Dissociative electron detachment

A Dalton is defined as l/12tli of the mass of a C atom. It differs from an atomic mass unit (amu), which is defined as l/16th of the mass of a atom. Electron capture dissociation Electron detachment dissociation Electron-induced dissociation... 

PES photoelectron spectrometer SF space focus L1-4 laser for different purposes LI resonant multiphoton ionization secondary ion excitation mass selective dissociation, electron detachment from anions anion formation, laser desorption TOF-sep mass separation by time-of-flight... 

The discussion earfier implicitly assumes that the anion M is bound, that is, lower in energy than the neutral molecule Mat the geometry of M . At the heart of dissociative electron detachment, however, are temporary anion resonances that are metastable only with respect to autodetachment. This is the case, for example, when the anion M is formed at the neutral molecule s geometry in the example depicted in Figure 1. Flere, the anion is higher in energy at the neutral molecule s most stable geometry. 

In the studies outlined below, collisionally activated dissociation (MS/MS) experiments were attempted as a means of helping establish the identities of a number of anions. In every case tried, however, the parent anions appeared to undergo electron detachment with no daughter ion production observed. 

Processes (19.61)-(19.63) are similar to electron-impact processes (19.33), (19.34) and (19.25), except that in the present case the intermediate (H-7) resonant state is formed not by capture of a free electron but rather by capture of loosely bound electron from 11. There may be also other mechanisms for electron detachment processes (19.61) and (19.63) and for the dissociation process (19.62). 

Warman JM, Fessenden RW, Bakale G. (1972) Dissociative attachment of thermal electrons to NjO and subsequent electron detachment. J Chem Phys 57 2702-2711. 

Similar arguments apply to the results (see Fig. 18) of the electric deflection analysis of the M/z = 254 ion beam. Both l2 and the formed as a result of the repulsive electron detachment from in the first field-free region (that is, I(R) ) will be transmitted by the magnetic sector. Kinetic energy analysis of this ion beam requires 2U(. to bring the I(R) species to Faraday cup 2 and to detect the l2 ion. Any formed by dissociation of I2 in the second field-free region, I(D), will be detected at Faraday cup 2 by application oiUJl to the electric deflection plates. 

In this chapter the experimental ECD and NIMS procedures for studying the reactions of thermal electrons with molecules and negative ions are described. Gas phase electron affinities and rate constants for thermal electron attachment, electron detachment, anion dissociation, and bond dissociation energies are obtained from ECD and NIMS data. Techniques to test the validity of specific equipment and to identify problems are included. Examples of the data reduction procedure and a method to include other estimates of quantities and their uncertainties in a nonlinear least-squares analysis will be given. The nonlinear least-squares procedure for a simple two-parameter two-variable case is presented in the appendix. 

The specific rate constants of interest to the ECD and NIMS are dissociative and nondissociative electron attachment, electron detachment, unimolecular anion dissociation, and electron and ion recombination. The reactions that have been studied most frequently are electron attachment and electron and ion recombination. To measure recombination coefficients, the electron concentration is measured as a function of time. The values are dependent on the nature of the positive and negative ions and most important on the total pressure in the system. Thus far few experiments have been carried out under the conditions of the NIMS and ECD. However, the values obtained under other conditions suggest that there is a limit to the bimolecular rate constant, just as there is a limit to the value of the rate constant for electron attachment. The bimolecular rate constants for recombination are generally large, on the order of 10 7 to 10-6 cc/molecule-s or 1014 to 1015 1/mole-s at about 1 atm pressure. Since the pseudo-first-order rate constants are approximately 100 to 1,000 s 1, the positive-ion concentrations in the ECD and NIMS are about 109 ions/cc. 

Electron detachment (5-126) together with dissociative attachment (5-125) creates a chain reaction of water decomposition. One electron is able to participate in the H2O dissociation process many times in this case, which makes the whole kinetic mechanism energy effective. Cross sections of the electron detachment by electron impact (5-126) are shown in Fig. 5-58 a typical value of rate coefficient for the detachment process is very high (kd = 10 cm /s Smirnov Chibisov, 1965 Tisone Branscome, 1968 Inocuti et al., 1967). The chain mechanism, (5-125) and (5-126), is initiated by ionization of H2O molecules. The chain termination is related to fast ion-ion recombination,... 

The continuous spectrum is also present, both in physical processes and in the quantum mechanical formalism, when an atomic (molecular) state is made to interact with an external electromagnetic field of appropriate frequency and strength. In conjunction with energy shifts, the normal processes involve ionization, or electron detachment, or molecular dissociation by absorption of one or more photons, or electron tunneling. Treated as stationary systems with time-independent atom - - field Hamiltonians, these problems are equivalent to the CESE scheme of a decaying state with a complex eigenvalue. For the treatment of the related MEPs, the implementation of the CESE approach has led to the state-specific, nonperturbative many-electron, many-photon (MEMP) theory [179-190] which was presented in Section 11. Its various applications include the ab initio calculation of properties from the interaction with electric and magnetic fields, of multiphoton above threshold ionization and detachment, of analysis of path interference in the ionization by di- and tri-chromatic ac-fields, of cross-sections for double electron photoionization and photodetachment, etc. 

In some cases, the consequence of photon absorption is electron detachment rather than dissociation, as shown in Equation 9.2. Electron detachment is straightforward to monitor in a mass spectrometer because the charge state number increases by one. Note that for monoanions, the products of electron detachment are neutral molecules and electrons because the neutral species escape the trap undetected, the dissociation yield must be calculated from the disappearance of the parent ion. 

Budnik, B.A., Haselmann, K.F., and Zubarev, R.A. (2001) Electron detachment dissociation of peptide di-anions an electron-hole recombination phenomenon. Chem. 

While most peptide dissociation is carried out in the positive ion mode, the negative ion mode is often better suited for acidic peptides, particularity those carrying acidic modifications (e.g., phosphorylations). There are a number of equivalent ion activation methods for peptide anions, involving ion-electron and ion-ion reactions, such as electron detachment dissociation (EDD) [49], negative electron transfer dissociation (NETD) [50, 51], and negative electron capture dissociation (nECD) [52]. 

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