The system is based on a quartz reaction cell within a bench-mounted, high pumping capacity vacuum system, through which a carrier gas flows at ~1 mbar pressure. The carrier gas also contains other key components. A 10Hz photolysis laser is guided into this reaction cell to create radicals R° from a precursor. Following this photolysis step, the radicals react with a further component X, which is also mixed at low concentration with the He or Ar.

The RX component and carrier effuse from a small orifice (~1mm diameter) in the reaction cell. Components in the emerging plume are then ionised with a vacuum ultraviolet (VUV) laser pulse, creating RX+ ions that can be pulsed with a Kore TOF source and then measured by the TOF mass spectrometer. The counting system is analogue, owing to the expected data rate, and the results are recorded on a fast digital oscilloscope.

By varying the delay time between the first laser pulse to create radicals and the second laser pulse to create RX ions, the kinetics of the R°+X=RX reaction can be investigated. The time constants for the reactions of interest range between 0.1 microseconds and 1000 microseconds (1 millisecond).

For an example of the work that is being done with this apparatus, please click here

The Observation Of Transient Species Using a TOF-MS

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.

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

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.

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.

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

[Radical] = [Radical]₀ x exp(-k_obst)

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