Resonance-enhanced harmonics for probing molecular electrons

Abstract

Electrons in molecules rearrange on an attosecond timescale, much faster than the nuclear rearrangement associated with chemical bond breaking, which takes tens to hundreds of femtoseconds. In spite of these dramatically different timescales, it has been shown that the electron dynamics can influence the ensuing nuclear motion.1 Capturing, and ultimately controlling, this electronic motion may therefore open up the possibility of dictating the outcome of a chemical reaction before the nuclei even begin to move,2 by shaping the spectral amplitude and phase of the attosecond pulse to select a particular reaction pathway. Despite widespread interest, application of attosecond science to molecular targets has proceeded slowly. For pump-probe experiments, it would be ideal to generate intense isolated attosecond ‘pump’ pulses3 from below-threshold harmonics (i.e., with photon energies below the ionization threshold of the generation medium, which can excite ground state electrons to a higher molecular orbital). Such harmonics lie in the relatively low-photon-energy vacuum-UV (VUV) spectral region, where molecules are most electronically active. However, it is difficult to generate below-threshold harmonics, and until now the low conversion efficiency of attosecond pulse generation has severely limited the achievable photon flux (number of photons). Macroscopic buildup of high-order harmonics can be enhanced through phase-matching techniques. In these, the interaction between a driving laser and the medium in which the high-order harmonics are to be generated is experimentally tuned to maintain a fixed phase relationship between the driving laser and the generated harmonic fields. Phase matching techniques have been used effectively in high-order harmonic generation in the above-threshold region to scale the achievable photon flux, primarily in the extreme-UV and soft x-ray spectral regions.4 We have demonstrated a new mechanism for phase Figure 1. (a) Spectrally and angularly resolved resolution-enhanced structure (RES) generation. The atomic resonances in the vicinity of the 9th harmonic (3p6 ! 3p5ns and 3p5nd, 15–16eV) are indicated. The 11th harmonic (H11, 19–20eV) is comparatively featureless.