Photolysis of ethyl bromide via two electronic excited states at 163.3 nm

Abstract

The present investigation is concerned with the 163.3 nm photolysis of ethyl bromide in the pressure range of 2.1-100 Torr and at room temperature. The principal products were found to be $C_2H_4,\;C_2H_6,\;C_2H_3Br,\; CH_3CHBr_2$, and small quantity of n-$C_4H_{10}$. Addition of NO as a radical scavenger completely suppressed the formation of $C_2H_6,\;C_2H_3Br,\;CH_3CHBr_2$, and n-$C_4H_{10}$, and partially reduced that of $C_2H_4$. Increasing the total pressure of the system, the ratio of the quantum yield of $C_2H_6$ to that of $C_2H_4$ was increased, and this trend was reduced as the collision partner changed from $C_2H_5Br$ to $CF_4$ and $N_2$. And the quantum yield of $C_2H_3Br$ was decreased with the irradiation time. The results were interpreted in terms of two different excited stated, i.e., the first and the second excited states. These two excited states can be formed at the same time with a discrete ratio when photon was absorbed. The first excited state could decompose to $C_2H_4$ and HBr by molecular elimination or experience collision induced cross over to the second excited state. The second excited state decomposed by C-Br bond fission. The secondary radical reaction mechanism through which $C_2H_6,\;C_2H_3Br,\;CH_3CHBr_2$ scavengable $C_2H_4$, and n-$C_4H_{10}$ were formed as well as the primary processes were proposed. The life time of the first excited state was presumed to be $\sim 5 \times 10^{-10}$ sec, assuming k$_1$ to be the usual value, $\sim 10^7$ Torr${-1}$ sec$^{-1}$. The rate constant ratio of $C_2H_5$ radical with NO to $C_2H_5Br,$k_{NO}/k_4$, was also determined to be $5.0 \pm 0.4 \times 10^4$.