The electronic spectrum of Si₃ I: Triplet D_3h system

We report the measurement of a jet-cooled electronic spectrum of the silicon trimer. Si₃ was produced in a pulsed discharge of silane in argon, and the excitation spectrum examined in the 18000-20800cm^-1 region. A combination of resonant two-color two-photon ionization (R2C2PI) time-of-flight mass spectroscopy, laser-induced fluorescence/dispersed fluorescence, and equation-of-motion coupled-cluster calculations have been used to establish that the observed spectrum is dominated by the 1³A ₁ ^″ - ã ³ A^′ ₂transition of the D_3h isomer. The spectrum has an origin transition at 18600± 4cm^-1 and a short progression in the symmetric stretch with a frequency of ∼445cm^-1, in good agreement with a predicted vertical transition energy of 2.34eV for excitation to the 1³ A^″ ₁state, which has a calculated symmetric stretching frequency of 480cm^-1. In addition, a ∼505cm^-1 ground state vibrational frequency determined from sequence bands and dispersed fluorescence is in agreement with an earlier zero-electron kinetic energy study of the lowest D_3h state and with theory. A weaker, overlapping band system with a ∼360cm^-1 progression, observed in the same mass channel (m/z=84) by R2C2PI but under different discharge conditions, is thought to be due to transitions from the (more complicated) singlet C_2v ground state (¹A ₁) state of Si₃. Evidence of emission to this latter state in the triplet dispersed fluorescence spectra suggests extensive mixing in the excited triplet and singlet manifolds. Prospects for further spectroscopic characterization of the singlet system and direct measurement of the energy separation between the lowest singlet and triplet states are discussed.