Photodetachment Process Research Articles

Induced dipole effect in strong-field photodetachment of atomic negative ions

The process of strong-field photodetachment of an atomic negative ion is considered with a simple analytical account for the ac dipole moment induced in the atomic core by the external laser field. The method is based on a Keldysh-like saddle-point theory, modified by using dipole-spherical angular wave functions in the initial state of the outer electron. The field-induced dipole results in an increase of the detachment rate and essential modifications in its angular dependence. These modifications are interpreted as destruction of the quantum interference originated from the coherent superposition of the two saddle-point contributions to the transition amplitude.

Negative ion photoelectron spectroscopy of acridine molecular anion and its monohydrate

Negative ion photoelectron spectroscopy was employed to investigate the electronic structure of the acridine molecular anion and its monohydrated anion in the gas phase. Their adiabatic electron affinities were measured to be 0.896+/-0.010 and 1.18+/-0.05 eV, and the low-lying electronic excited states in both neutral acridine and in its monohydrate were revealed. The photoelectron spectra clearly exhibit the presence of low-lying singlet and triplet states having a (pi,pi*) configuration in an uncomplexed acridine molecule. Comparison of the photoelectron spectrum of acridine with that of anthracene shows that photodetachment processes into the excited states of (n,pi*) configuration have little intensity, implying a relatively large intramolecular structural relaxation in the (n,pi*) states.

The effect of electron detachment on the structure and properties of the chlorine-acetonitrile anionic complex.

The results of the theoretical study of ground state potential energy surfaces for the chlorine-acetonitrile anion and its photodetachment product are presented. The shallow potential surfaces allow for the nondefinitive position of the chlorine within the complex. The dissociation energy of the neutral complex, estimated through the thermodynamic cycle, indicates significant structural changes due to the photodetachment process. The excess negative charge is localized mostly on the chlorine atom, and the electron detachment proceeds as an electron is removed from chlorine. The process leads to drastic changes in the electrostatic interactions within the complex. The first electronic excited state corresponds to the excess electron transfer from chlorine to acetonitrile fragment. This state is a precursor of the observed charge-transfer-to-solvent state.

Photodetachment in a strong circularly polarized laser field

The saddle-point analysis of the transition amplitude shows that the quantum interference effect does not occur in the direct process of photodetachment by a circularly polarized laser field. This fact is interpreted in terms of classical electron trajectories. Comparison between the length and velocity gauges is performed by simulating spectra of photoelectrons produced in a focus of a laser pulse. A substantial discrepancy is found in the kinetic energy distribution of photoelectrons. At low energies, only the predictions in the length gauge are consistent with the Wigner threshold law.

The geometry of the NO 2− anion: ab initio calculations and Franck–Condon analysis

Geometry optimization and harmonic vibrational frequency calculations have been performed on the X ̃ 2 A 1 state of NO 2 and X ̃ 1 A 1 state of NO 2 −. Franck–Condon analyses and spectral simulations were carried out on the NO 2( X ̃ 2 A 1 )–NO 2 −( X ̃ 1 A 1 ) photo-detachment process. In addition, the equilibrium geometry parameters, r(NO)=1.248 ± 0.005 Å and ∠(ONO)=116.8 ± 0.5°, of the X ̃ 1 A 1 state of NO 2 −, are derived by employing an iterative Franck–Condon analysis procedure in the spectral simulation. Our conclusions regarding the anion geometry suggest a reinterpretation of the results of Woo et al. [Phys. Rev. A 24 (1981) 1380].

Inner-shell photodetachment of excited 4S° and 6S° metastable bound states of Be−

The total cross sections for photodetachment of the metastable 1s22p3 4S° and 1s2s2p3 6S° excited bound states of the negative ion of beryllium are presented for a range of initial photon energies across and beyond the 1s detachment threshold. A multichannel close-coupling R-matrix approximation is used to compute the cross sections, with sophisticated configuration-interaction wavefunctions being used to represent the initial and final states. At present there are no other theoretical or experimental data available with which to compare the cross sections for these two photodetachment processes.

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.

Static electric-field effects in the photodetachment ofCs−at the3Presonance region

We calculated near-threshold photodetachment cross sections for ${\mathrm{Cs}}^{\ensuremath{-}}$ in the presence of a dc electric field using three different approaches: the frame-transformation method with and without rescattering effects and the Kirchhoff-integral approach. Radial wave functions for the electronic motion were obtained using the Pauli equation method with a model potential describing the effective electron-atom interaction. Our results demonstrate the inadequacy of the frame-transformation method in the ${}^{3}P$ resonance region even for weak fields $(E<10\mathrm{k}\mathrm{V}/\mathrm{c}\mathrm{m}).$ We show that the triplet and singlet contributions to the total cross section can be manipulated by varying the external electric field. This can enhance spin-orbit effects in the photodetachment process and create more favorable conditions in the experiment for the observation of the lowest ${}^{3}P$ resonance in ${\mathrm{Cs}}^{\ensuremath{-}}.$

Short-Time Water Caging and Elementary Prehydration Redox Reactions in Ionic Environments

This article deals with the direct probing of water cages that assist two well-defined prehydration electron transfers (PHETs) with the reactive metal cation Cd2+. Early electron photodetachment processes are triggered by a two-photon UV excitation of aqueous halide ion Cl- (R = [H2O]/[CdCl2] = 110). Concomitant with an ultrafast Cd2+ reduction by IR p-like excited electron (J. Phys. Chem. A 1998, 102, 7795), a subpicosecond oxidoreduction reaction occurs in caged three-body complexes {Cl··e-··Cd2+}aq. Near-IR spectroscopic measurements give a PHET frequency of 1.38 ± 0.02 1012 s-1. This reduction reaction is 70 times faster than a diffusion-controlled bimolecular reaction between aqueous Cd2+ ions and fully hydrated electrons (s-state). Femtosecond spectroscopic data indicate that preexisting bridging water-molecule-bonded Cl-···Cd2+ pairs (SSIP-like configurations) assist efficient prehydration electron transfer. Because the 4s-like orbital radius of nascent {Cl··e-··Cd2+}aq configurations is larger tha...

Auger recombination and charge-carrier thermalization in Hg - n -cluster photoelectron studies.

Photoelectron spectra (PES) of Hg(-)(n) show strong dependence of spectral features on photon energy, i.e., peak tailing and band gap filling. This dependence suggests the existence of complex photoinduced processes in parallel with the direct photodetachment process. The "corrupted" PES, taken with intermediate photon energy, carry the signature of interband absorption followed by charge-carrier thermalization and Auger electron ejection in Hg(-)(n). These processes, so significant in the photophysics of bulk semiconductors and nanoparticles, have not yet been identified in clusters.

Ab initio simulations of the KrO− anion photoelectron spectra

The photoelectron spectra of the weakly bound KrO− anion are simulated using a theory which combines the atoms-in-molecule model for molecular electronic wave functions and the Rau–Fano model for photodetachment intensities [J. Chem. Phys. 112, 5852 (2000)]. The nonrelativistic potential energy curves of the anion are obtained from ab initio calculations. The calculated spectra and their temperature variation agree with the experimental data presented in the preceding article. The strong selectivity of the photodetachment process with respect to the symmetry of fine-structure components is rationalized and quantified.

Photodetachment of FSi(OMe) 4− and FGe(OMe) 4− anions: an experimental and theoretical study of gas-phase hypervalent Si and Ge species

The structure and thermochemistry of the hypervalent FSi(OMe) 4 −, FGe(OMe) 4 − and FGe(H)(OMe) 3 − anions have been investigated by a combination of photodetachment spectroscopy and ab initio calculations. All three of these anions undergo photodetachment at wavelengths between 266 and 285 nm but not at 355 nm. The data suggest that the observed photodetachment process for these ions is a one-photon process leading to dissociation of a MeO group for the first two anions, and presumably dissociation of a H atom in the last case. Theoretical calculations are consistent with these observations and predict a fluoride binding energy of 47.5 kcal mol −1 and a methoxide binding energy of 52 kcal mol −1 in FSi(OMe) 4 −. The most stable structure for FSi(OMe) 4 − is predicted to be a trigonal bipyramid with the fluorine occupying an axial position. This is similar to what has been found for the corresponding Ge(OMe) 4 species. These results, the gas-phase chemical behavior of these anions, and the calculated charge distribution are consistent with anions in which a high degree of covalent bonding exists between the electronegative ligand and the central Si or Ge atom.

Tunneling photodetachment of negative ions exposed to a bichromatic laser field

Tunneling photoionization by a bichromatic field consisting of the coherent superposition of the fundamental laser field frequency and its second harmonic is studied theoretically within the framework of the adiabatic Landau–Dykhne approach applied to the process of negative-ion photodetachment. Analytical expressions for the probability of photodetachment were derived at two fixed specified values of relative phase shift, Φ=0 and Φ=π/2, between the harmonics of the incident bichromatic field. The derived expressions are thoroughly analyzed for two possible limiting cases of tunneling and multiphoton ionization and are demonstrated to be different from the respective expressions for the monochromatic case because of the presence of an effective field strength, which is a function that strongly depends on the parameters of the incident bichromatic field.

Multielectron system in an ultrashort, intense laser field: A nonperturbative, time-dependent two-active-electron approach

We present a two-active-electron (TAE) approach for solving the time-dependent Schr\"odinger equation (TDSE) for the interaction of a multi-electron system with an ultrashort, intense, and linearly polarized laser pulse [Lagmago Kamta and Starace, Phys. Rev. Lett. 86, 5687 (2001)]. A technique for obtaining angular distributions for double ionization by such pulses is also described. The approach for solving the TDSE in the TAE approximation is full dimensional and accounts for correlations between the two electrons, as well as the polarization of the core. It is based on a configuration-interaction expansion of the time-dependent wave function in terms of one-electron atomic orbitals. Applying the method to the lithium negative ion $({\mathrm{Li}}^{\ensuremath{-}}),$ we display the time-dependent dynamics of the photodetachment process. For low intensities, our results for the detachment yield follow expectations from lowest-order perturbation theory and agree satisfactorily with $\mathcal{R}$-matrix calculations. Our results for angular distributions indicate that following multiphoton double ionization by an intense laser field, electrons are predominantly ejected along the laser polarization axis; however, a significant number are ejected perpendicularly to this axis. An angular momentum-based analysis of these angular distributions indicates that, in the dipole approximation and for an initial ${}^{1}{S}^{e}$ state interacting with a linearly polarized laser field, double ejection of both electrons along the direction perpendicular to the laser polarization axis can only occur following absorption of an even number of photons, whereas multiphoton absorption of an odd number of photons does not lead to double ejection at these angles.

Comment on “Photodetachment in combined static and dynamic electric fields”

The theoretical analysis of C. Rangan and A. R. P. Rau [Phys. Rev. A 61, 033405 (2000)] on the process of photodetachment of ${\mathrm{H}}^{\ensuremath{-}}$ in a strong static electric field, which calls into question the predictions of B. Gao and A. F. Starace [Phys. Rev. A 42, 5580 (1990)] and also of M. Q. Bao et al. [Phys. Rev. A 58, 411 (1998)], is shown in this Comment to be incorrect. First, we point out that a number of assumptions of Rangan and Rau's analysis rest on tenuous theoretical grounds. Second, we adduce two completely independent and different analyses of the problem which precisely confirm the results of Gao and Starace. These independent analyses also provide the interpretation that Gao and Starace's predicted strong-field effects are due to the exact account of the influence of the static electric field, not only on the final state of the detached electron, but also on the initial bound-electron state.

Measurement and theoretical simulation of the HCCO− anion photoelectron spectrum

The photoelectron spectrum of HCCO− at the photodetachment wavelength of 355 nm is reported. A theoretical model for the simulation of the photodetachment process is described and the influence of various parameters is discussed. The experimental spectrum is compared with the simulation and an assignment of the spectrum is given.

Ab initio zero electron kinetic energy spectroscopy of the ArCl− and KrCl− anions

The theory for calculating the energies and relative intensities of the photodetachment transitions of the weakly bound diatomic anions is implemented for simulations of the zero electron kinetic energy spectra of the ArCl− and KrCl− anions using high-quality ab initio potentials. Its key features are the reduction of the molecular electronic photodetachment transition dipole moment matrix elements to the combination of atomic ones within the atoms-in-molecule model and estimation of the latter by well-developed approaches to the atomic photodetachment processes. The difference in the electronic photodetachment transition dipole moments for distinct electronic states of the neutral, importance of vibrational–rotational coupling, and rotational structure of vibronic bands are analyzed. It is shown that ab initio simulations provide almost the same level of agreement with measured spectra as did the empirical potentials adjusted to reproduce the experimental data. The predictive power of the theory is demonstrated by the discovery of the hidden transitions to the I3/2 state of KrCl which strongy overlap with those to X1/2 state.

Studies of angular distributions and cross sections for photodetachment from the oxygen molecular anion

We have studied the photodetachment from molecular oxygen anion O2− leading to eleven O2 electronic states. Both single- and multichannel scattering calculations were performed using the Schwinger variational method with the Hartree–Fock or configuration interaction approximations for the target states. Our calculated total cross sections agree well with the existing experimental data. One Feshbach resonance was found at about 5 eV above the photodetachment threshold. This resonant state has the character of the 1πu31πg4 configuration, and decays to several final channels. Our calculated partial cross sections suggest that shape resonances exist in different channels at higher energies, and the structures of these shape resonances differ from what we have seen in the photoionization from neutral molecules. The most significant feature in this photodetachment process is the channel coupling effect due to electron correlation.

An Experimentalist's View of the H− Ion

AbstractWe present a short survey of selected experimental studies of the negative ion of hydrogen over the past twenty‐three years at the 800 MeV linear accelerator at Los Alamos. Experimental techniques are described and heuristic models are used to explain and motivate experimental results. We discuss the first observations of the 1P resonances just below n = 2, the so‐called “Feshbach” and “shape” resonances, seen in photodetachment. As one experimental question is answered, another question becomes exposed, for example, the effects of electric fields on the spectroscopy of the ion. Techniques used to convert the H− ion into a dual‐beam interferometer moving near the speed of light are described. Experimental data taken near thresh old in an electric field are displayed on a universal plot. Multiphoton detachment extends the spectroscopy of the ion to singlet states that would be unreachable by single photodetachment processes. A thin foil also is used to probe the ion structure. Using ion beams in the 35 kV range, 2‐photon measurements of an isotope shift between Hand D− reveal two‐electron correlation consequences. Finally we ad dress the question, “Why has the shape resonance not been seen in astrophysical observations?

Dissociation Dynamics and Stability of Cyclic Alkoxy Radicals and Alkoxide Anions

Photodetachment and dissociative photodetachment processes of cyclopropoxide, c-C3H5O-, and cyclobutoxide have been studied at 532 nm. Photodetachment of c-C3H5O- produces both the ground X(2A‘ ‘) state and the first excited A(2A‘) state of cyclopropoxy radical, c-C3H5O. The X(2A‘ ‘) state is stable at lower levels of excitation, but with increasing internal energy, dissociation into HCO + C2H4 is observed. The A(2A‘) state completely dissociates into HCO + C2H4. Correlated measurements of photoelectron and photofragment kinetic energies provide dissociation energies c-C3H5O- and c-C3H5O into HCO- + C2H4 and HCO + C2H4 of 0.85 ± 0.07 and −0.26 ± 0.07 eV, respectively. Ab initio calculations have been performed to aid the interpretation of the dissociation mechanism. Cyclobutoxide, c-C4H7O-, undergoes only dissociative photodetachment to ground-state vinoxy radical and ethylene. The adiabatic electron affinity (AEA) of c-C4H7O is estimated to be 1.7 ± 0.1 eV. c-C4H7O- and c-C4H7O are both found to be therm...