Sr Lab in Modern Physics
Sr Lab in Modern Physics PHYS 407
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This 3 page Class Notes was uploaded by Sonny Breitenberg on Monday September 28, 2015. The Class Notes belongs to PHYS 407 at George Mason University taught by Staff in Fall. Since its upload, it has received 7 views. For similar materials see /class/215191/phys-407-george-mason-university in Physics 2 at George Mason University.
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Date Created: 09/28/15
A Conceptual Tour of TeachSpin s Optical Pumping Optical Pumping allows us to examine the phenomenon of Zeeman Splitting a spreading of atomic energy sublevels in the presence of a magnetic eld This experiment uses two isotopes of rubidium gas Rb85 and Rb 87 buffered with neon Light given off by a rubidium lamp is first collimated into a beam by a 50 cm focal length lens and then passed through a lter which transmits only the wavelength which excites an atom from the 812 to the PM energy level This light has a wave length of 7948nm and carries an energy of 156eV The beam proceeds through a 45 degree linear polarizer followed by a 1A wave plate This creates circularly polarized light each photon of which carries one unit of angular momentum The circularly polarized light is sent through a cell containing the two isotopes of rubidium gas The cell is heated to keep the rubidium vaporized and the temperature can be both monitored and varied The usual operating temperature is 500 C Of course the effect of temperature on optical pumping can be examined The light that has passed through the sample cell is again collimated by a lens and aimed into a photodetector The intensity of the light reaching the detector is transformed into a voltage which can be displayed on an oscilloscope As the photons in the beam of light pass through the gas they are absorbed by the rubidium atoms causing transitions in the atoms from the 812 to P1 2 levels As the atoms return to the lower energy state the light is reradiated in all directions not just in the direction of the beam The intensity of the beam emerging from the sample cell is significantly less than the intensity of the beam that entered If however the sample cell is in a magnetic field interesting things happen The 81 and PM energy levels subdivide each into 8 sublevels characterized by among other things a distinct quantum of angular momentum Because the circularly polarized light entering the gas contains one unit of angular momentum the selection rules demand that any transition from the 812 to the PM level be to an energy state with one more unit of angular momentum than the state it left Barbara L WolffReichert August 2002 ENERGY LEVELS FOR I 12 J 32 IN AN APPLIED MAGNETIC FIELD stream wwmc 15W mm M r t mm 1 Emma z 2 222 F mew WWWW ammo downstream 5 sum es mom a Energwegv mama 3 29quot 25 Ifyou examine the energy diagram shown in Figure 1 you can see that this constraint means that atoms from the F2 M2 level ofthe sin energy level cannot move into any ofthe Pm energy states Atoms that have moved from the 7 available sin to the various pm levels reradiate their absorbed energy and return equally to all 8 ground level sp2 sesn t t7t the pm and then in returning are spread amongst all ofthe sp2 levels the atoms in the gas quickly become pumped into that eighth sp2 level With disproportionate number ofthe atoms in the sin F2 M2 state the gas is no longer in the thermal equilibrium The number of atoms of gas which are in sin levels capable ofabsorbing the rubi um light has greatly de reased The entire sample becomes signi cantly more transparent The light beam emerging from the cell actually becomes brighter L 394 quot 39 quot r r39 pioce u tumin the onbeam 39 L very suddenly Initially the light reaching the photodetector would be relatively dim as the rubidium atoms ofthe gas absorbed the 7948 nm light In times on the order of milliseconds however the atoms would reach their nonthermal equilibrium state The gaswou 39 39 f equot 39 39 At t would increase signi cantly To begin our experiment the light beam through the Optical pumping apparatus is aligned along the nonhrsouth axis ofthe local eartth magnetic eld The set of quot l L l quot L quot 39 thembidium cellare A 39 39 eitieal r Ean L 39 9 cell TL39 39 the horizontal component will be of interest The large set ofHelmholtz coils with its axis along the light path creates ahorizontal eld opposite to that ofthe eanh We start our examination 39 39 39 39 C L quot L 39 39 r39 quot ithth Page 2 opucal Pumping Conceptual Tuur BarbaraWulffLRelcherL August znnz swept through zero andtncreases tn the atrectton of the earth39s magnettc eld u HAAp attuwt 5 I E m g t Intually because of the spltttang caused by the magnettc fleld alarge proportton of the atoms have been pumped tnto the SmF1M2 state and the gas ts transparent At ftrst As long as there ts a netmagnetac eld the gas rematns transparent At some potnt the magneuc eld The Zeeman levels collapse The atoms escape from the SmF1 We begtn to separate The atoms are agatn pumped tnto anonrthermal equlllbnum dtstrtbutaon wtth an tnordtnate proporuon tn the SmF1M2 state The gas agatn ts further as we tncrease the magnettc eld across the cell It bathes the cell tn photons wtth an energy tn thelo399 ev range the t F r l ts 2 9 x 1039 ev requlnng an rf stgnal of 71mm As the Zeeman levels separate due to the tncreastng magnettc eld we reach amoment between the Zeeman levels Thts resonant energy allows the atoms tn the pumped each wtth tts own charactertsttc way ofspreadlng outtts Zeeman levels The ftrst dlp ts Prge z Opttcrlpumptng Concepttunour Em uan Wowaetchert Augustznnz