Periodic Table of Elements
Taking fingerprints of
stars, galaxies, and
interstellar gas clouds
Absorption and emission from
atoms, ions, and molecules
• The universe is mostly hydrogen H and helium He (97%)
• These (and a little lithium, Li) were only elements created in Big Bang
– ALL heavier elements have been (and are still being) manufactured in
stars, via nuclear fusion
• Each element has own characteristic set of energies at which it
absorbs or radiates electromagnetic radiation
Planck’
Planck’s Theory, 1901
• Light with wavelength λ has frequency
ν = c/λ
• can “exchange” energy with matter
(atoms) in units of:
E = hν
• h is “Planck’s constant”
h = 6.625 × 10-34 Joule-seconds
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
+
The Bohr Atom
Bohr Atom
• Electron “orbits” have fixed sizes ─ “orbitals”
– Not Like Planets in a “Solar System”
– atomic orbitals are “QUANTIZED”
Absorption of Photon “kicks” electron
to “higher” orbital
-
+
• Movement of electron e- between orbitals requires
absorption or radiation of energy
-
• only some orbital radii are “allowed”
– was very confusing to physicists
– first deduced by physicist Neils Bohr
– jump from lower to higher orbital ⇒ atom absorbs energy
– jump from higher to lower orbital ⇒ atom emits energy
1
Bohr Atom
• Atom “absorbs” photon if electron “kicked” up
to a “higher” energy state
-
Emission of Photon makes Electron
“drop” to “lower” orbital
Absorption vs. Emission
+
-
• 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
“Ensembles”
Ensembles” (Groups) of Atoms
Ensemble of Atoms in “Low”
Low”
States
Ready to Absorb, SIR!
Ensemble of Atoms in “Low”
Low” States
+
-
+
-
+
-
+
-
+
Absorption “lines”
lines”
Photons from Star at “correct”
λ are absorbed, and thus
removed from the observed
light
-
-
+
-
+
-
+
-
• What determines the “distribution” of
states of a group of atoms?
+
– Electrons in some atoms are in “low” states
and are more likely to absorb photons
– Electrons in some atoms are in “high”
states and more likely to emit photons
+
-
• Individual H atoms in a group of H
atoms have different states (are in
different “orbitals”)
+
Absorption Line
• Discovered in Solar spectrum by Fraunhofer
+
-
+
-
+
-
– called “Fraunhofer Lines”
• “Lines” because they appear as dark bands
superimposed on “rainbow” of visible
spectrum
2
Ensemble of Atoms in “High”
High”
States
Ensemble of Atoms in “High”
High”
States
Ready to Emit, SIR!
+
-
+
-
+
-
+
-
Photons at “correct” λ are emitted,
and thus added to any observed
light
+
-
+
-
+
-
+
-
+
-
+
-
+
-
+
-
+
-
+
-
Emission Line
Dark Background
Emission line spectrum
Some Atoms are in Both States
(but “one”
one” usually dominates)
More absorption if more atoms
in “low” state
More emission if more atoms
in “high” state
+
+
-
Appear as Bright Bands on “Faint Background Spectrum”
Why the Background??
+
-
+
-
+
-
+
-
+
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Absorption & Emission
Why Would Ensemble of Atoms
be in “High”
High” or “Low”
Low” State?
• Some other mechanism (besides light)
must be at work! But what?
TEMPERATURE T
Effect of Thermal Energy
• If T ≈ 0K (ensemble of atoms is very cold), most
atoms are in “low” state
– can easily absorb light
• If T >> 0K (ensemble of atoms is hot), the
thermal energy “kicks” most atoms into “high”
state
– can easily emit light
3
Sidebar: LASER
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:
• External source maintains “energy
inversion”
– more electrons in “high” state, even during
and after emission
high
– electrical
– optical (external light source, flash lamp)
Emission
Absorption
low
After “Driving”
Geometries for producing
absorption lines
1
After Emission
Sun’
Sun’s Fraunhofer absorption lines
The Observer
2
• Absorption lines require cool matter (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
(wavelengths listed in Angstroms; 1 Å = 0.1 nm)
Emission line spectra
1
The Observer
2
Insert various emission line spectra here
• Emission lines require hot matter (gas) viewed against
colder background
– scenario 1: hot “corona” of a star
– scenario 2: cold gas cloud seen against “empty” (colder) space
4
What Wavelengths are Emitted
and/or Absorbed?
Energies of H Orbitals
• Depends on Size of “Gaps” between
Energy States in the atoms
Limiting Energy
Energies of Orbitals of H
“Transitions” between
Orbitals
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
Relate Size of “Jump”
Jump” to the
λ Absorbed or Emitted
Relate Size of “Gap”
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
Sidebar: A Common Transition
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
Low-E State
• ∆E = hc/λ ≈ 9.4 ×
Joules
• ⇒ λ ≈ 0.21 m = 21 cm
• ⇒ ν ≈ 1420.4 MHz ⇒ RADIO Wave
10-25
5
Sidebar: 2121-cm Radio Wave of H
• First observed in 1951
– Simultaneously Discovered at 3
observatories!! (Harvard, Leiden, Sydney)
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. ...
• Measures the H in “interstellar matter”
• More electrons (and protons) ⇒ more
complicated absorption/emission spectrum
– Map of Spiral Arms in Milky Way Galaxy
Emission line images
Intensity
Optical EmissionEmission-Line Spectrum of
“Young Star”
Star”
Planetary nebula NGC 6543
(blue: X Rays)
λ (in Angstroms Å, or units of 10 nm)
Spectra of
ions
Neon
Orion Nebula
green ⇒ oxygen
red ⇒ hydrogen
Iron
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
• Molecules are more complicated than atoms
– atoms stripped of one
or more electrons
– Spectra are VERY complicated
• Ions of certain heavier
elements (e.g., neon
and iron with only one
electron) behave
much like
“supercharged” H and
He
• 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)
6
Molecular Spectra
• Transitions between different “orbitals” of
molecules (“electronic” states) (large ∆E)
– mostly in ultraviolet (UV), optical, and infrared (IR)
regions of spectrum
• Transitions between different “Vibrational”
states (“middlin” ∆E)
Rank Molecular Transitions by
Energy
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 (small ∆E)
– mostly in the radio region
Molecular Transitions in
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 to Measure Spectra?
•
1. Filter Spectrometer
A: With a “Spectrum Measurer”
– “SPECTROMETER”
– “Splits” light into its constituent wavelengths
and measures them
•
Mechanisms for “Splitting” Light
1. Optical Filters: “Block” light except in desired
band
2. “Dispersion” of Glass = “Differential Refraction”
-
Prism
3. Diffraction Grating
Filters in Rotating “Filter Wheel”
Sequence of “Monochrome” Images thru Different Colors
(How the images in the laboratory were created)
7
Multispectral Imaging
used for Manuscripts
2. Prism Spectrometer
Recall: Optical Dispersion
n
ultraviolet
450 nm
550 nm
650 nm
near infrared
λ
2. Prism Spectrometer
2. Prism Spectrometer
• “Refractive Index” n measures the
velocity of light in matter
• Refractive index n of glass decreases
with increasing wavelength λ
• Make a glass device that uses optical
dispersion to “separate” the
wavelengths
c
n=
v
c = velocity in vacuum ≈ 3 ×108
meters/second
v = velocity in medium measured in same
units
n ≥ 1.0
– a PRISM
2. Prism Spectrometer
Problems:
2. Prism Spectrometer
• Glass absorbs some light
– Ultraviolet light
θRed
White Light In
Long λ “dispersed” by smallest angle θ
θBlue
• Why you can’t get a suntan through glass
– Infrared light
• Images taken in different λ will “overlap”
• Dispersion Angle θ is complicated function
of wavelength λ
⇒ Spectrometer is difficult to “calibrate”
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3. Grating Spectrometer
“Interference” of Light
θRed
3. Grating Spectrometer
White Light In
θRed
θ
Different λ Interfere
at Different θ
λ
θBlue
λ
θBlue
Long λ “diverges: at
largest angle θ
Long λ “dispersed” by largest angle θ
Can be constructed for all wavelengths
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 θ is proportional to λ
– Easy to calibrate!
• Images at different λ can still overlap
9