Rotational Coherence Dominates Early-Time Dynamics and Produces Long-Time Revivals in the S2 State of Azulene

TL;DR

This work resolves a long-standing question about azulene's $S_2$ dynamics by showing that the early $2-5$ ps decay is due to rotational dephasing rather than intramolecular vibrational redistribution (IVR). Using polarization-resolved TR-REMPI with a nanosecond delay window, the authors demonstrate a consistent rotational anisotropy signature across vibronic origins and observe long-lived rotational coherence manifested as $J$-type and $C$-type revivals out to $1.3$ ns. The findings connect short- and long-time dynamics under a single rotational-coherence framework, establishing azulene and PAHs as model systems for quantum-coherent wavepacket dynamics and offering a framework to study coherence, decoherence, and rotational control in electronically rich molecules. The results challenge prior IVR-based interpretations and pave the way for broader vibronic-rotation studies across larger PAHs and different excitation conditions.

Abstract

The ultrafast dynamics of azulene have been debated for decades, with reported picosecond decay constants variously attributed to intramolecular vibrational redistribution (IVR), internal conversion, or rotational dephasing. Using polarization and femtosecond time-resolved Resonance Enhanced Multi-photon Ionization Spectroscopy with a nanosecond delay window, we disentangle this long-standing inconsistency and show that the early 2-5 ps decay component arises entirely from rotational dephasing of an excited-state wavepacket. Identical time constants extracted from the decay of the parallel signal and rise of the perpendicular signal across multiple vibronic origins provide an unambiguous rotational anisotropy signature, eliminating the need for IVR-based interpretations. Extending the measurement window to 1.3 ns reveals well-structured J-type and C-type rotational coherence revivals in S2 azulene on top of the well-documented fluorescence decay, demonstrating that both the short- and long-time dynamics contain information about the coherent rotational dynamics. These results show that azulene, and by extension polycyclic aromatic hydrocarbons (PAH), can sustain structured rotational coherence deep into the nanosecond regime, positioning PAHs as model systems for quantum-coherent wavepacket dynamics and providing a framework for probing coherence, decoherence, and rotational control in electronically rich molecular systems.

Figures (7)

• Figure 1: Measured REMPI Spectrum and calculated Franck-Condon Herzberg-Teller spectrum at 0 K. The FWHM of the calculated spectrum is 1 $cm^{-1}$ and shifted to a lower energy by 0.091 eV. This work specifically studied the photon energies indicated by the red arrows. The inset shows the structure and principle axes of azulene.
• Figure 2: a) Normalized decay kinetics (grey markers) measured for early time dynamics with pump energy of 3.56 eV, 3.68 eV and 3.72 eV. Red curve shows exponential fit result and the black dashed traces are the fitted IRF. b) Time constants from the fit results for all seven pump energies (grey circles with error bars), The average time constants of the early time dynamics (red squares).
• Figure 3: a) Decay kinetics pumped at 3.56 eV with different pump polarization. b) Normalized Time-Resolved REMPI spectra pumped at 3.56, 3.68 and 3.72 eV scanned up to 1.3 ns with J type and C type rotational coherence transients labeled. J type transients labeled in red indicating a positive signal and blue indicating a negative signal. The results are calculated within the near-prolate top approximation.
• Figure S1: Gaussian fit for early time dynamics for $S_2$ azulene when pumped at 3.57 eV. The fitted width is $0.1712 \pm 0.0049~\mathrm{ps}$, corresponding to an FWHM of 0.4032 ps. This FWHM lies within the expected IRF range, indicating the absence of measurable absorption at off-resonance pump energies and demonstrating that the experiment achieves vibronic-state–selective resolution.
• Figure S2: Raw Data of early time dynamics at magic angle. Pumped at 3.56 eV.