First hyperpolarizability of molecules excited by optical pump

Usually hyperpolarizabilities are studied for molecules in their ground state. However the real population of the initial state for the virtual electronic excitations induced by incident light can include not only the ground state but also excited states populated owing to an external source. We report results of numerical calculations and experimental study of the first hyperpolarizability /spl beta/(-2/spl omega/;/spl omega/,/spl omega/) in pora-nitroaniline (PNA) with excited states obtained by optical pump.