Photoelectron imaging and spectroscopy of MI(2)(-) (M = Cs, Cu, Au): evolution from ionic to covalent bonding.

We report a combined experimental and theoretical investigation of MI(2)(-) (M = Cs, Cu, Ag, Au) to explore the chemical bonding in the group IA and IB diiodide complexes. Both photoelectron imaging and low-temperature photoelectron spectroscopy are applied to MI(2)(-) (M = Cs, Cu, Au), yielding vibrationally resolved spectra for CuI(2)(-) and AuI(2)(-) and accurate electron affinities, 4.52 ± 0.02, 4.256 ± 0.010, and 4.226 ± 0.010 eV for CsI(2), CuI(2), and AuI(2), respectively. Spin-orbit coupling is found to be important in all the diiodide complexes and ab initio calculations including spin-orbit effects allow quantitative assignments of the observed photoelectron spectra. A variety of chemical bonding analyses (charge population, bond order, and electron localization functions) have been carried out, revealing a gradual transition from the expected ionic behavior in CsI(2)(-) to relatively strong covalent bonding in AuI(2)(-). Both relativistic effects and electron correlation are shown to enhance the covalency in the gold diiodide complex.

Photoelectron imaging of doubly charged anions, (-)O2C(CH2)nCO2(-) (n = 2-8): observation of near 0 eV electrons due to secondary dissociative autodetachment.

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.

Evidence of significant covalent bonding in Au(CN)(2)(-).

The Au(CN)(2)(-) ion is the most stable Au compound known for centuries, yet a detailed understanding of its chemical bonding is still lacking. Here we report direct experimental evidence of significant covalent bonding character in the Au-C bonds in Au(CN)(2)(-) using photoelectron spectroscopy and comparisons with its lighter congeners, Ag(CN)(2)(-) and Cu(CN)(2)(-). Vibrational progressions in the Au-C stretching mode were observed for all detachment transitions for Au(CN)(2)(-), in contrast to the atomic-like transitions for Cu(CN)(2)(-), revealing the Au-C covalent bonding character. In addition, rich electronic structural information was obtained for Au(CN)(2)(-) by employing 118 nm detachment photons. Density functional theory and high-level ab initio calculations were carried out to understand the photoelectron spectra and obtain insight into the nature of the chemical bonding in the M(CN)(2)(-) complexes. Significant covalent character in the Au-C bonding due to the strong relativistic effects was revealed in Au(CN)(2)(-), consistent with its high stability.

Probing the electronic stability of multiply charged anions: sulfonated pyrene tri- and tetraanions.

The strong intramolecular Coulomb repulsion in multiply charged anions (MCAs) creates a potential barrier that provides dynamic stability to MCAs and allows electronically metastable species to be observed. The 1-hydroxy-3,6,8-pyrene-trisulfonate {[Py(OH)(SO(3))(3)](3-) or HPTS(3-)} was recently observed as a long-lived metastable MCA with a large negative electron binding energy of -0.66 eV. Here we use Penning trap mass spectrometry to monitor the spontaneous decay of HPTS(3-) --> HPTS(*2-) + e(-) and have determined the half-life of HPTS(3-) to be 0.1 s. To explore the limit of electronic metastability, we tried to make the related quadruply charged pyrene-1,3,6,8-tetrasulfonate {[Py(SO(3))(4)](4-)}. However, only its decay product, the triply charged radical anion [Py(SO(3))(4)](*3-), as well as the triply charged ion-pairs [Py(SO(3))(4)H](3-) and [Py(SO(3))(4)Na](3-), was observed, suggesting that the tremendous intramolecular Coulomb repulsion makes the [Py(SO(3))(4)](4-) anion extremely short-lived. Photoelectron spectroscopy data showed that [Py(SO(3))(4)](*3-) is an electronically stable species with electron binding energies of +0.5 eV, whereas [Py(SO(3))(4)H](3-) and [Py(SO(3))(4)Na](3-) possess electron binding energies of 0.0 and -0.1 eV, respectively. Ab initio calculations confirmed the stability of these triply charged species and further predicted a large negative electron binding energy (-2.78 eV) for [Py(SO(3))(4)](4-), consistent with its short lifetime.

Experimental and theoretical studies of complexes of [Pb(m)Ag](-) (m = 1-4).

The metal clusters [Pb(m)Ag](-) (m = 1-4) are studied by photoelectron (PE) spectra and density functional theory (DFT). The adiabatic electron affinity (EA) and vertical detachment energy (VDE) of [Pb(m)Ag](-) are obtained from PE spectra at 308 nm. Theoretical calculation is carried out to search for the lowest-energy geometry and elucidate their structures and bonding mode. By comparing the theoretical results, including EA, VDE and simulated density of state (DOS) spectra, with the experimental determination, the lowest-energy structures for each species are obtained. The analysis of the molecular orbital composition provides evidence that the silver atom binds on lead clusters through an Ag-Pb sigma bond. Moreover, the clusters of [Pb(3)](2-), [Pb(4)](2+), Pb(4) and [Pb(4)](2-) are investigated for aromaticity.

Photoelectron spectroscopy of cold hydrated sulfate clusters, SO4(2-)(H2O)n (n = 4-7): temperature-dependent isomer populations.

Sulfate is an important inorganic anion and its interactions with water are essential to understand its chemistry in aqueous solution. Studies of sulfate with well-controlled solvent numbers provide molecular-level information about the solute-solvent interactions and critical data to test theoretical methods for weakly bounded species. Here we report a low-temperature photoelectron spectroscopy study of hydrated sulfate clusters SO(4)(2-)(H(2)O)(n) (n = 4-7) at 12 K and ab initio studies to understand the structures and dynamics of these unique solvated systems. A significant increase of electron binding energies was observed for the 12 K spectra relative to those at room temperature, suggesting different structural isomers were populated as a function of temperature. Theoretical calculations revealed a competition between isomers with optimal water-solute and water-water interactions. The global minimum isomers all possess higher electron binding energies due to their optimal water-solute interactions, giving rise to the binding energy shift in the 12 K spectra, whereas many additional low-lying isomers with less optimal solvent-solute interactions were populated at room temperature, resulting in a shift to lower electron binding energies in the observed spectra. The current work demonstrates and confirms the complexity of the water-sulfate potential energy landscape and the importance of temperature control in studying the solvent-solute systems and in comparing calculations with experiment.

Photoelectron imaging of multiply charged anions: Effects of intramolecular Coulomb repulsion and photoelectron kinetic energies on photoelectron angular distributions.

Multiply charged anions possess strong intramolecular Coulomb repulsion (ICR), which has been shown to dictate photoelectron angular distributions (PADs) using photoelectron imaging. Here we report the effects of photoelectron kinetic energies on the PADs of multiply charged anions. Photoelectron images on a series of dicarboxylate dianions, (-)O(2)C(CH(2))(n)CO(2) (-) (D(n) (2-), n=3-11) have been measured at two photon energies, 532 and 266 nm. The first photoemission band of D(n) (2-), which is a perpendicular transition in the absence of the ICR, comes from electron detachment of an O lone pair orbital on the -CO(2) (-) end groups. Recent photoelectron imaging studies at 355 nm show that the PADs of D(n) (2-) peak in the directions parallel to the laser polarization for small n due to the ICR, which directs the outgoing electrons along the molecular axis. The current data show much stronger parallel peaking at 532 nm, but much weaker parallel peaking in the 266 nm data, relative to the 355 nm data. These observations indicate that the ICR has greater influence on the trajectories of slow photoelectrons and much reduced effects on faster photoelectrons. This study demonstrates that the PADs of multiply charged anions depend on the interplay between ICR and the outgoing photoelectron kinetic energies.

Photoelectron angular distribution and molecular structure in multiply charged anions.

Photoelectrons emitted from multiply charged anions (MCAs) carry information of the intramolecular Coulomb repulsion (ICR), which is dependent on molecular structures. Using photoelectron imaging, we observed the effects of ICR on photoelectron angular distributions (PAD) of the three isomers of benzene dicarboxylate dianions C6H4(CO2)2(2-) (o-, m- and p-BDC(2-)). Photoelectrons were observed to peak along the laser polarization due to the ICR, but the anisotropy was the largest for p-BDC(2-), followed by the m- and o-isomer. The observed anisotropy is related to the direction of the ICR or the detailed molecular structures, suggesting that photoelectron imaging may allow structural information to be obtained for complex multiply charged anions.

Observation of H2 aggregation onto a doubly charged anion in a temperature-controlled ion trap.

Hydrogen is the second most difficult gas to be condensed due to its weak intermolecular interactions. Here we report observation of H(2) aggregation onto a doubly charged anion, (-)O(2)C(CH(2))(12)CO(2)(-)(DC(2-)). Weakly bound DC(2-)(H(2))(n) clusters were formed in a temperature-controlled ion trap and studied using photoelectron spectroscopy. The onset of clustering was observed at 30 K, whereas extensive condensation was observed at 12 K with n up to 12. Photoelectron spectra were obtained for DC(2-)(H(2))(n) (n = 0-6) at 193 and 266 nm. The spectra of DC(2-)(H(2))(n) were observed to be identical to that of the bare DC(2-) dianion except a slight blue shift, indicating the weak interactions between H(2) and the parent dianion. The blue shift on average amounts to approximately 34 meV (3.3 kJ/mol) per H(2), which represents the lower limit of the H(2) binding energy to DC(2-).

Imaging intramolecular coulomb repulsions in multiply charged anions.

The properties of multiply charged anions are dominated by intramolecular Coulomb repulsion (ICR). Using photoelectron imaging, we show the effect of ICR on photoelectron angular distributions for a series of dianions, -O2C(CH2)_{n}CO_{2};{-} (D_{n};{2-}). The observed photoemission band of D_{n};{2-} was due to a perpendicular transition from the charged end group. However, photoemission intensities were observed to peak along the laser polarization for smaller n due to the strong ICR that forces electrons to be emitted along the molecular axis. This emission pattern weakens with increasing n and at D112- the angular distribution reverses back to peak at the perpendicular direction due to the reduced ICR.

Photoelectron spectroscopy of anions at 118.2 nm: observation of high electron binding energies in superhalogens MCl4- (M=Sc, Y, La).

High energy photon is needed for photoelectron spectroscopy (PES) of anions with high electron binding energies, such as superhalogens and O-rich metal oxide clusters. The highest energy photon used for anion PES in the laboratory has been 157 nm (7.866 eV) from F2 eximer lasers. Here, we report an anion PES experiment using coherent vacuum ultraviolet radiation at 118.2 nm (10.488 eV) by tripling the third harmonic output (355 nm) of a Nd:YAG laser in a XeAr cell. Our study focuses on a set of superhalogen species, MCl(4) (-) (M=Sc, Y, La), which were expected to possess very high electron binding energies. While the 157 nm photon can only access the ground state detachment features for these species, more transitions to the excited states at binding energies higher than 8 eV are observed at 118.2 nm. The adiabatic detachment energies are shown to be, 6.84, 7.02, and 7.03 eV for ScCl(4) (-), YCl(4) (-), and LaCl(4) (-) eV, respectively, whereas their corresponding vertical detachment energies are measured to be 7.14, 7.31, and 7.38 eV.

Negative electron binding energies observed in a triply charged anion: photoelectron spectroscopy of 1-hydroxy-3,6,8-pyrene-trisulfonate.

We report the observation of negative electron binding energies (BEs) in a triply charged anion, 1-hydroxy-3,6,8-pyrene-trisulfonate (HPTS(3-)). Low-temperature photoelectron spectra were obtained for HPTS(3-) at several photon energies, revealing three detachment features below 0 electron BE. The HPTS(3-) trianion was measured to possess a negative BE of -0.66 eV. Despite the relatively high excess energy stored in HPTS(3-), it was observed to be a long-lived anion due to its high repulsive Coulomb barrier (RCB) ( approximately 3.3 eV), which prevents spontaneous electron emission. Theoretical calculations were carried out, which confirmed the negative electron BEs observed. The calculations further showed that the highest occupied molecular orbital in HPTS(3-) is an antibonding pi orbital on the pyrene rings, followed by lone pair electrons in the peripheral -SO(3) (-) groups. Negative electron BE is a unique feature of multiply charged anions due to the presence of the RCB. Such metastable species may be good models to study electron-electron and vibronic interactions in complex molecules.

Experimental and theoretical study on the structure and formation mechanism of [C6H5Cu(m)]- (m = 1-3).

The important intermediate phenyl-copper metal complexes [C(6)H(5)Cu(m)]- (m = 1-3), which are produced from the reactions between copper metal clusters formed by laser ablation and the benzene molecules seeded in argon carrier gas, are studied by photoelectron spectroscopy(PES) and density functional theory (DFT). Their structures and bonding patterns are investigated, which results in the conclusion that C(6)H(5) groups bond perpendicularly on copper clusters through Cu-C sigma bond. The formation mechanism of these complexes has been studied at B3LYP//6-311G(d, p)/Lanl2dz level. Direct insertion reaction between [Cu(m)]- and C(6)H(6) yields intermediate complex [C(6)H(5)Cu(m)H]-, and then eliminates the H atom, or releases the H atom to other neutral Cu atoms or anionic Cu ions via H abstraction reaction. The first step is the rate-limiting step with C-H activation and cleavage, and H abstraction by neutral Cu atom is the most energetically favorable pathway for the final step. Moreover, the complex [C(6)H(5)Cu(2)]- is ascertained to be easier to be generated than [C(6)H(5)Cu(3)]- and [C(6)H(5)Cu]-, which are in excellent agreement with the experimental results.