Does the “Reef Structure” at the Ozone Transition State towards the Dissociation Exist? New Insight from Calculations and Ultrasensitive Spectroscopy Experiments

Since the discovery of anomalies in ozone isotope enrichment, several fundamental issues in the dynamics linked to the shape of the potential energy surface in the transition state region have been raised. The role of the reeflike structure on the minimum energy path is an intricate question previously discussed in the context of chemical experiments. In this Letter, we bring strong arguments in favor of the absence of a submerged barrier from ultrasensitive laser spectroscopy experiments combined with accurate predictions of highly excited vibrations up to nearly 95% of the dissociation threshold.

Figure 1

The shape of the ozone PESs obtained from ab initio calculations. Panel (b) shows a bird’s-eye view on the valence-angle section from the equilibrium to the dissociation along the MEP (dashed green line). The TS range (encircled) is blown up at panel (a) where two PES versions are compared: one with the reef barrier at the right-hand side (R_PES) and another one without reef structure (NR_PES) at the left-hand side. Here $r_1$, $r_2$, and $\theta$ are bond length and bond angle internal coordinates: $\theta =117°$ at (a); $r=r_1$ and $r_2=2.4\mathrm{a}.\mathrm{u}.$ at (b).

Figure 2

Log scale summary for experimental intensities of ozone vibration-rotation transitions showing a necessity of extremely sensitive methods to access the dissociation threshold $D_0$. Previously observed data are shown in black with extrapolations in gray [26]. Our CRDS measurements are shown in blue including yet unpublished bands in red. The upper panel gives an example of an agreement of experimental spectra and theoretical simulation resulting from the analyses.

Figure 3

(a) ${}^{16}{\mathrm{O}}_3$ band centers (${\nu }_0$) deduced from CRDS spectra and their rectilinear normal mode inflation coefficients: contributions beyond the first major term (left panel) and those beyond two and three major terms (right panel). (b) The shape of 3D vibrational wave function $\mathrm{\Psi }$ corresponding to our highest observed ${}^{16}{\mathrm{O}}_3$ band at ${\nu }_0=7860{\mathrm{cm}}^{-1}$. Though high states do not have pure normal mode character, it is clearly seen that $\mathrm{\Psi }$ probes nonlinear MEP [17] corresponding to asymmetric stretch (AS) orbit in the TS range: dashed green curve.

Figure 4

Discrepancies (Calc. - Obs.) between ${}^{16}{\mathrm{O}}_3$ band centers observed in our CRDS spectra and theoretical predictions from two ab initio PESs [17] in the range approaching the dissociation threshold for the same energy ranking numbers. Circles correspond to NR_PES and triangles to R_PES. Centers of three newly observed bands at the highest interval are given in Table 1.