Transient Negative-Ion States in Alicyclic and Aromatic Fluorocarbon Molecules

Abstract

The formation of short‐lived (lifetime ∼10⁻¹⁵ − 10^− 13 sec) and long‐lived (lifetime ∼10^− 6−10⁻³ sec) temporary negative‐ion states in 12 cyclic fluorocarbon molecules is studied in the gas phase with monoenergetic electron beams. The long‐lived molecular ions exhibit a systematic increase of autoionization lifetime from 7 μsec for C₄F₆⁻ possessing 24 vibrational degrees of freedom to 800 μsec for C₇F₁₄⁻ having 57 vibrational degrees of freedom. This 100‐fold increase of lifetime with increasing number of degrees of freedom qualitatively conforms to the previous theoretical model describing nondissociative electron attachment in polyatomic molecules. All of the long‐lived ions (C₄F₆⁻, C₆F₆⁻, C₄F₈⁻, C₅F₈⁻, C₇F₈⁻, C₆F₁₀⁻, and C₆F₁₂⁻) have a maximum attachment cross section at some energy less than 0.05 eV, and the widths of the attachment resonances observed in the beam experiments were instrumental with the exception of C₇F₁₄⁻, whose width at $\frac{1}{2}$ maximum was approximately 0.2 eV. The SF₆ scavenger technique is employed to detect short‐lived transient negative‐ion states in the fluorinated benzenes, and the peak of the resonances occurred at 1.35 eV for C₆H₅F, 0.6 eV for 1,3‐C₆H₄F₂, 0.3 eV for 1,3,5‐C₆H₃F₃, and at ∼0 eV for 1,2,3,4‐C₆H₂F₄ and C₆HF₅. The peak energies of the temporary negative‐ion resonances show an approximately linear decrease as a function of the number of fluorines added to the ring, and evidence is given that addition of each fluorine to the benzene ring increases the electron affinity by ∼0.4 eV. The completely fluorinated benzene ring attaches thermal energy electrons into a negative‐ion state which has a lifetime of 12 μsec.