Properties of gaseous closo-[B6X6]2- dianions (X = Cl, Br, I).

A photoelectron imaging study of the gas-phase dianion of pyrromethene-556 is presented. The photoelectron spectra and angular distributions following resonant excitation of the S1 excited state with nanosecond and femtosecond laser pulses are compared, and the influence of the repulsive Coulomb barrier (RCB) in both cases evaluated. Photoelectron angular distributions show the effect of molecular alignment due to an allowed electronic excitation and can be understood qualitatively based on the calculated RCB surface using the Local Static Approximation. Comparison between femtosecond and nanosecond excitation reveals marked differences in the photoelectron spectra. While femtosecond experiments confirm that tunneling through the RCB is adiabatic, nanosecond experiments show a broad photoelectron feature peaking near the RCB maximum. This is explained in terms of the lifetime of internal conversion, which has been determined by time-resolved photoelectron spectroscopy to be ~120 ps: as this is faster than the nanosecond laser pulses, multiple photons can be absorbed through the S1 ← S0 transition which leads to large amounts of internal energy and enables electron detachment directly above the RCB. Fragmentation and detachment from the monoanion are also inferred by the presence of photoelectrons emitted at very low kinetic energy. Our results highlight the difficulty in interpreting photoelectron spectra of polyanions in which a resonant state is excited.

Photoelectron spectroscopy and theoretical investigations have been performed to systematically probe the intrinsic electronic properties of [Mo6X14]2- (X = halogen). All three PE spectra of gaseous [Mo6X14]2- (X = Cl, Br, I) dianions, which were generated by electrospray ionization, exhibit multiple resolved peaks in the recorded binding energy range. Theoretical investigations on the orbital structure and charge distribution were performed to support interpretation of the observed spectra and were further extended onto [Mo6F14]2-, a dianion that was not available for the experimental study. The measured adiabatic (ADE) and vertical detachment energies (VDE) for X = Cl-I were well reproduced by density functional theory calculations (accuracy ∼0.1 eV). Corresponding ADE/VDE values for the dianions were found to be 1.48/2.13 (calc.) and 2.30/2.65, 2.30/2.62, and 2.20/2.42 eV (all expt.) for X = F, Cl, Br, and I, respectively, showing an interesting buckled trend of electron binding energy (EBE) along the halogen series, i.e., EBE (F) ≪ EBE (Cl) ∼ EBE (Br) > EBE (I). Molecular orbital analyses indicate different mixing of metal and halogen atomic orbitals, which is strongly dependent on the nature of X, and suggest that the most loosely bound electrons are detached mainly from the metal core for X = F and Cl, but from halide ligands for X = Br and I. The repulsive Coulomb barrier (RCB), estimated from the photon energy dependent spectra, decreases with increasing halogen size, from 1.8 eV for X = Cl to 1.6 eV for X = I. Electrostatic potential modeling confirms the experimental RCB values and predicts that the most favorable electron detaching pathway should lie via the face-bridging halide ligands.

Doubly deprotonated adenosine 5'-diphosphate ([ADP-2H](2-)) and adenosine 5'-triphosphate ([ATP-2H](2-)) dianions were investigated using infrared multiple photon dissociation (IR-MPD) and photoelectron spectroscopy. Vibrational spectra acquired in the X-H stretch region (X = C, N, O) and augmented by isotope-labelling were compared to density functional theory (DFT) calculations at the B3LYP/TZVPP level. This suggests that in [ATP-2H](2-) the two phosphate groups adjacent to the ribose ring are preferentially deprotonated. Photoelectron spectra recorded at 4.66 and 6.42 eV photon energies revealed adiabatic detachment energies of 1.35 eV for [ADP-2H](2-) and 3.35 eV for [ATP-2H](2-). Repulsive Coulomb barriers were estimated at ~2.2 eV for [ADP-2H](2-) and ~1.9 eV for [ATP-2H](2-). Time-dependent DFT calculations have been used to simulate the photoelectron spectra. Photodetachment occurs primarily from lone pair orbitals on oxygen atoms within the phosphate chain.

The long-range electronic structure of polyanions is defined by the repulsive Coulomb barrier (RCB). Excited states can decay by resonant electron tunnelling through RCBs, but such decay has not been observed for electronically excited states other than the first excited state, suggesting a Kasha-type rule for resonant electron tunnelling. Using action spectroscopy, photoelectron imaging, and computational chemistry, we show that the fluorescein dianion, Fl2–, partially decays through electron tunnelling from the S2 excited state, thus demonstrating anti-Kasha behavior, and that resonant electron tunnelling adheres to Koopmans’ correlations, thus disentangling different channels.

Two polyoxometalate Keggin-type anions, alpha-PM12O40(3-) (M = Mo, W), were transferred to the gas phase by electrospray; their electronic structure and stability were probed by photoelectron spectroscopy. These triply charged anions were found to be highly stable in the gas phase with large adiabatic electron detachment energies of 1.7 and 2.1 eV for M = Mo and W, respectively. The magnitude of the repulsive Coulomb barrier was measured as approximately 3.4 eV for both anions, providing an experimental estimate for the intramolecular Coulomb repulsion present in these highly charged anions. Density functional theory calculations were carried out and compared with the experimental data, providing insight into the electronic structure and valence molecular orbitals of the two Keggin anions. The calculations indicated that the highest occupied molecular orbital and other frontier orbitals for PM12O40(3-) are localized primarily on the mu2-oxo bridging ligands of the polyoxometalate framework, consistent with the reactivity on the mu2-oxo sites observed in solution. It was shown that the HOMO of PW12O40(3-) is stabilized relative to that of PMo12O40(3-) by approximately 0.35 eV. The experimental adiabatic electron detachment energies of PM12O40(3-) (i.e., the electron affinities of PM12O40(2-)) are combined with recent calculations on the proton affinity of PM12O40(3-) to yield O-H bond dissociation energies in PM12O39(OH)2- as approximately 5.1 eV.

Photoelectron imaging as a probe of the repulsive Coulomb barrier in the photodetachment of antimony tartrate dianions

We present ab initio calculations of the repulsive Coulomb barrier for several geometrically stable isomers of the BeC(2-)(4) dianion. We describe how the deformation of certain isomers can account for the experimental Coulomb explosion images of the dianion. For the most stable linear isomer, C(-)(2)BeC(-)(2), we examined the electron tunneling process along the dissociation path to obtain C(-)(2) plus BeC(-)(2). We found the crossing point for autodetachment to be R(c)(dis)= 3.25 A. R(dis) is the bond length between C(-)(2) and BeC(-)(2); at this point, the electron tunneling energy is equal to the maximum of the repulsive Coulomb barrier. In the framework of the Wenzel-Kramer-Brioullin theory, the electron-loss lifetime of the metastable C(-)(2)BeC(-)(2) dianion at the equilibrium geometry, R(dis) = 1.64 A, was estimated to be about 5 ms. This lower limit is in agreement with the experimental results in which the BeC(2-)(4) dianion has a lifetime much longer than 5 micros.

We report photodetachment photoelectron spectroscopy (PES) of gaseous copper phthalocyanine (CuPc) tetrasulfonate quadruply charged anions, [CuPc(SO3)4]4-, and its monoprotonated and -sodiumated triply charged anions, [CuPc(SO3)4H]3- and [CuPc(SO3)4Na]3-. The [CuPc(SO3)4]4- tetraanion was found to possess a negative electron binding energy of −0.9 eV, whereas the trianions have binding energies of 1.0 and 1.2 eV for the sodiumated and protonated species, respectively. The PES spectral features of the three multiply charged anions were observed to be similar to that of the parent CuPc neutral molecule, except that the anions have lower binding energies due to the presence of the negative charges (−SO3-). The data thus suggested a stepwise tuning of the molecular electronic energy levels of the CuPc molecule through charging, wherein the molecular orbital energies of the parent molecule were systematically pushed up by the negative charges. We further carried out semiempirical calculations, which provided insight into the nature of the localized charges on the peripheral −SO3- groups and the intramolecular electrostatic interactions in the multiply charged anions and confirmed the interpretation of the stepwise tuning of molecular energy levels by charging. Photon energy-dependent studies revealed the effects of the repulsive Coulomb barriers on the photodetachment PES spectra of the multiply charged anions. The barrier heights were estimated to be about 3.5 and 2.5 eV for the tetra- and trianions, respectively. We also observed excited states for the multiply charged anions and resonant tunneling through the repulsive Coulomb barriers via the excited states.

We have used femtosecond time-resolved photoelectron spectroscopy to examine the photodissociation dynamics of doubly charged anions IrBr(6)(2-) after excitation at h nu(pump) = 1.6 eV and with a detachment photon energy of h nu(probe) = 4.8 eV. Excited state dynamics proceed by successive decay of the initially excited state, by way of an intermediate and back to the electronic ground state. This is associated with lifetimes of tau(1) = 2.1+/-0.3 ps and tau(2) = 21+/-2 ps, respectively. After nonadiabatic relaxation, the internal energy of the dianion is sufficiently large to induce fragmentation and delayed emission of Br(-) over the repulsive Coulomb barrier with a 79+/-21 ps time constant. As both fragments are negatively charged, Coulomb repulsion at early times (and correspondingly small separations) is reflected in the transient photoelectron spectra. Analysis of both shifts and intensities of the time-dependent bromide detachment features allows determination of the shape of the dissociation barrier. A lower limit of the outer height was retrieved from the kinetic energy release of KER > or = 1.6 eV. Modeling of the dissociation rate with statistical rate theory results in an inner barrier height of E(RCB) = 0.95 eV.

Production of dianionic and trianionic noble metal clusters in a Penning trap

Fullerene dianions in the range C702- to C902- have been created by subjecting trapped fullerene monoanions to low energy electrons in a Penning trap. The dianion production was found to be a function of the trapping-potential depth and the time of interaction between the simultaneously stored monoanions and electrons. Under similar conditions the dianion yield depends on the size of the fullerenes with more than 10% of the trapped C90- ions forming dianions while the corresponding relative yield for C702- was less than 0.1%. The large difference can be explained by the repulsive Coulomb barrier and the second electron affinity of the fullerenes.

We report a photodetachment photoelectron spectroscopy study of a doubly charged anion, ZrF62-, in the gas phase at two photon energies: 6.424 (193 nm) and 7.865 (157 nm) eV. The adiabatic detachment energy, i.e., the second electron affinity of ZrF6, was measured to be extremely high, 2.9(0.2) eV. The vertical detachment energy (VDE) was measured to be 3.4(0.1) eV. Two excited states were also observed for the ZrF6- singly charged anion with VDEs at 4.0 and 4.6 eV, respectively. The repulsive Coulomb barrier, and hence the intramolecular Coulomb repulsion in ZrF62-, was estimated from the photon-energy-dependent spectra to be ∼2.4 eV. The ZrF62- dianion was observed to be metastable with a half-life of ∼4 s under our experimental conditions, confirming theoretical predictions that ZrF62- is thermodynamically unstable against loss of an F-, despite its extremely high electronic stability. We also performed experiment for the singly charged ZrF5-, but could not observe photodetachment even at 157 nm, the highest photon energy available to us. Thus, we estimated that the electron affinity of ZrF5 is larger than 7.8 eV, the highest electron affinity yet confirmed experimentally.

The hallmark of multiply charged anions is the repulsive Coulomb barrier (RCB), which prevents low-energy electrons from being emitted in photodetachment experiments. However, using photoelectron imaging, we have observed persistent near 0 eV electrons during photodetachment of doubly charged dicarboxylate anions, (-)O(2)C(CH(2))(n)CO(2)(-) (D(n)(2-), n = 2-8). Here we show that these low-energy electron signals are well structured and are independent of the detachment photon fluxes or energies. The relative intensities of these signals are dependent on n, with maxima at n = 2, 4, and 6. These near 0 eV electrons cannot come from direct photodetachment of the dianions and are proposed to come from decarboxylation of the product radical anions upon photodetachment of the parent dianions [(*)O(2)C(CH(2))(n)CO(2)(-) --> CO(2) + (*)(CH(2))(n)CO(2)(-)], followed by dissociative autodetachment [(*)(CH(2))(n)CO(2)(-) --> (CH(2))(n) + CO(2) + e] or hydrogen-transfer-induced electron detachment [(*)(CH(2))(n)CO(2)(-) --> CH(2)=CH(CH(2))(n-2)CO(2)H + e]. Energetic considerations suggest that these processes are exothermic. It is further observed that solvation by one water molecule quenches the low-energy electron signals in the spectra of D(n)(2-)(H(2)O), consistent with the proposed mechanisms. These indirect dissociative autodetachment processes are expected to involve cyclic transition states for n > 2, which is in agreement with the dependence on the chain length due to the anticipated strains in the intermediate steps. The quenching of the low-energy electron signals by one water molecule demonstrates the importance of solvation on chemical reactions.

The relevant interaction energies for astrophysical radiative capture reactions are very low, much below the repulsive Coulomb barrier. This leads to low cross sections, low counting rates in $\gamma$-ray detectors, and therefore the need to perform such experiments at ion accelerators placed in underground settings, shielded from cosmic rays. Here, the feasibility of such experiments in the new shallow-underground accelerator laboratory in tunnels VIII and IX of the Felsenkeller site in Dresden, Germany, is evaluated. To this end, the no-beam background in three different types of germanium detectors, i.e. a Euroball/Miniball triple cluster and two large monolithic detectors, is measured over periods of 26-66 days. The cosmic-ray induced background is found to be reduced by a factor of 500-2400, by the combined effects of, first, the 140 meters water equivalent overburden attenuating the cosmic muon flux by a factor of 40, and second, scintillation veto detectors gating out most of the remaining muon-induced effects. The new background data are compared to spectra taken with the same detectors at the Earth's surface and at other underground sites. Subsequently, the beam intensity from the cesium sputter ion source installed in Felsenkeller has been studied over periods of several hours. Based on the background and beam intensity data reported here, for the example of the $^{12}$C($\alpha$,$\gamma$)$^{16}$O reaction it is shown that highly sensitive experiments will be possible.

DNA multiply charged anions stored in a quadrupole ion trap undergo one-photon electron ejection (oxidation) when subjected to laser irradiation at 260 nm (4.77 eV). Electron photodetachment is likely a fast process, given that photodetachment is able to compete with internal conversion or radiative relaxation to the ground state. The DNA [6-mer]3- ions studied here show a marked sequence dependence of electron photodetachment yield. Remarkably, the photodetachment yield (dG6 > dA6 > dC6 > dT6) is inversely correlated with the base ionization potentials (G < A < C < T). Sequences with guanine runs show increased photodetachment yield as the number of guanine increases, in line with the fact that positive holes are the most stable in guanine runs. This correlation between photodetachment yield and the stability of the base radical may be explained by tunneling of the electron through the repulsive Coulomb barrier. Theoretical calculations on dinucleotide monophosphates show that the HOMO and HOMO-1 orbitals are localized on the bases. The wavelength dependence of electron detachment yield was studied for dG63-. Maximum electron photodetachment is observed in the wavelength range corresponding to base absorption (260-270 nm). This demonstrates the feasibility of gas-phase UV spectroscopy on large DNA anions. The calculations and the wavelength dependence suggest that the electron photodetachment is initiated at the bases and not at the phosphates. This also indicates that, although direct photodetachment could also occur, autodetachment from excited states, presumably corresponding to base excitation, is the dominant process at 260 nm. Excited-state dynamics of large DNA strands still remains largely unexplored, and photo-oxidation studies on trapped DNA multiply charged anions can help in bridging the gap between gas-phase studies on isolated bases or base pairs and solution-phase studies on full DNA strands.