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1105-Sel = 4 (i) N la #3574 LEYBOLD Atomic and Nuclear Physics Physies Atomic shell Leaflets P6.2.4.1 Franck-Hertz experiment Franck-Hertz experiment with mercury Recording with the oscilloscope, the XY-recorder and point by point Objects of the éxperimient \" -Jo-record:a Franck-Heitz.curve for.meréury. ‘ME To measure the discontinuous energy. etnission of free:electrons:for inelastic collision: To interpret the Theasurement results a8 representing disctéte enérgy absorption by mércury atoms. Principles In 1914, James Franck and Gustav Hertz reported an energy loss occurring in distinct “steps” for electrons passing through mercury vapor, and a corresponding emission at the ultraviolet line (\ = 254 nm) of mercury. Just a few months later, Niels Bohr recognized this as evidence confirming his model of the atom. The Franck-Hertz experiment is thus a classic experiment for confirming quantum theory. Ina previously evacuated glass tube, mercury atoms are held at a vapor pressure of about 15 hPa, which is kept constant by The electron current flowing to the collector as a function of the temperature control. This experiment investigates the energy acceleration voltage in the Franck-Hertz experiment with mercury loss of free electrons due to inelastic scattering, and thus due {schematic representation} to collision excitation of mercury atoms. a eeAU=E,, ! kAU--AU AU AU P6.2.4.1 LEYBOLD Physics Leaflets Apparatus 4 Franck-Hertztube,Hg ... 02.0.0. 555 85 KG,G,A 1 Socket for Franck-Hertz tube, Hg with multi-pin plug 2... 6... ee 555 861 1 Electric oven, 220V....0..... 555 81 if 4 Franck-Hertz supply unit... 6.6... 55588 1 Temperature sensor, NiCr-Ni . 1... 2. | 666 193 Recommended for optimizing the Franck-Hertz curve: 575211 $76 24 1 Two-channel oscilloscope 303... .... 2 Screened cables BNC/4 mm Recommended for recording the Franck-Hertz curve: 1 XY-Yt recorder SR 720 575 663 Connecting leads The glass tube contains a cylindrically symmetrical system of four electrodes (see Fig. 1). The cathode K is surrounded by a gtid-type control electrode G, at a distance of a few tenths of a millimeter, an acceleration grid G2 at a somewhat greater distance and finally the collector electrode A outermost. The cathode is heated indirectly, in order to prevent a potential differential along K. Electrons are emitted by the hot electrode and form a charge cloud. These electrons are attracted by the driving potential U, between the cathode and grid G1. The emission current is practically independent of the acceleration voltage U2 between grids Gy and Gy, if we ignore the inevitable punch-through. A braking voltage U3 is present between grid G2 and the collector A. Only electrons with sufficient kinetic energy can teach the collector electrode and contribute to the collector current, In this experiment, the acceleration voltage U2 is increased from 0 to 30 V while the driving potential U, and the braking voltage U; are held constant, and the corresponding callectar current fa is measured. This current initially Increases, much as in a conventional tetrode, but reaches a maximum when the kinetic energy of the electrons closely in front of grid Gz is just sufficient to transfer the energy required to excite the mercury atoms (Eig = 4.9 €V) through collisions. The collector current drops off dramatically, as after collision the electrons can no longer overcome the braking voltage Us. As the acceleration voltage U2 increases, the electrons attain the energy level required for exciting the mercury atoms at ever greater distances from grid Gz. After collision, they are accel- erated once more and, when the acceleration voltage is suffi- cient, again absorb so much energy from the electrical field that they can excite a mercury atom. The result is a second Thaximum, and at greater voltages & further maxima of the collector currents ia. dit Uy uU, U, U; Fig. 1: Schematic diagram of the mercury Franck-Hertz tube Preliminary remark The complete Franck-Hertz curve can be recorded manually. For a quick overview, e.g. for optimizing the experiment para- meters, we recommend using a two-channel oscilloscope. However, note that at a frequency of the acceleration valtage Up such as is required for producing a stationary oscilloscope pattern, capacitances of the Franck-Hertz tube and the holder become significant. The current required to reverse the charge of the electrode causes a slight shift and distortion of the Franck-Hertz curve. An XY-recorder is recommended for recording the Franck- Hertz curve, a) Manual measurement: = Set the operating-mode switch to MAN. and slowly in- crease U2 by hand from 0 V to 30 V, ~ Read voltage U2 and current /, from the display; use the selector switch to toggle between the two quantities for each voltage. b) Representation on the oscilloscope: ~- Connect output sockets U2/10 to channel I! (0.5 V/DIV) and output sockets Ua to channel | (2 V/DIV) of the oscillo- scope. Operate the oscilloscope in XY-mode. — Set the operating-mode switch on the Franck-Hertz supply unit to “Sawtooth”. - Set the Y-position so that the top section of the curve is displayed completely. ce) Recording with the XV-recorder: - Connect output sockets 4/2/10 to input X (0.2 Vicm CAL) and output sockets Ua to input Y (1 V/cm CAL) of the XY-recorder. — Set the operating-mode switch on the Franck-Hertz supply unit to RESET. P6.2.4.1 LEYBOLD Physics Leaflets B) Optimizing Uy: A higher driving potential Uy results in a greater electron emission current. 1 U=27,0V I, If the Franck-Hertz curve rises too steeply, i.e. the overdrive nA limit of the current measuring amplifier is reached at values a) below Up = 30 V and the top of the Franck-Hertz curve is cut 10 off (Fig. 35) - Reduce U; until the curve steepness corresponds to that shown in Fig. 3d. if the Franck-Hertz curve is too flat, i.e. the collector current fs, remains below § nA in all areas (see Fig. 3¢): — Increase U4 {max. 4.8 V) until the curve steepness corre- sponds to that shown in Fig. 3d. If the Franck-Hertz curve is flat even after increasing U,: — Reduce the set value 4 for the oven temperature using the screwdriver potentiometer. ¢) Optimizing Us: A greater braking voltage Uy causes better-defined maxima and minima of the Franck-Hertz curve; at the same time, however, the total collector current is reduced. If the maxima and minima of the Franck-Hertz curve are insuffi- ciently defined (see Fig. 3d): — Alternately increase first the braking voltage U3 (maximum 4.5 V) and then the criving potential U; until you obtain the curve form shown in Fig, 31. Ifthe minima of the Franck-Hertz curve are cut off at the bottam (see Fig. 3¢): - Alternately reduce first the braking voltage Us (maximum 4.8 V) and then the driving potential U1 until you obtain the curve form shown in Fig. 3f. Carrying out the experiment — Record the Franck-Hertz curve (see preliminary remark). - To better demonstrate the first maxima, yor can increase the sensitivity of the Y-input and repeat the recording process. Measuring example and evaluation th =1.58V Uy =3.95V 95 = 180°C In Fig, 4, the average of the intervals between the successive maxima gives us the value AU2 = 5.1V, This corresponds to an energy transfer of AE=5.1eV U,=21,8V b), U=N6V > 10 20 30 U, Vv Fig. 4: a) Franck-Hertz curve of mercury (recorded with X¥-recorder) b) Curve section, with ordinate enlarged five times We can compare this value with the literature value Fug = 4.9 eV for the transition energy of the mercury atoms from the ground state 'Sq to the first 3P, state. The kinetic energy of the electrons at grid G2 can be calculated as Ekin = (Uy + Up) On the basis of this, we would expect the first maximum of the collector current at Uy + U2 = 4.9 V. In fact, the first maximum is not registered until U; + U> = 8.1 V. The difference between the two values is the effective contact potential between cathode K and grid Gp. Supplementary information A number of factors influence the effective contact potential, The most important of these deserve mention here. The actuat contact patential is caused by the different work of emission of the cathode and grid materials, The emission properties of the mixed-oxide cathode and the gas charge resp. the mercury coating of the grid play an important role here. The electrons emitted by the hot cathode have an initial veloc- ity which depends on the temperature of the cathode. LEYBOLD DIDACTIC GMBH - Leyboldstrasse 1 - D-80354 Hurth . Phone (02233) 604-0 - Telefax (02233) 604-222 - Telex 17 223 232 LHPCGN D © by Leybold Didactic GmbHt Printed in the Federal Republic of Germany Technical alterations reserved 
