CONCENTRATION, MATRIX, AND WAVELENGTH DEPENDENCE IN THE PHOTOLYSIS EFFICIENCY OF MATRIX-ISOLATED BIACETYL
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Date
2008
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Ohio State University
Abstract
The ultraviolet irradiation of matrix-isolated biacetyl (C$_4$H$_6$O$_2$) results in the formation of a complex of \textit{trans}-methylhydroxycarbene (CH$_3$COH) and ketene (CH$_2$CO). Studies in this laboratory using pulsed sources have confirmed that these products arise from a two-photon process: the first photon ($\lambda <$ 450 nm) places the molecule in the \~{A} $^1$A$_u$ state, followed by relaxation to the long-lived \~{a} $^3$A$_u$ state. Triplet-triplet excitation($\lambda <$ 550 nm) from this state leads to the observed products. The apparent quantum efficiency for the formation of these products was measured as a function of matrix (N$_2$, O$_2$, Ar, Kr, and Xe), dilution ratio for the parent compound, and wavelength of irradiation (using a continuous source). For all matrices, the apparent efficiency decreases with increasing C$_4$H$_6$O$_2$ dilution, indicating that the products may arise from a bimolecular process. No such effect was seen in C$_4$D$_6$O$_2$ experiments, which exhibit much lower levels of efficiency at all dilutions. The matrix has an effect on the high-dilution efficiency (N$_2$, O$_2 >$ Kr, Xe $>$ Ar), which suggests that the matrix participates in the H-transfer process. Increasing the wavelength of irradiation from 436 to 450 nm results in an increase in the measured efficiency. At 436 nm, the reverse intersystem crossing (\~{a} $^3$A$_u$ $\rightarrow$ \~{A} $^1$A$_u$) channel is available to drain the triplet population. At 450 nm, this channel is no longer available, and the resulting increase in the steady-state triplet population leads to the observed enhancement.
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Author Institution: Department of Chemistry, The University of Scranton, Scranton, PA 18510-4626