Absolute cross sections for electronic excitations of cytosine by low energy electron impact

摘要

The absolute cross sections (CS) 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 ³A′(π→π*) and 2 ³A′(π→π*) valence states of the molecule. Their energy dependent CS exhibit essentially a common maximum at about 6 eV with a value of 1.84 × 10⁻¹⁷ cm² for the former and 4.94 × 10⁻¹⁷ cm² 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 ¹A′(π→π*) valence state, shows only a steep rise to about 1.04 × 10⁻¹⁶ cm² 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 ¹A′(π→π*), 5 ¹A′(π→π*), and 6 ¹A′(π→π*) valence states, respectively. The CS for the 3 ^{3, 1}A′ and 4 ¹A′ 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 cm², 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 cm² 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 cm² at the incident energy of 13 eV.