Radiation effects in solid nitrogen and nitrogen-containing matrices: fingerprints of N4⁺ species.

The radiation effects and relaxation processes in solid N2 and N2-doped Ne matrices, preirradiated by an electron beam, have been studied in the temperature range of 5-40 and 5-15 K, respectively. The study was performed using luminescence methods: cathodoluminescence CL and developed by our group nonstationary luminescence NsL, as well as optical and current activation spectroscopy methods: spectrally resolved thermally stimulated luminescence TSL and exoelectron emission TSEE. An appreciable accumulation of N radicals, N(+), N2(+) ions, and trapped electrons is found in nitrogen-containing Ne matrices. Neutralization reactions were shown to dominate relaxation scenario in the low-temperature range, while at higher temperatures diffusion-controlled reactions of neutral species contribute. It was conceived that in α-phase of solid N2, the dimerization reaction (N2(+) + N2 → N4(+)) proceeds: "hole self-trapping". Tetranitrogen cation N4(+) manifests itself by the dissociative recombination reaction with electron: N4(+) + e(-) → N2*(a'(1)Σ(u)(-)) + N2 → N2 + N2 + hν. In line with this assumption, we observed a growth of the a'(1)Σ(u)(-) → X(1)Σ(g)(+) transition intensity with an exposure time in CL spectra and the emergence of this emission in the course of electron detrapping on sample heating in the TSL and NsL experiments.

Experimental and theoretical studies of the electronic transitions of BeC.

Electronic spectra for BeC have been recorded over the range 30,500-40,000 cm(-1). Laser ablation and jet-cooling techniques were used to obtain rotationally resolved data. The vibronic structure consists of a series of bands with erratic energy spacings. Two-color photoionization threshold measurements were used to show that the majority of these features originated from the ground state zero-point level. The rotational structures were consistent with the bands of (3)Π-X(3)Σ(-) transitions. Theoretical calculations indicate that the erratic vibronic structure results from strong interactions between the four lowest energy (3)Π states. Adiabatic potential energy curves were obtained from dynamically weighted MRCI calculations. Diabatic potentials and coupling matrix elements were then reconstructed from these results, and used to compute the vibronic energy levels for the four interacting (3)Π states. The predictions were sufficiently close to the observed structure to permit partial assignment of the spectra. Bands originating from the low-lying 1(5)Σ(-) state were also identified, yielding a (5)Σ(-) to X(3)Σ(-) energy interval of 2302 ± 80 cm(-1) and molecular constants for the 1(5)Π state. The ionization energy of BeC was found to be 70,779(40) cm(-1).

Reactions of CH3SH and CH3SSCH3 with gas-phase hydrated radical anions (H2O)n(•-), CO2(•-)(H2O)n, and O2(•-)(H2O)n.

The chemistry of (H(2)O)(n)(•-), CO(2)(•-)(H(2)O)(n), and O(2)(•-)(H(2)O)(n) with small sulfur-containing molecules was studied in the gas phase by Fourier transform ion cyclotron resonance mass spectrometry. With hydrated electrons and hydrated carbon dioxide radical anions, two reactions with relevance for biological radiation damage were observed, cleavage of the disulfide bond of CH(3)SSCH(3) and activation of the thiol group of CH(3)SH. No reactions were observed with CH(3)SCH(3). The hydrated superoxide radical anion, usually viewed as major source of oxidative stress, did not react with any of the compounds. Nanocalorimetry and quantum chemical calculations give a consistent picture of the reaction mechanism. The results indicate that the conversion of e(-) and CO(2)(•-) to O(2)(•-) deactivates highly reactive species and may actually reduce oxidative stress. For reactions of (H(2)O)(n)(•-) with CH(3)SH as well as CO(2)(•-)(H(2)O)(n) with CH(3)SSCH(3), the reaction products in the gas phase are different from those reported in the literature from pulse radiolysis studies. This observation is rationalized with the reduced cage effect in reactions of gas-phase clusters.

Communication: spectroscopic measurements for HfF+ of relevance to the investigation of fundamental constants.

The properties of the HfF(+) cation are thought to be well-suited for investigations of the electron electric dipole moment (eEDM) and temporal variations of the fine structure constant. Precision spectroscopic measurements involving the X(1)Σ(+) and low-lying (3)Δ(1) states have been proposed to measure both. Due to the lack of data for HfF(+), the design of these experiments has relied entirely on the predictions of electronic structure calculations. Spectroscopic characterizations of the X(1)Σ(+), (3)Δ(1), (3)Δ(2) and (3)Δ(3) states are reported. The results further support the contention that HfF(+) is a viable candidate for eEDM measurements. The spacings between adjacent X(1)Σ(+) and (3)Δ(1) levels are found to be less favorable for the proposed studies of the fine structure constant.

Charging effects in an electron bombarded Ar matrix and the role of chemiluminescence-driven relaxation.

The relaxation processes in pure and doped Ar films preirradiated by an electron beam are studied with the focus on charging effects. Correlated real time measurements have been performed applying current and optical activation spectroscopy methods. Thermally stimulated exoelectron emission and thermally stimulated luminescence are detected in the vacuum ultraviolet and visible range. An appreciable accumulation of electrons in the matrix is found, and prerequisites for negative space charge formation are ascertained. The part played by pre-existing and radiation-induced defects as well as dopants is considered and the temperature range of the electron trap stability is elucidated. It is shown that laser-induced electron detachment from O(-) centers results in an enhancement of electron detrapping via the chemiluminescence mechanism, viz. neutralized and thermally mobilized O atoms recombine. Formation of O(2)* results in the emission of visible photons. These photons act as a stimulating factor for electron release and transport, terminating in exoelectron emission and charge recombination. Chemiluminescence therefore plays an important role in the decay of charged centers.

Bonding in beryllium clusters.

Beryllium clusters provide an ideal series for exploring the evolution from discrete molecules to the metallic state. The beryllium dimer has a formal bond order of zero, but the molecule is weakly bound. In contrast, bulk-phase beryllium is a hard metal with a high melting point. Theoretical calculations indicate that the bond energies increase dramatically for Be(n) clusters in the range n=2-6. A triplet ground state is found for n=6, indicating an early emergence of metallic properties. There is an extensive body of theoretical work on smaller Be(n) clusters, in part because this light element can be treated using high-level methods. However, the apparent simplicity of beryllium is deceptive, and the calculations have proved to be challenging owing to strong electron correlation and configuration interaction effects. Consequently, these clusters have become benchmark systems for the evaluation of a wide spectrum of quantum chemistry methods.

Spectroscopic characterization of Be(2) (+) X (2)Sigma(u) (+) and the ionization energy of Be(2).

Rotationally resolved spectra for Be(2) (+) have been recorded using the pulsed-field ionization zero kinetic energy photoelectron technique. Vibrational levels in the range v(+)=0-6 were observed. The rotational selection rules confirmed that the ground state is (2)Sigma(u) (+), resulting from the removal of an electron from the sigma(u) antibonding orbital of Be(2). The bond energy and equilibrium distance for Be(2) (+) were found to be D(e) (+)=16 438(5) cm(-1) and R(e) (+)=2.211(8) A. The ionization energy for Be(2) [59 824(2) cm(-1)] was also refined by these measurements. Comparisons with high-level theoretical results indicate that the bonding in Be(2) (+) is adequately described by multi reference singles and doubles configuration interaction (MRDCI) calculations that employ moderate to large scale basis sets.

Spectroscopy, structure, and ionization energy of BeOBe.

Electronic transitions of BeOBe have been investigated using laser-induced fluorescence and resonance enhanced multiphoton ionization techniques in the 27000-33000 cm-1 range. Vibronic progressions observed in these spectra were assigned to the symmetric and antisymmetric stretching vibrations in the excited electronic state. The nuclear spin statistics of the ground state, observed in the intensity patterns of rotationally resolved spectra, confirmed that the molecule is symmetric (BeOBe) and has 1 Sigma(g)+ symmetry. Analysis of the rotational structure yielded a value of 1.396(3) A for the BeO bond length. Ground state vibrational frequencies were determined using stimulated emission pumping. Photoionization efficiency curves were recorded that yielded a value of 8.119(5) eV for the BeOBe ionization energy. Multireference electronic structure calculations have been used to predict molecular constants and explore the orbital compositions of the ground and excited states.

Beryllium dimer--caught in the act of bonding.

The beryllium dimer is a deceptively simple molecule that, in spite of having only eight electrons, poses difficult challenges for ab initio quantum chemical methods. More than 100 theoretical investigations of the beryllium dimer have been published, reporting a wide range of bond lengths and dissociation energies. In contrast, there have been only a handful of experimental studies that provide data against which these models could be tested. Ultimately, the uncertain extrapolation behavior associated with the available data has prevented quantitative comparisons with theory. In our experiment, we resolve this issue by recording and analyzing spectra that sample all the bound vibrational levels of the beryllium dimer molecule's electronic ground state. After more than 70 years of research on this problem, the experimental data and theoretical models for the dimer are finally reconciled.

Ionization energy measurements and spectroscopy of HfO and HfO+.

Rotationally resolved spectra for the HfO(+) cation have been recorded using the pulsed field ionization zero electron kinetic energy (PFI-ZEKE) technique. Resonant excitation of the F(0(+))<--X (1)Sigma(+) band system of HfO was used as an intermediate level providing molecule and rovibrational state selectivity in the ionization process. The ionization energy (IE) of HfO, derived from the PFI-ZEKE spectrum, was determined to be 7.916 87(10) eV, which is 0.37 eV higher than the value reported from electron impact measurements. Underestimation of the IE in the previous studies is attributed to ionization of thermally excited states. A progression in the HfO(+) stretch vibration up to nu(+)=4 was observed in the PFI-ZEKE spectrum, allowing for the determination of the ground electronic state vibrational frequency of omega(e)(+)=1017.7(10) cm(-1) and anharmonicity of omega(e)x(e)(+)=3.2(2) cm(-1). The rotational constant of HfO(+) was determined to be 0.403(5) cm(-1). Benchmark theoretical ab initio calculations were carried out in order to explore the effects of electron correlation on the predicted molecular properties. Survey scans utilizing laser induced fluorescence and resonance enhanced multiphoton ionization detection revealed many previously unassigned bands in the region of the F-X and G-X bands of HfO, which we attribute to nominally forbidden singlet-triplet transitions of HfO.

Experimental and theoretical study of the electronic spectrum of BeAl.

The electronic structure of BeAl was investigated by laser induced fluorescence and resonance enhanced multiphoton ionization spectroscopy. BeAl was formed by pulsed laser ablation of a Be/Al alloy in the presence of helium carrier gas, followed by a free jet expansion into vacuum. In agreement with recent ab initio studies, the molecule was found to have a (2)Pi(1/2) ground state. Transitions to two low lying electronic states, (2)2Pi1/2(v') <-- X 2Pi1/2 (v'' = 0) and (1)2Delta(v') <-- X 2Pi1/2 (v'' = 0,1), were observed and rotationally analyzed. An additional band system, identified as (4)2Sigma+(v') <-- X 2Pi1/2, was found in the 28 000-30 100 cm(-1) energy range. This transition exhibited an unusual pattern of vibrational levels resulting from an avoided crossing with the (5)2Sigma+ electronic state. New multi-reference configuration interaction calculations were carried out to facilitate the interpretation of the UV bands. An ionization energy of 48 124(80) cm(-1) was determined for BeAl from photoionization efficiency (PIE) measurements. Fine structure in the PIE curve was attributed to resonances with Rydberg series correlating with vibrationally excited states of the BeAl+ ion. Analysis of this structure yielded a vibrational frequency of 240(20) cm(-1) for the cation.

The ionization energy of Be2, and spectroscopic characterization of the (1)3Sigmau+, (2)3Pig, and (3)3Pig states.

Low lying electronic states of the beryllium dimer were investigated by laser induced fluorescence (LIF) and resonance enhanced multiphoton ionization (REMPI) techniques. Be(2) was formed by pulsed laser ablation of Be metal in the presence of helium carrier gas, followed by a free jet expansion into vacuum. Several previously unobserved states of the dimer were characterized. These included transitions of the triplet manifold (2)(3)Pi(g) <-- (1)(3)Sigma(u)+ and (3)(3)Pi(g) <-- (1)(3)Sigma(u)+, for which rotationally resolved bands were obtained. In addition, transitions to the v' = 10-18 vibrational levels of the A (1)Pi(u) state were recorded. Photoionization efficiency (PIE) measurements were used to determine an accurate ionization energy (IE) for Be(2) of 7.418(5) eV and the term energy for (1)(3)Sigma(u)+. Above the ionization threshold the PIE spectrum was found to be highly structured, consisting of overlapping Rydberg series that converged on excited vibrational levels of Be(2)+. Analysis of these series yielded a vibration frequency for the X(2)Sigma(u)+ state of 498(20) cm(-1). The bond dissociation energy for Be(2)+, deduced from the IE measurement, was 16 072(40) cm(-1). Multi-reference configuration interaction (MRCI) calculations were carried out for Be(2) and Be(2)+, yielding results that were in excellent agreement with the experimental observations.

Radiation effects, energy storage and its release in solid rare gases.

An irradiation of solid argon sample by electrons ionizes the Ar atoms, and part of the beam energy is stored in the solid mainly in the form of self-trapped Ar(2)(+) holes. The pre-irradiated samples are investigated by methods of the so called "activation spectroscopy". During their controlled warm-up three thermally stimulated effects are observed and, in our experiments, simultaneously monitored: a VUV emission resulting from neutralization of the Ar(2)(+) holes by electrons, an anomalous desorption of surface atoms, and an exoelectron emission. A comparison of experiments with linear and step-wise sample heating shows clearly that all three processes are intimately connected. The heating detraps electrons, which neutralize the Ar(2)(+) holes resulting in a bound-free emission of argon dimers, centered around 9.7 eV. The excess energy set free during this process may dislodge surface atoms leading to an anomalous, low temperature, pressure rise. Some of the electrons can also be directly extracted from the sample and detected as an exoelectron current. The experiments provide information about the depth of electron traps, and indicate that there is a nearly continuous distribution of trapping energies.

Reactions of large water cluster anions with hydrogen chloride: formation of atomic hydrogen and phase separation in the gas phase.

The reactions of water cluster anions (H2O)n-, n = 30-70, with hydrogen chloride have been studied by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. The first HCl taken up by the clusters is presumably ionically dissolved. The solvated electron recombines with the proton, which is thereby reduced to atomic hydrogen and evaporates from the cluster. This process is accompanied by blackbody radiation and collision induced loss of water molecules. Subsequent collisions lead to uptake of HCl and loss of H2O, yielding mixed clusters Cl-(HCl)m(H2O)n until they are saturated with HCl. Those saturated clusters lose H2O and HCl in a characteristic sequence. The final stage of the reaction, involving clusters with m = 0-4 and n = 0-6, is studied in detail with density functional theory calculations. The Cl-(HCl)4(H2O)6 cluster represents an example for supramolecular self-organization in the gas phase: it consists of a tetrahedral Cl-(HCl)4, connected on one side of the tetrahedron to a compact water hexamer.

Ab initio molecular dynamics studies of ionic dissolution and precipitation of sodium chloride and silver chloride in water clusters, NaCl(H2O)n and AgCl(H2O)n, n = 6, 10, and 14.

An ab initio molecular dynamics method was used to compare the ionic dissolution of soluble sodium chloride (NaCl) in water clusters with the highly insoluble silver chloride (AgCl). The investigations focused on the solvation structures, dynamics, and energetics of the contact ion pair (CIP) and of the solvent-separated ion pair (SSIP) in NaCl(H(2)O)(n) and AgCl(H(2)O)(n) with cluster sizes of n = 6, 10 and 14. We found that the minimum cluster size required to stabilize the SSIP configuration in NaCl(H(2)O)(n) is temperature-dependent. For n = 6, both configurations are present as two distinct local minima on the free-energy profile at 100 K, whereas SSIP is unstable at 300 K. Both configurations, separated by a low barrier (<10 kJ mol(-1)), are identifiable on the free energy profiles of NaCl(H(2)O)(n) for n = 10 and 14 at 300 K, with the Na(+)/Cl(-) pairs being internally solvated in the water cluster and the SSIP configuration being slightly higher in energy (<5 kJ mol(-1)). In agreement with the low bulk solubility of AgCl, no SSIP minimum is observed on the free-energy profiles of finite AgCl(H(2)O)(n) clusters. The AgCl interaction is more covalent in nature, and is less affected by the water solvent. Unlike NaCl, AgCl is mainly solvated on the surface in finite water clusters, and ionic dissolution requires a significant reorganization of the solvent structure.

Reactions of hydrated electrons (H2O)n- with carbon dioxide and molecular oxygen: hydration of the CO2- and O2- ions.

The gas-phase reactions of hydrated electrons with carbon dioxide and molecular oxygen were studied by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Both CO2 and O2 react efficiently with (H2O)n- because they possess low-lying empty pi* orbitals. The molecular CO2- and O2- anions are concurrently solvated and stabilized by the water ligands to form CO2(-)(H2O)n and O2(-)(H2O)n. Core exchange reactions are also observed, in which CO2(-)(H2O)n is transformed into O2(-)(H2O)n upon collision with O2. This is in agreement with the prediction based on density functional theory calculations that O2(-)(H2O)n clusters are thermodynamically favored with respect to CO2(-)(H2O)n. Electron detachment from the product species is only observed for CO2(-)(H2O)2, in agreement with the calculated electron affinities and solvation energies.

Thermally stimulated exoelectron emission from solid neon.

In spite of the negative electron affinity of Ne atoms, appreciable concentrations of electrons can be trapped in solid neon layers formed by depositing the gas on a cold substrate with concurrent electron irradiation. These are trapped at defect sites, and can be promoted into the conduction band in an annealing experiment. They can then recombine with positive charges producing vacuum ultraviolet "thermoluminescence," but can also be extracted from the solid, and detected as an "exoelectron" current. The thermally stimulated exoelectron emission profiles of the electron current versus temperature reveal two broad features near 7.5 and 10 K. These are shown to correspond to two distributions of electron trapping sites with slightly differing activation energies. For the narrower, higher temperature maximum, an average activation energy of about 23 meV is deduced, in good agreement with predictions based on the theory of electronic defect formation.

Generation of C6H4+* by laser vaporization of magnesium with o-C6H4F2 in argon carrier gas.

A route to efficient generation of C6H4+*, potentially the benzyne radical cation, is presented. Laser vaporization of Mg+* and supersonic expansion in helium doped with o-, m-, or p-C6H4F2 yields, among other ions, o-, m-, p-C6H4F2Mg+* complexes, but no C6H4+*. Collision-induced dissociation experiments show that the o-C6H4F2Mg+* complex can be converted into C6H4+* in a mildly energetic collision, with a center-of-mass energy around 1-2 eV. These conditions can also be reached in the ion source when argon is used as a carrier gas. In this way, mass spectra containing the desired m/z 76 peak, i.e. C6H4+*, are obtained.