CHAPTER 7 COMPONENTS OF OPTICAL INSTRUMENTS - based on absorption, fluorescence, phosphorescence, scattering, emission and chemiluminescence - 5 components: stable source of radiation, transparent sample holder, monochromator (for isolating restricted region of spectrum), radiation detector (converts energy to electrical signal) and a signal processor/readout - absorption, fluorescence and phosphorescence require external sources of energy - in phosphorescence and fluorescence, emitted radiation is measured at an 90 angle w.r.t the source - no external source is - can be used with flame and thermal conductivity detectors (universal) for emission and chemiluminescence spectroscopy since the sample itself is the emitter

Sources of radiation - must be stable and powerful enough - continuum source (intensity changes slowly as a function of wavelength); used widely in absorption and fluorescence spectroscopy - line source (bands of radiation each spanning a limited range of wavelengths); used widely in atomic absorption, atomic and molecular fluorescence and Raman spectroscopy

Lasers - lasers are highly useful sources because of their high intensities, narrow bandwidths, and coherence - light amplification by stimulated emission of radiation

- the lasing medium is pumped (activated) by external radiation to form a cascade of photons that are passed back and forth by a pair of mirrors => amplification of highly parallel radiation

- laser action: pumping, spontaneous emission, stimulated emission and absorption - pumping: excites the active species in lasers, causing several higher electronic and vibrational energy levels to become populated - spontaneous emission: random, yields incoherent monochromatic radiation - stimulated emission: the excited laser species is struck by photons with the same energies as those produced by spontaneous emission, causing it to relax and emit a photon of the same phase and energy as the one that stimulated this process => stimulated emission is totally coherent with incoming radiation - absorption: produces the same result as pumping and competes with stimulated emission

- for there to be light amplification in a laser, # photons from stimulated emission > # photons lost to absorption => population inversion via pumping

Wavelength selectors - filters and monochromators - the effective bandwidth is an inverse measure of the quality of the device

Filters - interference filters: relies on optical inerference to provide narrow bands of radiation; consists of a transparent dielectric sandwiched between two glass plates => thickness determines the wavelength of transmitted radiation; follows Bragg's rule

- absorption filters: absorb selected portions of the spectrum; consists of colored glass or of a dye sandwiched between glass plates

Monochromators - designed for spectral scanning => continuously varying the wavelength of radiation over a broad range - monochromators for UV, visible and IR all use slits, lenses, mirrors, windows, gratings or prisms - consist of: an entrance slit, collimating lens/mirror, prism/grating, focusing element, exit slit

Sample containers - needed for all spectroscopic studies except emission spectroscopy - the cells or cuvettes that hold the samples must be transparent to radiation in the spectral region of interest - NaCl is the most commonly used substance for the IR region Radiation transducers - real transducers exhibit a small constant response in the absence of radition => dark current S= kP + k' - respond to either heat or photons (photoelectric or quantum) - photon transducers are used mostly for UV, visible and near-IR - although the relative sensitivity of thermal transducers is independent of wavelength, it is significantly lower than that of photoelectric transducers Photon transducers - photovoltaic cells: energy generates current at the interface of a semiconductor layer and a metal - phototubes: radiation causes emission of electrons from a photosensitive solid surface - photomultiplier tubes: photoemissive surface that emits cascades of electrons when struck by electrons from photosensitive area - photoconductivity transducer: absorption of radiation by a semiconductor produces electrons and holes, thus enhancing conductivity - Si photodiodes: photons cause the formation of electron-hole pairs and a current across a reverse-biased pn junction - charge transfer transducers: charge developed in a silicon crystal as a result of photon absorption is collected and measured

Thermal transducers - used for the IR region since photons from phototransducers do not have enough energy to cause photoemission in the IR region - radiation is absorbed by a small blackbody and the resultant temperature rise is measured; since the power from an IR beam is extremely small, the heat capacity of the absorbing element must be as small as possible for the temperature change to be detected; housed in a vacuum to avoid thermal noise from surroundings - thermocouple: pair of junctions formed when two pieces of metals are fused to each end of a dissimilar metal, causing a voltage that changes with the temperature difference to develop between the two junctions - thermopile: thermocouples connected in series; higher sensitivity - bolometer: resistance thermometer constructed from strips of metal that exhibit large changes in resistance as a function of temperature - thermistor: semiconductor bolometer - pyroelectric transducer: temperature-dependent retention of electric field polarization that is temperature => pyroelectric crystal sandwiched between two electrodes; change in charge distribution upon irradiation produces a current that is proportional to the rate of change in polarization with temperature; lose their polarization when heated above the Curie point Signal processors and readouts - electronic device that amplifies and processes the signal from the transducer - include digital meters, cathode ray tubes, LCD panels, computer displays Types of optical instruments - a spectroscope is an optical instrument used for the visual identification of atomic emission lines - a colorimeter is an instrument for absorption measurements in which the human eye serves as the detector - a photometer consists of a source, filter and a photoelectric transducer as well as a signal processor and readout; fluorometers are photometers designed for measuring fluorescence - a spectrometer provides information about radiation intensity as a function of wavelength or frequency - polychromators are multichannel dispersing modules that allow >2 frequencies to be viewed simultaneously - spectrophotometer is a spectrometer equipped with >1 exit slits and photoelectric transducers that permit the determination of the ratio of the radiant power of two beams as a function of wavelength FT Optical Measurements - throughput (Jaquinot) advantage: few optical elements and no slits to attenuate the radiation => greater radiant power and S/N ratio - high resolving power and wavelength reproducibility

- rapid data acquisition since all elements reach the detector simultaneously => Fellgett/multiplex advantage - frequency domain to time domain

- signal power usually decreases with time because the closely spaced wavelengths become more and more out of phase - time and frequency domains can be interconverted via:

- the time domain data is acquired using a Michelson interferometer

Michelson interferometer consists minimally of mirrors M1 & M2 and a beam splitter M. In Fig 2, a source S emits light that hits the beam splitter (in this case, a plate beamsplitter) surface M at point C. M is partially reflective, so part of the light is transmitted through to point B while some is reflected in the direction of A. Both beams recombine at point C' to produce an interference pattern incident on the detector at point E (or on the retina of a person's eye). If there is a slight angle between the two returning beams, for instance, then an imaging detector will record a sinusoidal fringe pattern as shown in Fig. 3b.

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