EVIDENCE FOR COHERENT ELECTRON MOTIONS IN MULTIPHOTON X-RAY-PRODUCTION FROM KR AND XE CLUSTERS

Studies of multiphoton-induced x-ray generation in Kr and Xe clusters give direct information concerning (1) the atom-specific energy transf er rate, (2) the dependence of the x-ray yield on the strength of the intra-cluster inelastic electron scattering cross section, and (3) the threshold intensity for x-ray generation. Measurements of these three classes of observables with subpicosecond (similar to 300 fs) ultravi olet (248 nm) radiation at a maximum intensity of similar to 10(19) W cm(-2) all indicate that the non-linear coupling to the cluster has an anomalous strength with respect to that derivable from conventional s ingle-particle interactions. Five cases have been examined [Kr(M), Kr( L), Xe(N), Xe(M) and Xe(L)], spectrally spanning the range from simila r to 80 eV to similar to 5 keV. In order to reconcile theoretical esti mates with these experimental findings, three generalizations of the o riginal formulation of the interaction were necessary. The most import ant involves an enhancement in the coupling arising from the coherent motion of the (Z) field-ionized electrons induced by the external driv ing field. The coherently energized electrons act like a quasi-particl e possessing a charge Ze and a mass Zm, thereby presenting a sharply a ugmented coupling resembling that associated with energetic ion-atom c ollisions. The second involves the process of multiple electron ejecti on from an inner-shell, a mechanism that was found imperative to inter pret the high level of ionization observed in the Xe M-shell. The thir d modification concerns multiple transits of the driven electrons in t he cluster. The inclusion of these considerations consistently brings the theoretical analysis into agreement with the three measured proper ties. These results also indicate that energy deposition rates exceedi ng similar to 1 W/atom are feasible in appropriately designed molecule s incorporating heavy atoms. The limiting magnitude of the excitation energy Delta E(max) characteristic of the coherent coupling is estimat ed to be in the range Zamc(2) less than or equal to Delta E(max) less than or equal to Z(2) amc(2), a bound that can considerably exceed the K-shell binding energy of the heaviest atoms.