The rubidium $5S\text{\ensuremath{-}}5D$ Doppler-free two-photon transition has been observed in a thermal vapor cell by detecting the infrared radiation emitted during the decay from the
5D excited level to the
5P intermediate level. Different to the usual detection scheme based on blue light emission this approach does not suffer from reabsorption and still works at high densities. This not only allows for a dramatically enhanced detectable emission rate but also for investigating collisional effects such as collisional broadening and collisional energy transfer. Furthermore, we observe a yet unknown temperature dependence of the resonance linewidth for transitions into the states of the
5D5/2 level. Consequences for the design of an optical Rb-frequency standard are discussed.
Investigations are made of hyper-Raman scattering and four-photon parametric processes in two-photon excitation of metal vapor. The laws of propagation of infrared and ultraviolet radiation generated in a metal vapor medium by these processes are found for the steady-state interaction. The nonlinear polarization of the medium is obtained in terms of elements of the atomic density matrix allowing for saturation by the pump field and for the influence of collisions. The spectral and tuning characteristics of the converted radiation are studied and a comparison is made with experimental data. A dip observed in the infrared radiation spectrum for hyper-Raman scattering and its broadening are explained. A detailed discussion is given of the role of collisions that substantially narrow the tuning range of the infrared radiation in hyper-Raman scattering.