  Laser induced thermal desorption of Xe atoms into vacuum from a metal surface
following the nano-second pulsed laser heating was investigated by the
time-of-flight (TOF) measurement. The desorption flow was studied at a wide
range of desorption flux by varying the initially prepared Xe coverage {\Theta}
(1 ML = 4.5*10^18 atoms/m^2). At {\Theta} = 0.3 ML, the TOF of Xe was well
represented by a Maxwell-Boltzmann velocity distribution, which is in good
agreement with thermal desorption followed by collision-free flow. At {\Theta}
> 0.3 ML, the peak positions of the TOF spectra were shifted towards the
smaller values and became constant at large {\Theta}, which were well fitted
with a shifted Maxwell-Boltzmann velocity distribution with a temperature T_D
and a stream velocity u. With T_D fixed at 165 K, u was found to increase from
80 to 125 m/ s with increasing {\Theta} from 1.2 to 4 ML. At {\Theta} > 4 ML,
the value of u become constant at 125 m/ s. The converging feature of u was
found to be consistent with analytical predictions and simulated results based
on the Knudsen layer formation theory. We found that the Knudsen layer
formation in laser desorption is completed at Knudsen number Kn <0.39.
