Direct observation of the two-photon generation of free excitonic molecules in CuCl

Two-photon absorption spectra due to excitonic molecule generation in thin CuCl samples at 4.2°K are directly measured using the intense light from a pulsed, frequency tunable dye laser. One narrow induced absorption peak is observed at 3893 Å. This peak exhibits a Fano interference line profile at low laser light intensities. High laser light flux densities result in a saturation, broadening, and blue shift of the induced absorption peak. The maximum value of the two-photon absorption cross section is directly measured to be 2 x 10^-43 cm⁴ sec, and the binding energy of the molecule is found to be 39 meV.