Photoelectron imaging of hydrated carbon dioxide cluster anions

The effects of homogeneous and heterogeneous solvation on the electronic structure and photodetachment dynamics of hydrated carbon dioxide cluster anions are investigated using negative-ion photoelectron imaging spectroscopy. The experiments are conducted on mass-selected [(CO(2))(n)()(H(2)O)(m)()](-) cluster anions with n and m ranging up to 12 and 6, respectively, for selected clusters. Homogeneous solvation in (CO(2))(n)()(-) has minimal effect on the photoelectron angular distributions, despite dimer-to-monomer anion core switching. Heterogeneous hydration, on the other hand, is found to have the marked effect of decreasing the photodetachment anisotropy. For example, in the [CO(2)(H(2)O)(m)()](-) cluster anion series, the photoelectron anisotropy parameter falls to essentially zero with as few as 5-6 water molecules. The analysis of the data, supported by theoretical modeling, reveals that in the ground electronic state of the hydrated clusters the excess electron is localized on CO(2), corresponding to a (CO(2))(n)()(-).(H(2)O)(m)() configuration for all cluster anions studied. The diminishing anisotropy in the photoelectron images of hydrated cluster anions is proposed to be attributable to photoinduced charge transfer to solvent, creating transient (CO(2))(n)().(H(2)O)(m)()(-) states that subsequently decay via autodetachment.     

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.     

Quantum dynamics study of the dissociative photodetachment of HOCO-

Six-dimensional wave packet calculations are carried out to study the behavior of HOCO subsequent to the photodetachment of an electron from the negative anion, HOCO-. It is possible to form stable and/or long-lived HOCO complexes, as well as the dissociative products OH+CO and H+CO2. A variety of observables are determined: the electron kinetic energy (eKE) distributions associated with the OH+CO and H+CO2 channels, the correlated eKE and product translational energy distribution for the OH+CO channel, and product branching ratios. Most of our results are in good accord with the experimental results of Clements, Continetti, and Francisco [J. Chem. Phys. 117, 6478 (2002)], except that the calculated eKE distribution for the H+CO2 channel is noticeably colder than experiment. Reasons for this discrepancy are suggested.     

Electronic relaxation dynamics of water cluster anions

The electronic relaxation dynamics of water cluster anions, (H(2)O)(n)(-), have been studied with time-resolved photoelectron imaging. In this investigation, the excess electron was excited through the p<--s transition with an ultrafast laser pulse, with subsequent electronic evolution monitored by photodetachment. All excited-state lifetimes exhibit a significant isotope effect (tau(D)2(O)/tau(H)2(O) approximately 2). Additionally, marked dynamical differences are found for two classes of water cluster anions, isomers I and II, previously assigned as clusters with internally solvated and surface-bound electrons, respectively. Isomer I clusters with n > or = 25 decay exclusively by internal conversion, with relaxation times that extrapolate linearly with 1/n toward an internal conversion lifetime of 50 fs in bulk water. Smaller isomer I clusters (13 < or = n < or = 25) decay through a combination of excited-state autodetachment and internal conversion. The relaxation of isomer II clusters shows no significant size dependence over the range of n = 60-100, with autodetachment an important decay channel following excitation of these clusters. Photoelectron angular distributions (PADs) were measured for isomer I and isomer II clusters. The large differences in dynamical trends, relaxation mechanisms, and PADs between large isomer I and isomer II clusters are consistent with their assignment to very different electron binding motifs.     

Solvation effects on angular distributions in H- (NH3)n and NH2(-) (NH3)n photodetachment: role of solute electronic structure

We report 355 and 532 nm photoelectron imaging results for H(-)(NH(3))(n) and NH(2)(-)(NH(3))(n), n = 0-5. The photoelectron spectra are consistent with the electrostatic picture of a charged solute (H(-) or NH(2)(-)) solvated by n ammonia molecules. For a given number of solvent molecules, the NH(2)(-) core anion is stabilized more strongly than H(-), yet the photoelectron angular distributions for solvated H(-) deviate more strongly from the unsolvated limit than those for solvated NH(2)(-). Hence, we conclude that solvation effects on photoelectron angular distributions are dependent on the electronic structure of the anion, i.e., the type of the initial orbital of the photodetached electron, rather than merely the strength of solvation interactions. We also find evidence of photofragmentation and autodetachment of NH(2)(-)(NH(3))(2-5), as well as autodetachment of H(-)(NH(3))(5), upon 532 nm excitation of these species.     

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.     

Electronic states of thiophenyl and furanyl radicals and dissociation energy of thiophene via photoelectron imaging of negative ions

We report photoelectron images and spectra of deprotonated thiophene, C(4)H(3)S(-), obtained at 266, 355, and 390 nm. Photodetachment of the α isomer of the anion is observed, and the photoelectron bands are assigned to the ground X(2)A(') (σ) and excited A(2)A(") and B(2)A(") (π) states of the thiophenyl radical. The photoelectron angular distributions are consistent with photodetachment from the respective in-plane (σ) and out-of-plane (π(?)) orbitals. The adiabatic electron affinity of α-(●)C(4)H(3)S is determined to be 2.05 ± 0.08 eV, while the B(2)A(") term energy is estimated at 1.6 ± 0.1 eV. Using the measured electron affinity and the electron affinity/acidity thermodynamic cycle, the C-H(α) bond dissociation energy of thiophene is calculated as DH(298)(H(α)-C(4)H(3)S) = 115 ± 3 kcal/mol. Comparison of this value to other, previously reported C-H bond dissociation energies, in particular for benzene and furan, sheds light of the relative thermodynamic stabilities of the corresponding radicals. In addition, the 266 nm photoelectron image and spectrum of the furanide anion, C(4)H(3)O(-), reveal a previously unobserved vibrationally resolved band, assigned to the B(2)A(") excited state of the furanyl radical, (●)C(4)H(3)O. The observed band origin corresponds to a 2.53 ± 0.01 eV B(2)A(") term energy, while the resolved vibrational progression (853 ± 42 cm(-1)) is assigned to an in-plane ring mode of α-(●)C(4)H(3)O (B(2)A(")).     

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 fragment's 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.     

Solvent resonance effect on the anisotropy of NO(-)(N(2)O)(n) cluster anion photodetachment

Photodetachment from NO(-)(N(2)O)(n) cluster anions (n< or =7) is investigated using photoelectron imaging at 786, 532, and 355 nm. Compared to unsolvated NO(-), the photoelectron anisotropy with respect to the laser polarization direction diminishes drastically in the presence of the N(2)O solvent, especially in the 355 nm data. In contrast, a less significant anisotropy loss is observed for NO(-)(H(2)O)(n). The effect is attributed to photoelectron scattering on the solvent, which in the N(2)O case is mediated by the (2)Pi anionic resonance. No anionic resonances exist for H(2)O in the applicable photoelectron energy range, in line with the observed difference between the photoelectron images obtained with the two solvents. The momentum-transfer cross section, rather than the total scattering cross section, is argued to be an appropriate physical parameter predicting the solvent effects on the photoelectron angular distributions in these cluster anions.     

Slow electron velocity-map imaging photoelectron spectra of the methoxide anion

High resolution anion photodetachment spectra are presented for the methoxide anion and its fully deuterated counterpart. The spectra were obtained with slow electron velocity-map imaging. Improved electron affinities are determined for CH3O as 1.5690+/-0.0019 eV and for CD3O as 1.5546+/-0.0019 eV. The spectra resolve many features associated with spin-orbit and vibronic coupling that were not seen in previous photodetachment studies. Photoelectron angular distributions taken as a function of detachment wavelength for the ground vibronic state transitions are recorded and are consistent with the removal of a nonbonding, p-type electron localized on the oxygen atom. Several hot bands and sequence bands are observed for the first time, providing insight into the vibrational structure of the methoxide anion. The results are compared to recent calculations of the anion photoelectron spectra that incorporate bilinear coupling terms among the methoxy vibrational modes and are found to be in reasonable agreement.     

The identification of a solvated electron pair in the gaseous clusters of Na(-)(H2O)n and Li(-)(H2O)n

By first principles calculations, we explore the possibility that Na(-)(H(2)O)(n) and Li(-)(H(2)O)(n) clusters, which have been measured previously by photoelectron experiments, could serve as gas-phase molecular models for the solvation of two electrons. Such models would capture the electron-electron interaction in a solution environment, which is missed in the well-known anionic water clusters (H(2)O)(n) (-). Our results show that by n = 10, the two loosely bound s electrons in Li(-)(H(2)O)(n) are indeed detached from lithium, and they could exist in either the singlet (spin-paring) or the triplet (spin-coupling) state. In contrast, the two electrons would prefer to stay on the sodium atom in Na(-)(H(2)O)(n) and on the surface of the cluster. The formation of a solvated electron pair and the variation in solvation structures make these two cluster series interesting subjects for further experimental investigation.     

Vibronic coupling in the superoxide anion: the vibrational dependence of the photoelectron angular distribution

We present a comprehensive photoelectron imaging study of the O(2)(X (3)Σ(g)(-),v(')=0-6)←O(2)(-)(X (2)Π(g),v(")=0) and O(2)(a?(1)Δ(g),v(')=0-4)←O(2)(-)(X (2)Π(g),v(")=0) photodetachment bands at wavelengths between 900 and 455 nm, examining the effect of vibronic coupling on the photoelectron angular distribution (PAD). This work extends the v(')=1-4 data for detachment into the ground electronic state, presented in a recent communication [R. Mabbs, F. Mbaiwa, J. Wei, M. Van Duzor, S. T. Gibson, S. J. Cavanagh, and B. R. Lewis, Phys. Rev. A 82, 011401(R) (2010)]. Measured vibronic intensities are compared to Franck-Condon predictions and used as supporting evidence of vibronic coupling. The results are analyzed within the context of the one-electron, zero core contribution (ZCC) model [R. M. Stehman and S. B. Woo, Phys. Rev. A 23, 2866 (1981)]. For both bands, the photoelectron anisotropy parameter variation with electron kinetic energy, β(E), displays the characteristics of photodetachment from a d-like orbital, consistent with the π(g)(?) 2p highest occupied molecular orbital of O(2)(-). However, differences exist between the β(E) trends for detachment into different vibrational levels of the X (3)Σ(g)(-) and a?(1)Δ(g) electronic states of O(2). The ZCC model invokes vibrational channel specific "detachment orbitals" and attributes this behavior to coupling of the electronic and nuclear motion in the parent anion. The spatial extent of the model detachment orbital is dependent on the final state of O(2): the higher the neutral vibrational excitation, the larger the electron binding energy. Although vibronic coupling is ignored in most theoretical treatments of PADs in the direct photodetachment of molecular anions, the present findings clearly show that it can be important. These results represent a benchmark data set for a relatively simple system, upon which to base rigorous tests of more sophisticated models.     

Solvation of O(2)(-) and O(4)(-) by p-difluorobenzene and p-xylene studied by photoelectron spectroscopy

Anion photoelectron spectra of the O(2)(-) . arene and O(4)(-) . arene complexes with p-xylene and p-difluorobenzene are presented and analyzed with the aid of calculations on the anions and corresponding neutrals. Relative to the adiabatic electron affinity of O(2), the O(2)(-) . arene spectra are blueshifted by 0.75-1 eV. Solvation energy alone does not account for this shift, and it is proposed that a repulsive portion of the neutral potential energy surface is accessed in the detachment, resulting in dissociative photodetachment. O(2)(-) is found to interact more strongly with the p-difluorobenzene than the p-xylene. The binding motif involves the O(2)(-) in plane with the arene, interacting via electron donation along nearby C-H bonds. A peak found at 4.36(2) eV in the photoelectron spectrum of O(2)(-) . p-difluorobenzene (p-DFB) is tentatively attributed to the charge transfer state, O(2)(-) . p-DFB(+). Spectra of O(4)(-) . arene complexes show less blueshift in electron binding energy relative to the spectrum of bare O(4)(-), which itself undergoes dissociative photodetachment. The striking similarity between the profiles of the O(4)(-) . arene complexes with the O(4)(-) spectrum suggests that the O(4)(-) molecule remains intact upon complex formation, and delocalization of the charge across the O(4)(-) molecule results in similar structures for the anion and neutral complexes.     

Multidimensional Franck-Condon simulations of photodetachment spectra for the formate-water cluster anion: investigating H atom transfer along the HCOOH+OH reaction coordinate

A new multidimensional Franck-Condon (FC) simulation methodology was applied to an anionic-neutral cluster transition for the first time to investigate the use of photodetachment spectroscopy of the HCOO(-).H(2)O anion as a means to study the HCOO.H(2)O and HCOOH.OH neutral clusters. For the HCOO(-).H(2)O to HCOO.H(2)O transition, vibrationally resolved simulated spectra were obtained across the threshold detachment region, indicating that photodetachment spectroscopy of the respective anionic cluster should provide detailed structural information on the bifurcated HCOO.H(2)O neutral cluster. The simulations predict that the photodetachment spectra should display prominent progressions of both the intermolecular stretch and the in-plane OCO bending mode. In contrast, for the HCOO(-).H(2)O to HCOOH.OH transition, the vibronic FC simulations resulted in transitions with negligible intensities, despite the fact that the geometries of the respective anionic and neutral systems were similar. The low FC intensities were traced to the large off-diagonal elements of the Duschinsky matrix for this transition, which arise due to the considerable differences in the vibrational wave functions following hydrogen transfer.     

Photoelectron angular distributions in negative-ion photodetachment from mixed sp states

We describe an approach for constructing analytical models for the energy-dependence of photoelectron angular distributions in the one-electron, non-relativistic approximation. We construct such a model for electron emission from an orbital described as a superposition of s- and p-type functions, using linearly polarized light. In the limits of pure s or pure p electron photodetachment or photoionization, the model correctly reproduces the familiar Cooper-Zare formula. The model predictions are compared to experimental results for strongly solvated H(-) and NH(2)(-), corresponding to predominantly s and predominantly p character parent states, respectively.     

The C-H bond dissociation energy of furan: photoelectron spectroscopy of the furanide anion

Using photoelectron spectroscopy, we interrogate the cyclic furanide anion (C(4)H(3)O(-)) to determine the electron affinity and vibrational structure of the neutral furanyl radical and the term energy of its first excited electronic state. We present the 364-nm photoelectron spectrum of the furanide anion and measure the electron affinity of the X?(2)A(') ground state of the α-furanyl radical to be 1.853(4) eV. A Franck-Condon analysis of the well-resolved spectrum allows determination of the harmonic frequencies of three of the most active vibrational modes upon X?(2)A(') ← X?(1)A(') photodetachment: 855(25), 1064(25), and 1307(40) cm(-1). These modes are ring deformation vibrations, consistent with the intuitive picture of furanide anion photodetachment, where the excess electron is strongly localized on the α-carbon atom. In addition, the A?(2)A(') excited state of the α-furanyl radical is observed 0.68(7) eV higher in energy than the X?(2)A(') ground state. Through a thermochemical cycle involving the known gas-phase acidity of furan, the electron affinity of the furanyl radical yields the first experimental determination of the C-H(α) bond dissociation energy of furan (DH(298)(C(4)H(3)O-H(α))): 119.8(2) kcal mol(-1).     

On the stability of glycine-water clusters with excess electron: implications for photoelectron spectroscopy

Calculations are presented for the glycine-(H(2)O)(n) (-) (n=0-2) anionic clusters with excess electron, with the glycine core in the canonical or zwitterion form. A variety of conformers are predicted, and their relative energy is examined to estimate thermodynamic stability. The dynamic (proton transfer) pathways between the anionic clusters with the canonical and the zwitterion glycine core are examined. Small barrier heights for isomerization from the zwitterion glycine-(H(2)O)(2) (-) anion to those with canonical glycine core suggest that the former conformers may be kinetically unstable and unfavorable for detection of neutral glycine zwitterion-(H(2)O)(n) (n=1,2) clusters by photodetachment, in accordance with the photoelectron spectroscopic experiments by Bowen and co-workers [Xu et al., J. Chem. Phys. 119, 10696 (2003)]. The calculated stability of the glycine-(H(2)O)(n) (-) anion clusters with canonical glycine core relative to those with zwitterion core indicates that the observation of the anionic conformers with the canonical glycine core would be much more feasible, as revealed by Johnson and co-workers [Diken et al. J. Chem. Phys. 120, 9902 (2004)].     

Photoelectron spectroscopy of hydrated adenine anions

We report the observation of hydrated adenine anions, A(-)(H(2)O)(n), n=1-7, and their study by anion photoelectron spectroscopy. Values for photoelectron threshold energies, E(T), and vertical detachment energies are tabulated for A(-)(H(2)O)(n) along with those for hydrated uracil anions, U(-)(H(2)O)(n), which are presented for comparison. Analysis of these and previously measured photoelectron spectra of hydrated nucleobase anions leads to the conclusion that threshold energies significantly overstate electron affinity values in these cases, and that extrapolation of hydrated nucleobase anion threshold values to n=0 leads to incorrect electron affinity values for the nucleobases themselves. Sequential shifts between spectra, however, lead to the conclusion that A(-)(H(2)O)(3) is likely to be the smallest adiabatically stable, hydrated adenine anion.     

Photoelectron imaging of Ag(-)(H2O)x and AgOH(-)(H2O)y (x=1,2, y=0-4)

The photoelectron images of Ag(-)(H(2)O)(x) (x=1,2) and AgOH(-)(H(2)O)(y) (y=0-4) are reported. The Ag(-)(H(2)O)(1,2) anionic complexes have similar characteristics to the other two coinage metal-water complexes that can be characterized as metal atomic anion solvated by water molecules with the electron mainly localized on the metal. The vibrationally well-resolved photoelectron spectrum allows the adiabatic detachment energy (ADE) and vertical detachment energy (VDE) of AgOH(-) to be determined as 1.18(2) and 1.24(2) eV, respectively. The AgOH(-) anion interacts more strongly with water molecules than the Ag(-) anion. The photoelectron spectra of Ag(-)(H(2)O)(x) and AgOH(-)(H(2)O)(y) show a gradual increase in ADE and VDE with increasing x and y due to the solvent stabilization.     

Dissociative photodetachment dynamics of the iodide-aniline cluster

The photodetachment dynamics of the iodide-aniline cluster, I-(C6H5NH2), were investigated using photoelectron-photofragment coincidence spectroscopy at several photon energies between 3.60 and 4.82 eV in concert with density functional theory calculations. Direct photodetachment from the solvated I- chromophore and a wavelength-independent autodetachment process were observed. Autodetachment is attributed to a charge-transfer-to-solvent reaction in which incipient continuum electrons photodetached from I- are temporarily captured by the nascent neutral iodine-aniline cluster configured in the anion geometry. Subsequent dissociation of the neutral cluster removes the stabilization, leading to autodetachment of the excess electron. The dependence of the dissociative photodetachment (DPD) and autodetachment dynamics on the final spin-orbit electronic state of the iodine fragment is characterized. The dissociation dynamics of the neutral fragments correlated with autodetached electrons were found to be identical to the DPD dynamics of the I atom product spin-orbit state closest to threshold at a given photon energy, lending support to the proposed sequential mechanism.