Average Kinetic Energy Release Research Articles

Dissociation mechanism of C2H42+ induced by 18-keV/u Ne8+ impact: Symmetric and isomeric cleavage of the C-C bond

The C-C bond cleavage dissociation of ethylene dication produced by 18-keV/u ${\mathrm{Ne}}^{8+}$ impact is investigated by combining experimental measurement and theoretical calculation. Using cold target recoil ion momentum spectroscopy, two channels, i.e., the symmetric fragmentation, ${\mathrm{C}}_{2}\mathrm{H}{{}_{4}}^{2+} \ensuremath{\rightarrow} \mathrm{C}\mathrm{H}{{}_{2}}^{+}+\mathrm{C}\mathrm{H}{{}_{2}}^{+}$, and the isomeric one, ${\mathrm{C}}_{2}\mathrm{H}{{}_{4}}^{2+} \ensuremath{\rightarrow} \mathrm{C}{\mathrm{H}}^{+}+\mathrm{C}\mathrm{H}{{}_{3}}^{+}$, are clearly identified and thus their kinetic-energy release (KER) distributions are determined. The average KER values are then compared with the theoretical results obtained by quantum chemical calculations, which provide reaction paths of both channels on the potential-energy surfaces of different molecular states. It is found that the hydrogen transfer process and excited-state dynamics still play crucial roles in the formation of C-C bond cleavage channels, but in a different way in comparison with the cases of acetylene ${\mathrm{C}}_{2}{\mathrm{H}}_{2}$ and ethane ${\mathrm{C}}_{2}{\mathrm{H}}_{6}$.

Dependence of average total kinetic energy of fission fragments on the excitation energy of the compound nucleus

The expression of dependence of average total kinetic energy release of fission fragments on the excitation energy of the compound nucleus is obtained in the framework of a simple statistical model. The experimental data for the neutron-induced fission of $^{233,235,238}\mathrm{U}$ and $^{239}\mathrm{Pu}$ measured in wide ranges of the excitation energy of the compound nuclei are well described using the obtained expression. The origin of the energy dependence of the average total kinetic energy is discussed in detail.

The total kinetic energy release in the fast neutron-induced fission of 232Th

The post-emission total kinetic energy release (TKE) in the neutron-induced fission of 232Th was measured (using white spectrum neutrons from LANSCE) for neutron energies from $ E_{n} = 3$ to 91MeV. In this energy range the average post-neutron total kinetic energy release decreases from $ 162.3 \pm 0.3$ at $ E_n = 3$ MeV to $ 154.9 \pm 0.3$ MeV at $ E_n = 91$ MeV. Analysis of the fission mass distributions indicates that the decrease in TKE with increasing neutron energy is a combination of increasing yields of symmetric fission (which has a lower associated TKE) and a decrease in the TKE release in asymmetric fission.

Fission-fragment total kinetic energy and mass yields for neutron-induced fission of 235U and 238U with En =200 keV – 30 MeV

The average Total Kinetic Energy (TKE ) release and fission-fragment yields in neutron-induced fission of 235 U and 238 U was measured using a Frisch-gridded ionization chamber. These observables are important nuclear data quantites that are relevant to applications and for informing the next generation of fission models. The measurements were performed a the Los Alamos Neutron Science Center and cover E n = 200 keV – 30 MeV. The double-energy (2E) method was used to determine the fission-fragment yields and two methods of correcting for prompt-neutron emission were explored. The results of this study are correlated mass and TKE data.

Metastable decay of nitrogen clusters ions, and determination of the average kinetic energy release and binding energy values

We have carried out measurements on metastable fragmentation of mass selected nitrogen cluster ions, (N2)n+, which are produced by electron impact ionization of a neutral nitrogen cluster beam. We have derived binding energies for n=5–10, from the kinetic energy released in metastable decay reactions using finite heat bath theory. Our results are compared with other experimental results and with quantum chemical calculations.

Cross sections for the CH2+ formation pathways via CH4 fragmentation by electron impact

SynopsisThe mechanisms for CH2+ production, due to methane fragmentation by electron impact in the 22 to 400 eV energy range, were investigated using the DETOF technique. Absolute cross sections for the two possible channels, CH4+ → CH2+ + H2 and CH4+ → CH2+ + 2H, and the corresponding kinetic energy distributions of CH2+ were obtained. Furthermore, the average kinetic energy release of H2 and H neutrals for these two reactions were estimated as 0.04 eV and 2.1 eV, respectively. The dynamics governing these two pathways is discussed.

Cross sections for the formation pathways via CH4 fragmentation by electron impact

The mechanisms for production due to methane fragmentation by electron impact in the 22 to 400 eV energy range were investigated using the delayed extraction time-of-flight technique. Absolute cross sections for the two possible channels, + H2 and + 2 H, and the corresponding kinetic energy distributions of were obtained. Furthermore, the average kinetic energy release of H2 and H neutrals for these two reactions were estimated as 0.3 eV and 2.1 eV, respectively. The dynamics governing these two pathways is discussed.

Kinetic energy distribution for the ionization and dissociation process of C2H4 by electron impact

The dissociative ionization of ethylene molecules following impact by electrons in the energy range of 20 to 200 eV has been studied with a cold target recoil-ion momentum spectrometer (COLTRIMS). The relative partial cross sections for the ionic fragments C2Hn+ (n=0~3) with respect to C2H4+ were plotted as a function of the incident energy. By measuring the time of flight and position of the fragment ions, the kinetic energy distribution has been deduced. Taking advantage of the supersonic jet expansion, the thermal motion contributing to the kinetic energy distribution of the recoil-ions was reduced. The average kinetic energy of the recoil ions as a function of the incident electron energy has also been plotted. Furthermore, the average kinetic energy release for the dissociation to C2H3+ and C2H2+ were estimated as 0.76 eV and 0.73 eV respectively.

Excitation energy dependence of the total kinetic energy release in 235U (n , f)

The total kinetic energy release in the neutron induced fission of $^{235}$U was measured (using white spectrum neutrons from LANSCE) for neutron energies from E$_{n}$ = 3.2 to 50 MeV. In this energy range the average post-neutron total kinetic energy release drops from 167.4 $\pm$ 0.7 to 162.1 $\pm$ 0.8 MeV, exhibiting a local dip near the second chance fission threshold. The values and the slope of the TKE vs. E$_{n}$ agree with previous measurements but do disagree (in magnitude) with systematics. The variances of the TKE distributions are larger than expected and apart from structure near the second chance fission threshold, are invariant for the neutron energy range from 11 to 50 MeV. We also report the dependence of the total excitation energy in fission, TXE, on neutron energy.

Dynamics of the C/H and C/F exchanges in the reaction of 3P carbon atoms with vinyl fluoride

Two product channels C3H2F + H and C3H3 + F were identified in the reaction of C((3)P) atoms with vinyl fluoride (C2H3F) at collision energy 3.7 kcal mol(-1) in a crossed molecular-beam apparatus using selective photoionization. Time-of-flight (TOF) spectra of products C3H2F and C3H3 were measured at 12-16 laboratory angles as well as a TOF spectrum of atomic F, a counter part of C3H3, was recorded at single laboratory angle. From the best simulation of product TOF spectra, translational-energy distributions at seven scattering angles and a nearly isotropic (forward and backward peaked) angular distribution were derivable for exit channel C3H2F + H (C3H3 + F) that has average kinetic-energy release of 14.5 (4.9) kcal mol(-1). Products C3H2F + H and C3H3 + F were estimated to have a branching ratio of ~53:47. Furthermore, TOF spectra and photoionization spectra of products C3H2F and C3H3 were measured at laboratory angle 62° with photoionization energy ranging from 7 eV to 11.6 eV. The appearance of TOF spectra is insensitive to photon energy, implying that only single species overwhelmingly contributes to products C3H2F and C3H3. HCCCHF (H2CCCH) was identified as the dominant species based on the measured ionization threshold of 8.3 ± 0.2 (8.6 ± 0.2) eV and the maximal translational-energy release. The C/H and C/F exchange mechanisms are stated.

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.

Exploring the Dynamics of Reaction C(3P) + C2H4 with Crossed Beam/Photoionization Experiments and Quantum Chemical Calculations

We investigated the title reaction at collision energy 3.5 kcal mol(-1) in a crossed molecular beam apparatus using undulator radiation as an ionization source. Time-of-flight (TOF) spectra of product C(3)H(3) were measured in laboratory angles from 20° to 100° using two photoionization energies 9.5 and 11.6 eV. These two sets of experimental data exhibit almost the same TOF distributions and laboratory angular distributions. From the best simulation, seven angle-specific kinetic-energy distributions and a nearly isotropic angular distribution are derived for product channel C(3)H(3) + H that has an average kinetic-energy release of 15.5 kcal mol(-1), corresponding to an average internal energy of 33.3 kcal mol(-1) in C(3)H(3). Furthermore, TOF spectra of product C(3)H(3) were measured at laboratory angle 52° with ionizing photon energies from 7 to 12 eV. The appearance of TOF spectra remains almost the same, indicating that a species exclusively contributes to product C(3)H(3); the species is identified as H(2)CCCH (propargyl) based on the ionization energy of 8.6 ± 0.2 eV and the maximal kinetic-energy release of 49 kcal mol(-1). Theoretical calculations indicate that the rapid inversion mechanism and rotation in intermediate H(2)CCCH(2) can result in a forward-backward symmetric angular distribution for product C(3)H(3) + H. The present work avoids the interference of reactions of C((1)D) and C(2) radicals with C(2)H(4) and rules out the probability of production of other isomers like c-C(3)H(3) and H(3)CCC proposed in the previous work at least at the investigated collision energy.

Mass-analyzed velocity map imaging of doubly charged photofragments from C70

The velocity distributions of the fragments produced by dissociative photoionization of C(70) have been measured at several photon energies in the extreme UV region, by using a flight-time resolved velocity map imaging (VMI) technique combined with a high-temperature molecular beam and synchrotron radiation. Average kinetic energy release was estimated for the six reaction steps of consecutive C(2) emission, starting from C(70)(2+) → C(68)(2+) + C(2) to C(60)(2+)→ C(58)(2+) + C(2). The total kinetic energy generated in each step shows a general tendency to increase with increasing hν, except for the first and fifth steps. This propensity reflects statistical redistributions of the excess energy in the transition states for the above fragmentation mechanism. Analysis based on the finite-heat-bath theory predicts the detectable minimum cluster sizes at the end of the C(2)-emission decay chain. They accord well with the minimum sizes of the observed ions, if the excess energy in the primary C(70)(2+) is assumed to be smaller by ~15 eV than the maximum available energy. The present VMI experiments reveal remarkably small kinetic energy release in the fifth step, in contradiction to theoretical predictions, which suggests involvement of other fragmentation mechanisms in the formation of C(60)(2+).

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.

Dynamics of multi-channel dissociation of tetrahydrofuran photoexcited at 193 nm: distributions of kinetic energy, angular anisotropies and branching ratios

We investigated the photodissociation dynamics of tetrahydrofuran (c-C(4)H(8)O) at 193.3 nm in a molecular-beam apparatus using photofragment-translational spectroscopy and direct vacuum-ultraviolet (VUV) photoionization. Five dissociation channels leading to products with m/z ratios appropriate for CH(2)CH(2)CH(2) + H(2)CO, CH(2)CHCH(2) + CH(2)OH, H + CH(2)CH(2) + CH(2)CHO, CH(2)CH(2) + CH(3) + HCO and CH(2)CH(2) + CH(2)CO + H(2) were identified; their branching ratios were determined to be 0.40, 0.25, 0.04 0.29 and 0.02, respectively. Secondary dissociations from nascent products CH(2)CH(2)CH(2)CHO to CH(2)CH(2) + CH(2)CHO and from CH(2)CH(2)O to CH(3) + HCO and likely to CH(2)CO + H(2) were observed. We measured distributions of product kinetic energy, average kinetic-energy release, and fractions in translation for each dissociation channel. The formation of CH(2)CHCH(2) + CH(2)OH indicates that hydrogen migration occurs before complete fragmentation. All photofragments have nearly isotropic angular distributions, with |beta| values less than 0.05. The photodissociation of tetrahydrofuran into five channels is proposed to proceed mainly on the ground state potential-energy surface following ring opening and efficient internal conversions.

Experimental Evidence for the Existence of an Electronically Excited State of the Proposed Dihydrogen Radical Cation He‐H‐H‐He+

Survival of the weakest: The existence of a new class of centrosymmetric radical cations in which H(2) bridges two identical main group elements was recently proposed in this journal by Uggerud and co-workers. By growing complexes inside helium nanodroplets at subkelvin temperatures, we obtained experimental evidence for the existence of the most weakly bound member of this class, He-H-H-He(+) (see picture), although in a metastable, electronically excited state.In a recent report, Uggerud and co-workers (A. Krapp et al., Chem. Eur. J. 2008, 14, 4028) proposed the existence of a new class of radical cations in which a dihydrogen bridges two identical main group elements. Upon electron impact ionization of helium nanodroplets doped with one or more H(2) molecules we observe various He(x)H(y) (+) cluster ions, including He(2)H(2) (+), which would belong to the proposed class of radical cations. Mass-analyzed kinetic energy scans reveal that the ion is metastable; it dissociates in the field-free region of the mass spectrometer. One reaction is into HeH(2) (+) + He with a low kinetic energy release of 15+/-4 meV. Surprisingly, another unimolecular reaction is observed, into HeH(+) + HeH (or He + H). The probability of this reaction is an order of magnitude higher, and the average kinetic energy release is four times larger. These findings suggest the presence of a metastable electronically excited state; they are consistent with the proposed linear, centrosymmetric ion structure of He-H-H-He(+).

UV photodissociation of N-methylpyrrole: The role of 1πσ ∗ states in non-hydride heteroaromatic systems

The photophysics of jet-cooled N-methylpyrrole molecules following excitation to their first excited singlet state (the 1A 2 state, arising from a 3s/σ ∗←π electron promotion) has been investigated by resonance enhanced multiphoton ionisation spectroscopy, by measurements of wavelength resolved ‘action’ spectra for forming CH 3 photoproducts, and by velocity map imaging studies of these CH 3 products (in their v = 0 and v 2 = 1 vibrational levels). CH 3 products are observed at all excitation wavelengths within the NMP ( 1 A 2 ← X ∼ 1 A 1 ) absorption band. Direct dissociation on the 1A 2 potential energy surface (PES) yields ‘fast’ CH 3 fragments, with an average total kinetic energy release (TKER) of ∼6500 cm −1, but this product channel is only observed in a narrow wavelength range near the absorption band origin. All of the measured CH 3 images also show a broad component, peaking at lower TKER (∼1700 cm −1); this component extends beneath the ‘fast’ feature in images recorded at wavelengths near the 1 A 2 ← X ∼ 1 A 1 origin, and accounts for all of the CH 3 products observed at shorter photolysis wavelengths. These products are attributed to decay of highly vibrationally excited ground state molecules formed by radiationless transfer from the 1A 2 state. Similarities and differences with the results of previous studies of the H + pyrrolyl products arising in the UV photodissociation of pyrrole are discussed in terms of the likely nuclear motions on the relevant ground and excited PESs (along R N – CH 3 / R N – H ), and the possible couplings between these surfaces. The present study confirms that the proposed model of 1πσ ∗ state induced bond fission in heteroaromatic molecules [A.L. Sobolewski, W. Domcke, Chem. Phys. 259 (2000) 181] is also applicable to non-hydride substituted heteroaromatics, but that mass effects can have an important influence on the subsequent nuclear dynamics.

Fragmentation and mean kinetic energy release of the nitrogen molecule

Ionization and fragmentation of the N2 molecule in coincidence with the final projectile charge state have been measured for the impact of 0.188–0.875MeV/amu He+ projectiles. The average kinetic energy release (KER) of the target ionic fragments is derived from the peak widths of their time-of-flight distributions. It is shown that the KER’s for singly-charged products follow scaling laws irrespectively to the collision channel.

Metastable C 3H 52+ produced by electron impact of propane

Metastability of C 3H 5 2+ ions formed by electron impact ionization of propane was monitored with the help of a magnetic mass spectrometer of reversed geometry (BEE geometry). In the present paper, we report decay reactions resulting in C 3H 4 ++H +, and C 3H 3 ++H 2 +. We observed fragment ions which are formed with high kinetic energy. Mass analyzed ion kinetic energy (MIKE) scan technique in the third field free region of the mass spectrometer was applied to identify the fragment ions and determine their kinetic energy release (KER). An average KER of 1.6±0.3 eV for the first reaction and 0.67±0.15 eV for the second reaction are reported.