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Published online: 9 September 2009 | doi:10.1038/nchina.2009.179

Atomic physics: Photoionization is simple man

Felix Cheung

Introduction

© APS

Under intense laser fields, atoms may absorb more photons than necessary for ionization. This process, known as above-threshold ionization (ATI), generates a photoelectron energy spectrum that consists of a series of peaks separated by a gap equivalent to the photon energy. Although the semiclassical 'simple man' model can give a good qualitative description of the ATI processes in the long-wavelength and high-intensity regimes, it is unable to explain many of the prominent features in other regimes.

Thanks to advances in ultrafast laser technology and precise detection systems, researchers led by Zhizhan Xu and Ya Cheng at the Chinese Academy of Sciences in Shanghai¹ have recently obtained high-resolution ATI spectra for noble gas atoms (see image for xenon gas) at mid-infrared wavelengths. They discovered two humps in the very low-energy regime: the first at electron energies below 1 eV; and the second, broader hump at slightly higher electron energy, displaying a strong dependence on laser wavelength and intensity. The most striking feature of these two humps is that they can be qualitatively explained using the simple man model.

The simple man model interprets ATI in terms of the classical trajectories of electrons in the external laser field, as well as quantum tunnelling through the potential barrier formed by the Coulomb interaction between electrons and parent ions. The fact that the model can reproduce the experimentally observed features means that the Coulomb potential plays a prominent role in ATI.

The researchers believe their findings will contribute to our understanding of the ionization dynamics of atoms.

The authors of this work are from:
State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan, China; Center for Applied Physics and Technology, Peking University, Beijing, China; Institute of Applied Physics and Computational Mathematics, Beijing, China; State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai, China.

Reference

1. Quan, W. et al. Classical aspects in above-threshold ionization with a midinfrared strong laser field. Phys. Rev. Lett. 103, 093001 (2009).  | Article | OpenURL