Level-Crossing Studies of Na2 Using Laser-Induced Fluorescence

The technique of using laser excitation to study level crossings in molecules has been developed and used to determine the radiative lifetime of an excited state of Na₂. Optical detection of level crossing in the $\text{υ′\hspace{0.17em}=\hspace{0.17em}10, J′\hspace{0.17em}=\hspace{0.17em}12}$ level of the $B^1{\Pi }_u$ electronic state of Na₂, excited by the 4765‐Å argon‐ion laser line, has resulted in a precise measurement of the product ${\text{gτ\hspace{0.17em}=\hspace{0.17em}4.11\hspace{0.17em}±\hspace{0.17em}0.12\hspace{0.17em}×\hspace{0.17em}10}}^{\text{−11}}\sec$ , corresponding to a magnetic field half‐width of 1385 ± 42 G. If the $g$ value is calculated assuming Hund's coupling case (a), the radiative lifetime is determined to be ${\text{τ\hspace{0.17em}=\hspace{0.17em}6.41\hspace{0.17em}±\hspace{0.17em}0.38\hspace{0.17em}×\hspace{0.17em}10}}^{\text{−9}}\sec$ . The influence of molecular hyperfine structure on this measurement is discussed and is found in general to contribute little uncertainty to the value of the radiative lifetime, provided the rotational angular momentum is much larger than the nuclear spin and/or provided the hyperfine interaction is much smaller than the natural width of the excited state.