Pulsed ejection technique

A better approach is to pulse the reactant ion beam and to eject the product ions by pulsing a repeller electrode within the collision chamber as soon as the reactant ions have left the chamber. Figure 3 shows some data obtained by this pulsed ejection technique for the asymmetric charge-transfer reaction Ar -I- CH4-> Ar -I- CHj" -I- The minor... 

The longitudinal tandem technique, effective above energies of 1 eV, will surely continue to be used, but it must produce uncertain results to the extent that product ion detection efficiencies are uncertain. The pulsed ejection technique provides a hopeful, but difficult means of circumventing this problem. ... 

Fig. 22. Excitation function for the total charge-transfer cross section for the reactants Ar + CH4. Open circles refer to data obtained by the longitudinal tandem/pulsed ejection technique illustrated in Fig. 3. Solid circles refer to data obtained by the single-source impulse technique discussed in Section 3.4.4c. The relative excitation function of Koski is also shown and is normalized to Masson s absolute excitation function at 10 eV. Shown as a dashed line is the close-collision cross section predicted from the Langevin theory.

Slow dissociation rates (10 -10 s ) have been measured in Dunbar s laboratory by time-resolved photodissociation, which consists of trapping ions in an ICR cell during a variable delay time after a phot-odissociating photon pulse. The technique called time-resolved photoionization mass spectrometry , developed by Lifshitz, consists of trapping photoions in a cylindrical trap at very low pressure to avoid bimolecular collisions, and then ejecting them into a mass filter after a variable delay covering the microsecond to millisecond range. When the dissociation rate constant becomes lower than ca. 10 s competition with infrared fluorescence takes place and limits the lifetime of the decomposition process. This has to be taken into account to extract the dissociation rate constant from the experimental data. 

FT-ICR mass spectrometers belong to the tandem-in-time category of instruments. The stage of precursor ion selection (MSI) is accomplished by selectively storing the ions of interest, whereas all others are ejected by means of a suitably tailored excitation pulse, e.g., using the SWIFT technique. [206] FT-ICR mass spectrometers are also capable of MS . 

Dorfman and collaborators have recently developped a very promising technique for the production of carbenium ions as transient species in halocarbon sdvents based on the dissociative ionisation of suitable precursors induced by pulse radiolysis of the solvent. While the extremely interesting kinetic results vdiich this group is obtaining will be discussed in Sect. II-G4, it is emphasised here that the fast time response of the apparatus used allows the characterisation of carbenium ions hitherto unobservable because of their excessive reactivity. The ultraviolet absorption spectrum and some reactions of the benzylium ion have been studied for the first time wdth this powerful tool. From the point of view of cationic pdymerisation, the information obtained in this type of work is particularly relevant, since it deals vrith the identification and reactivity of carbenium icais formed in very low concentration in the nght kind of medium. Cation radicals had already been prepared by pulse radiolysis involving nondissociative ionization (electron ejection or transfer), as will be discussed in Sect. II-K. 

When the device is operated as a mass spectrometer, ions produced by an electron-impact or chemical ionization source are admitted through a grid in the upper end cap. The ionization source is pulsed so as to create a burst of ions. The ions over a large mass range of interest are trapped simultaneously. The ion trap can store ions for relatively long times, up to l.o minutes for some stable ions. A technique called mns.s-.te/ct/ri C ejeaion is then used to sequentially eject the trapped ions in order of ma,ss by increasing the radio-frequency... 

It is clearly difficult to employ this technique at high pressures since reactant and product ions ejected by the ejection pulse may be scattered differently on their passage out of the source. Under such conditions, it is apparently preferable to rely on thermal effusion, provided high enough pressures are employed. 

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