Two-pulse orientation dynamics and high-harmonic spectroscopy of strongly-oriented molecules

We present the detailed analysis of a new two-pulse orientation scheme that achieves macroscopic field-free orientation at the high particle densities required for attosecond and high-harmonic spectroscopies (Kraus et al 2013 arXiv:1311.3923). Carbon monoxide molecules are oriented by combining one-colour and delayed two-colour non-resonant femtosecond laser pulses. High-harmonic generation is used to probe the oriented wave-packet dynamics and reveals that a very high degree of orientation ( N up / N total = 0.73–0.82) is achieved. We further extend this approach to orienting carbonyl sulphide molecules. We show that the present two-pulse scheme selectively enhances orientation created by the hyperpolarizability interaction whereas the ionization-depletion mechanism plays no role. We further control and optimize orientation through the delay between the one- and two-colour pump pulses. Finally, we demonstrate a complementary encoding of electronic-structure features, such as shape resonances, in the even- and odd-harmonic spectrum. The achieved progress makes two-pulse field-free orientation an attractive tool for a broad class of time-resolved measurements.