Threshold photoelectron studies of isoxazole over the energy range 9.9–30eV

Abstract

The threshold photoelectron spectrum of the isoxazole molecule, C 3 H 3 NO has been measured over the photon energy range 9.9eV to 30eV with the use of synchrotron radiation. In the 9.9-10.8 eV range, corresponding to photoionization from the highest occupied molecular orbital 3a"(π 3 ), seven well resolved vibrational series have been observed and their modes are tentatively assigned. A strong adiabatic ionization, with an energy of 11.132 ± 0.003eV corresponding to the 2a'' (π 2 ) band, has also been observed. This is followed by a single vibrational series of the v C-H stretching mode.Photoelectron bands in the energy region 13-30 eV have also been identified, some for the first time.

Superexcited states in the vacuum-ultraviolet photofragmentation of isoxazole molecules

“…The vertical excitation energies (centres) of the first and second bands in the yield curves obtained from the fittings show an increase with increasing n of the H(n) fragment. The superexcited states, contributing to the first and second bands in the spectra of figure 5, may involve states converging to the ionic states observed in the threshold photoelectron spectra of isoxazole at 26.44 eV [20] and those, which could be predicted around 34 eV from photoelectron spectra of furan [21], the isoelectronic molecule to isoxazole. For the two remaining bands, the obtained average excitation energies are 32.2 ± 0.5 eV and 38.8 ± 0.9 eV.…”

Fragmentation of isoxazole molecules by electron impact in the energy range 10–85eV

“…Comparison of the measured appearance energies with the known energies of the photoelectron bands enable a tentative identification of these ion cores. The formation of CH in excited states may correspond to the ionic bands observed in the very recent threshold photoelectron studies in the energy range 14-16 eV [22] which include the (13a 0 ) À1 and (12a 0 ) À1 bands. The states dissociating into H(n = 4, l) atoms may be attributed to an ionic band detected at 22.95 eV in the photoelectron spectrum [22] while those related to H(n, l) atoms with n = 5-7 to two bands at 24.49 and 26.44 eV.…”

“…The appearance energies of the observed fragmentation species are above the adiabatic ionization energy of isoxazole, which was measured at 9.976 eV [22]. In the fragmentation process the higher-lying (superexcited) states of the molecule are excited and next undergo decomposition following energetically and dynamically favourable routes on the hyperdimensional potential energy surface.…”

Vacuum ultraviolet photoionization and ionic fragmentation of the isoxazole molecules

“…The TIY was differentiated to extract the underlying thresholds; the result is displayed as a red curve in Figure 3. In order to interpret the peaks in the differentiated TIY, Figure 3 shows for comparison the threshold photoelectron spectrum (TPES) of isoxazole measured in the same photon energy range [13]. To facilitate the comparison, the TPES was shifted by about 0.006 eV toward lower energies and scaled to display similar peak heights at the maximum with the differentiated TIY.…”

“…This ring has the properties of an aromatic system with a fully conjugated set of π-electrons, the two highest occupied orbitals being π2 and π3. The outermost occupied molecular orbitals of isoxazole in the X 1 A' ground state are (11a') 2 (1a") 2 (12a') 2 (13a') 2 (14a') 2 (15a') 2 (2a") 2 (3a") 2 [11], [12], [13]. The 15a' orbital is the σ nitrogen lone pair (LPN) and the 1a", 2a", and 3a" orbitals are labeled π1, π2, and π3, respectively.…”