Global empirical potentials from purely rotational measurements

The recent advent of chirped-pulse FTMW technology has created a plethora of pure rotational spectra for molecules for which no vibrational information is known. The growing number of such spectra demands a way to build empirical potential energy surfaces for molecules, without relying on any vibrational measurements. Using ZnO as an example, we demonstrate a powerful technique for efficiently accomplishing this. We first measure eight new ultra-high precision ($pm2$ kHz) pure rotational transitions in the $X$-state of ZnO. Combining them with previous high-precision ($pm50$ kHz) pure rotational measurements of different transitions in the same system, we have data that spans the bottom 10% of the well. Despite not using any vibrational information, our empirical potentials are able to determine the size of the vibrational spacings and bond lengths, with precisions that are more than three and two orders of magnitude greater, respectively, than the most precise empirical values previously known, and the most accurate emph{ab initio} calculations in todays reach. By calculating the $C_{6},$ $C_{8},$ and $C_{10}$ long-range constants and using them to anchor the top of the well, our potential is emph{globally} in excellent agreement with emph{ab initio} calculations, without the need for vibrational spectra and without the need for emph{any} data in the top 90% of the well.

High resolution photoassociation spectroscopy of the $^{6}$Li$_2$ $A(1^1Sigma_u^+)$ state

We present spectroscopic measurements of seven vibrational levels $v=29-35$ of the $A(1^1Sigma_u^+)$ excited state of Li$_2$ molecules by the photoassociation of a degenerate Fermi gas of $^6$Li atoms. The absolute uncertainty of our measurements is $pm 0.00002$ cm$^{-1}$ ($pm 600$ kHz) and we use these new data to further refine an analytic potential for this state. This work provides high accuracy photo-association resonance locations essential for the eventual high resolution mapping of the $X(1^1Sigma_g^+)$ state enabling further improvements to the s-wave scattering length determination of Li and enabling the eventual creation of ultra-cold ground state $^6$Li$_2$ molecules.