Astronomy 171
Solar System Astronomy
Prof. Paul Martini

Lecture 24: Spectroscopy

Key Ideas:

Every atom, ion, and molecule has a unique spectral signature: Reflects its internal electron orbital structure
Absorption and Emission of Photons: Excitation and De-excitation
Ionization: Eject electrons or add extra electrons

Looking inside the Atom

Electrons cannot orbit just anywhere around a nucleus: Can only orbit in discrete orbitals; Each orbital corresponds to a particular energy of the orbiting electron; If an electron does not have exactly the right energy, it cannot be in an orbital
Details are dictated by Quantum Mechanics

Hydrogen: The Simplest Atom

An atom of Hydrogen (¹H) is:: Nucleus of 1 proton; Orbited by 1 electron
First orbital: "Ground State" (n = 1): Lowest energy orbital
Higher orbitals: "Excited States" (n = 2, 3, ...): Higher orbits around the nucleus; Come at specific, exact energies

Emission Lines

An electron jumps from a higher to a lower energy orbital: One photon is emitted with exactly the energy difference between orbitals
Electrons get into excited states by: Colliding with atoms or free electrons; Absorbing photons of specific energies...

Absorption Lines

An electron absorbs a photon with exactly the energy needed to jump from a lower to a higher orbital: Only photons with the exact excitation energy are absorbed; All others pass through unabsorbed
The excited states decay by emitting photons in random directions

Fingerprinting Matter

Atoms other than Hydrogen have different spectra: More complex electron orbital structures; More complex line spectra
A unique spectrum for each element: Tell elements apart by their spectra; Isotopes show the same lines, but slightly shifted in wavelength

Molecules

Molecules are even more complex: Compounds of two or more atoms; Share some electrons in common orbitals
Results in very complex spectra: Broad "bands" of many lines together; Bands span large wavelength regions
Get strong lines at infrared, microwave, and radio wavelengths.

Ionization

If an electron absorbs enough energy from a photon or a collision, it can be ejected: Get a positive ion (net positive charge)
Can also add extra electrons: Get a negative ion (net negative charge)
Ions differ from their parent neutral forms: Different spectral line signatures; Different chemical properties

Importance of Spectroscopy

From an object's spectrum, we can learn:: Which atoms and molecules are present, and in what proportions; Which atoms are ionized, and in what proportions; How excited (or not) the atoms are, which tells us the object's state (e.g. hot or cold)
These data give us a nearly complete picture of the physical conditions in the object.

See A Note about Graphics to learn why some of the graphics shown in the lectures are not reproduced with these notes.