Collision-free multiple photon ionization of atoms and molecules at 193 nm

Abstract

The nonlinear coupling of 193-nm radiation to a range of atomic and molecular materials has been experimentally explored up to a maximum intensity on the order of -10' W/cm . Studies of collision-free ion production clearly exhibit anomalous behavior which strongly implies that the atomic shell structure is the principal determinant in the observed response. On the basis of the observed coupling strength and the measured atomic-number (Z) dependence, the experimental evidence points to a coherent atomic motion involving several electrons, possibly an entire shell, as the main physical mechanism enabling the scale of energy transfers seen. Therefore, states representing multiple excitations appear to play a central role in the coupling, a consideration that fundamentally distinguishes the nonlinear interaction of a multielectron atom from that of a single-electron system. Comparison of the experimental findings with standard theoretical treatments, of either a perturbative or nonperturbative nature, does not produce satisfactory agreement. Conversely, the formulation of a simple classical estimate qualitatively conforms to several features of the observed behavior including the shell character of the interaction, the maximum energy transfer, the dependence of the average energy transfer on the intensity of irradiation, the frequency dependence of the observed energy transfer, and the weak influence of polarization.

Limiting cross sections for multiphoton coupling

“…Secondly, it is believed possible than an understanding of these physical interactions will provide a basis for an efficient means for the generation of stimulated emission in the X-ray range [4] by direct, highly nonlinear coupling of ultraviolet radiation to atoms and molecules. This view follows since (a) the fundamental task in the production of coherent short wavelength radiation is the controlled deposition of energy at high specific power and since (b) there is accumulating physical evidence [4,5,[9][10][11][12][13][14][15][16][17] [4,[18][19][20]. This situation stems directly from basic physical reasoning [4, 21, [4, 9, 10], a model which bears an analogy to certain atom-atom and ion-atom collisional processes [9].…”

Independent electron theory of multiple ionization of xenon by intense laser pulses

“…It is worth stressing that this test of Geltman theory does not rest on the introduction of a particl]l~r SEIP function. In this respect it seems to be more reliable than the best fit [7] of the ionization probabilities of Xe § Xe 2+, Xe 3 § taken from the Saclay's [1] and Chicago [2][3][4] works.…”

“…An increasing volume of data relative to the production of charge states of several atoms is available over a wide range of photon energies, for pulse widths ranging from nanoseconds to picoseconds. The Saclay group [1] and Rhodes and co-workers [2][3][4] have detected the presence of Xe ions with charge up to q = 8. The motivation for these studies is that the highly charged ions appear somewhat more readily than expected on the basis of the increasing number of photons needed to ionize the higher charge states.…”

Institutional Research and Development: (Annual report), FY 1986

“…The interaction of intense (>10 1Z W/cm z ) electromagnetic radiation with gaseous targets under collisionless conditions has produced some surprising and unexpected phenomena (Luk et al, 1985: Kruit etal., 1983). Among these phenomena are the generation of up to the seventeenth harmonic of incident laser light and the apparent "closing" of the low-energy electron channels in multiphoton ionization due to "ponderomotive" forces.…”