  We measured the fluorescence photon yield of neon upon soft x-ray ionization
(~1200 eV) from the x-ray free electron laser at Linac Coherent Light Source,
and demonstrated the usage of a grazing incidence spectrometer with a variable
linespacing grating to perform x-ray fluorescence spectroscopy on a gas phase
system. Our measurements also allowed us to estimate the focal size of the beam
from the theoretical description developed, in terms of the rate equation
approximation accounting for photoionization shake-off of neutral neon and
double Auger decay of single core holes.
