Photoluminescence excitation (PLE) spectra at 77 K have been measured over the range 1.201.75 eV using the Ti sapphire cw tunable laser as the excitation source. Two undoped a-Si:H samples on rough substrates have been investigated. The first sample has a very high dangling bond (DB) density; the second one has low DB density. The PLE spectrum of photoluminescence (PL) at 0.8 eV for the first sample follows the shape of the absorption spectrum measured by photothermal deflection spectroscopy (PDS) at room temperature. This behavior can be understood within the context of the existing models as due to recombination through defects which produces PL centered around 0.8 eV. However the PLE spectrum of PL at 0.8 eV for the second sample drops very rapidly with decreasing energy for energies less than about 1.3 eV. This behavior, which differs dramatically from that of the absorption spectrum, is consistent with earlier results and suggests that the PL measured at 0.8 eV for the second sample may be largely due to a contribution of the tail of the PL band which peaks near 1.3 eV. The PLE spectra for PL at 1.0 eV and 1.1 eV for the second sample approach the PLE spectrum previously obtained using the integrated PL intensities.