Dissociative electron attachment excitation

Dissociative electron attachment (DEA) occurs when the molecular transient anion state is dissociative in the Franck-Condon (FC) region, the localization time is of the order of or larger than the time required for dissociation along a particular nuclear coordinate, and one of the resulting fragments has positive electron affinity. In this case, a stable atomic or molecular anion is formed along with one or more neutral species. Dissociative electron attachment usually occurs via the formation of core-excited resonances since these possess sufficiently long lifetimes to allow for dissociation of the anion before autoionization. 

Figure 2 Excitation (broken line, observed wavelength = 475 nm) and fluorescence (solid line, excitation wavelength = 320 nm) spectra of the benzyl radical generated in y-irradiated MTHF at 77 K by dissociative electron attachment to C2H5COOCH2C6H5.

The adsorbate substrate complex excitation mechanism predicts an action spectrum analagous to the absorption spectra of the complex. The direct mechanism predicts a photochemical action spectrum similar to that of the gas phase molecule. The final mechanism, dissociative electron attachment (DEA), suggests that the action spectrum should be referenced to the absorbance of the substrate, modified by the surface work function and electron attachment cross section of the adsorbate. The DEA mechanism appears to be of importance for many metal and semiconductor substrates, especially for the case of photochemistry induced by anomalously low energy radiation. 

An estimate of the activation barrier for this reaction is 0.2 eV, comparable to the energy of a COj laser photon. Excitation of the 10 jam rj mode of SFg is seen to enhance the dissociative electron attachment process. 

N. Itoh, A.M. Stoneham, Materials Modification by Electronic Excitation, Cambridge University Press, Cambridge (2001) L. Sanche, Excess Electrons in Dielectric Media, p. 1, CRC Press, Boca Raton (1991) A.D. Bass and L. Sanche, Dissociative electron attachment and charge transfer in con densed matter, Radiat. Phys. Chem. 68(1 2), 3 13 (2003). 

Similar experiments with more tightly held adsorbates require higher bias voltages [54—59]. In this way, chlorobenzene adsorbed on Si(l 11) was subjected to the selective dissociation of C—Cl bonds, and it was concluded that a two-electron mechanism is operating that couples vibrational excitation and dissociative electron attachment processes. As can be seen from Fig. 4.9, the yield of desorption increases linearly with the electron current indicating a single-electron process, while for dissociation the yield increases with the second power. 

Good examples that illustrate direct photon adsorption for an adsorbate are the photodissociation of Mo(Co)g on Cu(lll) and Ag(lll). Here, the wavelength dependence of photodissociation is nearly the same as that in the gas phase. Direct absorption can also mediate charge-transfer surface reactions. Dissociative electron attachment of adsorbates is an important process in surface chemistry induced by laser excitation this will be the topic of Section 27.2. 

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. ... 

The increase in the electron energy may have several consequences. It may lead to dissociative or nondissociative electron attachment (54, 61). This would give rise to a step enhancement as in N20 (8). The most important possibility is electronic excitation of the molecular species, which should manifest itself by an increased yield of all products arising from excited intermediates as the mean energy of the electron swarm rises with field strength. 

FIG. 13. Cross sections for electron collisions for SiH4-H2 (a) SiH4. (b) Si2H (dotted lines) and H2 (solid lines). Abbreviations are ion, ionization dis. dissociation vib. vibrational excitation att, attachment. See Table II for details and references. (Adapted from G. J. Nienhuis. Ph.D. Thesis. Uni-versiteit Utrecht. Utrecht, the Netherlands. 1998. with permission.)... 

The observed femtosecond dynamics of this dissociative CT reaction is related to the nature of bonding. Upon excitation to the CT state, an electron in the highest occupied molecular orbital (HOMO) of benzene (ir) is promoted to the lowest occupied molecular orbital (LUMO) of I2 (a ). Vertical electron attachment of ground state I2 is expected to produce molecular iodine anions in some high vibrational levels below the dissociation limit. In other words, after the electron transfer, the I—I bond is weakened but not yet broken. While vibrating, the entire I2 and benzene complex begins an excursion motion within die coulombic field and the system proceeds... 

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