Photodissociation of PbI₂ in the ultraviolet: Analysis of the A → X band of PbI

Emission and absorption studies of lead moniodide (PbI) have been carried out by photodissociation of PbI₂ vapor at one of several wavelengths (193, 248, 266, 308, and 351 nm) in the ultraviolet. Strong emission on the A → X ²Π_1/2 band (14 400-22 800 cm^-1; 440 ≲ λ ≲ 695 nm) occurs when PbI₂ is photodissociated at 248 or 266 nm. Also, absorption bands attributed to the B ← X ²Π_1/2,3/2, D ← X ²Π_1/2,3/2, and E ← ²Π_1/2,3/2 transitions of the diatomic molecule have been observed at 290 and 380, 225 and 265, and 203 and 240 nm, respectively, as have emission bands peaking at 397.6, 531.6, 582.0, 595.1, 639.7, 685.9, and 707.9 nm that appear to arise from PbI₂ itself. Analysis and computer simulations of the A → X ²Π_1/2 emission spectra have resulted in identifications for virtually all (>120) of the observed vibrational bandheads. Several spectroscopic constants for the A and the X ²Π_1/2 states of PbI have been determined to be T_e(A) = 20 659 ± 130 cm^-1, ω_e′ = 132.2 ± 1.0 cm^-1, ω_e′X_e′ = 1.91 ± 0.06 cm^-1, ω_e″ = 160.3 ± 0.6 cm^-1, and ω_e″x_e″ = 0.24 ± 0.03 cm^-1. Also, the difference between the equilibrium internuclear separations for the X ²Π_1/2 and the A states has been determined to be ΔR_e = 0.45 ± 0.05 Å. The spontaneous-emission lifetime for the PbI(A) state and the rate constant for quenching of this state by PbI₂ (in two-body collisions) have been measured to be (94.3 ± 8.8) ns and (4.3 ± 0.4) × 10-¹⁰ cm³ s^-1, respectively.