LASER SPECTROSCOPY OF THE (ELUSIVE) JET-COOLED SiCF FREE RADICAL 8 16.00 90 232.0 92 238.0 17 35.45 68 167.3 53 126.9 & 62 150.4 10.81 5 32.1 16 57 138.9 LASER SPECTROSCOPY OF THE (ELUSIVE) JET-COOLED SiCF 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 SiCX (X=H, F, Cl) free radicals have been suggested as intermediates in semiconductor growth and etching processes. 2. These radicals have a Si-C double bond in the ground state but an unusual SiC triple bond in the excited state. 3. The radicals are spectroscopically interesting with a substantial Renner-Teller effect in the 2P ground states. Si C H 1.0677(4) Å 1.69252(8) Å Si C H 1.0625(5) Å 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 (SiCH), 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 2IIi Ground State of Silicon Methylidyne (SiCH)," 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 SiCH 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 (SiCX; 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 SiCCl Free Radical," J. Chem. Phys., 117, 6446-6449 (2002).

π σnb ELECTRONIC SPECTROSCOPY OF SiCH & SiCF 2+ 21/2 23/2 Excited State Electron Configuration: (•••2b 1nb 4) Ground State (•••2b 2nb 3) SiCH 850-600 nm Ab initio SiCF 585 nm 2+ 21/2 23/2 π σnb

PULSED DISCHARGE JET SPECTROSCOPY Precursor + Argon

PRECURSORS Commercially available (CH3)3SiCF3: Yielded very weak spectra of SiCF along with overlapping bands of SiCH, SiH2, HCF, HSiF, SiCH2 etc.

ALTERNATE PRECURSORS Extensive and rather involved syntheses of CF3SiCl3 and CF3SiF3: No spectra of SiCF!! Theorize that the strong Si-F and Si-Cl bonds are not being broken in the discharge. Reduction of CF3SiCl3 to CF3SiH3: Fewer impurities but SiCF spectra still very weak.

EXPT’L vs AB INITIO SiCF BAND CONTOURS SiCF Ab Initio: T0 = 17104 cm-1

0-0 BAND EMISSION SPECTRUM 665 cm-1 105 cm-1 104 cm-1 SiCF Ab Initio: SiC stretch = 672 cm-1 ASO = -93 cm-1

RENNER-TELLER ANALYSIS OF EMISSION SPECTRA Expt. Theory e -0.146(5) -0.161 w2 260.5(9) 272 w3 665.7(8) 667 ASO -103.4(5) -93

SiCF POTENTIALS FROM EXPT’L  = -0.146 In-Plane Orbital a 2A (𝒂′) 𝟐 ( 𝒂) 𝟏 2A (𝒂′) 𝟏 ( 𝒂) 𝟐 Out-of-Plane Orbital a 2 Linear Electron Configuration: (•••2b 2nb 3)

Si C F F C Si AB INITIO MOLECULAR STRUCTURES CCSD(T)/aug-cc-p(V+d)5Z 1.592 Å 1.293 Å A2Σ+ ~ 1.693 Å 1.285 Å F C Si X2Πi ~ CCSD(T)/aug-cc-p(V+d)5Z

Si-C BOND LENGTHS SiCHa SiCFb SiCClb SiC (Å) 1.612 1.592 1.610 1.693 1.692 aExperimental determination bCCSD(T)/aug-cc-pVTZ value

CONCLUSIONS The elusive SiCF free radical has been identified in the gas phase. The radical is formed in small quantities from an electric discharge through CF3Si(CH3)3 and CF3SiH3 but not CF3SiF3. Ab initio calculations indicate that SiCF has a Si-C double bond in the ground state but a triple bond in the excited state. We have now spectroscopically characterized the SiCH, SiCCl and SiCF free radicals in the gas phase.