Knowledge Bank

Radiationless transitions of isolated molecules have been observed using coaxial laser and molecular beams. A molecular beam is periodically irradiated along its axis at 5145 {\AA} with a tunable argon-ion laser. Highly monochromatic radiation ($\Delta \lambda —\sim 10-4$ {\AA}), obtained by exciting a single axial mode of the laser, produces molecules in selectively excited states with well characterized energies. Transitions of the laser excited beam molecules leading to spontaneous predissociation and other unimolecular processes are detected as a modulation of the molecular beam. In initial experiments with isotopic bromine we are able to identify spontaneous predissociation of the B($\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Pi ou+$) state as well as direct photodissociation processes. The peak in the $Br2$ modulated mass signal amplitude corresponding to a predissociation process have a width of approximately 500 MHz, and occur at wavelengths which produce selective excitation of the individual isotopic species. The measured predissociation to direct photodissociation cross-section ratio, $\sigma p/\sigma d$ is $103$. For $\mathrm{<mrow\; data-mjx-texclass="ORD">79,79</mrow>}$ $Br2$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">81,81</mrow>}Br2$ predissociation cross-section is of the order of $6\times 10-17cm2$. A relatively stronger predissociation signal is observed for the mixed isotope$\mathrm{<mrow\; data-mjx-texclass="ORD">79,81</mrow>}Br2$.