Deng Zongwu, Imhoff Marjorie, Huels Michael A
Ion Reaction Laboratory, Department of Nuclear Medicine and Radiobiology, Faculty of Medicine, University of Sherbrooke, Quebec, Canada.
J Chem Phys. 2005 Oct 8;123(14):144509. doi: 10.1063/1.2046671.
We report measurements of the formation and desorption of ionic fragments induced by very low-energy (10-200 eV) Ar(+) irradiation of thymine (T) films, deposited on a polycrystalline Pt substrate. A multitude of dissociation channels is observed, among which the major cation species are identified as HNCH(+), HNC(3)H(4) (+), C(3)H(3) (+), OCNH(2) (+), T-OCN, T-OCNH(2), T-O, and T+H and the major anions as H(-), O(-), CN(-),and OCN(-). Cation fragment desorption appears at much lower threshold energies (near 15 eV) than anion fragment desorption, where the latter depends strongly on the film thickness. It is proposed that anion fragment formation and desorption results from projectile impact-induced excitation of either (1) a neutral thymine molecule, followed by fragmentation and charge exchange between the energetic neutral fragment and the substrate (or film) and/or (2) a deprotonated monoanionic thymine molecule to a dissociative state, followed by a unimolecular fragmentation of the excited thymine anion. The H(-) and O(-) fragment formations may have a further contribution from dipolar dissociation, e.g., formation of electronically excited neutral thymine, followed by dissociation into O(-)+T-O, due to their reduced sensitivity to the film thickness. Positive-ion fragment desorption exhibits no significant dependence on film thickness before the emergence of surface charging, and originates from a kinetically assisted charge-transfer excitation. The results suggest that the potential energy of the incident ion plays a significant role in lowering the threshold energy of kinetic fragmentation of thymine. Measurements of the time-dependent film degradation yields for 100-eV Ar(+) suggest a quantum efficiency for degradation of about six thymine molecules per incident ion.
我们报告了对沉积在多晶铂衬底上的胸腺嘧啶（T）薄膜进行极低能量（10 - 200 eV）氩离子（Ar⁺）辐照所诱导的离子碎片形成和解吸的测量结果。观察到众多解离通道，其中主要的阳离子种类被鉴定为HNCH⁺、HNC₃H₄⁺、C₃H₃⁺、OCNH₂⁺、[T - OCN]⁺、[T - OCNH₂]⁺、[T - O]⁺和[T + H]⁺，主要的阴离子种类为H⁻、O⁻、CN⁻和OCN⁻。阳离子碎片解吸出现在比阴离子碎片解吸低得多的阈值能量（接近15 eV）处，后者强烈依赖于薄膜厚度。有人提出，阴离子碎片的形成和解吸是由于以下两种情况之一的弹丸撞击诱导激发：（1）一个中性胸腺嘧啶分子，随后进行碎片化以及高能中性碎片与衬底（或薄膜）之间的电荷交换，和/或（2）一个去质子化的单阴离子胸腺嘧啶分子激发到解离态，随后激发的胸腺嘧啶阴离子进行单分子碎片化。H⁻和O⁻碎片的形成可能因偶极解离而有进一步贡献，例如形成电子激发的中性胸腺嘧啶，随后由于它们对薄膜厚度的敏感性降低而解离为O⁻ + [T - O]⁺。在表面充电出现之前，正离子碎片解吸对薄膜厚度没有显著依赖性，并且源于动力学辅助的电荷转移激发。结果表明，入射离子的势能在降低胸腺嘧啶动力学碎片化的阈值能量方面起着重要作用。对100 eV Ar⁺的随时间变化的薄膜降解产率的测量表明，每个入射离子的降解量子效率约为六个胸腺嘧啶分子。
