Detachment Channels Research Articles

Photoelectron spectroscopy of the deprotonated tryptophan anion: the contribution of deprotomers to its photodetachment channels.

Photoelectron spectroscopy and electronic structure calculations are used to investigate the electronic structure of the deprotonated anionic form of the aromatic amino acid tryptophan, and its chromophore, indole. The photoelectron spectra of tryptophan, recorded at different wavelengths across the UV, consist of two direct detachment channels and thermionic emission, whereas the hν = 4.66 eV spectrum of indole consists of two direct detachment features. Electronic structure calculations indicate that two deprotomers of tryptophan are present in the ion beam; deprotonation of the carboxylic acid group (Trp(I)-) or the N atom on the indole ring (Trp(II)-). Strong similarities are observed between the direct detachment channels in the photoelectron spectra of tryptophan and indole, which in conjunction with electronic structure calculations, indicate that electron loss from Trp(II)- dominates this portion of the spectra. However, there is some evidence that direct detachment of Trp(I)- is also observed. Thermionic emission is determined to predominantly arise from the decarboxylation of Trp(I)-, mediated by the ππ* excited state near λ = 300 nm, which results in an anionic fragment with a negative electron affinity that readily autodetaches.

Photoelectron Imaging Study of the Diplatinum Iodide Dianions [Pt2I6]2- and [Pt2I8]2.

Photoelectron spectroscopy has been used to study the electronic structure, photodetachment, and photodissociation of the stable diplatinum iodide dianions [Pt2I6]2– and [Pt2I8]2–. Photoelectron spectra over a range of photon energies show the characteristic absence of low kinetic energy photoelectrons expected for dianions as a result of the repulsive Coulomb barrier (RCB). Vertical detachment energies of ∼1.6 and ∼1.9 eV and minimum RCBs of ∼1.2 and ∼1.3 eV are reported for [Pt2I6]2– and [Pt2I8]2–, respectively. Both of the diplatinum halides exhibit three direct detachment channels with distinct anisotropies, analogous to the previously reported spectra for PtI2– and PtI–, suggesting a platinum-centered molecular core that dominates the photodetachment. Additionally, evidence for two-photon photodissociation and subsequent photodetachment channels producing I– are observed for both dianions. Finally, an unexplained feature is observed at photon energies around 3 eV, whose origin is considered. Our work highlights the complex electronic structure of the heavy platinum-halide dianions that are characterized by a dense manifold of electronic states.

The three-center two-positron bond.

Computational studies have shown that one or more positrons can stabilize two repelling atomic anions through the formation of two-center positronic bonds. In the present work, we study the energetic stability of a system containing two positrons and three hydride anions, namely 2e+[H3 3-]. To this aim, we performed a preliminary scan of the potential energy surface of the system with both electrons and positrons in a spin singlet state, with a multi-component MP2 method, that was further refined with variational and diffusion Monte Carlo calculations, and confirmed an equilibrium geometry with D 3h symmetry. The local stability of 2e+[H3 3-] is demonstrated by analyzing the vertical detachment and adiabatic energy dissociation channels. Bonding properties of the positronic compound, such as the equilibrium interatomic distances, force constants, dissociation energies, and bonding densities are compared with those of the purely electronic H3 + and Li3 + systems. Through this analysis, we find compelling similarities between the 2e+[H3 3-] compound and the trilithium cation. Our results strongly point out the formation of a non-electronic three-center two-positron bond, analogous to the well-known three-center two-electron counterparts, which is fundamentally distinct from the two-center two-positron bond [D. Bressanini, J. Chem. Phys., 2021, 155, 054306], thus extending the concept of positron bonded molecules.

Reactivity of Hydrated Hydroxide Anion Clusters with H and Rb: An ab Initio Study.

We present a theoretical investigation of the hydrated hydroxide anion clusters, OH(H2O)n-, and of the collisional complexes, H-OH(H2O)n- and Rb-OH(H2O)n- (with n = 1-4). The MP2 and CCSD(T) methods are used to calculate interaction energies, optimized geometries, and vertical detachment energies. Parts of the potential energy surfaces are explored with a focus on the autodetachment region. We point out the importance of diffuse functions to correctly describe the latter. We use our results to discuss the different water loss and electronic detachment channels, which are the main reaction routes at room temperature and below. In particular, we have considered a direct and an indirect process for the electronic detachment, depending on whether water loss follows or precedes the detachment of the excess electron. We use our results to discuss the implications for astrochemistry and hybrid trap experiments in the context of cold chemistry.

Photoelectron Spectroscopy of the Hexafluorobenzene Cluster Anions: (C6F6) n- ( n = 1-5) and I-(C6F6).

Frequency-resolved (2D) photoelectron (PE) spectra of the anionic clusters (C6F6) n-, for n = 1-5, and time-resolved PE spectra of I-C6F6 are presented using a newly built instrument and supported by electronic structure calculations. From the 2D PE spectra, the vertical detachment energy (VDE) of C6F6- was measured to be 1.60 ± 0.01 eV, and the adiabatic detachment energy (ADE) was ≤0.70 eV. The PE spectra also contain fingerprints of resonance dynamics over certain photon energy ranges, in agreement with the calculations. An action spectrum over the lowest resonance is also presented. The 2D spectra of (C6F6) n- show that the cluster can be described as C6F6-(C6F6) n-1. The VDE increases linearly (200 ± 20 meV n-1) due to the stabilizing influence on the anion of the solvating C6F6 molecules. For I-C6F6, action spectra of the absorption just below both detachment channels are presented. Time-resolved PE spectra of I-C6F6 excited at 3.10 eV and probed at 1.55 eV reveal a short-lived nonvalence state of C6F6- that coherently evolves into the valence ground state of the anion and induces vibrational motion along a specific buckling coordinate. Electronic structure calculations along the displacement of this mode show that at the extreme buckling angle the probe can access an excited state of the anion that is bound at that geometry but adiabatically unbound. Hence, slow electrons are emitted and show dynamics that predominantly probe the outer-turning point of the motion. A PE spectrum taken at t = 0 contains a vibrational structure assigned to a specific Raman- or IR-active mode of C6F6.

Channel branching ratios in CH2CN- photodetachment: Rotational structure and vibrational energy redistribution in autodetachment.

We report photoelectron spectra of CH2CN-, recorded at photon energies between 13 460 and 15 384 cm-1, which show rapid intensity variations in particular detachment channels. The branching ratios for various spectral features reveal rotational structure associated with autodetachment from an intermediate anion state. Calculations using equation-of-motion coupled-cluster method with single and double excitations reveal the presence of two dipole-bound excited anion states (a singlet and a triplet). The computed oscillator strength for the transition to the singlet dipole-bound state provides an estimate of the autodetachment channel contribution to the total photoelectron yield. Analysis of the different spectral features allows identification of the dipole-bound and neutral vibrational levels involved in the autodetachment processes. For the most part, the autodetachment channels are consistent with the vibrational propensity rule and normal mode expectation. However, examination of the rotational structure shows that autodetachment from the ν3 (v = 1 and v = 2) levels of the dipole-bound state displays behavior counter to the normal mode expectation with the final state vibrational level belonging to a different mode.

Sensitivity of Photoelectron Angular Distributions to Molecular Conformations of Anions.

An anion photoelectron imaging study probing the sensitivity of the photoelectron angular distribution (PAD) to conformational changes is presented. The PADs of a series of para-substituted phenolate anions is compared with those calculated using the Dyson orbital formalization. Good agreement was attained for the two observed direct detachment channels of all anions, except for the lowest-energy detachment channel of para-ethyl phenolate for which two conformations exist that yield very different PADs. The conformational freedom leads to an observed PAD that is the incoherent sum of the PADs from all conformers populated under experimental conditions. In contrast, a second detachment channel shows no sensitivity to the conformational flexibility of para-ethyl phenolate. Our results show that PADs can provide detailed information about the electronic structure of the anion and its conformations.

Photodetachment spectroscopy of the beryllium oxide anion, BeO.

The X2Σ+→X1Σ+ anion to neutral ground state photodetachment of BeO- has been studied by means of photoelectron velocity-map imaging spectroscopy in a newly constructed apparatus. Vibrational intervals, rotational constants, and the electron detachment threshold of BeO- were determined for the first time. The small moment of inertia of beryllium oxide allowed for the observation of partially resolved rotational contours. Analyses of these contours provided evidence of several detachment channels resulting from changes in molecular rotational angular momenta of ΔN = 0, ±1, ±2, and ±3. The relative intensities of these detachment channels were found to be a function of the electron kinetic energy. Experimental results are compared to the predictions of high level ab initio calculations.

Anion resonances of para-benzoquinone probed by frequency-resolved photoelectron imaging.

The resonant attachment of a free electron to a closed shell neutral molecule and the interplay between the following electron detachment and electronic relaxation channels represents a fundamental but common process throughout chemical and biochemical systems. The new methodology of anion frequency-resolved photoelectron imaging is detailed and used to map out molecular excited state dynamics of gas-phase para-benzoquinone, which is the electron accepting moiety in many biological electron-transfer chains. Three-dimensional spectra of excitation energy, electron kinetic energy, and electron ejection anisotropy reveal clear fingerprints of excited and intermediate state dynamics. The results show that many of the excited states are strongly coupled, providing a route to forming the ground state radical anion, despite the fact that the electron is formally unbound in the excited states. The relation of our method to electron impact attachment studies and the key advantages, including the extension to time-resolved dynamics and to larger molecular systems, are discussed.

Femtosecond Photodetachment of Silver Anions

The two- and three-photon detachment of negative silver ions in a femtosecond infrared laser field is studied using photoelectron velocity map imaging methods. Photoelectron angular distributions (PADs) are obtained for these detachment channels; these PADs change dramatically when the laser wavelength and intensity are changed. Theoretical predictions, which are based on the adiabatic Keldysh-Faisal-Reiss saddle point method, are in good agreement with our experiment. The dependence of the PAD on the laser wavelengths and intensities is due to the interference between the different partial wave functions. The relative contributions of the different partial waves to the detachment amplitude are altered by changing the laser parameters and, as a result, the shape of the PAD. Close to the detachment threshold, the two-photon detachment process also follows the Wigner threshold law. Near the detachment threshold, the large differences between the calculated results and our experimental results indicate that the ponderomotive energy shifts caused by the femtosecond laser fields must be taken into account in the theoretical model. The three-photon detachment of Ag(-) is also observed and compared with theoretical calculations.

Inter-channel effects in monosolvated atomic iodide cluster anion detachment: Correlation of the anisotropy parameter with solvent dipole moment

Photoelectron imaging results are presented for I(-)[middle dot]X cluster anions (X = CO(2), C(4)H(5)N [pyrrole], (CH(3))(2)CO, CH(3)NO(2)). The available detachment channels are labeled according to the neutral iodine atom states produced (channel I ≡ (2)P(3/2) and channel II ≡ (2)P(1/2)). At photon energies in the vicinity of the channel II threshold these data are compared to previously reported results for I(-)[middle dot]X (X = CH(3)CN, CH(3)Cl, CH(3)Br, and H(2)O). In particular, these results show a strong connection between the dipole moment of the solvent molecule and the behavior of the channel I photoelectron angular distributions in this region, which is consistent with an electronic autodetachment process. The evolution of the channel II:channel I branching ratios in this excitation regime supports this contention.

Theoretical study of photodetachment processes of anionic boron clusters. II. Dynamics

Photodetachment bands of anionic boron clusters, B(n) (n = 4,5) are theoretically examined here. The model Hamiltonians developed through extensive ab initio quantum chemistry calculations in Paper I are employed for the required nuclear dynamics study. While the precise location of vibronic lines and progression of vibrational modes within a given electronic band is derived from time-independent quantum mechanical studies, the broadband spectral envelopes and the nonradiative decay rate of electronic states are calculated by propagating wave packets in a time-dependent quantum mechanical framework. The theoretical results are in good accord with the experiment to a large extent. The discrepancies between the two can be partly attributed to the inadequate energy resolution of the experimental results and also to the neglect of dynamic spin-orbit interactions and computational difficulty related with detachment channels involving multi-electron transitions in the theoretical formalism.

Competitive processes in the associative ionization ofC−withC+,N+, andO+

Absolute integral cross sections have been measured for associative ionization reactions involving the ${\mathrm{C}}^{\ensuremath{-}}$ and ${\mathrm{C}}^{+}$, ${\mathrm{N}}^{+}$, and ${\mathrm{O}}^{+}$ reactants. These measurements, obtained using a merged-beam setup in the keV range, provide us with useful experimental information on the efficiency and mechanisms of molecular ion formation from ionic reactants. The relative magnitudes of the different cross sections are rationalized by considering the spin multiplicities of initial and final states, and the exothermicities of the detachment and transfer ionization channels. The very different production efficiencies of $\mathrm{C}{\mathrm{O}}^{+}$ ions via the ${\mathrm{O}}^{\ensuremath{-}}+{\mathrm{C}}^{+}$ and ${\mathrm{C}}^{\ensuremath{-}}+{\mathrm{O}}^{+}$ channels are explained by statistical and energetic considerations. The potential energy curves of CO and $\mathrm{C}{\mathrm{O}}^{+}$ have been calculated by quantum ab initio methods in order to characterize the reactive pathways leading to autoionization. Thermal rate coefficients are derived to serve the plasma physics community.

Stable Small Gas‐Phase Dianion C7H22−

AbstractThe geometries and vibrational frequencies of C7H22− and C7H32− dianions, as well as their corresponding anions were investigated theoretically using HF, MP2, CCSD and CISD methods. The lowest energy structures are C2C(H2)C42− and C2CHCHCHC22− for C7H22− and C7H32− dianions, respectively. Analysis of possible detachment channels of the two structures indicates that the two isomers are stable with respect to fragmentation into two monoanions. However, the vertical and adiabatic detachment energies of the two structures show that C2C(H2)C42− is electronically stable, while C2CHCHCHC22− is not.

Temperature-Dependent Photoelectron Spectroscopy of Methyl Benzoate Anions: Observation of Steric Effect in o-Methyl Benzoate

Temperature-dependent photoelectron spectra of benzoate anion (C6H5CO2(-)) and its three methyl-substituted isomers (o-, m-, p-CH3C6H4CO2(-)) have been obtained using a newly developed low-temperature photoelectron spectroscopy apparatus that features an electrospray source and a cryogenically controlled ion trap. Detachment channels due to removing electrons from the carboxylate group and benzene ring pi electrons were distinctly observed. Well-resolved vibrational structures were obtained in the lower binding energy region due to the OCO bending modes, except for o-CH3C6H4CO2(-), which yielded broad spectra even at the lowest ion trap temperature (18 K). Theoretical calculations revealed a large geometry change in the OCO angles between the anion and neutral ground states, consistent with the broad ground-state bands observed for all species. A strong steric effect was observed between the carboxylate and the methyl group in o-CH3C6H4CO2(-), such that the -CO2(-) group is pushed out of the plane of the benzene ring by approximately 25 degrees and its internal rotational barrier is significantly reduced. The low rotational barrier in o-CH3C6H4CO2(-), which makes it very difficult to be cooled vibrationally, and the strong coupling between the OCO bending and CO2 torsional modes yielded the broad PES spectra for this isomer. It is shown that there is no C-H...O hydrogen bond in o-CH3C6H4CO2(-), and the interaction between the carboxylate and methyl groups in this anion is found to be repulsive in nature.

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.

Photodissociation of Bromoform at 248 nm: Single and Multiphoton Processes

We have performed photodissociation experiments on CHBr 3 at 248 nm using VUV ionization photofragment translational spectroscopy. Prompt C-Br bond fission is the dominant single-photon dissociation channel. In addition to primary Br and CHBr 2 signals, we observe Br, CHBr, CBr, HBr, and Br 2 products attributed to secondary photodissociation of CHBr 2 and CHBr. There are three competing fragmentation channels from the photodissociation of CHBr 2 : CHBr + Br, CH + Br 2 , and CBr + HBr. The conclusion that Br 2 fragments do not arise from a single-photon channel in appreciable yield is supported by transient FM absorption measurements of the CHBr radical. Because the molecular HBr and Br 2 detachment channels are multiphoton processes, they will have very little impact on the atmospheric chemistry of CHBr 3 . We conclude that the most important photodissociation channel of CHBr 3 in the UV region is C-Br bond breaking.

Hepta- and octacoordinate boron in molecular wheels of eight- and nine-atom boron clusters: observation and confirmation.

Clusters of atoms can adopt different atomic arrangements from bulk materials. They often exhibit novel structures and properties, which provide opportunities for a new types of chemical bonding and stoichiometry. Herein we report experimental and theoretical evidence that 8- and 9-atom boron clusters are perfectly planar molecular wheels, with a hepta- or octacoordinated central boron atom, respectively, despite the predominance of three-dimensional structures normally found in bulk boron and its compounds.

Production of energetic electrons in the process of photodetachment of F-.

An imaging technique is used to record an energy and angle resolved spectrum of electrons produced by photodetachment of F- in a strong infrared laser pulse. The spectrum involves contributions from more than 23 excess photon detachment channels. Its higher energy part extends beyond the classical cutoff value, and it appears as a pronounced plateau localized within a small angle along the laser polarization axis. A Keldysh-like theory is able to qualitatively reproduce the spectrum without taking into account the rescattering mechanism. The role of the parity of the initial bound state is discussed.

Dissociation pathways of benzene trication

Density functional B3LYP calculations have been performed to investigate dissociation pathways of benzene trication in order to understand the mechanism of dissociative ionization of benzene after irradiation in intense laser fields. Numerous decomposition products can be formed: H 2CCCH 2++C 3H 3 + (with the activation barrier of 38.4 kcal/mol), cyclobutadiene dication C 4H 4 2++C 2H 2 + (53.0 kcal/mol), C 6H 5 2++H + (64–65 kcal/mol), etc. The C 2H 2 + detachment channels have the highest exothermicity (−131 to −138 kcal/mol), followed by C 3H 3 + detachment (−125.7) and finally H + detachment (−83.7). The fragments are expected to be produced with high-translational energy due to high Coulomb repulsion energy barriers.