Laser isotope separation and the multiphoton dissociation of formic acid using a pulsed HF laser

Abstract

The focussed beam from a single line [P 2 (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 P 2(5) photons (260 kJ/mole). For pressures above about 0.6 kPa, Y H 2 = (−0.6 ± 1.7) × 10 12 + (2.4 ± 1.0) × 10 12 P and Y CO = (−0.5 ± 6.1) × 10 13 × (8.7 ± 3.7) × 10 13 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.