Vacuum ultraviolet photoionization and ionic fragmentation of the isoxazole molecules

The photofragmentation of the isoxazole molecules producing ionized atomic and molecular fragments was investigated in the photon energy range of 9-32 eV, using synchrotron radiation excitation combined with ion time-of-flight spectrometry. Twenty-one well resolved cations were identified in the mass spectra of the isoxazole, and their appearance energies were determined. The yield curves of these cations were obtained in the photon energy ranges from their appearance energies up to 32 eV. Moreover, the total ion yield of isoxazole was recorded with high precision in the photon energy range of 9.9-10.5 eV. This allowed the determination of the adiabatic ionization energy of 9.96 (0.02) eV in excellent agreement with earlier spectroscopic studies. Our results show that the dissociative ionization of isoxazole starts from the ring-opening and isomerization of isoxazole, and further it follows strictly through its ionic states. Possible ionic fragmentation channels yielding particular ions are discussed.