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In this work, we study the photoelectron spectra of the diatomic molecule O2 in the direction perpendicular to the laser polarization. Compared with atoms, molecules have additional parameters, e.g., the orientation of the molecule and the symmetry of the initial electronic state, and thus give rise to richer strong-field phenomena. Our simulations show that for parallel and perpendicular alignments of the laser polarization with respect to the molecular axis, the photoelectron spectra display above-threshold ionization (ATI) peaks with a specific 2ħε separation in which the absorption of an even number of photons is absent. However, for other alignment angles, the even-order ATI peaks appear. According to our analysis, the physical mechanism of the alignment effect of the even-order ATI peaks can be attributed to the symmetry of the molecular ground-state wave function with respect to the photoelectron emission direction. [ABSTRACT FROM AUTHOR]

Rresonancelike enhancement of groups of adjacent peaks in the photoelectron spectrum of high-order above-threshold ionization has been well documented for noble-gas atoms subjected to intense infrared laser pulses. However, its physical origin is still under debate. In this Rapid Communication, we investigate experimentally and theoretically high-order above-threshold ionization of diatomic nitrogen and oxygen molecules in order to shed more light on the underlying mechanism. The resonancelike enhancement is experimentally observed for N2 but is absent for O2 molecules. A simulation on the basis of S-matrix theory and the strong-field approximation reproduces the experimental observations. This implies that the resonancelike enhancement can be attributed to the channel-closing effect. The specific molecular structure plays a decisive role for the presence or absence of this enhancement in molecular systems. [ABSTRACT FROM AUTHOR]