The Isotopic Effects in the Ir-multiphoton Excitation of Molecules with C_3V Symmetry

A stretch vibrtion Hamiltonian of XY₃ molecule has been derived by using the second quantization,unitary transformations and the optimization of coefficients.The energy spectrum of NH₃ and ND₃ molecules obtained from this Hamiltonian is in good agreement with that of experments.The multiphoton excitation of NH₃ and ND₃ molecules in laser fields have been calculated.The long time average of the transition probabilities and the isotopic dffects are discussed in detail.