Dissociative electron attachment ionization

Although the irradiation of 200 kGy decomposes about 80% of polystyrene in toluene by the dissociative electron attachment, the yield of the decomposition is only 20% for solid toluene. Because of its low efficiency of scission, the coupled polystyrene may not be a polymer suitable as a radiation resist. However, the present study has shown that a polymer that can be decomposed into two equivalent skeletons by ionizing radiation is possible to be... 

At cryogenic temperatures, alkane radical cations are stable when fully isolated in the CCI3F matrix, because electrons formed in the ionization process react with trichlorofluoromethane by dissociative electron attachment. 

Neutralization by electrons remains an important process in the radiolysis of alkanes containing chloroalkanes or CO2 as solute. Many of the electrons formed in the ionization process have insufficient energy to escape the Coulomb field of their associated cation and, on returning, neutralize the corresponding radical cations, carbenium ions or carbonium ions (geminate recombination). Part of the electrons formed do not return, however, but react with the chloroalkane solute by dissociative electron attachment. 

Below the first ionization potential, NH2 radical production has been proposed and attributed to a dissociative electron attachment occurring near 3.9 e.v. (9). 

Photoionization of the hydrocarbon followed by dissociative electron attachment (Reaction 1) should be considered since the ionization potential of a molecule is less in the liquid phase than it is in the gas phase. For hydrocarbons the ionization potential is 1 to 1.5 e.v. less in the liquid phase (24). The photon energy at 1470 A. is about 1.4 e.v. below the gas-phase ionization potentials of cyclohexane and 2,2,4-trimethylpentane (14). Some ionization may therefore occur, but the efficiency of this process is expected to be low. Photoionization is eliminated as a source of N2 for the following reasons. (1) If photoionization occurred and the electron reacted with nitrous oxide, then O" would be formed. It has been shown in the radiolysis of cyclohexane-nitrous oxide solutions that subsequent reactions of O result in the formation of cyclohexene and dicyclohexyl (I, 16, 17) and very little cyclohexanol (16, Table III). In the photolysis nitrous oxide reduces the yield of cyclohexene and does not affect the yield of dicyclohexyl. This indicates that O is not formed in the photolysis, and consequently N2 does not result from electron capture. (2) A further argument against photoionization is that cyclohexane and 2,2,4-trimethylpentane have comparable gas-phase ionization potentials but exhibit quite different behavior with respect to N2 formation. 

Guertin, Christe, and Pavlath (1966) considered another mechanism of NF4ArF6 synthesis in plasma through a set of ion-molecular reactions. The mechanism starts with the formation of positive ions (NF and Fj through ionization, and the formation of negative ions (F ) through dissociative electron attachment to NF3, AsFs, or F2 molecules. The initial positive ions lead to the formation of an ion radical NF4 ... 

The radical cation is similarly obtained by using a halogenated molecule as a solvent. RX (solvent molecule, X = Cl or Br) + ionizing radiation RX - - e e + RX R + X (dissociative electron attachment)... 

The physical processes when collecting negative ions from the same analytical GD are different and could provide complementary information. The ionization mechanisms may be two- or three-body electron attachment (for species with a suitable electron affinity) or dissociative electron attachment (i.e., electron attachment followed rapidly by unimolecular fragmentation) or charge transfer from existing negative ions [56]. 

The generation of SO/ involves a variety of reactions and the specific method can influence the ultimate ion chemistry. Dissociative electron attachment to SO2, which exhibits an energy-dependent cross-section (SO2 + e" —> SO" + O) [8], is the typical preparation method for SO". The sulfur dioxide radical anion, S02 , is easily produced via three-body electron attachment following ionization of dilute mixtures of SO2 in a chemically inert buffer gas [9]. Both O/ and CO," have been used [9J as efficient O" donors to form the sulfite radical anion, SO/, from sulfur dioxide, reactions (1) and (2) ... 

Dissociative excitation of molecules by electrons is a key process in many industrially important plasmas because it is the mechanism that provides the activated radicals that initiate the surface chemistries of interest. For example, many of the gases used in the etching of silicon do not display any reactivity in the absence of plasma. The construction of detailed models of these plasmas relies on a reliable data base of cross sections. Unfortunately, electron-impact dissociation cross sections are extremely difficult to measure and there are only a handful of cases where good data exist. Chlorine gas, which is widely used in the plasma etching of semiconductors, is one such example. Cross sections for ionization and dissociative electron attachment were measured during the 1970s and there has been one experimental study of electron impact dissociation. Cross sections for other dominant electron collision processes have been derived from Boltzmann analysis and early swarm measurements. ... 

Notably, sub-ionization low-energy electrons have been found to cause singlestrand breaks in DNA, which is thought to be consistent with a dissociative electron-attachment mechanism, according to Scheme 5.19 [93,103]. 

These halogenated solvents act as electron scavengers by dissociative electron attachment, leading to complexes between chloride ions and radicals that persist indefinitely in solids at low temperature. This leaves the holes to migrate through the solvent and attach to substrates that are easier to ionize than the solvent molecules, thus forming the targeted radical cation. The main limitation of the above methods is that the spectroscopic window for the observation of radical ions is usually limited to 300 -1400 nm (at the UV end by the electronic absorption of the neutral, in the NIR due to vibrational overtones of the solvent molecules). 

---