Abstract
We study experimentally and theoretically the high harmonic emission from aligned samples of nitrogen and carbon dioxide, in an elliptically polarized laser field. The ellipticity induces a lateral shift of the recombining electron wavepacket in the generation process. We show that this effect, which is well known from high harmonic generation (HHG) in atoms, can be useful to maintain the plane wave approximation in the case of HHG from molecules whose orbitals contain nodal planes. The study of the harmonic signal as a function of molecular alignment also reveals the role of the ellipticity on the recollision angle of the electron wavepacket, which can be used to accurately track the position of resonances in harmonic spectra.
Highlights
Frequency is given by = ε + Ip, where Ip is the ionization potential
We present a theoretical model of the harmonic emission in the plane wave approximation and compare it to the experimental results
Two important effects have to be present in the model: the transverse dimensions of the recolliding wavepacket, which is set by tunnel ionization, and the recollision angle
Summary
The experimental set-up has been described previously [11, 21]. Briefly, N2 or CO2 gas was introduced into the vacuum chamber through a pulsed supersonic valve providing a gas density of about 1017 cm−3 with a rotational temperature of about 30 K. HHG spectra were recorded for molecules aligned in 5◦ steps in the range of ±100◦ relative to the intense laser’s polarization. Influence of the recollision angle In atomic media, the harmonic signal is always maximum at zero ellipticity This is not the case in aligned molecules, as shown in figure 2(b). The harmonic signal increases if the molecule is rotated towards the generating laser polarization, i.e. if the recollision direction decreases. At a fixed molecular alignment, the harmonic signal from N2 will increase if the ellipticity decreases the recollision angle, i.e. if it is positive. The ellipticity 0 that maximizes the harmonic signal (figure 2(b)) corresponds to a trade off between these two effects
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