Multiple photon transitions induced by a dichromatic laser field

Three-photon transitions induced by a dichromatic C${\mathrm{O}}_2$ laser operating on the $P\left(10\right)\mathrm{}$ and $P\left(20\right)\mathrm{}$ lines of the 961-${\mathrm{cm}}^{-1\mathrm{}}$ band have been observed in a helium plasma. Laser photons interact resonantly with the $4{}_{}{}^3{}_{}{}^{}S$ and $4{}_{}{}^3{}_{}{}^{}P$ helium levels, resulting in new spectral lines which flank the 4713-Å, $2{}_{}{}^3{}_{}{}^{}P-4\mathrm{}{}_{}{}^3{}_{}{}^{}S$ He line. The multiple photon transition rates are calculated from a numerical diagonalization of the interaction Hamiltonian and are shown to be in agreement with experimental results.