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Raman Spectroscopy: Introductory Tutorial Daniel T. Schwartz Department of Chemical Engineering Box 351750 University of Washington Seattle, WA 98195-1750 [email protected] Goal of the tutorial is to answer the questions, “What is Raman spectroscopy and can the new Raman microscope at NTUF help with my sample?” Raman Spectroscopy: Some Sources General Principles and Instrumentation: Principles of Instrumental Analysis, by Douglas A. Skoog, F. James Holler, Timothy A. Nieman Inorganic: Infrared and Raman Spectra of Inorganic and Coordination Compounds : Theory and Applications in Inorganic Chemistry (Volume A) by Kazuo Nakamoto Infrared and Raman Spectra of Inorganic and Coordination Compounds : Applications in Coordination, Organometallic, and Bioinorganic Chemistry (Volume B) by Kazuo Nakamoto Organic: The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules by Daimay Lin-Vien, et al Raman Spectroscopy: Overview • A vibrational spectroscopy - IR and Raman are the most common vibrational spectroscopies for assessing molecular motion and fingerprinting species - Based on inelastic scattering of a monochromatic excitation source - Routine energy range: 200 - 4000 cm–1 • Complementary selection rules to IR spectroscopy - Selection rules dictate which molecular vibrations are probed - Some vibrational modes are both IR and Raman active • Great for many real-world samples - Minimal sample preparation (gas, liquid, solid) - Compatible with wet samples and normal ambient - Achilles Heal is sample fluorescence Raman Spectroscopy: General • IR and Raman are both useful for Fingerprinting • Symmetry dictates which are active in Raman and IR Raman Spectroscopy: General • Group assignments identify characteristic vibrational energy Raman Spectroscopy: Classical Treatment • Number of peaks related to degrees of freedom DoF = 3N - 6 (bent) or 3N - 5 (linear) for N atoms • Energy related to harmonic oscillator or c 2 k(m1 m2 ) m1m2 • Selection rules related to symmetry Rule of thumb: symmetric=Raman active, asymmetric=IR active CO2 Raman: 1335 cm–1 IR: 2349 cm–1 IR: 667 cm–1 H2O Raman + IR: 3657 cm–1 Raman + IR: 3756 cm–1 Raman + IR: 1594 cm–1 2nd Electronic Excited State Impurity 25,000 1st Electronic Excited State 4,000 emit emit emit Vib. states 0 fluorescence Elastic Scattering (Raleigh) fluorescence Excitation Energy, (cm–1) Main Optical Transitions: Absorption, Scattering, and Fluorescence IR Electronic Ground State UV/Vis Fluorescence Excitation Energy, (cm–1) Raman Spectroscopy: Absorption, Scattering, and Fluorescence 2nd Electronic Excited State 25,000 1st Electronic Excited State Stokes Anti-Stokes emit 4,000 0 ±∆ IR Raman ∆=emit– Vib. states ∆ Electronic Ground State Resonance Raman ∆=emit– Raman Spectroscopy: At NTUF, you pick the Laser Excitation Visible 514 nm Intensity Near IR 785 nm Stokes –∆ 11,000 Stokes Anti-Stokes +∆ 13,000 Anti-Stokes –∆ 15,000 17,000 19,000 Excitation Energy, (cm–1) +∆ 21,000 Impurity 25,000 Anti-Stokes emit 4,000 ∆ IR 1st Electronic Excited State Raman ∆=emit- fluorescence Stokes 0 2nd Electronic Excited State fluorescence Excitation Energy, (cm–1) Raman Spectroscopy: Absorption, Scattering, and Fluorescence Vib. states Fluorescence = Trouble Electronic Ground State Raman Spectroscopy: Coping w/ Fluorescence 1. Use NTUF 785 nm laser line (excites many fewer fluorophores) 2. Photobleach with long exposure laser irradiation. Raman Intensity Poly (diallyl phthalate) lex = 514.5 nm Without Bleaching After 2 hours Bleaching 1000 2000 3000 Raman Shift (cm-1) Raman Spectroscopy: Summary 1. Raman is a vibrational spectroscopy akin to IR - Good for fingerprinting, probing molecular symmetry 2. Scattering-based, not transmission/reflection - Means no need for fancy sample preparation…gas, liquid, or solid Virtually always use anti-Stokes lines due to stronger signal 3. You need to pick excitation energy (laser line) - 785 nm: Fluorescence less probable; Lower Raman signal 514 nm: Fluorescence more probable; Resonance more likely; Higher signal 4. Other things not talked about - SERS: Surface Enhanced Raman Spectroscopy Quantum origins of selection rules and scattering cross-section Raman Spectroscopy: Dan’s trip to NTUF Fuel Cell Materials --- picked because I had little Raman experience with these materials From http://www.fueleconomy.gov/feg/fc_pics/fuel_cell_still.gif Raman Spectroscopy: Dan’s trip to NTUF Flow Field Plate - Graphite Ig 3.98 Id Nanocrystalline graphite has graphitic (g) and disorder (d) peaks. The characteristic dimension of graphitic domains is given by: Ig (Å) 44 Id 175Å From early literature on graphitic materials Tuinstra and Koenig, J. Chem Phys. 53, 1126 (1970). Raman Spectroscopy: Gas Diffusion Layers (graphite paper) (Å) 161Å (Å) 76Å Dan’s trip to NTUF Raman Spectroscopy: Gas Diffusion Layers (Woven Fibers) (Å) 55Å Dan’s trip to NTUF Raman Spectroscopy: Dan’s trip to NTUF Nafion Fluorescence problems! Maybe try photobleaching as next option.