High-Harmonic Spectroscopy of Oriented OCS Molecules: Emission of Even and Odd Harmonics

We study the emission of even and odd high-harmonic orders from oriented OCS molecules. We use an intense, nonresonant femtosecond laser pulse superimposed with its phase-controlled second harmonic field to impulsively align and orient a dense sample of molecules from which we subsequently generate high-order harmonics. The even harmonics appear around the full revivals of the rotational dynamics. We demonstrate perfect coherent control over their intensity through the subcycle delay of the two-color fields. The odd harmonics are insensitive to the degree of orientation, but modulate with the degree of axis alignment, in agreement with calculated photorecombination dipole moments. We further compare the shape of the even and odd harmonic spectra with our calculations and determine the degree of orientation.

Figure 1

Experimental setup for high-harmonic spectroscopy of oriented molecules. The far-field spectrally resolved high-harmonic profile is a false-color logarithmic plot obtained at maximal orientation of the OCS molecules.

Figure 2

(a) High-harmonic spectra recorded at pump-probe delays of 82.3 ps (maximal orientation, red line), showing the presence of the even harmonics $H10–H20$ emitted from oriented molecules and 80.5 ps (near-random distribution of molecules, blue dashed line). (b) Variation of the intensity of the odd harmonics (blue lines) and even harmonics (red lines) as a function of the subcycle temporal delay between the 800 nm and 400 nm laser fields forming the pump pulse, measured at a pump-probe delay of 82.30 ps. The intensity of the odd harmonics has been normalized to their value at the left end of the panel and the intensity of each even harmonic $H2n$ has been divided by the intensity at the left end of the panel of the next lower odd harmonic $H2n-1$. The error bars represent rms variations of the measured intensities. (c) Illustration of the electric field of a $800+400\mathrm{nm}$ laser pulse for different two-color delays.

Figure 3

Variation of the intensity of the odd harmonics ($H9–H19$, blue lines) and even harmonics ($H10–H18$, red lines) with pump-probe delay. The intensity of the odd harmonics has been normalized to their value at $\Delta t=80.5\mathrm{ps}$ and the intensity of each even harmonic $H2n$ has been divided by the intensity, at $\Delta t=80.5\mathrm{ps}$, of the next lower odd harmonic $H2n-1$. The calculated evolution of the degree of axis alignment is shown in the upper left panel.

Figure 4

(a) and (b) Calculated photorecombination matrix elements for the HOMO of OCS using ab initio quantum scattering theory. The inset shows the HOMO of OCS as an isocontour plot and color-coded sign. (c) Predicted high-harmonic spectra for the odd orders (blue line) and even orders (red line) using the matrix elements from (a) and (b) and the axis distribution shown as inset ($⟨{cos}^2\theta ⟩=0.63$, perfect head-to-tail order). (d) Measured and calculated intensities of the odd and even harmonic orders using the axis distribution shown in the inset ($⟨{cos}^2\theta ⟩=0.63$, 16% head-to-tail order), obtained by fitting the head-to-tail order in the calculation to the experimental data.