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We theoretically investigate the Doppler effect on optical bistability in an N-type active Raman gain atomic system inside an optical ring cavity. It is shown that the Doppler effect can greatly enhance the dispersion and thus create the bistable behaviour or greatly increase the bistable region, which has been known as the positive Doppler effect on optical bistability. In addition, we find that a positive Doppler effect can change optical bistability from the hybrid dispersion-gain type to a dispersive type. [ABSTRACT FROM AUTHOR]

By developing a full quantum scattering theory of high-order above-threshold ionization, we study the energy spectra and the angular distributions of photoelectrons from atoms with intense laser fields shining on them. We find that real rescattering can occur many times, and even infinite times. The photoelectrons from the rescattering process form a broad plateau in the kinetic-energy spectrum. We further disclose a multiple-plateau structure formed by the high-energy photoelectrons, which absorb many photons during the rescattering process. Moreover, we find that both the angular distributions and the kinetic-energy spectra of photoelectrons obey the same scaling law as that for directly emitted photoelectrons. [ABSTRACT FROM AUTHOR]