The authors measured the ultrafast response of triangular silver nanoparticles upon femtosecond excitation of their plasmon resonance. After a fast electron relaxation, the signature of a bimodal mech. vibration of the particle is apparent as a spectral modulation. The authors identify the two lowest frequency, totally sym. vibrations of the particle as responsible for this modulation, through their influence on the plasmon peak position and width, in full agreement with the results of a variational elastodynamic model that is also presented. From the anal. of the phase the authors conclude that thermal expansion and electron pressure, resp., are responsible for the excitation of the two vibrations. [on SciFinder (R)]