High-order harmonic generation in laser-aligned molecules.

Strong-field interactions with molecules are not as well understood as for atoms. In atoms, both fully quantum-mechanical calculations [1] and semiclassical models [2] reproduce single-electron ionization and related phenomena—high-harmonic generation (HHG) and above-threshold ionization (ATI )—-with good accuracy. In contrast, for many molecules there is now consistent experimental evidence indicating that ionization rates are significantly lower than those in atoms with the same ionization potential (IP) [3], evidencing the critical role played by the ground state wavefunction in such processes, thus suggesting the possibility to use HHG to probe this ground state wave-function. In order to make progresses in the understanding the molecular electron dynamics in strong laser field the knowledge of the angular position of the molecules is required; to this end the molecular alignment by laser field has been shown to be effective in producing a macroscopic ensemble of ordered molecules [4]. We analyze the electron dynamics and high-order harmonic generation in molecules exposed to high intensity (1013-1015 W cm-2) ultrashort pulse laser fields. The use of adiabatic laser alignment techniques, using relatively long duration (300 ps) laser pulses of moderately high intensity (1011-1012 W cm2) enabled us to perform the first experiments investigating the role of the angle between the laser polarization vector and the molecular axis [5-7]. We have shown that alignment of a sample can enhance the harmonic yield by a small but significant factor. The unique influence of the dipole phase in molecular HHG is clearly demonstrated and is shown to offer a consistent explanation for our experimental observations. The electron wavepacket dynamics exhibit strong interference effects that result in the reduction or enhancement of harmonic yield at certain alignment angles. Experiments to further investigate these effects and to increase the modulation of the harmonics by molecular alignment are discussed.[1] M. Lewenstein, Ph. Balcou, M. Yu. Ivanov, A. L'Huillier, and P. Corkum, Phys. Rev. A 49, 2117 (1994).[2] P. B. Corkum, Phys. Rev. Lett. 71, 1994 (1993).[3] E. Wells, E. Merrick, J. DeWitt, and R. R. Jones, Phys. Rev. A 66, 013409 (2002).[4] J. Larsen, I. Wendt-Larsen, and Henrik Stapelfeldt, Phys. Rev. Lett. 83, 1123 (1999). [5] R. Velotta, N. Hay, M. Mason, M. Castillejo, and J. P. Marangos, Phys. Rev. Lett. 87, 183901 (2001).[6] N. Hay, R. Velotta, M. Lein, R. de Nalda, E. Heesel, M. Castillejo, and J. P. Marangos, Phys. Rev A, 65 053805 (2001).[7] R. de Nalda, E. Heesel, M. Lein, N. Hay, R. Velotta, E. Springate, M. Castillejo, and J. P. Marangos, Phys. Rev.A 69, 031804(R) (2004).