nitrogen heterocycles to self-induce an effect analogous to an internal heavy atom effect in the presence of CDs has been rep~r t e d .~J~ Certain nitrogen heterocycles possessing lone-pair electrons have a tendency to deactivate through radiationless intersystem crossing. As a direct consequence of the electrondonating ability of the ring nitrogen in ACR, deactivation of the ACR molecule, if manifested through the lowest n i * singlet state, would necessarily involve transitions due to the nitrogen heteroatom. Such a mechanism seems plausible, especially given the involvement of unprotonated ACR in the quenching scheme. However, if the nitrogen heteroatom on ACR is oriented in the B-CD cavity in close proximity to the glycosidic oxygens, it seems that the role of the nitrogen heteroatom would be more compatible as one of electron acceptor. Indeed, fluorescence lifetime and absorbance quenching studies of 10-methylacridinium chloride in various nucleosidic environments demonstrate the electronaccepting ability of ACR dyes.9 Lifetime decay times were also used to demonstrate the decreased fluorescence decay time of ACR resulting from adjustment of pH with increasing concentrations of NaOH. ConclusionIn view of our findings, the enhanced quenching of ACR's fluorescence may be attributed to the specific inductive effects due to the location of nitrogen within the ring system. It seems certain that the driving force for complexation of ACR with 0-CD involves the direct interaction of the nitrogen heteroatom with (39) Turro, N. J.; Cox, G. S.; Li, X. Photochem. Phorobiol. 1983,37 (2), 149.the higher electron density provided by the glycosidic oxygens. Additionally, it is apparent that the mechanism of deactivation for ACR is affected by the relative energy differences between SI, S2, and T3, all of which depend on the solvent matrix. The probable enhancement of intersystem crossing as a result of interactions with P-CD, in turn, decreases the number of fluorescing species in solution, resulting in the observed quenching. Furthermore, the presence of a heteroatom within the ring system not only affects the overall solubility of the compound in aqueous systems but also appears to be responsible for the relatively weaker binding constants calculated for ACR to &CD. In contrast, the binding of 0-CD to structurally similar compounds that do not incorporate a heteroatom is often considerably stronger.The neutral species, as determined through pH studies, appears to be most important to the quenching observed for ACR, particularly if compared to protonated heterocycles, such as indole, which exhibit fluorescence enhancement upon addition of BCD. This ability of ACR to reflect changes in the microenvironmental polarity and pH of the 0-CD system suggests its potential application as a probe in other organized media.Low-energy (0-20 eV) electron impact on thin condensed hydrocarbon films is observed to produce, via dissociative attachment and dipolar dissociation, significant yields of H-and much lower yields of CH; ( n = 1-3) ...

Purpose-Determine experimentally the absolute cross sections (CS) to deposit various amount of energies into DNA bases by low-energy electron (LEE) impact.Materials and methods-Electron energy loss (EEL) spectra of DNA bases are recorded for different LEE impact energies on the molecules deposited at very low coverage on an inert argon (Ar) substrate. Following their normalisation to the effective incident electron current and molecular surface number density, the EEL spectra are then fitted with multiple Gaussian functions in order to delimit the various excitation energy regions. The CS to excite a molecule into its various excitation modes are finally obtained from computing the area under the corresponding Gaussians.Results-The EEL spectra and absolute CS for the electronic excitations of pyrimidine and the DNA bases thymine, adenine, and cytosine by electron impacts below 18 eV are reported for the molecules deposited at about monolayer coverage on a solid Ar substrate.Conclusions-The CS for electronic excitations of DNA bases by LEE impact are found to lie within the 10 −16 -10 −18 cm 2 range. The large value of the total ionisation CS indicates that ionisation of DNA bases by LEE is an important dissipative process via which ionising radiation degrades and is absorbed in DNA.

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 3 A′(π→π*) and 2 3 A′ (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 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 CS 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.

Electron stimulated desorption of NO from step sites on Pt(112): The role of chemisorption site geometry on the cross section Energy analysis of CI-ions produced by dissociative attachment in electron stimulated desorption from Cl 2 condensed on a platinum substrate is reported. The electron energy dependence ofthe Clsignal exhibits two peaks around 2 and 5 eV which arise, respectively, from the 2ng and 2nu core-excited C1 2 -* resonant states. At higher Cl 2 coverages, a third peak is observed around 11.5 eV. From kinetic energy distributions, it is possible to ascribe this latter peak to CI-ions formed via the 2nu resonance by electrons which have suffered energy losses through the excitation of low-lying electronic states of molecular chlorine. In the energy range of the 2nu C1 2 -* resonance, we observe that multiple scattering processes are also important and that the curve representing the kinetic energy of CI-ions formed via a single scattering process as a function of incident electron energy is a straight line with a slope 1/2. This indicates that the chlorine lattice is not involved in the dissociation dynamics.wH. Sambe, D. E. Ramaker, L. Sanche, and L. Parenteau (to be published).

No abstract