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Atomic Spectra: Identifying Atoms by Their Light

Copyright (c) The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

NIR spectroscopy

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Basics of Spectroscopy:

Entrance Slit

DiffractionGrating

Dispersed images of entrance slit

Seeing Spectra

• Seeing the Sun’sspectrum requires a fewspecial tools, but it isnot difficult– A narrow slit only lets a

little light into theexperiment

– Either a grating or aprism splits the lightinto its componentcolors

– If we look closely at thespectrum, we can seelines, corresponding towavelengths of lightthat were absorbed.

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Telescope

detector

collimator

grating

!

Dtelescope !

2Dcol

• tan(")

!

Dcol

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Example spectrographs

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Types of Spectra

• Kirchoff’s Laws:– If the source emits light that is

continuous, and all colors arepresent, we say that this is acontinuous spectrum.

– If the molecules in the gas are well-separated and moving rapidly (havea high temperature), the atoms willemit characteristic frequencies oflight. This is an emission-linespectrum.

– If the molecules of gas are well-separated, but cool, they will absorblight of a characteristic frequencyas it passes through. This is anabsorption line spectrum.

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Spectra of Astronomical Objects

Emission Spectra

• Imagine that we have a hothydrogen gas.

– Collisions among the hydrogenatoms cause electrons to jumpup to higher orbitals, or energylevels

– Collisions can also cause theelectrons to jump back to lowerlevels, and emit a photon ofenergy hc/λ

– If the electron falls from orbital3 to orbital 2, the emittedphoton will have a wavelengthof 656 nm

– If the electron falls from orbital3 to orbital 2, the emittedphoton will have a wavelengthof 486 nm

• We can monitor the gas, and count how manyphotons of each wavelength we see. If wegraph this data, we’ll see an emissionspectrum!

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Emission spectrum of hydrogen

• This spectrum isunique to hydrogen– Like a barcode!

• If we were lookingat a hot cloud ofinterstellar gas inspace, and sawthese lines, wewould know thecloud was made ofhydrogen!

Different atom, different spectrum!

• Every element has itsown spectrum. Note thedifferences betweenhydrogen and heliumspectra below.

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Absorption Spectra

• What if, instead of hothydrogen gas, we had a cloudof cool hydrogen gas betweenus and a star?– Photons of an energy that

corresponds to theelectronics transitions inhydrogen will be absorbedby electrons in the gas

– The light from those photonsis effectively removed fromthe spectrum

– The spectrum will have darklines where the missing lightwould be

– This is an absorptionspectrum!

– Also like a barcode!