Electron Scattering from Methyl Formate (HCOOCH3): A Joint Theoretical and Experimental Study.

Elastic low-energy electron collisions with methyl formate have been studied theoretically at the level of various theories. The elastic integral cross section was calculated using Schwinger multichannel and R-matrix methods, in the static-exchange and static-exchange plus polarization levels of approximations for energies up to 15 eV. The absolute total cross section for electron scattering from methyl formate has been measured in a wide energy range (0.2-300 eV) using a 127° electron spectrometer working in the linear transmission configuration. The integral elastic and the absolute total cross sections display a π* shape resonance at around 1.70-1.84 eV, which can be related to the resonance visible for formic acid, and a broad structure located at 7-8 eV, which can be associated to a superposition of σ* shape resonances. Our results were compared with theoretical and experimental results available in the literature and with the results of electron collisions with formic acid. The additivity rule was used to estimate the total cross section of methyl formate and the results agree well with the experimental data.

Joint experimental and theoretical study on electron scattering from titanium tetrachloride (TiCl4) molecule.

Absolute grand-total cross section for electron scattering from titanium tetrachloride, TiCl4, molecule was measured at electron-impact energies ranging from 0.3 to 300 eV, in the linear electron-transmission experiment. The elastic integral, differential, momentum transfer, and total ionization cross sections for TiCl4 molecule were also calculated for low and intermediate collisional energies at the level of various theories. The low-energy elastic integral, differential, and momentum transfer cross sections were calculated with the Schwinger multichannel method implemented with pseudopotentials, in the static-exchange and static-exchange plus polarization levels of approximation, for energies up to 30 eV. The integral cross section calculated for low-energy electron scattering with the R-matrix method within the static-exchange and static-exchange plus polarization approximations for energies up to 15 eV are also reported. By the inspection of the cross sections, the presence of resonances is discussed. In particular, the calculated integral cross sections and the measured total cross section display a minimum at around 1 eV, which is consistent with the presence of a Ramsauer-Townsend minimum and a sharp increase at low energies, which is consistent with the presence of a virtual state. Additionally, interactions in elastic and ionization channels for intermediate collision energies were investigated with the additivity rule and the binary-encounter-Bethe methods.

Electron collisions with X(CH3)4 molecules (X = C, Si, Ge).

Absolute grand-total cross sections (TCSs) for electron scattering from tetramethylmethane [C(CH3)4], tetramethylsilane [Si(CH3)4], and tetramethylgermane [Ge(CH3)4] molecules have been measured at electron-impact energies extending from around 0.5 to 300 eV in the linear electron-transmission experiment. The measured TCS energy dependences show very pronounced broad enhancement, peaking near 5.5 eV for Si(CH3)4 and Ge(CH3)4 molecules and around 6.5 eV for C(CH3)4. Additional weak structures are also located at higher electron energies. We attributed the TCS features to the resonant processes involved in the electron-molecule scattering. To examine the role of permethylation in the scattering, the measured TCS energy functions for X(CH3)4 compounds (X = C, Si, Ge) have been compared to the TCS curves for XH4 molecules. Additionally, the integral elastic cross section (ECS) and ionization cross section (ICS) have been calculated from intermediate to high electron-impact energies using model methods. At energies above 50 eV, the sum of ECS and ICS for the investigated targets is in satisfactory agreement with the respective measured TCS. The computed ECS+ICS values can be used as rough estimation of TCS at energies above 300 eV.

Electron collisions with methyl-substituted ethylenes: Cross section measurements and calculations for 2-methyl-2-butene and 2,3-dimethyl-2-butene.

We report electron-scattering cross sections determined for 2-methyl-2-butene [(H3C)HC = C(CH3)2] and 2,3-dimethyl-2-butene [(H3C)2C = C(CH3)2] molecules. Absolute grand-total cross sections (TCSs) were measured for incident electron energies in the 0.5-300 eV range, using a linear electron-transmission technique. The experimental TCS energy dependences for the both targets appear to be very similar with respect to the shape. In each TCS curve, three features are discernible: the resonant-like structure located around 2.6-2.7 eV, the broad distinct enhancement peaking near 8.5 eV, and a weak hump in the vicinity of 24 eV. Theoretical integral elastic (ECS) and ionization (ICS) cross sections were computed up to 3 keV by means of the additivity rule (AR) approximation and the binary-encounter-Bethe method, respectively. Their sums, (ECS+ICS), are in a reasonable agreement with the respective measured TCSs. To examine the effect of methylation of hydrogen sides in the ethylene [H2C = CH2] molecule on the TCS, we compared the TCS energy curves for the sequence of methylated ethylenes: propene [H2C = CH(CH3)], 2-methylpropene [H2C = C(CH3)2], 2-methyl-2-butene [(H3C)HC = C(CH3)2], and 2,3-dimethyl-2-butene [(H3C)2C = C(CH3)2], measured in the same laboratory. Moreover, the isomeric effect is also discussed for the C5H10 and C6H12 compounds.

Electron scattering by trimethylene oxide, c-(CH2)3O, molecules.

Electron-scattering cross sections have been determined for trimethylene oxide, cyclic (CH(2))(3)O molecule, both experimentally and theoretically. The absolute total cross section (TCS) has been measured over energies from 1 to 400 eV using a linear electron-transmission method. The obtained TCS generally decreases with rising energy, except for the 3-10 eV range, where some resonantlike structures are discernible. Integral elastic cross section (ECS) and ionization cross section (ICS) have been also calculated up to 3 keV in the additivity rule approximation and the binary-encounter-Bethe approach, respectively. Their sum, ECS+ICS, is in a good agreement with the measured TCS. Comparison of the TCS energy dependence for trimethylene oxide with that for its isomeric open-chain counterpart--acetone, (CH(3))(2)CO, has also been made. Moreover, examination of experimental TCSs for the cyclic (CH(2))(n)O, n=2-4, ether series reveals that the intermediate-energy molecular TCSs for members of that family can be nicely represented as a sum of the effective TCSs for particular constituents of the molecule, i.e., methylene groups and oxygen atom. Finally, based on these partial TCSs, the TCS for the c-(CH(2))(5)O--the next member of the series--has been determined and compared with the respective ECS+ICS values computed here for this compound.

Intrinsic and extrinsic factors in anion electron-stimulated desorption: D- from deuterated hydrocarbons condensed on Kr and water ice films.

The results of D(-) ion desorption induced by 3-20 eV electrons incident on condensed CD(4), C(2)D(6), C(3)D(8), C(2)D(4), and C(2)D(2) are presented. These compounds were deposited in submonolayer amounts on the surfaces of multilayer solid films of Kr and nonporous and porous amorphous ice. While desorption of the D(-) anions proceeds via well-known processes, i.e., dissociative electron attachment (DEA) and dipolar dissociation, significant perturbations of these processes due to presence of the different film substrates are observed. We have shown that it is possible to distinguish between the character and nature of these perturbations. The presence of the nonporous ice perturbs the D(-) desorption intensity by affecting the intrinsic properties of the intermediate anion states through which dissociation proceeds. On the other hand, the presence of the porous ice introduces extrinsic effects, which can affect electron energy losses prior to their interaction with the hydrocarbon molecule and/or the energies and intensities of the fragment species after dissociation. Simple mechanisms responsible for the observed variations in the intensities of desorbed anionic signals are proposed and discussed. Electron transfer from transient anion states to electron states of the substrate film or nearby hydrocarbon molecules appear as the most efficient mechanism to reduce the magnitude of the DEA process.