We measure the energy distribution of a slow Cesium atomic beam when it is chopped into a short pulse and we find results which agree well with the time-energy uncertainty principle. The chopper consists in an atomic mirror formed by a laser evanescent wave whose intensity is pulsed. We use the temporally diffracted beam to design a Young-slit-type interferometer, in which the interfering paths consist of atomic trajectories bouncing at two different times on the mirror. By changing the mirror intensity, we can scan the atomic phase difference between the two arms. [S0031-9007(96)00519-4] When a beam of particles with a well defined energy is chopped into a short pulse, the outcoming energy distribution is broadened according to the time-energy uncertainty relation. This effect is very well known for photons, and it is at the basis of spectroscopy with ultrashort pulses of light. For matter waves, the phenomenon of diffraction by a time slit has been studied theoretically by several authors [1]. Its observation constitutes a test of time-dependent quantum mechanics, while usual diffraction phenomena can be described using the stationary formulation of the Schrodinger equation. We report here the observation of this temporal diffraction effect for de Broglie atomic waves, and we show that our results are in good agreement with the quantum mechanics prediction. We also use the coherence of the diffracted pulse to realize a Young interferometer using temporal slits. This interferometer is a very flexible device in which the temporal positions of the diffracting slits can