A one-parameter theory for the energy shifts experienced by the electronic levels of dipolar rare-gas--halide excimers dissolved in liquid rare gases is presented. The theory, based on Onsager's solute-solvent interaction, yields good results for both diatomic and triatomic species. The radius of the Onsager cavity is found to be dependent only on the dielectric characteristics of the solvent while the strength of the reaction electric field appears to be determined exclusively by solute molecular properties, namely, the dipole moment and the polarizability. These empirical rules lead to the determination of a fundamental constant, characteristic of these interactions, which is conserved to within approximately 2% for the materials studied. In addition, the use of this constant in the description of the interaction, which has the units of a polarizability, permits a drastic reduction of parameters, from one cavity radius per liquid host, to only a single fundamental constant. From this analysis, very simple expressions for the cavity radius and the reaction electric field are deduced for argon, krypton, and xenon.

The potential role of multiply excited states in multiphoton ionization of atoms under high-intensity ultraviolet laser radiation in the range of 1015–1017 W/cm2 is discussed. Since the density of multiply excited states is sufficiently great to form a quasi-continuum, the coupling can be described by an average one-photon absorption cross section. A numerical fit with experimental data from xenon, produced by 193-nm radiation, assuming an autoionization rate of 1013 sec−1, yields a cross section of 4.5 × 10−19 cm2. The resulting transition rates indicate that the motion of the electrons is highly coherent, with a ratio of dephasing rate to excitation rate of ~10−2. At 1017 W/cm2, the transition rate exceeds the optical frequency for 193-nm radiation by a factor of 30. This indicates that even for the shortest optical pulses, atoms start to interact violently with the optical field long before the peak intensity is reached and that unconventional theories will have to be developed if the observed phenomena are to be understood fully.

The energy spectra of electrons generated by collision-free multiphoton ionization of Xe, Kr, Ar, Ne, and He irradiated at intensities up to \ensuremath{\sim}${10}^{15}$ W/${\mathrm{cm}}^{2}$ with picosecond 193-nm (6.41-eV) radiation have been studied with an energy resolution of \ensuremath{\sim}50 meV. The formation of multiply charged ions by a sequential process of ionization has been directly detected in the electron spectra by the observation of a characteristic pattern of interwoven above-threshold-ionization ladder line series. The appearance and relative intensity of specific electron lines depends strongly on the presence of near-resonances and features of the interaction involving the laser pulse shape, saturation, and the shift of the ionization threshold arising from the influence of the ponderomotive potential. The experimental results are compared qualitatively with data from ion time-of-flight experiments and with differing models of multiphoton ionization.

Cryogenic rare‐gas halide solutions, excited optically with radiation at 351, 248 and 193 nm, exhibit fluorescence bands corresponding to rare‐gas halide dimer and trimer species. For liquid Ar, Kr, and Xe hosts, these emissions display a systematic trend of wavelength shifts and state lifetimes. Due to their strong dipolar character, these excimers radiate in the liquid at wavelengths which are considerably red shifted1 with respect to the gas phase values, interaction. In the case of XeF* , stimulated emission has been observed on the B1/2→X1/2 transition at 404 nm. Energies of ∼110 μJ have been measured in 5 nsec pulses.

Stimulated emission on the B ? X band of the XeF* molecule in the liquid phase at 404 nm was observed following transverse optical pumping at 351 nm. The energy of this emission was measured to be ~ 70 microJ, and the pulse had a FWHM of ~5 nsec. The stimulated-emission spectrum showed considerable narrowing compared with the spontaneous- emission spectrum. The temporal behavior of the 404-nm pulse was investigated, and gain saturation of the lasing medium was observed.

Intense stimulated emission in the 119 to 149 nm region is observed from HD excited by two quanta at 193 nm. For certain transitions, the conversion efficiency approaches 1% so that peak powers of ∼10 MW are produced. Electron collisions with the HD (E,F) level are found to efficiently transfer population to the HD(C) state which produces stimulated emission on several C → X Werner band lines in the vicinity of 120 nm. The data on the pressure dependence of the stimulated signals strongly support the conclusion that, in comparison to H2, HD exhibits a large isotopic dependence on the cross section for rotationally inelastic electron collisions in electronically excited states. The rate constant for this rotationally inelastic process is estimated as ∼2×10−7 cm3/s. This is the first observation of an isotopically sensitive electron collisional process involving an electronically excited level.

Cryogenic rare–gas–halogen liquids have been considered as potential candidates for optical energy storage media. The conditions allowing optical pumping in the liquid phase are substantially different from those in the gas phase,1,2 permitting linear absorption processes to be exploited with commercially available excimer lasers as pump sources.

Studies of multiphoton ionization of atoms have revealed several unexpected characteristics. The confluence of the experimental evidence involving studies of ion production and electron energy spectra leads to the hypothesis that the basic character of the atomic response involves highly organized coherent motions of entire atomic shells. This physical picture provides a basis for the expectation that stimulated emission in the x-ray range can be produced by direct highly nonlinear coupling of ultraviolet radiation to atoms.

The nonlinear coupling of 193-nm radiation to a range of atomic and molecular materials has been experimentally explored up to a maximum intensity on the order of \ensuremath{\sim}${10}^{17}$ W/${\mathrm{cm}}^{2}$. Studies of collision-free ion production clearly exhibit anomalous behavior which strongly implies that the atomic shell structure is the principal determinant in the observed response. On the basis of the observed coupling strength and the measured atomic-number (Z) dependence, the experimental evidence points to a coherent atomic motion involving several electrons, possibly an entire shell, as the main physical mechanism enabling the scale of energy transfers seen. Therefore, states representing multiple excitations appear to play a central role in the coupling, a consideration that fundamentally distinguishes the nonlinear interaction of a multielectron atom from that of a single-electron system. Comparison of the experimental findings with standard theoretical treatments, of either a perturbative or nonperturbative nature, does not produce satisfactory agreement. Conversely, the formulation of a simple classical estimate qualitatively conforms to several features of the observed behavior including the shell character of the interaction, the maximum energy transfer, the dependence of the average energy transfer on the intensity of irradiation, the frequency dependence of the observed energy transfer, and the weak influence of polarization.

Cryogenic rare-gas fluorine solutions are studied with optical excitation provided by excimer laser radiation at 193, 248, and 351 nm. Fluorescences corresponding to dimer [$\mathrm{Kr}{\mathrm{F}}^{*}(D\frac{1}{2}\ensuremath{\rightarrow}X\frac{1}{2})$, $\mathrm{Xe}{\mathrm{F}}^{*}(D\frac{1}{2}\ensuremath{\rightarrow}X\frac{1}{2})$, $\mathrm{Xe}{\mathrm{F}}^{*}(B\frac{1}{2}\ensuremath{\rightarrow}X\frac{1}{2})$, $\mathrm{Xe}{\mathrm{F}}^{*}(C\frac{3}{2}\ensuremath{\rightarrow}A\frac{3}{2})$] and trimer (${\mathrm{Ar}}_{2}$${\mathrm{F}}^{*}$, ${\mathrm{Kr}}_{2}$${\mathrm{F}}^{*}$, ${\mathrm{Xe}}_{2}$${\mathrm{F}}^{*}$) transitions are detected in several rare-gas liquid hosts. All of the emission bands detected exhibit red shifts with respect to the gas-phase spectra. With the exception of the $\mathrm{Xe}{\mathrm{F}}^{*}(C\ensuremath{\rightarrow}A)$, ${\mathrm{Kr}}_{2}$${\mathrm{F}}^{*}$, and ${\mathrm{Xe}}_{2}$${\mathrm{F}}^{*}$ bands, the shifts increase in magnitude with an increase of the atomic number of the host.

High spectral brightness KrF* (249 nm) and ArF* (193 nm) excimer laser sources have been used in a variety of experiments to test the generation of coherent short wavelength radiation (λ ti 100 nm) using nonlinear processes. These experiments include harmonic generation, sum frequency mixing, two and four photon pumping of lasers as well as spectroscopic application of such short wavelength radiation and the study of nonlinear coupling at intensities of up to ≈ 1017 W/cm2.

Multiphoton absorption of picosecond ArF (193 nm) laser radiation has resulted in the production of highly charged ions and in the observation of stimulated emission in the vacuum ultraviolet (VUV) and extreme ultraviolet (XUV).

The properties of dimer and trimer emissions following optical excitation of cryogenic rare-gas chlorine solutions—Kr(Cl2), Xe(Cl2), Ar(Kr, Cl2), Ar(Xe, Cl2), and Kr(Xe, Cl2)—at 351, 248, and 193 nm are reported. Fluorescences corresponding to dimer XeCl*(C3/2 → A3/2) and trimer (Kr2Cl*, KrXeCl*, Xe2Cl*) transitions are detected.

The interaction of atomic and molecular species with picosecond ArF* laser radiation is studied at intensities up to 1015 W/cm2. Anomalously strong, collision-free multiple ionization is observed. Standard theoretical models of stepwise ionization fail to describe the results. The experimental findings point to a collective response of the atom. At intensities of ~1013 W/cm2, selective multiquantum excitation of autoionizing states in Kr, followed by stimulated emission at wavelengths as short as 91.6 and 93 nm, is observed. The 93-nm radiation is tunable over a 600-cm−1 interval, whereas the 91.6-nm frequency is fixed. It appears that electron collisions redistribute energy among excited states.