Topics You Know By Now! • From Physics – EM radiation and its properties – Diffraction – Refraction – Reflection – Coherent and incoherent radiation – Polarization of radiation – Scattering of radiation • From Chemistry – Photoelectric effect – Electromagnetic spectrum – Beer’s Law, etc. – Quantized states in atoms – lead to line spectra – Quantized states in molecules – lead to broad or continuum spectraComponents of Optical Instruments Chapter 7 2Components of Optical Instruments: The generic spectrometer n Sources n Sample Holders n Wavelength Separators and Slits n Detectors n Signal processor and readoutComponents of Optical Instruments: The generic spectrometer n Sources n Sample Holders n Wavelength Separators and Slits n Detectors n Signal processor and readout5 Absorption measurements Fluorescence, Phosphorescence measurements Emission, Chemiluminescence measurements6 Generated beam with sufficient, stable radiant power7 Continuum Source Line Source Continuum + Line Source Ingle and Crouch, Spectrochemical Analysis Al + Mg Spectroscopic Sources8 Continuum sources: absorption and fluorescence spectroscopy Line sources: atomic absorption spectroscopy, atomic and molecular fluorescence spectroscopy, Raman spectroscopy Laser sources: Raman spectroscopy, molecular absorption spectroscopy, emission spectroscopy, as part of instruments for Fourier transform infrared spectroscopy Application9 Optical Continuum Source Characteristics Ingle and Crouch, Spectrochemical Analysis10 Tungsten Filament Heated to 2870 K. Useful Range: 350 – 2500nm11 Tungsten / Halogen Iodine added. Reacts with gaseous W near the quartz wall to form WI2. W is redeposited on the filament. Smooth spectral curve, stable output and little UV radiation Gives longer lifetimes Allows higher temperatures (~3500 K).Intensity(Spectrum(of(the(Tungsten4Halogen(Lamp(12 • Weak intensity in UV range • Good intensity in visible range • Very low noise • Low driftDeuterium (D2) Lamps 13 Discharge of excited deuterium gas. Continuum of UV light (185 – 400 nm). Ultraviolet source for UV-Vis and HPLC.14 Arc Lamps Electrical discharge is sustained through a gas or metal vapor. Continuous emission due to rotational/vibrational energy levels and pressure broadening. An electric arc lamp An mercury arc lamp An krypton long arc lampIntensity Spectrum of the Xenon Arc Lamp 15 • High intensity, smooth continuum from UV-Vis range • Small radiating arc region – point source • Closely match the sun’s spectrum • Absorbance and fluorescence applications16 Hg Arc Lamp 1. Continuum + Line Source – strong UV output with discrete lines. 2. High Power Source. 3. Often used in photoluminescence -simultaneous activation of several fluorochromes“LASER” • Light Amplification by Stimulated Emission of Radiation • Emits very intense, monochromatic light at high power (intensity) • Coherent nature of their outputs - all waves in phase (unique), and parallel • Narrow bandwidths • High-resolution spectroscopy, Raman spectroscopy, molecular absorption/emission spectroscopy, Fourier transform infrared spectroscopy • Not useful for scanning wavelengthsLaser SetupMechanism of Laser action 1. Pumping 2. Spontaneous emission 3. Stimulated emission 4. AdsorptionPumpingSpontaneous: Incoherent radiation Differs in direction and phase Spontaneous emissionStimulated emission 1 2 3 Stimulated emission: coherent radiation Same direction and phaseStimulated Emission • A photon incident on an excited state species causes emission of a second photon of the same frequency, which travels in exactly the same direction, and is precisely in phase with the first photo. Excited level Ground levelAbsorptionPopulation Inversion is Necessary for Light Amplification Population inversions are obtained by pumping 3 2 2 3Why four level laser systems are more efficient as compared to three level lasers? Easy population inversion Completely empty to start withAdvantages of Lasers • High intensities (“Small dot”) • Nearly Monochromatic (“narrow bandwidth”) • Coherent (“constructive interference”)Types of Lasers • Solid state lasers – Ruby (a three-level device) – Nd:YAG (a four-level laser source) • Gas lasers – lines w/ specific λs in UV/vis/IR • Neutral atom - He/Ne • Ion lasers - Ar+, Kr+ • Molecular lasers - CO2, N2 • eximers (XeF+,….) • Dye lasers – Solution of organic compounds – limited tunability in the visible • Semiconductor diode lasers – Light-emitting diodes with various λ– emitted limited tunability in the red and IRSample Holders (Cells) • Must: – contain the sample without chemical interaction – be more-or-less transparent to the wavelengths of light in use – be readily cleaned for reuse – be designed for the specific instrument of interest…. • Examples – quartz is good from about 190-3000 nm – glass is a less expensive alternative from about 300-900 nm – Plastic containers for visible region – NaCl and KBr are good for cell window in the IR region • Cells can be constructed to: – transmit light absorbed at 180 degrees to the incident light – allow emitted light to exit at 90 degrees from the incident light – contain gases (lower concentrations) and have long path lengths (1.0 and 10.0 cm cells are most common)Absorbance: usually in a matched pair! Fluorescence, Phosphorescence, ChemiluminescenceSub-Micro fluorescence cells (10-160 microliter) Spectrophotometer cells Cells for flow-through measurementsDisposable cuvettes for UV and VIS applications BRAND™ UV cuvettes - 220-900 nm spectral rangeWavelength Selectors….. • Used to select the wavelength (or wavelength range) of light that either – impinges on the sample (fluorescence and phosphorescence) – is transmitted through the sample (absorption and emission) • This selected wavelength then strikes the detector – the ability to select the wavelength helps you to discriminated between phenomena caused by your analyte and that caused by interfering or non-relevant species. • Are often combined with a set of SLITS