Vacuum-Ultraviolet Photoionization Measurement and ab Initio Calculation of the Ionization Energy of Gas-Phase SiO2

In this work we report on the detection and vacuum-ultraviolet (VUV) photoionization of gas-phase SiO2 generated in situ via laser ablation of silicon in a CO2 molecular beam. The resulting species are investigated by single-photon ionization with tunable VUV synchrotron radiation and mass analyzed using reflectron mass spectrometry. Photoionization efficiency (PIE) curves are recorded for SiO and SiO2, and ionization energy estimates are revealed from such measurements. A state-to-state ionization energy of 12.60 +/- 0.05 eV is recorded by fitting two prominent peaks in the PIE curve for the following process: (1)Sigma O-Si-O -> (IIg)-I-2 [O-Si-O](+). Electronic structure calculations aid in the interpretation of the photoionization process and allow for identification of the symmetric stretch of (IIg)-I-2 [O-Si-O](+), which is observed in the PIE spectrum to be 0.11 eV (890 cm(-1)) above the ground state of the cation and agrees with the 892 cm(-1) symmetric stretch frequency calculated at the CCSD(T)/aug-cc-pVTZ level.