  We experimentally study the excitation modes of bright matter-wave solitons
in a quasi-one-dimensional geometry. The solitons are created by quenching the
interactions of a Bose-Einstein condensate of cesium atoms from repulsive to
attractive in combination with a rapid reduction of the longitudinal
confinement. A deliberate mismatch of quench parameters allows for the
excitation of breathing modes of the emerging soliton and for the determination
of its breathing frequency as a function of atom number and confinement. In
addition, we observe signatures of higher-order solitons and the splitting of
the wave packet after the quench. Our experimental results are compared to
analytical predictions and to numerical simulations of the one-dimensional
Gross-Pitaevskii equation.
