Negative ions, long-lived

Organolead compounds may be detected by turning off the heating to the thermionic source and running in the FTID mode. In this mode, the combustion of organolead compounds lead to long-lived negative-ion products which are detected at the TID collector. 

Transient Attachment and Long-Lived Negative Ions. 99... 

Some molecules, such as BCI3 and C-C4F8, are able to attach an electron and form a long-lived negative-ion complex (Christophorou, 1978). The interpretation of electron attachment data is made difficult in such gases because the measured attachment coefficient t]/N becomes a function of pressure as well as of the size of the experimental apparatus. In the following I will describe attachment issues in BCI3. 

Eventually, the protonated monomer peak intensity declines, and the reactant ion peak is restored to the original intensity. Although this pattern is commonly associated with positive-polarity IMS, similar patterns may be observed with negative ions, such as MCE and M2CE, as observed with volatile halogenated anesthetics " and other chemicals like trichloroanisole that form long-lived negative-ion clusters. 

Other species isoelectronic with He", e. g., Li ° have been treated in detail by many of the same investigators who have examined He. A particularly important species tliat is isoelectronic with He and is of interest to this discussion is H The H ion is known to have at least one bound state (a very extensive literature exists concerning the H" ion). Recently, HiUs has reported that the singlet state of H" has only one bound state and that the triplet state of H" has no bound state within the approximations he employed. Further comments on the doubly-charged negative H " ion, however, will be held is abeyance until the experimental observations of a long-lived H ion by Schnitzer and Anbar and a much shorter-lived H ion by Walton, Peart and Dolder ° are considered. 

Figure Bl.6.11 Electron transmission spectrum of 1,3-cyclohexadiene presented as the derivative of transmitted electron current as a fiinction of the incident electron energy [17]. The prominent resonances correspond to electron capture into the two unoccupied, antibonding a -orbitals. The negative ion state is sufficiently long lived that discrete vibronic components can be resolved.

Carbon disulfide is isovalent to carbon dioxide and it also has a bent monomer anion. While gas-phase CO2 has negative EAg of —0.6 eV [24], for CS2, EAg is +0.8 eV [34]. Despite this very different electron affinity, Gee and Freeman [34] observed long-lived electrons in CS2 (with lifetime > 500 psec) with mobility ca. 8 times greater than that of solvent cations. Over time, these electrons converted to secondary anions whose mobility was within 30% of the cation mobility. Between 163 and 500 K, the two ion mobilities scaled linearly with the solvent viscosity, as would be expected for regular ions. Of course, Gee and Freeman s identification of the long-lived high-mobility solvent anions as electron is just a manner of speech Obviously, quasifree or solvated electrons cannot survive for over a millisecond in a positive-EAg liquid. 

The product recoil and angular distributions from the reaction Csl + CI2 were found to be consistent with a long-lived collision complex [625]. The reaction is pictured as proceeding by insertion of the I of the Csl into the Cl2 bond to give a Cs+ ion and a trihalide negative ion (C1IC1)- (which is stable because the least electronegative atom occupies the central position). 

Although for the molecules in Fig. 4 gissociative attachment processes are expected to be fast (<<10 sec), for C2C1 at 0.0 eV (and for a number of other autodissociating long-lived parent negative ions) both the autodissociation and the autodetachment processes are slow, as a result of vibrational redistribution of internal energy ( ). 

Ar . Long-lived metastable states of negative ions of the noble gas atoms other than He are unknown to the present, although Bethge indicated he has extracted very low intensity Xe ion beams from his dense plasma Penning source. Since so little other information is available at this time, it is not inappropriate to adopt He as a model for the isoelectronic monatomic ions of Group VII and for other species. 

The literature records a variety of statements to the effect that long-lived (> 10 s) doubly-charged negative ions do not exist. The remainder of this discussion basically involves descriptions and evaluations of experiments and interpretations that demonstrate that doubly-charged negative ions do exist. 

Therefore it was essential for Stuckey and Kiser to determine the ionization efficiency curves for the negative ions, both singly- and doubly-charged, that they had observed. These investigations would yield additional information about the formation of the long-lived doubly-charged negative ions. 

Chupka, Spence and Stevens, in recognizing the fundamental importance of the long-lived doubly-charged negative ions reported by Stuckey and Kiser, also have expressed the belief that the observed ion peaks may be experimental artifacts. 

Ahearn and Hannay studied mass spectrometrically the decay of SF to SFs and indicated that a long-lived metastable state of SFg was involved. Spence and Schulz considered the effect of temperature on electron attachment of molecules at low (< 0.2 eV) energies. They generalized that for many polyatomics the total cross section for negative ion production tends to an absolute maximum as the gas temperature is increased, in agreement with Fox and CurranSpence and Schulz also postulated that a common SF ion is involved in decompositions that yield F, SFs and the stable SF6. Henis and Mabie have studied an SF species formed by capture of very low energy electrons in an ICR spectrometer and found its lifetime to be approximately 0.5 ms. 

In the final step, electron attachment occurs to form the observed long-lived doubly-charged negative ion ... 

Chupka, W. A., Spence, D., Stevens, C. M. Search for Long-lived Doubly-Charged Negative Ions, pp. 126-127, Radiological and Environmental Research Division Annual Report, ANL-75-3-PI, July 1973-June 1974, Argonne National Laboratory, Atgonne, 111., 1974 Colburn, C. B., Kennedy, A. J. Amer. Chem. Soc. 80, 5004 (1958)... 

One of the important advantages of a tandem accelerator is that apparently does not form negative ions that live long enough to pass through the accelerator to the terminal (18,31). Thus a primary source of background is essentially eliminated. Another important characteristic of a TAMS is the fact that it is a relatively simple operation to sequence the acceleration of the various isotopes of carbon through the system in a reproducible manner. Thus measurements can be made of and... 

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