Above-threshold ionization of diatomic molecules by few-cycle laser pulses

Above-threshold ionization of diatomic molecules by infrared carrier-envelope phase (CEP) stable few-cycle laser pulses is analyzed both experimentally and theoretically. The theoretical approach is based on the recently developed molecular improved strong-field approximation (ISFA), generalized to few-cycle pulses. Instead of using the first Born approximation, the rescattering matrix element in the ISFA is now calculated exactly. This modification leads to the appearance of characteristic minima in the differential cross section as a function of the scattering angle. Experimental angle-resolved photoelectron spectra of N${}_2$ and O${}_2$ molecules are obtained using the velocity map imaging technique. A relatively good agreement of experimental and simulated angle-resolved spectra, CEP-dependent asymmetry maps, and extracted electron-molecular ion elastic scattering differential cross sections is obtained.

Figure 1

Ratios of exact and 1BA differential cross sections for elastic electron-nitrogen atom scattering as functions of the scattering angle for the fixed electron energies denoted in the legends.

Figure 2

Ratios of exact and 1BA differential cross sections for elastic scattering of electrons off N atoms as functions of the angle $\theta$ for the various fixed electron energies $E_{\mathbf{p}}$ denoted in the legends. The connection between the energies $E_{\mathbf{p}}$ and the angles $\theta$ of the electron detected in HATI experiments and the scattering energies $E_s$ and the angles ${\theta }_s$ is given in the text.

Figure 3

Comparison of experimental (upper panel) with theoretical (lower panel) angle-resolved photoelectron spectra of randomly oriented N${}_2$ molecules ionized by an ultrashort (pulse duration 6 optical cycles) CEP-stabilized (${\varphi }_{\mathrm{CEP}}=0$) laser pulse with a wavelength of 750 nm and a peak intensity of $4.3\times {10}^{13}\mathrm{W}/{\mathrm{cm}}^2$. On the abscissa the electron emission angle with respect to the laser polarization axis is denoted, while the photoelectron energy in electronvolts is shown on the ordinate. The same colorbar which covers 3.4 orders of magnitude is used in both panels. The arrows point toward the minima.

Figure 4

Same as in Fig. 3, but for O${}_2$ molecules and for the peak intensity $4.0\times {10}^{13}\mathrm{W}/{\mathrm{cm}}^2$. The colorbar covers 3.2 orders of magnitude.

Figure 5

Comparison of experimental (upper panel) with theoretical (lower panel) asymmetries (expressed using false colors with the corresponding colorbar), for randomly oriented N${}_2$ molecules. Asymmetries are presented as a function of the phase ${\varphi }_{\mathrm{CEP}}$ (abscissa) and the electron energy in electronvolts (ordinate). Laser pulse parameters are as in Fig. 3.

Figure 6

Same as in Fig. 5, but for the O${}_2$ molecule and with laser pulse parameters as for Fig. 4.

Figure 7

Comparison of angular distributions of elastic differential cross sections for electron-scattering energy of 5.6 eV, extracted from experimental and from theoretical CEP-averaged photoelectron spectra for N${}_2$, with laser pulse parameters as for Fig. 3, and random molecular orientation. The position of the minima of theoretical and experimental DCSs agree well.

Figure 8

Same as in Fig. 7, but for O${}_2$ molecules, for electron scattering energy of 6 eV and with laser pulse parameters as for Fig. 4.