Chapter 4: Spectroscopy
• What is spectroscopy?
• Characteristics of spectra
– continuous, emission, absorption
– Kirchoff’s law
• Structure of the atom
– Bohr model
• Transitions and spectra
– atoms, molecules
• Astronomical information from spectra
– Fraunhofer lines
– wavelengths, intensity, broadening
Information from Spectra
Almost all that
we know about
planets, stars,
and galaxies is
obtained from
studies of the
light received
from them.
Spectral Lines
• Newton:
–sunlight through pinhole to prism
–spectrum = continuous rainbow
• Blackbody radiation:
–radiation at all wavelengths
–spectrum = continuous rainbow
Blackbody Radiation
• Consider an idealized object that absorbs
all the electromagnetic radiation that falls
on it - called a “blackbody.”
• A blackbody will absorb all energy
incident on it and heat up until it is
emitting energy at the same rate that it
absorbs energy.
• The equilibrium temperature reached is a
function of the total energy striking the
blackbody each second.
Characteristics of Blackbody Radiation
• Three characteristics of a blackbody :
1. A blackbody with a temperature higher
than absolute zero emits some energy
at all frequencies (or wavelengths).
2. A blackbody at higher temperature emits
more energy at all frequencies
(or wavelengths) than does a cooler one.
3. The higher the temperature of a
blackbody, the higher the frequency
(the shorter the wavelength) at which the
maximum energy is emitted.
Blackbody Radiation
• Blackbody radiation:
the distribution of
radiation emitted by any
heated object.
• The curve peaks at a
single, well-defined
frequency and falls off to
lesser values above and
below that frequency.
The overall shape (intensity vs frequency) is characteristic
of the radiation emitted by any object, regardless of its
size, shape, composition, or temperature.
Planck Spectrum
• As an object is heated,
the radiation it emits
peaks at higher and
higher frequencies.
• Shown here are curves
corresponding to
temperatures of
300 K (room temperature),
1000 K (begin to glow deep red)
4000 K (red hot), and
7000 K (white hot).
Spectroscope
Spectral Lines from Sunlight
Wollaston (1802)
– sunlight through slit to prism
– spectrum = rainbow with holes
Fraunhofer (~1812)
– cataloged over
600 dark lines
Another Type of Spectra
• Further study revealed
that if gases are heated
until they emit light,
neither a continuous
nor a spectrum with
dark lines is produced.
– A spectrum made up of
bright lines appears.
• Also, each element
was discovered to
produce its own
distinctive pattern of
bright lines.
– Used as a way to
identify the
composition of an
unknown substance.
Continuous Spectra
Radiation is distributed over all
frequencies, not just a few specific
frequency ranges.
Emission Spectra
Pattern of bright spectral lines
produced by an element.
Absorption Spectra
Pattern of dark spectral lines
where light within a number of
narrow frequency ranges has been
removed.
When are Each of the Three
Types of Spectra Observed?
• The situation in which each of the
three types of spectra is observed
was summarized in a set of rules by
Gustav Kirchoff in the 1860’s.
• These rules are known as
“Kirchoff’s laws”.
Kirchoff’s Laws
• 1st law: A luminous solid or
liquid, or a sufficiently dense
gas, emits light of all
wavelengths and produces a
continuous spectrum of
radiation.
• 2nd law: A low-density hot
gas emits light whose
spectrum consists of a series
of bright emission lines which
are characteristic of the
chemical composition of the
gas.
• 3rd law: A cool thin gas
absorbs certain wavelengths
from a continuous spectrum,
leaving dark absorption lines
in their place superimposed
on the continuous spectrum.
Observed Spectra and Background
Type of spectrum seen depends on the temperature of the
thin gas relative to the background temperature.
TOP: thin gas cooler than background, absorption lines seen.
BOTTOM: thin gas hotter than background, emission lines seen.
How are Spectral Lines Created?
• In the last chapter, stated that
– electromagnetic waves are
created by moving charges and
– all matter is composed of atoms,
which are in turn composed of
• protons (+ charge),
• electrons (- charge), and
• neutrons (neutral charge)
• Is there a connection between
composition of matter and
the spectral lines produced?
Structure of the Atom
• For at least 25 centuries, matter believed
to be made of tiny particles -- atoms.
• Newton thought that atoms were hard
and indivisible.
• Complex structure of the atom not observed
until 20th century.
–In 1897, J.J. Thomson discovered the electron.
•
plum pudding model
–In 1911, Ernest Rutherford detected atomic nucleus.
•
alpha particles shot at thin gold foil
Rutherford Model
of the Atom
• Early models of the atom are like a miniature solar
system with the electrons orbiting the nucleus, just as
the planets orbit the Sun.
• The electrons must be in motion.
(e and p  attract each other;
stationary e would fall into nucleus.)
• Most of the atom is empty space.
• Problem:
Since e are in motion, why don’t atoms emit
E-M radiation continuously?
Why don’t e lose energy and spiral into nucleus?
Bohr Model of the Atom
• “Planetary model” of the atom.
– Neutrons and protons occupy a dense
central region called the “nucleus”.
– Electrons orbit the nucleus much like
planets orbiting the Sun.
• Modifications
– Only certain select radii are possible
for the electron orbits.
– If an electron moves in an allowed orbit,
it radiates no energy.
– The amount of energy required to move
from one orbit to another is fixed.
E
Photons
E
• Electrons may exist only in orbitals
having certain specified energies.
• Atoms can absorb only specific amounts of energy as
their electrons are boosted to excited states;
atoms emit only specific amounts of energy when
their electrons fall back down to lower energy states.
• The light absorbed or emitted must be in
“packets” of electromagnetic radiation containing
a specific amount of energy.
• These packets are called “PHOTONS.”
• The energy of a photon is related to the frequency of
the electromagnetic energy absorbed or emitted.
Frequency and Energy
The energy of a photon is related to the
frequency of light emitted or absorbed by
E = hf
where h = Planck’s constant
Recall that wave speed relates frequency and wavelength:
v = f
and for light,
so,
E f
c = f
or E  1/
“Every physicist thinks he
knows what a photon is.
I spent my life to find out what a
photon is and I still don’t know.”
Albert Einstein
Emission
Electrons drop
to lower energy
levels
releasing energy
in the form of
EMR
Hydrogen Atom
• The energy of particles in
Bohr atom are restricted to
certain discrete values.
• The energy is quantized.
– only certain orbits with
certain radii are allowed;
– orbits in between simply
don't exist.
• Energy levels labeled by an
integer n - called a
quantum number.
• The lowest energy state is
generally termed the
ground state.
• The states with successively
more energy than the ground
state are called the first excited
state, second excited state, etc.
Terminology
• The “normal” condition of an atom is called the
ground state.
– minimum energy configuration of the atom
• If an orbiting electron is given enough energy to
escape the atom, the atom is said to be ionized.
• Between ground state and ionization, the electron can
only exist in certain well-defined excited states.
– Each excited state has a specific energy (quantized).
– Electrons moving from one energy level to another, absorb
or emit an amount of energy equal to the difference
between the energy levels.
– The energy is absorbed or emitted in the form of a photon.
– The energy of the photon is proportional to frequency.
Excitation and Emission
• If an atom is given
energy, electron
may jump to a
more distant orbit.
• Atoms do not stay
in this energized
state long.
• Electron will fall
down to a lower
orbit, emitting a
photon.
Hot Gas Spectra
• The visible spectrum of
the hot gases in a nearby
star-forming region
known as the Omega
nebula (M17).
• Shining by the light of
several very hot stars,
the nebula produces a
complex spectrum of
bright and dark lines
(bottom).
• Spectrum also shown as
an intensity trace from
red to blue (center).
What type of spectra is this
(continuous, emission, or absorption)?
From Atoms to Molecules
If two or more atoms combine
to form a molecule, do the electron
energy levels and, consequently,
the observed spectra change?
If so, how much?
Molecular Motions
Three ways molecules can change
to emit or absorb E-M radiation.
change electron arrangement
(e.g., electron in the outermost
orbital of the oxygen atom
drops to lower-energy state)
change rotational state to
a lower energy mode
change vibrational state to
a lower energy mode
Hydrogen Spectra
(a) Molecular Hydrogen
(b) Atomic Hydrogen
Information from Spectra
• High temperatures, atoms mainly ionized.
– Spectrum is continuous.
• Cooler temperatures, more bound electrons
making transitions.
– Spectral lines help determine chemical composition.
• Spectral line strength depends on
– the number of a specific type atoms in the gas
– the temperature of gas containing atoms
• number in each excited state
• collisions
Spectral Linewidth
• May expect narrow, distinct
spectral lines.
• Physical mechanisms can
broaden spectral lines.
– Doppler Effect
• Thermal motion
• Rotation
• Gas Turbulence
– Collision Broadening
– Magnetism
Doppler Effect:
Thermal Motion of Atoms
• Atoms moving randomly.
• Redshifted, blueshifted, and
unshifted emission lines
created with respect to the
observer.
• In the detector,
individual redshifted
and blueshifted
emission lines merge
with the unshifted lines
to produce broadened
spectral lines.
Doppler Effect: Rotation
• Rotation of star/gas
will produce a
broadening of
spectral lines.
• Photons emitted
from side spinning
toward us,
blueshifted.
• Photons emitted
from side spinning
away from us,
redshifted.
Observed spectral line
is broadened.
Doppler Effect: Turbulence
• If gas in a cloud is churning in eddies and
vortices of different sizes, spectral lines from
each of those parts of the cloud will be
Doppler-shifted randomly wrt another part
of the cloud.
• If the cloud is far away or small, light from
all parts of the cloud will be blended in the
detector.
• Overall effect similar to thermal broadening,
but NOT related to temperature of the gas.
Spectral Linewidth
• Physical mechanisms
can broaden spectral
lines.
– Doppler Effect
• Thermal motion
• Rotation
• Gas Turbulence
– Collision Broadening
– Magnetism
Spectral Information from Starlight
Observed Spectral
Characteristics
Information Provided
Peak frequency or wavelength
(continuous spectra only)
Temperature
Lines present
Composition, Temperature
Line intensity
Composition, Temperature
Line width
Temperature, Turbulence,
Rotation Speed, Density,
Magnetic Field
Doppler shift
Line of sight velocity
What’s important?
• spectroscopy definition
• spectra: continuous, emission, absorption
– How is each of the above formed?
– Kirchhoff’s Laws
• relationship between type of spectra observed and
necessary conditions for that observation.
• “fingerprinting” composition
• atoms
• protons, electrons, neutrons
• structure: Bohr model of the atom; quantization
• relationship between structure and spectra; photons
• spectral line broadening: possible causes
• information from spectra