Excitation of the Gd(0001) surface state by 800 nm femtosecond laser pulses generates transient coherent surface and
bulk vibrations. These are observed by time-resolved linear and second harmonic reflectivity, simultaneously probing bulk
and surface dynamics. Initially, the surface vibration frequency of 2.8 THz lies ~18% below that of the bulk frequency.
On a picosecond time scale the surface vibration frequency increases and the bulk one shifts to a lower value. At delays
>2 ps these frequencies converge to an asymptotic value of 3.15 THz which agrees with theoretical predictions for the G
3 phonon mode. Both surface and bulk vibrations were found to persist only during non-equilibrium between electron and
lattice temperature and are damped within the electron-phonon relaxation time. We conclude that the observed frequency
shifts cannot be accounted for by anharmonic effects but originate from time-dependent changes of the potential energy
due to non-equilibrium electron distributions.
The publication is available at The American Physical Society- ©2004;
also cited in: Virt. J. Ultra Science 3, 7 (Jul 2004)