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The response of a SERF atomic magnetometer to a repetitive short-pulsed pump was investigated. Quantum sub-resonances at a repetition rate of $1/n$ of the Larmor frequency of the magnetic field inside the shield are experimentally observed and theoretically explained. This is a type of synchronization phenomenon. Investigations in single alkali atoms cells as well as mixed alkali atoms of K and Rb are presented. In the later, one species is pumped while the probe is on the other specie polarized by spin exchange. The effect of spin destruction, spin exchange and collisions are studied in order to account for the width of the resonances. Quantum calculations of a three levels $\Lambda$ model for this phenomenon exhibit a dip at the resonance frequency in the absorption spectrum for both cases of pulsed and CW pump modes and an evidence for EIT., Comment: 14 pages, 11 figures

We show that it is possible to get gain without inversion at high frequency in a three-level ladder type scheme driven by a single driving laser. The transition from the ground state to first excited state is driven by a low-frequency resonant laser field. The next excited state lies far above the first one; therefore its resonant frequency can be much higher than that of the lower transition, and hence one should go beyond the most commonly used rotating wave approximation consisting of retaining only resonant terms (slowly oscillating) in the Hamiltonian. We solve the master equation, both analytically (under proper approximations) and numerically, with the full Hamiltonian keeping counter-rotating highly non-resonant terms. We show a dressed state picture explaining spectral features of gain-absorption profiles. Also we present a kind of phase diagram in the plane γ–Λ, where γ and Λ are spontaneous emission rate and incoherent pumping rate, respectively. The whole γ–Λ plane is divided into four areas: gain-inversion, absorption-no inversion, gain without inversion and absorption with inversion. The first two are usual regimes of incoherently pumped lasers, while the last two stem from the quantum interference phenomena. We analyse the parameter areas where such quantum regimes are possible. The exact numerical solution of the time-dependent master equation shows that keeping counter-rotating terms (no rotating wave approximation) gives rise to new spectral features in the form of additional peaks at combination frequencies. Actually the considered system works as a coherent frequency up-converter.

In a recent publication we have studied counterintuitive pulse sequence applied to a three level ladder scheme, utilizing rectangular-shaped pulses. In this research we investigate amplification without population inversion in a three level ladder system interacting with two electromagnetic fields, namely, the probe and coupling fields. We take both probe and drive fields in the form of Gaussian shaped pulses. In a counterintuitive sequence scheme, the short probe pulse is introduced prior to the application of the coupling field, in contrast to a regular sequence scheme, where both fields are introduced at the same time. The influence of varying the probe pulse width and time delay between the initiation times of the probe and coupling fields on transient probe gain is investigated. It is found that the system exhibits a kind of memory about previously applied weak probe pulse. This may allow for detection of the past event by applying a strong drive pulse in the future.

A three level scheme interacting with three laser fields is considered. It is found that two competing processes contribute to the absorption/dispersion on a particular transition: (i) one-photon processes governed by the field applied to that transition, and (ii) two-photon processes governed by the two laser fields applied to two other transitions. Interference between the two processes can be both constructive and destructive. It is shown that the net absorption/gain and dispersion can be controlled by the relative phases of the laser fields.

In this research we study lasing without population inversion from a three-level atom interacting with two laser fields, in the ladder or cascade scheme. We investigate counterintuitive sequencing as well as regular sequencing of the time of laser fields application. In a counterintuitive sequence scheme a short probe pulse is introduced prior to the application of the coupling field, in contrast, to a standard sequence scheme, where both fields are introduced at the same time. The influence of varying the probe pulse width and time delay between the initiation of probe and coupling fields on transient probe gain is investigated. The calculations indicate the potential of producing gain without inversion via counterintuitive sequence scheme.

Ultrashort pulse counterintuitive sequencing of three level lambda scheme shows five fold enhancement of the amplification without inversion on the probe transition. Introducing a short pulse weak probe field prior to the strong pump field results in an enhanced dispersion and stronger amplification on the probe field. The presence of an incoherent pump populating the upper level is crucial to the amplification. Parametric study of this effect is briefly presented.

A dressed-state study of lasing without population inversion from a three level atom interacting with a bi-chromatic laser field, in the ladder configuration, is formulated. We allow the atomic system to be dressed by both laser filed photons (double dressing). The evolution of the system under consideration is being explored both analytically and numerically, within the transient regime. Time dependent approximate analytic solutions for dressed-state populations and coherences are derived, within the so called “secular approximation,” under resonant conditions. We also present time dependent numerical solutions for population and coherences in the off-resonance regime. A spectral analysis is also performed revealing the structure of various dressed states transitions. These are shown to be composed of quintets centered about the frequencies of the coupling and probe laser fields and having sidebands located symmetrically at positions shifted from line center at the Rabi and double Rabi frequencies.

A dressed-state study of lasing without population inversion from a three-level atom interacting with a bichromatic laser field in the ladder configuration is being formulated. The system is allowed to be dressed by both laser-field photons (double dressing). The temporal behavior of the system under consideration is being explored numerically. Although the use of dressed-basis density matrix equations introduces major complexity into the equations, this procedure has crucial importance to the understanding of the atomic spectrum and the underlying physics of the processes involved. The system is found to possess a gain without population inversion, as well as a regular gain with inversion for various transitions, both in the transient and steady-state regimes. Enhancement of the refractive index accompanied by vanishingly small absorption was also found. The frequency response of various transitions is explored via numerical Fourier analysis. Some transitions are found to exhibit a dispersion-like line shape, clear evidence of a quantum-coherent interference effect. The spectrum consists of two distinct quintets centered about the coupling and probe-laser frequencies. The spectral features consisting of each quintet are located symmetrically (at resonance conditions only) around the line center at positions shifted from the center by the generalized Rabi and double Rabi frequencies.

A scheme for obtaining a laser without inversion in the deep X-ray regime is proposed based on the principle of gain without inversion. A ladder scheme is proposed in the Ar8+ system. In this system, conventional lasing with population inversion in a neonlike scheme is used. This lasing field is proposed for the drive field on the higher transition of the ladder. A coherent field obtained in the high harmonics ultrashort laser provides the probe field to be amplified. The ground level is incoherently pumped into the highest level, while the latter is incoherently depumped into the ground level, at a different adjustable rate. A detailed density matrix calculation of the ladder configuration is performed [3], and the conditions under which the system exhibits inversionless gain, as well as regular gain, are derived. Numerical calculations in the bare state picture exhibits gain without inversion both at resonance and at the generalized Rabi side bands. The probe lineshape is shown to be constructed of two symmetrical dispersionlike features each centered at the generalized Rabi side bands. This is evidence of the quantum interference occurring at these locations and the basis for the gain without inversion.

A scheme for obtaining up-conversion lasing without inversion in the visible range is proposed using a Doppler-broadened He-Ne mixture as active medium. We consider a cascade scheme (2s2↔2p2↔1s3) with a frequency up-conversion ratio of 1.91. The upper transition is inverted and driven by an external field and lasing without inversion occurs at 616.4 nm provided that the collisional decay rates of the 1s3 and 1s5 levels overcome a certain threshold.

We propose a concept of fractional quantum resonances based on an idea of driving a quantum system by an external field in the form of a repetitive pulse sequence. We show that, in addition to intuitively expected resonance at repetition rate ${\ensuremath{\omega}}_{\text{Rep}}$ equal to the internal quantum frequency $\ensuremath{\omega}$, the system exhibits less intuitive fractional quantum resonances at ${\ensuremath{\omega}}_{\text{Rep}}=\ensuremath{\omega}/n$, with $n$ being an integer. It is a nonintuitive phenomenon since the external repetition rate has no quantum character, yet the atom responds to it, if the rate is equal to $1/n$ of its eigenfrequency. This result is in good agreement with recent magnetometer experiments. It is also found that other resonances are possible at pulse-repetition-rate frequencies equal to $1/n$ of combination frequencies involving the internal system frequencies and Rabi frequencies. We show that quantum-interference-related phenomena of gain without population inversion and absorption with inversion are observed at these unique resonances. Fractional quantum resonances provide a spectroscopy tool, allowing us to measure optical frequency on one transition through detection of resonances on another transition. We believe that our results will have implications in other quantum-related processes, such as laser-assisted resonant enhancement of chemical reactions and biological processes.

A scheme for active second harmonics generation is suggested. The system comprises N three-level atoms in ladder configuration, situated into a resonant cavity. The system generates the field whose frequency is twice the frequency of the pumping laser, and the field phase is locked to the phase of the pumping field. It is found that the system can lase in either superradiant or subradiant regime, depending on the number of atoms N. When N passes some critical value the transition from the super to subradiance occurs in a phase-transition-like manner. Stability study of the steady state supports this conclusion. For experimental realization of the super/subradiant second harmonics generation we propose semiconductor quantum well structures, superconducting quantum circuits, and evanescently coupled waveguides in which equally spaced levels relevant to this study exist.

The line profile of several Cu autoionization levels have been measured experimentally by the optogalvanic technique. The optogalvanic signals related to the autoionization states have a distinct signature, namely a fast initial response which is the result of an essentially immediate ionization of these levels. As a result, the initial response overlaps the temporal profile of the laser. This is followed by a usual optogalvanic signal (OGS) reflecting the relaxation of the plasma to its equilibrium conditions on plasma timescale. A tunable pulsed dye laser is used to obtain OGS and to scan the resonance profile of several Cu autoionization levels. The transitions show the characteristic asymmetric Fano line profiles. The detection of fast autoionization processes as well as measurements of saturation broadening in some of these levels are demonstrated.

Dynamic properties of closed three-level laser systems are investigated. Two schemes of pumping\char22{}$\ensuremath{\Lambda}$ and V\char22{}are considered. It is shown that the nonlinear behavior of the photon number as a function of pump both near and far above threshold is crucially different for these two configurations. In particular, it is found that in the high pump regime laser can turn off in a phase-transition-like manner in both $\ensuremath{\Lambda}$ and V schemes.

A scheme for active frequency doubling is suggested. The system comprises N semiconductor quantum wells situated into resonant cavity. Theoretical model for such systems is based on three-level virtual atoms in ladder configuration. It is found that the system can lase in either superradiant or subradiant regime, depending on the number of atoms N. When N passes some critical value the transition from the super to subradiance occurs in a phase-transition-like manner. Stability study of the steady state supports this conclusion.

An explicit relationship between the Lorenz-Mie resonances and nonlinear optics of a microsphere has been found. We have constructed an approximate analytic method which allows an adequate assessment of the threshold for stimulated Raman scattering in microspheres. Systematic investigation of the characteristic equation, using approximate analytic expressions, reveals that not all modes have the same quality. Some resonances with very high quality correspond to a physical situation in which the spherical waves can be envisioned as whispering-gallery light waves that are totally internally reflected by the sphere boundary, without suffering almost any loss. \textcopyright{} 1996 The American Physical Society.

A model of a superradiant laser comprising of N three-level systems driven by two pumping lasers is considered in semiclassical approximation. Steady-state equations for density matrix elements are solved both analytically and numerically. Various properties of generated light are investigated. It is found that such a laser can exhibit both superradiance, when the intensity scales as N(2), and subradiance when the intensity does not depend on N. The phase of the laser field is locked to the relative phase of the pumping lasers.

The dressed-atom approach to the fluctuation problem in lasers is described. Such an approach allows one to analyze the fluctuations in various kinds of laser models. The intensity fluctuations in a regularly pumped laser are considered in the framework of the dressed-atom approach. The Fokker-Plank equation for the field density matrix is derived from first principles without heuristic assumptions. An alternative interpretation of the noise suppression in the regularly pumped laser is given. It is based upon the nonstationary picture for atomic variables. In such a regime the statistics of the output laser field may be controlled by the inuput statistics of the atomic initial conditions. It is the regular pump that provides the best input statistics

The intrinsic property of laser systems with many internal degrees of freedom to generate a squeezed light is studied. The dressed-atom approach to fluctuations, described in the preceding paper [Khazanov, Koganov, and Shutter, Phys. Rev. A 48, 1661 (1993)], is employed. A minimization principle which governs noise quenching in laser systems is described comprehensively. This principle allows one to avoid complicated quantum-mechanical calculations to assess the squeezing capacity of the system. It is shown that noise in a laser system can be decomposed into noise states. These states interact coherently. For instance, each nonactive level in a multilevel lasing scheme may represent separate noise states under certain conditions while both lasing levels make only one noise state

The dynamics of a photoemitted charged pulse in a one dimensional photocathode is investigated. At a very short pulse duration, on the order of a few picoseconds, short with respect to the transit time, interesting features are revealed. After a transient period, the charged pulse spreads and becomes uniformly distributed. The pulse spreads approximately to the extent of the transit time in the gap. This behavior is the result of the self-action electric field and depends on the total charge emitted under the laser power, on the pulse length and on the electric field. A one dimensional simulation and an analytic model describe the pulse propagation and are in excellent agreement. At high acceleration fields, in the relativistic regime, different time evolutions result. In the present simulation we make the simplified assumption that the relativistic effect only changes the electron mass. The result is that the self-field is overcome by the relativistic dynamics, and the electron pulse steepens.

In this paper we present the preliminary results from a new experimental technique to synchronize the pulses from a mode-locked Nd-YAG laser to the light pulses in the VUV storage ring at the National Synchrotron Light Source (NSLS). We describe a method to electronically change the delay time between the laser pulses and the synchrotron pulses. We also illustrate a method to overlap the synchrotron pulses with the laser pulses in space and time. Preliminary results will be presented for two experiments.

The amplitude fluctuations in a single-mode laser for the case of regular pumping when the atoms are injected in the cavity at fixed, unfluctuating rate are discussed. The dressed atom approach is employed. A comparison is made of such pumping scheme with the conventional one, when the number of active atoms is fixed. The results of the present work agree well with other approaches.

A new principle of laser coherence found recently provides an example of the most correlated quantum state. We discuss the relation between squeezing phenomena and quantum correlations (QC). These new results can be understood only in the framework of a special QC approach.

Laser schemes with one and two lasing transitions are compared with respect to amplitude squeezing capacity. It is shown that the squeezing capacity of schemes with one lasing transition is larger. The hierarchy of the two different photon statistics is established.

We show that initially dipole-forbidden atomic transitions can be effectively controlled by bichromatic laser fields. Such transitions become dynamically allowed due to ac-Stark effect. Gain, dispersion and spectral properties of these transitions are discussed.

The {ital L}{sub 2,3} emission spectrum of {ital c}-Si, excited by monochromatized synchrotron radiation, has been recorded with a 5-m Rowland spectrometer. Dramatic spectral changes are observed as the excitation energy is varied from the 2{ital p} binding energy up to 144 eV. It is proposed that a spectator electron, close to the bottom of the conduction band, influences the emission spectrum. The observations suggest that intraband shakeup is important in the excitation process, and that a population of low-lying levels, via initial-state shakeup, influences the high-energy-excited Si {ital L} emission spectrum.

The dynamics and quantum statistical properties of the three-level laser ladder model are investigated. A comparison between this model to the other models, V and {lambda} ones, is presented. We found that the ladder-pumped lasers have both the highest intensity and the lowest linewidth, whereas the maximum noise reduction is more pronounced in the V and {lambda} models. We also show that at high pumping rate the ladder scheme behaves as the two-level one.

We describe an autodyne lidar with multiple external mirrors by using an analytical approach presented previously [ Appl. Opt.41, 7087 ( 2002)]. The use of multiple target mirrors allows one to detect and locate the concentration of species between any pair of neighboring mirrors.

A scheme for obtaining laser in the deep x-ray regime is proposed based on the principle of gain without inversion. A ladder scheme is proposed in ${\mathrm{Ar}}^{+8}$ system. In this system, conventional lasing with population inversion has been obtained in a neonlike fashion. This lasing field is proposed for the drive field on the higher transition of the ladder. A coherent field obtained in the high harmonics ultrashort laser provides the probe field to be amplified. This will be selected to best match the frequency needed for the transition from lower level of the present x-ray lasing to the ground state. Numerical calculations in the bare state picture exhibit possible gain at resonance, and at the Rabi side bands, for appropriate choice of system parameters, i.e., the strength of the driving field, the strength of the incoherent pump that provides the required energy needed for the inversionless amplification and the deexcitation rate. The latter rate is detrimental to lasing as it increases. It is quite interesting to note that the system exhibits surprisingly high stability for gain without inversion even for quite high rate of the incoherent deexcitation processes. Real and imaginary parts of the coherences were also obtained for detuned drive and probe fields. These provide insight into the role played by the quantum interference. The present theoretical calculations give high confidence in the quantum interference process as a candidate for realizing very short wavelength laser.

A three-level atom in the V-configuration is treated under two coherent laser fields at resonance with the two allowed transitions. The master equation for the system and full quantum mechanical Hamiltonian are derived, and equations of motion for the elements of the density matrix are obtained by projecting into the bare- and dressed-states schemes. Analysis of light amplification without population inversion in this V-type system is accomplished within the framework of the dressed-state basis. The physical content of the equations of motion for the density matrix elements, namely the coherences and the populations is provided by identifying effective decay and relaxation rates and gain/absorption constants with the appropriate composite coefficients. These depend on the Rabi frequencies of both the drive and probe coherent fields. Time-dependent solutions for both the coherences and the populations are obtained both numerically and analytically under the secular approximation. We have shown how to construct these analytical approximate time-dependent solutions for dressed-state populations and coherences. These approximate solutions, as well as the full numerical calculation quantities, exhibit oscillations at the generalized Rabi frequency.

The use of the optogalvanic technique to obtain the line profile of the Cu 5s' $^{4}$${\mathit{D}}_{5/2}$ autoionization level with different degrees of saturation broadening is demonstrated. The experimental results point out that the growth of the autoionization-level breadth follows the square root of the increase in the laser intensity. The measured value of the natural breadth of the Cu5s' $^{4}$${\mathit{D}}_{5/2}$ autoionization level is 7.2\ifmmode\pm\else\textpm\fi{}0.6 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. The laser saturation intensity was measured by two different methods, and its value is about 0.39\ifmmode\pm\else\textpm\fi{}0.09 MW/${\mathrm{cm}}^{2}$.

Temporal character of cone emission from Sr vapor excited with \ensuremath{\sim}3-ns, near-resonant laser pulses is studied with \ensuremath{\sim}0.5-ns resolution. The cone pulses appear significantly narrower than the laser pulse. Their properties (amplitude, width, energy, delay, and angular distribution) systematically depend on the laser energy and detuning in a way that contradicts present, steady-state models of cone emission.

Light amplification without population inversion and absorption with inversion driven by quantum coherent interferences in a closed three level V-type system is studied from the point of view of two color dressed state basis. Approximate analytic time dependent solutions, for coherences and population distribution are derived, and compared with full numerical solutions. Steady state quantities are also calculated, and the conditions under which the system exhibits gain with and without inversion, in the dressed state representation are derived. It is found that for a weak probe laser field light amplification without inversion may occur, for a range of system parameters, in both the bare and the dressed state representations.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

An interpretation of quantum fluctuations sources in quantum optical systems in terms of correlation of recyclingtimes is suggested. An appropriate recycling time operator is defined. It is shown that the degree of correlation of therecycling times is a measure of quantum feature ofthe field.1. INTRODUCTIONTime plays a major role in fluctuation dynamics and correlation effects in lasers and squeezing. However, as will beseen later it is not just a parameter, but it is rather possible to impart an operator character to recycling time of atomiclevels. This is a plausible conjecture as time can be a conjugate to the energy operator. Such a view involves relaxationprocesses as encountered in lasers. Thus, one may propose to deal with the correlation of the times of recycling of atomic states in lasing process. Moreover, such correlation approach would be helpful in understanding and interpreting the origin of noise quenching in systems with essentially cooperative behaviour as encountered in therecently proposed superradiant laser'.

Synchrotron radiation sources have opened a new window on the century-old use of x rays as a scientific tool. X-ray fluorescence, excited by the photoabsorption process, has been a part of this research picture almost since the day that x rays were first discovered. However, the investigation of multi-photon processes in gases and solids had to wait until the second half of the 20th century. The advent of intense synchrotron radiation sources based on the use of specialized insertion devices will provide many new scientific opportunities for the 21st century. This presentation will outline some of the recent exciting discoveries in soft x-ray fluorescence spectroscopy and discuss a new type of laser-synchrotron hybrid experimental technique based on the time structure of the synchrotron radiation.

We present a time-dependent study of a three-level ladder scheme interacting with two electromagnetic fields, utilizing a dressed-state approach. Analytical expressions for time-dependent populations and coherences are obtained under the secular approximation. These are found in good agreement with exact numerical solutions. It is found that the system exhibits gain both with and without inversion. The calculated temporal gain is found to be essentially larger than the steady-state gain. The frequency response of density matrix elements has been studied. Both normal and abnormal dispersion?absorption relations have been obtained.

Conditions for noise minimization in laser systems are studied and formulated as a principle. It involves the correlation of the internal degrees of freedom of the system and the relative rates of transitions between them. The minimization principle proposed is appropriate for a variety of lasing schemes including nonunidirectional pump processes. It allows one to interpret a wide scope of experimental results. It is expected to be serviceable in further exploring of the squeezing phenomenon

A dressed-state study of lasing without population inversion from a three-level atom interacting with a bi-chromatic laser field in the ladder configuration is presented. In our model we allow the atom to be dressed by both of the coupling and probe fields. The system under consideration is explored both analytically and numerically within the steady-state regime. A parameter study is performed in which we explore the influence of the change of control parameters (Rabi frequencies and incoherent pump rates, etc) on the system behaviour. The parameter study also applies to situations where both coherent fields have comparable intensities. Expressions for steady-state populations and coherences are derived, from which the constraints on the system control parameters allowing gain or absorption of various types, are calculated. The system is demonstrated to possess gain without inversion (GWI), as well as regular gain with inversion, for the appropriate choice of system control parameters. Inversion without lasing (IWL) is also found. The dressed-state approach offers a different insight into the system dynamics in which Rabi oscillations and spectral features appear natural. We believe that the dressed-state approach provides a more adequate description of the system dynamics than that provided by the bare state basis.

In spite of the fact that cone emission was first observed over a decade ago, there is still no generally accepted theory of the effect. Here we give an outline of a unified approach and point out a mechanism that supports Cherenkov-type radiation due to vacuum fluctuations as a possible source of cone emission. This can be considered as an additional mechanism to the previously discussed four-wave mixing, and initial encoding and follow-up refraction effects. We compare our treatment to previously considered models.

The observations of anomalous emission near the D1 transition of a dense (>or=1014 cm-3) sodium vapour illuminated by a laser detuned to the blue with respect to the D2 transition is studied and reported. A discussion of existing models is presented. The authors propose a type of parametric down-conversion process to explain this emission. This process involves a laser photon splitting into two photons by the three-level system of sodium, assisted by a magnetic dipole transition.

Red shifted conical emission induced by a laser blue shifted with respect to the D2 line of sodium is observed and interpreted in terms of Cherenkov emission. Measured cone angles and wavelength shifts agree well with model calculations taking into account saturation and dispersion effects.

Observations of Cooper minima in excited-state photoionization cross sections for the Ne $3s$ and Ar $4s$ levels are reported. Prebreakdown Ne and Ar discharges under irradiation by light in the 200-400-nm wavelength range yield spectral dependences of photoionization cross sections with minima at the ionization threshold of these states in Ne and slightly distant from them in Ar. They are in close agreement with certain theoretical predictions. Autoionization structure which exists between the two ionization limits $^{2}p_{\frac{3}{2}}$ and $^{2}p_{\frac{1}{2}}$ was not observed in any of the Ne or Ar photoionization spectra due to both low spectral resolution and the high density of levels in this spectral region. Application of these measurements to ultraviolet-radiation detection is examined. High detection sensitivity of ultraviolet light is indicated.

Following recent reports on strong uv and ir laser transitions in noble-gas mixtures, we attempted to determine the dominant precursor reactions leading to laser action. In this paper we present experimental results of time-dependent spectroscopy of the various species existing in He/Xe mixtures excited by fast transverse electrical discharges. We also present some parametric measurements of the He/Xe laser in the ir region. A plasma model is suggested for explaining the time behavior of the helium and xenon emissions. According to this model, which is justified by the experimental results, ir laser action in the He/Xe mixture is excited mainly by energy transfer from metastable helium atoms and molecules by Penning ionization of xenon atoms.

Time-resolved spectroscopy of excited mediums following electrical discharges in helium/xenon mixtures, in which xenon is the minority constituent, show enhancement in the emission of all the ionic-xenon levels relative to that observed in pure-xenon discharges. The time characteristics depend on the helium pressure and the decay follows the molecular helium emission. The total pressure ranges between 20 and 300 Torr, of which the xenon pressure is up to 1 Torr. The discharge has a peak current density of 500 A/${\mathrm{cm}}^{2}$ and it flows for about 100 nsec. It is suggested that helium plays an active role in the generation of excited xenon ions by transferring excitation energy to xenon. The acceptors can be ${\mathrm{Xe}}^{+}$ in its ground state and neutral Xe in its metastable state. In the investigated discharges both, ${\mathrm{Xe}}^{+}$ and metastable Xe* exist in high densities for up to 5-10 \ensuremath{\mu}sec into the early afterglow. During this period, atomic helium metastables and molecular helium states such as $a^{3}\ensuremath{\Sigma}_{u}^{+}$, $A^{1}\ensuremath{\Sigma}_{u}^{+}$, and $B^{1}\ensuremath{\Pi}_{g}$ are also highly populated and in fact constitute an energy store. The molecular states of helium can resonantly transfer energy through the bound-free transitions to ground-state ${\mathrm{Xe}}^{+}$ to form excited xenon ions ${\mathrm{Xe}}^{+*}$ with an estimated cross section of 4 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}14}$ ${\mathrm{cm}}^{2}$---a resonable order of magnitude considering the large enhancement observed in Xeii emission. Penning ionization and charge transfer have only little effect on observed enhancement.

We present a calculation of the dielectric correlation function in glasses showing how the assumption of short correlation length for normal modes breaks the momentum selection rules and leads to expressions for the first-order Raman-scattering intensity in terms of the density-of-states functions and known frequency-dependent amplitudes.

Four-wave mixing in the vicinity of a sodium doublet is reported. The three-level system allows collinear phase-matching. The FWM is resonantly enhanced by a high gain on the blue side of the D1 transition.

Two-photon enhancement of the optogalvanic signal is shown to be possible, both theoretically and experimentally, even in the case of population inversion of the first transition. This is shown, in particular, for the neon 1s 2 → 2p i transitions.

The behavior of a laser intensity as a function of a pump rate is studied. We consider a three-level lasing scheme with two different types of pumping. In the first case, the lower lasing level is depleted to the ground state, and then the pumping is used to excite the atom from the ground state to the upper lasing level. In the second case the pumping is used to excite the atom directly from the lower lasing level to one of the excited states, which is then depleted to the upper lasing level. We refer these two types of pumping as type 1 and type 2, respectively. It is shown that although these two schemes are equivalent mathematically, i.e., they are formally described by the same set of equations, physically their dynamical properties are completely different. It is noted that the pumping of the type 1 does not affect either of lasing levels directly, whereas the pumping of the type 2 does connect the lower lasing level with one of the excited states. It is this difference of the two ways of pumping which is responsible for the crucial difference in the dynamic behavior of the lasing schemes of types 1 and 2.