Rotational excitation of molecules in the regime of strong ro-vibrational coupling: Comparison between an optical centrifuge and a transform-limited pulse

We investigate theoretically the ability of an optical centrifuge - a laser pulse whose linear polarization is rotating at an accelerated rate, to control molecular rotation in the regime when the rigid-rotor approximation breaks down due to coupling between the vibrational and rotational degrees of freedom. Our analysis demonstrates that the centrifuge field enables controlled excitation of high rotational states while maintaining relatively low spread along the vibrational coordinate. We contrast this to the rotational excitation by a linearly polarized Gaussian pulse of equal spectral width and pulse energy which, although comparable to the centrifuge-induced rotation, is unavoidably accompanied by a substantial broadening of the vibrational wavepacket.

💡 Research Summary

In this theoretical study the authors compare two non‑resonant laser schemes for driving rotational excitation in a regime where the rigid‑rotor approximation breaks down because of strong ro‑vibrational coupling. The model system is the heavy diatomic molecule Rb₂ in its lowest triplet electronic state (a³Σ⁺_u), which supports about 41 vibrational levels and a large number of rotational states (up to N≈152 for the lowest vibrational band). The full rovibrational Hamiltonian includes the vibrational kinetic energy, the rotational kinetic term N²/(2µR²), the electronic potential V(R), and the laser‑induced interaction −I(t)/(2cε₀)