Kinetic Energy Release Values Research Articles

Formation, structure, and dissociation dynamics of CO2q+(q≤3) ions due to impact of 12-keV electrons

The electron-impact multiple ionization and subsequent dissociation of CO${}_{2}$ is studied for 12-keV electron energy using a linear time-of-flight mass spectrometer coupled with a multihit, position-sensitive detector. The complete as well as incomplete Coulomb explosion pathways for CO${}_{2}$${}^{2+}$ and CO${}_{2}$${}^{3+}$ ions are examined and identified. The kinetic energy release distributions for these precursor ions are obtained. The experimental kinetic energy release values for the complete Coulomb fragmentation channels are found to be overestimated by those calculated from the Coulomb explosion model. From the angular correlation studies, it is inferred that bent geometrical states are involved for most of the fragmentation channels of CO${}_{2}$${}^{2+}$ and CO${}_{2}$${}^{3+}$ ions. The concerted and/or sequential nature of all the dissociation pathways is also assigned. This study provides the first results on energetics associated with the charge separation in dissociative ionization of CO${}_{2}$ under the impact of electrons at a subrelativistic energy.

Electronic Properties and Dissociative Photoionization of Thiocyanates. Part II. Valence and Shallow-Core (Sulfur and Chlorine 2p) Regions of Chloromethyl Thiocyanate, CH2ClSCN

A combination of photoelectron spectroscopy and synchrotron based photoelectron photoion coincidence (PEPICO) spectra has been applied to investigate the electronic structure and the dissociative ionization of the CH(2)ClSCN molecule in the valence region. The PES is assigned with the electronic structure calculations at the outer-valence Green's function and symmetry adapted cluster/configuration interaction (SAC-CI) levels offer an explanation of our experimental results. Upon vacuum ultraviolet irradiation the low-lying radical cation, located at 10.39 eV is formed. The molecular ion is observed in the time-of-flight mass spectra, together with the CH(2)SCN(+) and CH(2)Cl(+) daughter ions. The total ion yield spectra have been measured in the S 2p and Cl 2p regions and several channels have been determined in dissociative photoionization events for the core-excited species. Thus, by using time-of-flight mass spectrometry and synchrotron radiation the relative abundances of the ionic fragments and their kinetic energy release values were obtained from both PEPICO and photoelectron photoion photoion coincidence spectra. Possible fragmentation processes are discussed and compared with that found for the related CH(3)SCN species.

Communication: Mass-analyzed velocity map imaging of thermal photofragments from C60

The velocity distributions of the fragments produced by dissociative photoionization of C(60) have been measured in the extreme UV region for the first time, by using a flight-time resolved velocity map imaging technique combined with a high-temperature molecular beam and synchrotron radiation. Values of the average kinetic energy release were estimated at six different photon energies with respect to five reaction steps of sequential C(2) ejection, starting from C(60)(2+)-->C(58)(2+)+C(2) to C(52)(2+)-->C(50)(2+)+C(2). The translational temperatures of the fragment ions were found to be lower than those obtained by laser multiphoton absorption of C(60). The kinetic energies released in the first to fourth steps increase with increasing hnu and reach 0.35-0.5 eV at hnu=102 eV, reflecting statistical redistribution of the excess energy in the transition state, whereas that in the fifth step leading to C(50)(2+) was exceptionally small.

Auger decay of1σgand1σuhole states of theN2molecule: Disentangling decay routes from coincidence measurements

Results of the most sophisticated measurements in coincidence of the angular resolved K-shell photo- and Auger-electrons, and of two atomic ions produced by dissociation of N2 molecule, are analyzed. Detection of photoelectrons at certain angles allows separating the Auger decay processes of the 1{\sigma}g and 1{\sigma}u core hole states. The Auger electron angular distributions for each of these hole states are measured as a function of the kinetic energy release of two atomic ions and are compared with the corresponding theoretical angular distributions. From that comparison one can disentangle the contributions of different repulsive doubly charged molecular ion states to the Auger decay. Different kinetic energy release values are directly related to the different internuclear distances. In this way one can trace experimentally the behavior of the potential energy curves of dicationic final states inside the Frank-Condon region. Presentation of the Auger electron angular distributions as a function of kinetic energy release of two atomic ions opens a new dimension in the study of Auger decay.

Separation of Auger transitions into different repulsive states afterK-shell photoionization ofN2molecules

The Auger transitions to different repulsive doubly charged molecular ion states are separated by measuring the angular resolved photoelectrons and Auger electrons in coincidence in the molecular fixed frame. The separation is achieved by comparing the experimental Auger-electron angular distributions at different kinetic-energy release values with theoretical curves calculated for different final dicationic states.

Perchloromethyl Mercaptan, CCl3SCl, Excited with Synchrotron Radiation in the Proximity of the Sulfur and Chlorine 2p Edges: Dissociative Photoionization of Highly Halogenated Species

We have investigated the dissociative photoionization of shallow-core excited CCl3SCl by using multicoincidence time-of-flight mass spectrometry and synchrotron radiation in the S 2p and Cl 2p edges. The relative abundances of the ionic fragments and their kinetic energy release values were obtained from both PEPICO (photoelectron photoion coincidence) and PEPIPICO (photoelectron photoion photoion coincidence) spectra. The dynamic of the ionic fragmentation of S and Cl 2p excited CCl3SCl has been studied and compared with those of CCl4. Features determined in the present study seem to be relevant aspects for explaining the dissociation of highly chlorinated species under the action of VUV irradiation. The fragmentation pattern shows that chlorine ion (Cl+) is prominently formed upon both S and Cl 2p excitations.

Dissociative Photoionization of Methyl Thiocyanate, CH3SCN, in the Proximity of the Sulfur 2p Edge

The dissociative photoionization of gaseous CH3SCN has been investigated at the S 2p core level using time-of-flight mass spectrometry and synchrotron radiation. The total ion yield spectrum could be successfully assigned by comparison with available data from electron energy loss spectra. The relative abundances of the ionic fragments and their kinetic energy release values were obtained from both PEPICO (photoelectron photoion coincidence) and PEPIPICO (photoelectron photoion photoion coincidence) spectra. The dynamics of the ionic fragmentation of S 2p excited CH3SCN is dominated by the rupture of both carbon-sulfur bonds. This process may be related with electronic excitations from the ground electronic state to vacant sigma* molecular orbitals.

Dissociation mechanism of electronically excited CH 2X 2 (X = Cl, Br) formed by near-resonant neutralization using charge-inversion mass spectrometry

The dissociation mechanism of excited CH 2X 2 (X = Cl, Br) was investigated using charge-inversion mass spectrometry, in which positive ions collide with an alkali metal target to generate neutral fragments, and the negative ions formed from the neutral fragments are mass analyzed. Different relative abundances of the negative ions were observed in the charge-inversion spectra for CH 2Cl 2 and CH 2Br 2. The kinetic energy release values calculated from analysis of the peak associated with CH Cl 2 - in the charge-inversion mass spectrum of the parent CH 2 Cl 2 + ion indicate that the excited CH 2Cl 2 formed by neutralization dissociates spontaneously into CHCl 2 + H.

Kinetic energy distributions in ion-inducedCOfragmentation: Signature of shallow states in multiply chargedCO

Ion-induced molecular fragmentation of $\mathrm{CO}$ has been studied using time-of-flight mass spectroscopy with position sensitive detectors in multihit coincidence mode. ${\mathrm{Ar}}^{8+}$ ions having a velocity of 1.1 a.u. were used as projectiles. The features observed in the kinetic energy release (KER) spectra for all the detected fragmentation channels are discussed in light of the existing and calculated ab initio potential energy curves. The preference of the symmetric breakup over the asymmetric one is clearly observed. For fragmentation channels originating from the same parent molecular ion, it is observed that the most probable KER value is higher for the dissociation channel having a higher charge on the oxygen ion. Occurrence of sharp peaks in KER spectra of some of the fragmentation channels hints towards the existence of shallow (possibly metastable) excited states of ${\mathrm{CO}}^{q+}$ $(q=4,5)$ molecular ions and calls for further theoretical investigations.

Electron ionization-induced fragmentation ofN- andO-alkoxymethylated carbostyril and phenanthridinone

Unimolecular fragmentation patterns of N-alkoxymethylated carbostyril and phenanthridinone and their O-alkoxymethyl isomers were studied. The main fragmentation reaction observed for the studied compounds is the elimination of an aldehyde molecule. The main products of this reaction are the appropriate N-methyl derivatives, but ions with other structures are also formed. This reaction is supposed to proceed via 1,3-H shift in the alkoxymethyl group in the case of the N-alkoxymethyl derivatives and by a multi-step mechanism for O-alkoxymethylated compounds. Another important fragmentation common for all studied compounds is the loss of an alkyl radical from N- and O-alkoxymethyl groups, yielding the appropriate stable isomeric cations, which, according to the results of the further fragmentation, undergo fast equilibration reaction via an ion–neutral complex. This process is accompanied by the unusually high kinetic energy release value. Copyright © 2004 John Wiley & Sons, Ltd.

Kinetic energy release of protonated methanol clusters using the low-temperature fast-atom bombardment: experiment and theory combined.

Low-temperature fast-atom bombardment was found to be an excellent method for generating large protonated methanol clusters, (CH(3)OH)(n)H(+) (n = 2 to 15). Metastable dissociations of these clusters, involving elimination of one methanol molecule, were studied using mass-analyzed ion kinetic energy spectra (MIKES). From metastable peak profiles kinetic energy release (KER) distributions were obtained, even for clusters as large as (CH(3)OH)(15)H(+). The results were analyzed by a simple thermal model, by the finite heat bath theory (FHBT) and by the RRKM-based MassKinetics algorithm. The KER distribution was shown to correspond to a three-dimensional translational energy distribution, implying statistical energy partitioning in the transition state. The mean KER values and transition state temperatures were found to increase with cluster size, reaching 25 meV and approximately 210 K for large clusters (n = 10).

Theoretical calculation of isotope effects, kinetic energy release and effective temperatures for alkylamines

The kinetic isotope effect (KIE) and kinetic energy release (KER) of protonated alkylamine dimers were studied by theoretical modeling. In the calculations on reaction kinetics one empirical parameter was used to describe the looseness of the transition state. Calculations are compared to experiments described by Norrman and McMahon [Int. J. Mass. Spetrom. 182/183 (1999) 381]. In the case of experiments using a high-pressure ion source the Rice–Ramsperger–Kassel–Marcus (RRKM) model, taking into account energy distributions and the time scale of metastable ion fragmentation, accurately describes the experimentally observed KIE’s of α-deuterated amines. The KER (available experimentally in one case only) is also correctly calculated, using no further parameters. In the case of low-pressure ion source, the internal energy distribution (IED) is not thermal, so it was empirically estimated based on the experimentally observed KIE. Using this estimate, it was possible to calculate the KER of α-deuterated amines accurately. Based on theoretical expectations it was found that the mean KER value is equal to 3/2 kT eff. This allows estimation of the KER in cases where it is determined by statistical factors. Energy distributions of various fragmenting ion populations are discussed in some detail. These may be helpful for a qualitative understanding of mass spectrometric processes and the theoretical basis of the kinetic method.

Electron impact mass spectral interpretation for some thiophosphoryl‐p‐acetylaminobenzenesulfonamides

This work discusses the synthesis and the fragmentation patterns for 2-(p-acetylaminosulfonamido)-2-thiono-(5,5-dimethyl-1,3,2-dioxaphosphorinane)(1) and for the p-acetylaminosulfonylamides of O,O-diethylthiophosphoric acid (2), O,O-diphenylthiophosphoric acid (3), dimethylaminocyclohexylthiophosphoric acid (4), and diethylaminophenylthiophosphoric acid (5). A thionamidic-thiolimidic structure was attributed to compounds 1-5, consistent with their IR and NMR spectra. EI mass spectra at 70 eV, high resolution (HR) mass measurements and metastable ion spectra were used to elucidate the fragmentation processes and to determine the kinetic energy release values associated with the metastable ion dissociations. HR accurate mass measurements were used to confirm the compositions of the more abundant ions.

電荷逆転質量分析法におけるピーク形状と運動エネルギー放出

A trapezoidal peak shape of dissociative ions is observed in the charge inversion mass spectra. The relation between the kinetic energy release (KER) and the peak shape was simulated. Dissociation with a specific KER value having very narrow distribution for both ionic and neutral species affords the trapezoidal peak shape in a mass spectrum. The KER value was evaluated from the full width at half maximum (FWHM) of the trapezoidal peak associated with the CH3O- ions formed from CH3OH+ ions in the charge inversion mass spectrum. The KER value showed the good agreement with that evaluated from the velocity distribution of H atom formed from the photo-dissociation of neutral methanol. This agreement demonstrated that the excited neutral methanol dissociated predominantly into the methoxy radical and the hydrogen atom, and that charge inversion mass spectrometry gave the dissociation of energy-selected neutral species.

Metastable decompositions of gem‐dialkoxyalkanes upon electron impact. III. Diethoxymethane (CH2(OCH2CH3)2)

Unimolecular metastable decomposition of diethoxymethane (CH2(OCH2CH3)2, 1) upon electron impact has been investigated by means of mass-analyzed ion kinetic energy (MIKE) spectrometry and theD-labeling technique in conjunction with thermochemistry. The m/z 103 ion ([M − H]+ : CH(OCH2CH3) = O+CH2CH3) decomposes into the m/z 47 ion (protonated formic acid, CH(OH) = O+H) by consecutive losses of two C2H4 molecules via an m/z 75 ion. The resulting product ion at m/z 47 further decomposes into the m/z 29 and 19 ions by losses of H2O and CO, respectively, via an 1,3-hydroxyl hydrogen transfer, accompanied by small kinetic energy release (KER) values of 1.3 and 18.8 meV, respectively. When these two elimination reactions are suppressed by a large isotope effect, however, another 1,1-H2O elimination with a large KER value (518 meV) is revealed. The m/z 89 ion ([M − CH3]+ : CH2(OCH2CH3)O+ = CH2) decomposes into the m/z 59 ion (CH3CH2O+ = CH2) by losing CH2O in the metastable time window. The source-generated m/z 59 ion ([M − OCH2CH3]+ : CH2 = O+CH2CH3) decomposes into the m/z 41 (CH2 = CH+CH2) and m/z 31 (CH2 = O+H) ions by losses of H2O and C2H4, respectively, with considerable hydrogen scrambling prior to decomposition. Copyright © 2000 John Wiley & Sons, Ltd.

Metastable decompositions of gem-dialkoxyalkanes upon electron impact. III. Diethoxymethane (CH(2)(OCH(2)CH(3))(2))

Unimolecular metastable decomposition of diethoxymethane (CH(2)(OCH(2)CH(3))(2), 1) upon electron impact has been investigated by means of mass-analyzed ion kinetic energy (MIKE) spectrometry and theD-labeling technique in conjunction with thermochemistry. The m/z 103 ion ([M - H](+) : CH(OCH(2)CH(3)) = O(+)CH(2)CH(3)) decomposes into the m/z 47 ion (protonated formic acid, CH(OH) = O(+)H) by consecutive losses of two C(2)H(4) molecules via an m/z 75 ion. The resulting product ion at m/z 47 further decomposes into the m/z 29 and 19 ions by losses of H(2)O and CO, respectively, via an 1,3-hydroxyl hydrogen transfer, accompanied by small kinetic energy release (KER) values of 1.3 and 18.8 meV, respectively. When these two elimination reactions are suppressed by a large isotope effect, however, another 1,1-H(2)O elimination with a large KER value (518 meV) is revealed. The m/z 89 ion ([M - CH(3)](+) : CH(2)(OCH(2)CH(3))O(+) = CH(2)) decomposes into the m/z 59 ion (CH(3)CH(2)O(+) = CH(2)) by losing CH(2)O in the metastable time window. The source-generated m/z 59 ion ([M - OCH(2)CH(3)](+) : CH(2) = O(+)CH(2)CH(3)) decomposes into the m/z 41 (CH(2) = CH(+)CH(2)) and m/z 31 (CH(2) = O(+)H) ions by losses of H(2)O and C(2)H(4), respectively, with considerable hydrogen scrambling prior to decomposition. Copyright 2000 John Wiley & Sons, Ltd.

Decomposition of metastable methyl trifluoroacetate and ethyl trifluoroacetate upon electron impact

Fragmentation mechanism of metastable ions generated from methyl trifluoroacetate, CF 3COOCH 3 (1), and ethyl trifluoroacetate, CF 3COOCH 2CH 3 (2), upon electron impact was investigated by means of mass analyzed ion kinetic energy (MIKE) spectra and ab initio molecular orbital (MO) calculations. The metastable [1–H] + ion decomposes to the m/z 99 ion by losing CO. This fragmentation proceeds with CF 3 migration via an ion-neutral complex. The metastable [1–F] + ion decomposes to m/z 81 ion by losing CO, too. In this case, however, the fragmentation proceeds with CH 3O migration to CF 2 group via a conventional three-membered ring transition state. The kinetic energy release (KER) values for these CO loss reactions are very different from each other, and are rationalized based on the different fragmentation mechanisms. The fragmentation pathways of 2 +· are similar to those of 1 +·. However, the loss of F 2CO from [2–H] + accompanies a very large KER value compared with that from [1–H] +.

Molecular Dynamics Simulation of Coulomb Explosion Processes

Irradiation of clusters with intense femtosecond laser pulses has been found to oftentimes produce highly charged ion fragments resulting from Coulomb explosion of the cluster under investigation. When a time-of-flight mass spectrometer is used for detection, double peaks can be observed for these multicharged species due to the different arrival times of backward and forward ejected ions. Kinetic energy release values can be calculated from these peaks splittings and also can be directly measured from cutoff potentials using a reflectron as an energy analyzer. The experimental values determined with both of these methods are surprisingly large, typically on the order of several hundred electronvolts. The purpose of the work reported herein is to model the temporal evolution of a cluster system after the ionization event, using MD simulations on a level of pure electrostatic interactions between the formed ion cores. The results give insight into how composition, size, structure, and charge distribution i...

Kinetic energy release and intercharge distance of the protonitronium dication (HONO 2+)

The usefulness of the determination of kinetic energy release (KER) distributions instead of single KER values is not widely known, though the KER distributions provide more detailed and accurate information about the studied process. To confirm this fact the KER distribution has been determined from the peak shape of the metastable peak corresponding to the charge separation reaction HONO 2+ → H + + NO 2 + which was published recently (J. Am. Chem. Soc. 115 (1993) 6312–6316). The most probable KER value yields an intercharge distance of 3.3 Å, in very good agreement with the previous theoretical estimate of 3.0 Å.

Conformation of Doubly Protonated Peptides Studied by Charge-separation Reactions in Mass Spectrometry

Charge-separation metastable processes were investigated by mass spectrometry and kinetic energy release (KER) distributions were determined from the measured metastable peak profiles. Benzene, p-N,N-dimethylaminoaniline and pipecuronium bromide (Arduan) were investigated as reference compounds to confirm that there is a well defined relationship between the KER distribution and the distance of the two charged centers. For these ‘rigid’ systems relatively narrow KER distributions were found, and the average values were in good agreement with the intercharge distances determined by a semi-empirical (MNDO) molecular orbital calculation. Two metastable processes of bradykinin, a nonapeptide and one of trilysine were studied. The distribution of KER suggests that the conformation of bradykinin is fairly rigid, whereas that of trilysine is flexible. The observed KER values clearly indicate far smaller intercharge distances than those corresponding to extended geometries. This is clear experimental evidence that medium-sized doubly charged peptides have a folded and not an extended geometry in the gas phase. © 1997 by John Wiley & Sons, Ltd.