution of molecular threshold photoelectron spectroscopy and threshold photoelectron photoion coincidence spectroscopy (TPEPICO) over the last fifty years are reviewed. Emphasis is placed on instrumentation and the extraction of dynamical information about energy selected ion dissociation, not on the detailed spectroscopy of certain molecules. Three important advances have expanded greatly the power of the technique, and permitted its implementation on modern synchrotron radiation beamlines.

Controlling tunnelling in methane loss from acetone ions by deuteration.

Energetic acetone cations decay by methane or methyl radical loss. Although the methane-loss barrier to form the ketene cation is higher and the activation entropy is lower, it has a significant branching ratio at low energies thanks to quantum tunnelling. H-atom tunnelling can be selectively quenched and the methane-loss channel suppressed quantitatively by deuteration.

Tunneling in H loss from energy selected ethanol ions.

The H/D loss and CH(3)/CD(3) loss reactions from energy selected ethanol isotopologue ions C(2)H(5)OH(+), C(2)D(5)OD(+), CD(3)CH(2)OH(+), and CH(3)CD(2)OH(+) have been studied by imaging threshold photoelectron photoion coincidence (iPEPICO) spectroscopy. In the lowest energy dissociation channel, the α-carbon loses a hydrogen or a deuterium atom. Asymmetry in the daughter ion time-of-flight (TOF) peaks, an ab initio study of the reaction rates, and shifts in the phenomenological onsets between isotopologues revealed that H/D loss is slow at its onset.

One- and two-dimensional translational energy distributions in the iodine-loss dissociation of 1,2-C2H4I2+ and 1,3-C3H6I2+: what does this mean?

Threshold photoelectron photoion coincidence (TPEPICO) has been used to study the sequential photodissociation reaction of internal energy selected 1,2-diiodoethane cations: C(2)H(4)I(2)(+) → C(2)H(4)I(+) + I → C(2)H(3)(+) + I + HI. In the first I-loss reaction, the excess energy is partitioned between the internal energy of the fragment ion C(2)H(4)I(+) and the translational energy. The breakdown diagram of C(2)H(4)I(+) to C(2)H(3)(+), i.

Dissociation of energy selected Sn(CH3)4(+), Sn(CH3)3Cl+, and Sn(CH3)3Br+ ions: evidence for isolated excited state dynamics.

The dissociation dynamics of Sn(CH(3))(4)(+), Sn(CH(3))(3)Cl(+), and Sn(CH(3))(3)Br(+) were investigated by threshold photoelectron photoion spectrometry using an electron imaging apparatus (iPEPICO) at the Swiss Light Source. The tetramethyltin ion was found to dissociate via Sn(CH(3))(4)(+) → Sn(CH(3))(3)(+) + CH(3) → Sn(CH(3))(2)(+) + 2CH(3), while the trimethyltin halide ions dissociated via methyl loss at low energies, and a competitive halogen loss at somewhat higher energies. The 0 K methyl loss onset for the three ions was found to be 9.

Understanding the complex dissociation dynamics of energy selected dichloroethylene ions: neutral isomerization energies and heats of formation by imaging photoelectron-photoion coincidence.

The dissociative photoionization of 1,1-C(2)H(2)Cl(2), (E)-1,2-C(2)H(2)Cl(2), and (Z)-1,2-C(2)H(2)Cl(2) has been investigated at high energy and mass resolution using the imaging photoelectron photoion coincidence instrument at the Swiss Light Source. The asymmetric Cl-atom loss ion time-of-flight distributions were fitted to obtain the dissociation rates in the 10(3) s(-1) < k < 10(7) s(-1) range as a function of the ion internal energy. The results, supported by ab initio calculations, show that all three ions dissociate to the same C(2v) symmetry ClC═CH(2)(+) product ion.

Dissociation dynamics and thermochemistry of tin species, (CH3)4Sn and (CH3)6Sn2, by threshold photoelectron-photoion coincidence spectroscopy.

The dissociative photoionization of tetramethyltin (Me₄Sn) and hexamethylditin (Me₆Sn₂) has been investigated by threshold photoelectron-photoion coincidence (TPEPICO). Ions are energy-selected, and their 0 K dissociation onsets are measured by monitoring the mass spectra as a function of ion internal energy. Me₄Sn(+) dissociates rapidly by methyl loss, with a 0 K onset of E₀ = 9.

Heats of formation of C(6)H(5)(•), C(6)H(5)(+), and C(6)H(5)NO by threshold photoelectron photoion coincidence and active thermochemical tables analysis.

Threshold photoelectron photoion coincidence has been used to prepare selected internal energy distributions of nitrosobenzene ions [C(6)H(5)NO(+)]. Dissociation to C(6)H(5)(+) + NO products was measured over a range of internal energies and rate constants from 10(3) to 10(7) s(-1) and fitted with the statistical theory of unimolecular decay. A 0 K dissociative photoionization onset energy of 10.

Binding energies and isomerization in metallocene ions from threshold photoelectron photoion coincidence spectroscopy.

Metallocene ions (Cp(2)M(+), M = Cr, Co, Ni) were studied by threshold photoelectron photoion coincidence spectroscopy (TPEPICO) to investigate the mechanism, energetics, and kinetics of the ionic dissociation processes. The examined energy-selected Cp(2)M(+) ions fragment by losing the neutral cyclopentadienyl ligand. In addition, CH and C(2)H(2) losses appear as minor channels, while the cobaltocene ion also loses an H atom.

Modeling unimolecular reactions in photoelectron photoion coincidence experiments.

A computer program has been developed to model and analyze the data from photoelectron photoion coincidence (PEPICO) spectroscopy experiments. This code has been used during the past 12 years to extract thermochemical and kinetics information for almost a hundred systems, and the results have been published in over forty papers. It models the dissociative photoionization process in the threshold PEPICO experiment by calculating the thermal energy distribution of the neutral molecule, the energy distribution of the molecular ion as a function of the photon energy, and the resolution of the experiment.

Tunneling in a simple bond scission: the surprising Barrier in the H loss from HCOOH(+).

The dissociation dynamics of gas phase formic acid ions (HCOOH(+), DCOOD(+), HCOOD(+), DCOOH(+)) are investigated by threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy and high level ab initio calculations. The slow rate constants for this seemingly simple H loss reaction and the large onset energy shifts due to isotopic substitution point to a substantial exit barrier through which the H or D atoms tunnel. Modeling of the HCOOH(+) experimental data using RRKM theory with tunneling through an Eckart potential are best fitted with a barrier of about 17 kJ mol(-1).

Dissociation dynamics of energy selected, propane, and i-C(3)H(7)X(+) Ions by iPEPICO: accurate heats of formation of i-C(3)H(7)(+), i-C(3)H(7)Cl, i-C(3)H(7)Br, and i-C(3)H(7)I.

The dissociation dynamics of energy selected i-C(3)H(7)X (X = H, Cl, Br, and I) ions have been investigated by imaging photoelectron-photoion coincidence (iPEPICO) spectroscopy using synchrotron radiation from the X04DB VUV beamline in the Swiss Light Source of the Paul Scherrer Institut. The 0 K dissociation energy (E(0)) for i-C(3)H(8) was determined to be 11.624 ± 0.

Ion spectroscopy: where did it come from; where is it now; and where is it going?

The ASMS conference on ion spectroscopy brought together at Asilomar on October 16-20, 2009 a large group of mass spectrometrists working in the area of ion spectroscopy. In this introduction to the field, we provide a brief history, its current state, and where it is going. Ion spectroscopy of intermediate size molecules began with photoelectron spectroscopy in the 1960s, while electronic spectroscopy of ions using the photodissociation "action spectroscopic" mode became active in the next decade.

Heats of formation of t-butyl peroxy radical and t-butyl diazyl ion: RRKM vs SSACM rate theories in systems with kinetic and competitive shifts.

The dissociations of energy-selected di-t-butyl peroxide and di-t-butyl diazene ions have been studied by threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy. Di-t-butyl peroxide ions dissociate via two parallel channels: (1) methyl loss at a 0 K onset (E0) of 9.58 +/- 0.

On the ionization and dissociative photoionization of iodomethane: a definitive experimental enthalpy of formation of CH3I.

The dissociative photoionization onset energy of the CH(3)I --> CH(3)(+) + I reaction was studied at the vacuum ultraviolet (VUV) beamline of the Swiss Light Source (SLS) using a new imaging photoelectron photoion coincidence (iPEPICO) apparatus operating with a photon resolution of 2 meV and a threshold electron kinetic energy resolution of about 1 meV. Three previous attempts at establishing this value accurately, namely a pulsed field ionization (PFI)-PEPICO measurement, ab initio calculations and a mass-analyzed threshold ionization (MATI) experiment, in which the onset energy was bracketed by state-selected excitation to vibrationally excited (2)A(1) A states of the parent ion, have yielded contradictory results. It is shown that dimers and adducts formed in the supersonic molecular beam affected the PFI-PEPICO onset energy.

Dissociative photoionization study of neopentane: a path to an accurate heat of formation of the t-butyl ion, t-butyl iodide, and t-butyl hydroperoxide.

The dissociation dynamics of energy selected neopentane, t-butyl iodide, and t-butyl hydroperoxide ions have been investigated by threshold photoelectron-photoion coincidence (TPEPICO) spectrometry. Although the methyl loss reaction from neopentane ions producing the t-butyl ion is in competition with a lower energy methane loss channel, modeling these two channels with the statistical theory of unimolecular decay provides a 0 K dissociation onset for methyl loss of 10.564 +/- 0.

Heat of formation of the allyl ion by TPEPICO spectroscopy.

The 0 K onset of C(3)H(6) --> C(3)H(5)(+) + H(*) is measured by threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy. From the onset (11.898 +/- 0.

Experimental thermochemistry of SiCl3R (R = Cl, H, CH3, C2H5, C2H3, CH2Cl, SiCl3), SiCl3+, and SiCl3*.

The 0 K onsets (E(0)) of a series of trichlorosilane derivatives SiCl(3)R --> SiCl(3)(+)+ R(*) (R = Cl, H, CH(3), C(2)H(5), C(2)H(3), CH(2)Cl, SiCl(3)) are measured by threshold photoelectron-photoion coincidence spectroscopy. The well-known heat of formation of SiCl(4) is used as an anchor to determine the heat of formation of SiCl(3)(+), which is, in turn, used as an anchor to determine the heats of formation of the other alkyltrichlorosilanes investigated. A series of isodesmic reactions at the G3 and CBS-QB3 levels are shown to accurately reproduce the experimental heats of formation, and this scheme is used to calculate the heat of formation of Si(2)Cl(6), from which the measured E(0) determines the SiCl(3)(*) heat of formation.

Dissociative photoionization of X(CH(3))(3) (X = N, P, As, Sb, Bi): mechanism, trends, and accurate energetics.

Threshold photoelectron photoion coincidence spectroscopy is used to study the dissociation of energy-selected X(CH(3))(3)(+) ions (X = As, Sb, Bi) by methyl loss, the only process observed up to 2 eV above the ionization energy. The ion time-of-flight distributions and the breakdown diagrams are analyzed in terms of the statistical RRKM theory to obtain accurate ionic dissociation energies. These experiments complement previous studies on analogous trimethyl compounds of the N group where X = N and P.

Imaging photoelectron photoion coincidence spectroscopy with velocity focusing electron optics.

An imaging photoelectron photoion coincidence spectrometer at the vacuum ultraviolet (VUV) beamline of the Swiss Light Source is presented and a few initial measurements are reported. Monochromatic synchrotron VUV radiation ionizes the cooled or thermal gas-phase sample. Photoelectrons are velocity focused, with better than 1 meV resolution for threshold electrons, and also act as start signal for the ion time-of-flight analysis.

Specific rate constants k(E) of the dissociation of the halobenzene ions: analysis by statistical unimolecular rate theories.

Specific rate constants k(E) of the dissociation of the halobenzene ions C6H5X+ --> C6H5+ + X* (X* = Cl, Br, and I) were measured over a range of 10(3)-10(7) s-1 by threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy. The experimental data were analyzed by various statistical unimolecular rate theories in order to derive the threshold energies E0. Although rigid activated complex RRKM theory fits the data in the experimentally measured energy range, it significantly underestimates E0 for chloro- and bromobenzene.

Heats of formation of HCCl3, HCCl2Br, HCClBr2, HCBr3, and their fragment ions studied by threshold photoelectron photoion coincidence.

The dissociative photoionization onsets for Cl and Br loss reactions were measured for HCCl3, HCCl2Br, HCClBr2, and HCBr3 by threshold photoelectron photoion coincidence (TPEPICO) in order to establish the heats of formation of the mixed halides as well as the following fragment ions: HCCl2(+), HCClBr(+), HCBr2(+). The first zero Kelvin onsets were measured with a precision of 10 meV. The second onsets, which are in competition with the lower energy onsets, were established with a precision of 60 meV.

Ozonolysis of oleic acid adsorbed to polar and nonpolar aerosol particles.

Single-particle kinetic studies of the reaction between oleic acid and O 3 have been conducted on two different types of core particles: polystyrene latex (PSL) and silica. Oleic acid was found to adsorb to both particle types in multilayer islands that resulted in an adsorbed layer of a total volume estimated to be less than one monolayer. The rate of the surface reaction between surface-adsorbed oleic acid and O 3 has been shown for the first time to be influenced by the composition of the aerosol substrate in a mixed organic/inorganic particle.