  We have used transient absorption spectroscopy to measure carrier lifetimes
in the multiband band semiconductor GaPAsN. These measurements probe the
electron populations in the conduction band, intermediate band and valance band
as a function of time after an excitation pulse. Following photoexcitation of
GaP0.32As0.67N0.01 we find that the electron population in the conduction band
decays exponentially with a time constant 23ps. The electron population in the
intermediate band exhibits bimolecular recombination with recombination
constant r = 2 10^-8 cm-3/s. In our experiment an optical pump pulse excited
electrons from the valance band to the intermediate and conduction bands, and
the change in interband absorption due to absorption saturation and induced
absorption was probed with a delayed white light pulse. We modeled the optical
properties of our samples using the band anti-crossing model to extract carrier
densities as a function of time. These results indicate that the minority
carrier lifetimes are too short for efficient solar power conversion and that
improvements in material quality will be required for practical applications of
GaPAsN based intermediate band solar cells.
