No abstract

We report on the single and double multiphoton ionization of ground state Sr atoms observed in an atomic beam experiment with laser pulses of ∼5 ns duration, maximum intensity ∼4 × 10 11 W cm −2 and within the 710-740 nm wavelength range. The Sr + spectrum consists of two strong lines originating from three-photon resonant four-photon ionization of bound states, a number of weak autoionizing resonances and a broad line due to four-photon excitation of the doubly excited 5p 2 1 S 0 state. The latter, along with a strong, broad and structured spectral feature, is also evident in the wavelength dependence of the doubly charged Sr 2+ ion. A weakly evident but reproducible inflection point ('knee' structure) appears in the intensity dependence of the Sr 2+ yield at the location of the 5p 2 1 S 0 resonance. A complementary fluorescence experiment revealed the accumulation of population in the 5p 1/2 , 3/2 , 6s 1/2 and 5d 3/2 , 5/2 excited Sr + states during the laser pulse. All fluorescence signals depend on laser wavelength in a manner similar to the recorded Sr + spectrum. The population accumulation in the 5p 1/2 , 3/2 ionic states unambiguously proves the absorption of two photons above the first 5s 1/2 atomic threshold while that of the 5d 3/2 , 5/2 and 6s 1/2 ones suggest the absorption of at least two more. Since under our laser pulse duration and intensity the absorption of such a number of photons in an unstructured continuum is highly improbable, it may be concluded that the process is mediated by dense manifolds of near-resonant doubly excited states, their role in the dynamics of laser-atom interaction under our conditions being far more crucial than in studies conducted using intense ultrashort pulses.

Discharges in He-CO, He-CO-O2 mixtures are studied in the range of experimental parameters typical of the action of a high-power CO laser with fast gas flow and liquid N2 cooling. Gas and vibrational temperatures, their radial profiles, CO vibrational population and E/N values are measured. The values of E/N are also calculated by solving the electron balance equation. Comparison of the measured and calculated data shows that the dominant ionization process in He-CO mixtures is the reaction CO+e to CO++2e. For He-CO-O2 mixtures, the reaction CO( nu >14)+CO*(I1 Sigma ) to C2O2++e is proposed as the main ionization process. Such a mechanism allows one to explain the marked influence of a small O2 admixture on He-CO plasma parameters.

The influence of molecular oxygen admixture on the vibrational distribution function (VDF) of CO molecules in a dc electric discharge in He : CO : O2 mixtures was studied. It was revealed that for O2 concentrations exceeding 10% of CO content the processes of VV′ exchange between CO and O2 molecules led to essential deformation of the CO VDF shape for vibrational states near v = 20 and higher. The comparison of experimentally measured and theoretically calculated VDFs of CO gave us a chance to realize the verification of CO–O2 VV′ rate constants proposed earlier.