Excited Target States Research Articles

Nuclear spin blockade of laser ignition of intramolecular rotation in the model boron rotor .

The boron rotor consists of a tri-atomic inner "wheel" that may rotate in its pseudo-rotating ten-atomic outer "bearing"-this concerted motion is called "contorsion." in its ground state has zero contorsional angular momentum. Starting from this initial state, it is a challenge to ignite contorsion by a laser pulse. We discover, however, that this is impossible, i.e., one cannot design any laser pulse that induces a transition from the ground to excited states with non-zero contorsional angular momentum. The reason is that the ground state is characterized by a specific combination of irreducible representations (IRREPs) of its contorsional and nuclear spin wavefunctions. Laser pulses conserve these IRREPs because hypothetical changes of the IRREPs would require nuclear spin flips that cannot be realized during the interaction with the laser pulse. We show that all excited target states of with non-zero contorsional angular momentum have different IRREPs that are inaccessible by laser pulses. Conservation of nuclear spins thus prohibits laser-induced transitions from the non-rotating ground to rotating target states. We discover various additional constraints imposed by conservation of nuclear spins, e.g., laser pulses can change clockwise to counter-clockwise contorsions or vice versa, but they cannot stop them. The results are derived in the frame of a simple model.

Seniority based energy renormalization group (Ω-ERG) approach in quantum chemistry: Initial formulation and application to potential energy surfaces

This investigation combines the concept of the seniority number Ω (defined as the total number of singly occupied orbitals in a determinant) with the energy renormalization group (ERG) approach to obtain the lowest-energy electronic states on molecular potential energy surfaces. The proposed Ω-ERG method uses Slater determinants that are ordered according to seniority number Ω in ascending order. In the Ω-ERG procedure, the active system consists of M (N-electron) states and K additional complement (N-electron) states (complement-system). Among the M states in the active system the lowest-energy m states represent target states of interest (target-states), thus m ≤ M. The environment consists of Full Configuration Interaction (FCI) determinants that represent a reservoir from which the complement-states K are being selected. The goal of the Ω-ERG procedure is to obtain lowest-energy target states m of FCI quality in an iterative way at a reduced computational cost. In general, the convergence rate of Ω-ERG energies towards FCI values depends on m and M, thus, the notation Ω-ERG(m, M) is used. It is found that the Ω-ERG(m, M) method can be very effective for calculating lowest-energy m (ground and excited) target states when a sufficiently large number of sweeps is used. We find that the fastest convergence is observed when M > m. The performance of the Ω-ERG(m, M) procedure in describing strongly correlated molecular systems has been illustrated by examining bond-breaking processes in N2, H8, H2O and C2. The present, proof-of-principle study yields encouraging results for calculating multiple electronic states on potential energy surfaces with near Full CI quality.

Communication: Transfer of more than half the population to a selected rovibrational state of H2 by Stark-induced adiabatic Raman passage

By using Stark-induced adiabatic Raman passage (SARP) with partially overlapping nanosecond pump (532 nm) and Stokes (683 nm) laser pulses, 73% ± 6% of the initial ground vibrational state population of H(2) (v = 0, J = 0) is transferred to the single vibrationally excited eigenstate (v = 1, J = 0). In contrast to other Stark chirped Raman adiabatic passage techniques, SARP transfers population from the initial ground state to a vibrationally excited target state of the ground electronic surface without using an intermediate vibronic resonance within an upper electronic state. Parallel linearly polarized, co-propagating pump and Stokes laser pulses of respective durations 6 ns and 4.5 ns, are combined with a relative delay of ~4 ns before orthogonally intersecting the molecular beam of H(2). The pump and Stokes laser pulses have fluences of ~10 J/mm(2) and ~1 J/mm(2), respectively. The intense pump pulse generates the necessary sweeping of the Raman resonance frequency by ac (second-order) Stark shifting the rovibrational levels. As the frequency of the v = 0 → v = 1 Raman transition is swept through resonance in the presence of the strong pump and the weaker delayed Stokes pulses, the population of (v = 0, J = 0) is coherently transferred via an adiabatic passage to (v = 1, J = 0). A quantitative measure of the population transferred to the target state is obtained from the depletion of the ground-state population using 2 + 1 resonance enhanced multiphoton ionization (REMPI) in a time-of-flight mass spectrometer. The depletion is measured by comparing the REMPI signal of (v = 0, J = 0) at Raman resonance with that obtained when the Stokes pulse is detuned from the Stark-shifted Raman resonance. No depletion is observed with either the pump or the Stokes pulses alone, confirming that the measured depletion is indeed caused by the SARP-induced population transfer from the ground to the target state and not by the loss of molecules from photoionization or photodissociation. The two-photon resonant UV pulse used for REMPI detection is delayed by 20 ns with respect to the pump pulse to avoid the ac Stark shift originating from the pump and Stokes laser pulses. This experiment demonstrates the feasibility of preparing a large ensemble of isolated molecules in a preselected single quantum state without requiring an intermediate vibronic resonance.

Laser-assisted electron momentum spectroscopy

We consider theoretically ionization of an atomic target by fast electron impact at large energy and momentum transfer and in the presence of laser radiation. The laser electric-field amplitude is weak compared to the typical field in the target. Two frequency regimes are investigated according to whether the laser frequency is (i) much smaller than or (ii) resonant to the frequency of the transition from the ground to the first excited target state. Fast incident, scattered, and ejected electrons are described using Volkov solutions. The dressing of the bound-electron state by the laser field is accounted for within time-dependent perturbation theory in the case of the low-frequency regime and within the rotating wave approximation in the case of a resonant one. The interaction of the incident electron with the target is treated in the first Born approximation. For atomic hydrogen embedded in a linearly or circularly polarized laser field, we discuss how the polarization-vector orientation influences the momentum-dependent $(e,2e)$ differential cross sections assisted by exchange of few photons between the colliding system and the field. In addition, we inspect the dependence of the cross sections on the dressing of the hydrogen state.

Symmetry- and multiplet-resolved N1sphotoionization cross sections of theNO2molecule

We present a joint theoretical and experimental investigation of the N 1s photoionization of NO{sub 2} in the shape resonance region. The theoretical calculations, based on a single-channel relaxed-core Hartree-Fock approximation, predict that the shape resonance appears only in the A{sub 1}{yields}B{sub 2} transition and that the shape resonance energy of the N 1s{sup -1} {sup 1}A{sub 1} channel is about 2.6 eV lower in kinetic energy than that of the N 1s{sup -1} {sup 3}A{sub 1} channel, suggesting that the potential for the {sup 1}A{sub 1} channel is much more attractive than that for the {sup 3}A{sub 1} channel. Symmetry-selected cross sections measured by means of a multiple-ion coincidence imaging prove that the shape resonance appears only in the A{sub 1}{yields}B{sub 2} transition, as predicted by the calculation. The experimental partial cross sections for the N 1s{sup -1} {sup 1}A{sub 1} and {sup 3}A{sub 1} channels measured by means of conventional electron spectroscopy exhibit the shape-resonance maxima at photon energies of 416.3 and 415.9 eV, respectively, at corresponding kinetic energies of 3.0 and 3.3 eV, respectively, implying that the attractive potential for the {sup 1}A{sub 1} channel is overestimated in the single-channel approximation. The possible role played by correlationmore » effects on the K-shell ionization of NO{sub 2} is discussed in terms of interchannel coupling between the main-line channels and, possibly, with additional excited target states.« less

Simulation of collisionally excited atomic states

A Mathematica application for the production of QuickTime movies and GIF animations (angle to time conversion) to represent collisionally excited target states is presented. These states are described by scattering amplitudes (input) for the excitation of individual magnetic sublevels as a function of the projectile scattering angle or the ejected electron angle in ionization processes. Examples for electron-impact excitation and ionization processes are presented, but the program can also be used, for example, to visualize the results of ion-atom collisions, including charge transfer processes.

Transfer excitation in slow collisions between ions of very high charge and two-electron targets.

We argue that a three-step transfer-excitation (capture and target excitation) mechanism enhances (depletes) the cross section for one- (two-) electron removal from a two-electron target. This mechanism, which becomes very important at high projectile charge (q), is mediated through quasimolecular couplings between diabatic double-capture and transfer-excitation channels, in which the inner of two active electrons is transferred to an excited target state. The inclusion of this process in the extended classical over-barrier model gives excellent agreement with unexplained experimental cross sections for one- and two-electron removal from He in slow collisions with Xe ions of high charge (q\ensuremath{\le}31).

Interference effects in electron-ion recombination. III. Excited target states and continuum-continuum coupling.

Provision is made for the existence of excited target states and projectile continuum-continuum coupling, within the framework of the interacting resonance theory of electron-ion recombination described previously (Phys. Rev. A 36, 4662 (1987); 38, 1820 (1988)). This ''extended'' theory makes explicit what was merely implicit in these earlier works. Dielectronic recombination probabilities, which include the effects of interaction and interference between resonances, are derived for two model systems: (a) A target ground state supporting two coupled projectile continua, and one excited target state as a closed channel; (b) a ground and first excited target state, each supporting one projectile continuum, and a second excited target state as a closed channel. Explicit calculations are performed for model (b), wherein are included Rydberg series of fully interacting resonances built on each of the two excited target states. The probability of excitation from the ground to the first excited target state is also obtained for model (b). This probability includes the effect of the interaction and interference between resonances, as well as the interference between resonance and direct excitation processes.

R -matrix theory of electron scattering at intermediate energies

A new R -matrix theory of electron-atom and electron-molecule scatter­ing at intermediate energies is described. In this theory both the outer valence electron of the target atom or molecule and the scattered electron are expanded in terms of a continuum R -matrix basis. This enables target eigenstates as well as pseudostates representing inelastic effects to be accurately represented in an internal region. In addition, a two-dimensional R -matrix propagator approach is developed that enables the internal region to be subdivided and highly excited target states that extend out to large distances to be treated. This new theory is then combined with the T -matrix energy averaging technique introduced earlier by Burke et al . (1981) to yield accurate cross sections at inter­mediate energies. The method is illustrated by applying it to the elastic s-wave scattering of electrons by atomic hydrogen from threshold to 60 eV.

A theoretical study of coherence effects in charge transfer collisions: Application to Na–Li+

We have carried out a theoretical study of charge exchange in the Na–Li+ system and have examined the effects of coherent laser preparation of excited target states. Accurate potential energy curves and nonadiabatic coupling matrix elements were obtained from ab initio, all-electron configuration-interaction calculations. These were used in determining the cross sections over a range of collision energies. Significant differences between coherent and incoherent target preparation were observed in both total and partial cross sections.

Total inelastic-scattering cross sections by optical-potential methods

Total excitation cross sections for electrons scattering from atomic hydrogen are calculated by using a complex optical potential. Distorted-wave propagators as well as free-particle propagators are used in computing the optical potential. It is found that using an attractive potential to generate the propagator results in poorer agreement with experiment than using a free-particle propagator. The effect of neglecting coupling between excited target states in computing the optical potential is also studied. It is concluded that it is necessary to include coupling between excited target states if the calculation is to be reliable.

On the valence model for radiative capture

We give several parametrizations for the elastic scattering and radiative capture cross sections for low neutron bombarding energy and discuss the relationship between the corresponding resonance parameters. We then perform an extensive investigation of the valence radiative capture model of Lane and Lynn. This model is formulated here in the frame of the shell-model approach. We exhibit the similarities and differences between our results and those derived from the R-matrix approach by Lane and Lynn on the one hand and from the optical-model approach by Lane and Mughabghab on the other hand. Particular attention is paid to the choice of the average potential well in the shell-model approach, in relation to the proper way to identify theoretical quantities and phenomenological parameters. We show that practically equivalent results can be obtained from a complex average potential well and from a suitably chosen real potential well, respectively. The following topics are investigated formally and numerically: dependence of the various theoretical expressions on the choice of the (real or complex) average potential well; relative importance of external and internal capture; dependence of photon widths and background cross section on mass number (for thermal energy and for E = 100 keV); dependence of the resonance parameters and background cross sections on energy, for A = 60; comparison between experimental data and theoretical values for radiative capture on 56Fe and 60Ni. We discuss the conditions of validity of the valence capture model. In particular, we investigate the role of the giant dipole resonance and of the closed channels. We argue that the success of the valence capture model is intimately related to the importance of external capture. The contribution of the low-lying excited target states is investigated formally and numerically; it increases with mass number and tends to diminish the correlation between neutron and photon widths, which is implied by the valence capture model.

On scattering by nuclei at high energies

We study how nucleon-nucleon correlations influence high-energy scattering by nuclei. Using the multiple scattering theory of Kerman, McManus, and Thaler, we formulate the scattering problem in terms of an infinite system of coupled equations. For the calculation of elastic scattering, we propose replacing the infinite system by a pair of coupled equations, containing the elastic channel, and another effective one which carries the inelastic strength. The coupling potential is proportional to the nuclear pair correlation function, and a first approximation to the potential is obtained. These equations are accurate to the extent that the pair correlations and not higher-order correlations are important. There are no restrictions to forward scattering, nor are any “on the energy shell” approximations made. In order to obtain insight into the structure of the many-channel S-matrix for high energies, the infinite system of coupled equations is solved by semi-classical methods, and explicit formulae for the S-matrix elements as a function of various potentials and the nuclear pair correlation function are obtained. We show how properties of excited target states may complicate a reliable extraction of correlations in high-energy scattering by nuclei. A close relation between our semi-classical solution and Glauber's multiple scattering is established. A numerical study of high-energy nucleon-nucleus scattering using the methods developed in this paper is under way.

A potential model for isobaric analogue states

Isobaric analogue states observed as resonances in (pp) scattering are explained by an optical model for which the isovector potential gives a coupling between the low excited target states and their isobaric analogue states. The model is exemplified for 48Ca(pp).