Multiphoton Isotope-Selective Dissociation of Formic Acid Molecules under Action of a Free Electron Laser

The infrared multiphoton dissociation (IR MPD) of formic acid molecules (HCOOH → CO + H 2 O) with free electron laser radiation in the 3 μm region (C−H stretch vibrational absorption band) and in the 6 μm region (C O stretch vibrational absorption band) has been achieved using the same laser. Results indicate that initial excitation of this dimer is more efficient for MPD then initial monomer excitation. Carbon isotope-selective MPD has been performed with an isotopic mixture, H 12 COOH + H 13 COOH (50% + 50%), at an irradiating wavelength of 5.56 μm. For this mixture, isotopic selectivity s = k 12 / k 13 = 23 has been attained as a ratio of the rate constants of the H 12 COOH MPD and the H 13 COOH MPD, respectively. The isotopic selectivity for 13 C in the naturally occurring mixture (1.1% of 13 C-containing molecules) s = k 13 / k 12 = 22 has been attained at an irradiating wavelength of 5.90 μm.