Département de Médecine Nucléaire et Radiobiologie, Faculté de Médecine et Sciences de la Santé, Université de Sherbrooke, Sherbrooke, Québec J1H 5N4, Canada.
J Chem Phys. 2010 Oct 21;133(15):155104. doi: 10.1063/1.3505301.
The absolute cross sections (CSs) for electronic excitations of cytosine by electron impact between 5 and 18 eV were measured by electron-energy-loss (EEL) spectroscopy of the molecule deposited at low coverage on an inert Ar substrate. The lowest EEL features found at 3.55 and 4.02 eV are ascribed to transitions from the ground state to the two lowest triplet 1 (3)A(')(π→π(∗)) and 2 (3)A(')(π→π(∗)) valence states of the molecule. Their energy dependent CSs exhibit essentially a common maximum at about 6 eV with a value of 1.84×10(-17) cm(2) for the former and 4.94×10(-17) cm(2) for the latter. In contrast, the CS for the next EEL feature at 4.65 eV, which is ascribed to the optically allowed transition to the 2 (1)A(')(π→π(∗)) valence state, shows only a steep rise to about 1.04×10(-16) cm(2) followed by a monotonous decrease with the incident electron energy. The higher EEL features at 5.39, 6.18, 6.83, and 7.55 eV are assigned to the excitations of the 3 (3,1)A(')(π→π(∗)), 4 (1)A(')(π→π(∗)), 5 (1)A(')(π→π(∗)), and 6 (1)A(')(π→π(∗)) valence states, respectively. The CSs for the 3 (3,1)A(') and 4 (1)A(') states exhibit a common enhancement at about 10 eV superimposed on a more or less a steep rise, reaching, respectively, a maximum of 1.27 and 1.79×10(-16) cm(2), followed by a monotonous decrease. This latter enhancement and the maximum seen at about 6 eV in the lowest triplet states correspond to the core-excited electron resonances that have been found by dissociative electron attachment experiments with cytosine in the gas phase. The weak EEL feature found at 5.01 eV with a maximum CS of 3.8×10(-18) cm(2) near its excitation threshold is attributed to transitions from the ground state to the 1 (3,1)A(")(n→π(∗)) states. The monotonous rise of the EEL signal above 8 eV is attributed to the ionization of the molecule. It is partitioned into four excitation energy regions at about 8.55, 9.21, 9.83, and 11.53 eV, which correspond closely to the ionization energies of the four highest occupied molecular orbitals of cytosine. The sum of the ionization CS for these four excitation regions reaches a maximum of 8.1×10(-16) cm(2) at the incident energy of 13 eV.
利用沉积在惰性 Ar 衬底上的低覆盖率分子的电子能量损失 (EEL) 光谱，测量了胞嘧啶在 5 至 18 eV 电子碰撞电子激发的绝对截面 (CSs)。在 3.55 和 4.02 eV 处发现的最低 EEL 特征归因于从基态到分子的两个最低三重态 1(3)A(')(π→π())和 2(3)A(')(π→π())价态的跃迁。它们的能量相关 CSs 在大约 6 eV 处表现出基本相同的最大值，对于前者，其值为 1.84×10(-17)cm(2)，对于后者，其值为 4.94×10(-17)cm(2)。相比之下，下一个 EEL 特征在 4.65 eV 处的 CS，归因于到 2(1)A(')(π→π())价态的光学允许跃迁，仅显示出大约 1.04×10(-16)cm(2)的急剧上升，随后随着入射电子能量的增加单调下降。在 5.39、6.18、6.83 和 7.55 eV 处的较高 EEL 特征分别分配给 3(3,1)A(')(π→π())、4(1)A(')(π→π())、5(1)A(')(π→π())和 6(1)A(')(π→π())价态的激发。3(3,1)A(')和 4(1)A(')态的 CSs 在大约 10 eV 处表现出共同增强，叠加在或多或少的陡峭上升上，分别达到 1.27 和 1.79×10(-16)cm(2)的最大值，随后单调下降。后一种增强和在最低三重态中约 6 eV 处看到的最大值对应于在气相中用胞嘧啶进行的离解电子附加实验中发现的核心激发电子共振。在激发阈值附近，在 5.01 eV 处发现的最大 CS 为 3.8×10(-18)cm(2)的微弱 EEL 特征归因于从基态到 1(3,1)A(")(n→π())态的跃迁。在 8 eV 以上的 EEL 信号单调上升归因于分子的电离。它被分为四个激发能量区域，约为 8.55、9.21、9.83 和 11.53 eV，这些区域与胞嘧啶的四个最高占据分子轨道的电离能密切对应。这四个激发区域的总离子 CS 在 13 eV 的入射能量下达到最大值 8.1×10(-16)cm(2)。
