Two-Photon Absorption (2PA) Spectroscopy
Two-photon absorption (2PA) spectroscopy measures the wavelength-dependent cross-sections for simultaneously absorbing two photons that are individually non-resonant with the electronic transitions of a molecule. We measure the continuous 2PA spectrum using the pump-probe technique with intense, tunable pump pulses and broadband probe pulses. When the pump and probe pulses overlap in space and time, the attenuation of probe light is proportional to the 2PA cross-section at the total energy of the two photons. Unlike other 2PA techniques, we measure the full spectrum simultaneously.
The unit for 2PA cross-sections is called a Goeppert-Mayer (1 GM = 10-50 cm4 s molecules-1 photons-1).
Two-photon absorption transitions can also be used to excite molecules in a two-photon pump-probe experiment. In this case, two photons are simultaneously absorbed from the intense pump beam to promote the molecule to an excited state. Due to the different symmetry-selection rules for one- versus two-photon excitation, it is sometimes possible to access different excited states using the two methods.