Two different approaches to multiphoton excitation of an anharmonic multilevel system are presented. Both methods result in nearly complete population of the upper state. The first a~proach involving phase shifting portions of the coherent excitation is treated analytically With square pulses. The second approach uses sequences of several pulses with selected relative phase and delays. Some numerical examples are presented for three-and four-photon excitation of the vibrational level structure of CH3F. This particular molecule is again taken as model to demonstrate four-photon excitation with a five pulse sequence, taking into account the rotational line structure. The rotational line distribution in the upper state is seen to correspond to a lower temperature than in the ground state.

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We theoretically analyze the absorption of media excited by a pair of coherent pulses, as a function of their relative phase, in conditions of multiphoton resonance and negligible dephasing. It is shown that the periodicity of transmission vs delay and phase is characteristic of the nature and order of the process. Also a multilevel system may evolve from pure muhiphoton resonance to cascade resonance under specific conditions depending on the detunings of the levels involved.(1)In Eq. (1), A. j and xkj are, respectively, the eigenvalues and eigenvector components of the tridiagonal matrix n, and A . J