ABSTRACT The present work explores a new phenomenon that not all the transition probability of two photon processes is negligible at low irradiance. The irreducible representation 2B2 of C2v is unexpected, for there is no much deviation in oscillator strength for two-photon and single-photon process A1 to 2B2. This new phenomenon is only possible to be explored by the symmetrical consideration: the necessary and sufficient condition is molecular plane coincident with yz plane or the operation σ ’v(yz) for group C2v. It is only possible to be evaluated out by use of the full relativistic quantum mechanical theory.
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