Determination of hydrogen concentration in solids by transmission ERDA under nuclear-elastically enhanced recoiling of H by 8 and 9 MeV He.

Hydrogen concentrations in thin self-supporting samples of polyphenylene sulfide (PPS) and muscovite have been determined by nuclear-elastic recoil detection analysis (ERDA) of transmission layout. The analysis procedure is based only on the database of stopping power and recoil cross section for material analysis, without using any reference sample of known H content. For the PPS sample, the determined value of(2.87±0.26)×1022H cm-3is in good agreement with the calculated value of3.01×1022H cm-3. For the muscovite sample, the H concentration originating each from bound water and absorbed water is uniform over the entire thickness of the sample. The determined concentration(9.43±0.75)×1021H cm-3of the muscovite agrees excellently with the value of9.36×1021H cm-3obtained from other quantitative analyses typically applied for minerals. The present results demonstrate the capability of accurate determination of H contents in materials and minerals by transmission ERDA.

Depth resolution of transmission ERDA for H in Al under nuclear-elastically enhanced recoiling of H by 8 MeV He.

Using stacked samples of Al foil and H-containing resin film, we have carried out elastic recoil detection analysis with transmission layout (T-ERDA) to investigate the depth resolution in the measurements of H distribution in Al. For narrow and wide acceptance conditions of the detector, the depth resolutions of 1.5-4.9µm at several depths in Al of 50 and 80µm thicknesses have been determined for incidence of 8 MeV4He. While the main factor to degrade the depth resolution is the energy straggling of recoil H for narrow acceptance conditions, it is the extended low-energy side of the H spectrum for wide acceptance conditions. The knowledge obtained in this work is useful for analysis of 3D images of H distribution measured by T-ERDA, for example, future analysis of minerals or natural glass samples to determine abundances and distributions of water or OH in the samples.

Kinetic-energy-sensitive mass spectrometry for separation of different ions with the same m/z value.

A double-focusing mass spectrometer (MS) equipped with a superconducting-tunnel-junction (STJ) detector has been applied to measure relative ionization cross-sections for the production of ions that are accompanied by different ion species with the same mass-to-charge (m/z) value. The STJ detector fabricated for this study enables kinetic energy (E) measurement of incoming individual ions at a counting rate of up to approximately 100 k ions/s and an energy resolution (DeltaE/E) of 15%. Both high counting rate and high-energy resolution are necessary to independently determine both m and z and not the m/z value only in ion-counting MS experiments. Ions such as (14)N(2) (2+) and (14)N(+) with the same m/z value can be clearly discriminated using a kinetic-energy-sensitive MS. This fine discrimination capability allows direct determination of relative ionization cross-sections of the homonuclear diatomic ions (14)N(2) (2+)/(14)N(2) (+) and (16)O(2) (2+)/(16)O(2) (+), which are difficult to measure due to the strong interference by the signals of their dissociated atomic ions with noticeably large ionization cross-sections. The new instrument requires no low-abundance heteronuclear diatomic molecules of the forms (14)N(15)N or (16)O(17)O to carry out ionization studies and thus, is expected to be useful in fields such as atmospheric science, interstellar science, or plasma physics.

Stabilities of multiply charged dimers and clusters of fullerenes.

The authors find even-odd variations as functions of r (...+[C60]2(r+)([C60C70](r+)) electron-transfer collisions. This even-odd behavior is in sharp contrast to the smooth one for fullerene monomers and may be related to even-odd effects in dimer ionization energies in agreement with results from an electrostatic model. The kinetic energy releases for dimer dissociations [predominantly yielding intact fullerenes [C60]2(r+)-->C60(r1+)+C60(r2+) in the same (r1=r2) or nearby (r1=r2+/-1) charge states] are found to be low in comparison with the corresponding model results indicating that internal excitations of the separating (intact) fullerenes are important. Experimental appearance sizes for the heavier clusters of fullerenes [C60]n(r+) (n>3 and r=2-5) compare well with predictions from a new nearest-neighbor model assuming that r unit charges in [C60]n(r+) are localized to r C60 molecules such that the Coulomb energy of the system is minimized. The system is then taken to be stable if (i) two (singly) charged C60 are not nearest neighbors and (ii) the r C60(+) molecules have binding energies to their neutral nearest neighbors which are larger than the repulsive energies for the (r-1) C60(+)-C60(+) pairs. Essential ingredients in the nearest-neighbor model are cluster geometries and the present results on dimer stabilities.

Differences in electronic properties of fluorinated and trifluoromethylated fullerenes revealed by their propensity for dianion formation.

The cross sections for electron transfer from sodium to C(60)F(n) (-) and C(60)(CF(3))(n) (-) anions in 50-keV collisions as a function of the number of functional groups n are reported. There are clear differences between derivatives of fluorine and trifluoromethyl due to the different electron withdrawing properties of F and CF(3). The role of inductive effects and pi electron delocalization on the electron affinity is discussed, assuming a correlation between the cross section and the electron affinity of the anion.

Formation of long-lived fluorofullerene trianions in collisions with Na.

The first experimental observation of long-lived triply charged fluorofullerene anions in the gas phase obtained from C60F48 is reported. The existence of a Coloumb barrier trapping the third electron in the trianion is supposed to be responsible for detection of the species which is estimated to have negative third electron affinity.

Hydrogen loss from nucleobase nitrogens upon electron attachment to isolated DNA and RNA nucleotide anions.

Electron transfer to isolated nucleotide monoanions in collisions with Na vapor induces hydrogen loss from nitrogen of the transient nucleobase anion. The cross section for this process is linearly correlated with the number of N-H hydrogens and is highest for guanine. The process is much faster than microseconds since only dehydrogenated dianions survived for mass spectrometric detection. The lifetime of the adenosine 5(')-monophospate dianions was measured to be 0.2 ms in an electrostatic ion storage ring but also a longer-lived component with a lifetime of at least 10 ms was identified. Implications of dissociation along the N-H coordinate for a nucleotide in DNA are briefly discussed in terms of Watson-Crick base pairs.

Photophysics of protoporphyrin ions in vacuo: triplet-state lifetimes and quantum yields.

Lifetimes of triplet-state molecules and triplet quantum yields are important parameters in photobiology as they determine the generation of singlet-oxygen upon irradiation with visible light. Here we report lifetimes of protoporphyrin IX (pp) in vacuo measured in an ion storage ring. We find that after 532 nm photon absorption, pp(-) (free base and negatively charged carboxylate) and pp(+) (single protonation of ring nitrogen) have triplet-state lifetimes of 12 and 6 ms, respectively. After 415 or 390 nm absorption the lifetime of the anion is shorter (1.5 and 0.6 ms) as expected from the increase in temperature. Triplet quantum yields of pp(-) and pp(+) are similar, 0.6-0.7, close to values reported for the free base and monocation in solution. The other channel, direct decay to the electronic ground state and subsequent dissociation of vibrationally excited ions, is much faster than triplet-singlet intersystem crossing. We measured lifetimes of 63 micros, 96 micros, and 0.3 ms after 390, 415, and 532 nm excitation, respectively. A fit of a statistical model to the pp(-) decay results in an Arrhenius activation energy of 0.5+/-0.2 eV for CO(2) loss and a low preexponential factor (10(6)-10(10) s(-1)), indicative of an entropic barrier.

Formation of C60(2-) dianions in collisions between C60- and Na atoms.

We have observed the formation of C2-60 and C2-70 in collisions between C(-)(60)/C(-)(70) and Na atoms. Cross sections for the electron transfer to the monoanion are determined to be 36+/-9 and 57+/-14 A(2) for C-60 and C-70, respectively. A simple model investigation suggests that the electron is transferred from a Na atom to a low-lying electronic state of the fullerene to form a dianion. The method leads to pico-ampere energetic beams of C60 dianions that can be used for spectroscopy and lifetime studies.

Coulomb explosion upon electron attachment to a four-coordinate monoanionic metal complex.

Electron capture to monoanionic metal complexes in high-energy collisions with sodium vapor is shown to occur with the formation of dianions. In this way, we prepared the small dianions Cr(SCN)42-, Fe(CN)42-, Pt(NO2)22-, and Pt(C2O4)22- in the gas phase. The Cr(SCN)42- dianion Coulomb explodes into Cr(SCN)3- and SCN- with a release of kinetic energy (3.2 +/- 0.4 eV) into translational energy of the fragments. The scheme provides a way to study charge dissociation reactions of molecular dianions that are too short-lived to survive extraction from the ion source.