The cooling of the electrons and thereby the heating of the lattice takes place
on a 1-ps timescale. The lattice will undergo thermal expansion, but the
timescale of expansion is in the order of tens of picoseconds (for particles
larger than 30 nm). Lattice heating is therefore much faster than lattice ex-
pansion, which will lead to an overshoot of the lattice expansion [43]. Addi-
tionally, due to the strong and fast increase of the electron temperature, there
is a sudden increase of the electron pressure, which induces a force on the lat-
tice and contributes to the sudden lattice expansion [44]. This has the same
effect on the nanoparticle as the beat of a clapper has on a bell. The particle
will start to 'ring'; a periodic expansion of the lattice is launched, which is
called an acoustic vibration. The timescale of the vibration is directly related
to the sound velocity of the material of the particle and to its size