The electronic spectrum of Si3 I : triplet D3h 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 18 000–20 800 cm−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 1A1″3 – ã A2′3 transition of the D3h isomer. The spectrum has an origin transition at 18 600± 4 cm−1 and a short progression in the symmetric stretch with a frequency of ∼445 cm−1, in good agreement with a predicted vertical transition energy of 2.34 eV for excitation to the 1A1″3 state, which has a calculated symmetric stretching frequency of 480 cm−1. In addition, a ∼505 cm−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 D3h state and with theory. A weaker, overlapping band system with a ∼360 cm−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 C2v ground state (1A1) 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.