HIGH-RESOLUTION SPECTROSCOPY OF {$n$p} RYDBERG STATES OF He$_2$: 1. RYDBERG-STATE-RESOLVED THRESHOLD IONIZATION SPECTRA OF METASTABLE He$_2$

{M. Raunhardt, M. Schafer, N. Vanhaecke, F. Merkt{R. Seiler, U. Hollenstein, G. M. Greetham, F. Merkt{R. Seiler, Th. A. Paul, M. Andrist, F. Merkt{M. Raunhardt, M. Schafer, N. Vanhaecke, F. MerktAuthor Institution: ETH Zurich, Laboratorium fur Physikalische Chemie; Wolfgang-Pauli-Strasse 10, 8093 Zurich, SwitzerlandA supersonic beam of metastable He$_2^*$ a $^3\Sigma_{\rm u}^+$ molecules was generated using a pulsed discharge at the exit of a pulsed valve prior to the gas expansion into vacuum.% , \emph{J. Chem. Phys.} \textbf{128}, 164310 (2008).} Transitions to high $n$p Rydberg states were recorded using photoionization and Rydberg-state-resolved threshold ionization spectroscopy.% , \emph{Chem. Phys. Lett.} \textbf{346}, 201 (2001).} Overview scans at moderate resolution (0.3 cm$^{-1}$) were obtained with ionization fields ranging from 1.3 to 133 V/cm, lowering the ionization thresholds by 5.5 and 55 cm$^{-1}$, respectively. Using a solid-state UV laser system% , \emph{Rev. Sci. Instr.} \textbf{76}, 103103 (2005).} with a 20 MHz bandwidth, high-resolution spectra of Rydberg series with $n$ up to 150 and with resolved fine structure of the initial He$_2^*$ a $^3\Sigma_{\rm u}^+$ ($N''$) state were recorded. The assignment of the observed Rydberg states is based on multichannel quantum defect theory calculations from a recent study$^a$ of pulsed-field-ionization zero-kinetic-energy (PFI-ZEKE) photoelectron and photoionization spectra of He$_2$ (see following talk). %, %\emph{J. Chem. Phys.} \textbf{128}, 164310 (2008).}. The extrapolation of the observed Rydberg series to their limits enabled the determination of the ionization energy of the a $^3\Sigma_{\rm u}^+$ state and the rotational structure of the He$_2^+$ ion with a precision of better than 20 MHz