Details

Autor(en) / Beteiligte

• Wade, Elisabeth Ayn

Titel

Photodissociation of ketene: Rates and dynamics

Ort / Verlag

ProQuest Dissertations & Theses

Erscheinungsjahr

1996

Link zum Volltext

• ProQuest

Quelle

ProQuest Dissertations & Theses A&I

Beschreibungen/Notizen

• The rotational energy release in the dissociation of ketene (CH$\sb2$CO) along its singlet potential energy surface is observed and compared with several statistical and dynamical theories. Rotational distributions for the product, CO($\rm\~X\sp1\Sigma\sp{+}$)(v=1), are measured from the threshold for production of $\rm CH\sb2(\~a\ \sp1A\sb1)(0,0,0)+CO(\~X\sp1\Sigma\sp{+})$(v=1) to 1720 cm$\sp{-1}$ above. Near threshold ($\rm E\le 200\ cm\sp{-1}$ over threshold), phase space theory (PST) matches the observed distributions. At 357 and 490 cm$\sp{-1}$, PST constrained by the measured state distributions of the methylene fragment, provides a good fit to these CO(v=1) rotational distributions. For $\rm E>490\ cm\sp{-1}$, the constrained PST matches the average rotational energy observed but predicts distributions which are broader than observed. This contrasts to the rotational distributions of the $\rm\sp1CH\sb2$ fragment which become shifted to lower rotational states than PST as energy increases from 200 cm$\sp{-1}$ above threshold. Dynamical models, the impulsive model and Franck-Condon mapping, do not account for the product rotational state distributions. The CO(v=1) rotational distributions for $\rm E>200\ cm\sp{-1}$ contain no measurable product from triplet channel fragmentation. Therefore, they can be compared with the previously determined CO(v=0) rotational distributions in order to partition the CO(v=0) yield between singlet and triplet channels and recalculate the singlet yield. This new yield is found to be at the upper limits of the range previously reported. Rate constants and quantum yields have been determined for the photodissociation of ketene to produce $\rm CH\sb2(\~a\ \sp1A\sb1)\ (0,0,0)+CO(\~X\ \sp1\Sigma\sp{+})$ (v=1). At 57, 110, 200, 357, and 490 cm$\sp{-1}$ above this product threshold, vibrational branching ratios for the singlet products were measured and compared to phase space theory (PST), separate statistical ensembles (SSE), and variational RRKM (var. RRKM). Above 100 cm$\sp{-1}$ above that threshold, the experimental values are consistent with SSE and var. RRKM. CO(v=1,J$\sb{\rm CO}$) photofragment excitation (PHOFEX) spectra were observed up to 357 cm$\sp{-1}$ over the threshold for production of CO(v=1) and used to calculate the total yield of the state probed. Up to 350 cm$\sp{-1}$ over threshold, this yield is statistical, consistent with the observed $\rm\sp1CH\sb2$ rotational distributions. The singlet channel vibrational branching ratios and PHOFEX spectra are combined with the previously determined singlet yield and total rate constant to determine the singlet rate constant for CO(v=l) production as a function of energy. Rate constants are given accurately by PST from threshold up to 35 $\pm$ 5 cm$\sp{-1}$ above. From 35 cm$\sp{-1}$, the transition state begins to tighten as energy increases; the rate increases more slowly than the PST rate, as predicted by var. RRKM. For energies up to 2500 cm$\sp{-1}$, it appears that both the outer, PST transition state and an inner transition state at a C-C distance of 2.2-3.1 A affect the rate. Above 2500 cm$\sp{-1}$, the measured rate is consistent with var. RRKM with a single transition state at 2.2-3.1 A. From the measured rate constants, an experimental vibrational density of states is calculated to be 1.82 $\pm$ 0.12 times the Whitten-Rabinovich density of states. This indicates that the degeneracy of the coupled triplet channels, g$\sb{\rm t}$, is 1.6 $\pm$ 0.1. The vibrational branching ratios and product yields for the vibrationally excited triplet products were also estimated, and found to be nearly constant over this energy region. These values were about 17% of the predicted value, assuming that the triplet fragmentation dynamics are vibrationally adiabatic.