TWO-PHOTON THERMAL LEMSING SPECTROSCOPY
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Date
1981
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Ohio State University
Abstract
For centrosymmetric molecules, two-photon selection rules provide for the direct observation of even parity excited states. It allots us to study many forbidden or weekly allowed states not observable by one-photon spectroscopy. For fluorescent molecules, two-photon excitation (TPE) is one of the most convenient detection methods. Far molecules, with low fluorescence quantum yields, TPE might not be applicable. These molecules dissipate their excitation energy through radiationless processes, thus converting the input energy into hest in the sample. The temperature rise induced by two-photon absorption from a well-behaved (Gaussian) laser beam creates a transverse gradient in the refractive index which can be probed optically as a thermal lens. It has been shown that the observation of the two-photon thermal lens effect is determined by the competition between one- and two-photon absorption for the energy from the pump laser. Estimates have show that the two-photon thermal lens might play a more significant role than one-photon thermal lens if the laser frequency is coincident with a two-photon allowed band but with little one-photon absorption. This indicates that for centrosymmetric molecule, the two-photon thermal lens is useful for studying low-lying g* states but might not be appropriate for high-lying $g^{*}$ states. In this report, two-photon absorption spectra measured with a dual-beam thermal thermal lensing technique are studied, for naphthalene and several polyenes. A near-Gaussian mode YAG/DYE laser is used as the excitation source.
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