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Journal of Physics B Atomic Molecular and Optical Physics

Relevance of dissociative molecular states for resonant two-center photoionization of heteroatomic dimers

F. Grüll, Alexander B Voitkiv, Carsten Müller

Published Nov 7, 2022• 4 citations
Publisher

Abstract

Abstract Photoionization of a weakly bound van-der-Waals dimer, composed of two atomic speciesAandB, is studied theoretically. AtomAcan be ionized either directly or via an indirect two-center channel, where atomBis first resonantly photoexcited and, afterwards, transfers the excitation energy via long-range electron–electron correlations radiationlessly to atomA, causing its ionization. By taking the vibrational nuclear motion of the dimer during the process into account, we analyze the contributions to the photoionization cross section from bound and dissociative molecular states in the final A+ –Bsystem. For the specific example of a LiHe dimer we show (a) that substantial contributions to both ionization mechanisms result from dissociative Li ++ He states and (b) that the two-center pathway strongly dominates over the direct photoionization channel for photon energies close to one of the molecular LiHe resonances.

DOI
10.1088/1361-6455/aca0d3

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Abstract Photoionization of a weakly bound van-der-Waals dimer, composed of two atomic speciesAandB, is studied theoretically. AtomAcan be ionized either directly or via an indirect two-center channel, where atomBis first resonantly photoexcited and, afterwards, transfers the excitation energy via long-range electron–electron correlations radiationlessly to atomA, causing its ionization. By taking the vibrational nuclear motion of the dimer during the process into account, we analyze the contributions to the photoionization cross section from bound and dissociative molecular states in the final A+ –Bsystem. For the specific example of a LiHe dimer we show (a) that substantial contributions to both ionization mechanisms result from dissociative Li ++ He states and (b) that the two-center pathway strongly dominates over the direct photoionization channel for photon energies close to one of the molecular LiHe resonances.
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Photoionization of an atom AAA by a strong laser field in the presence of a spatially well-separated neighboring atom BBB is considered. The laser field frequency is assumed to lie below the ionization potential of atom AAA and be resonant with a dipole-allowed transition in atom BBB. In this situation, the ionization may occur either directly by multiphoton absorption from the laser field at the first atomic center or via an indirect pathway involving two-center electron-electron correlations, where the neighbor atom BBB is first photoexcited and, afterwards, transfers its energy upon deexcitation radiationlessly to atom AAA. Considering monochromatic as well as bichromatic laser fields, we study various coupling regimes of the photoionization process and identify experimentally accessible parameter domains where the two-center channel is dominant.
Abstract An additional channel for photoionization of an atom A by an electromagnetic field opens if it interacts with an atom B resonantly coupled to this field. In this channel, which is known to be very efficient when A and B constitute a bound system, A is ionized via resonant photoexcitation of B with subsequent energy transfer to A through two-center electron correlations. We show that it can strongly dominate the ionization of A also in collisions with B, even though the average distance between A and B exceeds the typical size of a bound system by orders of magnitude.
Abstract The double ionization of an atom by single-photon absorption in the presence of a neighbouring atom is studied. The latter is, first, resonantly photoexcited and, afterwards, transfers the excitation energy radiationlessly to the other atom, leading to its double ionization. The process relies on the combined effect of interatomic and intra-atomic electron correlations. It can dominate over the direct double photoionization by several orders of magnitude at interatomic distances up to a few nanometers. The relative position of the neighbouring atom is shown to exert a characteristic influence on the angular distribution of emitted electrons.
We report here the direct observation of a two-step, ionizing "optical collision" between crossed lithium beams in which very-long-range coupling, due to van der Waals' resonance force between identical particles, is followed by photoionization: ${\mathrm{Li}}^{*}(2p)+\mathrm{Li}(2s)+\ensuremath{\hbar}\ensuremath{\omega}\ensuremath{\rightarrow}{\mathrm{Li}}^{*}(3d)+\mathrm{Li}(2s),$ ${\mathrm{Li}}^{*}(3d)+\ensuremath{\hbar}\ensuremath{\omega}\ensuremath{\rightarrow}{\mathrm{Li}}^{+}+e.$ Cross-section magnitude and form of the intensity profile are described by a simple three-level model. These results suggest that polarization of the laser field together with collisional two-photon ionization may be used to delineate the shape of individual molecular-state potential curves at long range.
The two-photon excitation of cesium to high-n Rydberg levels through resonant dissociative states of the dimer is investigated over the wavelength range 6000-5900A. Highly vibrationally excited sp1Sigmasbrmgsp+\\sp1\\Sigma\\sb{\\rm g}\\sp+sp1Sigmasbrmgsp+ ground state molecules are excited to the continuum of sp1Pisbrmu\\sp1\\Pi\\sb{\\rm u}sp1Pisbrmu bound states which dissociate to give 5D atoms. The 5D atoms are subsequently photoexcited to high-n Rydberg states which are detected by selective field ionization techniques. Series corresponding to both (5D to\\toto nF) and (5D to\\toto nP) transitions with components up to n = 150 were observed. The photoexcitation process appears to be very efficient and produces copious quantities of Cs Rydberg atoms.