Eric Surber

Photoelectron imaging spectroscopy of molecular and cluster anions: CS2− and OCS−(H2O)1,2

We report the photoelectron imaging study of molecular and cluster anions CS2− and OCS−(H2O)1,2 at 800, 529, and 400 nm, comparing the results for the hydrated OCS− cluster ions to CS2−. The photoelectron angular distributions are interpreted qualitatively using group theory in the framework of the one-electron, electric-dipole approximations. The energetics of OCS−(H2O)1,2 are compared to the theoretical predictions. The vertical detachment energies of OCS−⋅H2O and OCS−(H2O)2 are determined to be 2.07±0.07 and 2.53±0.07 eV, respectively. An indirect estimate of the adiabatic electron affinity of OCS yields a value of −0.04 eV.

Photoelectron anisotropy and channel branching ratios in the detachment of solvated iodide cluster anions

Photoelectron spectra and angular distributions in 267 nm detachment of the I(-)Ar, I(-)H(2)O, I(-)CH(3)I, and I(-)CH(3)CN cluster anions are examined in comparison with bare I(-) using velocity-map photoelectron imaging. In all cases, features are observed that correlate to two channels producing either I((2)P(3/2)) or I((2)P(1/2)). In the photodetachment of I(-) and I(-)Ar, the branching ratios of the (2)P(1/2) and (2)P(3/2) channels are observed to be approximately 0.4, in both cases falling short of the statistical ratio of 0.5. For I(-)H(2)O and I(-)CH(3)I, the (2)P(1/2) to (2)P(3/2) branching ratios are greater by a factor of 1.6 compared to the bare iodide case. The relative enhancement of the (2)P(1/2) channel is attributed to dipole effects on the final-state continuum wave function in the presence of polar solvents. For I(-)CH(3)CN the (2)P(1/2) to (2)P(3/2) ratio falls again, most likely due to the proximity of the detachment threshold in the excited spin-orbit channel. The photoelectron angular distributions in the photodetachment of I(-), I(-)Ar, I(-)H(2)O, and I(-)CH(3)CN are understood within the framework of direct detachment from I(-). Hence, the corresponding anisotropy parameters are modeled using variants of the Cooper-Zare central-potential model for atomic-anion photodetachment. In contrast, I(-)CH(3)I yields nearly isotropic photoelectron angular distributions in both detachment channels. The implications of this anomalous behavior are discussed with reference to alternative mechanisms, affording the solvent molecule an active role in the electron ejection process.

Photoelectron imaging of carbonyl sulfide cluster anions: Isomer coexistence and competition of excited-state decay mechanisms

We investigate the structure and decay of (OCS)n− cluster ions (n=2–4) using photoelectron imaging spectroscopy. The results indicate the coexistence of isomers with OCS− and covalently bound (OCS)2− cluster cores. A several-fold decrease in the relative abundance of the dimer-based species is observed for n=3 and 4 compared to n=2. The OCS−(OCS)n−1 cluster ions undergo direct photodetachment similar to OCS−⋅H2O, while (OCS)2−(OCS)n−2 exhibits both direct electron detachment and cluster decomposition via ionic fragmentation and autodetachment. The autodetachment originates from either the excited states of the parent cluster or internally excited anionic fragments. It is described using a statistical model of thermionic emission, which assumes rapid thermalization of the excitation energy. A decrease in the relative autodetachment yield in the trimer and tetramer cluster ions, compared to the covalent dimer, is attributed to competition with ionic fragmentation.

Effects of solvation and core switching on the photoelectron angular distributions from (CO2)n− and (CO2)n−⋅H2O

Photoelectron images are recorded in the photodetachment of two series of cluster anions, (CO(2))(n)(-), n=4-9 and (CO(2))(n)(-).H(2)O, n=2-7, with linearly polarized 400 nm light. The energetics of the observed photodetachment bands compare well with previous studies, showing evidence for switching between two anionic core structures: The CO(2)(-) monomer and covalent (CO(2))(2)(-) dimer anions. The systematic study of photoelectron angular distributions (PADs) sheds light on the electronic structure of the different core anions and indicates that solvation by several CO(2) molecules and/or one water molecule has only moderate effect on the excess-electron orbitals. The observed PAD character is reconciled with the symmetry properties of the parent molecular orbitals. The most intriguing result concerns the PADs showing remarkable similarities between the monomer and dimer anion cluster-core types. This observation is explained by treating the highest-occupied molecular orbital of the covalent dimer anion as a linear combination of two spatially separated monomeric orbitals.

Nonexistent electron affinity of OCS and the stabilization of carbonyl sulfide anions by gas phase hydration

We report the formation of heterogeneous OCS–water cluster anions [(OCS)n(H2O)k]− (n⩾1,n+k⩾2), of which OCS−⋅H2O is the most interesting species in view of the near absence of unhydrated OCS− in the same ion source. The presence of OCS−⋅H2O indicates that the intra-cluster formation of OCS− does occur as part of the [(OCS)n(H2O)k]− formation mechanism. In this light, the near absence of unhydrated OCS− anions points towards their metastable nature, while the abundance of the hydrated anions is attributed to the stabilizing effect of hydration. These conclusions are supported by the results of an extensive theoretical investigation of the adiabatic electron affinity (EA) of OCS. We conclude that the EA of OCS is either negative or essentially zero. The best estimate based on the Gaussian-3 theory calculation puts the EA at −0.059±0.061 eV. A study of the structure and energetics of OCS−⋅H2O predicts the existence of four structural isomers. Using the coupled-cluster theory, we find that the most stable str...

Photoelectron imaging of negative ions: atomic anions to molecular clusters

The negative ion photoelectron imaging technique is illustrated using two relatively simple atomic and molecular anion systems, and then applied to the study of a cluster system. Photoelectron images of I- and CS2- at 267 nm and 800 nm respectively are presented. Photoelectron spectra and angular distributions are obtained from the images and the concepts underlying these and their interpretation are outlined. The imaging technique is then applied to (CS2)n - (n = 2-4) cluster anions, for which 400 nm images are presented. Features of these images are highlighted and discussed with reference to solvation effects and structural properties of the cluster anionic moiety. Photoelectron signatures of different forms of the cluster core are discussed. These core structures are anionic monomer units solvated by the remaining n - 1 CS2 molecules or covalent dimer units solvated by the remaining n - 2 molecules. Images of the n = 2 anion at 400, 530 and 800 nm reveal information about the electron detachment processes within the different cluster types and both direct detachment and autodetachment are seen. The direct transitions are seen from clusters with either core type, while autodetachment is only seen from clusters with the covalent dimer core. The imaging work also reveals evidence of a previously unreported electronic transition within the direct detachment band due to the covalently bound core type.

Time-resolved electron detachment imaging of the I- channel in I2Br- photodissociation.

The evolution of the I(-) channel in I(2)Br(-) photodissociation is examined using time-resolved negative-ion photoelectron imaging spectroscopy. The 388 nm photodetachment images obtained at variable delays following 388 nm excitation reveal the transformation of the excess electron from that belonging to an excited trihalide anion to that occupying an atomic orbital localized on the I(-) fragment. With increasing pump-probe delay, the corresponding photoelectron band narrows on a approximately 300 fs time scale. This trend is attributed to the localization of the excess-electron wave function on the atomic-anion fragment and the establishment of the fragments electronic identity. The corresponding band position drifts towards larger electron kinetic energies on a significantly longer, approximately 1 ps, time scale. The gradual spectral shift is attributed to exit-channel interactions affecting the photodetachment energetics, as well as the photoelectron anisotropy. The time-resolved angular distributions are analyzed and found consistent with the formation of the asymptotic I(-) fragment.