Vibrational analysis of the emission and transient absorption bands of Br₂ in the region 2950-2410 Å

Abstract

Transient absorption of natural Br₂ in the region 2870-2410 Å and emission spectra of isotopically enriched ⁸¹Br₂ and natural Br₂ in the region 2950-2600 Å were photographed at reciprocal dispersions of 1.2, 2.3 and 2.5 Å mm⁻¹ respectively. Vibrational analysis of the bands observed in these spectra shows that they, as well as most of the bands obtained earlier by Venkateswarlu and Verma (VV) and Sur and Tellinghuisen (ST) in emission and by Briggs and Norrish (BN) in transient absorption could be understood as belonging to one system. Weighted least squares fit of the bands observed in the present experiments together with those of ST and BN yielded the following vibrational constants (in cm⁻¹) for ⁸¹Br₂ which are valid in the range 0≤ v' ≤ 43 and 0≤ v" ≤ 23: Δ T_e= 35703.16 (32), ω '_e≤ 148.97 (4), ω '_ex'_e=3.870(25)×10⁻¹, ω '_ey'_e=6.732 (469)×10⁻⁴, ω _e=159.79 (15), ω "_ex"_e=1.516 (25), ω "_ey"_e= −4.918 (165)×10⁻² and ω "_ez"_e=6.599 (353)×10⁻⁴. Probable assignments of the two states are discussed. The transition probably involves for its lower state the metastable σ ²_gπ ⁴_uπ³_gσ _u³∏ (2_u) suggested by Rao in transient absorption and by Tellinghuisen in emission, the upper state being a 2_g state. The study indicates the probable overlap in emission of the neighboring transitions ³∏ (1_u)−1_g and ³∏(0⁺_u)-0⁺_g.