Multiphoton ionization of SiH3 and SiD3 radicals: Electronic spectra, vibrational analyses of the ground and Rydberg states, and ionization potentials

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Abstract
The electronic spectra of silyl radicals, SiH3 and SiD3, were observed between 310 and 430 nm (46 000–64 000 cm−1) by resonance enhanced multiphoton ionization (REMPI) mass spectroscopy. The spectra were generated through a 2+1 REMPI mechanism. Two Rydberg series originating from planar, D3h point group states were observed. One series, of quantum defect δ=1.45(2), is comprised of the Ẽ 2A‘2 (4p), J̃ 2A‘2(5p), and M̃ 2A‘2(6p) Rydberg states which have origins at ν0–0 =48 438, 56 929, and 60 341 cm−1 in SiH3 and at ν0–0 =48 391, 56 874, and 60 267 cm−1 in SiD3. In SiD3 the P̃ 2A‘2(7p) Rydberg origin was observed at ν0–0 =62 002 cm−1. The H̃, K̃, and Ñ states observed in the SiD3 spectrum comprise the second Rydberg series, δ=2.09, and were tentatively assigned as ns 2A’1 Rydberg states (n=5, 6, 7). The K̃ and Ñ origins were observed at ν0–0 =58 417 and 61 005 cm−1. A fit of the Rydberg formula to the np 2A‘2(n≥5) origins found the adiabatic ionization potential of the SiH3 and SiD3 radicals to be IPa=8.135(+5,−2) eV and IPa=8.128(1) eV, respectively. Detailed vibrational analyses of these Rydberg states are presented. Analysis showed that in the Ẽ 2A‘2 (4p) state of the SiH3 radical ω2 (a‘2 symmetric bend)=796(7) and 2ω4 /2(e’ degenerate bend)=870(5) cm−1 and that in SiD3 radical ω’1 (a1 SiH3 symmetric stretch)=1576(3), ω’2 =589(3), and 2ω4 /2=635(6) cm−1. The REMPI spectra exhibited ν‘2 hot bands from vibrational levels as high as Ev =2073 cm−1 in the X̃ 2A1state. Modeling calculations, which fit the numerous ν‘2 hot bands, predicted barriers to inversion of Binv =1935 cm−1 and Binv =1925 cm−1 for SiH3 and SiD3 X̃ 2A1 radicals, respectively.