  We review some known properties of the "emptiness formation probability"
correlation which we have calculated numerically for spin-1/2 XX chains with
constant (homogeneous) or alternating (dimerized) nearest-neighbor coupling and
an external field (in $z$ direction) for arbitrary temperature. The
long-distance asymptotic behavior of this correlation is known to be Gaussian
at zero temperature and exponential at finite temperature for the homogeneous
chain. By simple analytical arguments the exponential behavior at finite
temperature extends to the dimerized system. Numerical results for the
dimerized chain confirm the exponential decay at finite temperature and show
Gaussian decay at zero temperature.
