Knowledge Bank

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 ($\backslash lambda\; <$ 450 nm) places the molecule in the ~{A} $1$A state, followed by relaxation to the long-lived ~{a} $3$A state. Triplet-triplet excitation($\backslash lambda\; <$ 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 $\to$ ~{A} $1$A) 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.