  The dynamics of thermal and non-thermal lattice deformation of nanometer
thick polycrystalline aluminum film has been studied by means of femtosecond
(fs) time-resolved electron diffraction. We utilized two different pump
wavelengths: 800 nm, the fundamental of Ti: sapphire laser and 1250 nm
generated by a home-made optical parametric amplifier(OPA). Our data show that,
although coherent phonons were generated under both conditions, the diffraction
intensity decayed with the characteristic time of 0.9+/-0.3 ps and 1.7+/-0.3 ps
under 800 nm and 1250 nm excitation, respectively. Because the 800 nm laser
excitation corresponds to the strong interband transition of aluminum due to
the 1.55 eV parallel band structure, our experimental data indicate the
presence of non-thermal lattice deformation under 800 nm excitation, which
occurs on a time-scale that is shorter than the thermal processes dominated by
electron-phonon coupling under 1250 nm excitation.
