Multicomponent emission bands are recorded for the P-3(1)-->S-1(0) transition of atomic mercury isolated at single sites in solid Ar, Kr, and Xe matrices. A blueshift observed at elevated temperatures on the 273 nm emission of Hg/Xe is identified in line shape analysis as arising from decreasing intensity of the central component in the band profile. The origin of the multiple components in the emission bands is ascribed to the existence of several vibronic modes which lead to excited state stabilization in the Hg(P-3(1))/RG matrix systems. A detailed description of these modes and their energetics is presented in the paper directly following. Photoexcitation of the P-3(1) state also yields small amounts of P-3(0) state emission. Hg atom P-3(1) to P-3(0) state intramultiplet relaxation (IMR) is most efficient in Hg/Xe where the ratio of this relaxation channel to P-3(1) state radiative decay is 1/200 as established in time-integrated emission spectra. Despite the weakness of IMR, pulsed laser excitation combined with photon counting detection provide time-gated P-3(0) state emission spectra largely free of the more intense P-3(1) state emission. Such emission spectra recorded under high resolution for the P-3(0)-->S-1(0) transition of atomic mercury isolated in solid Xe provide the first example of the occurrence of a zero-phonon lines for a metal atom isolated in a rare gas matrix. Wp line shape analysis conducted on the emission bands recorded at specific temperatures, confirm this assignment. The electron-phonon coupling strength (Huang-Rhys, S factor) extracted in the line shape fits for the Hg/Xe transition is 1.3. Slightly stronger coupling is identified in Kr (S=2.2) and stronger still in Ar (S=3.3). Analysis of the diatomic Hg.RG potential energy curves reveal that the origin of the weak electron-phonon coupling lies primarily in the similarity in the ground and excited states, but also indicates the site size offered by the host solid plays a role. (C) 2003 American Institute of Physics.