The object of the studies summarised in this paper are mercury atoms isolated in low temperature matrices. Beside traditional rare gas hosts, the interest was focused at molecular hosts such as nitrogen, methane etc. and at the mixed (rare gas/molecule matrices) as e.g. nitrogen/krypton, methane/argon or ammonia/argon. The techniques used are the high resolution infrared spectroscopy, the electronic (absorption and emission) spectroscopy in ultraviolet and photochemistry induced by uv laser irradiation and detected by ir absorption spectra.

Inclusion of molecular hosts allows observations of unexplored (or practically unexplored) processes:

-     infrared absorption forbidden in the neat host crystal and induced in the vicinity    of the guest atom (Hg/N₂ centres in N₂ matrices),

-     transfer of the electronic excitation of Hg to host vibrations monitored by the    infrared fluorescence in the same Hg/N₂  system,

-     breakdown of selection rules in the processes of radiative and nonradiative relaxation of electronically excited Hg atoms,

-     formation of complexes strongly bound in excited electronic states (exciplexes) involving Hg and such molecules as N_2, H₂O or NH₃.

The photochemical processes of insertion of the excited Hg atom into X-H bond were investigated for different partners: H₂, CH₄, SiH₄ and their deuterated analogs.