Taking fingerprints of stars, galaxies, and interstellar gas clouds Absorption and emission from atoms, ions, and molecules 1
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 2
Planck s s Theory, 1901 Light with wavelength λ has frequency ν = c/λ can exchange energy with matter (atoms) in units of: E = hν h is Planck s constant h = 6.625 10-34 Joule-seconds 3
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 + 4
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 5
Bohr Atom Absorption of Photon kicks electron to higher orbital + - - 6
Bohr Atom Emission of Photon makes Electron drop to lower orbital + - - 7
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 8
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? 9
Ensemble of Atoms in Low States Ready to Absorb, SIR! 10
Ensemble of Atoms in Low States Photons from Star at correct λ are absorbed, and thus removed from the observed light Absorption Line 11
Absorption lines Discovered in Solar spectrum by Fraunhofer called Fraunhofer Lines Lines because they appear as dark bands superimposed on rainbow of visible spectrum 12
Ensemble of Atoms in High States Ready to Emit, SIR! 13
Ensemble of Atoms in High States Photons at correct λ are emitted, and thus added to any observed light Emission Line Dark Background 14
Emission line spectrum Appear as Bright Bands on Faint Background Spectrum Why the Background?? 15
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 16
Why Would Ensemble of Atoms be in High or Low State? Some other mechanism (besides light) must be at work! But what? TEMPERATURE T 17
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 18
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) 19
Sidebar: LASER External source maintains energy inversion more electrons in high state, even during and after emission high Absorption Emission low After Driving After Emission 20
Geometries for producing 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 21
Sun s s Fraunhofer absorption lines (wavelengths listed in Angstroms; 1 Å = 0.1 nm) 22
Geometries for producing emission lines 1 2 The Observer 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 23
Emission line spectra Insert various emission line spectra here 24
What Wavelengths are Emitted and/or Absorbed? Depends on Size of Gaps between Energy States in the atoms 25
Energies of H Orbitals Limiting Energy Energies of Orbitals of H Transitions between Orbitals 26
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 27
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 E2 E1 = E = hν = λ λ Larger E Shorter λ bluer light Smaller E Longer λ redder light 28
Relate Size of Jump to the λ Absorbed or Emitted Very Small E Very Long λ Radio Waves Very Large E Very Short λ X rays 29
Sidebar: A Common Transition Very Small E Very Long λ Due to spin flip of e - in Hydrogen Atom High-E State Low-E State E = hc/λ 9.4 10-25 Joules λ 0.21 m = 21 cm ν 1420.4 MHz RADIO Wave 30
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 31
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 32
Optical Emission-Line Spectrum of Young Star Intensity λ (in Angstroms Å, or units of 10 nm) 33
Emission line images Planetary nebula NGC 6543 (blue: X Rays) green oxygen red hydrogen Orion Nebula 34
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) 35
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 36
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 37
Rank Molecular Transitions by Energy 1. UV, Visible, IR Electronic 2. NIR Vibrational 3. Radio Rotational 4. Radio H spin flip @ ν = 1420 MHz 38
Molecular Transitions in Planetary Nebula NGC 2346 Electronic Transition (visible light) Vibrational Molecular Hydrogen Transition (IR) 39
Molecular Emission: Rotational Transition Rotational CO (carbon monoxide) Emission from Molecular Clouds in Milky Way 40
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 41
1. Filter Spectrometer Filters in Rotating Filter Wheel Sequence of Monochrome Images thru Different Colors (How the images in the laboratory were created) 42
Multispectral Imaging used for Manuscripts ultraviolet 450 nm 550 nm 650 nm near infrared Multispectral imaging will be used to differentiate between the two inks in the two sets of writing and separate them from the parchment and the mold. The goal is to read the erased writing underneath the obscuring overwriting and also to detect any ink in the moldy regions. The spectral response of the two inks is visible in this figure. The contrast of both inks decreases for longer, i.e. redder wavelengths, but the erased writing decreases in contrast more quickly. 43
2. Prism Spectrometer Recall: Optical Dispersion n λ 44
2. Prism Spectrometer 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 45
2. Prism Spectrometer Refractive index n of glass decreases with increasing wavelength λ Make a glass device that uses optical dispersion to separate the wavelengths a PRISM 46
2. Prism Spectrometer θ Red θ Blue White Light In Long λ dispersed by smallest angle θ 47
2. Prism Spectrometer Problems: Glass absorbs some light Ultraviolet light Why you can t get a suntan through glass Infrared light Images taken in different λ will overlap Dispersion Angle θ is complicated function of wavelength λ Spectrometer is difficult to calibrate 48
3. Grating Spectrometer Interference of Light θ Red θ λ Different λ Interfere at Different θ θ Blue λ 49
3. Grating Spectrometer White Light In θ Red θ Blue Long λ dispersed by largest angle θ Can be constructed for all wavelengths Long λ diverges: at largest angle θ 50
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 λ Easy to calibrate! Images at different λ can still overlap 51