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Transition Metal Carbides: V n C m and Cr n C m Clusters

Transition Metal Carbides: V n C m and Cr n C m Clusters. S. A. Bates Department of Physics & Astronomy TCU Fort Worth, TX 76129 14 November 2005. Motivation. Astrophysics ( e . g . identification of interstellar and circumstellar molecules in the infrared)

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Transition Metal Carbides: V n C m and Cr n C m Clusters

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  1. Transition Metal Carbides: VnCm and CrnCm Clusters • S. A. Bates • Department of Physics & Astronomy • TCU • Fort Worth, TX 76129 • 14 November 2005

  2. Motivation • Astrophysics (e.g. identification of interstellar and circumstellar molecules in the infrared) • Cn chains act as backbones for many molecules • Radio astronomy measures rotational spectra – homonuclear linear molecules cannot be detected • Space Infrared Telescope Facility (SIRTF) launched 08/2003

  3. Molecules Discovered in Space Number of Atoms 9 2 3 4 6 7 8 5 10 11 12 13 From the Cologne Database 09/2005 http://www.ph1.uni-koeln.de/vorhersagen

  4. Metal-Carbon Studies: Vanadium • Guo, Kerns, & Castleman (1992) discovered the Ti8C12 metallo-carbohedrene (“metcar”) using mass spectrometry • 1994: Guo & Castleman expand to M8C12 (M=Ti, V, Zr, Hf) • 2001: Castleman, etal., use mass spectrometry on large VnCm clusters (e.g. V4C6, V9C13, V10C15); M4C9 (M = V, Ti) observed using mass spectrometry (Pradeep, etal.) • 2004: DFT studies on V8C12 by Muckerman, etal.

  5. Metal-Carbon Studies: V & Cr • Photoelectron Spectroscopy (PES): MC2¯ (M = Cr, Ni, V, Sc, Fe, and Mn) (Li & Wang, 1999) • Predict metal-carbon stretch for VC2 at 550±40 cm-1 and 510±30 cm-1 for CrC2 • Suggest C2v structure for several MC2 clusters • PES: MC3¯ (M = Cr, V, Sc, Fe, and Mn) (Wang & Li, 2000) • Use DFT to predict vibrational modes and C2v geometries for neutral species • Metal carbon stretches at 600±30 cm-1 and 560±60 cm-1 for VC3 and CrC3 species, respectively

  6. Metal-Carbon Studies: Vanadium • 2002: DFT and PES studies on VC2, V2Cn¯ (n=2-4) (Tono, et al.) • 2003: Majumadar, et al. use a variety of caclulations to determine VC2, VC2+, VC2¯ have C2v structures VC2 V2C2 V2C3 V2C4

  7. Metal-Carbon Studies: Chromium • Chromium: PES and DFT studies on CrCn (n=2-8) (Zhai, et al., 2004) • DFT predictions for neutral species • CrC4, CrC6, CrC8 have linear structure • CrC5 “fan-like” structure; CrC7 cyclic structure • CrC2 with C2v structure • CrC3 with close-lying (0.3 eV) linear and C2v structures • PES results indicate that most anion and neutral species have the same geometries: n=4,6,8 is linear; n=7 is cyclic; n=5, spectrum not well-resolved – most likely not linear; n=2,3 linear and C2v isomers within 0.3 eV, predominantly C2v structure

  8. Samples • Vanadium • VnCm samples made from V, 12C, and 13C powders using new press • Best powder ratio was 60%V / 40% C • New unbaked (i.e. “soft”) rods using the small press with ~15% 13C enrichment • Chromium • Cr rod (ESPI, 99.9% purity), dual ablation with soft carbon rod

  9. Experimental Conditions • Vanadium • 60% V / 40% C rod • Low enrichment experiments have 15% 13C • Laser power 2.2 Watts, ~15 min. deposition • Samples annealed up to 26 K • Chromium • Dual ablation of Cr with a “soft” low-enrichment 13C rod (~15%), or a baked rod (~30% 13C) • Laser power 2.7 Watts (Cr), 0.7 Watts (C) • ~30 mins. of deposition • Samples annealed up to 20K

  10. Sample Chamber Nd-YAG 1064 nm pulsed laser laser focusing lens CsI window Quartz window gold mirror ~ 10K To pump 10-3Torr To pump 10-7Torr or better Bomem DA3.16 Fourier Transform Spectrometer • KBr beam splitter • liquid N2 cooled MCT detector (550-3900 cm-1) Carbon rod Transition metal rod Ar

  11. DFT (B3LYP/6-311G + 3df) calculations for (C2v) VC3 (doublet) DFT (B3LYP/6-311G + 3df) calculations for linear VC3 (sextet) DFT Predictions for VC3 C2v structure Linear structure a) Wang and Li, 2000

  12. DFT Predictions for linear VC5 DFT (B3LYP/6-311G + 3df) calculations for linear VC5 (sextet)

  13. 923.3 919.5 917.3 V + 15% 13C V + 12C C3 915 920 925 930 1700 1710 1720 2029.7 2032.8 1718.8 C3 2019.4 1697.7 1699.8 H2OV C3 2010 2020 2030 2040 Results: VnCm Candidates Frequency (cm-1) Frequency (cm-1) Frequency (cm-1)

  14. V + 15% 13C 2091.4 V + 12C 1974.5 1970 1980 2090 2070 2080 Results: VnCm Candidates - Possibly linear VC5? 2089.5 n1(s) n2(s) 2085.8 2074.3 2084.4 Frequency (cm-1) Frequency (cm-1)

  15. Preliminary DFT Predictions for linear CrC3 (scaled) DFT (B3LYP/6-311G*) calculations for linear CrC3 (quintet)

  16. 1700 1750 1800 1850 1900 1950 2000 2050 2100 2150 Results: linear CrC3 n5 n3 n4 n6 n7 n6 C7 C5 C7 C10 n3 C11 C9 C3 n9 1894.3 2164.1 2074.9 2127.8 1946.1 1998.0 C12 n5 n5 n7 2038.9 C6 C8 n8 C10 12C + Cr rod n5 1818.0 C11 C9 1952.5 2071.7 1915.8 1856.7 n9 2078.1 C8 1789.5 1710.5 Absorption 12C rod annealed 26 K Frequency (cm-1)

  17. Cr rod + 30% 13C rod Results: linear CrC3 (A’) (A) 1720.6 1789.5 (D’) 1721.5 (B’) (D) (B) (C’) (C) 1731.4 1733.5 Absorption 1777.8 1767.1 1779.5 1743.3 Cr rod + 15% 13C rod (D) 1735.1 1746.1 (B) (C) DFT simulation 10% 13C 1720 1730 1740 1750 1760 1770 1780 1790 Frequency (cm-1)

  18. Future Work: Vanadium • Apply low laser power technique (Kinzer) to carbon rod in low enrichment 13C (~10%) vanadium-carbon dual ablation experiments. • Low laser power on high 13C enrichment (~90%) rods in vanadium-carbon dual ablation experiments.

  19. Future Work: Chromium • Apply low laser power technique (Kinzer) to carbon rod with various 13C enrichments (e.g. 50%) to see if CrC3 spectrum looks better or if anything new appears. • Use a single “soft” CrnCm rod in chromium-carbon experiments (i.e. mix Cr, 12C, and 13C powders to form a single sample like with V samples)

  20. Acknowledgments • ADVISOR: Dr. W. R. M. Graham • THEORETICAL CALCULATIONS: Dr. C. M. L. Rittby • TECHNICAL SUPPORT: • Machine Shop: Mike Murdock, David Yale • Electronics Shop:Jerry Katchinska • FUNDING: • The W. M. Keck Foundation • The Welch Foundation • TCU Research and Creative Activities Fund (TCURCAF) • The Texas Space Grant Consortium (TSGC) • The Barnett Scholarship

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