Comment on ‘Rate coefficients for photoinitiated NO2 unimolecular decomposition: energy dependence in the threshold regime’ [Chem. Phys. Lett. 358 (2002) 71]

Abstract

Recently, Wittig and co-workers have published rate coefficients kðEÞ for the unimolecular decomposition of photoinitiated NO 2 close to the dissociation threshold [Chem. Phys. Lett. 358 (2002) 71]. They found out that kðEÞ for low angular momentum J exhibits a strong increase within 25 cm À1 of the reaction threshold. The authors emphasize that their experimental results are surprising and cannot be understood quantitatively on the basis of current theory on NO 2 . In this Comment we demonstrate that recent quantum mechanical calculations of NO 2 resonances on a global 3D-potential energy surface can indeed explain their data close to the dissociation threshold as well as for larger excess energies.

Photodissociation resonances of jet-cooled NO2 at the dissociation threshold by CW-CRDS

“…However, the disparate spectral resolution associated with each experimental setup has to be considered. Resonance width fluctuations, over an energy range of ∼20 cm −1 , are in agreement with the quantum mechanical calculations based on a 3D-PES of Schinke's group 44,48,59 and with PST. Despite these quantum fluctuations, we report on the unimolecular dissociation coefficients relatively to the opening of the two first dissociation channels NO 2 …”

“…We concluded that our data agree with those of Tsuchiya's group 38 and we mainly valid the PST approach, 40,104 and the hypothesis of loosely bound transition states in the probed excess energy range despite the fact that we did not study the photofragments. This conclusion may disagree with time-resolved measurements, 44,47,59 while the move of the transition states from loosely to tightly bounding has been suggested as a plausible way to reconcile the different experimental observations. However, the disparate spectral resolution associated with each experimental setup has to be considered.…”

“…Nevertheless, if this result agrees with Miyawaki and co-authors conclusions, it does not match the conclusions of Abel's group. 59 On the other hand, the energy distribution in the fragment NO (v = 1), i.e., from dissociation energies well above D 0 shows imperfect agreement with PST. 40 This latter group reported on dissociation rates k uni (E ‡ ) based on quantum mechanical calculations up to D 0 + 1000 cm −1 , and he has claimed that he could not rule out a contribution of long-range interactions to the dissociation rates at threshold ("they only play a minor role").…”

“…56 Another controversy concerns the interpretation of different experimental data sets. [57][58][59] One argued point stated about the observation (or not) of the opening of specific dissociation channels, and of the existence of loose (freely rotating fragments) or tight (hindered rotation motion of the two fragments) transition states (TS) along the reaction coordinate. This controversy probably indicates that temporal and spectral experiments do not necessarily provide data that are easy to reconcile.…”

Ab initio calculation of femtosecond-time-resolved photoelectron spectra of NO2 after excitation to the A-band

“…In the picosecond time range, the dynamics are known to be complex. Individual resonances show strongly varying dissociation constants close to the dissociation threshold, leading to excitation-energy-dependent dissociation times 62,63 . In picosecond and femtosecond time-resolved experiments, due to the bandwidth of the excitation pulse and the associated spectral averaging, a monotonic increase of the dissociation rates with increasing excitation energies 64,65 has been reported (see crosses in fig.…”