Bound-free spectroscopy ofCs2+

Bound-free transitions of ${\mathrm{Cs}}_2^{+}$ are studied in a fast-ion-beam photofragment spectrometer. Molecular cesium ions are produced by field ionization on a liquid cesium droplet and extracted as an ion beam. Their photodissociation is observed by monitoring the appearance and energy distribution of the charged photofragments ${\mathrm{Cs}}^{+}$. In the wavelength range between 4067 and 9500 Å, three photodissociation channels are identified of the type $\mathrm{Cs}_2^{}{}_{}{}^{+}\left(X{}_{}{}^2{}_{}{}^{}\Sigma _g^{+}{}_{}{}^{}\right)+h\nu \to \mathrm{Cs}_2^{}{}_{}{}^{+*}\to {\mathrm{Cs}}^{+}\left({}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}\right)+\mathrm{Cs}\mathrm{}\left(\mathrm{nl}\right)+E_{\mathrm{kin}}$. At the longest wavelengths studied, photodissociation produces ground-state atoms $\mathrm{Cs}6s$. In the range between 5309 and 4545 Å, excited-state atoms $\mathrm{Cs}6p$ are produced. Beginning at 4579 Å, photodissociation leads to formation of both $\mathrm{Cs}6p$ and $\mathrm{Cs}5d$. The $\mathrm{Cs}5d$ channel dominates at wavelengths around 4100 Å. The angular distribution of photofragments is investigated to help identify the excited molecular states involved in the photodissociation processes. An approximate relative cross section for photodissociation of ${\mathrm{Cs}}_2^{+}$ over this wavelength range is obtained. At 7992.2 Å the absolute cross section for photodissociation of ${\mathrm{Cs}}_2^{+}$ is measured to be 2×${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$. Our measurements establish, for the first time, a lower limit for the bond energy of $\mathrm{Cs}_2^{}{}_{}{}^{+}\left(X{}_{}{}^2{}_{}{}^{}\Sigma _g^{+}{}_{}{}^{}\right)$ of 0.59±0.06 eV. This places an upper limit of 3.76±0.06 eV for the ionization potential of ${\mathrm{Cs}}_2\left(X{}_{}{}^1{}_{}{}^{}\Sigma _g^{+}{}_{}{}^{}\right)$.