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Two-Photon Thermal Lensing Spectroscopy Using A Synchronously Pumped Dye Laser

Recently the thermal lens effect has been developed as a sensitive technique for measuring two-photon absorption spectra of molecules with low fluorescence yields, where the normal two-photon excitation technique is difficult to apply. For centrosymmetric molecules, the two-photon technique permits a direct examination of even-parity transitions in weakly fluorescent molecules adding a new dimension to the electronic spectroscopy. It has been shown by Twarowski and Kliger1that the two-photon thermal lens can be observed if the laser frequency is coincident with a two-photon allowed band but with little one-photon absorption in molecules. Since the two-photon process depends quadratically upon the laser intensity, the temperature profile induced by two-photon absorption possesses a narrower spatial distribution than is expected for the corresponding temperature profile induced by one-photon absorption. Then the two-photon process might play a significant role because only the center of the heat distribution leads to the formation of the lens signal.