There's antimony, arsenic, aluminum, selenium, And hydrogen and oxygen and nitrogen and rhenium, And nickel, neodymium, neptunium, germanium, And iron, americium, ruthenium, uranium, Europium, zirconium, lutetium, vanadium, And lanthanum and osmium and astatine and radium, And gold and protactinium and indium and gallium, And iodine and thorium and thulium and thallium. There's yttrium, ytterbium, actinium, rubidium, And boron, gadolinium, niobium, iridium, And strontium and silicon and silver and samarium, And bismuth, bromine, lithium, beryllium, and barium. There's holmium and helium and hafnium and erbium, And phosphorus and francium and fluorine and terbium, And manganese and mercury, molybdenum, magnesium, Dysprosium and scandium and cerium and cesium. And lead, praseodymium, and platinum, plutonium, Palladium, promethium, potassium, polonium, And tantalum, technetium, titanium, tellurium, And cadmium and calcium and chromium and curium. There's sulfur, californium, and fermium, berkelium, And also mendelevium, einsteinium, nobelium, And argon, krypton, neon, radon, xenon, zinc, and rhodium, And chlorine, carbon, cobalt, copper, tungsten, tin, and sodium. These are the only ones of which the news has come to Ha'vard, And there may be many others, but they haven't been discavard. Tom Lehrer, The Elements

Assignments: Problem Set #2 due Thursday 4 February 5pm

In Class: ------------------------- review: how atoms interact with light - individually, the can accept energy (aka photons) _only_ if they can accomodate it - change in energy budget of the atom - new, higher energy state must be stable - quantum mechanics says that there are only a few stable configurations for any atom --> other configurations can't happen (or equivalently, fall apart really quickly) - the number of different energy levels that an atom can stably occupy determines what kinds of photons it can absorb - can only accept photons whose energies will take the atom from it's current (stable) state to another stable state --> can accept photons whose energies equal the difference in energy between stable states staircase model - large collection of atoms - consider each atom as a ball on one tread of a staircase - each atom's position is a measure of the energy it has - some high energy atoms, some low energy atoms - consider one atom in particular - if you add just the right amount of energy to it, it can move up to the next tread - not quite right? --> nothing happens Absorption spectra: - spectrally-smooth, or continuous, emission passes through a cloud of these atoms -- specific photons are selected out -- the ones with energy = the difference in allowed energy states of the atom -- i.e., just enough energy to allow the electron to jump up to the next state -- the rest of the photons just cruise on through something like a filter -- some light is removed, but not all - orange filter lets through orange light - absorbs or scatters other colros - green liquid lets through green light - absorbs or scatters other colros - Neon filter substracts out photons with energies corresponding to differences in stable energy levels of the Neon atom (hard to make in a classroom; not so hard in outer space) --> RESULT: continuous spectrum with "bites" taken out at a few specific wavelengths, the photons have been removed >>>absorption lines<<< ----------------------------------- Emission spectra: - basically the same thing, only sort of backwards - emission of photon requires a lowering of the energy state of an atom -- electron moves closer to nucleus -- change in energy is the same as in absorption -- wavelength of photon is the same - instead of a smooth spectrum with bites taken out -- see only the bites -- i.e., emission at a few specific wavelengths -- and NO EMISSION at any other wavelengths - does not require a continuous spectrum shining through -- only need that for absorption so there's something to absorb -- instead, you just need a mechanism for getting the atoms into higher energy states so they can lower their energy and emit photons -- one way to do this is with collisions -- hit atoms hard enough to knock the electrons around -- some will get popped up into higher states -- when they try to get back down to the lower energy state (i.e., closer to the nucleus) they'll emit photons with wavelength = difference between energy states >>>>demo -- He discharge tube and diffration gratings<<<< spectral lines as fingerprints for atoms atoms differ from one another in how many protons, neutrons, and electrons they contain -- H: one P, one e- --> very simple -- He: two p, two n, two e- --> a little more complicated -- C: six p, six n, six e- --> a lot more complicated the different constituents of different atoms lead to differences in an atom's stable electronic energy levels -- the energy change between adjacent levels changes -- that means the energy required to get from one level to another is different for different atoms --> the wavelength of the photon absorbed/emitted will be different for different atoms i.e, atoms have different favorite photons use this as an ID procedure for atoms -- look at the He tube -- see blue, green, yellow, and red lines 400, 510, 590, 680nm -- wherever you see this pattern of lines, there must be glowing He -- this holds true for absorption, too -- wherever you see this pattern of absorption lines you must be looking at a continuous source through a veil of He ------------------- when you see spectral line emission vs. blackbody emission we've talked about two ways in which matter interacts with light way 1: blackbody emission only temperature is important creates smooth "continuous" spectrum emission over a wide range of wavelengths way 2: spectral line emission actual atomic composition is important different materials under the same conditions will emit different spectra creates discrete "line" spectrum the key element which determines whether a chunk of stuff emits in spectral line or blackbody mode is DENSITY high density --> blackbody emission low density --> spectral line emission Why? at high densities, atoms are crowded together in a solid, where the motion of atoms is constrained by neighbors even in a high pressure gas atoms can move around, but they're alway bumping into other atoms their structure is influenced by neighboring atoms electron orbits are screwed up a bit don't get to settle down to the stable states discussed above instead of a few stable states and a bunch of unacceptable or unstable ones, there are a large number of sort of stable, or meta-stable states therefore, transitions can occur between a large number of different energy levels, and a wide range of photons can be produced --> RESULT: broad spectrum of emission the rate at which atoms are jostled in a dense environment is governed by the temperature, and so T plays a key role in determining the structure of the broad spectrum --> RESULT: BB emission as we understood from previous lectures for high density stuff at low densities, (as in some gases) atoms don't collide as often their structure isn't disrupted very often at all they can settle down to the discrete energy level structure we described today and last time BOTTOM LINE: high density --> blackbody emission low density --> spectral line emission