An apparent predissociation rate k(pre) of 0.06 ps(-1) at 10 K which rises to 0.11 ps(-1) at 40 K is observed in pump-probe spectra of the populations arriving in predissociated states. We show that the observed rate is determined by a balance of the calculated energy-dependent predissociation rate k(p)(E), which rises in approaching the relevant curve crossing from higher energies, and the vibrational relaxation rate k(vib), which funnels population toward this crossing. k(vib) is explored via several probe windows using fluorescence emitted by three different ionic states. k(vib) increases exponentially with energy above the B state potential minimum. Predissociation occurs in a region where k(vib) equals k(p). The increase of k(vib) with matrix temperature T explains the rise of k(p), at 40 K. Disorder induced by double doping furthermore increases k(vib). The deduced effective curve crossing matrix element of 15-20 cm(-1) is in accordance with calculations.