Robert Abouaf

Low-energy electron attachment to fullerenes and in the gas phase

We have investigated the low-energy (0 - 10 eV) electron attachment on and , using an improved energy resolution close to zero incident energy (0.030 - 0.060 eV). We have found evidence of a zero-energy attachment process (within 0.030 eV), for both and , indicating that a s-wave capture process is present at low energy for these two molecules. For , although in contrast with symmetry arguments, and with previous free-electron attachment experiments, this result is in full agreement with the conclusions presented by several groups, using the Rydberg atom energy transfer technique to investigate the low-energy attachment in these systems. Energy positions of several resonant states, appearing between 0.5 and 5 eV in the ion yield versus electron energy spectrum, are clearly established in and , and assignments are discussed. Lifetime measurements are reported for the fullerene anions. In the incident energy range 7 - 11 eV the lifetime of was found to vary from 500 to , respectively, in agreement with previous measurements. The thermionic model for detachment does not seem to describe the values obtained, and their observed variation in this energy range. The lifetime of is longer and could not be measured precisely with our time-of-flight conditions. An estimation of has been made at . Below the lifetime of was larger than and could not be measured precisely.

The triplet state of cytosine and its derivatives: Electron impact and quantum chemical study

The excitation of the lowest electronic states and vibrational excitation of cytosine (C) have been studied using electron energy loss spectroscopy (EELS, 0-100 eV) with angular analysis. The singlet states have been found to be in good agreement with UV-VIS absorption results on sublimed films, slightly blueshifted by about 0.1 eV. The EEL spectra recorded at residual energy below 2 eV show clear shoulders at energy losses of 3.50 and 4.25 eV (+/-0.1 eV). They are assigned to the lowest triplet electronic states of cytosine. Energies and molecular structures of the lowest-lying triplet state of C and its methylated and halogenated 5-X-C, 6-X-C, and 5-X, 6-X-C substituted derivatives (X=CH3, F, Cl, and Br) have been studied using quantum chemical calculations with both molecular orbital and density functional methods, in conjunction with the 6-311++G(d,p), 6-311++G(3df,2p), and aug-cc-pVTZ basis sets. The triplet-singlet energy gap obtained using coupled-cluster theory [CCSD(T)] and density functional theory (DFT) methods agrees well with those derived from EELS study. The first Cs vertical triplet state is located at 3.6 eV, in good agreement with experiment. The weak band observed at 4.25 eV is tentatively assigned to the second Cs vertical triplet excitation. For the substituted cytosines considered, the vertical triplet state is consistently centered at 3.0-3.2 eV above the corresponding singlet ground state but about 1.0 eV below the first excited singlet state. Geometrical relaxation involving out-of-plane distortions of hydrogen atoms leads to a stabilization of 0.6-1.0 eV in favor of the equilibrium triplet. The lowest-lying adiabatic triplet states are located at 2.3-3.0 eV. Halogen substitution at both C(5) and C(6) positions tends to reduce the triplet-singlet separations whereas methylation tends to enlarge it. The vibrational modes of triplet cytosine and the ionization energies of substituted derivatives were also evaluated.

Fine structure in the dissociative attachment cross sections for HBr and HF

Abstract The dissociative attachment cross sections of F − -HF and Br − -HBr have been investigated in the region 0–4 eV. The vertical onsets observed revealed that the 2 Σ + low-lying resonant state is attractive. A well developed step-type structure observed in Br − (and weakly in F − is attributed to the opening of new decay channels for HX − .

Electron impact on free C60. Excited states below 10 eV

Abstract Electron energy loss spectra have been performed on C 60 vapour, both at very low ( E r = 0), and at intermediate residual energies (up to E r = 70 eV), along with an angular analysis of the scattered electrons. Besides the two strong bands at 3.78 and 4.84 eV, due to dipole-allowed transitions to 1 T 1u states, and the π-π* collective excitation at 6.1 eV, we have observed a band peaking at 2.26 eV, dominant at low residual energies, corresponding to superimposed transitions to triplet states, and dipole orbitally forbidden states. A new peak, likely to be of triplet character is observed at 2.96 eV. In contrast with similar studies on C 60 films, we have not found any evidence of the lowest triplet state 3 T 2g at 1.55 eV.

Low energy electron impact on C76 and C84: excitation, metastable anion formation, and lifetime

Abstract Low energy (0–15 eV) electron impact on gas phase fullerenes C76 and C84 has been investigated in an e− beam experiment (electron energy resolution 0.050 eV). Electron energy loss spectroscopy results are presented for C76 at intermediate and low incident energy and are compared with the UV–visible absorption spectrum. The lowest triplet state 3B3 has not been detected; however, two bands at 1.70 and 2.58 eV, clearly of triplet character are observed. The band at 1.70 eV is assigned to 3B1 and 3B2 states. Ion yields of C76− and C84− versus e− energy are presented in the range 0–10 eV. The capture threshold is shown to appear at zero energy (within 0.030 eV), indicating the occurrence of an s-wave capture. The nature of other features due to resonances appearing on the spectrum is discussed. Lifetimes τ of C76− and C84− are observed to be strongly dependent of the electron energy. They are reported in the range 10–15 eV. For the capture of a 14 eV electron we have measured τ(C76−) = 100 μs and τ(C84−) = 180 μs. Comparison between the lifetimes of C60−, C70−, C76−, and C84− shows that τ behaves exponentially with the number of carbon atoms involved in the fullerene anion.

Electron energy loss spectroscopy and anion formation in gas phase coronene

Electron collisions with gas phase coronene have been studied in the energy range 0-80 eV to investigate electronic and pure vibrational excitations, as well as the anion formation near zero energy. Quantum chemical calculations using time dependent density functional theory (TDDFT) have been performed for an improved data analysis. Results have been obtained on both dipole allowed and forbidden transitions to low lying electronic excited states up to 8 eV, by varying the incident energy and scattering observation angle. Calculated values for excitation energies are in good agreement with experiment and with previous data. Pure vibrational excitation, performed with an energy resolution of 0.025 eV, is presented at incident energies from 1.5 to 9 eV. Five groups of excited vibrational modes are observed with energy losses ranging from 0.065 to 0.375 eV. Several resonant states appear to contribute to these excitations. Calculations of the harmonic vibrational frequencies of coronene, and determination of their main character, are used for the analysis of the results. The vertical electron affinity of coronene is calculated to be 0.45 eV in very good agreement with previous measured or calculated results. The parent anion (C24H12)- yield versus electron energy presents 2 peaks at zero and 0.30 eV (within 0.030 eV), in contrast with previous results where only one peak at zero was reported

Vibrational excitation in e-NO2 collision at low energy (0.3–2.5 eV)

Abstract Resonant vibrational excitation by e− impact has been investigated in NO2. Differential cross sections for elastic and vibrational excitation present complex structures which are not simple, due to overlapping bands. Nevertheless, they indicate a long-lived resonance (1A1) below 1 eV and an intermediate-lifetime state above 1.1 eV (3B1 and/or 1B1).

Low energy electron collisions on OCS: differential vibrational cross sections and S− production around 1 eV

Abstract The low energy vibrational excitation in OCS has been re-examined. The angular dependences of the vibrational cross sections in the 2 Π resonance region are shown to be proportional to 2 - cos 2 θ for π vibrational modes, and to 1 + 7 cos 2 θ for σ modes. The threshold spectrum ( E residual = 0 eV) reveals that, in the bending series below 0.5 eV, the vibrational modes of σ symmetry are more excited than the π modes. Indeed the vibrational excitation cross sections recorded for a few vibrational modes, show a peak at threshold for the σ modes but not for the π ones. We have also observed a new structure in the S − /OCS dissociative attachment cross section.

Electronic excitation of C70 in the gas phase by electron energy loss spectroscopy

Abstract Electron energy loss spectroscopy of the C 70 molecule in the gas phase, with angular analysis, has been investigated in the intermediate energy range (from excitation thresholds up to 80 eV above threshold). The occurrence of a ‘plasmon like’ process, different from the one-electron dipole-allowed excitations, has been detected based on a different angular behaviour. Optically forbidden transitions appear only weakly and are observed at 1.9 and 4.8 eV. In contrast to the case of C 60 , a small vibrational excitation, possibly resonant in character, has been observed around 2 eV incident energy. Variations in the cross sections and angular behaviour of the main excitation processes observed are presented.