Quiz 1 Flashcards by Brittany Lazenby

Define spectroscopy.

interactions of radiation and matter

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what are photons

discrete particles of energy

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What is the QM description of EM radiation

permanent transfer of energy as stream of photons

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electron volts

kinetic energy gained from one electron accelerating from rest

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radiation source

generates beam with enough power to detect

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wavelength selector

isolates region of spectrum to analyze

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sample holder

transparent container

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radiation detector

converts energy to usable electrical signal

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signal processor

displays results

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what is a continuous source?

changes in intensity slowly as a function of wavelength

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What is a discontinuous source?

limited number of bonds, limited range

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What is the luminescence of a black-body source?

flux of radiation escaping from small hole

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What is the Wien Displacement Law?

increasing temperature increases emitted I, wavelength max to shorter wavelength

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What is the spectral emissivity of a black-body source?

ratio of spectral radiance of a source to that of a black body radiator; varies between 0 and 1 with 1 being exactly black body

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What is a gray body source?

real sources, emissivity less than 1

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gas discharge continuous source

low pressure, UV, D + H lamps, 160 - 380 nm

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In what types of spectroscopies are discontinuous sources commonly used?

atomic, fluorescent, raman

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What are the requirements for discontinuous sources?

sufficient radiant power in relevant wavelengths, stable and constant intensity

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How are discontinuous sources
stabilized?

regulated power, 2 beam instrument

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What are the various types of discontinuous sources?

metal vapor lamp, electrodeless discharge, hollow cathode

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Where are dispersive methods commonly used

UV/VIS/NIR and Raman; manufactor of lenses

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where are non-dispersive methods commonly used

Infrared and NIR, increasing in Raman and MS

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Dispersive elements

filter, prism, grating

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interference filters rely on

optical interference to provide narrow band

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absorbance filters

absorbs selected portions of spectrum

What is the physical principle behind interference filters?

optical interference

How are interference filters constructed?

thin, transparent dielectric material sandwiched between 2 thin semitransparent metal films

What controls the wavelengths of transmitted light through an interference filter?

constructive interference

What is the physical principle behind absorbance filters?

absorption of radiation by colored glass, crystals, dyes, solutions, etc. that allow radiation to be transmitted only in certain regions

What is a bandpass filter?

allows a certain region of radiation to be transmitted

What is a cut-off filter?

cuts off everything below or above a certain wavelength and passes all of the rest of the radiation

Absorbance filters governed by

wavelength, transmission and bandwidth

What is the physical principle behind the use of prisms as dispersive devices?

light from low to high refractive toward normal, high to low bends away

How does light reflect in prisms?

light enters toward normal and exits away

Which wavelength (red or blue) is more highly refracted in a prism? Why?

blue because its shorter

angular dispersion units

rad nm-1

linear dispersion units

mm nm-1 or mmA-1

What is the reciprocal linear dispersion?

the figure of merit for comparing the dispersion properties of optical elements

why is reciprocal linear dispersion useful

lower D-1 is better

What is a reasonable magnitude for resolving power in prisms?

What is the Rayleigh Criterion?

use to see if two adjacent are fully resolved

What is a grating?

disperses radiation into component wavelengths

What is the physical principle behind the use of gratings as dispersive devices?

dispersion is a result of diffraction at reflective surface

What is the angular dispersion of a grating?

dr/dλ

What is the resolving power of a grating?

limit of ability to separate adjacent images with similar wavelengths

What is a reasonable magnitude for resolving power in gratings?

10^3 to 10^4

What is a blazed grating?

concentrates large percent of diffracted into certain place

What is the blaze angle?

groved for broad reflective face and unused narrow

What is the blaze wavelength?

first order wavelength corresponding to m=1 diffracted

What are the 2 processes for producing diffraction gratings?

holographic and mechanical

advantages holographic

perfect grooves, no ghosts

advantages mechanical

accurate and efficient

disadvantages mechanical

hard to make

What are some common groove densities in gratings?

UV: 1800 or 2400 VIS: 1200 IR: 600 or 300

What are the components of a Czerny-Turner monochromator?

concave mirrors, reflection grating, focal plane

What are the components of a general monochromator?

entrance slit, collimating lense, grating, focusing, exit slit

What is monochromator dispersion?

influences ability to separate wavelengths

What is the effective bandwidth of a grating monochromator?

1/2 width wavelength distribution passed by slit

What is the resolving power of a monochromator?

ability to separate adjacent lines

How are different wavelengths passed through the exit slit of a monochromator?

rotating dispersive element

What is the take-off angle? φ

angle diffracted ray makes with optical axis (the light path)

What is the grating rotation angle? θ

angle grating makes with optical axis (light path)

How are take off and grating rotation angles related to the angles of incidence and diffraction from the grating?

α = incidence β = diffraction θ = α + φ φ = θ - β α = θ - φ β = θ + φ

What is the slit function of a monochromator?

triangular function where entrance and exit images overlap

How does the shape of the entrance and exit slits determine the instrumental spectral profile?

diffracted radiation from grating and collimating mirror produces image of entrance in focal plan of exit. slits must be same width for triangular shape.

propagation

how it moves through a medium

arc lamps continuous source

high temp, very intense source with good UV output. molecular fluorescence

filament lamps continuous source

glowing incandescent filaments, visible absorption. UV Vis and near IR

glowing inert solids continuous source

IR/Far IR range

gas discharge wavelengths

160 to 380

arc lamps wavelengths

250 to 600

filament lamp wavelengths

240-2500, 350-2200

glowing inert solid wavelengths

1200 to 40,000; 400-20,000

pros of prisms as dispersive devices

cheap, easy to make, adjustable, no overlap

cons of prisms

low dispersion, resolving power, light flux; no far-Ir or far-Uv, nonlinear wavelength

holographic production of gratings

coat flat glass with photosensitive material, laser projects pattern, photosensitive layer notes groove placement to be etched, metal coat to reflect

mechanical production of gratings

diamond grooves flat glass that is then covered with reflective metal via vapors. master made first, resin cast made, metal coat new form

pros of gratings

efficient for wavelength separation, no limiting optical, very wide spectral range, constant linear dispersion, size and shape varied to change results

cons of gratings

overlap, confusing resolution