  We show that light-induced atom desorption (LIAD) can be used as a flexible
atomic source for large Rb-87 and K-40 magneto-optical traps. The use of LIAD
at short wavelengths allows for fast switching of the desired vapor pressure
and permits experiments with long trapping and coherence times. The wavelength
dependence of the LIAD effect for both species was explored in a range from 630
nm to 253 nm in an uncoated quartz cell and a stainless steel chamber. Only a
few mW/cm^2 of near-UV light produce partial pressures that are high enough to
saturate a magneto-optical trap at 3.5 x 10^9 Rb atoms or 7 x 10^7 K atoms.
Loading rates as high as 1.2 x 10^9 Rb atoms/s and 8 x 10^7 K atoms/s were
achieved without the use of a secondary atom source. After the desorption light
is turned off, the pressure quickly decays back to equilibrium with a time
constant as short as 200 us, allowing for long trapping lifetimes after the MOT
loading phase.
