VERY HIGH-RESOLUTION SPECTROSCOPY OF HIGH RYDBERG STATES OF THE ARGONATOM

Very high resolution spectra of high Rydberg states of the argon atom with principal quantum numbers in the range n = 60-200 have been measu red in double-resonance experiments using a high resolution vacuum ult raviolet laser-and frequency Stabilized, millimeter Waves; The 250 kHz resolution achieved in the double-resonance spectra enables the deter mination bf accurate effective: quantum numbers and the precise measur ement of fine-structure intervals in l = 0-3 Rydberg;states at n value s much beyond 50. The high resolution is also used to detect spectral shifts induced by small electric fields. Analysis of these spectral sh ifts allows the determination of stray electric fields with uncertaint ies of-less than 1 mV/cm and their compensation to less than 1 mV/cm. The spectra of high Rydberg states are very strongly influenced by exp erimental conditions and the highest resolution can only be obtained w hen-the stray electric fields are reduced to less than l mV/cm and the intensity of the millimeter waves are reduced to much less than 1 mu W/cm(2). High resolution measurements on the very high Rydberg states probed; by pulsed-field-ionization zero-kinetic-energy (PFI-ZEKE) phot oelectron spectroscopy are also reported. These-spectra are strongly i nfluenced by the inhomogeneous electric held of ions and other Rydberg states located in the photoexcitation region. These fields induce a s trong mixing of the optically accessible low-l Rydberg states with non penetrating high-l Rydberg states. These;results conclusively demonstr ate the important role played by ions in PFI-ZEKE spectroscopy. (C);19 98 American Institute of Physics.