We report the first femtosecond spectroscopic investigation of ultrafast electronic processes in $a$-Si:H. By tuning the probe wavelength in a wide range around the optical gap, we are able to follow the time evolution of carrier trapping in band-tail states. At moderate injected carrier density, the band-tail states are populated by multiple trapping. As more carriers are injected, the shallow band-tail states saturate and deeper band-tail states capture carriers by direct trapping, which is a faster process (1-2 ps) than multiple trapping (10 ps).

• Received 1 August 1986

DOI:https://doi.org/10.1103/PhysRevLett.57.2438