FTIR Spectroscopy of Small Titanium-Carbides A Survey and Preliminary Results

1. FTIR Spectroscopy of Small Titanium-CarbidesA Survey and Preliminary Results Robin Kinzer TCU Molecular Physics Laboratory 31 October 2005

2. Background & Previous Research

3. Background - Titanium • Titanium exists in five stable isotopes, 46-50Ti. 48Ti is most common (73.7%); radioactive 44Ti is produced almost exclusively in supernova. • Ti composes 80 ppb/atom in the universe and 100 ppb/atom of the sun. Carbonaceous meteorites contain 230,000 ppb/atom. • By comparison, gold is 0.004 and 0.01 ppb/atom for universe and sun, respectively; iron is 20,000 and 30,000 ppb/atom, respectively. Iron is 7.7 x 106 ppb/atom in meteorites. Data from: www.webelements.com

4. Background - Titanium • M stars, the coolest stars, are notable for having very strong molecular absorption bands, particularly TiO (Carrol & Ostlie, p. 226).

5. Background - Ti8C12+ • Observation of C60 sparked extensive study of other fullerenes. • The first report of metallocarbohedrenes – cage-like molecules incorporating metals and carbon atoms – was made by Castleman, et al. in Science (1992, v.255, 1411). They reported the discovery of Ti8C12+ via mass spectrometry.

6. Background - Ti8C12+ • Ti8C12+ has dodecahedral (Th) symmetry and is very stable due to strong Ti-C & C-C s-bonding and its shape. • This ‘metcar’ was formed by reacting Titanium with CH4, C2H2, C2H4, C3H6, C6H6 vapors. • Ti7C13 +, Ti6C14 +et al. with similar dodecahedral symmetry are less stable.

7. Background - Research • Experimental and theoretical studies of smaller ‘metcar’ structures are needed to better understand how more complex ‘metcars’ form. • Studies include PES observation of TiCx- where x=2-5 and DFT study of the structure of TiC2, TiC3 and TiC4. • Though these studies concentrate on infrared, experimental studies have yet to observe the fundamental vibrational levels of neutral titanium-carbides.

8. 355 nm 532 nm Background – PES Studies • Wang, Ding & Wang researched vibrational PES of TiCx-, x=2-5, using Nd:YAG lasers . • Ground state for the TiCx- (X) and excited vibrational states of the neutral TiCx (A-D) were observed. • Fundamental vibrations of neutral molecules extrapolated from excited vibrational levels. Wang, X.B; Ding, C.F; Wang, L.S; J. Phys. Chem. A 1997, 101, 7699-7701.

9. Background – PES Studies • No previous calculations had been made for TiC3-5; prediction for their structures were made by comparing this data to previous YCx and LaCx research. • Since the C-C bond is strong and less likely to break than a TiC bond, they predicted ring-like structures for TiC3-5, similar to YCx and LaCx. Totally symmetric modes. Wang, X.B; Ding, C.F; Wang, L.S; J. Phys. Chem. A 1997, 101, 7699-7701.

10. Background – PES Study • Problem: The vibrational fundamental of the neutral molecule is extrapolated from the higher vibrational energy levels. • The reported observations have large margins of error, in the range of 30 - 60 cm-1. • Note: These are the only results available for TiC2-5 in the NIST Chemistry WebBook.

11. Background – DFT Study • R. Sumathi and M. Hendrickx published a DFT survey of TiCx, x=2-4, solving ab initio the energy levels of several isomers. • Vibrational frequencies for each isomer and electronic state were solved. • Calculated frequencies for the optimum isomer were in good agreement with those observed by Wang, et al. • Confirm TiC2 has C2v symmetry with a stretching mode at 587 cm-1. Note that Wang, et al. observed 560 ± 50 cm-1.

12. Background – DFT Study TiC3 Below: Optimized geometries for singlet, triplet ( ) and quintet [ ] isomers of TiC3 at the B3LYP level of theory. Bond length in Å. Above: Plot of relative energies (kcal/mol) of the various isomers of TiC3 in different electronic states.

13. Background – DFT Study TiC3 Singlet Calculated Frequencies: 1531.4; 1281.3; 833.6; 686.5; 591.2; 465.3 cm-1 Observed Frequencies (Wang): 650 ± 30 cm-1 TiC4 Triplet Calculated Frequencies: 1882.7; 1789.2; 1055.5; 601.1; 473.6; 472.2; 399.6; 391.1; 251.9 cm-1 Observed Frequencies (Wang): 440 ± 40 cm-1

14. Background – DFT Study • This study is exhaustive in considering several possible isomers and electronic states for the chosen molecules. • Unfortunately, this study does not provide estimated intensities for these different modes of TiC3 and TiC4.

15. Current Research & Experiment

16. Nd-YAG 1064 nm pulsed laser laser focusing lens CsI window Quartz window gold mirror ~ 10K To pump 10-7Torr or better To pump 10-3Torr Carbon rod Titanium rod Ar Experimental Apparatus Bomem DA3.16 Fourier Transform Spectrometer • KBr beam splitter • liquid N2 cooled MCT detector (400 - 4000 cm-1)

17. Survey Spectrum of Ti + 12C • The following (best) spectra of titanium and graphite ablation were obtained in June 2005. • Three bands of interest appear: 624.3 cm-1, 915.7 cm-1, 1484.1 cm-1. • These three bands are from one experiment where the laser window nearly broke (hitting the C-rod). Thus, less intense light was hitting the rod, and less Carbon deposited. • These are the strongest these bands appeared in Ti + 12C experiments.

18. 915.7 912.2 624.3 616.9 910 920 930 1484.1 620 640 1485.1 1480 1490 TiC Candidates Experiment: 90 min. deposition 12C < 2.0 W; Ti~ 2.7 W Anneal 14 – 26 K Spectra at 26 K.

19. Experimental Technique • Metal carbide experiments are best performed using a dual ablation technique. This allows more control over multiple evaporation rates than single-rod ablation. • Baked 13C low-enrichment (l.e.) rods, though stronger due to baking, take one month to prepare. Soft rods, which are non-baked, are weaker. • Experimental technique needs to be adjusted for limitations of the samples. Soft rods cannot be used at the same laser power or as long as baked rods.

20. Experimental Technique • Problem: Aforementioned TiC candidates appeared weakly using l.e. 13C rods (if at all!). • Baked l.e. 13C rods were previously used (power ~1.5 W). Signal diminishes quickly. Too little deposition of Ti. • New baked rods not readily available. Soft rods must be utilized. • Consider reducing laser power on Carbon rod, in spirit of the Ti-12C results.

21. C3 (Used) Baked. 10 min dep. ~1.3 Watts. 4 C6 C7 C9 C5 C3 C3 C7 3.5 (New) Soft. 25 min dep. ~0.8 Watts. 3 2.5 2 1800 1850 1900 1950 2000 2050 2100 2150 Experimental Technique Overall effect of reducing laser power on Carbon rod. No annealing. 1800 – 2200 cm-1. At low power. C3, C6, C9 significantly stronger. Other chains also stronger. Not correct! Weaker!

22. ~1541 cm-1 ~1530 ~1506 Theoretical Results for TiC3 ScaledValues for 1484 cm-1

23. Experimental Technique C4 Baseline corrected. 40 min dep. 18K. 1.92 1.9 1484.1 1.88 C4 (~8%) C5 1.86 1473.5* (~57% 1484) C4 ? 1450.9 (~9% 1484) C4 ?? 1.84 C5 C5 1.82 1440 1460 1480 1500 1520 1540

24. High Enrichment 13C Experiment Baseline corrected. 20 min dep. 22K. 1435.5 1.86 1.85 1.84 1.83 1.82 1410 1420 1430 1440 1450 1460 1470 1480 1490 1500

25. Results • TiC3 with C2v symmetry may have a vibrational mode at 1484.1 cm-1. Better resolution of the shift at 1473 and high enrichment 13C experiments are needed to confirm this. • As of yet, no shifts of the 915.7 cm-1 or 624.3 cm-1 band have been observed. • Laser powers of ~0.8 W on the l.e. carbon rod produces small carbon chains abundantly. Longer carbon chains may be constructed by annealing these. • Low laser power has proven to help in sharpening and obtaining shifts of small metal-carbides, as seen in CrC3 (Bates).

26. Further Work • Resolve the ‘feature’ at 1473 cm-1 as best as possible. • Additional high enrichment 13C experiments need to be run for the TiC3 candidate. Some of these will be soft rods. • Experiments to study and obtain shifts 624 cm-1 and 915 cm-1 bands are needed.