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) 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 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: to ground state 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 Electron transitions produce visible and ultraviolet lines b) Vibrational transitions produce infrared lines c) Rotational transitions produce radio-wave lines

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

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?

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 Total energy output Surface temperature/color Radius Chemical composition Velocity relative to Earth Rotation period