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53 62 150. 4 126. 9 90 232. 0 & 17 35. 45 8 16. 00 92 238. 0 90 232. 0 53 126. 9 68 167. 3 57 138. 9 5 16 10. 81 32. 1 LASER SPECTROSCOPY OF THE (ELUSIVE) JET-COOLED Si. CF FREE RADICAL TONY C. SMITH, Ideal Vacuum Products, LLC, Albuquerque, NM DENNIS J. CLOUTHIER, Department of Chemistry, University of Kentucky, Lexington, KY
MOTIVATIONS 1. The Si. CX (X=H, F, Cl) free radicals have been suggested as intermediates in semiconductor growth and etching processes. These radicals have a Si-C double bond in the ground state but an unusual Si. C triple bond in the excited state. 2. 3. The radicals are spectroscopically interesting with a substantial Renner-Teller effect in the 2 P ground states. H C Si 1. 0677(4) Å 1. 69252(8) Å H C 1. 0625(5) Å Si 1. 6118(1) Å
LITERATURE 1. T. C. Smith, H. Li, D. J. Clouthier, C. T. Kingston and A. J. Merer, "The Electronic Spectrum of Silicon Methylidyne (Si. CH), a Molecule with a Silicon-Carbon Triple Bond in the Excited State, " J. Chem. Phys. , 112, 3662 -3670 (2000). 2. T. C. Smith, H. Li, D. A. Hostutler, D. J. Clouthier and A. J. Merer, "Orbital Angular Momentum (Renner-Teller) Effects in the 2 IIi Ground State of Silicon Methylidyne (Si. CH), " J. Chem. Phys. , 114, 725 -734 (2001). 3. T. C. Smith, D. J. Clouthier and T. C. Steimle, "Hyperfine Structure and the Stark Effect in the Electronic Spectrum of the Si. CH Radical with Implications for Microwave Spectroscopy and Radioastronomy, " J. Chem. Phys. , 115, 817 -823 (2001). 4. C. J. Evans and D. J. Clouthier, "Ab initio Predictions of the Spectroscopic Parameters of the Silicon Halomethylidyne (Si. CX; X=F, Cl, Br) Free Radicals, " J. Chem. Phys. , 117, 6439 -6445 (2002). 5. T. C. Smith, C. J. Evans and D. J. Clouthier, "Spectroscopic Detection of the Si. CCl Free Radical, " J. Chem. Phys. , 117, 6446 -6449 (2002).
ELECTRONIC SPECTROSCOPY OF Si. CH & Si. CF Excited State Electron Configuration: ( • • • 2 b 1 nb 4) 2 + Si. CH 850 -600 nm Ab initio Ground State Electron Configuration: ( • • • 2 b 2 nb 3) σnb Si. CF 585 nm 2 1/2 2 3/2 π
PULSED DISCHARGE JET SPECTROSCOPY Precursor + Argon
PRECURSORS Commercially available (CH 3)3 Si. CF 3: Yielded very weak spectra of Si. CF along with overlapping bands of Si. CH, Si. H 2, HCF, HSi. F, Si. CH 2 etc.
ALTERNATE PRECURSORS Extensive and rather involved syntheses of CF 3 Si. Cl 3 and CF 3 Si. F 3: No spectra of Si. CF!! Theorize that the strong Si-F and Si-Cl bonds are not being broken in the discharge. Reduction of CF 3 Si. Cl 3 to CF 3 Si. H 3: Fewer impurities but Si. CF spectra still very weak.
EXPT’L vs AB INITIO Si. CF BAND CONTOURS Si. CF Ab Initio: T 0 = 17104 cm-1
0 -0 BAND EMISSION SPECTRUM 665 cm-1 104 cm-1 Si. CF Ab Initio: Si. C stretch = 672 cm-1 ASO = -93 cm-1
RENNER-TELLER ANALYSIS OF EMISSION SPECTRA Expt. -0. 146(5) Theory -0. 161 w 2 260. 5(9) 272 w 3 665. 7(8) 667 ASO -103. 4(5) -93
Si. CF POTENTIALS FROM EXPT’L = -0. 146 Potentials In-Plane Orbital a 2 A Out-of-Plane Orbital a 2 Linear Electron Configuration: ( • • • 2 b 2 nb 3)
AB INITIO MOLECULAR STRUCTURES ~ A 2 Σ+ ~ X 2 Πi F F 1. 293 Å 1. 285 Å C C 1. 592 Å 1. 693 Å CCSD(T)/aug-cc-p(V+d)5 Z Si Si
Si-C BOND LENGTHS Si. CHa Si. CFb Si. CClb Si C (Å) 1. 612 1. 592 1. 610 Si=C (Å) 1. 693 1. 692 a. Experimental determination b. CCSD(T)/aug-cc-p. VTZ value
CONCLUSIONS 1. The elusive Si. CF free radical has been identified in the gas phase. 2. The radical is formed in small quantities from an electric discharge through CF 3 Si(CH 3)3 and CF 3 Si. H 3 but not CF 3 Si. F 3. 3. Ab initio calculations indicate that Si. CF has a Si. C double bond in the ground state but a triple bond in the excited state. 4. We have now spectroscopically characterized the Si. CH, Si. CCl and Si. CF free radicals in the gas phase.