13C-selective infrared multiple photon dissociation of β-propiolactone by a free electron laser

The irradiation of a free electron laser at Tokyo University of Science (FEL-TUS) to β-propiolactone molecules in the gas phase has efficiently induced the infrared multiple photon dissociation of the compound in the wavenumber regions of 1000–1100 and 1700–2000 cm−1. The products were carbon dioxide and ethylene at equal amounts, where carbon dioxide was enriched with 13C under selected irradiation conditions. The relative yields of products and the selectivity of 13C were examined under various experimental conditions; the maximum 13C atom fraction of 59% was achieved when 2.5 Torr β-propiolactone was irradiated by FEL pulses at 1750 cm−1 with a fluence of 5.8 J cm−2. One of the reasons of this high selectivity is a large isotope shift of ∼47 cm−1 for the CO stretching vibration around 1880 cm−1, which is larger than the energy resolution of FEL-TUS. Another possible factor may be a shorter micropulse interval (∼350 ps) of FEL-TUS than the collisional frequency of β-propiolactone molecules in the gas phase, which prevents the collisional energy transfer from 13C molecules to 12C molecules during successive micropulses.