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To directly track real-time intramolecular dynamics during ground-state chemical reactions, the authors simulated Time-Resolved Coulomb Explosion Imaging (TR-CEI) of threshold $\text{NO}_2$ unimolecular dissociation using quasi-classical trajectory (QCT) calculations on an accurate potential energy surface. This approach bridges a longstanding experimental gap in verifying statistical reaction theories by mapping ion-pair kinetic energy releases directly to evolving neutral Jacobi separations. Supported by recurrence time distributions and Lyapunov exponent analyses, the results confirm that TR-CEI directly captures phase space evolution and establish that the system requires 2 to 3 ps to achieve statistical ergodicity.
Ground-state molecular decay takes a measurable 2 to 3 picoseconds to achieve statistical chaos, directly imaged by projecting evolving phase-space wavepackets into Coulomb explosion energy spectra.
Direct time-resolved observation of large amplitude dynamics during ground state statistical unimolecular decay remains an experimental challenge. Here, an experimental Time-Resolved Coulomb Explosion Imaging (TR-CEI) study (J. Chem. Phys. 151, 174301, 2019) of the canonical unimolecular reaction NO$_2$ $\rightarrow$ NO($^2\Pi, v=0, J)$ + O($^3P_j$) at threshold dissociation energies is modeled and analyzed. Quasi-classical trajectory (QCT) simulations on an accurate and validated ground state potential energy surface (PES), with energy and angular momentum constraints from known product state distributions, are used to characterize the dissociation dynamics. TR-CEI projects suddenly, via ultrafast Strong Field Ionization (SFI), the evolving neutral ground state wavepacket onto the repulsive doubly charged cationic state which then impulsively fragments into the NO$^{+}$ + O$^{+}$ ion pair. Within the Coulomb recoil approximation, which applies at longer NO--O (Jacobi) separations, the ion pair CEI-energy is inversely proportional to the scalar distance between the neutral ground state NO and O moieties at the moment of SFI, thus providing a direct measure of the evolving average NO--O distance during unimolecular decay. Results from QCT simulations agree favourably with the TR-CEI measurements. Phase space evolution, recurrence time distributions and Lyapunov exponent analysis are presented which support the conclusion that TR-CEI directly observes phase space evolution during unimolecular decay. For the present system, the time scale for the phase space distribution to achieve statisticality is estimated to be 2 to 3 ps.