We report the observation of collision-induced narrowing in the two-photon $6S-12S$ transition in Cs perturbed by argon using resonance ionization spectroscopy. The narrowing occurs at 25-50-Torr Ar buffer gas pressure. It is explained via the competition between collision-induced single- and two-photon processes.

In ordinary quantum-beat spectroscopy the excitation mechanism (e.g., laser pulse) is not expected to affect the results because the measurements are made on a freely evolving atom in the dark (after pulse ends). In this paper we point out an example where this is not so. As an example, we show how light shifts can alter the results of time-delayed measurements.

We present concise parametrized expressions for the angular distribution and polarization of the coherent fluorescence radiation following photoexcitation and photoionization including all multipole transitions. Many interesting cases are discussed, and their implications are pointed out.

Instabilities associated with bistable branches of positive differential gain are investigated numerically in a ring-cavity bistable device, allowing for transverse variation of the laser beam. Additional instability domains beyond those predicted by plane-wave analysis are shown to occur. The nature of the bifurcation sequences within each domain is different from the plane-wave case and is characterized by quasiperiodic and frequency-locked behavior. Some sequences are very similar to experimentally observed bifurcations in hydrodynamics.

Evidence from new "universal" four-dimensional computer simulations suggets that swept-gain superfluorescence is subject to an unexpected scaling relation as a function of Fresnel number.

The effective growth rate of critical fluctuations in the unstable region of the system polydimethylsiloxane-diethyl carbonate was found to be time independent for an appreciable time and the negative composition diffusivity $D^{*}$ to be represented by the power law of $D^{*}\propto {\epsilon }^{{\nu }^{*}}$ with ${\nu }^{*}\approx 0.62$, where $\epsilon =\left|\frac{(T-T_c)}{T_c}\right|$. The value of $D^{*}$ was approximately three times larger than the positive composition diffusivity $D$ at the same $\epsilon$ in the stable region.

An algorithm proposed by Takens, which can determine the capacity (generalized dimensionality) of a dynamical system from the time series of a single observable, is tested numerically for several intrinsically stochastic models. The algorithm is found to converge too slowly (if at all) to be useful for the analysis of experimental data.

Results on the relaxation dynamics of an initially localized distribution in the Chirikov and Rannou maps are described. These distributions are shown to approach a coarse-grained uniform distribution at a rate in accord with the Lyapunov exponent until relaxation is almost complete. The relationship to the generalized $H$ theorem is emphasized.

Equations for the first- and second-derivative coupling matrix elements between the adiabatic electronic wave functions of a canonical Van Vleck approach are derived for the case of a single nuclear coordinate. Expressions for their order-by-order evaluation, analogous to the multidimensional perturbative expansion of the effective Hamiltonian (given elsewhere) are presented. Diagrammatic notations useful for enumerating certain components of the derivatives are introduced, and the diagrams required through first order are shown.

An inhomogeneous integral equation has been derived for the bound-state wave functions. Iterative solutions to this equation provide wave functions for bound states of weakly coupled Hamiltonians in one and two dimensions.

The theory of interaction of atoms and molecules with radiation confined in a cavity is presented. The effect of the confinement on the field commutation relations is examined and the modified commutation relations are derived for an arbitrary cavity. It is shown that a canonical transformation on the minimal-coupling Hamiltonian for such systems leads to a new Hamiltonian containing explicitly neither the interatomic nor the atom image potentials. The new multipolar Hamiltonian is used to calculate energy shifts of an atom and a pair of atoms near a conducting wall. Changes in the rate of spontaneous emission from an excited atom are also evaluated.

We report a laboratory measurement of absolute emission cross sections of both the Lyman bands ($B{}_{}{}^1{}_{}{}^{}\Sigma _u^{+}{}_{}{}^{}\to X{}_{}{}^1{}_{}{}^{}\Sigma _g^{+}{}_{}{}^{}$) and Werner bands ($C{}_{}{}^1{}_{}{}^{}\Pi _u^{}{}_{}{}^{}\to X{}_{}{}^1{}_{}{}^{}\Pi _g^{+}{}_{}{}^{}$) of ${\mathrm{H}}_2$ by electron impact over the energy range from threshold to 400 eV with the same optical system. We find the emission cross section for the $B{}_{}{}^1{}_{}{}^{}\Sigma _u^{+}{}_{}{}^{}\to X{}_{}{}^1{}_{}{}^{}\Sigma _g^{+}{}_{}{}^{}$ transition at 100 eV to be (3.55±0.8) × ${10}^{-17\mathrm{}}$ ${\mathrm{cm}}^2$ (2.7 × ${10}^{-17\mathrm{}}$ ${\mathrm{cm}}^2$, direct excitation, 0.85 × ${10}^{-17\mathrm{}}$ ${\mathrm{cm}}^2$, cascading) and the emission cross section for the $C{}_{}{}^1{}_{}{}^{}\Pi _u^{}{}_{}{}^{}\to X{}_{}{}^1{}_{}{}^{}\Sigma _g^{+}{}_{}{}^{}$ transition at 100 eV to be (3.1±0.6) × ${10}^{-17\mathrm{}}$ ${\mathrm{cm}}^2$ (cascading is estimated to be not present). The cross-section ratio $\frac{Q_C}{Q_B}$ for direct excitation is 1.21±0.30 at 300 eV in excellent agreement with published values for this ratio from theoretical calculations and experimental data of the optical oscillator strengths. We measure the cross section for cascading to the $B$ state to be 24±10% of the total emission cross section both at 100 and 300 eV. We show that cascading increases to 51±20% of the total cross section of the $B$ state at 20 eV. The vibrational population distribution of the $B$ state is found to be a function of electron-impact energy as the importance of cascading relative to direct excitation changes with electron-impact energy.

Recent developments in the theory of the energy loss of charged particles in the gases ${\mathrm{H}}_2$, He, ${\mathrm{N}}_2$, ${\mathrm{O}}_2$, and C${\mathrm{H}}_4$, permit a calculation of the stopping power with only one free parameter—that needed in the description of the cubic correction term for the projectile charge. Experimental data for the stopping power of these gases for protons and alpha particles have been compared with calculated values. Once a value of the free parameter is assumed, it appears that no further improvements in the theory are required to explain the data for projectile energies above 0.5 MeV/u. The error of the theory is probably about ±2% for these energies.

Ratios of ionization cross sections for proton impact to those for ${}_{}{}^3{}_{}{}^{}\mathrm{H}\mathrm{e}-\mathrm{i}\mathrm{o}\mathrm{n}$ impact have been measured on $\mathrm{Al}K$ and $\mathrm{Y}L$ shells at projectile energies of 7, 9, 12, 16, and 21 MeV /amu. The ratio tends to unity with an increase in the projectile energy for both the $\mathrm{Al}K$ and $\mathrm{Y}L$ ionizations. This behavior can well be explained in terms of the Binstock-Reading theory combined with the distant-collision theory of Hill and Merzbacher: The polarization effect in distant collisions and the decreasing electron-density effect in close collisions cancel out each other in this high-energy region.

An investigation of electron — hydrogen-molecule scattering has been performed using the method of model potentials. A modified form of the Hara free-electron-gas exchange potential has been employed to take account of exchange. Two different adiabatic polarization potentials have been used to make the comparative study. Our model exchange potential is found to be suitable in predicting the cross sections. The results for the differential and total cross sections have been given in the energy range 0.3 to 10 eV. Comparisons are made with the available experimental and theoretical findings.

Energy losses of 7-MeV protons in metallic and organic thin foils have been obtained as a function of emergence angles. Angular distributions due to multiple scattering have been also measured. The energy loss is found to increase for all targets with increasing emergence angles. The increase of the energy loss cannot be explained by the following three effects: (1) the increase of the target thickness caused by the deflection of the particle, (2) the increase of the path length due to multiple scattering with atomic nuclei, and (3) the energy transfer to the recoil nuclei during the multiple-scattering process. From this consideration, the observed increase of the energy loss with increasing emergence angle is concluded to be due to a hitherto unknown effect. This effect is very likely the dependence of the energy loss on the average impact parameter with the atomic nucleus.

The complex potential-energy function for nuclear motion in the Born-Oppenheimer approximation for the lowest ${}_{}{}^2{}_{}{}^{}\Sigma _u^{+}{}_{}{}^{}$ resonance state of ${\mathrm{H}}_2^{-}$ has been calculated using the complex self-consistent-field (CSCF) method which treats the incident and target electrons equivalently. There is substantial disagreement among various determinations of the complex potential function for the broad ${}_{}{}^2{}_{}{}^{}\Sigma _u^{+}{}_{}{}^{}$ resonance. The CSCF results agree best with the potential used by Bardsley and Wadehra [Phys. Rev. Lett. 41, 1795 (1978)] to compute dissociative attachment cross sections. The calculated width of the resonance as a function of internuclear distance is in excellent agreement with the form used by Bardsley and Wadehra while the real part of the CSCF potential agrees less well with their function. It is suggested that correlation effects are important in determining the position of the resonance and in determining the width at least near the internuclear distance at which the resonance becomes a bound state.

We report here the direct observation of a two-step, ionizing "optical collision" between crossed lithium beams in which very-long-range coupling, due to van der Waals' resonance force between identical particles, is followed by photoionization:

${\mathrm{Li}}^{*}\mathrm{}\left(2p\right)+\mathrm{Li}\mathrm{}\left(2s\right)+\hslash \omega \to {\mathrm{Li}}^{*}\mathrm{}\left(3d\right)+\mathrm{Li}\mathrm{}\left(2s\right),$

${\mathrm{Li}}^{*}\mathrm{}\left(3d\right)+\hslash \omega \to {\mathrm{Li}}^{+}+e.\mathrm{}$

Cross-section magnitude and form of the intensity profile are described by a simple three-level model. These results suggest that polarization of the laser field together with collisional two-photon ionization may be used to delineate the shape of individual molecular-state potential curves at long range.

We have measured the optical-polarization parameters of the $2s{}_{}{}^3{}_{}{}^{}S-3p{}_{}{}^3{}_{}{}^{}P\mathrm{He}\mathrm{I}$ transition following tilted-foil excitation of fast-helium beams of 50- to 900-keV energy. From these measurements we have obtained the alignment and orientation parameters for the $3p{}_{}{}^3{}_{}{}^{}P$ state over foil-tilt angles of 0° — 70° for this beam-energy range. We compare our results with those for the $3p{}_{}{}^1{}_{}{}^{}P$ state and note that one alignment parameter is independent of foil-tilt angle.