The concept of intrinsic and operational observables in quantum mechanics is introduced. In any realistic description of a quantum measurement that includes a macroscopic detecting device, it is possible to construct from the statistics of the recorded raw data a set of operational quantities that correspond to the intrinsic quantum-mechanical observable. This general approach is illustrated by the example of an operational measurement of the position and the momentum of a particle, as well as by an analysis of the operational detection of the phase of an optical field. For the latter we identify the intrinsic phase operator and report its explicit form.

A method of measuring parity nonconservation in ${\mathrm{Ba}}^{+}$ to a very high degree of accuracy has recently been proposed [N. Fortson, Phys. Rev. Lett. 70, 2383 (1993)]. The result of such an experiment when combined with calculations of comparable accuracy can provide useful information about several extensions of the standard model. We use a relativistic configuration-interaction approach to determine the contributions to parity nonconservation in ${\mathrm{Ba}}^{+}$ arising from neutral weak currents and the nuclear anapole moment. Our work suggests that any reliable theory of parity nonconservation for that ion should incorporate the many-body effects arising from the low-lying configurations.

A simple configuration-interaction method to calculate accurate electronic energies of two-electron molecular ions at intermediate internuclear distances is presented. The correlated basis functions are constructed from exact ${\mathrm{H}}_2^{+}$ molecular orbitals, and include an explicit dependence on the interelectronic distance in order to describe the cusp conditions of the Coulombic field. An important simplification of the Hamiltonian matrix elements is deduced. The method is applied to determine new potential curves of highly excited states of ${\mathrm{He}}_2^{2+}$ molecular ions dissociating into two excited fragments. The ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{\mathit{g}}^{}{}_{}{}^{}$ state dissociating into ${\mathrm{He}}^{+}$(2p)+${\mathrm{He}}^{+}$(2p) presents a deep well located at R=5.0 a.u., where metastable states can survive. These states could be of interest in excimer lasers.

We report the results of calculations of the asymmetry parameter and of the cross section for the photodetachment of the metastable state ${\mathrm{Be}}^{\mathrm{-}}$ 1${\mathit{s}}^2$2s2${\mathit{p}}^{24}$P, for photoelectron energies above the Be 1${\mathit{s}}^2$2s2${\mathit{p}}^3$${\mathit{P}}^{\mathit{o}}$ threshold of 0.74–2.74 eV. The calculations were based on configuration-interaction-type wave functions, where the final scattering state is obtained via the solution to all orders of multichannel reaction (K) matrices. Very good agreement is observed with the recent measurement of Pegg et al. [Phys. Rev. A 50, 3861 (1994)].

We have calculated the coplanar triply differential cross section (TDCS) for double ionization of helium by one photon at total energies of 10, 20, and 52.9 eV above threshold. Our results agree very well with recently measured and calculated angular distributions [O. Schwarzkopf, B. Krässig, J. Elmiger, and V. Schmidt, Phys. Rev. Lett. 70, 3008 (1993); O. Schwarzkopf, B. Krässig, V. Schmidt, F. Maulbetsch, and J. S. Briggs, J. Phys. B 27, L347 (1994); F. Maulbetsch and J. S. Briggs, ibid. 26, L647 (1993)] for the cases of equal energy sharing (photoelectron energies ${\mathit{E}}_1$ and ${\mathit{E}}_2$, such that ${\mathit{E}}_1$=${\mathit{E}}_2$=5 eV and ${\mathit{E}}_1$=${\mathit{E}}_2$=10 eV) and unequal energy sharing (${\mathit{E}}_1$=5 eV, ${\mathit{E}}_2$=47.9 eV). In contrast to previous calculations of the TDCS, our calculation is based on a basis-set method that is, in principle, exact, and we obtain absolute values of the TDCS that are fairly well converged with respect to the basis size.

We have measured the elastic-scattering cross section of ${}_{}{}^{87}{}_{}{}^{}\mathrm{Rb}$ atoms in the ‖F=1,${\mathit{m}}_{\mathit{F}}$=-1〉 ground state at 25 μK. The cross section is almost purely s wave at these temperatures and has a value of (5.4±1.3)×${10}^{\mathrm{-}12}$ ${\mathrm{cm}}^2$. We have searched for the predicted Feshbach-type resonances in the elastic cross section [Tiesinga et al., Phys. Rev. A 46, 1167 (1992)] as a function of magnetic field. There are no resonances with a magnetic-field width ≥2 G over a magnetic-field range of 15–540 G.

It is shown that the contribution to the double-to-single ionization ratio ${\mathrm{\sigma }}^{2+}$(ω)/${\mathrm{\sigma }}^{+}$(ω) of helium caused by small momentum transfer breaks down the asymptotics of this ratio at the high-energy limit. The addition of up to 10% at ω=12 keV is expected to cause a slope in the ratio, which can be related to the properties of the ground state. The contribution is expected to be important also in the analysis of the double photoionization of ${\mathrm{H}}^{\mathrm{-}}$ and ${\mathrm{H}}_2$.

The ionization of Rydberg states by subpicosecond, unipolar electromagnetic-field pulses shows a strong orientation dependence. A small static electric field permits us to study Rydberg-Stark states with oriented permanent electric dipole moments. The probability for ionizing a Stark state depends on its energy and the orientation of its electric dipole moment with respect to the direction of the pulsed electric field. This orientation dependence is most prominent for pulse durations comparable to the classical orbit period. Classical simulations reproduce many of the observed features.

The success of recombination-pumped x-ray laser schemes is determined by the kinetics of ions in plasmas with relatively dense, cold-electron distributions. We examine how laser gain in such systems is affected by a multipeaked electron distribution generated by sequential ionization of atoms using high-intensity, ultrashort-pulse lasers. We also investigate the role of heating processes that modify electron energy distributions during the recombination and the accompanying collisional cascade. We find that conditions for the success of these schemes are critically modified by the inclusion of these effects.

We report the examination of the dynamics of dissociative tunnel ionization of ${\mathrm{H}}_2$ in a 10-μm laser field. The kinetic energy of the dissociated fragments along with ion yields were measured. The proposed model prediction agrees with the observed intensity dependencies, suggesting the dynamics of the process: the tunnel ionization of ${\mathrm{H}}_2$ is followed by the chaotic dissociation of ${\mathrm{H}}_2^{+}$ in the beyond-dissociation threshold field, and the process is completed by the associative tunnel ionization of the neutral fragments.

We report on a study to determine the efficiency of various buffer gases in quenching the population trapping states of laser-excited ytterbium ions. The ions were confined in an rf-dc hybrid trap, and their ${}_{}{}^2{}_{}{}^{}$${\mathit{S}}_{1/2}$${\mathrm{\leftrightarrows }}^2$${\mathit{P}}_{3/2}$ transition was excited with 329-nm laser light. Buffer gases used include ${\mathrm{H}}_2$, ${\mathrm{N}}_2$, ${\mathrm{CO}}_2$, Ar, and He at several different pressure regimes. We have identified ${\mathrm{N}}_2$ to be highly efficient in quenching the population of the trapping states and thus be the most suitable buffer gas to use with ytterbium for the development of a trapped-ion microwave frequency standard.

An electromagnetically induced transparency is observed at one arm of a three-level Λ-type system in a rubidium D1 line (794.8 nm) with an 85% reduction in absorption, when a pumping field is present at the other arm. This reduction in absorption for the weak probe field is due to the atomic coherence produced by the strong pumping field. This experiment is done in a Rb vapor cell at room temperature with cw diode lasers for both pumping and probe beams in a Doppler-free configuration. A simple theoretical treatment including Doppler broadening is in good qualitative agreement with the experimental measurement.

A theory of the recently discussed and demonstrated novel phenomenon of self-phase-matching [B. Glushko, B. Kryzhanovsky, and D. Sarkisyan, Phys. Rev. Lett. 71, 243 (1993)] is given by representing the conical beam, produced in the experiments, by a Bessel beam and as a superposition of Bessel beams. The predictions of the theory with the superposition of Bessel beams are in conformity with the observed behavior.

In a system of high-density excitons modeled as a system of anharmonic oscillators we demonstrate the (a) existence of the field-induced transparency, (b) changes in the dispersion characteristics, (c) the possibility of probe gain due to the presence of the pump laser, and (d) the possibility of gain enhancement in regions close to the bistability critical points.

The division of a scattering experiment into the preparation of a state and the registration of an observable in this state defines an obvious but commonly ignored quantum-mechanical arrow of time. This leads to irreversibility on the microphysical level. The time evolution semigroup for exponentially decaying Gamow states is an example of this microphysical irreversibility. A time-reversal operator exists which is compatible with the quantum-mechanical arrow of time.

The maximum-entropy principle is used to generalize the variational principle for the ground state (Rayleigh-Ritz minimization principle) to ensembles of unequally weighted states. A hierarchy of nested inequalities for the associated ‘‘free energies’’ is deduced. Applications of these inequalities to the estimation of the highest multiplet energy and a generalization of density-functional theory to ensembles of quantum states are discussed. Previous results given in the literature are obtained as special cases of our results under isothermal and adiabatic optimizations.

We describe an experimental configuration suitable for observing the geometric phase of Aharonov and Casher in atomic systems. Using this we have been able to show experimentally that the Aharonov-Casher phase is both independent of velocity and proportional to an electric field and we have verified the predicted size of the effect with an accuracy of 2%.

A particle moves in one spatial dimension in an attractive symmetric potential vf(x) and obeys nonrelativistic quantum mechanics. If ${\mathrm{F}}_{\mathrm{n}}$(v) is the trajectory function which describes how the nth energy eigenvalue depends on the coupling parameter, and ${\mathrm{G}}_{\mathrm{n}}$(u)=${\mathrm{uF}}_{\mathrm{n}}$(1/u), then an approximation f${\mathrm{̃}}_{\mathrm{n}}$(x) for the potential is given by the general inversion formula x=${\mathrm{\int }}_{{\mathrm{G}}_{\mathrm{n}}\mathrm{}\left(0\right)\mathrm{}}^{\mathrm{f}{\mathrm{̃}}_{\mathrm{n}}}$${\mathrm{P}}_{\mathrm{n}}$(z)dz/(f${\mathrm{̃}}_{\mathrm{n}}$-z${)}^{1/2}$, where ${\mathrm{P}}_{\mathrm{n}}$(z)=2n+1/${4\mathrm{G}}_{\mathrm{n}}^{\prime }$(${\mathrm{G}}_{\mathrm{n}}^{\mathrm{-}1}$(z))[${\mathrm{G}}_{\mathrm{n}}^{\mathrm{-}1}$(z)${]}^{1/2}$. Specific inversion formulas are derived for power-law potentials and also for the sech-squared potential. Every energy trajectory of the harmonic oscillator inverts to ${\mathrm{x}}^2$ exactly. In the other cases studied, ${\lim }_{\mathrm{n}\to \mathrm{\infty }}$f${\mathrm{̃}}_{\mathrm{n}}$(x)=f(x). These results obtained by the WKB approximation suggest that for attractive symmetric potentials, “geometric spectral inversion” ${\mathrm{F}}_{\mathrm{n}}$(v)→f(x) exists generally for every state n.

We compare the properties of four quantum phase distributions: the London distribution, the integrated Wigner function, the integrated Q function, and the quadrature-based phase distribution. Their utility in determining the accuracy of phase-shift measurements and their transformation properties under rotations and squeezing transformations are considered. We show that two of the distributions become the same for large-amplitude states that are sufficiently localized in phase space. We call these states quasiclassical phase states. Large-amplitude classical states fall into this class. Some restrictions on how peaked phase distributions of classical states can be are discussed. For quasiclassical phase states, the phase distribution shares many of the properties of classical phase distributions and is measurable. For other states, such as small-amplitude or some nonclassical states, there is no phase distribution with all the desired properties.

We consider the problem of a massive charged scalar particle bound in a given external Coulomb field of charge Z with varying strength parameter α=Zα’ (α’≊${137}^{\mathrm{-}1}$) in the Feshbach-Villars (FV) representation. It is shown how the expansion in α of the momentum space integral equations can be handled to reproduce the exactly known spectrum up to some given order in α. In particular, it is shown in detail how the O(${\mathrm{\alpha }}^5$) contributions to the ground-state energy that arise in the truncated FV equations from the kinetic and potential energies are canceled by virtual pair contributions. The findings are of relevance to quasipotential approaches to the relativistic few-body problem.