We show that the concept of phase-shifted laser pulse trains is meaningful only for purely amplitude-modulated pulses, and probably cannot be generalized to use the output of a normal pulsed laser. We demonstrate a laser system which gives picosecond pulse lengths, picosecond or nanosecond interpulse delays, and complete phase control. This makes most molecular transitions amenable to the coherent techniques previously restricted to nuclear magnetic resonance. Fluorescence suppression by π phase shifts is demonstrated for the first time with picosecond pulses and long delays. © 1987.