Knowledge Bank

New observations of the $b\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Pi u0$ state of $K2$ have been made by laser induced fluorescence spectroscopy, following single colour OODR excitation with a cw Ti:sapphire laser. Fourier transform spectra revealed two pieces of a progression of the $2\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Pi g\to b\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Pi u0$ system, beginning around 14250 $cm-1$, at $v=0$, and terminating at 7200 $cm-1$. The two pieces were unfortunately separated by very strong $A\to X$ emission, which dominated the region $8000-12000cm-1$, so that no measurements could be made for many intermediate vibrational levels. The triplet emission in the visible region was severely affected by spin-orbit interactions with $A1\Sigma u+$, whilst the infrared part of the spectrum was much less perturbed. The intermediate part of the potential is known only through the interactions of the $A1\Sigma u+$ and $b\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Pi u$ states. The spin-orbit interaction between the A and b states up to $vb=90$ has been modelled using an ab initio calculation of the spin-orbit function, which shows a notable dip as a function of R. This model allowed us to assign the vibrational quantum numbers in the infrared part of the fluorescence spectrum : the observations cover $89\le v\le 118$. We are fitting the available A and b state data to $Yij$ parameters. At present, we have achieved fits (with a standard deviation of $0.025cm-1$) of about 1000 term values or combination differences, corresponding to levels up to $5500cm-1$ above the minimum of the b state. We intend to continue the fits over a further $1500cm-1$ to incorporate all the data, reaching term values $500cm-1$ below the 4S + 4P threshold. An asymptotic calculation of the Hund's case c) $0u+$ states should be able to extend a deperturbed potential curve for the b state sufficiently accurately to allow the assignment of existing photoassociation data.