Rate coefficients for energy pooling ${\mathit{k}}_{\mathrm{EP}}$ of two Hg(6 ${}_{}{}^3{}_{}{}^{}\mathrm{P}_1^{}{}_{}{}^{}$) atoms to produce three observed states, Hg(9 ${}_{}{}^3{}_{}{}^{}\mathrm{S}_1^{}{}_{}{}^{}$), Hg(7 ${}_{}{}^3{}_{}{}^{}$${\mathit{D}}_3$), and Hg(7 ${}_{}{}^1{}_{}{}^{}\mathrm{D}_2^{}{}_{}{}^{}$), were measured directly following laser saturation of the resonance transition at 253.7 nm. That provided a reliable method for producing a known initial Hg(6 ${}_{}{}^3{}_{}{}^{}$${\mathit{P}}_1$) density without making a quantitative determination. The individual ${\mathit{k}}_{\mathrm{EP}}$ values summed to >4×${10}^{\mathrm{-}10}$ ${\mathrm{cm}}^3$ ${\mathrm{s}}^{\mathrm{-}1}$, a value that is essentially gas kinetic; their relative magnitudes showed a greater dependence on statistics than on the energy defect.

The uniform asymptotic description of the propagation of an input rectangle-modulated harmonic signal of fixed angular frequency ${\mathrm{\omega }}_{\mathit{c}}$ and initial pulse width T into the half-space z>0 that is occupied by a single-resonance Lorentz medium is presented. The asymptotic description is developed by representing the input rectangular pulse as the difference between two Heaviside-unit-step-function-modulated signals that are separated in time by the initial pulse width T. This representation clearly shows that the resultant pulse distortion in the dispersive medium is primarily due to the Sommerfeld and Brillouin precursor fields that are associated with the leading and trailing edges of the input pulse. The dynamical pulse evolution with increasing propagation distance z>0 is completely described for both long and very short initial pulse widths T. In both cases it is shown that the pulse distortion becomes severe when the propagation distance z is such that the precursor fields associated with the trailing edge of the pulse interfere with the precursor fields associated with the leading edge. Finally, the asymptotic theory clearly shows that the main body of the pulse propagates with the signal velocity in the dispersive medium.

Classical and quantum results for the strong-electromagnetic-field ionization of the ground state of a generic model are compared. Quantum results are also presented for the strong-field ionization of the hydrogen atom. These results demonstrate that ionization depends strongly on the phase of the field in such a way that the interaction potential acts as a barrier or well at large distances from the binding region, producing effectively a closed or open ‘‘gate’’ to the region of space outside the atom. The open gate is analogous to a strong, static electric field applied to an atom such that the atom ionizes classically. Quantum and classical ensemble results for the ionization probability are found to show close qualitative agreement. Other comparisons are made for classical versus quantum wave-packet trajectories.

We demonstrate how spontaneous-emission noise in lasers and masers can be reduced through memory effects of the lasing atoms. We show that for short measurement times the spontaneous-emission noise can be smaller than the usual Schawlow-Townes limit. We discuss a general model for atomic memory effects and derive expressions for the noise correlation function, phase diffusion, and the emitted spectrum. Furthermore, we allow for a frequency distribution over the lasing atoms and hence account for Doppler broadening or a selective excitation of the atoms.

Interferometers are the most promising sensors for large gravity-wave detectors. The coupling of the enclosure noise to the gravity-wave signal in a Fabry-Pérot interferometer is studied. The first part provides a theoretical framework, and in the second part two-limiting cases are discussed as an illustration. The relevance of this noise source for large gravity-wave interferometers is briefly discussed in the Conclusion.

We have studied trapping of resonance radiation under conditions of moderate to low line-center optical depths in sodium-argon mixtures. We report measured effective radiative decay rates, which are compared with predictions of the Post [Phys. Rev. A 33, 2003 (1986)] and the Milne [J. Lond. Math. Soc. 1, 40 (1926)] theories of radiation trapping. These theories are expected to be valid for spectral lines with low to moderate line-center optical depths, where the commonly used Holstein [Phys. Rev. 72, 1212 (1947); 83, 1159 (1951)] theory of radiation trapping is expected to break down. The experiment was performed under conditions in which the trapping was dominated by either the Doppler-broadened Gaussian or by the impact-broadened Lorentzian line-shape function. The measured effective radiative decay rates agree well with those predicted by the Milne and Post theories over the range of low to moderate optical depth.

A method is proposed to generate improved coherence in x-ray beams. The method leaves the photon source unchanged, and improves transverse coherence by reordering the emitted radiation through dynamical optical means (i.e., with at least one moving optical component). Since the method increases the transverse coherence of beams, it thus reduces the time needed to perform certain types of observations, such as in microscopy, and interference experiments such as holography. It does not increase the longitudinal coherence of beams. However, it can be supplemented by other means to increase longitudinal coherence. It does not increase the brightness, but allows one to more efficiently utilize beams of a given brightness. The use of this method with photons generated by pulsed electron beams, including those in storage rings, seems particularly appropriate.

Broadband intensity fluctuations from a heavily saturated source of amplified spontaneous emission (ASE) operating on the 3.51-μm transition of xenon show no evidence of a dynamical origin represented by a low-dimensional underlying chaotic attractor. The broadband coupled-mode fluctuations in ASE thus seem to be stochastic when contrasted with the recently reported deterministic nature of similar broadband fluctuations of single-mode lasers operating on the same transition.

We present the exact solution of the time-dependent Schrödinger equation describing an electron bound in a zero-range potential in three-dimensional space, subject to circularly polarized light. This solution is used to study the survival probability of the initial bound state and the above-threshold photodetachment spectra as well as the total ionization rate of the model negative ion. A monotonic intensity dependence of the total rate in circularly polarized light is predicted.

We calculate the intensity fluctuations of the light transmitted by a Fabry-Pérot interferometer when a broadband laser is incident upon it. The laser field is assumed to have a Lorentzian spectrum and to be broadened by random amplitude fluctuations (real or complex) or random frequency fluctuations. In each case we obtain closed-form expressions for the intensity autocorrelation function, from which we can obtain the magnitude of the intensity fluctuations, as well as the power spectrum. These results indicate a strong dependence of the transmitted intensity fluctuations on the higher-order statistical properties of the incident field. Agreement with similar results concerning atomic fluorescence fluctuations is very good.

A theoretical analysis of the optical coherent transients that arise when a sample of two-level atoms is irradiated by a sequence of three broad-bandwidth pulses is presented. The first two pulses have a relative delay time of order of the correlation time of the pulse fluctuations and are sent into an atomic vapor from different directions. These pulses, whose temporal width is much greater than the delay time, can be correlated with one another and can be strong enough to saturate the two-level atomic transition. The third pulse is weak, noncorrelated with the first two, and is delayed in time so that it does not overlap them. We present a detailed examination of the transient signal that is produced when the third pulse is scattered by the spatial gratings in the population difference of atoms created by the first two pulses. Taking into account the effects of inhomogeneous and homogeneous broadening, we calculate the intensity of the transient signal, emitted in different directions, as a function of delay time. The signal is found to depend dramatically on the intensities of the excitation pulses. It is shown that, for strong excitation pulses, there is a direct dependence of the signal on the cross-correlation time of pulses ${\mathrm{\tau }}_{\mathit{c}}^{12}$ that does not exist when the pulses are weak. In particular, the strongest signals exhibit a peak of width of order ${\mathrm{\tau }}_{\mathit{c}}^{12}$. This peak can have a very narrow dip near its maximum whose width is much smaller than ${\mathrm{\tau }}_{\mathit{c}}^{12}$, if the pulses are fully correlated. We develop a representation of the time evolution of the Bloch vector of a two-level atom, driven by time-delayed pulses, that enables us to explain our results. In this representation, the two time-delayed pulses are replaced by two fully overlapping pulses having some effective amplitudes and atomic field detunings.

Two-photon-resonant three-photon ionization of atomic phosphorus is studied in the 298–306-nm wavelength range. P atoms are produced by ${\mathrm{Ar}}^{+}$ ion sputtering of an InP solid sample and the photoions are detected by a quadrupole mass spectrometer. The laser power dependence of the 3${\mathit{p}}^4$${\mathit{S}}_{3/2}^0$–4 ${}_{}{}^4{}_{}{}^{}\mathrm{S}_{3/2}^0{}_{}{}^{}$ two-photon excitation and one-photon ionization of 4${\mathit{p}}^4$${\mathit{S}}_{3/2}^0$ phosphorus is determined in the 10–200 MW/${\mathrm{cm}}^2$ range. The cross section for the ionization step is estimated in the frame of the quantum-defect theory: σ∼3×${10}^{\mathrm{-}18}$ ${\mathrm{cm}}^2$. The two-photon Rabi frequency ${\mathrm{\Omega }}_{\mathit{R}}$ is deduced from the comparison of our experimental results with the extended two-level model developed by Eberly [Phys. Rev. Lett. 42, 1049 (1979)] to describe (m+n) resonant multiphoton ionization processes and with the rate equation approximation analysis. The best fit gives ${\mathrm{\Omega }}_{\mathit{R}}$(${\mathrm{s}}^{\mathrm{-}1}$)=8${\mathit{I}}_{\mathit{L}}$(W/${\mathrm{cm}}^2$). This value compares relatively well with the theoretical estimate ${\mathrm{\Omega }}_{\mathit{R}}$=5.5${\mathit{I}}_{\mathit{L}}$ obtained by limiting the perturbation-theory summation to the dominant intermediate states. The combination of well-characterized ${\mathrm{Ar}}^{+}$ ion sputtering of a solid sample with resonant photoionization is used to perform actual trace analysis of materials. For example, 0.5 ppm of phosphorus in a silicium sample was measured by this method with a lateral resolution of ∼300 μm.

It is shown that the basic symmetry of two-mode squeezed states is governed by the group Sp(4) in the Wigner phase space which is locally isomorphic to the (3+2)-dimensional Lorentz group. This symmetry, in the Schrödinger picture, appears as Dirac’s two-oscillator representation of O(3,2). It is shown that the SU(2) and SU(1,1) interferometers exhibit the symmetry of this higher-dimensional Lorentz group. The mathematics of two-mode squeezed states is shown to be applicable to other branches of physics including thermally excited states in statistical mechanics and relativistic extended hadrons in the quark model.

We report results of an experimental study of the evolution of small ensembles of atoms in a microwave cavity. The experiment employs an atomic beam of calcium Rydberg atoms and a split superconducting cavity operating at 35 GHz. At the ambient temperature of 2 K, the mean blackbody photon number is 0.8. Selective field ionization allows us to monitor simultaneously the populations of the initial and final states. The time evolution of the atomic system is probed by ‘‘Stark switching,’’ i.e., shifting the atomic resonance away from the cavity resonance by applying an electric field. The collective oscillations of energy between ensembles of atoms and a cavity with Q>${10}^7$ are studied for one to several hundred atoms. The results agree well with theory for ∼50 or more atoms, but some discrepancies are observed for smaller numbers of atoms. Possible experimental sources of these discrepancies are discussed.

The sub-Poissonian statistics and squeezing in the fluorescence field from N two-level atoms in a cavity are considered for the case for which the cavity mode is so intense that it can be treated as a classical field. It is shown that sub-Poissonian statistics of sidebands are present for a large number of atoms. Moreover, the sidebands become more sub-Poissonian with increasing number of atoms. The condition for obtaining a large degree of squeezing in the sidebands is discussed.

It is shown that a four-wave-mixing oscillator [R. Ghosh and G. S. Agarwal, Phys. Rev. A 39, 1582 (1989)] operating above threshold exhibits nonclassical effects in the average phase and the amplitude difference of two radiation fields and shows two-mode squeezing. A maximum of 50% intracavity squeezing is found in the fluctuations of two Hermitian operators corresponding to the average phase and the relative amplitude, which becomes 75% squeezing outside the cavity. The variance in the intensity difference between the two fields is half the shot-noise limit inside the cavity but is perfectly suppressed outside the cavity, and both variances inside and outside the cavity are independent of how far above threshold the oscillator is.

We report wavelength spectra of multiphoton ionization of rubidium, in which the intermediate state is a Rydberg state (35<n<45). The photoelectron yield is measured for a free atom, for an atom placed in an external electric field, and for an atom in a magnetic field. Both the magnetic- and the electric-field spectra show a dramatic decrease in yield above n≊40. Our analysis centers on the field-induced mixing of states. The threshold occurs when the resonant state, which has a large quantum defect, mixes with the manifold. The analysis is also applied to experiments reported previously.

Theoretical calculations of the two-photon ionization cross section of argon and of the two-photon detachment cross section of the negative fluorine ion are presented for the energy region from threshold to the single-photon ionization or detachment threshold. Detailed analyses are presented of the effects of various kinds of electron correlations, whose contributions to the two-photon transition amplitudes are evaluated using variationally stable procedures to sum implicitly over intermediate states. These variationally stable procedures enable us to give much more reliable predictions in the region of the first few intermediate-state resonances in argon than were provided by previous calculations of one of us [A.F. Starace and T. F. Jiang, Phys. Rev. A 36, 1705 (1987)]. In addition, we show here for both argon and ${\mathrm{F}}^{\mathrm{-}}$ the large corrections to the p${(}^1$S) final-state channel cross sections provided by certain intermediate-state shake-up interactions, which we show to be essential for obtaining good agreement of dipole length and velocity cross sections. We provide detailed comparisons of our results with those of previous workers; in particular, our results for the negative fluorine ion are in excellent agreement with the recent absolute measurement of Kwon et al. [Phys. Rev. A 40, 676 (1989)].

Using a variational method, we have developed an approximate model of the dynamics of soliton coupling described by coupled nonlinear Schrödinger equations. The results are analytical and can be recast into a suggestive particlelike description providing a physical insight into the problem. The particular case of the nonlinear coherent coupler is emphasized, and an estimate of the soliton switching peak power is obtained. A symmetry-breaking instability is also described, in a simple way, with the potential-well picture.

We examine the excitation of continuum-state resonances close to the threshold, paying particular attention to the breakdown of the Markov approximation and the consequences of this for initial-state time dependences and final-state spectra. Several theoretical dressed-state alternative descriptions are analyzed and compared: the nonperturbative coupling of a discrete state to a structured continuum with a threshold, the complete Fano diagonalization of the interacting system, and finally a partial diagonalization of the bound-state couplings to reveal the origin of dressed-state stabilization and partial decay suppression observed in this model.