Excited ions collisional dissociation

The principle of MS/MS for direct analysis of a multicomponent system is shown in Figure 6.18, in which the first mass spectrometer (MS I) operates with soft ionisation (FI, FD, Cl, LD), and thus produces an ensemble of molecular ions (M + H+, M — H+, or adducts). For identification of molecule ABC only ABC+ is allowed to enter an interface or fragmentation zone for excitation by collisional activation, laser radiation or surface-induced dissociation. Within the time of one vibration (10-13s), ABC+ dissociates into fragments characterising the original molecule. They are separated and detected by MS II [226]. Soft ionisation with FI/FD produces low ion yields, which may be insufficient for MS/MS LVEI (typically at 20 V) can be an alternative. Complete analysis of a multicomponent system is carried out in some 20 min. 

Excited states can be formed by a variety of processes, of which the important ones are photolysis (light absorption), impact of electrons or heavy particles (radiolysis), and, especially in the condensed phase, ion neutralization. To these may be added processes such as energy transfer, dissociation from super-excited and ionized states, thermal processes, and chemical reaction. Following Brocklehurst [14], it is instructive to consider some of the direct processes giving excited states and their respective inverses. Thus luminescence is the inverse of light absorption, super-elastic collision is the inverse of charged particle impact excitation, and collisional deactivation is the inverse of the thermal process, etc. 

C22a+) state, followed by predissociation from vibrational levels, v > j 218-220 Another mechanism for collisional dissociation entails direct excitation of the ion to the vibrational continuum of the ground state. Both of these mechanisms may operate in competition in the same system.217... 

Figure 22. Cross section for collisional dissociation reaction, 02 (02 02,0)0+, as function of energy of impacting electrons used to produce reactant. Solid curve represents cross section, bars indicate relative cross sections for various excited states, and arrows designate threshold energies for production of electronically excited states of O. Ion-beam energy in these experiments was 1.6 keV,38a...

Figure 26. Apparent cross section for collisional dissociation reaction, N2+(N2 N2,N)N+, as function of energy of electrons producing Nj" (solid curve and data points). Laboratory kinetic energy of primary ions was 10 eV. Cross section for radiative emissions from long-lived, excited states formed in electron impact on N- is also indicated (dashed line).36a...

The enhancement of collisional dissociation cross sections by reactant-ion vibrational excitation is also observed for interactions at relatively low translational energies, particularly in the energy region near threshold. This was demonstrated in studies of dissociation of ( = 0-5) in which... 

Since the velocities and angular distributions of products from collisional dissociations at low incident-ion energies have generally not been determined, the precise mechanism by which the products are formed is unknown. Thus in the collisional dissociation of H2+ with helium as the target gas, H+ may result from dissociation of H2+ that has been directly excited to the vibrational continuum... 

The yields calculated using these assumptions are in satisfactory agreement with the experimental values, as illustrated in Table 18. The observation that, among all the fragmentation processes induced by the decay, only reaction (27) is prevented by collisional deactivation, supports the view that the excitation level of the propyl ions that dissociate at 10 torr into allyl ions is low indeed, and that such decomposition is possible for its low energetic requirements. The situation is completely different in the liquid phase. This is indicated, in the first place, by the substantial decrease of the yield of HT, which provides a rough indication... 

Because of space limitations, we have limited ourselves to reactions of positive ions and have omitted processes like collisional dissociation which, so far as multiple collisions are concerned, are strongly related to vibrational excitation and deexcitation, and we have also omitted isotopic exchange processes, which show many similarities to proton-transfer reactions, isomerization processes, and switching reactions. 

Lastly, we mention one more excitation mechanism that has been observed in molecules. It is well-established that following strong field ionization in atoms and molecules, under certain conditions, the ionized electron can be driven back to the ion core where it can recombine to produce high-harmonic radiation, induce further ionization, or experience inelastic scattering. However, there is also the possibility of collisional excitation. Such excitation was observed in [43] in N2 and O2. In both molecules, one electron is tunnel ionized by the strong laser field. When the electron rescatters with the ion core, it can collisionally ionize and excite the molecular ion, creating either N + or Ol+ in an excited state. When the double ion dissociates, its initial state can... 

Collision induced dissociation the dissociation of ions after collisional excitation... 

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