Multicomponent emission bands are recorded for the 3P1→1S0 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(3P1)/RG matrix systems. A detailed description of these modes and their energetics is presented in the paper directly following. Photoexcitation of the 3P1 state also yields small amounts of 3P0 state emission. Hg atom 3P1 to 3P0 state intramultiplet relaxation (IMR) is most efficient in Hg/Xe where the ratio of this relaxation channel to 3P1 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 3P0 state emission spectra largely free of the more intense 3P1 state emission. Such emission spectra recorded under high resolution for the 3P0→1S0 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.