J. Mathias Weber

Electronic photodissociation spectroscopy of Au4+⋅Arn, n=0–4: Experiment and theory

We report experimental and theoretical studies of the electronic absorption spectra of mass selected Au4+⋅Arn (n=0–4) clusters in the photon energy range of 2.14–3.35 eV. Photodissociation spectra were recorded by monitoring ion depletion upon photon absorption, yielding absolute photodissociation cross sections. The experimental spectra were interpreted by comparing them with calculations of the optical response in the framework of time-dependent density functional theory using cluster structures calculated both by density functional theory (B3-LYP functional) and by ab initio calculations at the RI-MP2 level.

Unusual hydrogen bonding behavior in binary complexes of coinage metal anions with water

We have studied the interaction of atomic coinage metal anions with water molecules by infrared photodissociation spectroscopy of M-.H2O.Ar(n) clusters (M=Cu, Ag, Au; n=1, 2). We compare our observations with calculations on density-functional and coupled cluster levels of theory. The gold anion is bound to the water molecule by a single ionic hydrogen bond, similar to the halide-water complexes. In contrast, zero-point motion in the silver and copper complexes leads to a deviation from this motif.

Low-energy photoelectron imaging spectroscopy of nitromethane anions : Electron affinity, vibrational features, anisotropies, and the dipole-bound state

We present low-energy velocity map photoelectron imaging results for nitromethane anions. The photoelectron spectrum is interpreted with the aid of ab initio theory and Franck-Condon factor calculations. We obtain a new value for the adiabatic electron affinity of nitromethane of (172+/-6) meV and observe the dipole-bound state of nitromethane. The photoelectron angular distributions of the observed features are discussed in the context of threshold laws for photodetachment.

The infrared spectrum of Au−∙CO2

The Au-.CO2 ion-molecule complex has been studied by gas phase infrared photodissociation spectroscopy. Several sharp transitions can be identified as combination bands involving the asymmetric stretch vibrational mode of the CO2 ligand. Their frequencies are redshifted by several hundred cm(-1) from the frequencies of free CO2. We discuss our findings in the framework of ab initio and density-functional theory calculations, using anharmonic corrections to predict vibrational transition energies. The infrared spectrum is consistent with the formation of an aurylcarboxylate anion with a strongly bent CO2 subunit.

Infrared spectra of X−⋅CO2⋅Ar cluster anions (X=Cl,Br,I)

Ion-molecule clusters of the heavier halide anions X-.CO(2) (X=Cl-,Br-,I-) with CO2 have been studied by gas phase infrared photodissociation spectroscopy, using Ar evaporation from the complexes X-.CO2.Ar upon infrared excitation. We observe that the asymmetric stretch vibrational mode of the CO(2) molecule is red-shifted from the frequency of free CO2, with the red-shift increasing toward the lighter halide ions. A similar trend is repeated in the region of the Fermi resonance of the combination bands of the asymmetric stretch vibration with two quanta of the bending vibration and the symmetric stretch vibration. We discuss our findings in the framework of ab initio and density functional theory calculations.

Photoelectron spectroscopy of fullerene dianions C762−, C782−, and C842−

We report laser photoelectron spectra of the doubly negatively charged fullerenes C(76) (2-), C(78) (2-), and C(84) (2-) at 2.33, 3.49, and 4.66 eV photon energy. From these spectra, second electron affinities and vertical detachment energies, as well as estimates for the repulsive Coulomb barriers are obtained. These results are discussed in the context of electrostatic models. They reveal that fullerenes are similar to conducting spheres, with electronic properties scaling with their size. The experimental spectra are compared with the accessible excited states of the respective singly charged product ions calculated in the framework of time dependent density functional theory.

Electronically excited states and visible region photodissociation spectroscopy of Aum+⋅Arn clusters (m=7–9): Molecular dimensionality transition?

Photodissociation spectra were determined for Au(m)(+) . Ar(n) (m=7; n=0-3 and m=8,9; n=0,1) in the photon energy range of 2.14-3.02 eV. Experimental data were compared with predictions of dipole allowed transitions using time-dependent density functional theory (TDDFT) as applied to cluster structures from both DFT (B3-LYP functional) and ab initio calculations at the MP2 level. Argon adduct formation does not significantly perturb the bare metal cluster core structure, but it does change the metal cluster spectrum for highly symmetric cluster structures. The photodissociation spectra are consistent with a transition from planar to three-dimensional gold cluster core geometries between m=7 and m=8 for both n=0 and 1. TDDFT predictions for favored isomers describe experimental absorption features to within +/-0.25 eV. We also discuss size-dependent trends in TDDFT transition energies for the lowest energy two- and three-dimensional structures of Au(m)(+)(m=3-9).

Infrared spectroscopy of anionic hydrated fluorobenzenes.

We investigate the structural motifs of anionic hydrated fluorobenzenes by infrared photodissociation spectroscopy and density functional theory. Our calculations show that all fluorobenzene anions under investigation are strongly distorted from the neutral planar molecular geometries. In the anions, different F atoms are no longer equivalent, providing structurally different binding sites for water molecules and giving rise to a multitude of low-lying isomers. The absorption bands for hexa- and pentafluorobenzene show that only one isomer for the respective monohydrate complexes is populated in our experiment. For C6F6.-H2O, we can assign these bands to an isomer where water forms a weak double ionic hydrogen bond with two F atoms in the ion, in accord with the results of Bowen et al. [J. Chem. Phys. 127, 014312 (2007), following paper.] The spectroscopic motif of the binary complexes changes slightly with decreasing fluorination of the aromatic anion. For dihydrated hexafluorobenzene anions, several isomers are populated in our experiments, some of which may be due to hydrogen bonding between water molecules.

Electronic photodissociation spectra and decay pathways of gas-phase IrBr62−

We report photodissociation action spectra for the dianion IrBr(6)(2-) in the range of 1.08-5.6 eV. The photoproducts observed are IrBr(6)(-), IrBr(5)(-), IrBr(4)(-) and Br(-). Comparison of the action spectra to the aqueous absorption spectrum of K(2)IrBr(6) leads to the determination of solvatochromic shifts of between 0.02 and 0.16 eV in the visible region and approximately 0.3 eV in the ultraviolet. The fragmentation branching ratios vary greatly as a function of photon energy. This behavior can be attributed to differences in the fragmentation mechanisms as well as differences in the excited states that are accessed at different energies. Absorption in the visible region favors fragmentation into IrBr(5)(-) and Br(-), whereas a number of fragmentation channels and mechanisms are active in the ultraviolet region. These mechanisms include fragmentation as well as electron detachment and dissociative electron detachment.

Electronic structure and UV spectrum of hexachloroplatinate dianions in vacuo

We present a joint experimental and theoretical study of the electronic spectrum of hexachloroplatinate dianion. We have measured electronic photodissociation and photodetachment spectra of mass-selected PtCl6(2-) ions in vacuo and compare these with calculated band positions from time-dependent density functional theory and from relativistic calculations. Excitation of an electronic transition of the dianion leads to resonant enhancement of the photodetachment cross section superimposed on direct detachment. Photoexcitation results in loss of Cl(-) and Cl(0), depending on photon energy. The photofragmentation spectrum for formation of the PtCl4(-) fragment ion mirrors the UV∕vis absorption spectrum of PtCl6(2-) in solution with a small solvatochromic shift.