  We report new time-resolved data of quasiparticle relaxation and Cooper pair
recombination dynamics in YBa_2Cu_3O_{7-\delta}, measured as a function of
temperature and doping \delta using femtosecond optical spectroscopy. The data
show the existence of a normal state pseudo-gap for in-plane charge excitations
below T* and an unusual divergence of the quasiparticle relaxation time in the
superconducting state \tau_s with \delta towards optimum doping. In the
underdoped state, no change in the amplitude of the induced transmission (which
is proportional to the DOS at E_F), or relaxation time \tau_s is observed at
T_c. From the sum rule, both observations signify that no gap opens at T_c in
underdoped YBCO for \delta >0.15. T_c in this case only signifies the onset of
phase coherence. The presented data thus suggest pair formation with an
associated redistribution of the DOS starting at T* and the establishment of
phase coherence at T_c consistent with Bose-Einstein condensation. In the
optimally doped material, \delta ~ 0.1, on the other hand, both a divergence in
lifetime and a change of the DOS occur at T_c, signifying the opening of a gap
and the occurence of pairing takes place simultaneously.
