Direct observation of coherence transfer and rotational-to-vibrational energy exchange in optically centrifuged CO2 super-rotors

In this work the authors use coherent anti-Stokes Raman scattering to study collisional vibrational excitation in highly rotationally excited CO2 molecules prepared in an optical centrifuge. Optical centrifuges are laser-based molecular traps that can rotationally accelerate molecules to energies rivalling or exceeding molecular bond energies. Here we report time and frequency-resolved ultrafast coherent Raman measurements of optically centrifuged CO2 at 380 Torr spun to energies beyond its bond dissociation energy of 5.5 eV (J(max) = 364, E-rot = 6.14 eV, E-rot/k(B) = 71, 200 K). The entire rotational ladder from J = 24 to J = 364 was resolved simultaneously which enabled a more accurate measurement of the centrifugal distortion constants for CO2. Remarkably, coherence transfer was directly observed, and time-resolved, during the field-free relaxation of the trap as rotational energy flowed into bending-mode vibrational excitation. Vibrationally excited CO2 (nu(2) > 3) was observed in the time-resolved spectra to populate after 3 mean collision times as a result of rotational-to-vibrational (R-V) energy transfer. Trajectory simulations show an optimal range of J for R-V energy transfer. Dephasing rates for molecules rotating up to 5.5 times during one collision were quantified. Very slow decays of the vibrational hot band rotational coherences suggest that they are sustained by coherence transfer and line mixing.

Automated summary

The authors used coherent anti-Stokes Raman scattering to study collisional vibrational excitation in highly rotationally excited CO2 molecules in an optical centrifuge. They reported time and frequency-resolved ultrafast coherent Raman measurements of optically centrifuged CO2 at energies beyond its bond dissociation energy. They observed direct coherence transfer and time-resolved vibrational excitation due to rotational-to-vibrational energy transfer.