Knowledge Bank

We have built an apparatus to study the rotationally resolved photoelectron spectroscopy of small molecules and radicals. Molecules are prepared in a skimmed supersonic molecular beam and are excited to very high metastable Rydberg states ($n>200$). These states are then field ionized by the application of a small pulsed electric field (of the order of 0.4 V/cm) to the interaction region at times typically 1$\mu$s subsequent to excitation. Excitation is by means of a single photon transition in the extreme ultra-violet region of the spectrum using narrow bandwidth, coherent XUV radiation generated by four wave sum mixing in a pulsed jet of either krypton or xenon gases. Photon energies between 12 and 17 cV are obtained. Rotationally resolved threshold photoelectron spectra have been obtained for carbon monoxide ionized to various vibrational levels of the $CO+$ ion in both its ground electronic state, $X2\Sigma +$, and its first excited state, $A2\Pi$. Rotational transitions are observed which involve large changes in ion core rotation. $\Delta$J $\ge$ 4.5, associated with photoelectron partial waves with high orbital angular momentum. The intensities of the ratational branches have been calculated using the model developed by $Buckingham1$ in good agreement with experiment. The technique has been extended to the photoionization of he linear triatomic molecule $N2O$ to the first excited state of the $N2O+$ ion. $A2\Sigma +$. Work on other molecules is in progress with particular emphasis on excited electronic stats of the ion core.