High-order harmonic generation in molecules: signature of the ground state orbital.

We discuss the first experiment of high-order harmonic generation in molecules and companion atoms (atoms with approximately the same ionization potential) in the temporal regime of few optical cycles.1 A more extended HHG spectrum for molecules with an anti-symmetric HOMO (O2 and CO2), has been confirmed and explained in terms of ionization suppression. No such an effect has been observed in N2 with respect to Ar as previously observed for longer pulses.2 We have shown that the atom-like approach proposed in Ref. [2] is very powerful in explaining the main features of the experimental results. In particular, the higher contrast in HHG spectra for molecule is a clear signature of the role played by the single active electron orbital. Thus, the shape of HHG spectra is very similar for all the investigated molecules as they are produced by a d0-like orbital, if the molecular ensemble is not aligned. As a consequence, HHG in small molecules is strongly influenced by the partially d-like character of the orbitals that are ionized and this finding is of broad interest as the d-like character of the outermost orbitals is known to be responsible for other important phenomena such as continuum shape resonances or chemical bond. A strong angular dependence is predicted for HHG produced in aligned molecular systems. The few cycle laser pulse produces a broadened spectrum when the gas target is very soft to ionize (e.g. Xe), as a consequence of the time confinement in HHG production. The analysis has been subsequently extended to some alkanes (methane, Ip = 12.97 eV; propane, Ip = 11.07 eV; butane, Ip = 10.63 eV) whose HHG spectra were compared with those measured for xenon (Ip = 12.13 eV). The harmonic emission was obtained focusing femtosecond light pulses on a pulsed gas jet. Two different pulse durations were considered, namely 20 fs and 6 fs. In spite of the lower Ip, cutoff saturation in alkanes occurs at higher energies; it is worth noting that, among the alkanes, shorter cutoff wavelength corresponds to higher ionization potential as expected. In all the experimental range for a fixed driving energy, the cutoff scaling obeys to the relation c,Xe>c,butane>c,methane where c is the cutoff wavelength. Propane and butane have very similar cutoff position as expected from the small difference between their ionization potential. These results evidence that XUV emission is strongly affected by the nature of the involved ground state. The experimental results have been compared to numerical simulation based on an atom-like Lewenstein model and preliminary results reproduce the cutoff wavelength behavior of the investigated molecular species.References:[1] C. Altucci, R. Velotta, J. P. Marangos, E Heesel, E. Springate, M. Pascolini, L. Poletto, P. Villoresi, C. Vozzi, G. Sansone, M. Anscombe, J-P. Caumes, S. Stagira, M. Nisoli, Phys. Rev. A (in press).[2] B. Shan, X.-M. Tong, Z. Zhao, Z. Chang, and C.D. Lin, Phys. Rev., 66, R061401 (2002)