Vibrational splitting in Si 2p core-level photoelectron spectra of silicon molecules

Abstract

High-resolution (∼0.1 eV) Si 2p gas phase photoelectron spectra of the following twenty-three silicon compounds have been recorded: SiHXD4−X; Si(CH3)X(OCH3)4−X; Si(CH3)X[N(CH3)2]4−X; SiHX[Si(CH3)3]4−X (where x=0–4), and SiH3–CH3, SiH3–SiH3, SiH3–SiH2–SiH3, Si(CH3)3–Si(CH3)3, Ge[Si(CH3)3]4, and [Si(CH3)2]6. Vibrational fine structure has been resolved in the Si 2p spectra of the five SiHXD4−X (x=0–4) compounds, methyl silane (H3C–SiH3), disilane (H3Si–SiH3), and trisilane (SiH3–SiH2–SiH3). For the five mixed hydrogen/deuterium compounds and methyl silane, the Si 2p vibrational structure is determined by the totally symmetric Si–H, Si–D, or Si–C stretching vibrational mode. In contrast, the spectra of disilane and trisilane are dominated by the nontotally symmetric Si–H bending vibrations—the first example of this in core-level photoelectron spectroscopy. This unusual vibrational effect is interpreted in terms of vibronic coupling that results from core-hole localization in the ion states of molecules such as disilane and trisilane which have equivalent cores. In the remaining compounds the vibrational effects are not well resolved. However, the Si 2p peak widths increase in the order Si(CH3)4≤Si[Si(CH3)3]4≤Si[N(CH3)2]4≤Si(OCH3)4≤SiF4 showing that the size of the vibrational manifold increases in the same order. The Si 2p photoelectron spectra of the series SiHx[Si(CH3)3]4−x mimic the chemical shift effects of zero, one, two, and three hydrogens bonded to a silicon surface. Vibrational effects must now be considered for adsorbate systems such as H adsorbed on a silicon surface. The Si 2p spectra are also used to predict the overall C 1s linewidth of organic analogs in the gas phase and in polymers.