Fluorination effects on the inner-shell spectra of unsaturated molecules

Abstract

The applicability of the perfluoro effect to the X-ray spectra (300-800 eV) of unsaturated organic molecules is explored. The Cls and Fls (and 01s where appropriate) oscillator strength spectra of five fluoroethylenes, octafluorocyclopentene, formyl fluoride, carbonyl fluoride, hexafluorobutadiene, trifluoroacetic acid, heduorobutyne-2, hexafluoroacetone, and octatluoronaphthalene were derived from electron impact energy loss spectra recorded under electric-dipole scattering conditions. These spectra are analyzed and compared with those of their perhydro analogs, several of which (naphthalene, acetic acid, butyne-2) are reported for the first time. In unsaturated systems in which all the atoms lie in the molecular plane, such as ethylene, formaldehyde, benzene, etc., perfluorination results in approximately 10 eV shifts of the inner-shell energy loss spectra to higher energies, yet the term values for the Cls-, ln* excitations are shifted by only l eV and often less. In direct contrast, the term values for the equivalent C l s -+ l a * excitations in unsaturated systems having atoms out of the molecular plane, such as butene-2 and acetone, are shifted upward by up to 3 eV upon perfluorination. These different spectral behaviors of planar and nonplanar systems on fluorination quantitatively parallel those which were observed earlier for valence-level ionization potentials (10-20 eV) and attributed to the perfluoro effect. I t is observed for the first time that the Cls+ln* excitation energies in planar hydrocarbons are only very weakly dependent on the spatial extent of the n-electron system. An explanation involving a localized Cls hole is proposed to rationalize this behavior.The perfluoro effect also predicts that excitations to a* MO's will become relatively low-lying in highly fluorinated planar systems. Such low-lying inner-shell excitations induced by fluorination are observed in the fluoroethylene series and in the fluorocarbonyls. When the negative-ion spectra of the fluoroethylenes are assigned in a self-consistent manner, a a* MO is found to drop into the vicinity of In* upon fluorination. A similar intrusion of the lowest dr MO among the n+ MO's is also observed upon fluorinating benzene, while evidence for this in the case of naphthalene is less clear, on account of the complex pattern of multiple Cls-+nn* transitions in this molecule.

Photodissociation of organic molecules in star-forming regions

“…This small excess could be associated with the high stabilization of COOH + due to charge resonance (charge migration) (Silverstein & Webster 1998) or a possible dissociation of CH + 3 into minor fragments. Other possibility is that the methyl carbon IP occurs at 291.6, 5 eV less than for the carboxyl carbon (see Robin et al 1988). As a consequence, the number of resonances to excited orbitals involving the carbon atom in the COOH site is higher than for the other carbon site at photon energies above the IP of the methyl carbon.…”

“…Figure 2 shows the mass spectrum of acetic acid obtained at photon energy of 288.3 eV. The C1s → π * resonance energy from C = O bond is 288.6 eV (Robin et al 1988). We can see the methyl fragment group (mass from 12 to 15 a.m.u), the C 2 group (24 to 27 a.m.u), the HCO group (28 to 31 a.m.u) and the CCO group (40 to 44 a.m.u).…”

“…5 as a function of photon energy. The absolute absorption cross section of acetic acid (Robin et al 1988) is also shown for comparison. Those values are also shown in Table 3.…”

“…In order to put our data on an absolute scale, after subtraction of the linear background and false coincidences coming from aborted double and triple ionization (see Simon et al 1991), we have summed the contributions of all cationic fragments detected and normalized them to the photoabsorption cross sections measured by Robin et al (1988).…”

Understanding the hydrogen and oxygen gas pressure dependence of the tribological properties of silicon oxide–doped hydrogenated amorphous carbon coatings

“…To further investigate the structural transformations and surface reactions occurring on a-C:H:Si:O upon sliding under different environmental conditions, imaging near edge X-ray absorption fine structure spectroscopy (NEXAFS) measurements, which are highly surface sensitive (for electron yield NEXAFS spectroscopy of low atomic number elements, the information depth, which is the specimen thickness measured normal to the surface from which a specified percentage of typically 95% of the detected signal originates, is usually less shows characteristic absorption features at 285.0±0.1 eV (due to the C1s→π * transition for disordered carbon-carbon bonds), ~286.5 eV (due to the C1s→π * transition for carbonyl groups), 287.5±0.1 eV (assigned to the C1s→σ * transition for carbon-hydrogen bonds), and 288.9±0.1 eV (assigned to the C1s→σ * transition for carbon-oxygen and carbon-silicon bonds, as well as to the C1s→π * transition for carboxyl groups) 25,26 . These assignments are summarized in Table 1.…”