Knowledge Bank

Recently, two studies have been devoted to the low-lying electronic states of Cl$\mathrm{2+}$, one by PFI-ZEKE photoelectron (PE) spectroscopy of the $X+\leftarrow X$ transition, \emph{J.\ Chem.\ Phys.} \textbf{127}, 104307 (2007).} and one by laser-induced-fluorescence spectroscopy of the $A+\to X+$ band system, \emph{66 Int.\ Symposium on Molecular Spectroscopy}, Book of Abstracts, p.\ 169 (2011).}. To complement the information available on the X$+$ and A$+$ electronic states of Cl$\mathrm{2+}$, we have recorded partially rotationally resolved single-photon PFI-ZEKE PE spectra of the $X+ 2\Pi g,i\leftarrow X 1\Sigma g+$ and $A+ 2\Pi u,i\leftarrow X 1\Sigma g+$ ($i=3/2,1/2$) photoionizing transitions of Cl$2$ in the wavenumber ranges $92500-96500 \mathrm{cm}-1$ and $106750-115500 \mathrm{cm}-1$. These regions correspond to transitions to low-lying vibrational levels of the X$+$ state with $v+=0-5$ and to transitions to vibrational levels of the X$+$ state with $v+=25-45$ and vibrational levels of the A$+$ state with $v+\le 7$.\ The analysis of the rotational structure and the isotopic shifts of these spectra has enabled the derivation of an improved value of the first adiabatic ionization energy ($92647.7\pm 0.3 \mathrm{cm}-1$ for $\mathrm{<mrow\; data-mjx-texclass="ORD">35</mrow>}$Cl$2$). In combination with measurements of ion-pair states with $n\approx 1800$, \emph{Phys.\ Rev.\ A} \textbf{82}, 032510 (2010).}, new values for the dissociation energies $D0$ of Cl$2$ and Cl$\mathrm{2+}$ could also be derived. The potential energy function of the X$+$ state of Cl$\mathrm{2+}$ was determined in a least-squares fitting procedure. Spin-orbit splittings were derived for many vibrational levels of the X$+$ and A$+$ states. Combining our results with other results, \emph{Chem.\ Phys.} \textbf{83}, 203 (1984).}$\mathrm{<mrow\; data-mjx-texclass="ORD">,</mrow>}$, \emph{J.\ Mol.\ Spect.} \textbf{230}, 72 (2005).}$\mathrm{<mrow\; data-mjx-texclass="ORD">,</mrow>}$, \emph{Chem.\ Phys.} \textbf{89}, 313 (1984).}, several low-lying vibrational levels of the upper spin-orbit component of the A$+$ state could be assigned for the first time. The PFI-ZEKE PE spectra also contain a series of as yet unassigned lines and reveal numerous perturbations.