- - Taking fingerprints of stars, galaxies, and interstellar gas clouds Absorption and emission from atoms, ions, and molecules Periodic Table of Elements The universe is mostly hydrogen H and helium He (97%) These (and a little lithium, Li) were only elements created in Big Bang ALL heavier elements have been (and are still being) manufactured in stars, via nuclear fusion Each element has own characteristic set of energies at which it absorbs or radiates electromagnetic radiation Planck s s Theory, 1901 Light with wavelength has frequency ν = c/ can exchange energy with matter (atoms) in units of: E = hν h is Planck s constant h = 6.625 10-34 Joule-seconds The Bohr Atom Model of Hydrogen atom Introduced by Niels Bohr early in 1913 to explain emission and absorption of light by H 1 proton ( nucleus ) orbited by 1 electron + The Bohr Atom Electron orbits have fixed sizes orbitals Not Like Planets in a Solar System atomic orbitals are QUANTIZED only some orbital radii are allowed was very confusing to physicists first deduced by physicist Neils Bohr Movement of electron e - between orbitals requires absorption or radiation of energy jump from lower to higher orbital atom absorbs energy jump from higher to lower orbital atom emits energy Bohr Atom Absorption of Photon kicks electron to higher orbital + 1
Bohr Atom + - - Emission of Photon makes Electron drop to lower orbital Absorption vs. Emission Atom absorbs photon if electron kicked up to a higher energy state Atom emits photon if electron drops down to a lower state Again, only a certain set of energy states is allowed set of states depends on the atom or molecule Ensembles (Groups) of Atoms Individual H atoms in a group of H atoms have different states (are in different orbitals ) Electrons in some atoms are in low states and are more likely to absorb photons Electrons in some atoms are in high states and more likely to emit photons What determines the distribution of states of a group of atoms? Ensemble of Atoms in Low States Ready to Absorb, SIR! Ensemble of Atoms in Low States Absorption lines Photons from Star at correct are absorbed, and thus removed from the observed light Absorption Line Discovered in Solar spectrum by Fraunhofer called Fraunhofer Lines Lines because they appear as dark bands superimposed on rainbow of visible spectrum 2
Ensemble of Atoms in High States Ready to Emit, SIR! Ensemble of Atoms in High States Photons at correct are emitted, and thus added to any observed light Emission Line Dark Background Emission line spectrum Some Atoms are in Both States (but one usually dominates) Absorption & Emission More absorption if more atoms in low state More emission if more atoms in high state Appear as Bright Bands on Faint Background Spectrum Why the Background?? Why Would Ensemble of Atoms be in High or Low State? Some other mechanism (besides light) must be at work! But what? TEMPERATURE T Effect of Thermal Energy If T 0K (ensemble of atoms is very cold), most atoms are in low state can easily absorb light If T >> 0K (ensemble of atoms is hot), the thermal energy kicks most atoms into high state can easily emit light 3
Sidebar: LASER Electrons in the medium (gas, solid, or diode) of a LASER are driven to high state by external energy Emit simultaneously and with same phase External Energy: electrical optical (external light source, flash lamp) Sidebar: LASER External source maintains energy inversion more electrons in high state, even during and after emission high low Absorption Emission After Driving After Emission Geometries for producing absorption lines Sun s s Fraunhofer absorption lines 1 2 The Observer Absorption lines require cool matter (gas) between observer and hot source scenario 1: cooler atmosphere of star scenario 2: cool gas cloud between star and observer (wavelengths listed in Angstroms; 1 Å = 0.1 nm) Geometries for producing emission lines Emission line spectra 1 2 The Observer Insert various emission line spectra here Emission lines require hot matter (gas) viewed against colder background scenario 1: hot corona of a star scenario 2: cold gas cloud seen against empty (colder) space 4
What Wavelengths are Emitted and/or Absorbed? Depends on Size of Gaps between Energy States in the atoms Energies of H Orbitals Limiting Energy Energies of Orbitals of H Transitions between Orbitals Ionization of Hydrogen Limiting Energy If electron absorbs sufficient energy E to rise above the upper limit of energy for a bound electron, then the electron becomes ionized electron escapes the proton Relate Size of Gap to Wavelength of Light Larger gaps or jumps in energy (both absorbed and emitted) photon carries more energy Recall hc 1 E E = E = hν = 2 1 Larger E Shorter bluer light Smaller E Longer redder light Relate Size of Jump to the Absorbed or Emitted Sidebar: A Common Transition Very Small E Very Long Due to spin flip of e - in Hydrogen Atom Very Small E Very Long Radio Waves High-E State Low-E State Very Large E Very Short X rays E = hc/ 9.4 10-25 Joules 0.21 m = 21 cm ν 1420.4 MHz RADIO Wave 5
Sidebar: 21-cm Radio Wave of H First observed in 1951 Simultaneously Discovered at 3 observatories!! (Harvard, Leiden, Sydney) Measures the H in interstellar matter Map of Spiral Arms in Milky Way Galaxy Bohr Atom: Extension to other elements H is simplest atom, BUT concept of electron orbitals applies to all atoms Neutral atoms have equal numbers of protons (in nucleus) and electrons (orbiting nucleus) He has 2 protons & 2 electrons; Lithium (Li), 3 each; Carbon (C), 6 each, etc.... More electrons (and protons) more complicated absorption/emission spectrum Optical Emission-Line Spectrum of Young Star Emission line images Intensity (in Angstroms Å, or units of 10 nm) Planetary nebula NGC 6543 (blue: X Rays) green oxygen red hydrogen Orion Nebula Spectra of ions Emission lines from heavy ions dominate high-energy (X-ray) spectra of stars atoms stripped of one or more electrons Ions of certain heavier elements (e.g., neon and iron with only one electron) behave much like supercharged H and He Neon Iron Wavelength (in Angstroms) Molecules Also have characteristic spectra of emission and absorption lines Each molecule has particular set of allowed energies at which it absorbs or radiates Molecules are more complicated than atoms Spectra are VERY complicated Electrons shared by one (or more) atoms in molecule absorb or emit specific energies Changes in state of vibration and/or rotation are also quantized Vibration, rotation spectra unique to each molecule 6
Molecular Spectra Transitions between different orbitals of molecules ( electronic states) (large E) mostly in ultraviolet (UV), optical, and infrared (IR) regions of spectrum Transitions between different Vibrational states ( middlin E) mostly in the near-infrared (NIR) Transitions between different Rotational states (small E) mostly in the radio region Rank Molecular Transitions by Energy 1. UV, Visible, IR Electronic 2. NIR Vibrational 3. Radio Rotational 4. Radio H spin flip @ ν = 1420 MHz Molecular Transitions in Planetary Nebula NGC 2346 Molecular Emission: Rotational Transition Electronic Transition (visible light) Vibrational Molecular Hydrogen Transition (IR) Rotational CO (carbon monoxide) Emission from Molecular Clouds in Milky Way Q: How to Measure Spectra? A: With a Spectrum Measurer SPECTROMETER Splits light into its constituent wavelengths and measures them Mechanisms for Splitting Light 1. Optical Filters: Block light except in desired band 2. Dispersion of Glass = Differential Refraction - Prism 3. Diffraction Grating 1. Filter Spectrometer Filters in Rotating Filter Wheel Sequence of Monochrome Images thru Different Colors (How the images in the laboratory were created) 7
Multispectral Imaging used for Manuscripts Recall: Optical Dispersion n ultraviolet 450 nm 550 nm 650 nm near infrared Refractive Index n measures the velocity of light in matter c n = v c = velocity in vacuum 3 10 8 meters/second v = velocity in medium measured in same units n 1.0 Refractive index n of glass decreases with increasing wavelength Make a glass device that uses optical dispersion to separate the wavelengths a PRISM White Light In Long dispersed by smallest angle θ θ Red θ Blue Problems: Glass absorbs some light Ultraviolet light Why you can t get a suntan through glass Infrared light Images taken in different will overlap Dispersion Angle θ is complicated function of wavelength Spectrometer is difficult to calibrate 8
3. Grating Spectrometer Interference of Light 3. Grating Spectrometer θ θ Red Different Interfere at Different θ White Light In θ Blue θ Red θ Blue Long diverges: at largest angle θ Long dispersed by largest angle θ Can be constructed for all wavelengths 3. Grating Spectrometer Uses Diffraction Grating works by interference of light Regularly spaced transparent & opaque regions Can be made without absorbing glass Used at all wavelengths (visible, UV, IR, X-Rays, ) Dispersion angle θ is proportional to Easy to calibrate! Images at different can still overlap 9