Components of Optical Instruments: The generic spectrometer n Wavelength Separators – monochromators and slits n DetectorsMonochromatorsPrism monochromatorMonochromators Grating monochromatorBeam 2 travels a greater distance than beam 1, for constructive interferences to occur, CB + BD = nλ angle i = CAB, angle r = DAB CB = dsini, BD = dsinr nλ = d(sini + sinr) d: Spacing between the reflecting surfacesEchelle grating Higher dispersion and high resolution than a Echellette; nλ = 2dsiniPerformance Characteristics of Grating Monochromators 1. Spectral Purity - Scattered radiation - Stray radiation - Imperfections of monochromator components. 2. Dispersion of Grating Monochromators Dispersion is the ability of a monochromator to separate the different wavelengths. Angular dispersion is equal to the rate of change of the angle of deviation with respect to the change in wavelength. 3. Resolving power of a grating monochromator The resolving power R of monochromator – the limit of its ability to separate adjacent images that have a slight difference in wavelength. 4. Light Gathering Power f-number F – a measure of the ability of a monochromator to collect the radiation that emerges from the entrance slit. R =λΔλD−1=1fdλdγ=d cosγnfF =fd`Monochromator slitsSlits = hole in the wall • Control the entrance of light into and out from the monochromator. They control quality! • Entrance slits control the intensity of light entering the monochromator and help control the range of wavelengths of light that strike the grating – Less important than exit slits • Exit slights help select the range of wavelengths that exit the monochromator and strike the detector – More important than entrance slits • Can be: – Fixed (just a slot) – Adjustable in width (effective bandwidth and intensity) – Adjustable in height (intensity of light)Wider slits = greater intensity, Poorer resolution Narrower slits = lower intensity, Better resolutionDETECTORS Just photon transducers!Radiant'energy'''''''''''''''''''''''''Electrical'signal'• Proper3es'of'an'Ideal'Transducer:''1.'High%sensi)vity:%The'transducer'should'be'capable'of'detec3ng'very'''small'signals'2.'Signal%to%noise%ra)o%(S/N):%A'high'signal'to'noise'ra3o'is'an'important'characteris3c'of'a'good'transducer'3.'Constant%response:%When'radia3on'of'different'wavelengths'but'of'the'same'intensity'are'measured,'the'transducer'should'give'a'constant'response'4.'Fast%response:%A'short'response'3me'is'essen3al'especially'for'scanning'instruments.'5.'Zero%dark%current:%In'absence'of'illumina3on,'the'detector'output'should'read'zero'6.'Zero%dri?:%If'radia3on'of'constant'intensity'hits'the'transducer,'signal'should'be'constant'with'3me'7.'Signal%(S)''='kP'(P:'radiant'power)'Radiation transducers• Respond to Photons - the intensity of EMR striking them by changing a voltage or current emitted or required by themselves – for UV, VIS and near infrared. • Response to Heat (thermal detector), e.g. for IR radiation • Do NOT respond selectively to specific wavelengths (that is what the wavelength selector is for) but work over a range of wavelengths • Various types of photo transducers: – Photographic films (not widely in use any more) – Phototubes (used in simpler instruments) – Photomultiplier tubes (used in more complex instruments) – Multichannel transducers • Diode arrays • Charged coupled devices (CCD’s, like in many camcorders) Types of radiation transducersDifferent wavelengths require different detectors!! Most UV-VIS instruments have two photomultiplier tubes.1. Photovoltaic cells Operate in the visible region (350-750 nm) with maximum sensitivity at about 550 nm; Low sensitivity; Fatigue (its current output decreases with time although the intensity is constant;)2. Vacuum phototubes Good for the general detection of radiation intensity in the UV-Vis region; Reliable Function based on the photoelectric effect; A small dark current is always available;• Also function based on photoelectric effect • Additional signal is gained by multiplying the number of electrons produced by the initial reaction in the detector. • Each electron produces as series of photo-electrons, multiplying its signal. Thus the name PMT! • Very sensitive to incoming light. – Most sensitive light detector in the UV-VIS range. – VERY rugged. They last a long time. – Sensitive to excessive stray light (room light + powered PMT = DEAD PMT) • Always used with a scanning or moveable wavelength selector (grating) in a monochromator 3. Photomultiplier tubesDynodes D1 –D7 Photoemissive cathode ~ 104 – 107 electrons produced for every photo emitted from the cathode Modes of OperationsF undam entals of m odern U V -visible spectroscopy20F igure :Photomultiplier Tube DetectorAnode• High sensitivity at low light levels• Cathode material determines spectral sensitivity• Good signal/noise• Shock sensitivePMT Gain and Offset 8–19 dynodes (9-10 is most common). Gain is the amount of the amplifier multiplies the differential input voltage to produce the output voltage. (0 – 1000V) Offset is an error (either voltage or current) at the input that adds to the input signal before gain takes effect. (-100 – 100V) 1. Raising gain (voltage) on PMT can amplify weak signal but also amplify noise; 2. Raising offset on PMT (threshold) can cut off background noise but signal is equally affected,! the image quality won't improve. Improvements: increase photon number, such as average, accumulation; slow scan speed; lower scan format for bigger pixel size and longer pixel time; larger pinhole size, etc..Multichannel Photon Transducers1. Photodiode Array – Semiconductors (Silicon and Germanium) • Group IV elements • Formation of holes (via thermal agitation/excitation) • Doping • n-type: Si (or Ge) doped with group V element (As, Sb) to add electrons. As: [Ar]4S23d104p3 • p-type: Doped with group III element (In, Ga) to added holes In: [Kr]5S24d105p1• Semiconductor Diodes – Diode: is a nonlinear device that has greater conductance in one direction than in another – Adjacent n-type and p-type regions – pn junction: the interface between the two regionsForward biasing Reverse biasing High resistant e-Diode I-V diagram•