Lecture 4: Measuring Light:
Spectroscopy

Readings: Chapter 5, sections 5-6, 5-7, 5-8

Every atom, ion, and molecule has a unique spectral signature.

• Reflection of their the underlying electron orbital structure.

• Absorption and Emission of Photons
• Collisional excitation and de-excitation

• Remove one or more electrons, or add an extra electron.

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 that orbital (all or nothing).

The details are dictated by quantum mechanics.

An atom of Hydrogen (¹H) consists of:

• A single proton in the nucleus.
• A single electron orbiting the nucleus.

First orbital: Ground State (n=1)

• Lowest energy orbital the electron can reside in.

Higher orbitals: Excited States (n=2,3,...)

• Higher orbits around the nucleus.
• Come at specific, exact energies.

Emission Lines:

When an electron jumps from a higher to a lower energy orbital, a single photon is emitted with exactly the energy difference between orbitals. No more, no less.

Absorption Lines:

When an electron absorbs a photon with exactly the energy needed to jump from a lower to a higher orbital. No more, no less.

Other atoms have more electrons, and hence more complex electron orbital structures.

• Results in more complex line spectra.
• There is a unique spectrum for each element, reflecting its unique electron orbital structure.
• Isotopes show the same lines, but slightly shifted in wavelength.

Every element has its own, distinctive spectral signature.

Emission Spectra of Different Elements

Molecules are more complex still:

• Compounds of two or more atoms, often of different elements.
• Share some electrons in common orbitals.

Results in very complex spectra:

• Broad "bands" consisting of many lines.
• Bands often span large wavelength regions.
• Can get strong lines at infrared, microwave, and radio wavelengths.

If an atom or molecule absorbs enough energy from a photon or a collision, an electron can be ejected.

• Get a Positive Ion (atom or molecule with a net positive charge).

Similarly, you can also add extra electrons:

• Get a Negative Ion (atom or molecule with a net negative charge).

Ions differ from their parent neutral atoms or molecules:

• Diferent spectral line signatures.
• Different chemical properties.

From the emission or absorption lines in an object's spectrum, we can learn:

• Which elements are present, and in what proportions.
• Which elements are present in ionized form.
• Which elements are combined into molecules (if any).

These data give us a nearly complete picture of the physical conditions in the object.

Spectroscopy is one of the most important tools of the astronomer.