Knowledge Bank

Ab initio configuration interaction calculations using relativistic effective core potentials and flexible basis sets have been used to study and assign the electronic spectrum of $HgCl2$. The lowest dipole-allowed transition is $1I\Sigma +\to 11\Pi u$, which corresponds to a charge transfer from a $C13p\Pi$ orbital to the Hg 6s-like orbital. Upon dissociation this state correlates with $HgCl(A2\Pi )+Cl(2P)$. The second stronger dipole-allowed transition is $11\Sigma g++11\Sigma u+$, which corresponds to a charge transfer from a Cl $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}p\Pi$ orbital to a Hg $\mathrm{<mrow\; data-mjx-texclass="ORD">6</mrow>}p\Pi$ orbital. The $11\Sigma u+$ state will dissociate to $HgCl(B2\Sigma +)+Cl(2P)$. This explains why excitation of the second band ($\sim 180$ nm) in $HgCl2$ leads to $fluorescence2$ or $lasing3$ on the $B2\Sigma +\to X2\Sigma +$ transition in HgCl. The effect of bending on the energies of the excited $11\Pi u$ and $ll\Sigma u+$ states in $HgCl2$ is investigated and the implications for the photodissociation process are considered.