Measurement of Lifetimes and Quenching Cross Sections of the B State of I2

The phase‐shift method was used to measure lifetimes of selected vibrational regions from v′=10 to v′∼100, together with self‐quenching collision cross sections, of the $B^{{3}}{\Pi }_{0^{+}u}$ state of I₂. The reliability of the method was confirmed by good agreement with other measurements for the lifetime of the mercury ³P₁ state. At high optical densities, the observed increase in lifetime was shown to be due to radiation entrapment. Reasonable agreement was found between the lifetime values and absorption measurements, but, contrary to expectations, the self‐quenching cross sections varied only a factor of 1.3 with vibrational excitation whereas the lifetimes varied by as much as a factor of 8. The substantial variation of lifetime with v′, which cannot be merely a wavelength effect, indicates either that spontaneous predissociation of the B state of I₂ is an important process which has a rate at least as great as that for radiative decay and which varies considerably with v′ or that the transition moment is changing with vibrational stretching.