Kinetic-energy release in Coulomb explosion of metastable C3H52+

Low-Energy Electron Generation for Biomolecular Damage Inquiry: Instrumentation and Methods

Technological advancement has produced a variety of instruments and methods to generate electron beams that have greatly assisted in the extensive theoretical and experimental efforts devoted to investigating the effect of secondary electrons with energies approximately less than 100 eV, which are referred as low-energy electrons (LEEs). In the past two decades, LEE studies have focused on biomolecular systems, which mainly consist of DNA and proteins and their constituents as primary cellular targets of ionizing radiation. These studies have revealed that compared to other reactive species produced by high-energy radiation, LEEs have distinctive pathways and considerable efficiency in inducing lethal DNA lesions. The present work aims to briefly discuss the current state of LEE production technology and to motivate further studies and improvements of LEE generation techniques in relation to biological electron-driven processes associated with such medical applications as radiation therapy and cancer treatment.

Dissociation of cyclopropane in double ionization continuum

Dissociative double photoionization of cyclopropane is studied in the inner-valence region using tunable synchrotron radiation. With the aid of ab initio quantum chemical calculations the energies of dication states and their favoured fragmentation pathways are determined. These are compared to the experimental appearance energies of two-body fragmentation processes and to the kinetic energy released upon dissociation. Photon energy dependent state-selective dissociation in the 25-35 eV range is found. Calculations of dissociation pathways suggest that cyclopropane ring-deformation is selectively triggered at certain photon energies. The calculations suggest that initial ring deformation essentially determines the population of different dication states that function as gateways for particular dissociation channels. The measurements show that stepwise ionization processes populate dissociative 3e'^-2 states via ring-opening and Jahn-Teller active states at photon energies below the double-ionization threshold. For energies above the double-ionization threshold the kinematics indicate that double ionization takes place predominantly within the Franck-Condon region populating 3e'^-1 1e''^-1 states.

Ab Initio/RRKM Study of Dissociation Mechanism of Benzene Trication

Density functional B3LYP /6-31 G (d,p) calculations have been performed in order to investigate isomerization and dissociation of [Formula: see text] in the ground electronic state, which are relevant to the Coulomb explosion mechanism of benzene. The results demonstrate that the benzene-like isomer of [Formula: see text], 1, can decompose through various pathways leading to distinct fragmentation products. The most kinetically favorable channel involves ring opening in 1 accompanied with 1,2-shifts of two hydrogen atoms followed by rotation around the middle C–C bond and dissociation to H 2 CCCH 2+ + H 2 CCCH + with exothermicity and the highest barrier of 100.2 and 38.4 kcal/mol, respectively. Several other product channels, including [Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text], share the same highest barrier for the initial ring opening step with the pathway producing H 2 CCCH 2+ + H 2 CCCH + but exhibit higher second highest barriers. Elimination of [Formula: see text] to form [Formula: see text] is 137.9 kcal/mol exothermic but the highest barrier on the pathway leading to these products is 53.0 kcal/mol. A simple proton elimination to produce [Formula: see text] is computed to be 82–84 kcal/mol exothermic and to depict a 64–65 kcal/mol barrier. RRKM calculations of rate constants for individual reaction steps assuming that the initial internal energy of 1 is 110 kcal/mol and solving kinetic master equations to obtain relative branching ratios show that H 2 CCCH 2+ + H 2 CCCH + are the dominant products (81.5%) followed by [Formula: see text] (13.2%) and the other minor products include [Formula: see text] (2.6%), [Formula: see text] (1.1%), [Formula: see text] (0.55%), [Formula: see text] (0.49%), [Formula: see text] (0.20%), and [Formula: see text] (0.14%). The fragments are expected to be produced with high translational energy due to high Coulomb repulsion energy barriers.

High-resolution kinetic energy release distributions and dissociation energies for fullerene ions Cn+, 42 < or = n < or = 90

We have measured the kinetic energy released in the unimolecular dissociation of fullerene ions, Cn+ --> C(n-2)+ + C2, for sizes 42 < or = n < or = 90. A three-sector-field mass spectrometer equipped with two electric sectors has been used in order to ensure that contributions from isotopomers of different masses do not distort the experimental kinetic energy release distributions. We apply the concept of microcanonical temperature to derive from these data the dissociation energies of fullerene cations. They are converted to dissociation energies of neutral fullerenes with help of published adiabatic ionization energies. The results are compared with literature values.

Mass spectrometric investigation of anions formed upon free electron attachment to nucleobase molecules and clusters embedded in superfluid helium droplets

Here we report the first mass spectrometric study of negative ions formed via free electron attachment (EA) to nucleobases (NBs) embedded in helium clusters. Pure and mixed clusters of adenine and thymine have been formed by pickup of isolated NB molecules by cold helium droplets. In contrast to EA of isolated molecules in the gas phase we observe a long-lived parent anion NB- and, in addition, parent cluster ions NB-n up to size n=6. Moreover, we show that a low energy electron penetrating into a doped helium droplet causes efficient damage of the embedded nucleobases via resonant, site selective, dissociative electron attachment.

Cross sections and ion kinetic energy analysis for the electron impact ionization of acetylene

Using a Nier-type electron impact ion source in combination with a double focusing two sector field mass spectrometer, partial cross sections for electron impact ionization of acetylene are measured for electron energies up to 1000 eV. Discrimination factors for ions are determined using the deflection field method in combination with a three-dimensional ion trajectory simulation of ions produced in the ion source. Analysis of the ion yield curves obtained by scanning the deflectors allows the assignment of ions with the same mass-to-charge ratio to specific production channels on the basis of their different kinetic energy distributions. This analysis also allows to determine, besides kinetic energy distributions of fragment ions, partial cross sections differential in kinetic energy. Moreover a charge separation reaction, the Coulomb explosion of the doubly charged parent ions C2H2++ into the fragment ions C2H+ and H+, is investigated and its mean kinetic energy release (KER=3.88 eV) is deduced.

The anomalous shape of the cross section for the formation of SF3+ fragment ions produced by electron impact on SF6 revisited

The partial ionization cross section for the formation of SF(3) (+) fragment ions following electron impact on SF(6) is known to have a pronounced structure in the cross section curve slightly above 40 eV. We used the mass-analyzed ion kinetic energy (MIKE) scan technique to demonstrate the presence of a channel contributing to the SF(3) (+) partial ionization cross section that we attribute to the Coulomb explosion of doubly charged metastable SF(4) (2+) ions into two singly charged ions SF(3) (+) and F(+), with a threshold energy of about 45.5 eV. Thus the observed unusual shape of the SF(3) (+) partial ionization cross section is the result of two contributions, (i) the direct formation of SF(3) (+) fragment ions via dissociative ionization of SF(6) with a threshold energy of 22 eV and (ii) the Coulomb explosion of metastable SF(4) (2+) ions with a threshold energy of about 45.5 eV. A detailed analysis of the MIKE spectrum reveals an average kinetic energy release of about 5 eV in the Coulomb explosion of the SF(4) (2+) ions with evidence of a second channel corresponding to an average kinetic energy release of about 1.1 eV.