First infrared spectroscopic characterization of the disilyl (Si2H5) and d5-disilyl (Si2D5) radicals in low temperature silane matrices

The disilyl, Si2H5(X2A'), and the d5-disilyl, Si2D5(X2A'), radicals were observed for the first time via infrared spectroscopy in low temperature silane matrices at 10 K upon an irradiation of the silane matrices with mono energetic electrons. We were able to observe absorptions of the umbrella modes at 843 (silane matrix) and at 621 cm-1 (d4-silane matrix). Accounting for a scaling factor of 0.97, ab initio DFT data predict that the most intense absorption of the disilyl radical should be observable at 843 (Si2H5(X2A')) and 621 cm-1 (Si2D5(X2A')). These computed data are in excellent agreement with our observations. The blue shift of the Si-H and Si-D modes from 820 (Si2H6) to 843 cm-1 (Si2H5) and 606 (Si2D6) to 621 cm-1 (Si2D5) goes hand in hand with a change from a sp3 to an sp2 hybridized silicon atom. This knowledge of the infrared absorption features might help to follow the chemical evolution of silicon CVD process not only via mass spectrometry, but also through time resolved infrared spectroscopy monitoring the most intense umbrella mode of the disilyl species. Further, the infrared frequencies provide vital guidance to search for hitherto undetected silicon-bearing species in the circumstellar envelope of the carbon star IRC + 10216