Non-statistical Processes Research Articles

Investigation of (γ, xpxn)-reactions on titanium, lutetium, nickel and chromium nuclei at Ebr=37 MeV

For the first time, both the flux-weighted average yields of the 50Cr(γ,pn+2n)48V, natTi(γ,pxn)46Sc, 48Ti(γ,p)47Sc reactions and the isomeric yield ratios of the 175Lu(γ,n)174Lum,g reaction were measured at a bremsstrahlung end-point energy of 37 MeV. The flux-weighted average yields of the 52Сr(γ,n)51Cr, 58Ni(γ,n)57Ni and 58Ni(γ,p)57Co reactions were also measured and their values coincided with the results of the works of the other authors. According to the results of modeling within the framework of the TALYS-1.96 code, a significant contribution of non-statistical processes is ascertained to the mechanisms of the 48Ti(γ,p)47Sc and 50Cr(γ,pn+2n)48V reactions.

Dissociation processes of ionized freons: CHFCl2+ and CF2Cl2+ in the gas phase.

The present study reveals the effects of symmetry on how the distribution and flow of energy play out on the decomposition of small halocarbons. Unimolecular decay of the freons CHFCl2 and CF2Cl2 when ionized has been investigated. Mass spectrometric results that encompass isotope effects (peak heights) and energy distribution in the exit channel (peak shapes) are interpreted by computational methods. Non-statistical processes of electronic predissociation and isolated state decay are shown to be directly associated with molecular symmetry.

Ion Chemistry of Carbon Dioxide in Nonthermal Reaction with Molecular Hydrogen.

The exothermic hydrogen transfer from H2 to CO2·+ leading to H and HCO2+ is investigated in a combined experimental and theoretical work. The experimental mass/charge ratios of the ionic product (HCO2+) and the ionic reactant (CO2·+) are recorded as a function of the photoionization energy of the synchrotron radiation. Theoretical density functional calculations and variational transition state theory are employed and adapted to analyze the energetic and the kinetics of the reaction, which turns out to be barrierless and with nonthermal rate coefficients controlled by nonstatistical processes. This study aims to understand the mechanisms and energetics that drive the reactivity of the elementary reaction of CO2·+ with H2 in different processes.

Nonstatistical Photoinduced Processes in Gaseous Organic Molecules.

Processes that proceed in femtoseconds are usually referred to as being ultrafast, and they are investigated in experiments that involve laser pulses with femtosecond duration in so-called pump probe schemes, where a light pulse triggers a molecular process and a second light pulse interrogates the temporal evolution of the molecular population. The focus of this review is on the reactivity patterns that arise when energy is not equally distributed on all the available degrees of freedom as a consequence of the very short time scale in play and on how the localization of internal energy in a specific mode can be thought of as directing a process toward (or away from) a certain outcome. The nonstatistical aspects are illustrated with examples from photophysics and photochemistry for a range of organic molecules. The processes are initiated by a variety of nuclear motions that are all governed by the energy gradients in the Franck–Condon region. Essentially, the molecules will start to adapt to the new electronic environment on the excited state to eventually reach the equilibrium structure. It is this structural change that is enabling an ultrafast electronic transition in cases where the nuclear motion leads to a transition point with significant coupling between to electronic states and to ultrafast reaction if there is a coupling to a reactive mode at the transition point between the involved states. With the knowledge of the relation between electronic excitation and equilibrium structure, it is possible to predict how the nuclei move after excitation and often whether an ultrafast (and inherently nonstatistical) electronic transition or even a bond breakage will take place. In addition to the understanding of how nonstatistical photoinduced processes proceed from a given excited state, it has been found that randomization of the energy does not even always take place when the molecule takes part in processes that are normally considered statistical, such as for example nonradiative transitions between excited states. This means that energy can be localized in a specific degree of freedom on a state other than the one that is initially prepared. This is a finding that could kickoff the ultimate dream in applied photochemistry; namely light excitation that leads to the rupture of a specific bond.

Site-selective soft X-ray absorption as a tool to study protonation and electronic structure of gas-phase DNA.

The conformation and the electronic structure of gas-phase oligonucleotides depends strongly on the protonation site. 5'-d(FUAG) can either be protonated at the A-N1 or at the G-N7 position. We have stored protonated 5'-d(FUAG) cations in a cryogenic ion trap held at about 20 K. To identify the protonation site and the corresponding electronic structure, we have employed soft X-ray absorption spectroscopy at the nitrogen K-edge. The obtained spectra were interpreted by comparison to time-dependent density functional theory calculations using a short-range exchange correlation functional. Despite the fact that guanine has a significantly higher proton affinity than adenine, the agreement between experiment and theory is better for the A-N1 protonated system. Furthermore, an inverse site sensitivity is observed in which the yield of the nucleobase fragments that contain the absorption site appears substantially reduced, which could be explained by non-statistical fragmentation processes, localized on the photoabsorbing nucleobase.

Дослідження заселення 7Be в реакціях на ядрах берилію і бору з гальмівними гамма-квантами в широкому діапазоні величин енергії

The average cross-sections were measured at the first time for 7Bе population at bremsstrahlung end-point energies 40 and 55 MeV and for the 10B(γ,t)7Bе reaction at bremsstrahlung end-point energy 20 MeV. The theoretical calculations carried out within the framework of the software packages EMPIRE-3.2 and TALYS-1.9 demonstrate the domination of the nonstatistical processes.

Studying the Activation of 177Lu in (γ, рxn) Reactions

Flux-weighted cross section integrals and weighted average cross sections of 177Lu at the threshold energies of bremsstrahlung gamma rays of 17.5, 20, 37, and 55 MeV are measured for the first time in (γ, pxn)-reactions. Results from modeling with the TALYS-1.9 and EMPIRE-3.2 software packages demonstrate the dominance of nonstatistical processes.

Excitation of 177,178Lu in reactions with bremsstrahlung with escaping of charged particles

Isomeric yields ratio and flux-weighted average cross-sections of 177Lum,g in (γ,α)- and 178Lum,g in (γ,pn)-reactions were determined for the first time for the bremsstrahlung end-point energies of 37 and 55 MeV, properly by activation and off-line γ-ray spectrometric technique. The results of modeling in the framework of the EMPIRE-3.2 code demonstrate the dominance of non-statistical processes.

Excitation of 179Hfm2 with (γ,n)-reaction

Isomeric yields ratio, the integral and flux-weighted average cross-sections of 179m2Hf population were determined for the first time in (γ,n)-reaction for the bremsstrahlung end-point energies of 14.5 and 14.8 MeV by activation and off-line γ-ray spectrometric technique. The results of modeling in the framework of the TALYS-1.6 and EMPIRE-3.2 codes demonstrate the dominance of non-statistical processes.

Hydrogenated pyrene: Statistical single-carbon loss below the knockout threshold

An ongoing discussion revolves around the question of what effect hydrogenation has on carbon backbone fragmentation in polycyclic aromatic hydrocarbons (PAHs). In order to shed more light on this issue, we have measured absolute single carbon loss cross sections in collisions between native or hydrogenated pyrene cations (C16H+ 10+m , m = 0, 6, 16) and He as functions of center-of-mass energies down to 20 eV. Classical molecular dynamics (MD) simulations give further insight into energy transfer processes and also yield m-dependent threshold energies for prompt (femtoseconds) carbon knockout. Such fast, non-statistical fragmentation processes dominate CH x -loss for native pyrene (m = 0), while much slower statistical fragmentation processes contribute significantly to single-carbon loss for the hydrogenated molecules (m = 6 and m = 16). The latter is shown by measurements of large CH x -loss cross sections far below the MD knockout thresholds for C16H+ 16 and C16H+ 26.

Non-statistical fragmentation of large molecules in collisions with atoms

Non-statistical fragmentation processes are important when Polycyclic Aromatic Hydrocarbon (PAH) molecules, fullerenes, or other large molecules collide with atoms at center- of-mass energies from a few tens to a few hundreds of eV. The typical result is the prompt, billiard-ball-like knockout of single atoms (CHx-loss). This is distinct from the well-known statistical fragmentation patterns of these molecules, which are dominated by H- and C2H2-loss for PAHs and C2-loss for fullerenes. We have explored the role of non-statistical fragmentation of PAHs and fullerenes in a series of experimental and theoretical studies. In general, the yield of non-statistical fragments depends sensitively on their stability against secondary statistical fragmentation following knockout.

Collision Induced Dissociation of PAHs and Biomolecules

SynopsisWe have performed an extensive set of experiments with focus on collisions between PAH or biomolecular cations and He atoms at center-of-mass energies around 100 eV. In this energy range, non-statistical fragmentation processes are important.

Nonstatistical fragmentation of large molecules

We present experimental evidence for the dominance of prompt single-atom knockout in fragmenting collisions between large polycyclic aromatic hydrocarbon cations and He atoms at center-of-mass energies close to 100 eV. Such nonstatistical processes are shown to give highly reactive fragments. We argue that nonstatistical fragmentation is dominant for any sufficiently large molecular system under similar conditions.

Non-statistical fragmentation of PAHs and fullerenes in collisions with atoms

Non-statistical fragmentation processes may be important when Polycyclic Aromatic Hydrocarbon molecules (PAHs), fullerenes, or other large complex molecules collide with atoms and atomic ions. For collisions with hydrogen or helium this occurs for center-of-mass energies between a few tens to a few hundreds of electron volts and typically results in losses of single atoms. In such processes one forms much more reactive fragments than in statistical fragmentation, which instead are dominated by losses of C2- or C2H2-molecules (H-atoms) from fullerenes and PAHs, respectively. An enhanced reactivity has recently been demonstrated for van der Waals clusters of C60 molecules where prompt knockouts of single C-atoms from one of the fullerenes yield highly reactive C59+ fragments, which easily form covalent bonds with a C60 molecule inside the clusters.

Vibrational and electronic excitations in fluorinated ethene cations from the ground up

Valence threshold photoelectron spectra of four fluorinated ethenes; C2H3F, 1,1-C2H2F2, C2HF3, and C2F4 were recorded at the Swiss Light Source with 0.002 eV resolution. The adiabatic ionization energies were found to be 10.364 ± 0.007, 10.303 ± 0.005, 10.138 ± 0.007, and 10.110 ± 0.009 eV, respectively. The electronic ground state of each cation shows well-resolved multi-component vibrational progressions, the dominant transitions being in the C=C stretching mode. Density functional theory based Franck-Condon simulations are used to model the vibrational structure and assign the spectra, sometimes revising previous assignments. An additional vibrational progression in the first photoelectron band of 1,1-C2H2F2 indicates that the ground electronic state of the molecular ion is no longer planar. It is shown that ab initio vibrational frequencies together with the observed vibrational spacings do not always suffice to assign the spectra. In addition to symmetry rules governing the transitions, it is often essential to consider the associated Franck-Condon factors explicitly. Ionization to higher lying excited valence electronic states were also recorded by threshold ionization up to 23 eV photon energy. Equation-of-motion coupled cluster with single and double substitutions for ionization potential (EOM-IP-CCSD/cc-pVTZ) calculations confirmed historic electronic state assignments, and untangled the ever more congested spectra with increasing F-substitution. Previous attempts at illuminating the intriguing dissociative photoionization mechanism of fluorinated ethenes are reconsidered in view of new computational and experimental results. We show how non-statistical F-atom loss from C2H3F(+) is decoupled from the ground state dissociation dynamics in the energy range of its C̃ state. Both the statistical and the non-statistical dissociation processes are mediated by a plethora of conical intersections.

Statistical vs non-statistical dissociation processes in W(CO)62+ excited by collisions with 3.1 keV H+, F+, Cl+

Dissociation of W(CO)62+ has been investigated with the CIDEC method using 3.1 keV H+ F+ and Cl+ projectiles. The fragmentation schemes of W(CO)62+ are found drastically different depending on the projectile nature. These differences are explained in terms of different reaction energy windows leading to very small energy deposition in the case of Cl+ and also to close collisions with W(CO)62+ molecule (analog to nuclear stopping in solids).

A simple ‘statistical’ approach for fragmentation studies of doubly ionized cytosine, thymine and uracil bases

A simple statistical model describing the dissociation of molecular dications into correlated fragment pairs has been developed. This model is based on a combinatory approach in which all possible fragments are enumerated and is refined by taking into account the initial structure of the parent molecule, considering the number of chemical bonds to be broken to give rise to the fragments. We show how this model can be used as a tool to help interpret experimental results of coincidence experiments. It shows that dissociation of doubly ionized molecules upon 100 keV proton irradiation is dominated by statistical processes but it also enables easy identification of the dissociation products originating from non-statistical processes requiring further investigation, possibly conveying information on the radiation–molecule interaction itself.

Excitation and dissociation of tungsten hexacarbonyl W(CO)6: Statistical and nonstatistical dissociation processes

We have studied the excitation and dissociation processes of the molecule W(CO)(6) in collisions with low kinetic energy (3 keV) protons, monocharged fluorine, and chlorine ions using double charge transfer spectroscopy. By analyzing the kinetic energy loss of the projectile anions, we measured the excitation energy distribution of the produced transient dications W(CO)(6)(2+). By coincidence measurements between the anions and the stable or fragments of W(CO)(6)(2+), we determined the energy distribution for each dissociation channel. Based on the experimental data, the emission of the first CO was tentatively attributed to a nonstatistical direct dissociation process and the emission of the second or more CO ligands was attributed to the statistical dissociation processes. The dissociation energies for the successive breaking of the W-CO bond were estimated using a cascade model. The ratio between charge separation and evaporation (by the loss of CO(+) and CO, respectively) channels was estimated to be 6% in the case of Cl(+) impact.

Probing the Sub-microsecond Photodissociation Dynamics in Gas-Phase Retinal Chromophores

The photoinduced fragmentation of a retinal model chromophore (all-trans-n-butyl protonated Schiff-base retinal) was studied in vacuo using a new experimental technique. The apparatus is able to record the photodissociation yield of gas-phase biomolecular ions in the first microseconds after absorption. Together with the existing ion storage ring ELISA, which operates on the millisecond to second time scale, the complete decay dynamics of such molecules can now be followed. In the case of retinal, the time-dependent fragmentation yield observed after irradiation with a 410 nm laser pulse exhibits contributions from one- and two-photon absorption, which decay non-exponentially with lifetimes on the order of 1 ms and 1 micros, respectively. The decay can be simulated using a statistical model, yielding good agreement with the experimental findings on both the millisecond and the microsecond time scales. No indication for nonstatistical processes is found for this molecule, the upper limit for a possible direct rate being a factor of 10(4) below the observed statistical dissociation rate.

Photodissociation of Mg+–XCH3 (X=F, Cl, Br, and I) complexes. II. Fragment angular and energy distributions

Angular and energy distributions of photofragments from Mg+-XCH3 (X=F, Cl, Br, and I) were deduced from time-of-flight (TOF) profiles measured by rotating the polarization direction of the dissociation laser with respect to ion beam direction. The TOF profiles of ICH3+ and MgI+ fragment ions produced from Mg+-ICH3 complex with 266 and 355 nm photons showed clear but opposite recoil anisotropy to each other. In addition, BrCH3+ formed by a dissociation of the Mg+-BrCH3 complex at a photolysis wavelength of 266 nm also showed an anisotropic distribution in the TOF profile which had the same behavior as the profile of ICH3+. For Mg+-FCH3 complex, CH3+ and MgF+ formed with a 266 nm photon had also spatial anisotropy, in which the TOF profile of MgF+ was almost opposite to that of MgI+. These anisotropic distributions were explained by (1) local excitation on the Mg+ ion, (2) rapid dissociation compared with a rotational period of the parent complex, and (3) geometrical structures of the parent complexes. Anisotropy beta parameter values were determined to be +1.30(ICH3+), -0.50(MgI+), +0.74(BrCH3+), and +0.75(CH3+ and MgF+). This dependence on the halogen atom observed in beta values was qualitatively explained by both the geometrical parameters and classical rotational periods of parent complexes. In the product energy distribution, 46%, 40%, 21%, 16%, and 16% of available energies were found to be transferred into translational energies of ICH3+, MgI+, BrCH3 +, CH3+, and MgF+, respectively. These values were compared with energy distributions estimated by a statistical prior distribution and a nonstatistical impulsive model. For ICH3+ and MgI+, the translational energies determined from the measurement had values between those estimated from statistical and nonstatistical models. On the other hand, the energy partitioning for the product ions of BrCH3+, CH3+, and MgF+ was found to be almost statistical. From these considerations, we concluded that nonstatistical processes were more important in the dissociation of Mg+-ICH3 than in other systems.