  The probability of emission of a single photon via Raman scattering of laser
pulse on the three-level $\Lambda$ - type atom in microcavity is studied. The
duration of the pulse is considered to be short enough, so that the hyperfine
structure of the upper level remains totally unresolved, while that of the
lower level is totally resolved. The coherent laser pulse is assumed to be in
resonance with the transition between one hyperfine structure component of the
lower atomic level and all hyperfine structure components of the upper level,
while the quantized cavity field is assumed to be in resonance with the
transition between the other hyperfine structure component of the lower level
and all components of the upper one. The dependence of the photon emission
probability on the mutual orientation of polarization vectors of the cavity
mode and of the coherent laser pulse is analyzed. Particularly, the case is
investigated, when the total electronic angular momentum of the lower atomic
level equals 1/2, which is true for the ground states of alkali atoms employed
in the experiments on deterministic single photon emission. It is shown, that
in this case the probability of photon emission equals zero for collinear
polarizations of the photon and of the laser pulse, and the probability obtains
its maximum value, when the angle between their polarizations equals 60
degrees.
