O. Echt

Evolution of “magic numbers” in mass spectra of water clusters

Abstract The well-known intensity, anomalies in the mass spectra of water clusters evolve via evaporation of monomers. The depletion of the relative unstable 22-mer and 29-mer occurs surprisingly late, in the time window 4 t

Formation of long-lived CO−2, N2O−, and their dimer anions, by electron attachment to van der waals clusters

Abstract CO2− and N2O− anions, which cannot be formed by electron attachment to the corresponding molecules in the gas phase, are formed by dissociative attachment to clusters of CO2 and N2O, respectively. The relative yields of the monomer and dimer anions are presented and discussed.

Trapping of low energy electrons at preexisting, cold water clusters

Long‐lived water cluster anions [(H2O)−n, n≥11, and (D2O)−n, n≥12] can be formed by electron attachment to a beam of preformed, neutral clusters; collisional stabilization is not required. Attachment occurs resonantly at or very close to 0 eV incident electron energy; the ion yield is unexpectedly large.

Appearance and ionization energies of multiply-charged C70 parent ions produced by electron impact ionization

Abstract Using an improved crossed beams/mass spectrometer apparatus we have extended previous electron impact ionization studies concerning ionization cross sections and appearance energies of up to quadruply-charged ions of C 70 to ions with charge states up to 6. A novel refined data procedure, involving a simultaneous non-linear weighted least-squares fit of two functions, allows the extraction of the cross section threshold. The ionization energies obtained depend linearly on the precursor charge state and compare well with theoretical predictions and the molecular capacitor scaling law. The corresponding maximum cross sections of C 70 5+ and C 70 6+ are about 1.5 × 10 −18 and 1 × 10 −20 cm 2 , respectively.

Ultracold Water Cluster Anions

Attachment of free electrons to water clusters embedded in helium droplets leads to water-cluster anions (H2O)n(-) and (D2O)n(-) of size n > or = 2. Small water-cluster anions bind to up to 10 helium atoms, providing compelling evidence for the low temperature of these complexes, but the most abundant species are bare cluster anions. In contrast to previous experiments on bare water clusters, which showed very pronounced magic and anti-magic anion sizes below n = 12, the presently observed size distributions vary much more smoothly, and all sizes are easily observed. Noticeable differences are also observed in the stoichiometry of fragment anions formed upon dissociative electron attachment and the energy dependence of their yield. Spectroscopic characterization of these ultracold water-cluster anions promises to unravel the relevance of metastable configurations in experiments and the nature of the still controversial bonding sites for the excess electron in small water-cluster anions.

High-resolution kinetic energy release distributions and dissociation energies for fullerene ions Cn+,42⩽n⩽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.

Binding energy of C60+ revisited – What is the problem?

Abstract The binding energy of C 60 + (or C 60 ) has been the subject of numerous experimental reports. Published values range from E a =2–13 eV for C 2 loss from C 60 + . One reason for this enormous scatter is the fact that the transition state of metastable C 60 has unusual properties which, if not taken into account in the data analysis, lead to erroneous binding energies. We critically examine and re-analyze previously published reports within a common framework (with an A -factor of 5×10 19 s −1 or, equivalently, a Gspann factor of 33.8). The resulting binding energies of 17 previously reported gas-phase studies agree surprisingly well with each other, and their average, E a =10.0±0.2 eV , agrees with theory.

Dissociative Electron Attachment to DNA Bases Near Absolute Zero Temperature: Freezing Dissociation Intermediates

Solvation and temperature are two important variables for controlling chemical change. The rates and products of chemical reactions can alter dramatically in response to large changes in temperature or in moving from the gas to the condensed phase. Understanding such changes provides insight into fundamental aspects of chemistry. Herein, we explore a transition in chemistry brought about by a concomitant change in phase and temperature. We measure the dissociative attachment of electrons to biological molecules—a process important in the radiation damage of DNA—both as free molecules in the gas phase at 400 K and when embedded in superfluid helium at a temperature near absolute zero. The goal is to explore the extent to which the dissociation of the intermediate negative ion that is responsible for the initial attachment of the electrons can be frozen in the extreme environment of ultra-cold and superfluid helium in which molecular vibrations and rotations are in their lowest energy states. The pioneering work of Boudaiffa et al. has demonstrated that low-energy electrons have the potential effectively to induce strand breaks in plasmid DNA and this has motivated a wealth of electron attachment studies with building blocks of DNA in the gas phase 3] and when deposited on thin films , both experimental and theoretical. The unique capability of low-energy electrons to break specific bonds selectively has also been shown previously in gas-phase studies in our own laboratory with the isolated nucleobases (NBs) thymine (T), adenine (A) and uracil. The attachment of a free electron to an isolated nucleobase initially forms an unstable transient negative ion, [NBC ]*, with the same geometry as the neutral precursor (a vertical transition). The attached electron may occupy an antibonding orbital or the vertical transition may end up in the repulsive part of a potential energy curve that begins to separate parts of the transient negative ion. As long as the potential energy of [NBC ]* is higher than that of NB + e, the electron can be detached retaining its initial kinetic energy (elastic scattering) or a reduced kinetic energy (inelastic scattering). Moreover, autodetachment competes with dissociative electron attachment (DEA) until the internuclear separation between the charged and neutral fragments exceeds the intersection of the corresponding potential energy curves of the anionic and neutral system. The time to reach this point of no return towards DEA depends strongly on the mass of the lightest fragment and is shortest if one of the fragments is a hydrogen atom. This may be one reason for the high probability of DEA in NBs and other biomolecules to form a closed-shell anion [NB H] upon electron attachment. 9, 10] In the gas phase, the maximum cross section for hydrogen loss, illustrated in channel 1 a [Reaction (1 a)] , is at around 1 eV for all DNA bases . The H formation, channel 1 b [Reaction (1 b)] , is observed at higher electron energies and has a much lower cross section than neutral hydrogen loss. Furthermore, pronounced site selectivity as a function of the electron energy was discovered for H formation upon free electron attachment to NBs and other organic molecules, as shown in Reaction (1):

Thermionic emission from free, photoexcited tungsten clusters

We report on delayed electron emission from free tungsten clusters, excited by light from a Q‐switched YAG laser. Using a novel ion extraction lens, electron emission can be analyzed over a time range of 50 ns–5 μs after the laser pulse without interference from prompt ions. All clusters of size 5≤n≤40 exhibit delayed emission on this time scale, while delayed emission from smaller clusters does not occur. We analyze the time dependence and size dependence of the emission rate for different wavelengths and fluences. The yield of delayed ions may exceed the yield of prompt ions for intermediate laser fluences. A statistical model is proposed which is based on the assumption that energy randomization in the electronically excited clusters proceeds much faster than in 50 ns, i.e., that the observed phenomenon is the (cluster) analog of thermionic emission. Good agreement with all our experimental findings is achieved, although the model invokes only one adjustable parameter. We argue that other delayed deexc...

Kinetic energy release distributions and evaporation energies for metastable fullerene ions

Abstract Kinetic energy release distributions for spontaneous (metastable) decay of singly, doubly and triply charged fullerene ions have been measured for C 60 z + and C 58 z + . The data are analyzed in terms of generalized Maxwell distributions. Based on recently measured values of the Gspann parameter γ , energies for evaporation of C 2 are derived. An average evaporation energy E a =11.6 eV is obtained for γ =37.6; E a =10.1 eV is obtained for γ =33. No significant dependence of E a on the size or charge state of the ions is observed within the experimental error of about −11% to +5%.