Kinetic Energy Release Measurements Research Articles

Isomerization Dynamics in the Symmetric and Asymmetric Fragmentation of Ethane Dications.

Hydrogen- or proton-migration-induced isomerization has recently been of concern for its critical role in the dissociation of organic molecules of astrophysical or biological relevance. Herein we present a combined experimental and theoretical study of the two-body C-C bond breakdown dissociation of ethane dication. For the asymmetric fragmentation channel CH2+ + CH4+, the kinetic energy release measurements and ab initio quantum chemical calculations demonstrate that the reaction pathway involving hydrogen-migration-induced isomerization of [CH3-CH3]2+ to [CH2-CH4]2+ can be accessed via the lowest triplet state rather than the ground singlet state of ethane dication. Interestingly, it is found that a considerable proportion of the yield of symmetric fragmentation CH3+ + CH3+, which is usually considered from a direct Coulomb explosion and seemingly independent of isomerization, could come from the dissociation of ethane dication in the ground singlet state with the involvement of [CH3-CH3]2+ isomerization to intermediate [H2C(H2)CH2]2+ of the diborane-like double-bridged structure.

An improved quasi-diabatic representation of the 1, 2, 3(1)A coupled adiabatic potential energy surfaces of phenol in the full 33 internal coordinates.

In a recent work we constructed a quasi-diabatic representation, H(d), of the 1, 2, 3(1)A adiabatic states of phenol from high level multireference single and double excitation configuration interaction electronic structure data, energies, energy gradients, and derivative couplings. That H(d) accurately describes surface minima, saddle points, and also regions of strong nonadiabatic interactions, reproducing the locus of conical intersection seams and the coordinate dependence of the derivative couplings. The present work determines the accuracy of H(d) for describing phenol photodissociation. Additionally, we demonstrate that a modest energetic shift of two diabats yields a quantifiably more accurate H(d) compared with experimental energetics. The analysis shows that in favorable circumstances it is possible to use single point energies obtained from the most reliable electronic structure methods available, including methods for which the energy gradients and derivative couplings are not available, to improve the quality of a global representation of several coupled potential energy surfaces. Our data suggest an alternative interpretation of kinetic energy release measurements near λphot ∼ 248 nm.

Resonant multiphoton ionisation probe of the photodissociation dynamics of ammonia.

The dissociation dynamics of the Ã-state of ammonia have been studied using a resonant multiphoton ionisation probe in a photoelectron spectroscopy experiment. The use of a resonant intermediate in the multiphoton ionisation process changes the ionisation propensity, allowing access to different ion states when compared with equivalent single photon ionisation experiments. Ionisation through the E' 1A1' Rydberg intermediate means we maintain overlap with the ion state for an extended period, allowing us to monitor the excited state population for several hundred femtoseconds. The vibrational states in the photoelectron spectrum show two distinct timescales, 200 fs and 320 fs, that we assign to the non-adiabatic and adiabatic dissociation processes respectively. The different timescales derive from differences in the wavepacket trajectories for the two dissociation pathways that resonantly excite different vibrational states in the intermediate Rydberg state. The timescales are similar to those obtained from time resolved ion kinetic energy release measurements, suggesting we can measure the different trajectories taken out to the region of conical intersection.

Experimental binding energies for the metal complexes [Mg(CH3OH)n](2+), [Ca(CH3OH)n](2+), and [Sr(CH3OH)n](2+) for n in the range 4-20.

A supersonic source of clusters has been used to prepare neutral complexes of methanol in association with an alkaline earth metal atom. From these complexes the following metal-containing dications have been generated through electron ionization: [Mg(CH3OH)n](2+), [Ca(CH3OH)n](2+), and [Sr(CH3OH)n](2+), and for n in the range 4-20, kinetic energy release measurements following the evaporation of a single molecule have been undertaken using a high resolution mass spectrometer. Using finite heat bath theory, these data have been transformed into binding energies for individual methanol molecules attached to each of the three cluster systems. In the larger complexes (n > 6) the results exhibit a consistent trend, whereby the experimental binding energy data for all three metal ions are similar, suggesting that the magnitude of the charge rather than charge density influences the strength of the interaction. From a comparison with data recorded previously for (CH3OH)nH(+) it is found that the 2+ charge on a metal ion has an effect on the binding energy of molecules in complexes containing up to 20 solvent molecules. The results recorded for [Ca(CH3OH)n](2+) show evidence of a very marked transition between n = 6 and 7, which is thought to coincide with the completion of a primary solvation shell and the onset of molecules starting to occupy a second and most probably a third shell.

Experimental binding energies for the metal complexes [Mg(NH3)n]2+, [Ca(NH3)n]2+, and [Sr(NH3)n]2+ for n = 4-20 determined from kinetic energy release measurements.

A supersonic source of clusters has been used to prepare neutral complexes of ammonia in association with a metal atom. From these complexes the following metal-containing dications have been generated: [Mg(NH3)n](2+), [Ca(NH3)n](2+), and [Sr(NH3)n](2+), and for n in the range 4-20, kinetic energy release measurements following the evaporation of a single molecule have been undertaken using a high resolution mass spectrometer. Using finite heat bath theory, these data have been transformed into binding energies for individual ammonia molecules attached to each of the three cluster systems. In the larger complexes (n > 6) the results exhibit a consistent trend, whereby the experimental binding energy data for all three metal ions are very similar, suggesting that the magnitude of the charge rather than charge density influences the strength of the interaction. From a comparison with data recorded previously for (NH3)nH(+) it is found that the 2+ charge on a metal ion has an effect on the binding energy of molecules in complexes containing up to 20 solvent molecules. Although subject to comparatively large experimental errors, the results recorded for Ca(2+) and Sr(2+) when n ≤ 6 show evidence for the formation of an inner solvation shell containing up to 6 molecules. However, Mg(2+) exhibits relatively low binding energies when n = 5 and 6, which suggests that a second shell starts to form before there are 6 ammonia molecules bound to the metal ion. This conclusion is supported by DFT calculations and it is proposed that these complexes could take the form [Mg(NH3)4(NH3)](2+) when n = 5 and either [Mg(NH3)4(NH3)2](2+) or [Mg(NH3)5(NH3)](2+) when n = 6. In each case, additional molecules are hydrogen bonded to one or more molecules in the inner solvation shell.

Ion beam characterization in the Mini-Ring and kinetic energy release measurement of unimolecular decay of anthracene cations

The Mini-Ring is a compact electrostatic ion storage ring designed for molecular relaxation dynamics studies. In order to accomplish an injection of the incoming ion beam, the electrode voltage settings appear to be quite critical to avoid large betatron oscillations. The beam injection was controlled by taking pictures of a test Ar+ ion beam with a digital camera. The amplitude of the betatron oscillation was measured by the impact position of neutral fragments exiting the ring on a position-sensitive detector. The broad spot observed for anthracene was attributed to the kinetic energy release resulting from the delayed unimolecular dissociation of anthracene by emission of a neutral C2H2.

Reprint of “Binding energies determined from kinetic energy release measurements following the evaporation of single molecules from the molecular clusters H+(H2O)n, H+(NH3)n and H+ (CH3OH)n”

High resolution mass spectrometry in conjunction with a supersonic source of clusters has been used to record values for the average kinetic energy release that accompanies fragmentation of the molecular cluster ions: H+(H2O)n, H+(NH3)n and H+(CH3OH)n for n in the range 3–30. Using finite heat bath theory, these data have been transformed into binding energies for molecules attached to each of the three cluster systems. For n>6 the results show quantitative agreement with the strengths of hydrogen bonds expected to be present in the clusters and it is concluded that each cluster fragments via the breaking of a single hydrogen bond.

Binding energies determined from kinetic energy release measurements following the evaporation of single molecules from the molecular clusters H+(H2O)n, H+(NH3)n and H+(CH3OH)n

High resolution mass spectrometry in conjunction with a supersonic source of clusters has been used to record values for the average kinetic energy release that accompanies fragmentation of the molecular cluster ions: H+(H2O)n, H+(NH3)n and H+(CH3OH)n for n in the range 3–30. Using finite heat bath theory, these data have been transformed into binding energies for molecules attached to each of the three cluster systems. For n>6 the results show quantitative agreement with the strengths of hydrogen bonds expected to be present in the clusters and it is concluded that each cluster fragments via the breaking of a single hydrogen bond.

Interatomic Coulombic decay of fixed-in-space neon dimers

The detailed theoretical and experimental analysis of the angular distributions of electrons from interatomic Coulombic decay (ICD) of the Ne dimer in the molecular frame is performed. In the initial state the doubly charged dimer ion has one $2s$ vacancy and one $2p$ vacancy on one atom. After the ICD process the neutral neon atom is ionized and the triply charged molecular ion dissociates into singly and doubly charged atomic ions, ${\text{Ne}}^{2+}(2{p}^{\ensuremath{-}2})+{\text{Ne}}^{+}(2{p}^{\ensuremath{-}1})$. From the coincident measurement of kinetic energy release (KER) of the ions and the ICD electron the decay channel can be identified unambiguously. The most detailed experimental data have been obtained for the singlet dicationic state Ne${}^{2+}(2{p}^{\ensuremath{-}2}){[}^{1}\phantom{\rule{-0.16em}{0ex}}D]$. Different KER energies correspond to different internuclear distances at which the ICD process takes place. In experiment the data have been presented for three regions of KER energies, and the corresponding calculations have been performed for three fixed internuclear distances. In calculations we imply that all the electrons in Ne${}_{2}$ to a good approximation are localized. However, we need to retain the molecular character of the dimer wave functions which opens the possibility for the ICD process. To do it, we calculate at first the Hartree-Fock ground state wave functions of the neutral Ne${}_{2}$ dimer using the standard procedure for homonuclear diatomic molecules corresponding to the ${D}_{\ensuremath{\infty}h}$ symmetry group. For the doubly charged ion Ne${}_{2}{}^{2+}$ with two vacancies on one atom the symmetry is lowered to ${C}_{\ensuremath{\infty}v}$, and we are looking now for the set of one-electron Hartree-Fock wave functions which are localized either on the left or on the right atom as a linear combination of symmetry-adopted wave functions. The theory correctly reproduces the experimental data and predicts the sharp variation of the angular distributions as a function of internuclear distance.

Measurements of kinetic energy release and binding energy following the unimolecular fragmentation of molecular cluster ions

High resolution mass spectrometry has been used to record the fragmentation patterns of size-selected carbon dioxide, carbon disulphide and protonated acetonitrile cluster ions that have been produced by electron impact ionization of a beam of neutral clusters. From the shapes of fragment ion peaks it has been possible to extract information about the kinetic energy that is released in the decay reaction. Average kinetic energies have been recorded and using finite heat bath theory these data have been used to calculate the binding energies of single molecules to the decaying cluster ions.

Measurement of kinetic energy release in CO fragmentation by charge-changing collisions of fast heavy ions

We study ionization and fragmentation of CO in electron loss and capture collisions of B{sup 2+}, O{sup 2+}, and Si{sup 2+} ions at an energy of 71.4 keV/u (v=1.69 a.u.). Coincidence measurements of fragment ions from CO and charge-selected ions were performed by means of a momentum three-dimensional imaging technique. Production cross sections of CO{sup r+} and branching ratios into various fragmentation channels were obtained for r=1-4. We also measured kinetic energy release (KER) in individual fragmentation channels. The KER spectra for r =3. As a measure of the degree of molecular fragmentation, the magnitude of the binding energy of the relevant electronic states seems the important parameter both in loss and capture collisions.

Ultraviolet photodissociation of H3+

As a step towards in situ probing of the rotational-state distribution of H3+ ions in a stored beam, we have realized their photodissociation in a storage ring by a parallel ultraviolet laser beam. The decay of the photodissociation signal of H3+ for 4.9 eV photons into the channel H+2 + H was identified and showed a time constant of about 2 ms, typical for vibrational levels of H3+ over a wide range of binding energies. The photodissociation products of H3+ at this photon energy were further investigated for a single pass ion beam by kinetic energy release and charge-exchange measurements.

The stability of singly and multiply charged La@C 80 and La@C 82 ions determined from kinetic energy release measurements

We have measured the kinetic energy release distributions for unimolecular C 2 loss from singly and multiply charged endohedral La@C 80 z+ and La@C 80 z+ ions for charge states z = 1–3. Using finite heat bath theory, we deduce the dissociation energies for loss of C 2. The charge state z has no statistically significant effect on the dissociation energies. The dissociation energies of endohedral fullerene ions are larger than those of the corresponding empty fullerene ions by a few tenths of an electron volt, but the differences are barely statistically significant. The small differences also imply that the complexation energies of La@C n z+ and La@C n − 2 z+ are identical within the experimental errors, for n = 80 and 82 and all charge states.

Kinetic energies of charged fragments resulting frommultifragmentation and asymmetric fission of the C60molecule in collisions with monocharged ions(2–130 keV)

Multifragmentation and asymmetric fission (AF) of the C60 molecule inducedby H+,H2+,H3+and He+ions at medium collision energies (2–130 keV) areconsidered. Momenta and kinetic energies of Cn+ fragmentions (n = 1– 12)are deduced from an analysis of time-of-flight spectra. In multifragmentationprocesses, momenta are found to be approximatively constant when n > 2, abehaviour which explains that the most probable kinetic energy, as well as the width ofthe kinetic energy distributions, is found to be inversely proportional to the fragmentsize n;both momenta and kinetic energies are independent of the velocityand nature of the projectile, and hence of the energy deposit. Aspecific study of the AF shows that the kinetic energies of C2+, C4+ andC6+fragments are also independent of the collision velocity and projectile species; aquantitative agreement is found with values deduced from kinetic energy releasemeasurements by another group in electron impact experiments, and the observeddecrease when the mass of the light fragment increases is also reproduced.A quantitative comparison of AF and multifragmentation for the n = 2, 4and 6 fragment ions shows that kinetic energies in AF exceed that inmultifragmentation, a result which explains the oscillations observedwhen momenta or kinetic energies of fragments are plotted against then-value.The AF yield is also found to scale with the energy deposit in the collisionvelocity range extending below the velocity at the maximum of the electronicstopping power; except for protons, it remains negligible with respect tomultifragmentation as soon as the total energy deposit exceeds about 100 eV.

Photodissociation of water in the à band revisited with new potential energy surfaces

Theoretical calculations on the photodissociation of water in the first absorption band have been used to test the accuracy of three available potential energy surfaces for the first excited state of water: the well-known coupled electron pair approximation potential of Staemmler and Palma [Chem. Phys. 93, 63 (1985)], and two new multireference double excitation configuration interaction surfaces: the Dobbyn–Knowles surface (unpublished), and the Leiden surface [R. van Harrevelt and M. C. van Hemert, J. Chem. Phys. 112, 5777 (2000)]. Exact quantum mechanical calculations, using the wave packet approach, have been performed for J″>0, where J″ is the initial rotational state of the water molecule. The cross section was found not to depend strongly on the rotational state, so that it is reasonable to compare calculated cross sections for J″=0 with experimental room temperature cross sections. Small and simple corrections were applied to the potential energy surface to improve the agreement between theory and experiment for the cross section of H2O. Spectra for D2O and vibrationally excited water molecules calculated with all three corrected potential energy surfaces were in good agreement with experiments. A comparison between calculated OH(X) or OD(X) vibrational distributions, and recent kinetic energy release measurements of the H or D atoms produced in the 157.6 nm photodissociation of water and its isotopomers [Yang et al., J. Chem. Phys. 113, 10597 (2000)], however, suggests that the Leiden surface is more accurate than the two other surfaces.

Structure and fragmentation of b 2 ions in peptide mass spectra

In a number of cases the b 2 ion observed in peptide mass spectra fragments directly to the a 1 ion. The present study examines the scope of this reaction and provides evidence as to the of the b 2 ions undergoing fragmentation to the a 1 ion. The b 2 ion H–Ala–Gly + fragments, in part, to the a 1 ion, whereas the isomeric b 2 ion H–Gly–Ala + does not fragment to the a 1 ion. Ab initio calculations of ion energies show that this different behavior can be rationalized in terms of protonated oxazolone structures for the b 2 ions provided one assumes a reverse activation energy of ∼1 eV for the reaction b 2 → a 2; such a reverse activation energy is consistent with experimental kinetic energy release measurements. Experimentally, the H–Aib–Ala + b 2 ion, which must have a protonated oxazolone structure, fragments extensively to the a 1 ion. We conclude that the proposal by Eckart et al. (J. Am. Soc. Mass Spectrom. 1998, 9, 1002) that the b 2 ions which undergo fragmentation to a 1 ions have an immonium ion structure is not necessary to rationalize the results, but that the fragmentation does occur from a protonated oxazolone structure. It is shown that the b 2 → a 1 reaction occurs extensively when the C-terminus residue in the b 2 ion is Gly and with less facility when the C-terminus residue is Ala. When the C-terminus residue is Val or larger, the b 2 → a 1 reaction cannot compete with the b 2 → a 2 fragmentation reaction. Some preliminary results on the fragmentation of a 2 ions are reported.

Energy partitioning following the infrared photofragmentation of SF 6·(C 6H 6) + cluster ions

Kinetic energy release measurements have been performed following the metastable and infrared photoinduced decay of SF 6·(C 6H 6) n + cluster ions. Using a model due to Klots [J. Chem. Phys. 58 (1973) 5364], photofragment kinetic energy releases have been predicted on the assumption that energy from the photon (∼950 cm −1 deposited into the ν 3 of the SF 6 moiety) is partitioned statistically throughout the cluster ion. However, a comparison between experimental and calculated results suggests that incomplete energy randomisation may be taking place, and a better description of the photofragmentation process is provided on the assumption that the excited vibrational mode undergoes partial relaxation to either the ν 2 or ν 4 modes. The energy difference (∼300 cm −1) is then randomised and is reflected in the magnitudes of the measured kinetic energy releases.

Photodissociation spectroscopy and dynamics of MgC2H4+

The weakly bound ion–molecule complex MgC2H4+ has been studied by photodissociation spectroscopy in a reflectron time-of-flight mass spectrometer over the spectral range 218–510 nm. Mg+ is the major photofragment throughout this range, although for λ<270 nm, charge-transfer dissociation to C2H4+ is observed as a minor channel. We have identified five absorption bands of MgC2H4+. The spectral assignment is facilitated by results from ab initio calculations for the ground and low-lying excited states of MgC2H4+. Three of the bands, 1 2B2←1 2A1, 1 2B1←1 2A1, and 2 2A1←1 2A1, are based primarily in the metal-centered Mg+(3p 2P←3s 2S) atomic transition. One of the remaining bands is assigned as 2 2B2←1 2A1, a transition correlating with the a 3B1u←X 1Ag forbidden band of C2H4, with mixed charge-transfer character. The final band, 3 2A1←1 2A1, is assigned to a metal-to-ligand charge-transfer transition, enhanced by coupling with the nearby 2 2A1 state. The 1 2B2←1 2A1 band is a broad continuum, indicative of fast predissociation in the upper state. A nonadiabatic dissociation mechanism involving C=C π-bond activation by Mg+(3p) is suggested by ab initio calculations. The 1 2B1←1 2A1 band shows pronounced vibrational structure with a strong progression in the Mg+–CH4 intermolecular stretch (ν2), and weaker progressions assigned to combination bands built on the intermolecular out-of-plane wag (ν3), and a CH2–CH2 wag (ν7). The observed vibrational constants are ω2=329, x22=−2.3, ω3=439, and ω7=1024 cm−1. Measurement of the photofragment kinetic energy release determines the bond dissociation energies for the ground state [D0″(Mg+–C2H4)=0.7±0.2 eV], and for the 1 2B1 excited state, [D0′(Mg+–C2H4)=1.8±0.2 eV]. Spectroscopic constants are in good agreement with ab initio predictions.

Pathways to Immonium Ions in the Fragmentation of Protonated Peptides

The pathways leading to the formation of immonium (An) ions in the fragmentation of protonated peptides were investigated using metastable ion studies, including kinetic energy release measurements, and low-energy collision-induced dissociation studies. In addition to the established pathway Bn→An+CO, it is shown that B2 ions, in suitable circumstances, fragment directly to A1 ions. In addition, metastable ion studies show that A1 ions can be formed directly from protonated di- and tripeptides most likely by concerted elimination of CO and an amino acid or smaller peptide. A2 ions can be formed directly from protonated dipeptides in part through the sequential loss of H2O+CO, although kinetic energy release measurements suggest direct elimination of HCOOH also may be occurring. Internal immonium ions are shown to originate by further fragmentation of An ions and by further fragmentation of Yn″ ions. © 1997 by John Wiley & Sons, Ltd.

Pathways to Immonium Ions in the Fragmentation of Protonated Peptides

A 1 formed directly from protonated di- and tripeptides most likely by concerted elimination of CO and an amino acid or smaller peptide. ions can be formed directly from protonated dipeptides in part through the sequential loss of A 2 although kinetic energy release measurements suggest direct elimination of HCOOH also may be H 2 O + CO, occurring. Internal immonium ions are shown to originate by further fragmentation of ions and by further A n fragmentation of ions. 1997 by John Wiley & Sons, Ltd. Y n / (