Ionization, dissociative

Figure Bl.7.7. Summary of the other collision based experiments possible with magnetic sector instruments (a) collision-mduced dissociation ionization (CIDI) records the CID mass spectrum of the neutral fragments accompanying imimolecular dissociation (b) charge stripping (CS) of the incident ion beam can be observed (c) charge reversal (CR) requires the ESA polarity to be opposite that of the magnet (d) neutiiralization-reionization (NR) probes the stability of transient neutrals fonned when ions are neutralized by collisions in the first collision cell. Neutrals surviving to be collisionally reionized in the second cell are recorded as recovery ions in the NR mass spectrum.

Dissociative ionization. A process in which a gaseous molecule decomposes to form products, one of which is an ion. 

A book (B-71MS) and a review by Nishiwaki (74H(2)473) contain much information about the behaviour of pyrazoles under electron impact. The Nishiwaki review covers mainly the hydrogen scramblings and the skeletal rearrangements which occur. One of the first conclusions reached was that pyrazoles, due to their aromatic character, are extremely stable under electron impact (67ZOR1540). In the dissociative ionization of pyrazole itself, the molecular ion contributes about 45% to the total ion current thus, the molecular ion is the most intense ion in the spectrum. 

Sulfurous acid has never been isolated as a pure compound, although it has recently been detected in the gas phase by neutralization reionization mass spectrometry (NRMS) following the facile dissociative ionization (70 eV) of either diethyl sulfite or ethanesulfonic acid " ... 

Reactions 8 and 9 represent and are typical of all the dissociative ionizations producing the observed fragment alkyl ions. Reaction 9... 

In silane discharges, one observes the following when the discharge is off, the mass spectrometric signal at m/e = 31 amu/e (SiH ) as a function of electron energy is due to dissociative ionization of SiHa in the ionizer of the QMS, with an ionization potential of 12.2 eV [312]. The signal with the discharge on is due to ionization of the radical SiHa plus the contribution from dissociative ionization... 

Si2H4 is an ion that is created in the plasma by polymerization reactions. Several pathways may lead to this ion. The first pathway is the dissociative ionization of SiiHa that is formed in a radical-neutral reaction. The second pathway is the direct formation in the ion-molecule reaction [192] SiH + SiH4 SizH + Hi. 

The delay time between the pump and the probe laser pulses is usually very short in these experiments. The delay time is less than 5 ns when the pump and the probe laser pulses are the same, and the delay time is as long as several hundred nanoseconds when the pump and the probe laser pulses are from two different sources. The short delay time ensures that the fragments flying with different velocities are equally sampled before they leave the detection region. Since the delay time is much shorter than the lifetime of the excited molecules (.A ), most of these molecules do not dissociate into fragments when the probe laser pulse arrives. As a result, the probe laser can easily cause dissociative ionization of the vibrationally excited molecules due to their large internal energy. 

However, at very short delay time, some of the hot toluene which has not dissociated into fragments could absorb the VUV photon and result in dissociative ionization. 

Fig. 22. The product buildup times. The solid line is the simulation from reactions (27)-(36). The non-zero intensity at I, - 0 and the intensity decay of mje = 94 is due to the dissociative ionization (reaction (36)).

It would be advantageous to be able to lower the electron energy in the electron impact source of CMB instruments sufficiently to reduce strongly, or even eliminate, the problem of dissociative ionization from interfering... 

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