Fragment Kinetic Energy Research Articles

Kinetic energies of [formula omitted] ( n ⩽ 58) fragment ions produced by nanosecond laser impact on C 60

Laser-induced fragmentation of C 60 has been studied using a time-of-flight mass spectrometric technique. The average kinetic energies of fragment ions C n + ( n ⩽ 58) have been extracted from the measured full width at half maximum (FWHM) of ion beam profiles. The primary formation mechanism of small fragment ion C n + ( n < 30) is assumed to be a two-step fragmentation process: C 60 sequential decay to unstable C 30 + ion and the binary fission of C 30 + . Considering a second photo absorption process in the later part of laser pulse duration, good agreement is achieved between experiment and theoretical description of photoion formation.

Time-series analysis of vibrational nuclear wave-packet dynamics inD2+

We discuss the extent to which measured time-dependent fragment kinetic energy release (KER) spectra and calculated nuclear probability densities can reveal (1) the transition frequencies between stationary vibrational states, (2) the nodal structure of stationary vibrational states, (3) the ground-state adiabatic electronic potential curve of the molecular ion, and (4) the progression of decoherence induced by random interactions with the environment. We illustrate our discussion with numerical simulations for the time-dependent nuclear motion of vibrational wave packets in the $\text{D}_{2}{}^{+}$ molecular ion caused by the ionization of its neutral ${\text{D}}_{2}$ parent molecule with an intense pump laser pulse. Based on a harmonic time-series analysis, we suggest a general scheme for the full reconstruction, up to an overall phase factor, of the initial wave packets based on measured KER spectra. We apply this scheme in a numerical simulation for vibrational wave packets in $\text{D}_{2}{}^{+}$ molecular ions and show how this reconstruction allows the clear distinction between commonly assumed stationary vibrational state distributions of the molecular ion following the ionization of ${\text{D}}_{2}$.

Femtosecond time-resolved photoelectron-photoion coincidence imaging of multiphoton multichannel photodynamics in NO2

The multiphoton multichannel photodynamics of NO(2) has been studied using femtosecond time-resolved coincidence imaging. A novel photoelectron-photoion coincidence imaging machine was developed at the laboratory in Amsterdam employing velocity map imaging and "slow" charged particle extraction using additional electron and ion optics. The NO(2) photodynamics was studied using a two color pump-probe scheme with femtosecond pulses at 400 and 266 nm. The multiphoton excitation produces both NO(2) (+) parent ions and NO(+) fragment ions. Here we mainly present the time dependent photoelectron images in coincidence with NO(2) (+) or NO(+) and the (NO(+),e) photoelectron versus fragment ion kinetic energy correlations. The coincidence photoelectron spectra and the correlated energy distributions make it possible to assign the different dissociation pathways involved. Nonadiabatic dynamics between the ground state and the A (2)B(2) state after absorption of a 400 nm photon is reflected in the transient photoelectron spectrum of the NO(2) (+) parent ion. Furthermore, Rydberg states are believed to be used as "stepping" states responsible for the rather narrow and well-separated photoelectron spectra in the NO(2) (+) parent ion. Slow statistical and fast direct fragmentation of NO(2) (+) after prompt photoelectron ejection is observed leading to formation of NO(+)+O. Fragmentation from both the ground state and the electronically excited a (3)B(2) and b (3)A(2) states of NO(2) (+) is observed. At short pump probe delay times, the dominant multiphoton pathway for NO(+) formation is a 3x400 nm+1x266 nm excitation. At long delay times (>500 fs) two multiphoton pathways are observed. The dominant pathway is a 1x400 nm+2x266 nm photon excitation giving rise to very slow electrons and ions. A second pathway is a 3x400 nm photon absorption to NO(2) Rydberg states followed by dissociation toward neutral electronically and vibrationally excited NO(A (2)Sigma,v=1) fragments, ionized by one 266 nm photon absorption. As is shown in the present study, even though the pump-probe transients are rather featureless the photoelectron-photoion coincidence images show a complex time varying dynamics in NO(2). We present the potential of our novel coincidence imaging machine to unravel in unprecedented detail the various competing pathways in femtosecond time-resolved multichannel multiphoton dynamics of molecules.

Kinetic energy releases of small amino acids upon interaction with keV ions

In chromatin, DNA is tightly packed into one complex together with histone and non-histone proteins. These proteins are known to protect the DNA against indirect and to some extent even direct radiation damage. Radiation action upon amino acids is thus one of the primary steps in biological radiation action. In this paper we investigate the ionization and fragmentation of the gas-phase amino acids glycine, alanine and valine upon interaction with keV α-particles. High resolution coincidence time-of-flight mass spectrometry is used to determine the dominant fragmentation channels as well as fragment kinetic energies.

Description of deep inelastic transfer reactions at the Fermi energy within transport models

The possibility of studying the dissipative processes near the Fermi energy using the transport equations arising within the Boltzmann-Nordheim-Vlasov approach is discussed. It is shown that this approach makes it possible to reproduce all the main features of deep inelastic collisions of heavy ions: specifically, the deviation curves for reaction fragments and their Coulomb and nuclear rainbows and the strong correlation between the fragment exit angle and kinetic energy loss. The theoretical results are compared with the experimentally obtained data on the isotopic and velocity distributions of light fragments escaping at small angles in the 18O + 181Ta and 18O + 9Be reactions at an energy of 35 MeV/nucleon. It is established that the experimental velocity distributions contain two components: dissipative (with a velocity smaller than that of the projectile ion) and direct (having a maximum at the particle beam velocity). It is shown that the transport equations describe well the dissipative part of the reaction, which is about 30% of the total output. The direct component arises during fission of the projectile ion nucleus and is described well within the Goldhaber model.

Isomeric effects in ion-induced fragmentation of α- and β-alanine

We have investigated the dissociation of α- and β- alanine following impact of slow multicharged ions, namely He+, He2+, O5+ and Xe20+ at 10 keV per charge unit. The collision products were analyzed using a reflectron-type time-of-flight mass spectrometer. In general, for a given projectile, ionization of both alanine-isomers gives rise to similar fragmentation patterns. However, significantly different peak intensities are observed for fragments with mass over charge ratios (m/q) of 30 and 44 amu. These differences can be explained in terms of isomer geometry and fragment stability. By analyzing the fragment kinetic energies, we found that protons as well as ionic oxygen fragments are sufficiently energetic (up to 200 eV for Xe20+) to induce further damage in a biological environment.

Structure properties ofTh226andFm256,258,260fission fragments: Mean-field analysis with the Gogny force

The constrained Hartree-Fock-Bogoliubov method is used with the Gogny interaction D1S to calculate potential energy surfaces of fissioning nuclei $^{226}\mathrm{Th}$ and $^{256,258,260}\mathrm{Fm}$ up to very large deformations. The constraints employed are the mass quadrupole and octupole moments. In this subspace of collective coordinates, many scission configurations are identified ranging from symmetric to highly asymmetric fragmentations. Corresponding fragment properties at scission are derived yielding fragment deformations, deformation energies, energy partitioning, neutron binding energies at scission, neutron multiplicities, charge polarization, and total fragment kinetic energies.

Prompt fission neutron emission in neutron and proton induced reactions at intermediate energies

The generalized model for the description of the prompt fission neutron spectra and multiplicities at neutron and proton energies up to about 100 MeV is described. The three main emission mechanisms considered are the precompound emission, the pre-scission particle evaporation before the saddle point and at descent to the scission point, and the emission from excited fission fragments. The two-component exciton model is used for the description of the preequilibrium stage of the reaction. The time-dependent statistical model with inclusion of the nuclear friction effects describes particle evaporation starting just after the precompound emission stage and lasting for the duration of the evolution of the compound nucleus toward scission. The fragment mass distribution and fission fragment kinetic and excitation energies are determined from the properties of the composite system at the scission point. The particle spectra from the fission fragments are calculated within the statistical approach. These spectra are then transformed into the laboratory rest frame using the calculated fragment kinetic energies and are averaged over the calculated fragment mass distributions. The model was applied to analyze the prompt fission neutron characteristics in the proton and neutron induced fission of actinide nuclei, and comparisons with the experimental data in {supmore » 238}U(n,f), {sup 238}U(p,f), and {sup 242}Pu(p,f) reactions are presented.« less

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.

Fragmentation of multiply ionized fullerene molecules

The dissociation of C60 fullerene molecules has been studied by means of an analysis of the kinetic energies of charged fragments formed upon the capture of several electrons from C60 by multiply charged Ar6+ ions. The kinetic energies of fragment ions of a multiply ionized C60 molecule are distributed in accordance with the Coulomb explosion mechanism of dissociation.

A 193nm laser photofragmentation time-of-flight mass spectrometric study of chloroiodomethane

The photodissociation dynamics of chloroiodomethane (CH2ICl) at 193nm has been investigated by employing the photofragment time-of-flight (TOF) mass spectrometric method. Using tunable vacuum ultraviolet undulator synchrotron radiation for photoionization sampling of nascent photofragments, we have identified four primary dissociation product channels: CH2Cl+I(P1∕22)∕I(P3∕22), CH2I+Cl(P1∕22)∕Cl(P3∕22), CHI+HCl, and CH2+ICl. The state-selective detection of I(P3∕22) and I(P1∕22) has allowed the estimation of the branching ratio for I(P1∕22):I(P3∕22) to be 0.73: 0.27. Theoretical calculations based on the time-dependent density-functional theory have been also made to investigate excited electronic potential-energy surfaces, plausible intermediates, and transition structures involved in these photodissociation reactions. The translation energy distributions derived from the TOF measurements suggest that at least two dissociation mechanisms are operative for these photodissociation processes. One involves the direct dissociation from the 2A′1 state initially formed by 193nm excitation, leading to significant kinetic-energy releases. For the I-atom and Cl-atom elimination channels, the fragment kinetic-energy releases observed via this direct dissociation mechanism are consistent with those predicted by the impulsive dissociation models. Other mechanisms are likely predissociative or statistical in nature from the lower 1A′1 and 1A″1 states and/or the ground X̃A′1 state populated by internal conversion from the 2A′1 state. On the basis of the maximum kinetic-energy release for the formation of CH2Cl+I(P1∕22), we have obtained a value of 53±2kcal∕mol for the 0K bond dissociation energy of I–CH2Cl. The intermediates and transition structures for the CHI+HCl and CH2+ICl product channels have been also investigated by ab initio quantum calculations at the MP2(full)∕6-311G(d) and B3LYP(full)∕6-11G(d) levels of theory. The maximum kinetic-energy releases observed for the CHI+HCl and CH2+ICl channels are consistent with the interpretation that the formation of CHI and CH2 in their ground triplet states is not favored.

Dissociation of water molecules upon keV H+- and Heq+-induced ionization

The interaction of keV H+, He+ and He2+ ions with gas-phase H2O molecules leads to formation of H2Oq+ ions which subsequently can undergo dissociation into various fragment species. From coincident determination of the fragment-ion kinetic energies, kinetic energy releases (KER) for the different dissociation channels are obtained. We present the KER values for the various dissociation channels at different projectile kinetic energies. For He+, extraordinarily large relative dissociation yields are observed as compared to H+ and He2+. For both singly charged projectiles we observe 14.5 eV H+ fragments, most probably due to the two-step H2O2+→ HO2+ + H → H+ + O+ + H process predicted 5 years ago (Nobusada K and Tanaka K 2000 J. Chem. Phys. 112 7437).

Binary fragmentations of excited nuclear systems in the 372 MeVFe56+Th232reaction

The binary fragmentation of the excited superheavy system of Z=116 formed in the reaction 372 MeV {sup 56}Fe+{sup 232}Th has been investigated. The fragment masses and kinetic energies were determined through measurement of the fragment velocities by the time-of-flight method. Neutron spectra were measured in coincidence with the fragments at several angles with respect to the fragment direction, which were analyzed to deduce the total as well as the pre-scission neutron multiplicities. We analyzed the correlations between fragment mass and kinetic energy, gated and nongated by the coincidence with neutrons, to learn about the dynamics of the reaction with respect to the two-body exit channels. The events in the near-mass-symmetric valley region appear to receive a significant contribution from the asymmetric mode of fission of the superheavy compound nucleus Z=116, which can be due to the influence of the closed proton and neutron shells of Z=50 and N=82 in the light fragments. From the observed number of pre-scission neutrons, it is inferred that the time scales of the fissionlike reactions leading to near-mass-symmetric splits are rather large, of the order of several times 10{sup -20} s. The average number of prompt neutrons emitted in the spontaneous fission of such amore » superheavy nucleus is {nu}=(12{+-}1), as deduced from the neutron measurements.« less

Ground and Excited State Dissociation Dynamics of Ionized 1,1-Difluoroethene

The kinetic energy release distributions (KERDs) for the fluorine atom loss from the 1,1-difluoroethene cation have been recorded with two spectrometers in two different energy ranges. A first experiment uses dissociative photoionization with the He(I) and Ne(I) resonance lines, providing the ions with a broad internal energy range, up to 7 eV above the dissociation threshold. The second experiment samples the metastable range, and the average ion internal energy is limited to about 0.2 eV above the threshold. In both energy domains, KERDs are found to be bimodal. Each component has been analyzed by the maximum entropy method. The narrow, low kinetic energy components display for both experiments the characteristics of a statistical, simple bond cleavage reaction: constraint equal to the square root of the fragment kinetic energy and ergodicity index higher than 90%. Furthermore, this component is satisfactorily accounted for in the metastable time scale by the orbiting transition state theory. Potential energy surfaces corresponding to the five lowest electronic states of the dissociating 1,1-C2H2F2+ ion have been investigated by ab initio calculations at various levels. The equilibrium geometry of these states, their dissociation energies, and their vibrational wavenumbers have been calculated, and a few conical intersections between these surfaces have been identified. It comes out that the ionic ground state X2B1 is adiabatically correlated with the lowest dissociation asymptote. Its potential energy curve increases in a monotonic way along the reaction coordinate, giving rise to the narrow KERD component. Two states embedded in the third photoelectron band (B2A1 at 15.95 eV and C2B2 at 16.17 eV) also correlate with the lowest asymptote at 14.24 eV. We suggest that their repulsive behavior along the reaction coordinate be responsible for the KERD high kinetic energy contribution.

Dissociation and Ionization ofH2+by Ultrashort Intense Laser Pulses Probed by Coincidence 3D Momentum Imaging

Laser-induced dissociation and ionization of H(+)(2) were simultaneously measured using coincidence 3D momentum imaging, allowing direct separation of the two processes, even where the fragment kinetic energy is the same for both processes. The results for 45 and 135 fs 790 nm pulses with an intensity of approximately 2.5 x 10(14) W/cm(2) differ from each other much more than one would expect from previous measurements with longer pulses. Ionization was negligible for the longer pulse and was strongly aligned along the laser polarization for the shorter pulse, but showed no structure in its kinetic energy distribution. In addition, the ionization to dissociation ratio was found to be much smaller than theoretically predicted for H(+)(2).

Kinetic energy spectra for fragments and break-up density in multifragmentation

We investigate the possibility, in nuclear fragmentation, to extract information on nuclear density at break-up from fragment kinetic energy spectra using a simultaneous scenario for fragment emission. It is found that a decrease of peak centroids for kinetic energy spectra of fragments with increasing excitation energy can be observed at constant low density, which is different from recently published results of Viola et al. [V.E. Viola, K. Kwiatkowski, J.B. Natowitz, S.J. Yennello, Phys. Rev. Lett. 93 (2004) 132701].

Response of Polyatomic Molecules to Ultrastrong Laser- and Ion-Induced Fields

The exposure of molecules to short, ultrastrong electric fields leads to multiple ionization and a subsequent Coulomb explosion. We present a comparative study where uracil molecules are exposed to fields generated by high-power laser pulses (tau approximately 75 fs, I > 10(16) W/cm2) or swift highly charged ions (0.5 MeV Xe25+) representing a half-cycle pulse of less than 10 fs duration. Molecular dynamics and structural information contained in the fragmentation pathways can be assessed separately. Despite the similar field strengths large differences in fragment kinetic energies are found which are related to field shape and duration with the aid of molecular dynamics simulations.

Microscopic approach of fission dynamics applied to fragment kinetic energy and mass distributions inU238

The collective dynamics of low-energy fission in $^{238}\mathrm{U}$ is described within a time-dependent formalism based on the Gaussian overlap approximation of the time-dependent generator coordinate method. The intrinsic deformed configurations of the nucleus are determined from the self-consistent Hartree-Fock-Bogoliubov procedure employing the effective force D1S with constraints on the quadrupole and octupole moments. Fragment kinetic energy and mass distributions are calculated and compared with experimental evaluations. The effect of the collective dynamics along the fission paths and the influence of initial conditions on these distributions are analyzed and discussed.

Distributions of angular anisotropy and kinetic energy of products from the photodissociation of methanol at 157 nm

We investigated distributions of angular-anisotropy parameter beta and kinetic energy of fragments after photodissociation of methanol using time-of-flight (TOF) mass spectrometry. Fragments, in particular CH(3)O and CO, were successfully detected using tunable radiation from a synchrotron for photoionization. Following O-H bond fission, a CH(3)O fragment with internal energy greater than 104 kJ mol(-1) dissociates to CH(2)O+H. Elimination of two H(2) accompanies formation of CO. The beta value of hydroxyl hydrogen is -0.26 whereas that of methyl hydrogen is zero. H(2) has two distinct components in TOF spectra; these rapid and slow components have beta values -0.30 and -0.18, respectively. The CH(3)+OH dissociation exhibits a highly anisotropic angular distribution with beta= -0.75. The beta values of fragments from CD(3)OH photolysis are addressed. From measurements of angular-anisotropy parameters of various fragments, we surmise that the transition dipole moment mu is almost perpendicular to the C-O-H plane and that n-3p(x) (2 (1)A") is the major photoexcited state at 157 nm.

Temperature and fissility dependence of fragment mass variance in heavy ion induced fission

Fission fragment mass and kinetic energy distributions have been measured in {sup 12}C+{sup nat}Pb, {sup 209}Bi, {sup 235}U, and {sup 11}B+{sup 209}Bi reactions at above barrier energies. The variance of the fragment mass distribution ({sigma}{sub A}{sup 2}) has been studied as a function of the temperature and fissility of the compound nucleus by analyzing the present results along with the available data from literature. It is observed that {sigma}{sub A}{sup 2} increases exponentially with temperature as well as fissility of the compound nucleus. However, when data are analyzed with respect to the fragment temperature (T{sub F}), instead of the temperature of compound nucleus, the dependence of {sigma}{sub A}{sup 2} on fissility is greatly reduced for a given value of T{sub F}. The mass variance shows a universal dependence on T{sub F} irrespective of the fissility of the compound nucleus. This result implies that the mass variance is decided during the later stage of fission process and is related to the scission configuration of the fissioning nucleus.