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The Observation Of Transient Species Using a TOF-MS

Photo of Mark

Researcher: Dr. Mark Blitz, School of Chemistry, University of Leeds.

An example of the use of the Kore Mini-TOF for the measurement of bimolecular rate constants of reactions between molecules: NO2 + ClSO, and NO2 + SO.

Aim of experiment: to monitor the appearance and decay of short-lived radicals produced during photolysis of Cl2SO, and to determine the radical reaction rate constants with added reagents (NO2).

Method: use 248nm photolysis laser to create radicals from Cl2SO in the presence of NO2 and then probe the radical population using a second ionising laser of 118nm wavelength.

Experimental: when excited with 248nm light the molecule Cl2SO can produce short lived radicals in two ways:

Cl2SO = Cl + ClSO
Cl2SO = Cl2 + SO

97% of Cl2SO molecules convert according to the upper pathway, and 3% following the lower pathway. A mass spectrum shows the two molecules produced during irradiation.

Mass spectrum showing SO and ClSO peaks

Following a photolysis laser pulse to create the radical species, a second ionising laser with a frequency of 118nm was used to plot the relative concentration of these radicals as a function of elapsed time (out to 30 milliseconds). This is achieved by varying the delay time between the firing of the two lasers. The radicals were monitored by ionisation of these radical (R→R+) with their subsequent detection via the TOF-MS.

Decay curve for ClSO Decay curve for SO

From these graphs, rate constants, kobs, can be calculated:

[Radical] = [Radical]0 x exp(-kobst)

By varying NO2 in the system, the bimolecular rate constants, kbim, for reaction of ClSO + NO2 and SO + NO2 can be determined from kobs = kbim [NO2]; kbim equals the slope from plot of kobs vs [NO2].

Plot where slope gives Kbim(ClSO+NO2)=8E-12 molecule-1 cc-1 s-1 Plot where slope gives Kbim(SO+NO2)=1.52E-11 molecule-1 cc-1 s-1


Last updated: 25 February 2006 20:59

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