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Electric field ionization of excited Na atoms by microwave fields between 12 and 15 GHz exhibits several interesting phenomena. The $|m|=0 \mathrm{and} 1$ states may be ionized with a very low microwave field, $\ensuremath{\sim}\frac{1}{{3n}^{5}}$, corresponding to the point at which the $n$ and $n+1$ Stark manifolds of principal quantum numbers $n$ and $n+1$ intersect. The ionization occurs by an atom's making a succession of Landau-Zener transitions to higher-lying states, finally reaching the classical ionization limit. The $|m|=2$ states, on the other hand, ionize at a very high microwave field, $\ensuremath{\sim}\frac{1}{{9n}^{4}}$. Both these observations and the more familiar static or low-frequency result of ionization by a field of $\ensuremath{\sim}\frac{1}{{16n}^{4}}$ are all consistent with ionization in a completely classical fashion. The apparent differences are due to the dynamics of passing from zero field to a high ionizing field.

We report new, more extensive photoexcitation spectra of Ba autoionizing states which demonstrate that appearent interferences between discrete autoionizing states are in fact not interferences as previously proposed. A simple alternative interpretation of the "interference"-like line shapes as being due to variations in the spectral density of the autoionizing states and their overlap integrals with the bound initial state is presented and shown to agree with experiment. We argue that such spectra allow perhaps the most accurate determination of quantum defects, and should be a useful spectroscopic probe of autoionizing states.

A large enhancement in the resonant collisional energy-transfer process Na(ns)+Na(ns) ..-->..Na((n-1)p)+Na(nl) or Na(ns)+Na(ns)..-->..Na(np)+Na((n-1)l) is observed when the initial ns state lies midway between (n-1)p and an nl state or between np and an (n-1)l state. This resonance condition, achieved by the electric field tuning of the levels, leads to large cross sections approx.10/sup 9/ A/sup 2/ and narrow linewidths approx.1 GHz. These cross sections and linewidths are comparable to the previously observed resonant process Na(ns)+Na(ns)..-->..Na((n-1)p)+Na (np). We have also observed a similar collisional energy transfer starting from a completely different initial state, i.e., Na(15d)+Na(15d)..-->..Na(16f)+Na (15p). In this case the enhancement in the cross section (approx.10/sup 8/ A/sup 2/) is observed at a relatively low electric field approx.17 V/cm.

We describe the experimental observation of the resonant collisional energy transfer Na(ns)+Na(ns)+qh\ensuremath{\nu}\ensuremath{\rightarrow}Na(np)+Na[(n-1)p]+(q○)h\ensuremath{\nu} which occurs when the Na atoms in a strong microwave field are tuned in energy with a static field so that the energy resonance ${E}_{\mathrm{ns}}$+${E}_{\mathrm{ns}}$=${E}_{\mathrm{np}}$+${E}_{n\mathrm{\ensuremath{-}}1}$,p\ifmmode\pm\else\textpm\fi{}mh\ensuremath{\nu} occurs. We describe our experimental observations and provide a detailed description of a nonperturbative theory which we have developed to explain our observations. The theory is essentially developed in terms of the resonant dipole-dipole collisions between ``microwave dressed'' atoms. Using an extension of the approach we used previously to describe resonant dipole-dipole collisions in the absence of microwaves, we are able to take into account all the coherences in the problem in a completely straightforward way.