Photoionization of n-Butane at 1067–1048 Å. Decomposition of the Parent Ion and Superexcited Molecule

Abstract

n‐C₄H₁₀, n‐C₄D₁₀, CD₃CH₂CH₂CD₃, and C₄D₁₀—C₃H₆ mixtures have been irradiated in the presence of oxygen with the argon resonance lines at 1067 and 1048 Å. On the basis of saturation‐ion‐current measurements, the ionization‐efficiency ratios ηC₄D₁₀/η_NO and ηC₄H₁₀/η_NO were estimated to be 0.480±0.005 and 0.465±0.005, respectively. It is demonstrated that at the argon‐resonance lines the C₄H₁₀⁺ ion undergoes fragmentation according to the following two processes: $$\begin{array}{c}{\mathrm{C}}_4{\mathrm{H}}_{10}^{+}\to {\mathrm{C}}_3{\mathrm{H}}_7^{+}+{\mathrm{CH}}_3\\ \to {\mathrm{C}}_3{\mathrm{H}}_6^{+}+{\mathrm{CH}}_4.\end{array}$$ The isotopic analysis of the propane fraction formed in the irradiation of n‐C₄H₁₀–n‐C₄D₁₀–O₂ mixtures indicates that the propyl ion which reacts with n‐butane to form propane by a H⁻ or D⁻ ion transfer has the sec‐propyl ion structure. It is further shown that, in agreement with recent studies on ion—molecule reactions, the C₃H₆⁺ ion also reacts with C₄H₁₀ to form C₃H₈ (H₂⁻ transfer). The yield of the parent ion was estimated by making use of the recently established H₂‐transfer reaction: $${\mathrm{C}}_4{\mathrm{H}}_{10}^{+}+{\mathrm{C}}_3{\mathrm{D}}_6\to {\mathrm{CD}}_3{\mathrm{CDHCD}}_2\mathrm{H}+{\mathrm{C}}_4{\mathrm{H}}_8^{+}.$$ The fragmentation of the butane parent ion is strongly quenched by an increase in the pressure of butane, or upon addition of an inert gas. The following approximate relative efficiencies for the quenching of the fragmentation of the excited C₄D₁₀⁺ ion are obtained: He, 1; N₂, 5; C₄D₁₀, 35. It is also noted that the fragmentation of the C₄D₁₀⁺ ion is more readily quenched by collision with He than that of the C₄H₁₀⁺ ion by about a factor of 5. The fragmentation of the neutral excited molecule has been examined briefly on the basis of the isotopic analysis of products formed in the photolysis of CD₃CH₂CH₂CD₃—O₂ mixtures at 1470, 1236, and 1067–1048 Å, and of those formed in the photolysis of C₄D₁₀—H₂S mixtures at 1236 Å. It is concluded that the modes of decomposition of the superexcited molecule are essentially identical to those observed for molecules excited at photon energies below the ionization energy, although the elimination of an H₂ molecule or an alkane may be of lesser importance at the shorter wavelength.