  We applied Time-Resolved Two-Photon Photoemission spectroscopy to probe the
electron- phonon (e-ph) coupling strength in double-wall carbon nanotubes. The
e-ph energy transfer rate G(Te,Tl) from the electronic system to the lattice
depends linearly on the electron (Te) and lattice (Tl) temperatures for
Te>{\Theta}Debye. Moreover, we numerically solved the Two-Temperature Model. We
found: (i) a Te decay with a 3.5 ps time constant and no significant change in
Tl; (ii) an e-ph coupling factor of 2x10^16 W/m3; (iii) a mass-enhancement
parameter, {\lambda}, of (5.4=+/- 0.9)x10e-4; and (iv) a decay time of the
electron energy density to the lattice of 1.34+/-0.85 ps.
