Chapter 3: Quantum Nature of Light and Matter

Chapter 3 Quantum Nature of Light and Matter We understand classical mechanical motion of particles governed by New­ ton’s law. In the last chapter we examined in some detail the wave nature of electromagnetic fields. We understand the occurance of guided traveling modes and of resonator modes. There are characteristic dispersion relations or resonance frequencies associated with that. In this chapter, we want to summarize some experimental findings at the turn of the 19th century that ultimately lead to the discover of quantum mechanics, which is that matter has in addition to its particle like properties wave properties and electromag­ netic waves have in addition to its wave properties particle like properties. As turns out the final theory, which will be developed in subsequent chapters is much more than just that because the quantum mechanical wave function has a different physical interpretation than a electromagnetic wave only the mathematical concepts used is in many cases very similar. However, this is a tremendous help and guideance in doing and finally understanding quantum mechanics. 3.1 Black Body Radiation In 1900 the physicist Max Planck found the law that governs the emission of electromagnetic radiation from a black body in thermal equilibrium. More specifically Planck’s law gives the energy stored in the electromagntic field in a unit volume and unit frequency range, [f, f + df ] with df = 1Hz , when the CHAPTER 3. QUANTUM NATURE OF LIGHT AND MATTER electromagnetic field is in thermal equilibrium with its surrounding that is at temperature T. A black body is simply defined as an object that absorbs all light. The best implementation of a black body is the Ulbricht sphere, see Figure 3.1. Figure 3.1: The Ulbricht sphere, is a sphere with a small opening, where only a small amount of radiation can escape, so that the interior of the sphere is in thermal equilibrum with the walls, which are kept at a constant tremperature. The inside walls are typically made of diffuse material, so that after multiple scattering of the walls any incoming ray is absorbed, i.e. the wall opening is black. Figure 3.2 shows the energy density w(f) of electromagnetic radiation in a black body at temperature T . Around the turn of the 19th century w(f ) was measured with high precision and one was able to distinguish between various approximations that were presented by other researchers earlier, like the Rayleigh-Jeans law and Wien’s law, which turned out to be asymptotic approximations to Planck’s Law for low and high frequencies. In order to find the formula describing the graphs shown in Figure 3.2 Planck had to introduce the hypothesis that harmonic oscillators with fre­ quency f can not exchange arbitrary amounts of energy but rather only in discrete portions, so called quanta. Planck modelled atoms as classical oscillators with frequency f . Therefore, the energy of an oscillator must be quantized in energy levels corresponding to these energy quanta, which he 3.1. BLACK BODY RADIATION found to be equal to hf, where h is Planck’s constant h = 6.62620 ± 5 · 10−34 Js. (3.1) Black Body Energy Density w(f), fJs/m 3 1.0 Rayleigh-Jeans (T=5000K) Wien (T=5000K) 0.8 0.6 T=5000K 0.4 T=4000K T=3000K 0.2 T=2000K 0.0 0 2 4 6 Frequency f, 100THz 8 10 Figure 3.2: Spectral energy density of the black body radiation according to Planck’s Law. As a model for a black body we use now a cavity with perfectly reflecting walls, somewhat different from the Ulbricht sphere. In order to tap of a small but negligible amount of radiation from the inside, a small opening is in the wall. We can make this opening so small that it does essentially not change the internal radiation field. Then the radiation in the cavity is the sum over all possible resonator modes in the cavity. If the cavity is at temperature T all the modes are thermally excited by emission and absorption of energy quanta from the atoms of the wall. For the derivation of Planck’s law we consider a cavity with perfectly conducting walls, see Figure 3.3. CHAPTER 3. QUANTUM NATURE OF LIGHT AND MATTER Figure 3.3: (a) Cavity resonator with metallic walls. (b) Resonator modes characterized by a certain k-vector. If we extend the analysis of the plan parallel mirror waveguide to find the TE and TM modes of a three-dimensional metalic resonator, the resonator modes are TEmnp − and THmnp −modes characterized by its wave vector com­ ponents in x−, y−, and z−direction. The resonances are standing waves in three dimensions kx = mπ nπ pπ , ky = , kz = , for m, n, p = 0, 1, 2, ... Lx Ly Lz (3.2) An expression for the number of modes in a frequency interval [f, f + df ] can be found by recognizing that this is identical to the number of points in Figure 3.3(b) that are in the first octant of a spherical shell with thickness dk at k = 2πf /c.The volume occupied by one mode in the space of wave 3 numbers k is ∆V = Lπx · Lπy · Lπz = πV with the volume V = Lx Ly Lz . Then the number of modes dN in the frequency interval [f, f + df ] in volume V are dN = 2 · k2 dk 4πk2 dk = V , 3 π2 8 πV (3.3) where the factor of 2 in front accounts for the two polarizations or T E and T H-modes of the resonator and the 8 in the denominator accounts for the fact that only one eighth of the sphere, an octand, is occupied by the positive wave vectors. With k = 2πf /c and dk = 2πdf/c, we obtain for the number of modes finally 3.1. BLACK BODY RADIATION 8π 2 f df (3.4) c3 Note, that the same density of states is obtained using periodic boundary conditions in all three dimensions, i.e. then we can represent all fields in terms of a three dimensional Fourier series. The possible wave vectors would range from negative to positive values 2mπ 2nπ 2pπ , ky = , kz = f or m, n, p = 0, ±1, ±2. . . . (3.5) kx = Lx Ly Lz dN = V However, these wavevectors fill the whole sphere and not just one 8-th, which compensates for the 8-times larger volume occupied by one mode. If we imply periodic boundary conditions, we have forward and backward running waves that are independent from each other. If we use the boundary conditions of the resonator, the forward and backward running waves are connected and not independent and form standing waves. One should not be disturbed by this fact as all volume properties, such as the energy density, only depends on the density of states, and not on surface effects, as long as the volume is reasonably large. 3.1.1 Rayleigh-Jeans-Law The excitation amplitude of each mode obeys the equation of motion of a harmonic oscillator. Therefore, classically one expects that each of mode is in thermal equilibrium excute with a thermal energy kT according to the equipartition theorem, where k is Boltzmann’s constant with k = 1.38062 ± 6 · 10−23 J/K. (3.6) If that is the case the spectral energy density is given by the Rayleigh-JeansLaw, see Figure 3.2. 1 dN 8π w(f ) = kT = 3 f 2 kT. (3.7) V df c As can be seen from Figure 3.2, this law describes very well the black body radiation for frequencies hf ¿ kT but there is an arbitrary large deviation for high frequencies. This formula can not be correct, because it predicts infinite energy density for the high frequency modes resulting in an "ultravi­ olet catastrophy", i.e. the electromagnetic field contains an infinite amount of energy at thermal equilibrium. CHAPTER 3. QUANTUM NATURE OF LIGHT AND MATTER 3.1.2 Wien’s Law The high frequency or short wavelength region of the black body radiation was first empirically described by Wien’s Law 8πhf 3 −hf /kT e . (3.8) c3 Wien’s law is surprisingly close to Planck’s law, however it slightly fails to correctly predicts the asympthotic behaviour at low frequencies or long wave­ lengths. w(f) = 3.1.3 Planck’s Law In the winter of 1900, Max Planck found the correct law for the black body radiation by assuming that each oscillator can only exchange energy in dis­ crete portions or quanta. We rederive it by assuming that each mode can only have the discrete energie values Es = s · hf, for s = 0, 1, 2, ... (3.9) Thus s is the number of energy quanta stored in the oscillator. If the oscillator is a mode of the electromagnetic field we call s the number of photons. For the probability ps , that the oscillator has the energy Es we assume a Boltzmann­ distribution µ ¶ µ ¶ Es 1 hf 1 ps = exp − = exp − s , (3.10) Z kT Z kT where Z is a normalization factor such that the total propability of the os­ cillator to have any of the allowed energy values is ∞ X ps = 1. (3.11) s=0 Note, due to the fact th∠t the oscillator energy is proportional to the number of photons, the statistics are exponential statistics. From Eqs.(3.10) and (3.11) we obtain for the normalization factor ¶ µ hf 1 ¡ hf ¢ , Z= exp − s = kT 1 − exp − kT s=0 ∞ X (3.12) 3.1. BLACK BODY RADIATION which is also called the partition function. The photon statistics are then given by µ ¶ ∙ µ ¶¸ Es hf −1 ps = exp − (3.13) 1 − exp − kT kT hf or with β = kT X 1 1 e−βs , with Z (β) = ps = e−βs = . Z(β) 1 − e−β s=0 ∞ (3.14) Given the statistics of the photon number, we can compute moments of the probability distribution, such as the average number of photons in the mode ∞ ­ 1® X s1 ps . s = (3.15) s=0 This first moment of the photon statistics can be computed from the partition function, using the "trick" which is ­ 1® s = 1 ∂1 −β , 1 Z(β) = Z(β) e Z(β) ∂ (−β) hsi = 1 exp hf kT −1 . (3.16) (3.17) With the average photon number hsi , we obtain for the average energy stored in the mode hEs i = hsi hf, (3.18) and the energy density in the frequency intervall [f, f + df ] is then given by w (f ) = hEs i dN . V df (3.19) With the density of modes from Eq.(3.4) we find Planck’s law for the black body radiation 8π f 2 hf w (f ) = , (3.20) hf 3 c exp kT −1 which was used to make the plots shown in Figure 3.2. In the limits of low and high frequencies, i.e. hf ¿ kT and hf À kT , respectively Planck’s law asympthotically approaches the Rayleigh-Jeans law and Wien’s law. CHAPTER 3. QUANTUM NATURE OF LIGHT AND MATTER 3.1.4 Thermal Photon Statistics It is interesting to further investigate the intensity fluctuations of the thermal radiation emitted from a black body. If the wall opening in the Ulbricht sphere, see Figure 3.1, is small enough very little radiation escapes through it. If the Ulbricht sphere is kept at constant temperature the radiation inside the Ulbricht sphere stays in thermal equilibrium and the intensity of the radiation emitted from the wall opening in a frequency interval [f, f + df ] is I (f ) = c · w (f ) . (3.21) Thus the intensity fluctuations of the emitted black body radiation is directly related to the photon statistics or quantum statistics of the radiation modes at freuqency f , i.e. related to the stochastic variable s : the number of photons in a mode with frequency f . This gives us direclty experimental access to the photon statistics of an ensemble of modes or even a single mode when proper spatial and spectral filtering is applied. Using the expectation value of the photon number 3.17, we can rewrite the photon statistics for a thermally excited mode in terms of its average photon number in the mode as hsis 1 ps = s+1 = (hsi + 1) (hsi + 1) µ hsi (hsi + 1) ¶s , (3.22) The thermal photon statistics display an exponential distribution, see Figure 3.4. Before we move on, lets see how the average photon number in a given mode depends on temperature and the frequency range considered. Figure 3.5 shows the relationship between average number of photons in a mode with frequency f or wavelength λ and temperature T. 3.1. BLACK BODY RADIATION Image removed for copyright purposes. Figure 3.4: Photon statistics of a mode in thermal equilibrium with a mean photon number < s >= 10 (a) and < s >= 1000 (b). Image removed for copyright purposes. Figure 3.5: Average photon number in a mode at frequency f or wavelength λ and temperature. Figure 3.5 shows that at room temperature and micorwave frequencies CHAPTER 3. QUANTUM NATURE OF LIGHT AND MATTER large numbers of photons are present due to the thermal excitation of the mode. This is the reason that at room temperature the thermal noise over­ whelms eventual quantum fluctuations. However, quantum fluctuations are important at high frequencies, which start for room temperature in the far to mid infrared range, where on average much less than one photon is thermally excited. The variance of the photon number distribution is ­ ® σ 2s = s2 − hsi2 . (3.23) By generalizing Eq.(3.12) to the m-th moment by replacing the exponent 1 by m m hs i = = ∞ X sm ps (3.24) s=0 1 ∂m Z (β) , Z ∂ (−β)m we obtain for the second moment ­ 2® ­ ® s = 2Z (β) 2 e−2β − Z (β) 2 e−2β = 2 s2 + hsi . (3.25) (3.26) and therefore for the variance of the photon number using Eq.(3.23) is σ 2s = hsi2 + hsi . (3.27) As expected from the wide distribution of photon numbers the variance is larger than the square of the expectation value. This means that if we look at the light intensity of a single mode the intensity is subject to extremly strong fluctuations as large as the mean value. So why don’t we see this rapid thermal fluctuations when we look at the black body radiation coming, for example, from the surface of the sun? Well we don’t look at a single mode but rather at a whole multitude of modes. Even when we restrict us to a certain narrow frequency range and spatial direction, there is a multitude of transverse modes presence. We obtain for the average total number of photons in a group of modes and its variance hstot i = σ 2tot = N X i=1 N X i=1 hsi i , (3.28) σi2 . (3.29) 3.1. BLACK BODY RADIATION Since these modes are independent identical systems, we have (3.30) hstot i = N · hsi . ¡ ¢ 1 σ 2tot = N hsi2 + hsi = hstot i2 + hstot i . N Due to the averaging over many modes, the photon number fluctuations in a large number of modes is reduced compared to its mean value SNR = σ 2tot 1 1 + . 2 = N hstot i hstot i (3.31) Thus if one averages over many modes and has many photons in these modes the intensity fluctuations become small. 3.1.5 Mode Counting It is interested to estimate the number of modes one is averaging over given a certain emitting surface and a certain measurement time, see Figure 3.6. x Lx Ωc As Ac k AD z y Lz Ly Figure 3.6: Counting of longitudinal and transverse modes excited from a radiating surface of size As . If the area As is emitting light, it will couple to the modes of the free field. To count the modes we put a large box (universe) over the experi­ mental arrangement under consideration. The emitting surface is one side of the box. The light from this surface, i.e. specifying the transverse electric and magnetic fields, couples to the modes of the universe with wave vectors according to Eq.(3.5). CHAPTER 3. QUANTUM NATURE OF LIGHT AND MATTER Longitudinal Modes The number of longitudinal modes, that propagate along the positive zdirection in the frequency interval ∆f can be derived from ∆k = (2π/Lz ) ∆N and ∆k = (2π/c0 ) ∆f Lz ∆N = ∆f, (3.32) c0 or using the propagation or measurement time over which the experiment extends τ = Lz /c0 , (3.33) we obtain for the number of longitudinal modes that are involved in the measurement that is carried out over a time intervall τ and a frequency range ∆f ∆N = τ ∆f . (3.34) Transverse Modes The free space modes that arrive at the detector area AD will not only have wave vectors with a z−component, but also transverse components. Lets as­ sume that the detector area is far from the emitting surface, and we consider only the paraxial plane waves. The wave vectors of these waves at a given frequency or free space wave number k0 can be approximated by kmn = µ 2πm 2πn , , k0 Lx Ly ¶ with m, n = 0, 1, 2, ... (3.35) where m and n are transverse mode indices. Then one mode occupies the volume angle 4π 2 , Lx Ly k02 = λ20 /As . Ωc = (3.36) If the modes are thermally excited, the radiation in individual modes is uncorrelated. Therefore, if there is a detector at a distance r then only the field within an area Ac = r2 Ωc , (3.37) 3.2. PHOTO-ELECTRIC EFFECT is correlated. If the photodetector has an area Ad , then the number of transverse modes detected is Nt = Ad /Ac. (3.38) The total number of modes detected is Ntot = Ad τ ∆f = Ac Ad As τ ∆f . r2 λ20 (3.39) Note, that there is perfect symmetry between the area of the emitting and receiving surface. The emitter and the receiver could both be black bodies. If one of them is at a higher temperature than the other, there is a net flow of energy from the warmer body to the colder body until equilibrium is reached. This would not be possible without interaction over the same number of modes. Thus the formula which is completely unrelated to thermodynamics is necessary to fulfill one of the main theorems of thermodynamcis, that is that energy flows from warmer to colder bodies. 3.2 Photo-electric Effect Another strong indication for the quantum nature of light was the photoelec­ tric effect by Lenard in 1903. He discovered that when ultra violet light is radiated on a photo cathode electrons are emitted, see Figure 3.7. Figure 3.7: Photo-electric effect: (a) Schematic setup and (b) dependence of the necessary grid voltage to supress the electron current as a funtion of light frequency. CHAPTER 3. QUANTUM NATURE OF LIGHT AND MATTER Lenard surrounded the photo cathode by a grid, which is charged by the emitted electrons up to a voltage U, which blocks the emission of further electrons. Figure 3.7 shows the blocking voltage as a function of the fre­ quency of the incoming light. Depending on the cathode material there is a cutoff frequency. For lower frequencies no electrons are emitted at all. This frequency as well as the blocking voltage does not depend on the intensity of the light. In 1905, this effect was explained by Einstein introducing the quantum hypothesis for radiation. According to him, each electron emission is caused by a light quantum, now called photon. This photon has an energy hf and this quantum energy must be larger than the work function We of the material. The remainder of the energy me v2 /2 is transfered to the electron in form of kinetic energy. The resulting energy balance is 1 hf = We + me v 2 2 (3.40) The kinetic energy of the electron can be used to reach the grid surrounding the photo cathode until the charging energy due to the grid potential is equal to the kinetic energy of the electrons 1 1 eU = me v2 hf = We + me v 2 2 2 (3.41) or 1 −U = (hf − We ), for hf > We . (3.42) e This equation explains the empirically found law by Lenard explaining the cutoff frequency and the charge buildup as a function of light frequency. Einstein was first to introduce the idea that the electromagnetic field contains light quanta or photons. 3.3 Spontaneous and Induced Emission The number of photons in a radiation mode may change via emission of photon into the mode or absorption of a photon from the mode by atoms, molecules or a solid state material. Einstein introduced a phenomenological theory of these processes in order to explain how matter may get into thermal equilibrium by interaction with the modes of the radiation field. He consid­ ered the interaction of a mode with atoms modeled by two energy levels E1 and E2 , see Figure 3.8. 3.3. SPONTANEOUS AND INDUCED EMISSION Figure 3.8: Energy levels of a two level atom and populations. n1 and n2 are the population densities of these two levels considering a whole ensemble of these atoms. Transitions are possible in the atom between the two energy levels by emission of a photon at a frequency f= E2 − E1 h (3.43) Absorption of a photon is only possible if there is energy present in the radiation field. Einstein wrote for the corresponding transition rates, which should be proportional to the population densities and the photon density at the transition frequency ¯ ¯ dn2 ¯¯ dn1 ¯¯ = = B12 n1 w(f21 ). (3.44) − dt ¯Abs dt ¯Abs The coefficient B12 characterizes the absorption properties of the transition. Einstein had to allow for two different kind of processes for reasons that be­ come clear a little later. Transitions induced by the already present photons or radiation energy as well as spontaneous transitions ¯ ¯ dn1 ¯¯ dn2 ¯¯ =− =B21 n2 w (f21 ) + A21 n2 . (3.45) dt ¯Em dt ¯Em The coefficient B21 describes the induced and A21 the spontaneous emissions. The latter transitions occur even in the absence of any radiation and the corresponding coefficient determines the lifetime of the excited state τ sp = A−1 21 , (3.46) CHAPTER 3. QUANTUM NATURE OF LIGHT AND MATTER in the absence of the radiation field. The total change in the population densities is due to both absorption and emission processes ¯ ¯ dni dni ¯¯ dni ¯¯ = + , for i = 1, 2 (3.47) dt dt ¯Em dt ¯Abs Using Eqs.(3.44) and (3.45) we find − dn2 dn1 = = (B12 n1 − B21 n2 ) w (f21 ) − A21 n2 . dt dt (3.48) In thermal equilibrium the energy density of the radiation field must fulfill the condition A21 /B12 w (f21 ) = , (3.49) n1 /n2 − B21 /B12 while the atomic ensemble itself should also be in thermal equilibrium which again should be described by the Boltzmann statistics, i.e. the ratio between the population densities are determined by the Boltzmann factor µ ¶ E2 − E1 . (3.50) n2 /n1 = exp − kT And with it the energy density of the radiation field must be w (f21 ) = A21 /B12 ¡ hf21 ¢ . exp kT − B21 /B12 (3.51) A comparison with Planck’s law, Eq.(3.20), gives B21 = B12 , (3.52) and 3 8π hf21 B12 . (3.53) c3 Clearly, without the spontaneous emission process it is impossible to arrive at Planck’s Law in equilibrium. The spectral energy density of the radiation field can be rewritten with the average photon number in the modes at the transition frequency f21 as A21 = w (f21 ) = 2 8π f21 1 . hf21 hsi , with hsi = hsi = hf21 3 c exp kT − 1 (3.54) 3.3. SPONTANEOUS AND INDUCED EMISSION Or we can write w (f21 ) = A21 hsi . B12 With that relationship Eq.(3.45) can be rewritten as ¯ ¯ dn2 ¯¯ dn1 ¯¯ =− =A21 n2 (hsi + 1) , dt ¯Em dt ¯Em (3.55) which indicates that the number of spontaneous emissions is equvalent to in­ duced emissions caused by the presence of a single photon per mode. Having identified the coefficients describing the transition rates in the atom interact­ ing with the field from equilibrium considerations, we can rewrite the rate equations also for the non equilibrium situation, because the coefficients are constants depending only on the transition considered dn1 dn2 1 =− = [(n2 −n1 ) hsi + n2 ] . dt dt τ sp (3.56) With each transition from the excited state of the atom to the ground state an emission of a photon goes along with it. From this, we obtain a change in the average photon number of the modes which is d hsi dn1 , =V dt dt (3.57) d hsi V = [(n2 − n1 ) hsi + n2 ] . dt τ sp (3.58) Again the first term describes the stimulated or induced processes and the second term the spontaneous processes. As we will see later, the stimulated emission processes are coherent with the already present radiation field that is inducing the transitions. This is not so for the spontaneous emissions, which add noise to the already present field. For n1 > n2 the stimulated processes lead to a decrease in the photon number and the medium is absorbing. In the case of inversion, n2 > n1 , the photon number increases exponentially. Ac­ cording to Eq.(3.50) inversion corresponds to a negative temperature, which is an indication for a non equilibrium situation that can only be maintained by additional means. It is impossible to achieve inversion by simple irradia­ tion of the atoms with intense radiation. As we see from Eq.(3.58) in steady CHAPTER 3. QUANTUM NATURE OF LIGHT AND MATTER state the ratio between excited state and ground state population is n2 hsi = , n1 hsi + 1 (3.59) which at most approaches equal population for very large photon number. However, such a process can be exploited in a three or four level system, see Figure 3.9, to achieve inversion. Figure 3.9: Three level system: (a) in thermal equilibrium and (b) under optical pumping at the transition frequency f31 . By optical pumping population from the ground state can be transfered to the excited level with energy E3 . If there is a fast relaxation from this level to level E2 , where level two in contrast has a long lifetime, it is conceivable that an inversion between level E2 and E1 can build up. If inversion is achieved radiation at the frequency f21 is amplified. 3.4 Matter Waves and Bohr’s Model of an Atom By systematic scattering experiments Ernest Rutherford showed in 1911, that the negative charges in an atom are homogenously distributed in contrast to 3.4. MATTER WAVES AND BOHR’S MODEL OF AN ATOM the positive charge which is concentrated in a small nucleus about 10,000 times smaller than the atom itself. The nucleus also carries almost all of the atomic mass. Rutherford proposed a model of an atom where the electrons circle the nucleas similar to the planets circling the sun where the gravita­ tional force is replaced by the Coulomb force between the electrons and the nucleus. This model had many short comings. How was it possible that the elec­ trons, which undergo acceleration on their trajectory around the nucleus, do not radiate according to classical electromagnetism, loose energy and finally fall into the nucleus? Due to advances in optical instrumentation the light emitted from thermally excited atomic vapors was known to be in the form of discrete lines. Balmer found in 1885 that these lines could be expressed by the rule µ ¶ 1 1 1 = RH , with n = 3, 4, 5, ... (3.60) − λ 22 n2 where λ is the wavelength of light and RH = 10.968 · μm−1 is the Rydberg constant for hydrogen. For n = 3 this corresponds to the red Hα -line at λ = 656.3nm, for n → ∞ one obtains the wavelength of the limiting line in this series at λ = 364.6nm, see Figure 3.10. Image removed for copyright purposes. Figure 3.10: Balmer series on a wave number scale. . In the subsequent spectroscopy work further sequences where found: 1. Lyman Series: µ ¶ 1 1 1 = RH , with n = 2, 4, 5, ... (3.61) − λ 12 n2 CHAPTER 3. QUANTUM NATURE OF LIGHT AND MATTER 2. Balmer Series: 1 = RH λ µ 1 1 − 2 2 2 n ¶ , with n = 3, 4, 5, ... (3.62) µ 1 1 − 32 n2 ¶ , with n = 4, 5, 6, ... (3.63) µ 1 1 − 2 2 4 n ¶ , with n = 5, 6, 7, ... (3.64) µ 1 1 − 2 2 5 n ¶ , with n = 6, 7, 8, ... (3.65) 3. Paschen Series: 1 = RH λ 4. Brackett Series: 1 = RH λ 5. Pfund Series: 1 = RH λ The Lyman series in the UV-region of the spectrum, whereas the Pfund series is in the far infrared. These sequences can be represented as transitions between energy levels as shown in Figure 3.11. 3.4. MATTER WAVES AND BOHR’S MODEL OF AN ATOM Image removed for copyright purposes. Figure 3.11: Energy level diagram for the hydrogen atom. . In 1913, Niels Bohr found the quantization condition for the electron trajectories in the Hydrogen atom and he was able to derive from that the spectral series discussed above. He postulated that only those electron tra­ jectories are allowed that within one rountrip around the nucleus have an action equal to a multiple of Planck’s quantum of action h. I p · ds = n h, with n = 1, 2, 3.... (3.66) Second, he postulated that the electron can switch from one energy level or trajectory to another one by the emission or absorption of a photon with an CHAPTER 3. QUANTUM NATURE OF LIGHT AND MATTER energy equivalent to the energy difference between the two energy levels, see Figure 3.12. hf = ∆E. (3.67) Image removed for copyright purposes. Figure 3.12: Transition between different energy levels in the hydrogen atom. Assuming a circular trajectory of the electron with radius rn around the nucleus, the quantization condition for the electron trajectory (3.66) leads to 2πrn mvn = nh, with n = 1, 2, 3... (3.68) The other condition for radius and velocity of the electron around the nucleus is given by the equality of Coulomb and centrifugal force at radius rn , which leads to mvn2 e2 = , (3.69) 4πε0 rn2 rn or e2 vn2 = (3.70) . 4πε0 rn m Substituting this value for the electron velocity in the squared quantization condition (3.68), we find the radius of the electron trajectories ε0 h2 2 n. (3.71) πe2 m The radius of the first trajectory, called Bohr radius is r1 = 0.529 · 10−10 m. The velocities on the individual trajectories are rn = vn = e2 1 . 2ε0 h n (3.72) 3.4. MATTER WAVES AND BOHR’S MODEL OF AN ATOM The highest velocity is found for the tightest trajectory around the nucleus, i.e. for n = 1, which can be expressed in terms of the velocity of light as 1 e2 v1 = ·c= · c, 2ε0 hc 137 (3.73) 2 1 where 2εe0 hc = 137 is the fine structure constant. The energy of the electrons on these trajectories with the quantum num­ ber n is due to both potential and kinetic energy 1 2 me4 mvn = 2 2 2 , 2 8ε0 h n 2 e me4 = − = − 2 2 2. 4πε0 rn 4ε0 h n Ekin = (3.74) Epot (3.75) or En = Ekin + Epot me4 Epot = − 2 2 2 . 8ε0 h n (3.76) (3.77) Note, the energy of a bound electron is negative. For n → ∞, En = 0. The electron becomes detached from the atom, i.e. the atom becomes ionized. The lowest and most stable energy state of the electron is for n = 1 En = − me4 = −13.53eV, 8ε20 h2 (3.78) with correspondes to the ground state in hydrogen. When a transition be­ tween two of this energy eigenstates occurs a photon with the corresponding energy is released hf = Ek − En , µ ¶ me4 1 1 = − 2 2 . − 8ε0 h k2 n2 3.5 Wave Particle Duality (3.79) (3.80) CHAPTER 3. QUANTUM NATURE OF LIGHT AND MATTER Bohr’s postulates were not able to explain all the intricacies of the observed spectra and they couldn’t explain satisfactory the structure of the more com­ plex atoms. This was only achieved with the introduction of wave mechanics. In 1923, de Broglie was the first to argue that matter might also have wave properties. Starting from the equivalence principle of mass and energy by Einstein E0 = m0 c20 (3.81) he associated a frequency with this energy accordingly f0 = m0 c20 /h. (3.82) Since energy and frequency are not relativistically invariant quantities but rather components of a four-vector which has the particle momentum as its other components (E0 /c0 , px ,py , pz ) or (ω 0 /c0 , kx ,ky , kz ), it was a necessity that with the energy frequency relationship E = hf = ~ω, (3.83) there must also be a wave number associated with the momentum of a particle according to (3.84) p = ~k. In 1927, C. J. Davisson and L. H. Germer experimentally confirmed this prediction by finding strong diffraction peaks when an electron beam pene­ trated a thin metal film. The pictures were close to the observations of Laue in 1912 and Bragg in 1913, who studied the structure of crystaline and poly crystaline materials with x-ray diffraction. With that finding the duality between waves and particles for both light and matter was established. Duality means that both light and matter have simultaneous wave and particle properties and it depends on the experimental arrangement whether one or the other property manifests itself strongly in the experimental outcome. Bibliography [1] Fundamentals of Photonics, B.E.A. Saleh and M.C. Teich, [2] Optical Electronics, A. Yariv, Holt, Rinehart & Winston, New York, 1991. [3] Introduction to Quantum Mechanics, Griffiths, David J., Prentice Hall, 1995. [4] Quantum Mechanics I, C. Cohen-Tannoudji, B. Diu, F. Laloe, John Wiley and Sons, Inc., 1978.