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Spectroscopy

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Emission Line Spectrum ... All lines have widths, shown here for an emission line. Line shapes are an important tool in studying stars and gas in space ... – PowerPoint PPT presentation

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Title: Spectroscopy


1
Spectroscopy
Astronomers can learn an immense amount from the
details of lines in the spectrum of light coming
from planets and stars.
  • This tool is used by chemists, biologists,
    physicists and all of the other natural sciences.
  • It is based on the quantum or particle (photon)
    nature of light.

2
Spectral Lines
  • By the mid 19th century chemists noticed specific
    colors of light coming from particular gases.
  • Careful measurements indicated each element or
    compound produced a
    UNIQUE SET of EMISSION LINES

    equivalent to FINGERPRINTS identifying the
    element .
  • Spectra of the SUN and other STARS showed
    emission at most frequencies, but distinct dark
    bands, or ABSORPTION LINES, were also detected.

3
Stellar and Chemical Spectra
Introductory Spectroscopy Applet
4
What are the three basic types of spectra?
Continuous Spectrum
Emission Line Spectrum
Absorption Line Spectrum
Spectra of astrophysical objects are usually
combinations of these three basic types how they
arise is described by Gustav Kirchhofs Laws.
5
Continuous Spectrum
  • Example The spectrum of a common (incandescent)
    light bulb spans all visible wavelengths, without
    interruption. (stars interior)
  • A hot solid, liquid or dense gas produces a
    continuous thermal spectrum (Kirchhofs 1st Law)

6
Emission Line Spectrum
  • A thin or low-density cloud of gas emits light
    only at specific wavelengths that depend on its
    composition and temperature, producing a spectrum
    with bright emission lines (Kirchhofs 2nd Law).

7
Absorption Line Spectrum
  • A cloud of gas between us and a light bulb can
    absorb light of specific wavelengths, leaving
    dark absorption lines in the spectrum Kirchhofs
    3rd Law
  • Kirchoff's Laws Applet Viewing Continuum and
    Lines

8
What kind of spectrum does a hot solid produce?
  1. Emission (bright lines)
  2. Absorption (dark lines)
  3. Continuous (all the colors of the rainbow)
  4. Infrared
  5. Ultraviolet

9
What kind of spectrum does a hot solid produce?
  1. Emission (bright lines)
  2. Absorption (dark lines)
  3. Continuous (all the colors of the rainbow)
  4. Infrared
  5. Ultraviolet

10
By looking at the light of a hot, solid object,
you can tell
  1. Its temperature
  2. What it is made of
  3. Both 1 and 2
  4. Neither 1 nor 2, without some additional
    information

11
By looking at the light of a hot, solid object,
you can tell
  1. Its temperature
  2. What it is made of
  3. Both 1 and 2
  4. Neither 1 nor 2, without some additional
    information

12
Origin of Spectral Lines
  • Those in the radio, mm, IR, visible, UV and most
    X-ray are due to
    QUANTUM TRANSITIONS BY ELECTRONS IN ATOMS
    and MOLECULES (Gamma-rays
    usually come from quantum transitions in the
    nuclei of atoms.)
  • ABSORPTION LINES ARISE FROM PHOTONS BEING
    ABSORBED BY ATOMS AND EXCITING ELECTRONS TO
    HIGHER LEVELS.
  • EMISSION LINES ARISE FROM ELECTRONS DROPPING
    DOWN TO LOWER ENERGY LEVELS, EMITTING PHOTONS.

13
Atomic Structure
Helium 2 p 2 n Nuclei -- protons and neutrons
with essentially all the mass of the
atom. Electrons are in orbitals which define
atomic energy levels-- essentially fill the
volume. Carbon 6 p 6 n
14
Atomic Energy Levels
Classical model -- not a very good approximation
Probabalistic electron cloud-- wave function
15
Atomic Transitions
16
Energy Level Transitions
  • The only allowed changes in energy are those
    corresponding to a transition between energy
    levels (here, Hydrogen)
  • Atomic Energy Levels

Allowed
Not Allowed
17
Energy is Conserved in Atomic Transitions
  • E2 - E1 hf
  • Is the equation of CONSERVATION OF ENERGY FOR
    PHOTO-EXCITATION
    or PHOTOABSORPTION.
  • In denser gases frequent collisions between
    atoms shift observed wavelengths (Doppler effect)
    and smear out (broaden) the lines.
  • Once the density is high enough, the spectral
    lines blur into a CONTINUOUS SPECTRUM.

18
Atomic Transitions
19
Exciting Atoms
  • ELECTRONS can be EXCITED THROUGH
    either
  • PHOTO-EXCITATION (PHOTO-ABSORPTION), or
  • COLLISIONAL EXCITATION (atom collides with
    another atom or electron)
  • Here conservation of energy can be expressed
    as E1 KE1 E2 KE2,
    with E the
    electronic potential energy of the atom and KE
    the kinetic energy of the colliding atom or
    electron.

20
De-exciting Atoms
  • SPONTANEOUS (PHOTO-DE-EXCITATION) or
    PHOTO-EMISSION
  • COLLISIONAL DE-EXCITATION (no photon out)
  • STIMULATED PHOTOEMISSION, really requires 3
    energy levels
    a photon reminds an
    electron to drop to the middle level after
    another source of energy pumped many electrons
    the high level
  • amplifying the original photon via a chain
    reaction these photons are in phase, or
    COHERENT (via constructive interference) yields
    LASERS and MASERS.
  • Light or Microwave Amplification through
    Stimulated Emission of Radiation

21
Peer Instruction QuestionWhat can cause an
electron to jump from a low-energy orbital to a
higher-energy one?
  1. A photon of light is emitted
  2. A photon of light is absorbed
  3. The atom collides with an electron
  4. Both 2 and 3
  5. Both 1 and 3

22
What can cause an electron to jump from a
low-energy orbital to a higher-energy one?
  1. A photon of light is emitted
  2. A photon of light is absorbed
  3. The atom collides with an electron
  4. Both 2 and 3
  5. Both 1 and 3

23
Results of First Exam
  • Mean before curve 55.32
  • Curve 19
  • Mean after curve 74.32
  • Standard deviation 11.54
  • Distribution
  • 90 17 (high 102)
  • 80 15
  • 70 34
  • 60 22
  • lt60 15 (low50 lt49 random guessing 0)

24
Most Difficult Questions
  • A6C3 False, sidereal day23h 56m 4s
  • A15C10 False, p1/d1/500.02 NOT 0.05
  • A21C18 B 1 AU 1.5x108km1.5x1013cm
  • A25C29 B midnight, 4 min earlier/day 1 hr
    earlier/2 weeks 3 hours earlier in 6 weeks
  • A34C38 C FGM1M2/d2 so if d0.25 AU then
    1/d216 times as much
  • A37C41 Newton didnt determine distance between
    earth and sun he did everything else

25
Rest of Most Difficult Questions
  • A39C32 D, 20.00 AU because b2a2(1-e2) so b
    a(1-e2)1/2 30.00 (1-51/2/32)1/2
    b30.00 (1-5/9)1/230.00(4/9)1/230.00(2/3)
  • A41C34 B 5.6km/s because
  • VY/VE(2GMY/RY)1/2 / (2GME/RE)1/2
  • VY/VE(MY/ME)1/2 / (RY/RE)1/2 (2/8)1/21/2
  • So VYVE/211.2 km/s / 2 5.6 km/s

26
Spectral Lines Sodium
Emission and absorption lines from Na gas Yellow
Doublet (two nearby wavelengths)
27
Atomic Transitions Hydrogen
  • Lyman series involve transitions to the ground
    state (all UV lines)
  • Balmer series involve transitions to first
    excited state (visible and UV)

28
  • We should not expect to see an optical emission
    line spectrum from a very cold cloud of hydrogen
    gas because
  • Hydrogen gas does not have any optical emission
    lines.
  • The gas is too cold for collisions to bump
    electrons up from the ground state (lowest energy
    level).
  • Hydrogen gas is transparent to optical light.
  • Emission lines are only found in hot objects.
  • Cold objects only produce absorption lines.

29
  • We should not expect to see an optical emission
    line spectrum from a very cold cloud of hydrogen
    gas because
  • Hydrogen gas does not have any optical emission
    lines.
  • The gas is too cold for collisions to bump
    electrons up from the ground state (lowest energy
    level).
  • Hydrogen gas is transparent to optical light.
  • Emission lines are only found in hot objects.
  • Cold objects only produce absorption lines.

30
Chemical Fingerprints
  • Because those atoms can absorb photons with those
    same energies, upward transitions produce a
    pattern of absorption lines at the same
    wavelengths

31
Molecular Lines
Hydrogen spectra on top, molecular H2 On
bottom simpler atomic H
32
Molecular Transitions
  • In molecules there are many more possible quantum
    states, so many more spectral lines
  • Vibrational and rotational energy levels involve
    lower energies (longer wavelengths)

33
Doppler Effect
  • An observed wavelength or frequency will differ
    from the emitted one if there is a relative
    motion between the emitter and the observer.
  • RECESSION ? the OBSERVED ? IS LONGER,
  • or FREQUENCY IS LOWER --- REDSHIFT
  • APPROACH ? the OBSERVED ? IS SHORTER,
  • or FREQUENCY IS HIGHER --- BLUESHIFT

34
Doppler Shift Illustration
Cause of Doppler Effect
35
Measuring the Shift
Stationary
Moving Away
Away Faster
Moving Toward
Toward Faster
  • We generally measure the Doppler Effect from
    shifts in the wavelengths of spectral lines
  • Doppler effect for light

36
Doppler shift tells us ONLY about the part of an
objects motion toward or away from us
Star Motion Doppler Applet
37
Doppler Shift Formula
  • Ex ?em 400.000 nm, ?obs400.005 nm
  • What is the velocity of the star?
  • ?? 400.005 nm - 400.000 nm 0.005 nm
  • So, v c (??/ ?) 3.0x105 km/s (0.005 nm/400nm)
    3.0x105 km/s (1.25x10-5) 3.75 km/s
  • Or the star is moving 3.8 km/s AWAY from us.
  • Doppler shifts and velocities
  • We can much more easily HEAR the Doppler effect
    than SEE it.
    WHY?

38
Sound Speed vs Light Speed
  • The speed of sound in air is a little more than
    300 m/s (or 1000 ft/s) while the speed of light
    in air is 300,000,000 m/s or nearly 1,000,000
    times more!
  • A car traveling 62 mph (or 100 km/h) is moving
    roughly 30 m/s (really 27.8 m/s) or a little less
    than 10 of the speed of sound.
  • You can hear a pitch change of 10 very easily.
  • But the same car is traveling less than 10-7 of
    the speed of light -- that shift of a 500 nm
    visible spectral line would be only 0.00005 nm --
  • way too small to see and extraordinarily hard to
    measure only a few instruments around the world
    can do this they are used to find planets around
    OTHER stars.

39
Can the Doppler shift be measured with invisible
light?
  1. No
  2. Yes

40
Can the Doppler shift be measured with invisible
light?
  1. No
  2. Yes, Thats how you get a speeding ticketpolice
    use radar (microwaves) to measure your cars
    Doppler shift.

41
Thunder and Lightning
  • You see lightning the thunderclap comes later.
  • Lightning is seen at the speed of light
  • Thunder is heard at the speed of sound.
  • Since sound travels roughly 300 m/s or 1000 ft/s,
    if he sound arrives
  • about 3 seconds later the bolt was about 1 km
    away
  • 5 sec later, about a mile away
  • 0 sec later -- you may be dead from a lightning
    strike!

42
  • If a distant galaxy has a substantial redshift
    (as viewed from our galaxy), then anyone living
    in that galaxy would see a substantial redshift
    in a spectrum of the Milky Way Galaxy.
  • Yes, and the redshifts would be the same.
  • Yes, but we would measure a higher redshift than
    they would.
  • Yes, but we would measure a lower redshift than
    they would.
  • No, they would not measure a redshift toward us.
  • No, they would measure a blueshift.

43
  • If a distant galaxy has a substantial redshift
    (as viewed from our galaxy), then anyone living
    in that galaxy would see a substantial redshift
    in a spectrum of the Milky Way Galaxy.
  • Yes, and the redshifts would be the same.
  • Yes, but we would measure a higher redshift than
    they would.
  • Yes, but we would measure a lower redshift than
    they would.
  • No, they would not measure a redshift toward us.
  • No, they would measure a blueshift.

44
Spectral Line Shapes
  • In reality, lines are not at exactly one
    frequency or wavelength
  • All lines have widths, shown here for an emission
    line.
  • Line shapes are an important tool in studying
    stars and gas in space

45
Thermal and Rotational Broadening
  • Higher pressure gas produces broader lines
  • Rotation produces broadening of a different
    shape redshifts from receding side and
    blueshifts from approaching

46
Spectrometers
  • Most telescopes spend most of their time
    measuring spectra of stars, gas clouds and
    galaxies. WHY?
  • Spectra tell us
  • Compositions elements molecules (line
    positions)
  • Temperatures (line strengths)
  • Abundances (line strengths)
  • Pressures (line widths pressure broadening)
  • Radial velocities (Doppler shift)
  • Rotation (Doppler broadening)
  • Magnetic fields (Zeeman splitting)
  • And, indirectly, masses, ages, distances, sizes
    and MORE!

47
How do we interpret an actual spectrum?
  • By carefully studying the features in a spectrum,
    we can learn a great deal about the object that
    created it.

48
What is this object?
Reflected Sunlight Continuous spectrum of
visible light is like the Suns except that some
of the blue light has been absorbed - object must
look red
49
What is this object?
Thermal Radiation Infrared spectrum peaks at a
wavelength corresponding to a temperature of 225 K
50
What is this object?
Carbon Dioxide Absorption lines are the
fingerprint of CO2 in the atmosphere
51
What is this object?
Ultraviolet Emission Lines Indicate a hot upper
atmosphere
52
What is this object?
Mars!
53
Peer Instruction QuestionSuppose you observed
the spectrum of sunlight reflected from Mars.
Compared to the spectrum of the sun observed
directly, it would
  1. Have more emission (bright lines)
  2. Have more absorption (dark lines)
  3. Have more energy in the red part of the spectrum
  4. 1 and 3
  5. 2 and 3

54
Suppose you observed the spectrum of sunlight
reflected from Mars. Compared to the spectrum of
the sun observed directly, it would
  1. Have more emission (bright lines)
  2. Have more absorption (dark lines)
  3. Have more energy in the red part of the spectrum
  4. 1 and 3
  5. 2 and 3
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