Transitions induites entre deux niveaux d'énergie par une impulsion; effets de la durée

Abstract

From direct measurement of probabilities of molecule transitions induced, between two energy levels, by an electrical pulse field, the influence of different perturbations is here studied according to their time width D, at constant amplitude. When the sudden passage condition is attained, the transition probability oscillates with a period depending on the integral of action. We principally develop here the case in which this condition is not respected, with a weak electrical field, for some typical impulsion shapes: rectangular, triangular, trapezoidal, and sinusoidal. It is shown that rise time and fall time of the impulsion play an important role and that some effects can be explained when taking into account the evolution of the rect(t/D) function spectrum, characteristic of the finite impulsion duration. These results can be connected with resonance effects in physics of collisions: the D parameter is then equivalent to the interaction time. When the impulsion is a complete sinusoid, it is shown that the resonance frequency is lower than the transition frequency.