Resonance effects on two photon absorption processes of molecules

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Abstract
Effects of resonance on two photon absorption of molecules have been investigated. Using the expression for the photon processes which was derived by using the density matrix method in the Markoff approximation, the intensity of the two photon absorption can be separated into simultaneous, sequential, and their mixing terms. The last term vanishes in the case of zero pure dephasing constant between the initial and final electronic states. In order to study the resonance effects on the two photon absorption for a multilevel molecular system, an expression for the transition probability of the two photon absorption processes is derived in the Born–Oppenheimer basis set. Analytical expressions for both simultaneous and sequential processes are obtained for the displaced harmonic oscillator model. A temperature dependent simultaneous two photon absorption probability for the multimode case is formulated. Some model calculations of the vibrational structures in the low temperature limit for the molecular system are performed to illustrate the resonance effect.