Slow photoelectron velocity-map imaging spectroscopy of the phenoxide and thiophenoxide anions

High resolution anion photodetachment spectra of the phenoxide and thiophenoxide anions were obtained with slow electron velocity-map imaging. The spectra show transitions to the X(2)B(1) neutral states of both species and to the ?(2)B(2) state of the thiophenoxy radical. Comparison of the spectra with Franck-Condon simulations allows several gas-phase vibrations to be assigned. The adiabatic electron affinities are determined to be 2.2538(8) eV and 2.3542(6) eV for phenoxy and thiophenoxy, respectively. The term energy of the ?(2)B(2) state of thiophenoxy is found to be 0.3719(9) eV, higher than the values reported in photodissociation experiments of thiophenol.     

Slow electron velocity-map imaging spectroscopy of the 1-propynyl radical

High resolution photoelectron spectra of the 1-propynyl and 1-propynyl-d(3) anions acquired with slow electron velocity-map imaging are presented. The electron affinity is determined to be 2.7355+/-0.0010 eV for the 1-propynyl radical and 2.7300+/-0.0010 eV for 1-propynyl-d(3). Several vibronic transitions are observed and assigned using the isotopic shifts and results from ab initio calculations. Good agreement between experimental spectra and calculations suggests a C(3v) geometry for the 1-propynyl radical. No evidence is found for strong vibronic coupling between the ground electronic state and the low-lying first excited state.     

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.     

Vibronic structure in C2H and C2D from anion slow electron velocity-map imaging spectroscopy

The C2H and C2D radicals are investigated by slow electron velocity-map imaging (SEVI) of the corresponding anions. This technique offers considerably higher resolution (<0.5 meV) than photoelectron spectroscopy. As a result, SEVI spectra of the two isotopomers yield improved electron affinities and reveal many new structures that are particularly sensitive to vibronic coupling between the ground 2Sigma+ and low-lying excited 2Pi states. These structures, which encompass more than 5000 cm(-1) of internal excitation, are assigned with the aid of previous experimental and theoretical work. We also show that SEVI can be applied to photodetachment transitions resulting in ejection of an electron with orbital angular momentum l=1, a p wave, in contrast to anion zero-electron kinetic energy spectroscopy which is restricted to s-wave detachment.     

High-resolution rotational spectroscopy of the carbon chain anions C3N-, C4H-, and C4D(-)

The rotational spectra of C(3)N(-), C(4)H(-), and C(4)D(-) have been measured at high-spectral resolution by Fourier transform microwave spectroscopy. For both C(3)N(-) and C(4)D(-), hyperfine structure in the lowest-J transitions has been resolved and measured to better than 0.1 ppm. The quadrupole coupling constants eQq for both anions are close to those of the neutral counterparts C(3)N and C(4)D, and that of C(3)N(-) is in good agreement with theoretical calculations. Several properties of these anions, including their linewidths, drift velocities, and abundances, are systematically compared to similar-sized neutral molecules. The production of C(4)H(-) with different hydrocarbon precursor and buffer gases is also discussed.     

Photoelectron velocity-map imaging signature of structural evolution of silver-doped lead Zintl anions

A set of silver-doped lead Zintl anions, Ag@Pb(n) (-) (n = 5-12), have been studied using photoelectron velocity-map imaging spectroscopy and quantum chemical calculation. The structures of Ag@Pb(n) (-) (n = 7-9, 11) built upon a square pyramid base, hitherto not considered, were assigned. Overall agreement between the experimental and calculated photoelectron spectra as well as vertical detachment energies allows for structural evolution to be established. The silver atom prefers to stay outside in the n ≤ 6 clusters and intends to be encapsulated by the lead atoms in n > 6. A stable endohedral cage with bicapped square antiprism structure is formed at n = 10, the endohedral structure of which persists for the larger clusters. Especially, these Ag@Pb(n) (-) anions have been found to undergo a transition between square pyramid and pentagonal pyramid molecular structures at n = 11.     

Photoelectron imaging spectroscopy of nitroethane anions

We present low-energy velocity map photoelectron imaging results for bare and Ar solvated nitroethane anions. We report an improved value for the adiabatic electron affinity of nitroethane of (191 ± 6) meV which is used to obtain a C-NO(2) bond dissociation energy of (0.589 ± 0.019) eV in nitroethane anion. We assign a weak feature at (27 ± 5) meV electron binding energy to the dipole-bound anion state of nitroethane. Photoelectron angular distributions exhibit increasing anisotropy with increasing kinetic energies. The main contributions to the photoelectron spectrum of nitroethane anion can be assigned to the vibrational modes of the nitro group. Transitions involving torsional motion around the CN bond axis lead to strong spectral congestion. Interpretation of the photoelectron spectrum is assisted by ab initio calculations and Franck-Condon simulations.     

4D scanning transmission ultrafast electron microscopy: Single-particle imaging and spectroscopy

We report the development of 4D scanning transmission ultrafast electron microscopy (ST-UEM). The method was demonstrated in the imaging of silver nanowires and gold nanoparticles. For the wire, the mechanical motion and shape morphological dynamics were imaged, and from the images we obtained the resonance frequency and the dephasing time of the motion. Moreover, we demonstrate here the simultaneous acquisition of dark-field images and electron energy loss spectra from a single gold nanoparticle, which is not possible with conventional methods. The local probing capabilities of ST-UEM open new avenues for probing dynamic processes, from single isolated to embedded nanostructures, without being affected by the heterogeneous processes of ensemble-averaged dynamics. Such methodology promises to have wide-ranging applications in materials science and in single-particle biological imaging.     

Photoelectron spectroscopy of the molecular anions, ZrO-, HfO-, HfHO-, and HfO2H-

Negative ion photoelectron spectra of ZrO(-), HfO(-), HfHO(-), and HfO(2)H(-) are reported. Even though zirconium- and hafnium-containing molecules typically exhibit similar chemistries, the negative ion photoelectron spectral profiles of ZrO(-) and HfO(-) are dramatically different from one another. By comparing these data with relevant theoretical and experimental studies, as well as by using insights drawn from atomic spectra, spin-orbit interactions, and relativistic effects, the photodetachment transitions in the spectra of ZrO(-) and HfO(-) were assigned. As a result, the electron affinities of ZrO and HfO were determined to be 1.26 ± 0.05 eV and 0.60 ± 0.05 eV, respectively. The anion photoelectron spectra of HfHO(-) and HfO(2)H(-) are similar to one another and their structural connectivities are likely to be H-Hf-O(-) and O-Hf-OH(-), respectively. The electron affinities of HfHO and HfO(2)H are 1.70 ± 0.05 eV and 1.73 ± 0.05 eV, respectively.     

Characterization of cyclic and linear C3H- and C3H via anion photoelectron spectroscopy

Anion photoelectron spectroscopy of C3H- and C3D- is performed using both field-free time-of-flight and slow electron velocity-map imaging. We observe and assign transitions originating from linear/bent (l-C3H) and cyclic (c-C3H) anionic isomers to the corresponding neutral ground states and low-lying excited states. Transitions within the cyclic and linear manifolds are distinguished by their photoelectron angular distributions and their intensity dependence on the neutral precursor. Using calculated values for the energetics of the neutral isomers [Ochsenfeld et al., J. Chem. Phys. 106, 4141 (1997)], which predict c-C3H to lie 74 meV lower than l-C3H, the experimental results establish c-C3H- as the anionic ground state and place it 229 meV below l-C3H-. Electron affinities of 1.999+/-0.003 and 1.997+/-0.005 eV are determined for C3H and C3D from the X 2B2<--X 1A1 photodetachment transition of c-C3H. Term energies for several low-lying states of c-C3H and l-C3H are also determined. Franck-Condon simulations are used to make vibrational assignments for the bands involving c-C3H. Simulations of the l-C3H bands were more complicated owing to large amplitude bending motion and, in the case of the neutral A 2Pi state, strong Renner-Teller coupling.     

Characterization of the nanomorphology of polymer-silica colloidal nanocomposites using electron spectroscopy imaging

The internal nanomorphologies of two types of vinyl polymer-silica colloidal nanocomposites were assessed using electron spectroscopy imaging (ESI). This technique enables the spatial location and concentration of the ultrafine silica sol within the nanocomposite particles to be determined. The ESI data confirmed that the ultrafine silica sol was distributed uniformly throughout the poly(4-vinylpyridine)/silica nanocomposite particles, which is consistent with the "currant bun" morphology previously used to describe this system. In contrast, the polystyrene/silica particles had a pronounced "core-shell" morphology, with the silica sol forming a well-defined monolayer surrounding the nanocomposite cores. Thus these ESI results provide direct verification of the two types of nanocomposite morphologies that were previously only inferred on the basis of X-ray photoelectron spectroscopy and aqueous electrophoresis studies. Moreover, ESI also allows the unambiguous identification of a minor population of polystyrene/silica nanocomposite particles that are not encapsulated by silica shells. The existence of this second morphology was hitherto unsuspected, but it is understandable given the conditions employed to synthesize these nanocomposites. It appears that ESI is a powerful technique for the characterization of colloidal nanocomposite particles.     

Time-slice velocity-map ion imaging studies of the photodissociation of NO in the vacuum ultraviolet region

The time-slice velocity-map ion imaging and the resonant four-wave mixing techniques are combined to study the photodissociation of NO in the vacuum ultraviolet (VUV) region around 13.5 eV above the ionization potential. The neutral atoms, i.e., N((2)D(o)), O((3)P(2)), O((3)P(1)), O((3)P(0)), and O((1)D(2)), are probed by exciting an autoionization line of O((1)D(2)) or N((2)D(o)), or an intermediate Rydberg state of O((3)P(0,1,2)). Old and new autoionization lines of O((1)D(2)) and N((2)D(o)) in this region have been measured and newer frequencies are given for them. The photodissociation channels producing N((2)D(o)) + O((3)P), N((2)D(o)) + O((1)D(2)), N((2)D(o)) + O((1)S(0)), and N((2)P(o)) + O((3)P) have all been identified. This is the first time that a single VUV photon has been used to study the photodissociation of NO in this energy region. Our measurements of the angular distributions show that the recoil anisotropy parameters (β) for all the dissociation channels except for the N((2)D(o)) + O((1)S(0)) channel are minus at each of the wavelengths used in the present study. Thus direct excitation of NO by a single VUV photon in this energy region leads to excitation of states with Σ or Δ symmetry (ΔΩ = ±1), explaining the observed perpendicular transition.     

4D electron imaging: principles and perspectives

In this perspective we highlight developments and concepts in the field of 4D electron imaging. With spatial and temporal resolutions reaching the picometer and femtosecond, respectively, the field is now embracing ultrafast electron diffraction, crystallography and microscopy. Here, we overview the principles involved in the direct visualization of structural dynamics with applications in chemistry, materials science and biology. The examples include the studies of complex isolated chemical reactions, phase transitions and cellular structures. We conclude with an outlook on the potential of the approach and with some questions that may define new frontiers of research.     

Photoelectron spectroscopy of nickel-benzene cluster anions

(Nickel)(n)(benzene)(m) (-) cluster anions were studied by both mass spectrometry and anion photoelectron spectroscopy. Only Ni(n)(Bz)(m) (-) species for which n > or =m were observed in the mass spectra. No single-nickel Ni(1)(Bz)(m) (-) species were seen. Adiabatic electron affinities, vertical detachment energies, and second transition energies were determined for (n,m)=(2,1), (2,2), (3,1), and (3,2). For the most part, calculations on Ni(n)(Bz)(m) (-) species by B. K. Rao and P. Jena [J. Chem. Phys. 117, 5234 (2002)] were found to be consistent with our results. The synergy between their calculations and our experiment provided enhanced confidence in the theoretically implied magnetic moments of several nickel-benzene complexes. The magnetic moments of small nickel clusters were seen to be extremely sensitive to immediate molecular environmental effects.     

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.     

Communication: branching ratio measurements in the predissociation of 12C16O by time-slice velocity-map ion imaging in the vacuum ultraviolet region

The first direct branching ratio measurement of the three lowest energy dissociation channels of CO that produce C((3)P) + O((3)P), C((1)D) + O((3)P), and C((3)P) + O((1)D) is reported. Rotational resolved carbon ion yield spectra for two Π bands (W(3sσ)(1)Π (v(') = 3) at 108,012.6 cm(-1) and (1)Π(v(') = 2) at 109,017 cm(-1)) and two Σ bands ((4sσ)(1)Σ(+)(v(') = 4) at 109,452 cm(-1) and (4pσ)(1)Σ(+)(v(') = 3) at 109,485 cm(-1)) of CO were obtained. Our measurements show that the branching ratio in this energy region is strongly dependent on the electronic and vibrational energy but it is independent or just weakly dependent on the parity and rotational energy levels. To our knowledge, this is the first time that the triplet channel producing O((1)D) has been experimentally observed and this is also the first time that a direct measurement of the branching ratio for the different channels in the predissociation of CO in this energy region has been made.     

High-resolution threshold photoelectron study of the propargyl radical by the vacuum ultraviolet laser velocity-map imaging method

By employing the vacuum ultraviolet (VUV) laser velocity-map imaging (VMI) photoelectron scheme to discriminate energetic photoelectrons, we have measured the VUV-VMI-threshold photoelectrons (VUV-VMI-TPE) spectra of propargyl radical [C(3)H(3)(X?(2)B(1))] near its ionization threshold at photoelectron energy bandwidths of 3 and 7 cm(-1) (full-width at half-maximum, FWHM). The simulation of the VUV-VMI-TPE spectra thus obtained, along with the Stark shift correction, has allowed the determination of a precise value 70 156 ± 4 cm(-1) (8.6982 ± 0.0005 eV) for the ionization energy (IE) of C(3)H(3). In the present VMI-TPE experiment, the Stark shift correction is determined by comparing the VUV-VMI-TPE and VUV laser pulsed field ionization-photoelectron (VUV-PFI-PE) spectra for the origin band of the photoelectron spectrum of the X?(+)-X? transition of chlorobenzene. The fact that the FWHMs for this origin band observed using the VUV-VMI-TPE and VUV-PFI-PE methods are nearly the same indicates that the energy resolutions achieved in the VUV-VMI-TPE and VUV-PFI-PE measurements are comparable. The IE(C(3)H(3)) value obtained based on the VUV-VMI-TPE measurement is consistent with the value determined by the VUV laser PIE spectrum of supersonically cooled C(3)H(3)(X?(2)B(1)) radicals, which is also reported in this article.     

Photoelectron spectroscopy of hydrated hexafluorobenzene anions

We present a synergetic experimental/theoretical study of hydrated hexafluorobenzene anions. Experimentally, we measured the anion photoelectron spectra of the anions, C6F6(-)(H2O)n (n=0-2). The spectra show broad peaks, which shift to successively higher electron binding energies with the addition of each water molecule to the hexafluorobenzene anion. Complementing these results, we also conducted density functional calculations which link adiabatic electron affinities to the optimized geometric structures of the negatively charged species and their neutral counterparts. Neutral hexafluorobenzene-water complexes are not thought to be hydrogen bonded. In the case of C6F6(-)(H2O)1, however, its water molecule was found to lie in the plane of the hexafluorobenzene anion, bound by two O-H...F ionic hydrogen bonds. Whereas in the case of C6F6(-)(H2O)2, both water molecules also lie in the plane of and are hydrogen bonded to the hexafluorobenzene anion but on opposite ends. This study and that of Schneider et al. [J. Chem. Phys. 127, 114311 (2007), preceding paper] are in agreement regarding the geometry of C6F6(-)(H2O)1.