Photodetachment Studies Research Articles

A field ionizer for photodetachment studies of negative ions.

In this paper, we present an apparatus for studies into the photodetachment process of atomic negative ions. State-selective detection of the residual atom following the initial photodetachment step is achieved by combining resonant laser excitation of the photo-detached atom with electric field ionization. The resonance ionization technique in combination with a co-linear ion-laser beam geometry gives an experimental apparatus that has both high selectivity and sensitivity. In addition to measurements of a single selected partial photodetachment channel, the apparatus also can be used to study a manifold of photodetachment channels in which the residual atom is left in a high-lying Rydberg state and for investigation of the double electron-detachment process. Ion-optical simulations in SIMION are used to illustrate the operation of the apparatus for studying such processes. Successful performance of the apparatus against the simulation is demonstrated by a high resolution study of the photodetachment of cesium, where the sharp s-wave threshold of the photodetachment processes leaving the residual atom in the excited 6p state was investigated.

Opportunities for negative ions studies at the Frankfurt Low-energy Storage Ring (FLSR)

The room-temperature electrostatic heavy ion storage ring FLSR was originally designed to study the collision dynamics of atoms and molecules. Recently it has been equipped with a RF plasma ion source combined with a charge exchange cell to be able to perform studies with negative ions in the ring. In preliminary experiments beams of He−, O− and OH− could successfully be stored. The measured lifetime of the metastable He−–ion is in good agreement with previous results, showing that the lifetime measurement in this case is not limited by the storage time due to collisional detachment. In the case of O− and OH− storage times in the order of seconds have been achieved. In a next step laser beams will be introduced in the ring allowing photodetachment studies of vibrationally cold molecules.

A graphene-based neutral particle detector

A neutral particle detector is presented, in which the traditionally used target material, indium tin oxide (ITO), is replaced by graphene. The graphene-based detector enables collinear photodetachment measurements at a significantly shorter wavelength of light down to 230 nm compared to ITO-based detectors, which are limited at 335 nm. Moreover, the background signal from the photoelectric effect is drastically reduced when using graphene. The graphene based detector, reaching 1.7 eV further into the UV energy range, allows increased possibilities for photodetachment studies of negatively charged atoms, molecules, and clusters.

Dipole-bound excited states and resonant photoelectron imaging of phenoxide and thiophenoxide anions.

We report photodetachment and resonant photoelectron-imaging studies of cryogenically cooled phenoxide (C6H5O-) and thiophenoxide (C6H5S-) anions. In a previous study [H. T. Liu et al. Angew. Chem., Int. Ed. 52, 8976 (2013)], a dipole-bound excited state was observed for C6H5O- at 97 cm-1 below the detachment threshold. Eight resonant photoelectron spectra were obtained via excitations to eight vibrational levels of the dipole-bound state (DBS) followed by autodetachment. Here we present a complete photodetachment spectrum of C6H5O- covering a spectral range 2600 cm-1 above the detachment threshold and revealing nine additional vibrational resonances of the DBS. We also report the first observation of a dipole-bound excited state for C6H5S-, 39 cm-1 below its detachment threshold of 18 982 cm-1. Photodetachment spectroscopy covering a spectral range 1500 cm-1 above the threshold reveals twelve vibrational resonances for the DBS of C6H5S-. By tuning the detachment laser to the vibrational resonances in the DBS of C6H5O- and C6H5S-, we obtain highly non-Franck-Condon resonant photoelectron spectra, as a result of mode-selectivity and the Δv = -1 propensity rule for vibrational autodetachment. Five new fundamental vibrational frequencies are obtained for the ground state of the C6H5O (X 2B1) radical. Intramolecular inelastic scattering is observed in some of the resonant photoelectron spectra, leading to the excitation of the Franck-Condon-inactive lowest-frequency bending mode (ν20) of C6H5O. The first excited state of C6H5O (A 2B2) is observed to be 0.953 eV above the ground state. Twelve resonant photoelectron spectra are obtained for C6H5S-, allowing the measurements of seven fundamental vibrational frequencies of the C6H5S radical, whereas the non-resonant photoelectron spectrum exhibits only a single Franck-Condon active mode. The current study again demonstrates that the combination of photodetachment spectroscopy and resonant photoelectron spectroscopy is a powerful technique to obtain vibrational information about polar radical species.

Multiple Electron Ejection from Proteins Resulting from Single-Photon Excitation in the Valence Shell.

One-photon multiple ionization is a signature of dynamical electron correlations in atoms and small molecules, as observed in the Auger process when Auger electron emission follows core-shell ionization. In such a process, the high energy needed to remove several electrons is due to the strong Coulombic attraction between the last departing electron(s) and the ionic core. Multiply negatively charged molecules offer the possibility to overcome the Coulombic attraction, opening the way for multielectron photodetachment following valence shell excitation. Here photodetachment studies have been performed on electrosprayed protein polyanions using vacuum ultraviolet synchrotron radiation coupled to a radiofrequency ion trap. Double, triple, and quadruple electron emissions from protein polyanions resulting from single-photon excitation in the valence shell were observed with ionization thresholds below 20 eV photon energy. This suggests the existence of large electronic correlations in proteins between weakly bound electrons standing on distant sites. Besides, the resulting multiradical polyanions appear to be remarkably stable, an important issue in radiobiology.

On the Photodetachment from the Green Fluorescent Protein Chromophore

Motivated by the discrepancies in recent experimental and theoretical studies of photodetachment from isolated model chromophores of the green fluorescent protein (GFP), this study reports calculations of the electron detachment energies and photoelectron spectra of the phenolate and deprotonated p-hydroxybenzylidene-2,3-dimethylimidazolinone (HBDI) anions. The spectra were computed using double-harmonic parallel normal mode approximation. High-level coupled-cluster methods as well as density functional theory were used to compute vertical and adiabatic detachment energies of the phenolate anion serving as a model system representing anionic GFP-like chromophores (HBDI). The benchmark calculations reveal that the basis set has significant effect on the computed detachment energies, whereas the results are less sensitive to the level of electron correlation treatment. At least aug-cc-pVTZ basis set is required. The best ωB97X-D and CCSD(T) estimates of phenolate's adiabatic detachment energy are 2.12 and 2.19 eV; these values are very close to the experimental value, 2.253 eV [Gunion et al. Int. J. Mass Spectrom. Ion Proc. 1992, 117, 601]. The best estimate of the vertical detachment energy of deprotonated HBDI is 2.76 eV, which supports bound character of the bright excited state in the Franck-Condon region. The most intense transition in the computed photoelectron spectra of both phenolate and deprotonated HBDI is the 0-0 S0-D0 transition, which is 0.11 eV below vertical detachment energy. Therefore, the position of the maximum of the photoelectron spectrum does not represent vertical detachment energy, and the direct comparison between theory and experiment must involve spectrum modeling.

Inner-shell photodetachment: Shape and Feshbach resonances of anions

Inner-shell photoexcitation and photodetachment studies in strongly correlated systems are presented, with the focus on selected shape and Feshbach resonance studies, and inner-shell threshold measurements conducted on anions at the Advanced Light Source. We highlight the observations of simultaneous double-Auger decay from the He−2s2p2P4 hollow ion state and the formation of shape and Feshbach resonances when filling a vacancy in the valence shell with a core electron in oxygen-group elements and transition metal anions. Measurements of inner-shell thresholds have also led to the first measurements of core-excited neutral states in a number of species.

Role of Conical Intersections in Molecular Spectroscopy and Photoinduced Chemical Dynamics

This review describes how conical intersections affect measured molecular spectra and simple photofragmentation processes. We consider excitations that result in electron ejection, that is, photoionization or photodetachment, as well as photoinduced H-atom elimination. Section 1 presents a brief overview of the history of conical intersections and their rise from an arcane theoretical concept to a major paradigm in nonadiabatic chemistry. In Section 2, the generic properties of conical intersections are discussed, as well as their characterization with modern electronic-structure methods. Section 3 briefly discusses computational tools used to compute the nuclear motion involving conical intersections. Section 4 describes how the ideas of Sections 2 and 3 are combined to simulate molecular spectra impacted by conical intersections. Section 5 describes selected recent experimental and computational studies of photoelectron, photodetachment, and photofragment spectra. Rather than providing an encyclopedic bibliography of the previous and current literature, we illustrate significant problems currently being addressed and describe what can be accomplished with current computational techniques and how these results are achieved. Section 6 suggests future directions in this field.

Ion optical design of a collinear laser-negative ion beam apparatus

An apparatus for photodetachment studies on atomic and molecular negative ions of medium up to heavy mass (M ≃ 500) has been designed and constructed. Laser and ion beams are merged in the apparatus in a collinear geometry and atoms, neutral molecules and negative ions are detected in the forward direction. The ion optical design and the components used to optimize the mass resolution and the transmission through the extended field-free interaction region are described. A 90° sector field magnet with 50 cm bending radius in combination with two slits is used for mass dispersion providing a resolution of M∕ΔM≅800 for molecular ions and M∕ΔM≅400 for atomic ions. The difference in mass resolution for atomic and molecular ions is attributed to different energy distributions of the sputtered ions. With 1 mm slits, transmission from the source through the interaction region to the final ion detector was determined to be about 0.14%.

Dissociative Photodetachment Studies of Cooled HOCO¯ Anions Revealing Dissociation Below the Barrier to H + CO2

Dissociative photodetachment of internally cold HOCO¯ has been performed in a new cryogenically cooled photoelectron−photofragment coincidence apparatus, characterizing the HOCO potential energy surface in the Franck−Condon region determined by the anion. Three processes are observed in three distinct regions of this surface: detachment to stable HOCO, dissociative photodetachment to OH + CO, and dissociative photodetachment to H + CO2. Relative to earlier work on internally hot anions, the H + CO2 channel is significantly enhanced, with efficient dissociation to H + CO2 unambiguously detected below calculated barriers in the OH + CO → H + CO2 reaction. This finding shows that an important low-energy pathway exists to the exit channel in this reaction, reinforcing the need for further refinement of this important potential energy surface and providing benchmarks for further theoretical efforts.

Dissociative Photodetachment Studies of I2-·Ar: Coincident Imaging of Two- and Three-Body Product Channels

Van der Waals clusters serve as prototypical systems for studying processes of energy transfer. The I2.Ar system has attracted particular interest due to the wide array of decay processes occurring in competition with one another. Here, we present systematic dissociative photodetachment (DPD) studies of the I2- and I2-.Ar anions in the region 4.24-4.78 eV. The resulting neutral fragments are detected by time- and position-sensitive (TPS) coincident imaging. Photofragment mass distributions and translational energy distributions from the DPD of I2- are presented and facilitate understanding of the I2.Ar system. For the I2.Ar complex, channels resulting from two-body dissociation leading to I2+Ar photoproducts are observed at all photon energies employed. We also report the first direct observation of the previously inferred three-body dissociation channel leading to I+I+Ar photoproducts. The relative intensities of each decay channel are investigated in relation to the electronic state being accessed. Translational energy distributions of the I2.Ar complex lend further insight into the decay mechanism for each channel.

Photodetachment studies with the linear time of flight photoelectron spectrometer

A linear time of flight photoelectron spectrometer was built for photodetachment studies on anions. The design of the newly constructed photoelectron spectrometer is discussed briefly. Angular distribution of photoelectrons in the photodetachment from OH−, As− and Cu− were measured at a few distinct wavelengths. Energy dependence of the asymmetry parameter was studied for these anions. The relation between electronic configuration and the energy dependence of the asymmetry parameter is briefly discussed.

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.

Photoelectron–photofragment coincidence study of OHF−: transition state dynamics of the reaction OH + F → O + HF

A photoelectron-photofragment coincidence (PPC) study of the dissociative photodetachment of OHF at a photon energy of 4.80 eV is presented. The correlated electron kinetic energy (eKE) and translational energy release (E(T)) into the O + HF + e- products yield information on the potential energy surface close to the transition state of the neutral reaction OH + F --> O + HF. The correlation spectrum shows two different features in the energetically allowed O + HF product channel: (a) diagonal ridges, resulting from direct dissociative photodetachment (DPD) and (b) areas with higher E(T) in the neutral fragments from nonadiabatic dissociation. The total translational energy spectrum (E(TOT) = eKE + ET) reveals a vibrationally resolved product state distribution. These results are discussed in the context of recent theoretical studies of the dissociative photodetachment of OHF-.

Theoretical studies of photodetachment

A short survey of the model and ab initio many electron approaches applied to the study of negative ion photodetachment is presented. Primarily, attention is paid to the manifestation of dynamic many electron effects such as interference between different channels, polarization interaction, core relaxation and screening of the interelectron interaction in one-photon photodetachment processes.

Probing Negative Ions from Within

K-shell photodetachment studies of negative ions have been carried out using a collinear photon-ion apparatus coupled with the brightness and spectral resolution of the Advanced Light Source at Lawrence Berkeley National Laboratory. The experimental results, which revealed new phenomena, are pushing the limits of third generation photon sources and are stimulating various enhanced theories.

Dipole-bound anions of highly polar molecules: ethylene carbonate and vinylene carbonate.

Results of experimental and theoretical studies of dipole-bound negative ions of the highly polar molecules ethylene carbonate (EC, C3H4O3, mu=5.35 D) and vinylene carbonate (VC, C3H2O3, mu=4.55 D) are presented. These negative ions are prepared in Rydberg electron transfer (RET) reactions in which rubidium (Rb) atoms, excited to ns or nd Rydberg states, collide with EC or VC molecules to produce EC- or VC- ions. In both cases ions are produced only when the Rb atoms are excited to states described by a relatively narrow range of effective principal quantum numbers, n*; the greatest yields of EC- and VC- are obtained for n*(max)=9.0+/-0.5 and 11.6+/-0.5, respectively. Charge transfer from low-lying Rydberg states of Rb is characteristic of a large excess electron binding energy (Eb) of the neutral parent; employing the previously derived empirical relationship Eb=23/n*(max)(2.8) eV, the electron binding energies are estimated to be 49+/-8 meV for EC and 24+/-3 meV for VC. Electron photodetachment studies of EC- show that the excess electron is bound by 49+/-5 meV, in excellent agreement with the RET results, lending credibility to the empirical relationship between Eb and n*(max). Vertical electron affinities for EC and VC are computed employing aug-cc-pVDZ atom-centered basis sets supplemented with a (5s5p) set of diffuse Gaussian primitives to support the dipole-bound electron; at the CCSD(T) level of theory the computed electron affinities are 40.9 and 20.1 meV for EC and VC, respectively.

Studies of the Excited State Dynamics of N2O2 by Dissociative Photodetachment of N2O2-

The excited states of N2O2 have been characterized by studies of the dissociative photodetachment of N2O2- at 266 nm. Photoelectron−photofragment coincidence spectroscopy reveals the correlation between features observed in the photoelectron spectrum and different neutral dissociation pathways. Evidence for at least two isomers of N2O2- is presented and upper limits for their stabilities are determined. One isomer, ONNO-, is stable relative to NO + NO + e- by < 1.70 ± 0.05 eV, while the second isomer, trigonal N2O2-, is stable relative to O- + N2O by < 0.57 ± 0.05 eV. The observed dissociation channels are assigned to ONNO → NO + NO, N2O2 → O (3P) + N2O, and either O(1D) + N2O or N(4S) + NO2. No evidence for stable N2O2 was found, and the dissociation dynamics of the excited states of N2O2 are discussed.

Coincidence spectroscopy.

The application of coincidence techniques to the study of the reaction dynamics of isolated molecules is reviewed. Coincidence spectroscopy is a powerful approach for carrying out a number of measurements. At its most basic level, coincidence techniques can identify the source of a specific signal, as in the well-known photoelectron-photoion coincidence approach used for several years. By carrying out coincidence experiments in an increasingly differential manner, correlated energy and angular distributions of reaction products may be recorded. Completely energy- and angle-resolved measurements of photoelectrons and ionic or neutral products can reveal molecular-frame photoelectron and photofragment angular distributions and aid in the characterization of dissociative states of molecules and ions. Recent work in this area is reviewed, including examples from studies of dissociative photodetachment, dissociative photoionization, time-resolved studies of dissociative photoionization, and three-body dissociation processes.

Dissociative photodetachment studies of O−(H2O)2, OH−(H2O)2, and the deuterated isotopomers: Energetics and three-body dissociation dynamics

Photoelectron–photofragment coincidence spectroscopy was used to study dissociative photodetachment of the doubly hydrated clusters of oxide and hydroxide, M−(H2O)2→M+2H2O+e− (M=O, OH). These experiments yield information on the energetics of the parent anion and the dissociation dynamics of the photodetached neutral species. Photoelectron spectra and photoelectron–photofragment coincidence spectra are presented and compared to data for O−(H2O) and OH−(H2O). Unlike the singly hydrated species, no evidence of vibrationally resolved product translational energy distributions is observed. The second hydration energy of O− with both H2O and D2O was also measured to be 0.80±0.08 and 0.81±0.08 eV, respectively. The three-body dissociation dynamics of the neutral clusters produced by photodetachment were studied by measuring the velocities and recoil angles of all the particles in coincidence. The observed partitioning of momentum is consistent with a two-step mechanism or dissociation from a wide range of starting geometries.