The ion flux and ion velocity distribution function are studied using a capacitively coupled radio frequency ion flux probe and a multigrid retarding field analyzer in an HBr/O2 pulsed plasma process, dedicated to silicon etching in gate or shallow trench isolation applications. A decrease of the duty cycle of the pulsed plasma etch process affects strongly these parameters: the mean ion flux decreases almost by the square of the duty cycle. Furthermore, the contribution of high energy ions from the on-time of the plasma is strongly reduced while their energy is slightly increased. In average, this leads to a significantly reduced ion energy and a reduced ion flux in the pulsed plasma compared to the continuous wave plasma.