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AIP conference proceedings

Multiply charged ions produced by multiphoton absorption in rare gas atoms

Anne L’Huillier, Louis A. Lompre, G. Mainfray, C. Manus

Published Jan 1, 1984• 3 citations
Publisher

Abstract

This paper describes the production of multiply charged ions induced by collisionless multiphoton ionization of rare gases at 532 and 1064 nm. The production of multiply charged ions emphasizes atomic properties and laser characteristics. Multiphoton absorption is expected to selectively excite high ionic energy states, which could induce a stimulated emission in the extreme ultraviolet.

DOI
10.1063/1.34712

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This paper describes the production of multiply charged ions induced by collisionless multiphoton ionization of rare gases at 532 and 1064 nm. The production of multiply charged ions emphasizes atomic properties and laser characteristics. Multiphoton absorption is expected to selectively excite high ionic energy states, which could induce a stimulated emission in the extreme ultraviolet.
Multiphoton ionization of strontium in the wavelength range of \AA{}558-590 nm is studied via the fluorescence from excited Sr+(5p2P32,12){\mathrm{Sr}}^{+}(5p^{2}P_{\frac{3}{2},\frac{1}{2}})Sr+(5p2P23​,21​​) ions. The fine-structure sublevels in the singly charged ion are fully resolved and are found to be nonstatistically populated. Four- and five- (or more) photon processes are observed to populate Sr+(5p2P){\mathrm{Sr}}^{+}(5p^{2}P)Sr+(5p2P) in the intensity regime of $I\ensuremath{\sim}1\ensuremath{-}50$ GW/cm2{\mathrm{cm}}^{2}cm2, but no emission is detected from higher-lying states of Sr+{\mathrm{Sr}}^{+}Sr+.
Singly and doubly charged Ca ions have been produced by multiphoton excitation of Ca vapour with 5 ns, low intensity (⩽3 × 10 11 W cm −2 ) dye-laser pulses in the 675–685 nm wavelength range, including the four-photon 4s 2 1 S 0 → 4p 2 1 S 0 excitation. The intensity and wavelength dependence of the ion yields was recorded as well as the fluorescence emission from excited states of the Ca ion in an effort to identify the excitation pathways leading to single and double ionization. Unambiguous evidence for the absorption of at least two photons above the first ionization threshold was recorded, in agreement with earlier results for Mg and Sr obtained under similar conditions. However, certain characteristics of the process differ significantly from those of the earlier results, despite the apparent similarity in both the atomic structure and the excitation scheme. The most striking and unexpected finding is that for a certain wavelength, the Ca 2+ yield is observable well before the saturation intensity of Ca + and, moreover, it grows with intensity and saturates in parallel with the Ca + yield. Possible mechanisms behind this outcome are discussed in detail as well as their implications for the multiphoton multiple ionization of complex atoms. Our tentative conclusion is that the occurrence of doubly charged ion production before the singly charged ion saturation should not be considered as 'synonymous' with direct (or non-sequential) multiphoton double ionization.
Tunneling ionization, a fascinating quantum phenomenon, has played the key role in the development of attosecond physics. Upon absorption of a few tens of photons, tunneling ionization creates ions in different excited states and even enables the formation of population inversion between ionic states. However, the underlying physics is still being debated. Here, we demonstrate a significant enhancement in the efficiency of multiphoton excitation when ionization of neutral molecules and resonant excitation of ions coexist in strong laser fields. It facilitates the dramatic increase in population inversion and lasing radiation in N_{2}^{+} around 1000 nm pump wavelength. Utilizing the ionization-coupling theory, we discover that the synergistic interplay between tunneling ionization and multiphoton excitation enables the ionic coherence to be maximized by phase locking of the periodically created ionic dipoles and consistently maintain an optimal phase for the follow-up photoexcitation. This Letter provides new insights into the photoexcitation mechanism of ions in strong laser fields and opens up a route for optimizing ionic lasing radiations.
Multiphoton ionization of Ca and Sr was studied at a wavelength of 532 nm with optical pulses of 12-ns and 35-ps duration. Excess photon absorption followed by autoionizing decay leads to populations of excited states of singly charged ions that can be observed via their spontaneous emission. Relative measurements were performed on fluorescence from Ca+{\mathrm{Ca}}^{+}Ca+(4p), Ca+{\mathrm{Ca}}^{+}Ca+(5s), and Ca+{\mathrm{Ca}}^{+}Ca+(4d), and from Sr+{\mathrm{Sr}}^{+}Sr+(5p) and Sr+{\mathrm{Sr}}^{+}Sr+(6s). The contribution of the highly excited states was found to be favored under short-pulse (35-ps) irradiation.
Studies of multiphoton ionization of atoms have revealed several unexpected characteristics. The confluence of the experimental evidence leads to the hypothesis that the basic character of the atomic response involves highly organized, coherent motions of entire atomic shells. The important regime, for which the radiative field strength is greater than an atomic unit (e/a(2)(0)), can be viewed in approximate correspondence with the physics of fast (approximately 10 MeV per atomic mass unit) atom-atom scattering. This physical picture provides a basis for the expectation that stimulated emission in the x-ray range can be produced by direct, highly nonlinear coupling of ultraviolet radiation to atoms.