Chapter 3. Components of Optical Instruments

§1 General designs of optical instruments Source container Wavelength selector Radiation detector Signal processor and readout Absorption Fluorescence Phosphorescence Scattering Emission Chemiluminescence Components of typical spectroscopic instruments Phenomena for optical spectroscopic methods Three configurations

B. fluorescence, phosphorescence, scattering A. absorption (1) (2) (3) (4) (5) hv hv hv I Source lamp Or heated solid Sample holder Wavelength selector Photoelectric transducer Signal processor and readout B. fluorescence, phosphorescence, scattering (2) (3) (4) (5) hv hv I Sample holder Wavelength selector Photoelectric transducer Signal processor and readout hv (1) Source lamp Or heated solid C. emission, chemiluminescence (3) (4) (5) (1) and (2) hv hv I Wavelength selector Photoelectric transducer Signal processor and readout Source and Sample holder

§2 Sources of radiation Enough Energy & Stability Sources for spectroscopic instruments

Continuum Sources

Line Sources

• monochromatic (narrow bandwidths)(line sources) Laser Sources Light Amplification by Stimulated Emission of Radiation Advantages • intense • monochromatic (narrow bandwidths)(line sources) • pulsed (10-15-10-6 s) or continuous wave (cw) • coherent nature of outputs • small beam divergence

§3 Wavelength Selectors For spectroscopic analysis, a limited, narrow, continuous group of wavelength is required. 干涉光劈

Two types of wavelength selectors Filters monochromators Fig. 7-11 Output of a typical wavelength selector (p.155)

b. Absorption filter - colored glass or dye between two glass plates Filters a. Interference filter - two thin sheets of metal sandwiched between glass plates, separated by transparent material (Interference for transmitted wave through 1st layer and reflected from 2nd layer ) 100 Band-reject interference filter Transmittance b. Absorption filter - colored glass or dye between two glass plates 400 Wavelength (nm) 750 100 Useful in non-scanned situations and when a specific, known band of radiation needs to be monitored. Band pass interference filter Transmittance

Monochromators: Five components: • Entrance slit • Collimating lens or mirror • Dispersion element (prism or grating) • Focusing lens or mirror • Exit slit

Grating (most modern instruments) Echellette grating:

Performance characteristics of monochromators (1) Spectral purity scattered or stray light in exit beam Use entrance and exit windows, dust and light-tight housing, coat interior with light absorbing paint (2) Dispersion ability to separate small wavelength differences ( quantify the spatial separation of wavelengths on the exit focal plane)

Angular Dispersion: Da=dθ/dλ (property of dispersive element) Linear Dispersion: D=dy/dλ (property of dispersive device) Shown in instrument handbook

Resolving Power of monochromators

Resolving Power for a Grating So, in order to just resolve these two spectral lines: λ1 = 4501 Å λ2 = 4499 Å We need an instrument with a resolving power of: R = 4500/2 = 2250 The resolving power of a diffraction grating:

Effect of slit width on resolution Fig. 7-22 Illumination of an exit slit by monochromatic radiation 2 at various monochromator settings. Exit and entrance slits are identical. (p.165) Bandwidth: range of wavelengths exiting the monochromator Related to dispersion and entrance/exit slit widths Effective bandwidth (eff ): eff = When y is equal to slit width w,  is the effective bandwidth, we know

Effect of slit width on spectra (p.166) No spectral resolution of three wavelengths Partial resolution of three wavelengths Complete resolution of two lines, only if the slit width is adjusted so that the effective bandwidth of the monochromator is equal to one half the difference between l's

Example:

Example 7-2 (p.165): A grating monochromator with a reciprocal linear dispersion of 1.2 nm/mm is to be used to separate the sodium lines at 589.0 and 589.6. In theory, what slit width would be required? Solution: we know D-1 = 1.2 nm/mm (589.6-589.0) eff = = 0.3 nm 2 Since , then w = 0.3/1.2=0.25 mm

How Can We Improve Resolution?

Sample Containers and Optics: • Cuvettes • Lenses • Prisms, gratings, filters Made of suitable material (see table 7-2): Glass 400-3000 nm (vis-near IR) Silica/quartz 200-3000 nm (UV-near IR) NaCl 200-15,000 nm (UV-far IR)

Radiation Transducers: Ideally: • high sensitivity • low noise • wide wavelength response • fast response time • linear output (S=k·P) [Many real transducers exhibit dark current (kd) (S=k·P+kd)]

Detectors for spectroscopic instruments Two general types

Photon Transducers: (A) Photovoltaic cells - metal-semiconductor-metal sandwiches that produce voltage when irradiated (350-750 nm)

(B) Phototube - electrons produced by irradiation of cathode travel to anode. l response depends on cathode material (200-1000 nm) The basic phototube is packaged in vacuum and presents a photocathode (such as alkaline earth metals, Hg, Au, Ag... ) covered with a particular photoemissive material. A biased anode collects the ejected electrons and an external meter measures the current that flows. photon photocathode anode

Secondary Electron Emissive Materials for Dynodes (C) Photomultiplier tube (PMT) - irradiation of cathode produces electrons, series of anodes (dynodes) increases gain to 105-107 electrons per photon. Low incident fluxes only! light dynodes anode photochathode electrons voltage divider network high voltage Secondary Electron Emissive Materials for Dynodes BeO (Cs) GaP (Cs) MgO (Cs) Cs3Sb KCl

(D) Photodiode A photodiode is formed by sandwiching an undoped layer of Si between a heavily doped p-layer and a heavily doped n-layer. Photons can be absorbed in the intrinsic layer, producing an electron -hole pair. The bias potential sweeps these carriers to the opposite regions, producing a current in the external circuit. Photodiodes are more sensitive than phototubes, but less sensitive than PMT’s. (E) Photodiode Array

Thermal detectors - sensitive to IR (l>750 nm) thermocouples - junction thermometer bolometers - resistance thermometer pyroelectric devices - piezoelectric effect Charge-transfer device (CTD) Charge-coupled device (CCD) Charge-injection device (CID)