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Lecture Outlines

Astronomy Today

7th Edi t ion

Chaisson/McMillan

2011 Pearson Education, Inc.

Chapter 4

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2011 Pearson Education, Inc.

Chapter 4

Spectroscopy

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2011 Pearson Education, Inc.

4.1Spectral Lines

4.2Atoms and Radiation

The Hydrogen Atom

The Photoelectric Effect

4.3The Formation of Spectral Lines

4.4Molecules

4.5Spectral-Line Analysis

Units of Chapter 4

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Spectroscope: Splits light into componentcolors

4.1 Spectral Lines

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Emission lines:

Single frequencies

emitted by

particular atoms

4.1 Spectral Lines

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Emission spectrumcan be used to identify

elements

4.1 Spectral Lines

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Absorption spectrum:If a continuous spectrum

passes through a cool gas, atoms of the gas willabsorb the same frequencies they emit

4.1 Spectral Lines

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An absorption spectrum can also be used toidentify elements. These are the emission and

absorption spectra of sodium:

4.1 Spectral Lines

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Kirchhoffs laws:

Luminous solid, liquid, or dense gas

produces continuous spectrum

Low-density hot gas produces emissionspectrum

Continuous spectrum incident on cool, thin

gas produces absorption spectrum

4.1 Spectral Lines

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Kirchhoffs laws illustrated

4.1 Spectral Lines

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Existence of spectral lines required new

model of atom, so that only certain amountsof energy could be emitted or absorbed

Bohr modelhad certain allowed orbits for

electron

4.2 Atoms and Radiation

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Emission energiescorrespond to energy

differences between allowed levelsModern model has electron cloudrather than

orbit

4.2 Atoms and Radiation

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Energy levelsof the hydrogen atom, showing

two series of emission lines:

More Precisely 4-1: The Hydrogen Atom

The energies of

the electrons in

each orbit are

given by:

The emission

lines correspond

to the energy

differences

2

113.6 1 -- eV.nE

n

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The photoelectric effect:

When light shines on metal, electrons can be

emitted

Frequencymust be higher than minimum,characteristic of material

Increased frequencymore energetic

electrons

Increased intensitymore electrons, same

energy

Discovery 4-1: The Photoelectric Effect

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Photoelectric effectcan only be understood if

light behaves like particles

Discovery 4-1: The Photoelectric Effect

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Light particles each have energy E:

Here, his Plancks constant:

4.2 Atoms and Radiation

E hf

-34

6.63 10 joule seconds (J s)h

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Absorption can boost an electron to thesecond (or higher) excited state

Two ways to decay:

1. Directly to ground state

2. Cascadeone orbital at a time

4.3 The Formation of Spectral Lines

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(a) Direct decay (b) Cascade

4.3 The Formation of Spectral Lines

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Absorption spectrum: Created when atoms

absorb photons of right energy for excitation

Multielectron atoms: Much more complicated

spectra, many more possible states

Ionizationchanges energy levels

4.3 The Formation of Spectral Lines

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Emission linescan be used to identify atoms

4.3 The Formation of Spectral Lines

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Molecules can vibrate and

rotate, besides havingenergy levels

Electron transitions

produce visible andultraviolet lines

Vibrational transitions

produce infrared linesRotational transitions

produce radio-wave lines

4.4 Molecules

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Molecular spectraare much more complex

than atomic spectra, even for hydrogen

(a) Molecular hydrogen (b) Atomic hydrogen

4.4 Molecules

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Information that can be gleaned from

spectral lines:

Chemical composition

Temperature

Radial velocity

4.5 Spectral-Line Analysis

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Line broadening

can be due to a

variety of causes

4.5 Spectral-Line Analysis

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4.5 Spectral-Line Analysis

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4.5 Spectral-Line Analysis

The Doppler shift may

cause thermalbroadening of spectral

lines

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4.5 Spectral-Line AnalysisRotation will also cause broadening of

spectral lines through the Doppler effect

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Spectroscope splits light beam intocomponent frequencies

Continuous spectrum is emitted by solid,

liquid, and dense gas

Hot gas has characteristic emission spectrum

Continuous spectrum incident on cool, thin

gas gives characteristic absorption spectrum

Summary of Chapter 4

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Spectra can be explained using atomicmodels, with electrons occupying specific

orbitals

Emission and absorption lines result from

transitions between orbitals

Molecules can also emit and absorb

radiation when making transitions between

vibrational or rotational states

Summary of Chapter 4 (cont.)