Final Electronic States Research Articles

Theoretical study of the vertical electron excitation of linear carbon clusters C3, C5, and C7

AbstractThe vertical electron excitation spectra of linear carbon clusters C3, C5, and C7 have been investigated by means of polarization propagator calculations using the second‐order algebraic diagrammatic construction [ADC(2)] scheme. The structure of singlet and triplet excited‐state manifolds below the lowest ionization threshold has been analyzed in detail for the smallest prototype cluster C3. The electronic spectra of all three molecules contain numerous low‐lying single and double valence excitations, reflecting strongly the correlated nature in these clusters. The density of the excited states becomes increasingly high above the onset of Rydberg excitations. A substantial mixing of various valence and Rydberg configurations in this region does not allow for simple interpretation of the final electronic states. The spectroscopic and photophysical implications of this complex situation are discussed. Where possible the computed excitation energies and oscillator strength are compared with available experimental and previous theoretical data. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001

Orbital imaging for the valence shell of sulphur dioxide: comparison of EMS measurements with near Hartree–Fock limit and density functional theory

The momentum distributions of the valence orbitals of sulphur dioxide have been studied by electron momentum spectroscopy (EMS), Hartree–Fock (HF) and density functional theory (DFT) calculations. The experiments were performed using an energy dispersive multichannel EMS spectrometer at an impact energy of 1200 eV plus the binding energy and in symmetric non-coplanar kinematics. The valence-shell binding energy spectra have been measured for all valence orbitals over the energy range 8–60 eV. Calculated synthetic binding energy spectra derived from many-body Green’s function and DFT calculations are compared with experiment. In the inner valence region, strong splitting of the 3b 2 and 5a 1 ionization is observed due to final state electron correlation effects. The measured momentum profiles of the valence orbitals are compared with HF calculations at the level of the target HF approximation, and with DFT calculations using B3LYP functionals at the level of the target Kohn–Sham approximation, with basis sets ranging from minimum (STO-3G) to large (AUG-cc-pV5Z(-h)). Generally, the shapes of the experimental momentum profiles are well reproduced by HF and DFT calculations using large diffuse basis sets. However, small discrepancies still exist between all theoretical treatments and experiment in the low momentum region for the (4b 2+7a 1+2b 1) and 6a 1 orbitals.

A New Algorithm for Supernova Neutrino Transport and Some Applications

We have developed an implicit, multi-group, time-dependent, spherical neutrino transport code based on the Feautrier variables, the tangent-ray method, and accelerated ${\bf \Lambda}$ iteration. The code achieves high angular resolution, is good to O($v/c$), is equivalent to a Boltzmann solver (without gravitational redshifts), and solves the transport equation at all optical depths with precision. In this paper, we present our formulation of the relevant numerics and microphysics and explore protoneutron star atmospheres for snapshot post-bounce models. Our major focus is on spectra, neutrino-matter heating rates, Eddington factors, angular distributions, and phase-space occupancies. In addition, we investigate the influence on neutrino spectra and heating of final-state electron blocking, stimulated absorption, velocity terms in the transport equation, neutrino-nucleon scattering asymmetry, and weak magnetism and recoil effects. Furthermore, we compare the emergent spectra and heating rates obtained using full transport with those obtained using representative flux-limited transport formulations to gauge their accuracy and viability. Finally, we derive useful formulae for the neutrino source strength due to nucleon-nucleon bremsstrahlung and determine bremsstrahlung's influence on the emergent $\nu_{\mu}$ and $\nu_{\tau}$ neutrino spectra.

Ionization of multielectron atoms using Slater-type wave functions

The single ionization of multielectron atoms by the impact of protons, antiprotons, and highly charged bare ions is studied. The targets are treated as one-electron atoms, where the interaction of the active electron with the rest of the target is represented by a model potential. The final electronic state is described by the product of two Coulomb wave functions that put the distortion due to the projectile and the target on equal footing. We calculate the total and differential ionization cross sections for helium- and lithium-atom targets. The results thus obtained are compared with existing experimental data and other available theoretical predictions.

Measurement of the photon structure function at high Q2 at LEP

The structure functions of real and virtual photons are derived from cross section measurements of the reaction e +e −→e +e −+ hadrons at LEP. The reaction is studied at s ≃91 GeV with the L3 detector. One of the final state electrons is detected at a large angle relative to the beam direction, leading to Q 2 values between 40 GeV 2 and 500 GeV 2. The other final state electron is either undetected or it is detected at a four-momentum transfer squared P 2 between 1 GeV 2 and 8 GeV 2. These measurements are compared with predictions of the Quark Parton Model and other QCD based models.

Single ionization ofNe by intense laser fields

A three-dimensional classical Monte Carlo method has been used toexamine the final state electron dynamics after single ionization of neonin an electric field generated by a short-pulse high-intensity laser.Linearly and circularly polarized fields were studied. Intensities rangingfrom 3×1014 to 3×1015 W cm-2 wereused with a wavelength of 800 nm and a full-widthat half-maximum of30 fs. At these values, the Keldysh parameter is less than one, making theclassical approximation valid. A potential determined by Hartree-Fockcalculations has been used to model the Ne atom. A kinematicallycomplete picture of the final state momenta of Ne+ and the ionizedelectron is generated by the calculation. Differential cross sections werefound to be similar at electron energies < 2Up over a wide range ofintensities, butto vary greatly at higher electron energies, demonstrating thefield intensity dependence of rescattering of the electron by the Ne+ion. Comparisons have been made with experimental data and quantummechanical theory.

Laser-modified electron bremsstrahlung in a Coulomb field

We study spontaneous emission of electromagnetic radiation by an electron with given initial asymptotic momentum in interaction with a Coulomb field and with a homogeneous monochromatic electric field (laser field in dipole approximation). The intense field effects are taken into consideration for the initial and final continuum electron states, by using improved Volkov-Coulomb wave functions, including second-order corrections in the interaction with the electromagnetic field. For the evaluation of the transition amplitudes, the so-called three-$\ensuremath{\delta}$ approximation introduced by Korol [A. V. Korol, J. Phys. B 28, 3873 (1995)] is adopted. The numerical results are presented for a linearly polarized field of frequency of 0.117 eV, intensities up to ${10}^{\ensuremath{-}6}$ a.u., electron energies in the range 50--500 eV, and incident momentum orthogonal to the laser polarization direction. The study focuses on spontaneous emission accompanied by exchange of a fixed number $n=0,\ifmmode\pm\else\textpm\fi{}1,\ifmmode\pm\else\textpm\fi{}2$ of photons with the external field. Deviations from Born results are shown in several cases. We illustrate in graphs how the spontaneous emission is drastically reduced by the presence of the external field.

Strong potential second Born theory for low-energy electron emission in asymmetric collisions

The ejection of low-energy target electrons by heavy projectiles is calculated in a second Born approximation, allowing for propagation of the electrons in the strong projectile field. For neutral projectile impact this theory provides a satisfactory description of the collision process down to quite low impact velocities. This is shown by comparing the theory with experimental electron spectra from 0.1 MeV/amu Ne0 on He. However, when the projectile is charged the influence of its potential on the electronic final state may only be neglected for ejection of very low-energy electrons into the backward direction.

Auger decay of the C 1s−12π* resonance in carbon monoxide: Vibrationally and angularly resolved spectra

Auger electron spectra from the decay of the ν=0, 1, and 2 levels of the C 1s−12π* state were measured with sufficiently high electron and photon energy resolution to completely resolve the vibrational structure of the final electronic states. The results are compared with ab initio calculations with emphasis on the analysis of the spectator Auger transitions. The anisotropy parameters extracted from angle-resolved spectra show variations within the vibrational envelopes of the participator Auger decay and exhibit a complex oscillating behavior for the spectator Auger transitions. In addition, two-dimensional (2D) imaging of the electron emission across the C 1s−12π* resonance was performed. The ratio of resonant and nonresonant ionization processes is estimated from the 2D map. Strong vibrational lifetime interference effects apparent in the 2D imaging are discussed.

Quantum Control of Molecular Wavepackets: An Approximate Analytic Solution for the Strong-Response Regime

We derive a simple approximate analytic solution for the driving field that will generate a desired target vibrational wavepacket in a diatomic molecule. This solution is valid for arbitrary population transfer from the initial to the final electronic states. Analytic solutions to the control problem are possible when the control field is of shorter duration than the characteristic time scale of the system being manipulated. For target states that are localized wavepackets, our technique yields multiple solutions for the control field. We describe a procedure to choose from the set of solutions one that achieves both the target state and population transfer with high accuracy, for a large class of target states. The fidelity of each solution is tested by comparing the state obtained from a direct numerical integration of Schrodinger's equation using the analytic solution with the specified target state.

Use of a recursive expansion of the Green operator with absorbing boundary conditions to study the effect of non-adiabatic coupling on photodissociation cross sections: application to the CO molecule

The recently proposed iterative method (Grozdanov T P and McCarroll R 1996 J. Phys. B: At. Mol. Opt. Phys. 29 3373) for calculating the total photodissociation cross sections, based on a recursive expansion of the Green operator with absorbing boundary conditions, is applied to the case where the final electronic states are coupled leading to predissociation. The process considered is the transition of the CO molecule from the ground X 1+ state into Rydberg-valence predissociating B 1+-D´ 1+ states. Agreement with close coupling calculations is found.

Control and Characterization of Intramolecular Dynamics with Chirped Femtosecond Three-Pulse Four-Wave Mixing

Experimental control and characterization of intramolecular dynamics are demonstrated with chirped femtosecond three-pulse four-wave mixing (FWM). The two-dimensional (spectrally dispersed and time-resolved) three-pulse FWM signal is shown to contain important information about the population and coherence of the electronic and vibrational states of the system. The experiments are carried out on gas-phase I2 and the degenerate laser pulses are resonant with the X (ground) to B (excited) electronic transition. In the absence of laser chirp, control over population and coherence transfer is demonstrated by selecting specific pulse sequences. When chirped lasers are used to manipulate the optical phases of the pulses, the two-dimensional data demonstrate the transfer of coherence between the ground and excited states. Positive chirps are also shown to enhance the signal intensity, particularly for bluer wavelengths. A theoretical model based on the multilevel density matrix formalism in the perturbation limit is developed to simulate the data. The model takes into account two vibrational levels in the ground and the excited states, as well as different pulse sequences and laser chirp values. The analytical solution allows us to predict particular pulse sequences that control the final electronic state of the population. In a similar manner, the theory allows us to find critical chirp values that control the transfer of vibrational coherence between the two electronic states. Wave packet calculations are used to illustrate the process that leads to the observation of ground-state dynamics. All the calculations are found to be in excellent agreement with the experimental data. The ability to control population and coherence transfer in molecular systems is of great importance in the quest for controlling the outcome of laser-initiated chemical reactions.

Multiphoton ionization of the hydrogen atom by a circularly polarized electromagnetic field

This paper examines the multiphoton ionization of the ground state of the hydrogen atom in the field of a circularly polarized intense electromagnetic wave. To describe the states of photoelectrons, quasiclassical wave functions are introduced that partially allow for the effect of an intense electromagnetic wave and that of the Coulomb potential. Expressions are derived for the angular and energy distributions of photoelectrons with energies much lower than the ionization potential of an unperturbed atom. It is found that, due to allowance for the Coulomb potential in the wave function of the final electron states, the transition probability near the ionization threshold tends to a finite value. In addition, the well-known selection rules for multiphoton transitions in a circularly polarized electromagnetic field are derived in a natural way. Finally, the results are compared with those obtained in the Keldysh-Faisal-Reiss approximation.

Resonance enhanced multiphoton ionization spectroscopy of the NCI molecule: 1Σ Rydberg states studied by 2-photon excitation from the a1Δ state

Three 1Π Rydberg states of the NCI molecule, accessed from the NCl a1 ΔA state, have been investigated by multiphoton ionization spectroscopy. NCl was prepared from the Cl + N3 reaction and the excited 1Π states were observed via the 2-photon resonance enhancements they provide in the multiphoton ionization spectrum of NCl a1 Δ. Assignment of the initial and final electronic states associated with each band was achieved by rotational band contour simulations, least-squares fitting of the resolved rotational structure, recording spectra with linearly and circularly polarized radiation and recording spectra in the N35Cl+ and N37Cl+ mass channels. The Π+/Π− character of each excited state as well as the dominant electronic configuration at spectroscopically accessible bond lengths was established by multireference double excitation configuration interaction (MRDCI) calculations.

The electronic spectrum of pyrrole

The electronic spectrum of pyrrole has been investigated by performing calculations using a hierarchy of coupled-cluster models consisting of CCS, CC2, CCSD, and CC3. Basis-set effects have been investigated by carrying out calculations using correlation-consistent basis sets augmented with functions especially designed for the description of Rydberg states. Oscillator strengths, excited state dipole moments, and second moments of the electronic charge distributions have been used to characterize the electronic transitions and final states. Structures and vibrational frequencies have been calculated for a few selected states, and the importance of distinguishing between vertical and adiabatic transitions for accurate comparison with experiment has been emphasized. The experimental spectrum has been scrutinized in the relevant energy region, and the accuracy of recent calculations [CASPT2, MRMP, ADC(2)] has been critically reexamined.

Single and double K-shell vacancy production in N7+ + Ti collisions

Non-perturbative classical models were applied in the study of K-shell vacancy production in N7+ and Ti collisions in the 4–26 MeV/u projectile energy range. Total cross sections for the single and double K-shell vacancy productions are calculated by means of a simple binary encounter model, classical trajectory Monte–Carlo methods within and beyond the independent electron approximation. The experimental and theoretical ratios of the double- to single-vacancy production yields have been compared. It is demonstrated that a simple model for accounting the final state electron–electron correlation in the double capture and the capture+ionization channels significantly improves the agreement between experiment and theory compared to the independent electron approximation.

Orbital imaging and assessment of different orbital models for the valence shell of methanol: Comparison of electron momentum spectroscopy measurements with near-Hartree–Fock limit, MRSD-CI, localized valence bond and density functional theory

The momentum distributions of the valence orbitals of methanol have been studied by electron momentum spectroscopy (EMS) and Hartree–Fock (HF), multi-reference singles and doubles configuration interaction (MRSD-CI), localized valence bond (VB) and density functional theory (DFT) calculations. The experiment was performed using a multichannel EMS spectrometer at a total energy of 1200 eV plus the binding energy. Binding-energy spectra measured in the energy range of 6–47 eV are presented for the azimuthal angles φ=0° and φ=8°. Synthetic binding-energy spectra from Green's function and HF calculations for the azimuthal angles φ=0° and φ=8° in the 6–47 eV energy region are also compared to experiment. In the inner valence region strong splitting of the 4a′ and 3a′ ionization is observed due to final-state electron correlation effects. The measured momentum profiles are compared with HF calculations at the level of the target HF approximation using basis sets ranging from simple (STO-3G) to large (110-GTO and Trun-pV5Z). DFT calculations at the level of the target Kohn–Sham approximation employing the local density approximation or hybrid functional methods and the large Trun-pV5Z basis set are also compared to experiment. The effects of electron correlation and relaxation are also investigated in the outer valence region by MRSD-CI calculations of the full ion–neutral overlap amplitude using the 110-G(CI) basis set. The shapes of all momentum profiles are well predicted by higher level theory. Some small discrepancy still exists between all theoretical treatments and experiment in the low-momentum region for the HOMO 2a″ orbital. MRSD-CI or DFT (i.e. correlated) methods are needed to adequately describe the shape of the 7a′ and (6a′+1a″) momentum profiles. The s-type character in the 5a′ momentum profile is underestimated by HF theory and overemphasized by density functional theory (DFT). The 110-G(CI) calculation best predicts the shape for the 5a′ momentum profile. The shapes of the experimental momentum profiles for the 4a′ and 3a′ are well-reproduced by large basis set HF and DFT calculations. In addition, nearly all the inner valence 4a′ pole strength is found in the 20–28 eV region while the high-energy 28–47 eV region contains virtually all of the 3a′ pole strength. Lastly, the HOMO and NHOMO momentum profiles are compared to the corresponding localized molecular orbitals (LMO) (i.e. the `lone-pair' orbitals of VSEPR or qualitative VB theory), the canonical molecular orbitals (CMO) of molecular orbital (MO) theory and the Kohn–Sham orbitals (KSO) of DFT used in the present work. The experimental results unequivocally support the delocalized CMO and KSO models and are very different from the LMO description. Although electron correlation effects are also important at low momenta in the 7a′ NHOMO this orbital is also basically a CMO or KSO in character as shown by the CI and DFT calculations. It is noteworthy that the KSOs of DFT, often described as fictitious mathematical concepts, in fact very closely fit the experimental results and are very similar to the CMOs given by HF or CI calculations of the full ion–neutral overlap where necessary. These conclusions have profound implications for computer-aided molecular design, molecular modelling and molecular recognition.

The H 3O Rydberg radical

Ab-initio calculations on the CEPA level are presented for the wave functions of a number of electronic states of the Rydberg radical H 3O. We make predictions for wavelengths and transition probabilities of the vertical emission spectrum of Rydberg excited H 3O ∗ as well as for their equilibrium geometries. Furthermore we have investigated possible dissociation paths of some low-lying final electronic states. Three semivalence electronic states are thermodynamically unstable: The 3sa 1 ground state has an adiabatic IP of about 5.4 eV and a barrier to dissociation of less than 0.1 eV whereas for the components of 3pe, connected by conical intersections, the IP is 3.3 eV and the barrier about 0.8 eV.

Specific Fluorescence Properties and Picosecond Transient Absorption of 8-Azasteroids

The specific fluorescence properties as well as picosecond transient absorption features have been studied for two 8-azasteroids. It is shown that at various excitation wavelengths the essentially different final excited electronic states are realized. Because of the multicenter character of 8-azasteroids the spectroscopic data obtained may be analyzed on a basis of the “mesomeric tautomerism” model taking into account the dynamic combination of cis- and trans-configurations. The dependence of fluorescence spectral characteristics on the solvent nature is a manifestation of intermolecular H-bond interactions.

Influence of the polar medium fluctuation on the nonradiative electron transfer in molecules

The influence of the classical polar medium fluctuation on the nonradiative transition rate in molecules is considered. Change of the vibrational frequencies under electron excitation is taken into account. In the Gauss–Markov approximation for the fluctuation and the linear model of the vibrational term of the final electronic state, the formula for the rate of the process at low temperature is obtained. The formula reveals the temperature dependence arising from the influence of the environment medium on the barrier of the reaction. The theory is applied to the explanation of the value and temperature dependence of the rate of the low-potential cytochrome oxidation. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 74: 381–386, 1999