11922 J. Phys. Chem. A, Vol. 106, No. 49, 2002
Viggiano et al.
C8H10+) and 70 cm (52% C8H10+). This translates into R values
of 1.22 and 1.08, respectively, the latter being shown in the
graphs and tables. Thus, it appears that a small amount of
thermal dissociation is occurring. However, the difference is
small and barely detectable.
References and Notes
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At slightly higher temperatures, thermal dissociation of
+
C8H10 will probably be more important than the collisional
stabilization observed in the present study. Therefore, as the
temperature is raised, a competition between quenching and
thermal activation will occur. Eventually, the slopes of R should
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Conclusions
The first measurements taken in the upgraded TIFT as a
function of both temperature and pressure are reported. Few
other instruments are capable of such measurements,31 and this
is the only instrument to combine such capabilities with the
advantages inherent to flow tubes. For instance, the experiments
reported here would probably be limited by impurities in (quasi)
static systems. Flexibility in ion production is greater, and studies
involving reactive neutrals are possible.16 Low-pressure meas-
urements in conventional flow tubes and other still lower
pressure ion instruments generally measure true bimolecular rate
constants, although recent work has shown that exceptions may
occur regularly at pressures on the order of 1 Torr. Because
many plasmas occur at elevated pressure, it is important to
extend the pressure range upward.
The measurements reported here for the reaction of O2+ with
ethylbenzene confirm previous conclusions regarding the im-
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transfer and dissociative charge transfer for the reactions of
alkylbenzenes and simple ions.7,18 Increasing pressure dramati-
cally changes the amount of dissociation. The pressure depend-
ence of the ratio of nondissociative charge transfer products to
dissociative charge transfer, R, increases linearly over the
experimental range reported here. A simple model predicts such
a dependence. Increasing temperature decreases the importance
of the buffer, although the onset of a thermal dissociation process
is probably observed at the highest temperature. This leads to
a situation where the pressure dependence of R is steep and
positive at low temperature. The slope gradually decreases with
increasing temperature and is expected to become negative at
temperatures higher than the current upper limit. Thus, to fully
understand such reactions, studies must be conducted over as
wide a range of conditions as possible.
The present results will have an impact on the inputs to
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tion takes place at even higher temperatures and pressures than
the current experimental limitations allowed. Neither the ability
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impurities is an inherent obstacle to higher temperature and/or
pressure operation. Therefore, further upgrades to the TIFT
should allow for operation up to approximately 900 K and
pressures up to approximately one atmosphere. This will be the
second highest temperature of current generation ion-molecule
apparatus.34,35 Combining that capability with the extended
pressure variability will be unique and should allow for many
interesting studies.
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Acknowledgment. We thank John Williamson and Paul
Mundis for technical support. T.M.M. is supported through
Visidyne contract number F19628-99-C-0069. This work is
supported by AFOSR under Task 2303EP4.