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Experimental investigations of two-photon enhanced stimulated hyper-Raman and parametric four-wave-mixing emissions are described for tuning unfocused laser beams near two-photon resonance with 3D and 4D states in sodium vapor. Over wide ranges of vapor pressures, path lengths, and laser power densities, the gains for stimulated hyper-Raman (SHR) scattering and for parametric four-wave mixing (PFWM) can become very greatly reduced from values expected from simple considerations. The suppression of gain can arise from interferences produced at two-photon and three-photon resonances, and from ac Stark shifts produced by internally generated near-resonant fields. Quantitative comparisons are made between observed and theoretically predicted values for SHR and PFWM production. Of 12 predicted consequences of the interferences and Stark shifting on the nonlinear behavior, ten are demonstrated through the experiments.

Magnetic monopole detectors based on the premise that these particles can create the triplet metastable state in helium, He(${2}^{3}$S), exclusively and without ionization are believed to be feasible. In one of these, He(${2}^{3}$S) is detected in an ionization chamber array using both pure helium and a mixture of He+${\mathrm{N}}_{2}$ in which Penning ionization occurs. In the second version, resonance ionization spectroscopy is used to selectively ionize He(${2}^{3}$S). In both cases, monopoles with velocities greater than ${10}^{\mathrm{\ensuremath{-}}4}$c can be searched for with both large and reliable detectors.