Resonancelike enhancement in high-order above-threshold ionization of molecules

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 N${}_2$ but is absent for O${}_2$ 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.

Figure 1

Typical photoelectron kinetic energy spectra for Ar, N${}_2$, and O${}_2$. The laser intensity is given in units of ${10}^{13}$ W/cm${}^2$. For N${}_2$, the inset shows the spectra between 25 and 50 eV on a larger scale for improved resolution of the enhancement.

Figure 2

Photoelectron energy spectra in the direction along the laser polarization as functions of the laser intensity for the three gas targets Ar, N${}_2$, and O${}_2$, from left to right. The spectra are recorded with 30 fs, 800 nm laser pulses. The laser intensity is displayed in multiples of ${10}^{13}$ W/cm${}^2$.

Figure 3

Calculated spectra for Ar, N${}_2$, and O${}_2$. Note that the laser intensity is also displayed in terms of the parameter $\eta =(I_{\mathrm{p}}+U_{\mathrm{p}})/\omega$ [Eq. (1)] to demonstrate the channel-closing effect. Parameters in the simulation are chosen in accordance with the experiments.

Figure 4

Calculated PES of N${}_2$ and O${}_2$ averaged over all orientation angles at three intensities corresponding to those of Figs. 1b and 1c. Panels (a), (b), and (c) are for N${}_2$; panel (d) is for O${}_2$. The PES of panel (a) contains only $s$ states, that of (b) only $p$ states, while those of (c) and (d) are based on all of the LCAO states.