Investigation of the momentum distribution of an excited BEC by free expansion: coupling with collective modes

We investigate the evolution of the momentum distribution of a Bose-Einstein condensate subjected to an external small oscillatory perturbation as a function of the in-trap evolution of the condensate after the external perturbation is switched-off. Besides changing its momentum distribution, we observe that the cloud distributes the input energy among its normal collective modes, displaying center-of-mass dipolar mode and quadrupolar mode. While the dipolar mode can be easily disregarded, we show that the momentum distribution is closely tied to the quadrupolar oscillation mode. This convolution hinders the actual momentum distribution.