Dissociation, collisional

Although collision-stabilized reaction complexes take part in chain propagation, the complex spectra of ions observed for ethylene and acetylene suggest that this mechanism undoubtedly must compete with consecutive reactions of species produced by unimolecular dissociation of the complexes and by collisional dissociation of other ions. ... 

Collisional dissociation The collision between a molecule and another species with sufficient energy to break one or more bonds in the target molecule. 

Tandem MS provides a powerful approach to this problem. In this mode, an ion exiting Ql enters a cell containing a low pressure of a gas such as Ar or N2 where it is collisionally dissociated. The fragments are then detected using the second quadrupole (Q2). The fragments are characteristic of the ion selected using Ql and provide confirmation of the identity of the parent ion. 

FIGURE 11.46 Typical mass spectra of ambient air (a) without collisional dissociation of the adducts and (b) with collisional dissociation. The counts for NO, and its adducts with HNO, are shown in parentheses in (a). Note changes in scale from linear to logarithmic (adapted from Tanner et al., 1997). 

In addition to the fully LH reaction of + CO, C02 apparently produced by hot atoms is also observed by thermal dissociation [367], photochemical dissociation [368,369] or collisional dissociation [370] of 02 co-adsorbed with CO. Dosing a CO adsorbed layer with gas phase atoms also appears to give the same hot atom reaction, i.e., reaction prior to the atom fully equilibrating with the surface [371]. In all cases, the oxidation occurs at Ts far below the thermal LH... 

The nomenclature used here and throughout the tables presented in this review to designate ion-neutral reactions conforms to that currently in use (see Ref. Id, p. 104). For collisional dissociation and dissociative charge transfer, we have adopted the notations AB (X X B)A and A+(BC A,B)C, respectively. 

Alternatively, double or tandem mass spectrometers may be employed for collisional dissociation experiments.208 With the latter, the incident-ion kinetic energy can be much more effectively controlled and continuously varied. Such experiments, in which translational-energy thresholds for collision-induced dissociations have been measured,50-108c 209-212 have yielded useful ionic thermochemical data. 

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 24. Apparent cross sections for collisional dissociation reaction, N2+(N2 N2,N)N+, as function of reactant kinetic energy. Both laboratory (lab) and center-of-mass (CM) energy scales are shown. Energies of ionizing electrons producing N2+ in each case are indicated. Arrows indicate upper limits on cross section for reaction when is produced by 19.2-eV electrons.36 ...

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...

Figure 27. Cross section for collisional dissociation reactions of N/ ions in X22g and A2nu states as function of incident-ion kinetic energy are indicated by dotted and dashed lines, respectively. Solid line is collisional dissociation cross section for incident-ion beam produced by impact of 50 eV electrons, which includes a mixture of the two states.380...

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... 

Figure 29. Relative cross sections for collisional dissociation of CHjBr in selected vibrational-energy states as function of kinetic energy. Data were obtained by photoion photoelectron coincidence technique and were analyzed by assuming only backscattering in center of mass. Maxima of curves were normalized to same relative cross section.86b...

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... 

Figure 30, Relative cross section as function of nominal kinetic energy (center of mass), E0, for collisional dissociation of He on He. Data are presented for He formed at two different ion-source pressures, as indicated.97...

A third theoretical approach, which attempts to overcome the inadequacies of the previous two statistical theories to predict population inversion, is the so-called information-theory approach 477 The latter has recently been applied to the problem of energy disposal and consumption in elementary chemical reactions. To the best of our knowledge, it has been applied to ion-neutral interactions only in the case of the collisional dissociation of H2+ (Table VI). 

Translational and internal energy are equally effective in overcoming reaction endoergjcity of collisional dissociation... 

Figure 2. SIMS/FTMS/MS spectrum of gramicidin S. After Cs+ ionization, m/z 100-1100 ions are ejected from the cell and the (M + Na)+ ions, m/z 1164, are collisionally dissociated by admitting air.

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