  We use time- and angle-resolved photoemission to measure quasiparticle
relaxation dynamics across a laser-induced superconducting phase transition in
Bi2Sr2CaCu2O8+delta. Whereas low-fluence measurements reveal picosecond
dynamics, sharp femtosecond dynamics emerge at higher fluence. Analyses of data
as a function of energy, momentum, and doping indicate that the closure of the
near-nodal gap and disruption of macroscopic coherence are primary mechanisms
driving this onset. The results demonstrate the important influence of
transient electronic structure on relaxation dynamics, which is relevant for
developing an understanding of nonequilibrium phase transitions.
