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FTIR Spectroscopy of Vanadium-Carbon and Chromium-Carbon Clusters

FTIR Spectroscopy of Vanadium-Carbon and Chromium-Carbon Clusters. S. A. Bates Department of Physics and Astronomy Texas Christian University Fort Worth, TX 76129 27 January 2006. Motivation. Astrophysical

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FTIR Spectroscopy of Vanadium-Carbon and Chromium-Carbon Clusters

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  1. FTIR Spectroscopy of Vanadium-Carbon and Chromium-Carbon Clusters S. A. Bates Department of Physics and Astronomy Texas Christian University Fort Worth, TX 76129 27 January 2006

  2. Motivation • Astrophysical • Expect that transition-metal carbon species are important in circumstellar shells of late-type stars

  3. Formation of Metal-Carbon Species • Nuclear fusion in stellar cores creates heavier elements such as Mg, Si, Cr. • Atoms migrate from the core to the outer layers of the star. • Carbon is abundant in envelopes of late-type, carbon-rich stars. • Heavier atoms and carbon combine to form XnCm species, where X denotes another element.

  4. Observed Metal and Carbon Species • Detections in IRC +10216 and CRL2688 include • Pure carbon clusters (e.g. C3, C5) • Hydrocarbon species (e.g. CH4, C6H6, C7H) • Metal-carbon species containing Mg, Al, Na (e.g. MgNC,AlNC, NaCN) • Other metal-bearing free radicals (e.g. AlF, AlCl, KCl, NaCl) • Silicon-bearing species (e.g. SiC4, SiC2, SiNC, SiH4) Metals observed in stellar spectra

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

  6. Detection of Interstellar and Circumstellar Molecules • Cn chains act as “backbones” for many interstellar molecules. • Most detections made via radio astronomy (observe rotational spectra) – limitation: cannot detect molecules that do not have a permanent dipole moment, e.g. C5, C2H2 (acetylene). • Detections via IR vibrational spectroscopy found linear C3 and C5 in circumstellar shells of late-type carbon-rich stars (e.g. IRC +10216).

  7. Why Infrared? • Only way to observe molecules without a permanent dipole moment e.g. C3, C2H2, C5 • Most IR radiation is blocked by Earth’s atmosphere (because of H2O and CO2 absorption bands) • Must use space platform, e.g. Spitzer IR Space Telescope (‘03), ISO (’95), IRAS (’83) IR (left) and visible (right) images taken of the W5 region of Cassiopeia Images taken from http://www.spitzer.caltech.edu/Media/mediaimages/data.html

  8. Motivation • Astrophysical • Expect that transition-metal carbon species are important in circumstellar shells of late-type stars • Space-based IR telescopes observing in IR – need corresponding laboratory measurements

  9. Motivation • Spectroscopic • Measure vibrational fundamentals of novel species • Determine structures

  10. Spectroscopic: Vibrational Fundamentals • Measure vibrational fundamentals • Very few laboratory measurements exist • Most vibrational measurements from photoelectron spectroscopy (PES) • Accuracy insufficient for spectroscopic identification in astrophysical sources ( ± 50cm-1) • PES measurements thus far are low-frequency vibrations (borderline of far IR), which is more difficult for detections because absolute intensities of vibrations are lower

  11. Spectroscopic: Structure Determination • Theoretical studies propose a variety of structures for VnCm, CrnCm species – some species have competing predictions • Linear (CrC3)  Cyclic • Fan-like (C2v) (CrC3) • Complex structures (planar or 3-dimensional) (CrC7) (V2C4)

  12. Previous Work: Metcars • Mass Spectrometry • Ti8C12 metallo-carbohedrene (metcar), Guo, Kerns, & Castleman (1992) • M8C12 (M =Ti, V, Zr, Hf), Guo & Castleman (1994) • VnCm clusters (e.g. V4C6, V9C13, V10C15), Castleman etal. (2001) • M4C9 (M = V, Ti), Pradeep etal. (2001) • DFT Studies • V8C12, Muckerman etal. (2004) Proposed structure of Ti8C12 (Castleman, et al., 1992)

  13. Previous Work: MC2 Species • MC2¯ (M= Cr, Ni, V, Sc, Fe, & Mn): PES studies (Wang & Li, 1999) • Measured metal-carbon stretch for MC2 clusters including VC2 at 550±40 cm-1 and CrC2 at 510±30 cm-1. • PES measurements supported previous theoretical calculations for the vibrational modes of the C2v isomer of MC2 (M = Ti, Sc, Fe).

  14. Previous Work: MC3 Species • MC3¯ (M= Cr, Ni, V, Sc, Fe, & Mn): PES and DFT studies (Li & Wang, 2000) • Measured metal-carbon stretch for MC3 clusters including VC3 at 600±30 cm-1 and for CrC3 at 560±60 cm-1. • DFT predicted vibrational modes and C2v geometries for neutral species, which were supported by PES measurements.

  15. DFT Calculations: MC3 Species DFT (B3LYP/6-311G + 3df) calculations for and PES observations of (C2v) VC3 (doublet) and (C2v) CrC3 (triplet) (Wang & Li, 2000)

  16. VC2 V2C2 V2C3 V2C4 Previous Work: Small VnCm Species • DFT and PES studies on VC2, V2Cn¯ (n=2-4) (Tono et al., 2002) • Calculations with a variety of theoretical methods all confirm the prediction of C2v structure for VC2 and predict C2v structure for both VC2+ and VC2¯ as well (Majumadar et al., 2004). C2v “building block”

  17. Previous Studies on CrCn Species • Chromium: PES and DFT studies on CrCn (n=2-8) clusters (Zhai, Wang et al., 2004) Comparison of the DFT (B3LYP/6-311G*) predictions and observed (PES) geometries for CrCn (n=2-8) species

  18. Focus: Small Metal-Carbon Clusters and Techniques Employed • Focus of the present work: • Generation of small metal-carbon clusters, specifically VnCm and CrnCm clusters • Techniques • Laser ablation using Nd:YAG lasers → trap products in Ar at ~10K (matrix isolation) • Fourier-Transform infrared (FTIR) spectroscopy in conjunction with density functional theory (DFT) calculations • Isotopic substitution

  19. Key: metal atom 12C atom 13C atom a b a a a b unique central substitution site ← → equivalent substitution sites ← → 180° rotation 180° rotation g a b Each 12C atom is a unique substitution site! Isotopic Substitution Centrosymmetric molecules (e.g. C3, MC3M) Non-centrosymmetric molecules (e.g. MC3)

  20. Experimental Apparatus: Schematic gold-plated transfer optics laser beam evacuated connecting cone Nd:YAG laser mirror to guide laser beam to sample Bomem spectrometer Displex (~10K) lens to guide laser beam to sample to pump 10-7 Torr or better sample compartment sample chamber containing one or two rods CsI window Ar delivery system detectors gold-plated deposition mirror (~10K)

  21. New Old Experimental Apparatus: Construction Ar delivery system The new system incorporates the improvements: ½” diameter tubing and vacuum fittings instead of ¼” and new, faster diffusion pump – increased pumping speed (2 days evacuation time rather than 1 week) New frame – makes system more maneuverable, compact (arrow)

  22. Experimental Apparatus: Construction Displex closed-cycle refrigeration system • New frame – more maneuverable, • more compact Automated system for translation and rotation of rods during laser ablation (arrow) • Smaller volume and faster pump • provide more efficient pumping • New Displex and new temperature • controller provides better control over • annealing temperatures

  23. Experimental Apparatus: Sample Chamber Nd:YAG 1064 nm pulsed laser laser focusing lens CsI window quartz window Bomem DA3.16 Fourier Transform Spectrometer • KBr beam splitter • liquid N2 cooled MCT detector (550-3900 cm-1) gold mirror ~ 10K to pump 10-3Torr to pump 10-7Torr or better carbon rod transition metal rod 23 Ar

  24. Experimental Conditions • Vanadium • Single 60% V / 40% C unbaked lab rod • Laser power 2.2 watts • Products stable during annealing up to 26 K • Chromium • Dual ablation of Cr rod and C rod; single lump Cr3C2 pellet • Laser power 2.7 watts (Cr, Cr3C2), 1.5 watts (C) • Products stable during annealing up to 20 K

  25. Survey Spectrum: V+12C vs. 12C n6 C9 n9 C12 n5 C7 n4 c-C6 n6 n3 n7 n5 C10 C4 C11 C6 n3 n8 C3 n5 n7 C11 n5 C10 C9 n7 C8 C9 n9 C8 1000 900 1500 1600 1700 1800 1900 2000 2100 1998.0 1818.0 1694.9 1894.3 2074.9 1952.5 2038.9 1946.1 1543.4 2071.1 2078.1 12C rod 1915.8 1856.7 1601.1 1710.5 x10 Absorption 2019.4, 2029.7, 2032.8 1699.8, 1718.8 2091.4 917.3, 919.9, 923.3 1974.5 O2V 12C + V rod / / 25 Frequency (cm-1)

  26. 1700 1710 1720 915 920 925 1970 1980 VnCm Candidates V + 15% 13C 1718.8 923.3 1697.7 1699.8 H2OV 1974.5 919.5 917.3 Absorption V + 12C / / / / Frequency (cm-1)

  27. VnCm Candidates C3 2091.4 2029.7 2032.8 V + 15% 13C C3 2019.4 C3 Absorption V + 12C / / 2090 2070 2080 2010 2020 2030 2040 Frequency (cm-1)

  28. Linear VC5? 2091.4 1974.5 V + 15% 13C Absorption V + 12C / / 1970 1980 2090 2070 2080 Frequency (cm-1)

  29. DFT Predictions for VC5 Linear structure DFT (B3LYP/6-311G + 3df) calculationsa and matrix observations for linear VC5 (sextet) a Rittby, unpublished work (2005)

  30. 1700 1710 1720 Linear VC5? - Alternative C3 x10 2029.7 2032.8 1718.8 C3 1697.7 2019.4 1699.8 H2OV C3 V + 15% 13C Absorption V + 12C / / 2010 2020 2030 2040 Frequency (cm-1)

  31. DFT Predictions for VC5 Linear structure DFT (B3LYP/6-311G + 3df) calculationsaand matrix observations for linear VC5 (sextet) a Rittby, unpublished work (2005)

  32. Alternative Assignment- Linear VC3? 1974.5 V + 15% 13C Absorption V + 12C 1970 1980 Frequency (cm-1)

  33. DFT Predictions for VC3 Linear structure DFT (B3LYP/6-311G + 3df) calculationsa and matrix observations for linear VC3 (sextet) a Rittby, unpublished work (2005)

  34. Difficulties with Vanadium • The oxidation problem • V oxidized – contaminated powder • Solutions • Heat powder • O2 recombined with C powders • Could not reach temperature to bake out CO • V melts ~1900 °C causing cell failure • Ar environment (glove bag) + new powder – did not resolve CO problem • Powder possibly oxidized before arrival? • A new approach to dual ablation (Kinzer)

  35. n4 C7 n6 C9 n9 C12 n5 n3 C7 C5 n4 c-C6 n6 n3 n7 n5 C10 C4 C11 C6 n3 n8 C3 n5 n7 C11 n5 C10 C9 n3 n7 C8 C5 C9 n9 C8 Survey Spectrum: Cr+12C vs. 12C 2127.8 1998.0 1818.0 2164.1 1894.3 2074.9 1694.9 1543.4 1952.5 1946.1 2038.9 1856.7 1915.8 1710.5 2071.1 2078.1 12C rod 1446.4 1601.1 Absorption 1693.0 1554.5 1789.5 12C rod + Cr rod 1305.6, 1306.3 1300 1400 1500 1600 1700 1800 1900 2100 2000 35 Frequency (cm-1)

  36. n3 C4 CrnCm Candidates 1306.3 1305.6 1543.4 Lump Cr3C2 Absorption 1554.5 Cr rod + 12C rod / / 1500 1550 1600 1290 1300 1310 1320 Frequency (cm-1) 36

  37. CrnCm Candidates 1789.5 n4 c-C6 n6 1694.9 Cr rod + 12C rod C8 1693.0 1710.5 Absorption Cr rod + 12C rod, 20K Lump Cr3C2 Cr rod + 12C rod, 10K / / 1680 1700 1720 1770 1780 1790 Frequency (cm-1) 37

  38. CrC3 – C2v Structure? 1306.3 1305.6 Lump Cr3C2 Absorption Cr rod + 12C rod 1290 1300 1310 1320 Frequency (cm-1)

  39. DFT Predictions for CrC3 C2v structure DFT (B3LYP/6-311G + 3df) calculations,a PES observations,a and matrix observations for C2v CrC3 (triplet) Proposed structure (C2v) of CrC3 a Wang and Li (2000)

  40. 1789.5 1770 1780 1790 Frequency (cm-1) CrC3 – Linear Structure? Cr rod + 12C rod Absorption Lump Cr3C2

  41. DFT Predictions for CrC3 Linear structure DFT (B3LYP/6-311G + 3df) calculationsa and matrix observations for linear CrC3 (pentuplet) a Rittby, unpublished work (2005)

  42. Conclusions • Vanadium • Possible assignments of the n1(s) and n2(s) modes of linear VC5 to • 2091.5 and 1974.5 cm-1, or • 2032.8 and 1718.8 cm-1 • Possible assignment of the n1(s) mode of linear VC3 to 1974.5 cm-1 • Chromium • Possible assignments of the n1(a1) and n5(b2) modes of the C2v isomer of CrC3 to 1306.3, 1305.6 cm-1 • Possible assignment of the n1(s) mode of linear CrC3 to 1789.5 cm-1

  43. Future Work • Obtain 13C isotopic shifts • New technique to dual ablation using very low laser powers on carbon rod ( <1.0 watt) • Try various ratios of 13C and 12C in unbaked lab rod • Look for other VnCm and CrnCm species • Try to make longer Cn chains • Adjust laser powers on carbon rod (higher laser powers make longer chains) • For Cr: mix Cr and C powders together (Cr does not oxidize like V)

  44. Acknowledgments • ADVISOR: Dr. W. R. M. Graham • THEORETICAL CALCULATIONS: Dr. C. M. L. Rittby • TECHNICAL SUPPORT: • Machine Shop: Mike Murdock, David Yale • Electronics Shop:Gerry 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

  45. 1700 1750 1800 1850 1900 1950 2000 2050 2100 2150 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)

  46. Cr rod + 90% 13C rod (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)

  47. DFT and Matrix Measurement Comparison Comparison of observed vibrational frequencies (cm-1) of the 1() mode for single 13C-substituted isotopomers of linear CrC3 with the predictions of B3LYP/6-311G+(3df) calculations. aDFT calculations scaled by a factor of 1789.5/1947 0.9189.

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