  We separate a Bose-Einstein condensate into an array of 2D sheets using a 1D
optical lattice, and then excite quantized vibrational motion in the direction
normal to the sheets. Collisions between atoms induce vibrational
de-excitation, transferring the large excitation energy into back-to-back
outgoing atoms, imaged as rings in the 2D plane. The ring diameters correspond
to vibrational energy level differences, and edge-on imaging allows
identification of the final vibrational states. Time dependence of these data
provides a nearly complete characterization of the decay process including the
energies, populations, and lifetimes of the lowest two excited vibrational
levels. The measured decay rates represent a suppression of collisional
de-excitation due to the reduced dimensionality, a matter wave analog to
inhibited spontaneous emission.
