LASER Stimulated Emission Optical Resonance Types of Lasers
Dipping, Caroline, Food Writer has reference to this Academic Journal, PHwiki organized this Journal Topics You Know By Now! From Physics Courses EM radiation in addition to its properties Diffraction Refraction Reflection Coherent in addition to incoherent radiation Polarization of radiation Scattering of radiation From Chemistry Courses Photoelectric effect Electromagnetic spectrum Beers Law, etc. Quantized states in atoms lead to line spectra Quantized states in molecules lead to broad or continuum spectra Components of Optical Instruments: The generic spectrometer General Designs Sources in addition to Sample Holders Wavelength Separators Slits Detectors
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LASER Light Amplification by Stimulated Emission of Radiation Emits very intense, monochromatic light at high power (intensity) All waves in phase (unique), in addition to parallel All waves are polarized in one plane Used to be expensive Not useful as long as scanning wavelengths Laser Setup
Spontaneous: Incoherent radiation Differs in direction in addition to phase Stimulated 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, in addition to is precisely in phase with the first photo. M + hM + 2h Population Inversion is Necessary as long as Amplification Population inversions are obtained by pumping
Overall Easy population inversion Advantages of Lasers Low Beam Divergence (Small dot) Nearly Monochromatic (narrow b in addition to width) Coherent (constructive interference)
Eugene Hecht, Optics, Addison-Wesley, Reading, MA, 1998. Light Amplification in the Resonance Cavity. Optical Resonance Form a resonant cavity, with mirrors on each end Length of cavity is an integral multiple of the desired wavelength Types of Lasers Solid state lasers Nd:YAG neodymium yttrium aluminum garnet 1064 nm Gas lasers lines w/ specific s in UV/vis/IR He/Ne Ar+, Kr+ CO2 eximers (XeF+, .) Dye lasers limited tunability in the visible Semiconductor diode lasers limited tunability in the IR, red
Sample Holders (Cells) Must: contain the sample without chemical interaction be more-or-less transparent to the wavelengths of light in use be readily cleaned as long as reuse be designed as long as 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 NaCl in addition to KBr are good to much higher wavelengths (IR range) 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) in addition to have long path lengths (1.0 in addition to 10.0 cm cells are most common) Absorbance: usually in a matched pair! Fluorescence, Phosphorescence, Chemiluminescence
Wavelength Selectors Used to select the wavelength (or wavelength range) of light that either impinges on the sample (fluorescence in addition to phosphorescence) is transmitted through the sample (absorption in addition to emission) This selected wavelength then strikes the detector the ability to select the wavelength helps you to discriminated between phenomena caused by your analyte in addition to that caused by interfering or non-relevant species. Are often combined with a set of SLITS (discussed later) Various types based on filters (CHEAP COLORED GLASS) based on prisms (LIMITED APPLICATIONS) based on gratings . (GREAT STUFF) Filters Simple, rugged (no moving parts in general) Relatively inexpensive Can select some broad range of wavelengths Most often used in field instruments simpler instruments instruments dedicated to monitoring a single wavelength range. Two types of filters: Interference filters depend on destructive interference of the impinging light to allow a limited range of wavelengths to pass through them (more expensive) Absorption filters absorb specific wavelength ranges of light (cheaper, more common)
Fabry-Perot Filters (Interference Filters) Douglas A. Skoog in addition to James J. Leary, Principles of Instrumental Analysis, Saunders College Publishing, Fort Worth, 1992. Calcium or Magnesium Fluoride (FLUORITE!) t From 1 to 1: For rein as long as cement to occur at point 2, N is order of interference (a small whole number) A dielectric material is a substance that is a poor conductor of electricity, but an efficient supporter of electrostatic fields. Fabry-Perot Filters (Interference Filters) When approaches zero n = 2t Snells law: / = / then = is the wavelength passing the filter in addition to is the refractive index of the dielectric medium t Are we missing something
Absorption Filters Douglas A. Skoog in addition to James J. Leary, Principles of Instrumental Analysis, Saunders College Publishing, Fort Worth, 1992. Melles Griot Catalogue Typical Effective B in addition to widths = 30 250 nm BAND or CUT-OFF Filters Ingle in addition to Crouch, Spectrochemical Analysis B in addition to pass Cutoff Two basic functions . cutoff filters absorb light in a specific range of wavelengths. They cutoff this range from the detectors (e.g. cutoff as long as 550 nm) b in addition to pass filters absorb light outside of a specific range (e.g. 350-550 nm) often made of a combination of two cutoff filters!
Absorption Filters vs. Interference Filters Douglas A. Skoog in addition to James J. Leary, Principles of Instrumental Analysis, Saunders College Publishing, Fort Worth, 1992. Wavelength Selectors Filters Prisms Gratings Michelson Interferometer Czerny Turner Monochromator Douglas A. Skoog, F. James Holler in addition to Timothy A. Nieman, Principles of Instrumental Analysis, Saunders College Publishing, Philadelphia, 1998.
Types of Optical Instruments Spectroscope Optical instrument used as long as visual identification of atomic emission lines Colorimeter Human eye acts as detector as long as absorption measurements Photometer Contains a filter, no scanning function Fluorometer A photometer as long as fluorescence measurement Spectrograph Record simultaneously the entire spectrum of a dispersed radiation using plate or film Spectrometer Provides in as long as mation about the intensity of radiaition as a function of wavelength or frequency More (confusing )
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