Direct Observation, Study, and Control of Molecular Superrotors

Extremely fast rotating molecules whose rotational energy is comparable with the molecular bond strength are known as “superrotors.” It has been speculated that superrotors may exhibit a number of unique properties, yet only indirect evidence of these molecular objects has been reported to date. Here we demonstrate the first direct observation of molecular superrotors by detecting coherent unidirectional molecular rotation with extreme frequencies exceeding 10 THz. The technique of an “optical centrifuge” is used to control the degree of rotational excitation in an ultrabroad range of rotational quantum numbers, reaching as high as $N=95$ in oxygen and $N=60$ in nitrogen. State-resolved detection enables us to determine the shape of the excited rotational wave packet and quantify the effect of centrifugal distortion on the rotational spectrum. Femtosecond time resolution reveals coherent rotational dynamics with increasing coherence times at higher angular momentum. We demonstrate that molecular superrotors can be created and observed in dense samples under normal conditions where the effects of ultrafast rotation on many-body interactions, intermolecular collisions, and chemical reactions can be readily explored.

Figure 1

(a) Illustration of an optical centrifuge field propagating in the direction $\mathbf{k}$. The vector of linear polarization $\mathbf{E}$ undergoes an accelerated rotation. (b) Frequency spectra of a full (dashed) and truncated (solid) centrifuge pulse. (c) Time-frequency spectrogram of the centrifuge field, recorded by means of cross-correlation frequency resolved optical gating. (d) The speed and direction of molecular rotation is detected by measuring the magnitude and sign of coherent Raman scattering, respectively. (e) Experimental setup. Circular polarizer (CP), circular analyzer (CA), dichroic beam splitter (DB), delay line (DL), quarter-wave plate (QWP).

Figure 2

Time-dependent Raman shifts from the clockwise (a) and counterclockwise (b) centrifuged oxygen molecules. As the molecules spend a longer time in the centrifuge, the observed Raman frequency shift increases, providing direct evidence of accelerated molecular rotation in one well defined direction. An additional time-independent Raman signal originates from the molecules lost from the centrifuge.

Figure 3

(a) State-resolved Raman spectra of the centrifuged oxygen molecules. Higher curves correspond to longer spinning time inside the centrifuge. Red vertical arrows mark the rotational quantum numbers. (b) Experimental (dots with error bars) and calculated (dashed blue and solid green for the rigid and nonrigid rotor approximations, respectively) rotational energy spectrum expressed as a Raman frequency shift.

Figure 4

(a) Scattered light intensity as a function of time and frequency, converted to the rotational quantum number of oxygen. Dashed tilted line shows the increasing angular frequency of the centrifuge, terminated at about 70 ps. Dashed horizontal line marks the most populated rotational state of ${\mathrm{O}}_2$ at room temperature, $N=9$. Oscillations of the coherent rotational wave packet are shown in the inset, whereas their Fourier transform is plotted in panel (b) as a function of the inverse frequency. Lower red and middle green curves represent our experimental data and numerical simulations, respectively. The upper blue curve shows the experimental results for a weaker rotational excitation around $N=29$, and the vertical dashed line indicates the anticipated oscillation period of a rigid rotor. (c) State-resolved decay of rotational coherence in nitrogen.