Vibrational structure in atomic emission spectra: Hg atoms in molecular matrices

Mercury atoms excited to the P13 state in rare gas and molecular (CH4, CD4, CF4, and SF6) matrices relax by a direct P13→S01 fluorescence with a ca. 60 ns lifetime or by the P13⇝P03 radiationless transition followed by the P03→S01 long-lived (τ in the 10 to 500 ms range) emission. The main difference between atomic and molecular hosts is the appearance, in the latter case, of a well resolved vibrational structure in the spectrum of the P03→S01 transition involving the frequencies of nontotally symmetric modes of the host molecules. This structure, related to a shortening of the lifetime of the P03→S01 emission is explained by the intensity borrowing from the allowed P13→S01 transition, i.e., by the P13↔P03 mixing induced by nontotally symmetric vibrations of neighbor molecules (analog of the Herzberg–Teller effect). Relative intensities of bands corresponding to different vibrational modes may be deduced from estimation of electric fields induced in the Hg atom by displacements of electric charges of the vibrating molecule