Spectroscopy and
Atomic Structure Ch 04
Four essential questions:
• What is an atom?
• How do atoms interact with light?
• What kinds of spectra do you see when you
look at celestial objects?
• What can you learn from a star’s spectrum?
Objectives (Ch 04)
So far, l tells us ….
•
Color
•
Temperature
•
Energy Flux
•
Some Doppler
The Enigma, to understand how stars work  need to understand
how atoms work????
Explain the Formation of Spectral Lines
The Energy Levels of the Hydrogen Atom
The Photoelectric Effect
Chemical Composition
Explain how spectral lines apply to molecules
Explain what information Spectral-Lines provide
Stellar Spectra
The spectra of stars
are more complicated
than pure blackbody
spectra.
They contain
characteristic lines,
called absorption lines.
With what we
have learned
about atomic
structure, we
can now
understand
how those lines
are formed.
Kirchhoff’s Laws of Radiation (1)
1. A solid, liquid, or dense gas excited to
emit light will radiate at all wavelengths
and thus produce a continuous
spectrum.
Kirchhoff’s Laws of Radiation (2)
2. A low-density gas excited to emit light
will do so at specific wavelengths and
thus produce an emission spectrum.
Light excites electrons in
atoms to higher energy
states
Transition back to lower states
emits light at specific
frequencies
Kirchhoff’s Laws of Radiation (3)
3. If light comprising a continuous spectrum
passes through a cool, low-density gas, the
result will be an absorption spectrum.
Light excites electrons in
atoms to higher energy
states
transition energies are absorbed from the
continuous spectrum.
The Spectra of Stars
The inner, dense layers of
a star produce a
continuous (blackbody)
spectrum.
Cooler surface layers absorb light at specific
frequencies.
=> Spectra of stars are absorption
spectra.
Analyzing Absorption Spectra
• Each element produces a specific set of
absorption (and emission) lines.
• Comparing the relative strengths of these sets
of lines, we can study the composition of gases.
By far the
most
abundant
elements
in the
Universe
Spectral Lines
Emission spectrum can be used to
identify elements:
Spectral Lines
An absorption spectrum can also be used to
identify elements. These are the emission and
absorption spectra of sodium:
Spectral Lines … Bottom Line
Kirchhoff’s laws:
1. Luminous solid, liquid, or dense gas
produces continuous spectrum
2. Low-density hot gas produces emission
spectrum
3. Continuous spectrum incident on cool, thin
gas produces absorption spectrum
Formation of Spectral Lines
Existence of spectral lines required new model of
atom, so that only certain amounts of energy
could be emitted or absorbed.
Bohr model had certain allowed orbits for
electron:
Quantized Energy
• Continuous energy is like a
ramp.
• Quantized energy is like a
stair case.
• Each stair increases the
energy by the value of
Planck’s constant
• h = 6.63x10-34 J-s
• E = hf
• C = lf
• 1eV = 1.602 x 10-19 J
• En = 13.6 eV (1 – [1/n2])
The Dual Nature of Light
Light is a wave
–
–
–
–
Reflection
Refraction
Interference
Polarization
Light is a particle
– Photoelectric effect
– reflection
E = hf
Light is both a
wave and particle!!
http://jersey.uoregon.edu/vlab/elements/Elements.html;
Become familiar with the absorption and emission of various elemental
spectra. Pay particular attention to H, He, and the noble gases.
1. Set the applet to “absorption.” What do you notice about the spectral
lines? What is happening at the atomic level with respect to electron
transitions?
2. Set the applet to “emission.” What do you notice about the spectral
lines? What is happening at the atomic level with respect to electron
transitions?
The Bohr model is a primitive model of the hydrogen
atom. As a theory, it can be derived as a first-order
approximation of the hydrogen atom using the broader
and much more accurate quantum mechanics, and thus
may be considered to be an obsolete scientific theory.
However, because of its simplicity, and its correct results
for selected systems, the Bohr model is still commonly
taught to introduce students to quantum mechanics,
before moving on to the more accurate but more complex
valence shell atom. The quantum theory of the period
between Planck's discovery of the quantum (1900) and
the advent of a full-blown quantum mechanics (1925) is
often referred to as the old quantum theory.
In the model above, various electron transitions are
shown, where n = energy level. The ground state is given
by, n = 1. Transitions starting or ending at the ground
state are known as the “Lyman” series, discovered in
1914 by Theodore Lyman. The first is Lyman-a (n = 2 to
n = 1), then Lyman-b (n = 3 to n = 1), and so on. When n
= ∞, ionization occurs
Formation of Spectral Lines
Energy levels of the hydrogen atom, showing
two series of emission lines:
Formation of Spectral Lines … bottom line
Absorption can boost an electron to the second (or higher)
excited state
Two ways to decay:
1. to ground state
2. cascade one orbital at a time
Formation of Spectral Lines
(a) Direct decay
(b) cascade
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
Formation of Spectral Lines
Emission lines can be used to identify atoms:
Molecules
Molecules can vibrate and rotate,
besides having energy levels
a) Electron transitions produce
visible and ultraviolet lines
b) Vibrational transitions produce
infrared lines
c) Rotational transitions produce radiowave lines
Molecules
Molecular spectra are much more complex
than atomic spectra, even for hydrogen:
H2
(a) Molecular hydrogen
H
(b) Atomic hydrogen
Spectral-Line Analysis
Information that can be obtained from
spectral lines:
• Chemical composition
• Temperature
• Radial velocity:
Spectral-Line Analysis
Line broadening can be due to
Doppler shift
• from thermal motion
• from rotation
Zeeman Effect … It’s magnetic
http://phys.educ.ksu.edu/vqm/free/zeemanspec.html
The Doppler Effect
The Doppler Effect
Take l0 of the Ha (Balmer alpha)
line:
Assume, we observe a star’s spectrum with
the Ha line at l = 658 nm. What is the shift
and radial velocity?
Spectral-Line Analysis
Summary
• Spectroscope splits light beam into
component frequencies
• Continuous spectrum is emitted by solid,
liquid, and dense gas
• Hot gas has characteristic emission spectrum
• Continuous spectrum incident on cool, thin
gas gives characteristic absorption spectrum
Summary, cont.
• Spectra can be explained using atomic
models, with electrons occupying specific
orbitals
• Emission and absorption lines result from
transitions between orbitals
• Molecules can also emit and absorb
radiation when making transitions between
vibrational or rotational states
The Amazing Power of Starlight
Just by analyzing the light received from a
star, astronomers can retrieve information
about a star’s
1. Total energy output
2. Surface temperature/color
3. Radius
4. Chemical composition
5. Velocity relative to Earth
6. Rotation period