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Oxygen isotope fractionation in the vacuum ultraviolet photodissociation of carbon monoxide: Wavelength, pressure, and temperature dependency
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Image of FIG. 1.
FIG. 1.

Schematic and partial potential energy level diagram of CO showing the positions of every level used in these experiments along with their corresponding upper electronic states. A major phenomenon of accidental predissociation is schematically shown where the Rydberg state E 1Π crosses over another Rydberg state k 3Π and the latter finally dissociates through a 3Π repulsive state. E-X (1,0) and (0,0) transitions are demonstrated to follow the accidental predissociation pathways and unusual oxygen isotopic fractionation was observed for these two synchrotron bands.

Image of FIG. 2.
FIG. 2.

Schematic diagram of the experimental setup used in the chemical dynamics beamline at the Advanced Light Source for CO photodissociation experiment. Three stage differential pumping system was used with the reaction chamber of different lengths (48, 105, 130, and 182.5 cm) for different experimental runs to generate significant variation in column densities (and residence time) when combined with pressures used.

Image of FIG. 3.
FIG. 3.

The measured synchrotron beam profiles centered at six different energies within 90 through 110 nm wavelength regime as used in these experiments (left Y axis shows the beam current). The position of CO absorption bands are shown by vertical lines corresponding to their absorption cross section (absorption cross section values are taken from Ref. 2, right Y axis).

Image of FIG. 4.
FIG. 4.

Oxygen isotopic composition shown in three-isotope plot (δ 187O vs δ 18O) measured in different synchrotron bands at two different temperature settings of 20 °C (RT) and −78 °C (DI temperature) in logarithmic scale (a) centered at 105.17 and 107.61 nm, depicting a combined slope value of 1.36 for RT and 1.22 for 105.17 nm at DI temperature, respectively (b) centered at 97.03 nm showing a slope values of 1.24 and 0.89 for RT and DI temperature, respectively (c) centered at 94.12 nm showing a slope value of 0.76 and 0.80 for RT and DI temperature, respectively, and (d) centered at 91–92 nm showing a slope value of 0.72 and 0.76 for RT and DI temperature, respectively.

Image of FIG. 5.
FIG. 5.

Variation of slope values (δ ′17O/δ ′18O) measured in the experiment with energy. The slope values (both for RT and DI temperature) obtained from the regression lines of Figure 4 were plotted against the energy of the synchrotron bands. 1σ error of the regression lines is also shown. Isotope effects due to self-shielding (differential photon absorption) do not predict such a variation in slope values (energy dependence). The associated secondary mass-dependent fractionations are relatively small compared to the measured compositions and are insufficient to modulate the slope in a significant way (see text for detailed discussion). Therefore, addition mass-independent fractionation is required in photodissociation process to explain the results.


Generic image for table
Table I.

Tabulated CO absorption lines covered by the six different synchrotron VUV bands used in the experiments. A short description has been provided where available from the published literature.4,9–15,33,40

Generic image for table
Table II.

Compilation of all experimental parameters and conditions used in CO photodissociation experiments along with measured oxygen isotopic compositions (linear and logarithmic form). The calculated oxygen isotopic compositions of atomic oxygen (CO dissociation products) are tabulated in the last three columns.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Oxygen isotope fractionation in the vacuum ultraviolet photodissociation of carbon monoxide: Wavelength, pressure, and temperature dependency