Periodic Table of Elements Taking Fingerprints of Stars, Galaxies, and Other Stuff Absorption and Emission from Atoms, Ions, and Molecules Universe is mostly (97%) Hydrogen and Helium (H and He) The ONLY elements created in the Big Bang were H, He, and a little lithium, Li All heavier elements have been (and are still being) manufactured in stars via nuclear fusion Each element has characteristic set of energies where absorbs or radiates The Bohr Atom Model of Hydrogen atom Introduced by Niels Bohr early in 1913 to explain emission and absorption of light by H One proton (the nucleus ) orbited by an electron The Bohr Atom Electron orbits have fixed sizes orbitals Unlike Planets in a Solar System atomic structure is QUANTIZED 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: energy absorbed jump from higher to lower orbital: energy emitted Bohr Atom + - - Absorption of Photon kicks electron to higher orbital Bohr Atom + - - Emission of Photon makes Electron drop to lower orbital 1
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 States of individual H atoms in a group are not identical Some electrons are in low states and are more likely to absorb photons Some electrons 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 Ensemble of Atoms in Low States Ready to Absorb, SIR! Photons from Star at correct are absorbed, and thus removed from the observed light Absorption Line Absorption lines Ensemble of Atoms in High States Ready to Emit, SIR! 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 Emission line spectrum Photons at correct are emitted, and thus added to any observed light Emission Line Appear as Bright Bands on Faint Background Spectrum Why the Background?? Some Atoms are in Both States (but one dominates) Absorption & Emission More absorption if more atoms in low state More emission if more atoms in high state 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 0-K (ensemble of atoms is very cold), most atoms are in low state: can easily absorb light If T >> 0-K (ensemble of atoms is hot), the thermal energy kicks most atoms into high state: can easily emit light 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) 3
Sidebar: LASER External source maintains energy inversion more electrons in high state, even during and after emission Geometries for producing absorption lines 1 2 high low After Driving After Emission Emission Absorption lines require cool gas between observer and hot source scenario 1: cooler atmosphere of star scenario 2: cool gas cloud between star and observer Sun s Fraunhofer absorption lines Geometries for producing emission lines 1 2 (wavelengths listed in Angstroms; 1 Å = 0.1 nm) Emission lines require gas viewed against colder background scenario 1: the hot corona of a star scenario 2: cold gas cloud seen against empty (colder) space Emission line spectra Insert various emission line spectra here What Wavelengths are Involved? Depends on the Size of the Gaps between Energy States in the atoms 4
Energies of H Orbitals Energies of Orbitals of H Transitions between Orbitals 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 Transition with Very Small E Very Long Due to spin flip of e - in Hydrogen Atom Very Small E Very Long radio waves Very Large E Very Short X rays High-E State Low-E State E = hc/ 9.4 10-25 Joules 0.21 m = 21 cm ν 1420.4 MHz RADIO Wave 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.... In atoms with more electrons (and protons), the absorption/emission spectrum is more complicated 5
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) Spectra of 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 More on Molecular Spectra Transitions between different orbitals of molecules ( electronic states) mostly in ultraviolet (UV), optical, and infrared (IR) regions of spectrum Transitions between different Vibrational states mostly in the near-infrared (NIR) Transitions between different Rotational states 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 6
Molecular Emission: Vibrational Transition 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 Can We Measure Spectra? A: With a Spectrum Measurer A SPECTROMETER Splits light into its constituent wavelengths Common Mechanisms for Splitting Light 1. Optical Filters - Blocks 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) n 2. Prism Spectrometer Recall: Optical Dispersion 2. Prism Spectrometer Refractive Index n measures the velocity of light in matter n = c v c = velocity in vacuum 3 10 8 meters/second v = velocity in medium measured in same units n 1.0 7
2. Prism Spectrometer 2. Prism Spectrometer 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 θ Red θ Blue Long dispersed by smallest angle θ 2. Prism Spectrometer Problems: 3. Grating Spectrometer Interference of Light Glass absorbs some light Ultraviolet light Why you can t get a suntan under glass Infrared light Images taken in different will overlap Dispersion Angle θ is a complicated function of wavelength Spectrometer is difficult to calibrate θ θ Red θ Blue Different Interfere at Different θ 3. Grating Spectrometer White Light In Long dispersed by largest angle θ Can be constructed for all wavelengths θ Blue θ Red Long diverges: at largest angle θ 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 8