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

TL;DR

The paper addresses ro-vibrational dynamics of a diatomic molecule, specifically $$Rb_2$$ in the $a^3\Sigma_u^+$ state, under non-resonant light, and compares an optical centrifuge against a transform-limited Gaussian pulse with identical bandwidth and energy. Using a full quantum mechanical, grid-based approach, the authors solve the time-dependent Schrödinger equation in a rovibrational basis and analyze field-dressed populations, including dissociation into the continuum. They find that the optical centrifuge drives high rotational states (molecular super-rotors) while maintaining a relatively narrow vibrational distribution, whereas the Gaussian pulse yields broader vibrational excitation and greater coupling to dissociative channels. The results highlight the centrifuge’s potential for selective rotational control with partial vibrational decoupling, offering guidance for ultrafast spectroscopic experiments where separating rotational and vibrational excitations is desirable. These insights are especially relevant for heavier, softer-bonded molecules where ro-vibrational coupling is non-negligible and dissociation risks are significant.

Abstract

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.

Figures (12)

• Figure 1: For the initial state $(0,0,0)$, time evolution of the weights of the field-free states $|C_{0,0,0}(\nu,N,t)|^2$ with rotational quantum number $N\leq 14$ and vibrational quantum numbers a) and c) $\nu=0$; and b) and d) $\nu=1$. The peak intensities are $I_G^0=10^{12}~$W/cm$^2$ and $I_C^0=4.158\cdot10^{10}~$W/cm$^2$. The shaded areas represent the time profiles of the pulses.
• Figure 2: For the initial state $(0,0,0)$, final weights of the field-free rovibrational states $|C_{\nu,N}^{0}(t_f)|^2$ after the a) centrifuge and b) Gaussian pulses with peak intensities $I_G^0=10^{12}~$W/cm$^2$ and $I_C^0=4.158\cdot10^{10}~$W/cm$^2$, respectively.
• Figure 3: For the initial state $(0,0,0)$, final population distribution (dark green) according to rotational quantum number $N$ defined in \ref{['eq:rot_dist']} after the a) centrifuge and b) Gaussian pulses with peak intensities $I_C^0=4.158\cdot10^{10}~$W/cm$^2$ and $I_G^0=10^{12}~$W/cm$^2$, respectively. For the rigid-rotor approximation (light pink), we present the weights of each rotational state with the lowest vibrational band $\nu=0$.
• Figure 4: For the initial state $(0,0,0)$, time evolution of the alignment (dark-green thin line) induced by the a) centrifuge and b) Gaussian pulses with peak intensities $I_G^0=10^{12}~$W/cm$^2$ and $I_C^0=4.158\cdot10^{10}~$W/cm$^2$, respectively. The alignment computed within the rigid-rotor (light-pink thick line) approximation is also plotted.
• Figure 5: For the initial state $(0,0,0)$ in a CP, weights $|C_{0,0,0}(\nu,N,t)|^2$ of the field-free rotational and vibrational states into the field-dressed wavepacket at times a) $t=2.29$ ps (first maximum of $I_C(t)$); b) $t=6.86$ ps (fifth maximum); c) $t=10.97$ ps (twelfth maximum); and d) at the end of the pulse ($t=15$ ps), with peak intensity $I_C^0=1.8\cdot10^{11}~$W/cm$^2$.