Photoionization mass spectrometric study of formaldehyde H2CO, HDCO and D2CO

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Abstract
Photoionization efficiency curves for H2CO+, HDCO+, D2CO+ and HCO+, DCO+ and CO+ fragment ions are obtained. The ionization potentials of H2CO, HDCO, and D2CO thus determined are, respectively, 10.868, 10.879, and 10.882 eV. The appearance potential of HCO+ fragment is observed at 11.92±0.01 eV, from which the dissociation energy of H2CO+, the proton affinity of CO, and the ionization potential of HCO are inferred. The heat of formation ΔHf00 (HCO+) =196.2±0.5 kcal/mole is obtained, a value lower than any one previous determination. From the abundance ratios measurements the variation of the mass spectrum with photon energy is determined, and hydrogen isotope effects are measured. The observation of a metastable transition (D2CO+)m→DCO++D with a threshold some 2 eV above the dissociation limit is discussed along with the dissociation mechanisms with the help of correlation diagrams. Finally, the finer structure in the photoionization efficiency curves is analyzed in terms of preionized Rydberg series converging to the second and third electronic state of the ion.

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