Quantum Yield for Unimolecular Dissociation of I2 in Visible Absorption

An atomic fluorescence technique has been used to observe iodine atoms under conditions of steadystate irradiation of I₂ between 5000 and 6300 Å. Photodissociation quantum yields are reported for narrowband excitation at 12 wavelengths within this range. The yield exceeds 90% at both ends of this region with a minimum of 33% near 5900 Å and a subsidiary maximum of 72% at 5460 Å. These results are further resolved into contributions from direct dissociation via the ${\text{A[1u(}}^3{\mathrm{\Pi \; )]\leftarrow \; X[O}}^{+}{\mathrm{g(}}^1\mathrm{\Sigma \; )]}$ and ${\text{1u(}}^1\mathrm{\Pi \; )\leftarrow \; X}$ transitions, and a spontaneous predissociative decay, ${\mathrm{B[O}}^{+}{\mathrm{u(}}^3{\text{Π )]→ 1u(}}^1\mathrm{\Pi \; )}$ . The rate for this predissociation fluctuates with v′ in a manner which accounts for the previously reported erratic behavior of the B‐state lifetime. The total radiative decay rate varies smoothly but strongly with v′, implying that the $\mathrm{B\leftrightarrow \; X}$ electronic transition moment attains a maximum for some internuclear distance in the region 3.0–3.2 Å.