1. Taking “Fingerprints” of Stars, Galaxies, and Other Stuff
Absorption and Emission from Atoms, Ions, and Molecules

2. Periodic Table of Elements
Universe is mostly (97%) Hydrogen and Helium (H and He)
The ONLY elements created in the Big Bang were H, He, and a little lithium, Li
All heavier elements have been (and are still being) “manufactured” by stars via nuclear fusion
Each element has characteristic set of energies where absorbs or radiates

3. Planck’s Theory, 1901 Light with wavelength  has frequency  = c/
can “exchange” energy with matter (atoms) in units of:
E = h
h is “Planck’s constant”
h =  Joule-seconds

4. The Bohr Atom Model of Hydrogen atom
Introduced by Niels Bohr early in 1913
to explain emission and absorption of light by H
1 proton ( “nucleus”) “orbited” by 1 electron +

5. The Bohr Atom Electron “orbits” have fixed sizes ─ “orbitals”
Not Like Planets in a “Solar System”
atomic orbitals are “QUANTIZED”
only some orbital radii are “allowed”
was very confusing to physicists
first deduced by physicist Neils Bohr
Movement of electron e- between orbitals requires absorption or radiation of energy
jump from lower to higher orbital  atom absorbs energy
jump from higher to lower orbital  atom emits energy

6. Bohr Atom Absorption of Photon “kicks” electron to “higher” orbital +- - +

7. Bohr Atom Emission of Photon makes Electron “drop” to “lower” orbital- - +

8. Absorption vs. Emission
Atom “absorbs” photon if electron “kicked” up to a “higher” energy state
Atom “emits” photon if electron “drops” down to a “lower” state
Again, only a certain set of energy states is “allowed”
set of states depends on the atom or molecule

9. “Ensembles” (Groups) of Atoms
States of individual H atoms in a group are not identical
Some electrons are in “low” states and are more likely to absorb photons
Some electrons are in “high” states and more likely to emit photons
What determines the “distribution” of states of a group of atoms?

10. Ensemble of Atoms in “Low” States+ -

11. Ensemble of Atoms in “Low” States
Photons from Star at “correct”  are absorbed, and thus
removed from the observed light
Absorption Line + -

12. Absorption “lines” Discovered in Solar spectrum by Fraunhofer
called “Fraunhofer Lines”
“Lines” because they appear as dark bands superimposed on “rainbow” of visible spectrum

13. Ensemble of Atoms in “High” States+ -

14. Ensemble of Atoms in “High” States
Photons at “correct”  are emitted, and thus added to any observed light
Emission Line + -
Dark Background

15. Emission line spectrum
Appear as Bright Bands on “Faint Background Spectrum”
Why the Background??

16. All “real” gas clouds have atoms in both states, with one state usually dominant
Absorption & Emission + -
More absorption if more atoms
in “low” state
More emission if more atoms
in “high” state + - + - + - + - + - + -

17. Why would an ensemble of atoms (i. e
Why would an ensemble of atoms (i.e. a gas cloud) be in “High” or “Low” state?
Some other mechanism (besides light) must be at work! But what?
TEMPERATURE T

18. Effect of Thermal Energy
If T  0 K (ensemble of atoms is very cold), most atoms are in “low” state
can easily absorb light
If T >> 0 K (ensemble of atoms is hot), the thermal energy “kicks” most atoms into “high” state
can easily emit light

19. Sidebar: LASER
Electrons in the medium (gas, solid, or diode) of a LASER are “driven” to “high” state by external energy
Emit simultaneously and with same “phase”
External Energy:
electrical
optical (external light source, flash lamp)

20. Sidebar: LASER External source maintains “energy inversion”
more electrons in “high” state, even during and after emission
high
Emission
low
After “Driving”
After Emission

21. Geometries for producing absorption lines1 2
The Observer
Absorption lines require cool gas between observer and hot source
scenario 1: cooler atmosphere of star
scenario 2: cool gas cloud between star and observer

22. Sun’s Fraunhofer absorption lines
(wavelengths listed in Angstroms; 1 Å = 0.1 nm)

23. Geometries for producing emission lines1 2
The Observer
Emission lines require gas viewed against colder background
scenario 1: the hot “corona” of a star
scenario 2: cold gas cloud seen against “empty” (colder) space

24. Emission line spectra
Insert various emission line spectra here

25. What Wavelengths are Involved?
Depends on the Size of the “Gaps” between Energy States in the atoms

26. Energies of H Orbitals Limiting Energy Energies of Orbitals of H
“Transitions” between Orbitals

27. Ionization of Hydrogen
Limiting Energy
If electron absorbs sufficient
energy E to rise above the “upper
limit” of energy for a “bound”
electron, then the electron becomes
“ionized”
electron “escapes” the proton

28. Relate Size of “Gap” to Wavelength of Light
Larger “gaps” or “jumps” in energy (both absorbed and emitted)  photon carries more energy
Recall
Larger E  Shorter   “bluer” light
Smaller E  Longer   “redder” light

29. Relate Size of “Jump” to the  Absorbed or Emitted
Very Small E  Very Long   radio waves
Very Large E  Very Short   X rays

30. Sidebar: A Transition with Very Small E  Very Long 
Due to “spin flip” of e- in Hydrogen Atom
E = hc/  9.4 × Joules
   0.21 m = 21 cm
   MHz ─ RADIO Wave
High-E State
Low-E State

31. Sidebar: 21-cm Radio Wave of H
First observed in 1951
Simultaneously Discovered at 3 observatories!! (Harvard, Leiden, Sydney)
Measures the H in “interstellar matter”
Map of Spiral Arms in Milky Way Galaxy

32. Bohr Atom: Extension to other elements
H is simplest atom, BUT concept of electron orbitals applies to all atoms
Neutral atoms have equal numbers of protons (in nucleus) and electrons (orbiting nucleus)
He has 2 protons & 2 electrons; Lithium (Li), 3 each; Carbon (C) , 6 each, etc. ...
In atoms with more electrons (and protons), the absorption/emission spectrum is more complicated

33. Optical Emission-Line Spectrum of “Young Star”
Intensity
 (in Angstroms Å, or units of 10 nm)

34. Emission line images green  oxygen red  hydrogen
Planetary nebula NGC 6543
(blue: X Rays)
Orion Nebula
green  oxygen
red  hydrogen

35. Spectra of ions Neon Iron
Emission lines from heavy ions dominate high-energy (X-ray) spectra of stars
atoms stripped of one or more electrons
Ions of certain heavier elements (e.g., neon and iron with only one electron) behave much like “supercharged” H and He
Wavelength (in Angstroms)

36. Spectra of Molecules
Also have characteristic spectra of emission and absorption lines
Each molecule has particular set of allowed energies at which it absorbs or radiates
Molecules are more complicated than atoms
Resulting spectra are also more complicated
Electrons shared by one (or more) atoms in molecule absorb or emit specific energies
Changes in state of vibration and/or rotation are also quantized
Vibration, rotation spectra unique to each molecule

37. More on Molecular Spectra
Transitions between different “orbitals” of molecules (“electronic” states)
mostly in ultraviolet (UV), optical, and infrared (IR) regions of spectrum
Transitions between different “Vibrational” states
mostly in the near-infrared (NIR)
Transitions between different “Rotational” states
mostly in the radio region

38. Rank Molecular Transitions by Energy
UV, Visible, IR  Electronic
NIR  Vibrational
Radio  Rotational
Radio  H “spin  = 1420 MHz

39. Molecular Emission: Vibrational Transition
Planetary nebula NGC 2346
Electronic Transition
(visible light)
Vibrational Molecular Hydrogen
Transition (IR)

40. Molecular Emission: Rotational Transition
Rotational CO (carbon monoxide) Emission
from Molecular Clouds in “Milky Way”

41. Q: How Can We Measure Spectra?
A: With a “Spectrum Measurer”
A “SPECTROMETER”
“Splits” light into its constituent wavelengths
Common Mechanisms for “Splitting” Light
Optical Filters
“Blocks” light except in desired band
“Dispersion” of Glass = “Differential Refraction”
Prism
Diffraction Grating

42. 1. Filter Spectrometer Filters in Rotating “Filter Wheel”
Sequence of “Monochrome” Images thru Different Colors
(How the images in the laboratory were created)

43. 2. Prism Spectrometer
Recall: Optical Dispersion n 

44. 2. Prism Spectrometer
“Refractive Index” n measures the velocity of light in matter
c = velocity in vacuum  3 108 meters/second
v = velocity in medium measured in same units
n  1.0

45. 2. Prism Spectrometer
Refractive index n of glass DECREASES with increasing wavelength 
Make a glass device that uses optical dispersion to “separate” the wavelengths
a PRISM

46. Long  “dispersed” by smallest angle 
2. Prism Spectrometer
Red
Blue
White Light In
Long  “dispersed” by smallest angle 

47. 2. Prism Spectrometer Problems:
Glass absorbs some light
Ultraviolet light
Why you can’t get a suntan under glass
Infrared light
Images taken in different  will “overlap”
Dispersion Angle  is a complicated function of wavelength 
Spectrometer is difficult to “calibrate”

48. 3. Grating Spectrometer “Interference” of Light Different  Interfere
Red 
Different  Interfere
at Different  
Blue 

49. 3. Grating Spectrometer White Light In Red Blue Long  “diverges: at
largest angle 
Long  “dispersed” by largest angle 
Can be constructed for all wavelengths

50. 3. Grating Spectrometer Uses “Diffraction Grating”
works by “interference” of light
Regularly spaced “transparent” & “opaque” regions
Can be made without absorbing glass
Used at all wavelengths (visible, UV, IR, X-Rays, …)
Dispersion angle  is proportional to 
Easy to calibrate!
Images at different  can still overlap