  In order to determine the role of non-thermal transport of hot carriers which
is decisive for the dissipation of energy in condensed matter we performed
time-resolved broadband femtosecond transient reflectivity measurements on
$7-197 \mathrm{nm}$ thick Bi(111) films epitaxially grown on Si(111). We
monitored the behavior of the Fourier amplitude and the central frequency of
the coherent $A_\mathrm{1g}$ phonon mode as function of the incident fluence,
film thickness, and probe wavelength in the range of $580 -700 \mathrm{nm}$.
The frequency redshift that follows photoexcitation was used as a robust
quantity to determine the effective distribution of excited carriers that
governs the displacive excitation mechanism of coherent $A_\mathrm{1g}$ phonons
in Bi. For Bi films up to $50 \mathrm{nm}$ thickness a homogeneous excitation
due to the ultrafast transport of hot charge carriers is observed, limited by a
carrier penetration depth of $60 \mathrm{nm}$ independent of the totally
deposited laser energy.
