  We demonstrate selective excitation of a spin wave mode in a ferromagnetic
(Ga,Mn)As film by picosecond strain pulses. For a certain range of magnetic
fields applied in the layer plane only a single frequency is detected for the
magnetization precession. We explain this selectivity of spin mode excitation
by the necessity of spatial matching of magnon and phonon eigenfunctions, which
represents a selection rule analogous to momentum conservation for
magnon-phonon interaction in bulk ferromagnetic materials.
