Laser isotope separation and the multiphoton dissociation of formic acid using a pulsed HF laser |
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Authors: | DK Evans Robert D McAlpine FK McClusky |
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Institution: | Physical Chemistry Branch, Chalk River Nuclear Laboratories, Atomic Energy of Canada Limited, Chalk River, Ontario, Canada KOJ 1JO |
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Abstract: | The focussed beam from a single line P2 (5)] of a pulsed HF laser has been used to stimulate the decomposition of formic acid. The yield (Y is the number of product molecules per pulse / formic acid pressure) of the non-condensable (77 K) products, hydrogen and CO, has been studied as a function of laser radiant energy (from 25-115 mJ) and pressure (from 0.4-2.7 kPa). The intensity dependence of Y suggests that each dissociating formic acid requires the equivalent of at least 6 HF P2(5) photons (260 kJ/mole). For pressures above about 0.6 kPa, YH2 = (?0.6 ± 1.7) × 1012 + (2.4 ± 1.0) × 1012 P and YCO = (?0.5 ± 6.1) × 1013 × (8.7 ± 3.7) × 1013 P. The linear dependerrce of yields indicates that a collisionally assisted decomposition process is important at these pressures. The efficiency of the conversion of photon energy to reaction products at a pressure of 2.7 kPa is ? 7% for CO and ? 0.2% for hydrogen. Selective excitation of HCOOH in equimolar mixtures of HCOOH/HCOOD, at a total pressure of 0.6 kPa, has provided a physically separated product, hydrogen gas, which is isotopically enriched in H versus D 25 fold as compared to the formic acid mixture. The degree of enrichment decreases as the total pressure of the mixture is increased. A possible mechanism accounting for isotope enrichment and the collisionally assisted dissociation is outlined. |
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