Vacuum UV photoabsorption spectroscopic and photoionization mass spectrometric study of geminal dibromoethylene (1,1-Br2C2H2) in the 5–15eV range. Experiment and theory

Abstract

The vacuum UV photoabsorption spectrum (PAS) of 1,1-C2H2Br2 has been examined between 5 eV and 15 eV using the vacuum UV light of synchrotron radiation. For the first time the photoionization mass spectroscopy and efficiency of 1,1-C2H2Br2+ has been measured in the same energy range. A quantum chemical calculation is proposed for the investigation of the neutral and ionized states. The photoionization efficiency curve (PIEC) of 1,1-C2H2Br2+ provided the IEad(X˜2B1)= 9.617±0.005 eV. A vibrational structure gave ω3+=1336 cm−1 and most likely ω4+=483 cm−1. The PIEC of C2H2Br+ shows the lowest appearance energy to be at 11.14±0.02 eV. The onset at 11.56±0.01 eV likely corresponds to the excitation of Br in the 2P1/2 spin-orbit state. In the vacuum UV-PAS the broad band observed at 6.231 eV includes nσ→Rs, 2b1(π)→π* valence transitions and the 2b1→3 s Rydberg transition in agreement with the present calculations. An isolated weak continuum at 7.15 eV is assigned to the nπ→π* transition. The 2b1→3 s Rydberg transition is characterized by a short progression starting at 6.583 eV. A series of long progressions observed between 7.364 eV and 9.666 eV has been analyzed in terms of vibrational transitions to np- (δ= 0.544) and nd-type (δ= -0.04) Rydberg states all converging to the X˜2B1 ionic ground state. An analysis has been attempted providing average values of the vibrational wavenumbers ω3≈1300 cm−1 (or 162 meV), ω4≈480 cm−1 (or 60 meV) and ω5≈145 cm−1 (or 18 meV). By the same way several other Rydberg states were analyzed. For the first time the vacuum UV spectrum of 1,1-C2H2Br2 has been recorded above 10 eV and up to 15 eV. Several broad bands are tentatively assigned to transitions to Rydberg states converging to excited ionic states of 1,1-C2H2Br2+. For one of these a vibrational structure is observed and a tentative assignment is proposed.