Lecture PowerPoint Chapter 4

Astronomy Today, 5 th edition

Chaisson Last Revised: 8-Feb-09 McMillan © 2005 Pearson Prentice Hall This work is protected by United States copyright laws and is provided solely for the use of instructors in teaching their courses and assessing student learning. Dissemination or sale of any part of this work (including on the World Wide Web) will destroy the integrity of the work and is not permitted. The work and materials from it should never be made available to students except by instructors using the accompanying text in their classes. All recipients of this work are expected to abide by these restrictions and to honor the intended pedagogical purposes and the needs of other instructors who rely on these materials.

Chapter 4 Spectroscopy

Solar Spectrum Na D Lines

Units of Chapter 4 Spectral Lines The Formation of Spectral Lines The Energy Levels of the Hydrogen Atom The Photoelectric Effect Molecules Spectral-Line Analysis

Why Do We Care About Spectra?

Almost everything that we study has a spectrum we can use to analyze its composition, motions and many other properties Wollaston (1802) First Solar Spectrum - 8 lines seen Fraunhofer Fraunhofer (1817) Fraunhofer lines – A, B, C, D, etc.

Over 500 lines seen Modern Solar Spectrum Over one million lines seen!

Fra ünhofer Solar Line Chart

Identification Table Ca II H β Fe Na I H α T a b l e Original Fra ünhofer spectrum

4.1 Spectral Lines Spectroscope: splits light into its component colors Continuous spectrum Emits BB radiation

Blackbody Curve Spectrum

All colors ( λ’s) emitted

4.1 Spectral Lines Continuous Spectrum: all colors ( λ’s) present Emission lines : single frequencies emitted by particular atoms

H Na He Ne Hg 4.1 Spectral Lines Emission spectrum can be used to identify elements by wavelengths of lines: 600 500 400 nm

4.1 Spectral Lines Absorption spectrum: if a continuous spectrum passes through a cool gas, atoms of the gas will absorb the same frequencies they emit Dark line spectra Continuous spectrum

4.1 Spectral Lines An absorption spectrum can also be used to identify elements. These are the emission and absorption spectra of sodium (D lines): emission absorption

4.1 Spectral Lines Kirchhoff ’s laws (the 3 types of spectra):

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Luminous solid, liquid, or dense gas produces a continuous spectrum

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Low-density hot gas produces an emission spectrum

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Continuous spectrum incident on cool, thin gas produces an absorption spectrum

4.1 Spectral Lines Kirchhoff’s laws illustrated:

4.2 The Formation of Spectral Lines Existence of spectral lines required a new model of the atom , so that only certain amounts of energy could be emitted or absorbed. (Planck: ∆E = nhν) Bohr model electron: had certain allowed orbits for the (n=1) (n>1)

4.2 The Formation of Spectral Lines Emission energies correspond to energy differences between allowed levels.

Modern model has electron “cloud” rather than orbit:

4.2 The Formation of Spectral Lines Absorption can boost an electron to the second (or higher) excited state Two ways to decay: 1. to ground state 2.

cascade one orbital (energy level) at a time This second method of energy decay or relaxation can lead to the processes of fluorescence, phosphorescence and laser action

4.2 The Formation of Spectral Lines Energy levels of the hydrogen atom, showing two series of emission lines: Note: Balmer series transitions all start or end at n = 2 level

visible

Lyman series (n = 1)

ultraviolet

Paschen series (n = 3) Brackett series (n=4) Pfund series (n=5)

all in infrared

How Transitions Make Spectral Lines Transitions between energy levels of system Up: absorption Down: emission H β Emission H



Absorption

Energy (eV) Levels of H-atom

UV VIS IR

4.2 The Formation of Spectral Lines Absorption spectrum : created when atoms absorb photons of right energy for excitation Multielectron atoms : much more complicated spectra, many more possible states Ionization changes energy levels

4.2 The Formation of Spectral Lines Emission nebula (a) direct decay (b) cascade

4.2 The Formation of Spectral Lines Emission lines can be used to identify atoms in nebulae, stars and galaxies:

4.2 The Formation of Spectral Lines Light particles each have energy E: Here, h is Planck’s constant:

4.2 The Formation of Spectral Lines The photoelectric effect :

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When light shines on metal, electrons can be emitted

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Frequency must be higher than minimum, characteristic of material

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Increased frequency electrons – more energetic

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Increased intensity energy – more electrons, same

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Einstein won his Nobel prize for explaining this effect (NOT for relativity theory)

4.2 The Formation of Spectral Lines Photoelectric effect can be understood only if light behaves like particles

4.3 Molecules Molecules can vibrate and rotate, besides having energy levels

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Electron transitions produce visible and ultraviolet lines

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Vibrational transitions produce infrared lines

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Rotational transitions produce MW & radio-wave lines

4.3 Molecules Molecular spectra are much more complex than atomic spectra, even for hydrogen: (a) Molecular hydrogen (Band spectra) (b) Atomic hydrogen (Line spectra)

Astrophysical Molecular Spectra

• • •

These next few slides show spectra of astronomical objects revealing the presence of common organic and inorganic molecules “out there” These spectra are part of projects with our NASA colleagues that Dr. Blass, myself and our students work on together They are infrared molecular spectra taken by the Voyager , Galileo , Cassini and other spacecraft as well as by ground based, airborne and balloon telescopes

water germane

50 20 Wavelength ( μm) 15 12 10 9 Methane Phosphine Ammonia Hydrogen Ammonia Acetylene Ethane Wavenumber (cm -1 )

4.4 Spectral-Line Analysis Information that can be gleaned from spectral lines:

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Chemical composition (Wavelength pattern)

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Temperature (Wien’s law)

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Radial velocity (Doppler effect): moving away source at rest (lab) moving toward RED 650 nm 450 nm BLUE

4.4 Spectral-Line Analysis Line widths can be due to

Doppler (motional)

• •

broadening

caused by thermal motion rotation Or From

pressure broadening

due to collisions altering the velocities (

Lorentzian broadening

) Linewidth (FWHH)

4.4 Spectral-Line Analysis

Summary of Chapter 4

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Spectroscope splits light beam into component frequencies

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Continuous spectrum is emitted by solid, liquid, and dense gas

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Hot gas has characteristic emission spectrum

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Continuous spectrum incident on cool, thin gas gives characteristic absorption spectrum

Summary of Chapter 4, cont.

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Spectra can be explained using atomic models, with electrons occupying specific orbitals

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Emission and absorption lines result from transitions between orbitals

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Molecules can also emit and absorb radiation when making transitions between vibrational or rotational states