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

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.