Periodic Table of Elements
Taking “Fingerprints” of Stars,
Galaxies, and Other Stuff
Absorption and Emission from
Atoms, Ions, and Molecules
• 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”
• in stars via nuclear fusion
• Each element has characteristic set of energies where absorbs or radiates
The Bohr Atom
The Bohr Atom
• Model of Hydrogen atom
– Introduced by Niels Bohr early in 1913
– to explain emission and absorption of light by H
• One proton (the “nucleus”) “orbited” by an
electron
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• Electron “orbits” have fixed sizes ─ “orbitals”
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–
–
–
Unlike Planets in a “Solar System”
atomic structure is “QUANTIZED”
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: energy absorbed
– jump from higher to lower orbital: energy emitted
Bohr Atom
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Absorption of Photon “kicks” electron
to “higher” orbital
Bohr Atom
Emission of Photon makes Electron
“drop” to “lower” orbital
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Absorption vs. Emission
“Ensembles” (Groups) of Atoms
• Atom “absorbs” photon if electron “kicked”
up to a “higher” energy state
• States of individual H atoms in a group are
not identical
• Atom “emits” photon if electron “drops”
down to a “lower” state
• Again, only a certain set of energy states is
“allowed”
– 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?
– set of states depends on the atom or molecule
Ensemble of Atoms in “Low” States
Ensemble of Atoms in “Low” States
Ready to Absorb, SIR!
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Photons from Star at “correct” λ
are absorbed, and thus
removed from the observed light
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Absorption Line
Absorption “lines”
Ensemble of Atoms in “High” States
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• Discovered in Solar spectrum by Fraunhofer
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Ready to Emit, SIR!
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– called “Fraunhofer Lines”
• “Lines” because they appear as dark bands
superimposed on “rainbow” of visible spectrum
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Ensemble of Atoms in “High” States
Emission line spectrum
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Photons at “correct” λ are emitted,
and thus added to any observed
light
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Emission Line
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Appear as Bright Bands on “Faint Background Spectrum”
Why the Background??
Some Atoms are in Both States
(but “one” dominates)
More absorption if more atoms
in “low” state
More emission if more atoms
in “high” state
• Some other mechanism (besides light) must
be at work! But what?
TEMPERATURE T
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Absorption & Emission
Why Would Ensemble of Atoms
be in “High” or “Low” State?
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
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)
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Geometries for producing
absorption lines
Sidebar: LASER
• External source maintains “energy inversion”
– more electrons in “high” state, even during and
after emission
1
2
high
Emission
low
After “Driving”
After Emission
Sun’s Fraunhofer absorption lines
• 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
Geometries for producing
emission lines
1
2
• 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
(wavelengths listed in Angstroms; 1 Å = 0.1 nm)
Emission line spectra
What Wavelengths are Involved?
• Depends on the Size of the “Gaps” between
Energy States in the atoms
Insert various emission line spectra here
4
Energies of H Orbitals
Energies of Orbitals of H
“Transitions” between
Orbitals
Relate Size of “Gap” to
Wavelength of Light
• Larger “gaps” or “jumps” in energy (both
absorbed and emitted) ⇒ photon carries more
energy
• Recall
E2 − E1 = ∆E = hν =
hc
λ
∝
1
λ
• Larger ∆E ⇒ Shorter λ ⇒ “bluer” light
• Smaller ∆E ⇒ Longer λ ⇒ “redder” light
Relate Size of “Jump” to the
λ Absorbed or Emitted
Sidebar: A Transition with
Very Small ∆E ⇒ Very Long λ
• Due to “spin flip” of e- in Hydrogen Atom
• Very Small ∆E ⇒ Very Long λ ⇒ radio waves
High-E State
• Very Large ∆E ⇒ Very Short λ ⇒ X rays
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
Low-E State
• ∆E = hc/λ ≈ 9.4 × 10-25 Joules
• ⇒ λ ≈ 0.21 m = 21 cm
• ⇒ ν ≈ 1420.4 MHz ─ RADIO Wave
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
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Emission line images
Intensity
Optical Emission-Line Spectrum of
“Young Star”
Planetary nebula NGC 6543
(blue: X Rays)
λ (in Angstroms Å, or units of 10 nm)
Neon
Orion Nebula
green ⇒ oxygen
red ⇒ hydrogen
Iron
Spectra of ions
Spectra of Molecules
• Also have characteristic spectra of emission
and absorption lines
• Emission lines from
heavy ions dominate
high-energy (X-ray)
spectra of stars
– Each molecule has particular set of allowed
energies at which it absorbs or radiates
– atoms stripped of one or
more electrons
• Molecules are more complicated than atoms
• Ions of certain heavier
elements (e.g., neon and
iron with only one
electron) behave much
like “supercharged” H
and He
– Spectra are VERY 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
Wavelength (in Angstroms)
Rank Molecular Transitions by
Energy
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
1.
2.
3.
4.
UV, Visible, IR ⇒ Electronic
NIR ⇒ Vibrational
Radio ⇒ Rotational
Radio ⇒ H “spin flip” @ ν = 1420 MHz
– mostly in the near-infrared (NIR)
• Transitions between different “Rotational”
states
– mostly in the radio region
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Molecular Emission:
Vibrational Transition
Planetary nebula NGC 2346
Molecular Emission:
Rotational Transition
Rotational CO (carbon monoxide) Emission
from Molecular Clouds in “Milky Way”
Electronic Transition
(visible light)
Vibrational Molecular Hydrogen
Transition (IR)
Q: How Can We Measure Spectra?
•
1. Filter Spectrometer
A: With a “Spectrum Measurer”
– A “SPECTROMETER”
– “Splits” light into its constituent wavelengths
•
Common Mechanisms for “Splitting” Light
1. Optical Filters
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“Blocks” light except in desired band
2. “Dispersion” of Glass = “Differential Refraction”
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Prism
3. Diffraction Grating
2. Prism Spectrometer
Recall: Optical Dispersion
Filters in Rotating “Filter Wheel”
Sequence of “Monochrome” Images thru Different Colors
(How the images in the laboratory were created)
2. Prism Spectrometer
• “Refractive Index” n measures the velocity
of light in matter
n=
n
c
v
c = velocity in vacuum ≈ 3 ×108 meters/second
v = velocity in medium measured in same units
n ≥ 1.0
λ
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2. Prism Spectrometer
2. Prism Spectrometer
• Refractive index n of glass DECREASES
with increasing wavelength λ
• Make a glass device that uses optical
dispersion to “separate” the wavelengths
θRed
θBlue
White Light In
– a PRISM
Long λ “dispersed” by smallest angle θ
2. Prism Spectrometer
Problems:
3. Grating Spectrometer
“Interference” of Light
• Glass absorbs some light
– Ultraviolet light
θRed
• Why you can’t get a suntan under glass
θ
– Infrared light
Different λ Interfere
at Different θ
λ
• Images taken in different λ will “overlap”
• Dispersion Angle θ is a complicated
function of wavelength λ
θBlue
– Spectrometer is difficult to “calibrate”
λ
3. Grating Spectrometer
3. Grating Spectrometer
• Uses “Diffraction Grating”
White Light In
θRed
θBlue
Long λ “diverges: at
largest angle θ
– 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 θ is proportional to λ
– Easy to calibrate!
Long λ “dispersed” by largest angle θ
Can be constructed for all wavelengths
• Images at different λ can still overlap
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