A parameter-free spherical optical potential model is used to calculate the elastic differential, integral, and momentum transfer cross sections for the ${\mathit{e}}^{\mathrm{-}\mathrm{-}}$${\mathrm{H}}_2$S system in the energy range 50–1000 eV. The optical potential is constructed from a near-Hartree-Fock one-center expansion of a hydrogen sulfide (${\mathrm{H}}_2$S) wave function. The optical potential is then treated in a partial-wave analysis to extract various scattering cross sections.

The density-squared component of the depolarized-light-scattering spectrum of hydrogen at 50 K has been measured. This contribution is compared with the results of a theoretical model that considers both the wings of the rotational lines and the interaction-induced spectral components. The overall agreement is good, but some inconsistencies remain in the region between the rotational lines. Probable reasons for such a discrepancy are discussed.

A theory for impulsive resonant pump-probe and photon-echo spectroscopies of polyatomic dyes in solution is developed. A multimode calculation in which the intramolecular and solvation modes are modeled as Brownian oscillators is presented. Electronic dephasing, the time-dependent Stokes shift, and quantum beats are analyzed.

Due to a fundamental difference between the interference of particles and of light beams, it is proved that the phase observed in the latter will be the Hannay angle rather than the Berry phase. For systems characterized by quadratic Hamiltonians, the observed phase is shown to depend only on the group of transformations and not on the state of the field. The results are illustrated by several examples.

It is shown that the conjecture proposed in a recent communication [Chhajlany, Mal’nev, and Kumar, Phys. Rev. A 39, 5082 (1989)] regarding energy eigenvalues of the Coulomb-diamagnetic problem in two dimensions is not correct because it leads to non-normalizable solutions. Their results (i) disagree with an exact solution of the problem that exists for a specific relation between the associated coupling constants, (ii) disagree with the eigenvalues calculated by the method of Hill determinants for arbitrary values of these couplings, and (iii) violate strict variational upper bounds. We are able to verify numerically that the energy levels of the system calculated by the method of Hill determinants do indeed show a 3ħ${\mathrm{\omega }}_{\mathit{c}}$/2 spacing near the zero-field ionization threshold.

We reply to the preceding Comment by Pandey and Varma [Phys. Rev. A 42, 6928 (1990)].

Two recent calculations by Sen, Rai Dastidar, and Rai Dastidar [Phys. Rev. A 38, 841 (1988); 40, 735 (1989)] discuss intensity-dependent variations of photoelectron angular distributions resulting from (2+1)-multiphoton ionization of ${\mathrm{H}}_2$ via the E, ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{\mathrm{g}}^{+}{}_{}{}^{}$ state and from (3+1)-multiphoton ionization of ${\mathrm{H}}_2$ via the B’ ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{\mathit{u}}^{+}{}_{}{}^{}$ state. In this Comment, we show that most of the intensity-dependent effects presented are a result of an erroneous and unphysical generalization of the angle-independent multiphoton ionization rate to include the angular dependence.

Negative screening parameters for spin doublets have been reported for some atoms by using binding energies from Dirac-Fock calculations. The Dirac-Fock energies for spin doublets fail to converge to the same nonrelativistic limit because of unbalanced treatments of electron-electron correlations and give rise to spurious doublet splittings. We subtract from the Dirac-Fock doublet splittings the corresponding values in the nonrelativistic limit. The corrected doublet splittings form a smooth function of the atomic number and result in positive screening parameters. Detailed analyses are worked out for 3p spin doublets as an illustration.

We have investigated the direct reaction of ${\mathrm{Na}}^{\mathrm{*}}$(4 ${}_{}{}^2{}_{}{}^{}$P) with ${\mathrm{H}}_2$ to form the product NaH. Using far-wing absorption techniques, we have measured absorption into the ${\mathrm{NaH}}_2$ collision complex, followed by branching into nonreactive (formation of ${\mathrm{Na}}^{\mathrm{*}}$) or reactive (formation of NaH[X ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{}^{+}{}_{}{}^{}$(v’’,J’’)]) channels. We have observed the reaction to occur both via the attractive potential-energy surfaces and over a barrier on the repulsive surfaces. We have studied the effect of reactant orbital alignment on product rotational distribution for v’’=1. Specifically we find reaction on the repulsive surfaces leads preferentially to low rotational product states of NaH, while reaction on the attractive surfaces leads preferentially to high rotational states.

Higher multipole corrections to the dipole-approximation result for angular distributions of photoelectrons at low photon energies are discussed. The first-order corrections lead to a simplified formula for the angular distribution, which extends the dipole results for linearly, unpolarized, and partially polarized light. It is shown how atomic parameters can be obtained from experimental measurements, and relationships to previous work on photoelectron angular distributions are discussed briefly.

An energy-dependent vibrational frame transformation for electron-molecule scattering is reformulated to account for a class of nonadiabatic effects resulting when the electron time delay in the reaction zone is appreciable. The electron-molecule compound system is described by a special class of Born-Oppenheimer eigenstates, singled out by requiring their nuclear vibrational wave functions to coincide with those of the molecular target. Combined with the multichannel quantum defect theory, this treatment describes resonant and nonresonant electron-molecule scattering near threshold equally well, as well as molecular Rydberg spectra.

We measure several transitions between specific Rydberg levels in the nearly degenerate (ν=0, R=1) n=28 and (ν=0, R=3) n=16 Rydberg manifolds of ${\mathrm{H}}_2$ with microwave spectroscopy. These measurements can be combined with calculations of the Rydberg fine structure to deduce the energy difference between the two states of the free ${\mathrm{H}}_2$ core. The result, E(ν=0, R=3)-E(ν=0, R=1)=288.859 00(8) ${\mathrm{cm}}^{\mathrm{-}1}$, represents the most precise determination to date of any spectral property of the hydrogen molecular ion, to our knowledge.

We report the observation of electronically excited states of ${\mathrm{Li}}_4$, using depletion spectroscopy. The obtained spectra reveal five electronic bands between 380 and 760 nm. Energetic locations and absorption cross sections (ranging from 0.3 to 4 Å ${}_{}{}^2{}_{}{}^{}$) fully agree with previous molecular configuration-interaction ab initio calculations. Vibronic structures are also observed; they look promising for development of accessibility to adiabatic surfaces and equilibrium geometries in metallic tetramers.

The generalized multiphoton detachment cross sections for n=1–7 photons are evaluated for ${\mathrm{H}}^{\mathrm{-}}$ in a zero-range plane-wave approximation. Results are compared with the recent experiments of Tang et al. [Phys. Rev. A 39, 6068 (1989)] and Smith et al. [J. Opt. Soc. Am. (to be published)] on these processes using relativistic ${\mathrm{H}}^{\mathrm{-}}$ beams interacting with Doppler-shifted ${\mathrm{CO}}_2$ laser pulses. Many features of the data are qualitatively consistent with the present calculation, but significant differences in shape and magnitude still remain.

We report an observation of soft-x-ray lasing in Ti, at 326.5 Å, based on recent experiments conducted on the Glass Development Laser (1.054 μm) at the Laboratory for Laser Energetics and the Nova laser (0.53 μm) at Lawrence Livermore National Laboratory. This Ti laser, which has been identified as a 3p→3s transition in Ne-like Ti, is unusual in several ways; the J=0→1, 3p→3s transition is observed to be the dominant laser line in a Ne-like spectrum, the laser output has a very narrow divergence, and lasing is observed only in experiments where the main optical pulse is preceded by a low-intensity prepulse. We describe the experimental results and present evidence that suggests the gain is enhanced by resonant photopumping of the Ne-like Ti 2p→4d transitions by 3s→2p C-like and 3d→2p N-like Ti lines, which results in lasing on the 3p→3s transitions in Ne-like Ti.

We describe in this paper the dynamics of a bimode ${\mathrm{CO}}_2$ laser with an intracavity saturable absorber; more precisely the competition between the two modes when their single-mode regime is unstable and as simple as possible, i.e., with a single spike. The transition between the two modes occurs through a succession of periodic regimes separated by nonperiodic ones. The former are governed by the Farey arithmetic and lie on a staircase. Spectrum analyses and return maps suggest the existence of chaos in the nonperiodic states.

To describe multiphoton ionization in a strong field of low frequency ω, we develop an asymptotic expansion in powers of ${\mathrm{\omega }}^2$ of the ac quasienergy; the leading term is the (cycle-averaged) dc quasienergy. We conjecture that the ac width and ac shift (calculated within this framework) satisfy a dispersion relation, similar to that satisifed by the dc width and dc shift. We have calculated the first few terms of the expansion for the ground-state hydrogen atom in a circularly polarized field, and we find good agreement with the exact values for the shift and width down to wavelengths of about 300 nm, at intensities of order ${10}^{14}$ W/${\mathrm{cm}}^2$ or more.

The nature of the critical behavior in the triplet annihilation model—which exhibits a new kind of nonequilibrium phase diagram—is elucidated through scaling analysis of time-dependent Monte Carlo simulations. The results support the hypothesis that nonequilibrium critical points in single-component reaction diffusion systems are generically in the directed-percolation or Reggeon-field-theory class.

The diffusion Monte Carlo algorithm with and without importance sampling is analyzed in terms of the algorithm’s underlying transfer matrix. The crucial role played by the Langevin algorithm in the importance-sampling process is made explicit and emphasized. The failure of existing second-order algorithms to converge quadratically for atomic many-body problems is shown to be caused by nonperturbative convergence errors due to the intrinsic inability of the Langevin algorithm to sample Slater orbitals. This failure can be simply circumvented by enforcing attractive cusp conditions on the trial function. Various new second-order diffusion Monte Carlo algorithms are systematically derived and their quadratic convergence numerically verified in cases of He and ${\mathrm{H}}_2$.

Building upon previous work, several new thermodynamic properties are found for classical electromagnetic random radiation in thermal equilibrium with classical electric dipole harmonic oscillators. Entropy is calculated as a function of temperature and as a function of the positions of the dipole oscillators. In the process, a new derivation is obtained for what is often called Wien’s displacement law. The original derivation of this law makes a number of implicit assumptions not found in the present derivation, which prevents the original analysis from being sufficiently general to address an important class of thermal radiation spectrum candidates: namely, those that are nonzero at T=0. While leading up to the entropy calculation, a number of other thermodynamic properties are deduced. For example, a natural development is presented for reformulating the Stéfan-Boltzmann law to correspond to experimental observations about changes in thermal radiation energy. Also, the Rayleigh-Jeans spectrum is shown to conflict with basic concepts of thermodynamic processes, and asymptotic limits are found for the spectrum of classical electromagnetic thermal radiation. One asymptotic restriction arises from the demand of finite specific heat for thermal radiation. This restriction is sufficient to ensure that the classical electrodynamic system of dipole oscillators and thermal radiation must obey the third law of thermodynamics. The calculations described here include full nonperturbative evaluations of retarded van der Waals thermodynamic functions.