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arXiv:1809.06526 · New Journal of Physics

Dynamic two-center resonant photoionization in slow atomic collisions

Alexander B Voitkiv, C Müller, S F Zhang, X. Ma

Published Sep 19, 2019• 8 citations
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Abstract

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.

DOI
10.1088/1367-2630/ab4616

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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.
A free electron can form a bound state with an atomic center AAA upon photoemission (radiative recombination). In the presence of a neighboring atom BBB, such a bound state can, under certain conditions, be also formed via resonant transfer of energy to BBB, with its subsequent relaxation through radiative decay (two-center dielectronic recombination). This two-center process is very efficient in the ``static'' case where AAA and BBB form a weakly bound system, dominating over single-center radiative recombination up to internuclear distances as large as several nanometers. Here we study its dynamic variant in which recombination occurs when a beam of species AAA collides with a gas of atoms BBB and show that, even though the average distance between AAA and BBB in collisions is orders of magnitude larger than the typical size of a bound system, the two-center recombination can still outperform the single-center radiative recombination.
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 Ionization-excitation of an atom induced by the absorption of a single photon in the presence of a neighboring atom is studied. The latter is, first, resonantly photoexcited and, afterwards, transfers the excitation energy radiationlessly to the other atom, leading to its ionization with simultaneous excitation. The process relies on the combined effects of interatomic and intraatomic electron correlations. Under suitable conditions, it can dominate by several orders of magnitude over direct photoionization-excitation and even over direct photoionization. In addition, we briefly discuss another kind of two-center resonant photoionization with excitation where the ionization and residual excitation in the final state are located at different atomic sites.
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.