1. Spectroscopy and
Atomic Structure Ch 04

2. 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?

3. 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

4. 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.

5. Kirchhoff’s Laws of Radiation (1)
A solid, liquid, or dense gas excited to emit light will radiate at all wavelengths and thus produce a continuous spectrum.

6. 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

7. 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.

8. 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.

9. 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

10. Spectral Lines
Emission spectrum can be used to identify elements:

11. Spectral Lines
An absorption spectrum can also be used to identify elements. These are the emission and absorption spectra of sodium:

12. 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

13. 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:

14. 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 = x J
En = 13.6 eV (1 – [1/n2])

15. 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!!

16.
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?

17. The Bohr model is a primitive model of the hydrogen atom
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

18. Formation of Spectral Lines
Energy levels of the hydrogen atom, showing two series of emission lines:

21. Formation of Spectral Lines … bottom line
Absorption can boost an electron to the second (or higher) excited state
Two ways to decay:
to ground state
cascade one orbital at a time

22. Formation of Spectral Lines
(a) Direct decay (b) cascade

23. 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

24. Formation of Spectral Lines
Emission lines can be used to identify atoms:

25. Molecules
Molecules can vibrate and rotate, besides having energy levels
Electron transitions produce visible and ultraviolet lines
b) Vibrational transitions produce infrared lines
c) Rotational transitions produce radio-wave lines

26. Molecules
Molecular spectra are much more complex than atomic spectra, even for hydrogen: H2 H
(a) Molecular hydrogen (b) Atomic hydrogen

27. Spectral-Line Analysis
Information that can be obtained from spectral lines:
Chemical composition
Temperature
Radial velocity:

28. Spectral-Line Analysis
Line broadening can be due to Doppler shift
from thermal motion
from rotation

30. Zeeman Effect … It’s magnetic

31. The Doppler Effect

32. Take l0 of the Ha (Balmer alpha) line:
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?

33. Spectral-Line Analysis

34. 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

35. 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

36. The Amazing Power of Starlight
Just by analyzing the light received from a star, astronomers can retrieve information about a star’s
Total energy output
Surface temperature/color
Radius
Chemical composition
Velocity relative to Earth
Rotation period