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The 3p (3)P state of O and the 2s(2)2p(2)3p (4)D(0) states of N are populated by two-photon absorption at 226 and 211 nm, respectively, and the resulting near-IR fluorescence is detected. The exciting photons are provided by stimulated Raman frequency shifting, and the experiments are performed in a flow discharge. The measured lifetime of 39 (0) and 27 (N) nsec and quenching rate constants of 2.5 x 10(-10) cm(-3) sec(-1) for collisions of N(2) with each atom indicate promise for this method as a diagnostic tool in flames and plasmas.

Absolute bound-bound atomic two-photon cross sections have been measured using a singlefrequency laser for the first time, eliminating the usual uncertainties about unresolved temporal fluctuations.For the 3p PJ 2p P2 transition in atomic oxygen at 226 nm the integrated cross section is g aot2t(J' 2) 1.87+'0.60x10 35 cm . Doppler-free spectra have been used to measure relative fine-structure cross sections and energy spacings. Absolute and relative twophoton cross sections agree well with perturbation theory, validating this approach.The measurement of absolute two-photon absorption cross sections requires a thorough characterization of the radiation field. Because two-photon transition rates are proportional to the instantaneous square of the light intensity, unresolved temporal fluctuations can be a source of considerable uncertainty. ' With two exceptions ' absolute two-photon cross-section measurements have been carried out using pulsed, multimode laser systems subject to unresolvable temporal fluctuations. A single-frequency laser (one whose bandwidth and temporal profile are related by the uncertainty principle) has no such fluctuations. In this Rapid Communication we report the first absolute measurement of a bound-bound atomic twophoton cross section with a single-frequency laser, and compare it with ab initio calculation.Comparison with earlier experimental results yields a quantitative value of the second-order intensity correlation function for a tunable, multimode ultraviolet laser system. The first Doppler-free spectra of atomic oxygen have been obtained, allowing comparison with theoretical relative finestructure cross sections, leading to precise relative energy measurements in the 3p P electronic state, and extending Doppler-free spectroscopy to shorter wavelengths (226 nm) than ever before.The experimental procedure used to determine absolute two-photon cross sections for the 3p PJ. 2p PJtransition has been described in detail elsewhere. Oxygen atoms are generated in a microwave discharge in pure 02 at a total pressure of 0.5 Torr, with concentrations determined by titration with NO2. Tunable laser radiation near 226 nm is used for the two-photon excitation.Fluorescence at 845 nm from the 3p PJ. 3s S~transition (with the fine-structure components unresolved, collection axis perpendicular to the propagation direction of the laser), is used to determine the number of excited atoms after absolute calibration of the fluorescence collection system at this wavelength. The laser is linearly polarized in the vertical direction (perpendicular to the direction of fluorescence detection). Temporal and spatial profiles of the laser are measured in a gently focused geometry, and rate equations are used to extract the twophoton cross section. In all experiments reported here the peak laser intensity is less than 2 MW/cm2, well below the value at which photoionization by a third 226-nm photon can cause significant excited-state depletion. (The excited-state photoionization cross section is 5X 10 ' cmz. s)Becaus...

A computer algorithm is presented which allows simultaneous consideration of self-phase modulation and dispersion for predicting temporal shape changes during the propagation of plane-wave intense light pulses. The algorithm entails considering propagation alternately in regions where only one of the two above effects is operative. It is shown for clear materials that the parameters characterizing propagation are the nonlinear index change, the wavelength λ, the relaxation time of the nonlinearity, and the disperison parameter λ3(d2n/dλ2). The thickness of material over which a pulse will significantly reshape is found to be √λ times the geometric length of the pulse divided by the square root of the product of the dispersion parameter and the maximum nonlinear index. It is demonstrated that dispersion significantly modifies the self-steepening concept of DeMartini, Townes, Gustafson, and Kelley. Numerical simulations of propagation in CS2 indicate that, after sufficient travel, a shock can form on the leading edge of a mathematically smooth incident pulse. This is because of the retarded response of the nonlinearity; it is found that pulse features tend to evolve which are shorter than the relaxation time of the nonlinearity. With further propagation, the entire pulse develops violent amplitude features. For a 22-GW/cm2 5-ps 1.06-μm pulse in CS2, this shock appears after a propagation distance of 15 cm. Impressed amplitude noise on the pulse is shown to intensify this instability; under the same conditions, 10% peak-to-peak impressed ripple triples in approximately 3 cm. Because of the recently published experiments of Ippen, Shank, and Gustafson, propagation of 5-ps mode-locked dye laser pulses in CS2-filled fibers is also considered. In this case, shocks appear at 2-m propagation if the input peak intensity is 50 MW/cm2. Furthermore, attenuation present in the fibers tends to stabilize the shape of the shock as it forms. It is shown (for pulses which have not shocked) that dispersion can also modify the Fisher-Kelley-Gustafson pulse compression scheme. For the case of CS2, it is found for optimal compression that more dispersive delay is required in a compressor than would have been needed in the absence of CS2 dispersion. When the product of the peak intensity and the propagation length is 220 GW/cm, the proper compressor dispersive delay for optimum compression can be found by multiplying the optimally compressing dispersive delay in the absence of CS2 dispersion by the factor 1+0.1l, where l is the propagation length in cm. The calculation concerning compression of pulses emanating from the CS2-filled fibers was also carried out. It is found that although some temporal compression can occur, the subsequent compressed pulses are not much shorter than the temporal shock which had formed on the pulse prior to compression. We conclude that propagation distances in such experiments should be kept below the shock distance. The simulation of pulse propagation in Nd : glass laser amplifier chains is also studied, taking nonlinearity and dispersion into account. The glass dispersion and the glass nonlinearity were considered with the linear properties of the resonance (disperison and frequency-dependent gain). No provision was made to model a nonlinear response for the resonance. In the absence of gain, pulses temporally broaden and flatten because the glass dispersion is the wrong sign to compress the chirp which develops at the temporal center of the pulse. In pumped amplifiers a sharp temporal spike forms at the center because the chirp swings the pulse center frequency through the center frequency of the amplifying transition at that time. It is demonstrated that under typical operating conditions, pulses are relatively stable to amplitude modulation.