EP225 Lecture 31 Quantum Mechanical E ects 1 Why the Hydrogen Atom Is Stable In the classical model of the hydrogen atom, an electron revolves around a proton at a radius r = 5:3 10 11 m (Bohr radius) and velocity v = 2:2 10 6 m/sec so that the centrifugal force mv 2 =r is counterbalanced by the Coulomb force e 2 =4" 0 r 2 mv 2 r = e2 4" 0 r 2 : The acceleration on the electron is tremendously large, a = v2 r = 9 1022 m/s 2 and the electron should lose its energy to radiation at a rate P = (ea)2 6" 0 c 3 = 2:9 1011 ev/s. If such radiation were allowed, the orbit radius of the electron decreases very quickly (in 50 pico seconds!). The life time of the hydrogen atom should be very short. Of course, this conclusion is absurd, for the hydrogen atom is known to be very stable. The classical picture has thus to be modi ed. Classical picture of a hydrogen atom. In quantum mechanical picture, it is postulated that the orbit radius of r = 5:3 10 11 m is the possible minimum radius an electron can acquire in the hydrogen atom, where angualr momentum rmv & h 2 = ~ h = 6:63 10 34 J s = 4:14 10 15 ev s 1
is Planck constant. We will return to this important subject later. In quantum mechanics, the concept of well de ned particle position and velocity has to be abandoned. Discrete frequencies are frequently observed in nature. For example, standing waves in a clamped string are possible only for discrete oscillation frequencies. Such observation strongly suggests that discrete radiation frequencies from hydrogen atoms should have something to do with standing waves. Bohr s Model of Hydrogen Atom In the 19th century, spectroscopy as a means to study light emission from various gas discharges (plasmas) had been well established. The invention of the grating spectrometer and advancement in vacuum techniques played key roles in spectroscopic studies. Light emission from hydrogen gas was investigated in particular detail and the data accumulated prepared a bed for the birth of quantum mechanics. It had been well established that radiation spectrum from hydrogen gas was discrete and the radiation frequency obeyed the following empirical formula, 1 1 = const. n 2 m 2 where n and m are nonzero integers. The observed wavelengths in nm are: m n = 1 Lyman n = 2 Balmer n = 3 Paschen 2 122 3 103 658 (H-alpha) 4 98 488 (H-beta) 1881 5 95 435 1285 6 94 411 1097 and corresponding energy levels based on Bohr s model are shown. Hydrogen atom energy levels. 2
In 1913, Bohr proposed a radical idea that the angular momentum of the electron in the hydrogen atom is quantized, r n mv n = n~ n = 1 2 3 Then, the orbit radius is also quantized as seen from the force balance equation, mvn 2 = e2 r n 4" 0 rn 2 (mr n v n ) 2 = (n~) 2 = me2 r n 4" 0 ~ 2 r n = 4" 0 me 2 n2 = 5:3 10 11 (m) n 2 : The total energy of a hydrogen atom with electron orbit r n is E n = 1 2 mv2 n e 2 4" 0 r n = 1 n 2 e 2 8" 0 r 1 where ~ 2 r 1 = 4" 0 me = 5:3 10 11 m, 2 is the orbit radius of the ground state (n = 1): The negative energy E n < 0 means that an energy je n j must be given to the atom to dismantle the atomic bondage due to Coulomb force. In particular, for n = 1 E 1 = e 2 8" 0 r 1 = 13:6 ev, which means that an energy of 13.6 ev or more must be given to a hydrogen atom at the ground state to liberate the electron from the proton. This energy is known as the ionization potential (energy) of hydrogen. The discrete radiation spectrum from hydrogen discharge can now be understood as follows. In a gas discharge, ionization of a neutral gas forms a plasma in which electrons and ions coexist. The discharge itself is maintained by electron bombardment. Electrons acquire energy from the electric eld externally applied. In normal discharges with an electron temperature of order 1 ev (' 11600 K), not all hydrogen atoms are ionized and most atoms remains neutral but with an energy higher than at the ground state. This process is called excitation and provides a basic mechanism for laser emission from gases. Excited atoms are unstable because the electron has room to fall to a lower energy state. If it falls from m-th to n-th energy state, the energy di erence 1 1 E mn = 13:6 ev, m > n n 2 m 2 3
is carried o by radiation (photon) at a frequency = E mn h (Hz). Example. (a) What is the energy of a hydrogen atom excited to n = 3 state? (b) What is the electron orbit radius of the state? (c) If the atom is deexcited to n = 2 energy level, what is the wavelength of emitted radiation? Solution (a) The energy level of n = 3 state is (b) The orbit radius is E 3 = 13:6 3 3 = 1:5 ev. r 3 = n 2 r 1 = 9 5:3 10 11 m = 4:8 10 10 m. (c) The energy di erence is E = 1 1 13:6 ev 4 9 = 1:9 ev. The frequency of photon is f = E h = 1:9 1:6 10 19 J 6:63 10 34 J sec = 4:6 10 14 Hz. The wavelength is = c f = 658 nm. Photoelectric E ect 4
(a). Experimental setup of photoelectric e ect. (b). Current vs. voltage in Hertz s experiment. V s is the stopping potential, ev s = 1 2 mv2 max = h W: In 1886, Hertz discovered that certain metals emit electrons when illuminated by light. The electron was identi ed much later in 1900 by Thomson and at the time of Hertz s experiment, it was not clear what was really carrying electric current which was the quantity measured in the experiment. It is shown schematically in Fig. 3 (a). When light is on, the tube becomes a diode with the illuminated metal plate acting as an electron emitting cathode. The current in the tube disappears when light is o or else short wavelength (ultraviolet) components are ltered out. (In the demonstration used in the lecture, recall that if a glass plate is placed in front of the mercury lamp (strong UV source), nothing happened. Glass lters out ultraviolet light e ectively.) The amount of current decreases as the negative voltage increases in magnitude and eventually vanishes at the potential called the stopping potential V s as shown in Fig. 3 (b): This may be understood if emitted electrons have a kinetic energy and at su ciently large potential, all electrons are repelled back to the metal plate. A successful theoretical explanation for Hertz s observation of photoelectric e ect was 5
given by Einstein in 1905 in terms of energy conservation, h = 1 2 mv2 + W where 1 2 mv2 is the electron kinetic energy and W is the potential energy which keeps electrons in the metal under normal conditions. If an electron acquires a su cient energy from the photon to overcome the potential energy, it is liberated from the metal surface. The work function W is a constant for a given metal and normally ranges from 2 to 5 ev. Note that the kinetic energy of electrons 1 2 mv2 = h W is the possible maximum energy an electron an acquire. Some electrons are emitted with smaller energies between 0 and h W. The stopping potential is the potential to repel all electrons. Therefore, ev s = h W or W = h ev s which allows one to measure the work function W for a given photon energy. Einstein s explanation is based on energy conservation. What about momentum conservation? The momentum of photon is p p = h c = h in analogy to the momentum of electromagnetic wave (actually any wave), wave momentum = energy : c The photon pushes the metal with the momentum p p : Electrons also pushes the metal through recoil, and the total momentum given to the metal is p p + mv = h + mv: However, the recoil velocity of a metal plate is negligibly small under normal circumstance and momentum conservation does not a ect the basic equation of photoelectric e ect. (The situation is similar to re ection analysis of electromagnetic waves at an impedance discontinuity. The formula such as E r = Z 2 Z 1 Z 2 + Z 1 E i has been derived through energy conservation alone.) Example 2. Sodium is illuminated by light wave having = 450 m. What are the maximum electron energy emitted, stopping potential and cuto wavelength? The work function of sodium is 2.2 ev. Solution From 1 2 mv2 = h W = h c W = 4:14 10 15 ev sec 3 108 m/sec 450 10 9 m = 2:76 2:2 ev = 0:56 ev. 6 2:2 ev
The stopping potential is 0.56 V. The cuto wavelength to cause photoelectric e ect is = hc W = 4:14 10 15 3 10 8 2:2 m = 5:6 10 7 m. 7