1 PHYS 3313 – Section 001 Lecture #16 Monday, Mar. 24, 2014 Dr. Jaehoon Yu De Broglie Waves Bohr’s Quantization Conditions Electron Scattering Wave Packets and Packet Envelops Superposition of Waves Electron Double Slit Experiment Wave-Particle Duality Monday, Mar. 24, 2014PHYS , Spring 2014 Dr. Jaehoon Yu

Monday, Mar. 24, 2014PHYS , Spring 2014 Dr. Jaehoon Yu 2 Announcements Research paper template has been uploaded onto the class web page link to research Special colloquium on April 2, triple extra credit Colloquium this Wednesday at 4pm in SH101

De Broglie Waves Prince Louis V. de Broglie suggested that mass particles should have wave properties similar to electromagnetic radiation  many experiments supported this! Thus the wavelength of a matter wave is called the de Broglie wavelength: This can be considered as the probing beam length scale Since for a photon, E = pc and E = hf, the energy can be written as Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

What is the formula for De Broglie Wavelength? (a) for a tennis ball, m=0.057kg. (b) for electron with 50eV KE, since KE is small, we can use non-relativistic expression of electron momentum! What are the wavelengths of you running at the speed of 2m/s? What about your car of 2 metric tons at 100mph? How about the proton with 14TeV kinetic energy? What is the momentum of the photon from a green laser? Calculate the De Broglie wavelength of (a) a tennis ball of mass 57g traveling 25m/s (about 56mph) and (b) an electron with kinetic energy 50eV. Ex 5.2: De Broglie Wavelength Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

Bohr’s Quantization Condition One of Bohr’s assumptions concerning his hydrogen atom model was that the angular momentum of the electron-nucleus system in a stationary state is an integral multiple of h /2 π. The electron is a standing wave in an orbit around the proton. This standing wave will have nodes and be an integral number of wavelengths. The angular momentum becomes: Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

Electron Scattering Davisson and Germer experimentally observed that electrons were diffracted much like x rays in nickel crystals.  direct proof of De Broglie wave! George P. Thomson (1892–1975), son of J. Thomson, reported seeing the effects of electron diffraction in transmission experiments. The first target was celluloid, and soon after that gold, aluminum, and platinum were used. The randomly oriented polycrystalline sample of SnO 2 produces rings as shown in the figure at right. Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

Photons, which we thought were waves, act particle like (eg Photoelectric effect or Compton Scattering) Electrons, which we thought were particles, act particle like (eg electron scattering) De Broglie: All matter has intrinsic wavelength. –Wave length inversely proportional to momentum –The more massive… the smaller the wavelength… the harder to observe the wavelike properties –So while photons appear mostly wavelike, electrons (next lightest particle!) appear mostly particle like. How can we reconcile the wave/particle views? Monday, Mar. 24, 2014PHYS , Spring 2014 Dr. Jaehoon Yu 7

De Broglie matter waves suggest a further description. The displacement of a wave is This is a solution to the wave equation Define the wave number k and the angular frequency  as: The wave function is now: Wave Motion Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

Wave Properties The phase velocity is the velocity of a point on the wave that has a given phase (for example, the crest) and is given by A phase constant  shifts the wave:. Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu (When  =  /2)

Principle of Superposition When two or more waves traverse the same region, they act independently of each other. Combining two waves yields: The combined wave oscillates within an envelope that denotes the maximum displacement of the combined waves. When combining many waves with different amplitudes and frequencies, a pulse, or wave packet, can be formed, which can move at a group velocity : Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

Fourier Series Adding 2 waves isn’t localized in space… but adding lots of waves can be. The sum of many waves that form a wave packet is called a Fourier series : Summing an infinite number of waves yields the Fourier integral: Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

Wave Packet Envelope The superposition of two waves yields a wave number and angular frequency of the wave packet envelope. The range of wave numbers and angular frequencies that produce the wave packet have the following relations: A Gaussian wave packet has similar relations: The localization of the wave packet over a small region to describe a particle requires a large range of wave numbers. Conversely, a small range of wave numbers cannot produce a wave packet localized within a small distance. Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

A Gaussian wave packet describes the envelope of a pulse wave. The group velocity is Gaussian Function Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

Dispersion Considering the group velocity of a de Broglie wave packet yields: The relationship between the phase velocity and the group velocity is Hence the group velocity may be greater or less than the phase velocity. A medium is called nondispersive when the phase velocity is the same for all frequencies and equal to the group velocity. Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

Waves or Particles? Young’s double-slit diffraction experiment demonstrates the wave property of light. However, dimming the light results in single flashes on the screen representative of particles. Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

Electron Double-Slit Experiment C. Jönsson of Tübingen, Germany, succeeded in 1961 in showing double-slit interference effects for electrons by constructing very narrow slits and using relatively large distances between the slits and the observation screen. This experiment demonstrated that precisely the same behavior occurs for both light (waves) and electrons (particles). Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

Which slit? To determine which slit the electron went through: We set up a light shining on the double slit and use a powerful microscope to look at the region. After the electron passes through one of the slits, light bounces off the electron; we observe the reflected light, so we know which slit the electron came through. Use a subscript “ph” to denote variables for light (photon). Therefore the momentum of the photon is The momentum of the electrons will be on the order of. The difficulty is that the momentum of the photons used to determine which slit the electron went through is sufficiently great to strongly modify the momentum of the electron itself, thus changing the direction of the electron! The attempt to identify which slit the electron is passing through will in itself change the interference pattern. Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

Wave particle duality solution The solution to the wave particle duality of an event is given by the following principle. Bohr’s principle of complementarity : It is not possible to describe physical observables simultaneously in terms of both particles and waves. Physical observables are the quantities such as position, velocity, momentum, and energy that can be experimentally measured. In any given instance we must use either the particle description or the wave description. Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

Uncertainty Principle It is impossible to measure simultaneously, with no uncertainty, the precise values of k and x for the same particle. The wave number k may be rewritten as For the case of a Gaussian wave packet we have Thus for a single particle we have Heisenberg’s uncertainty principle : Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

Energy Uncertainty If we are uncertain as to the exact position of a particle, for example an electron somewhere inside an atom, the particle can’t have zero kinetic energy. The energy uncertainty of a Gaussian wave packet is combined with the angular frequency relation Energy-Time Uncertainty Principle:. Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

Probability, Wave Functions, and the Copenhagen Interpretation The wave function determines the likelihood (or probability) of finding a particle at a particular position in space at a given time. The total probability of finding the electron is 1. Forcing this condition on the wave function is called normalization. Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

The Copenhagen Interpretation Bohr’s interpretation of the wave function consisted of 3 principles: The uncertainty principle of Heisenberg The complementarity principle of Bohr The statistical interpretation of Born, based on probabilities determined by the wave function Together these three concepts form a logical interpretation of the physical meaning of quantum theory. According to the Copenhagen interpretation, physics depends on the outcomes of measurement. Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

Particle in a Box A particle of mass m is trapped in a one-dimensional box of width l.l. The particle is treated as a wave. The box puts boundary conditions on the wave. The wave function must be zero at the walls of the box and on the outside. In order for the probability to vanish at the walls, we must have an integral number of half wavelengths in the box. The energy of the particle is. The possible wavelengths are quantized which yields the energy: The possible energies of the particle are quantized. Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

Probability of the Particle The probability of observing the particle between x and x + dx in each state is Note that E0 E0 = 0 is not a possible energy level. The concept of energy levels, as first discussed in the Bohr model, has surfaced in a natural way by using waves. Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

Special Project #4 Prove that the wave function  =A[sin(kx-  t)+icos(kx-  t)] is a good solution for the time- dependent Schrödinger wave equation. Do NOT use the exponential expression of the wave function. (10 points) Determine whether or not the wave function  =Ae -  |x| satisfy the time-dependent Schrödinger wave equation. (10 points) Due for this special project is Monday, Oct. 28. You MUST have your own answers! Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

The Schrödinger Wave Equation Erwin Schrödinger and Werner Heinsenberg proposed quantum theory in 1920 The two proposed very different forms of equations Heinserberg: Matrix based framework Schrödinger: Wave mechanics, similar to the classical wave equation Paul Dirac and Schrödinger later on proved that the two give identical results The probabilistic nature of quantum theory is contradictory to the direct cause and effect seen in classical physics and makes it difficult to grasp! Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

The Schrödinger Wave Equation The Schrödinger wave equation in its time-dependent form for a particle of energy E moving in a potential V in one dimension is The extension into three dimensions is where is an imaginary number Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

The wave equation must be linear so that we can use the superposition principle to. Prove that the wave function in Schrodinger equation is linear by showing that it is satisfied for the wave equation      where a and b are constants and   and  describe two waves each satisfying the Schrodinger Eq. Ex 6.1: Wave equation and Superposition Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu Rearrange terms

General Solution of the Schrödinger Wave Equation The general form of the solution of the Schrödinger wave equation is given by: which also describes a wave propagating in the x direction. In general the amplitude may also be complex. This is called the wave function of the particle. The wave function is also not restricted to being real. Only the physically measurable quantities (or observables ) must be real. These include the probability, momentum and energy. Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

Show that Ae i(kx-  t) satisfies the time-dependent Schrodinger wave Eq. Ex 6.2: Solution for Wave Equation Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu So Ae i(kx-  t) is a good solution and satisfies Schrodinger Eq.

Determine  Asin(kx-  t) is an acceptable solution for the time- dependent Schrodinger wave Eq. Ex 6.3: Bad Solution for Wave Equation Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu This is not true in all x and t. So  (x,t)=Asin(kx-  t) is not an acceptable solution for Schrodinger Eq.

Normalization and Probability The probability P ( x ) dx of a particle being between x and X + dx was given in the equation Here  * denotes the complex conjugate of  The probability of the particle being between x1 x1 and x2x2 is given by The wave function must also be normalized so that the probability of the particle being somewhere on the x axis is 1. Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

Consider a wave packet formed by using the wave function that Ae -   where A is a constant to be determined by normalization. Normalize this wave function and find the probabilities of the particle being between 0 and 1/ , and between 1/  and 2/ . Ex 6.4: Normalization Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu Probabilit y density Normalized Wave Function

Using the wave function, we can compute the probability for a particle to be with 0 to 1/  and 1/  to 2/ . Ex 6.4: Normalization, cont’d Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu For 0 to 1/  : For 1/  to 2/  : How about 2/  : to ∞?

Properties of Valid Wave Functions Boundary conditions 1) To avoid infinite probabilities, the wave function must be finite everywhere. 2) To avoid multiple values of the probability, the wave function must be single valued. 3) For finite potentials, the wave function and its derivatives must be continuous. This is required because the second-order derivative term in the wave equation must be single valued. (There are exceptions to this rule when V is infinite.) 4) In order to normalize the wave functions, they must approach zero as x approaches infinity. Solutions that do not satisfy these properties do not generally correspond to physically realizable circumstances. Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

Time-Independent Schrödinger Wave Equation The potential in many cases will not depend explicitly on time. The dependence on time and position can then be separated in the Schrödinger wave equation. Let, which yields: Now divide by the wave function: The left side of this last equation depends only on time, and the right side depends only on spatial coordinates. Hence each side must be equal to a constant. The time dependent side is Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

We integrate both sides and find: where C is an integration constant that we may choose to be 0. Therefore This determines f to be by comparing it to the wave function of a free particle This is known as the time-independent Schrödinger wave equation, and it is a fundamental equation in quantum mechanics. Time-Independent Schrödinger Wave Equation(con’t) Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

Stationary State Recalling the separation of variables: and with f (t) = the wave function can be written as: The probability density becomes: The probability distributions are constant in time. This is a standing wave phenomena that is called the stationary state. Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

Comparison of Classical and Quantum Mechanics Newton’s second law and Schrödinger’s wave equation are both differential equations. Newton’s second law can be derived from the Schrödinger wave equation, so the latter is the more fundamental. Classical mechanics only appears to be more precise because it deals with macroscopic phenomena. The underlying uncertainties in macroscopic measurements are just too small to be significant due to the small size of the Planck’s constant Monday, Mar. 24, 2014PHYS , Spring 2014 Dr. Jaehoon Yu 39

Expectation Values In quantum mechanics, measurements can only be expressed in terms of average behaviors since precision measurement of each event is impossible (what principle is this?) The expectation value is the expected result of the average of many measurements of a given quantity. The expectation value of x is denoted by. Any measurable quantity for which we can calculate the expectation value is called a physical observable. The expectation values of physical observables (for example, position, linear momentum, angular momentum, and energy) must be real, because the experimental results of measurements are real. The average value of x is Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

Continuous Expectation Values We can change from discrete to continuous variables by using the probability P ( x, t ) of observing the particle at a particular x.x. Using the wave function, the expectation value is: The expectation value of any function g ( x ) for a normalized wave function: Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

Momentum Operator To find the expectation value of p, we first need to represent p in terms of x and t. Consider the derivative of the wave function of a free particle with respect to x:x: With k = p / ħ we have This yields This suggests we define the momentum operator as. The expectation value of the momentum is Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

Position and Energy Operators The position x is its own operator as seen above. The time derivative of the free-particle wave function is Substituting  = E / ħ yields The energy operator is The expectation value of the energy is Monday, Mar. 24, 2014PHYS , Spring 2014 Dr. Jaehoon Yu 43

Properties of Valid Wave Functions Boundary conditions 1) To avoid infinite probabilities, the wave function must be finite everywhere. 2) To avoid multiple values of the probability, the wave function must be single valued. 3) For finite potentials, the wave function and its derivative must be continuous. This is required because the second-order derivative term in the wave equation must be single valued. (There are exceptions to this rule when V is infinite.) 4) In order to normalize the wave functions, they must approach zero as x approaches infinity. Solutions that do not satisfy these properties do not generally correspond to physically realizable circumstances. Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

Infinite Square-Well Potential The simplest such system is that of a particle trapped in a box with infinitely hard walls that the particle cannot penetrate. This potential is called an infinite square well and is given by The wave function must be zero where the potential is infinite. Where the potential is zero inside the box, the time independent Schrödinger wave equation becomes where. The general solution is. Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

Quantization Since the wave function must be continuous, the boundary conditions of the potential dictate that the wave function must be zero at x = 0 and x = L. These yield valid solutions for B=0, and for integer values of n such that kL = n n  k=n  /L The wave function is now We normalize the wave function The normalized wave function becomes These functions are identical to those obtained for a vibrating string with fixed ends. Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

Quantized Energy The quantized wave number now becomes Solving for the energy yields Note that the energy depends on the integer values of n.n. Hence the energy is quantized and nonzero. The special case of n = 1 is called the ground state energy. Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

How does this correspond to Classical Mech.? What is the probability of finding a particle in a box of length L? Bohr’s correspondence principle says that QM and CM must correspond to each other! When? –When n becomes large, the QM approaches to CM So when n  ∞, the probability of finding a particle in a box of length L is Which is identical to the CM probability!! One can also see this from the plot of P! Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

Determine the expectation values for x, x 2, p and p2 p2 of a particle in an infinite square well for the first excited state. Ex 6.8: Expectation values inside a box Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu What is the wave function of the first excited state?n=?2

A typical diameter of a nucleus is about m. Use the infinite square-well potential to calculate the transition energy from the first excited state to the ground state for a proton confined to the nucleus. Ex 6.9: Proton Transition Energy Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu The energy of the state n is n=1What is n for the ground state? What is n for the 1 st excited state?n=2 So the proton transition energy is

Finite Square-Well Potential The finite square-well potential is The Schrödinger equation outside the finite well in regions I and III is for regions I and III, or using yields. The solution to this differential has exponentials of the form e α x and e - α x. In the region x > L, we reject the positive exponential and in the region x < 0, we reject the negative exponential. Why? Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu This is because the wave function should be 0 as x  infinity.

Inside the square well, where the potential V is zero and the particle is free, the wave equation becomes where Instead of a sinusoidal solution we can write The boundary conditions require that and the wave function must be smooth where the regions meet. Note that the wave function is nonzero outside of the box. Non-zero at the boundary either.. What would the energy look like? Finite Square-Well Solution Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

Penetration Depth The penetration depth is the distance outside the potential well where the probability significantly decreases. It is given by It should not be surprising to find that the penetration distance that violates classical physics is proportional to Planck’s constant. Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

The wave function must be a function of all three spatial coordinates. We begin with the conservation of energy Multiply this by the wave function to get Now consider momentum as an operator acting on the wave function. In this case, the operator must act twice on each dimension. Given: The three dimensional Schrödinger wave equation is Three-Dimensional Infinite-Potential Well Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu Rewrite

Consider a free particle inside a box with lengths L 1, L2 L2 and L3 L3 along the x, y, and z axes, respectively, as shown in the Figure. The particle is constrained to be inside the box. Find the wave functions and energies. Then find the ground energy and wave function and the energy of the first excited state for a cube of sides L. Ex 6.10: Expectation values inside a box Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu What are the boundary conditions for this situation? Particle is free, so x, y and z wave functions are independent from each other! Each wave function must be 0 at the wall!Inside the box, potential V is 0. A reasonable solution is Using the boundary condition So the wave numbers are

Consider a free particle inside a box with lengths L 1, L 2 and L 3 along the x, y, and z axes, respectively, as shown in Fire. The particle is constrained to be inside the box. Find the wave functions and energies. Then find the round energy and wave function and the energy of the first excited state for a cube of sides L. Ex 6.10: Expectation values inside a box Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu The energy can be obtained through the Schr ö dinger equation What is the ground state energy? When are the energies the same for different combinations of ni?ni? E 1,1,1 when n 1 =n 2 =n 3 =1, how much?

Degeneracy* Analysis of the Schrödinger wave equation in three dimensions introduces three quantum numbers that quantize the energy. A quantum state is degenerate when there is more than one wave function for a given energy. Degeneracy results from particular properties of the potential energy function that describes the system. A perturbation of the potential energy, such as the spin under a B field, can remove the degeneracy. Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu *Mirriam-webster: having two or more states or subdivisions having two or more states or subdivisions

The Simple Harmonic Oscillator Simple harmonic oscillators describe many physical situations: springs, diatomic molecules and atomic lattices. Consider the Taylor expansion of a potential function: The minimum potential at x=x 0, so dV/dx=0 and V 1 =0; and the zero potential V 0 =0, we have Substituting this into the wave equation: Let and which yields. Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

Parabolic Potential Well If the lowest energy level is zero, this violates the uncertainty principle. The wave function solutions are where H n ( x ) are Hermite polynomials of order n.n. In contrast to the particle in a box, where the oscillatory wave function is a sinusoidal curve, in this case the oscillatory behavior is due to the polynomial, which dominates at small x. The exponential tail is provided by the Gaussian function, which dominates at large x.x. Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

Analysis of the Parabolic Potential Well The energy levels are given by The zero point energy is called the Heisenberg limit: Classically, the probability of finding the mass is greatest at the ends of motion’s range and smallest at the center (that is, proportional to the amount of time the mass spends at each position). Contrary to the classical one, the largest probability for this lowest energy state is for the particle to be at the center. Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

Barriers and Tunneling Consider a particle of energy E approaching a potential barrier of height V 0 and the potential everywhere else is zero. We will first consider the case when the energy is greater than the potential barrier. In regions I and III the wave numbers are: In the barrier region we have Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

Reflection and Transmission The wave function will consist of an incident wave, a reflected wave, and a transmitted wave. The potentials and the Schrödinger wave equation for the three regions are as follows: The corresponding solutions are: As the wave moves from left to right, we can simplify the wave functions to: Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

Probability of Reflection and Transmission The probability of the particles being reflected R or transmitted T is: The maximum kinetic energy of the photoelectrons depends on the value of the light frequency f and not on the intensity. Because the particles must be either reflected or transmitted we have: R + T = 1 By applying the boundary conditions x → ±∞, x = 0, and x = L, we arrive at the transmission probability: When does the transmission probability become 1? Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

Tunneling Now we consider the situation where classically the particle does not have enough energy to surmount the potential barrier, E < V0.V0. The quantum mechanical result, however, is one of the most remarkable features of modern physics, and there is ample experimental proof of its existence. There is a small, but finite, probability that the particle can penetrate the barrier and even emerge on the other side. The wave function in region II becomes The transmission probability that describes the phenomenon of tunneling is Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

Uncertainty Explanation Consider when κL κL >> 1 then the transmission probability becomes: This violation allowed by the uncertainty principle is equal to the negative kinetic energy required! The particle is allowed by quantum mechanics and the uncertainty principle to penetrate into a classically forbidden region. The minimum such kinetic energy is: Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

Analogy with Wave Optics If light passing through a glass prism reflects from an internal surface with an angle greater than the critical angle, total internal reflection occurs. The electromagnetic field, however, is not exactly zero just outside the prism. Thus, if we bring another prism very close to the first one, experiments show that the electromagnetic wave (light) appears in the second prism. The situation is analogous to the tunneling described here. This effect was observed by Newton and can be demonstrated with two prisms and a laser. The intensity of the second light beam decreases exponentially as the distance between the two prisms increases. Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

Potential Well Consider a particle passing through a potential well region rather than through a potential barrier. Classically, the particle would speed up passing the well region, because K = mv 2 / 2 = E - V0.V0. According to quantum mechanics, reflection and transmission may occur, but the wavelength inside the potential well is shorter than outside. When the width of the potential well is precisely equal to half-integral or integral units of the wavelength, the reflected waves may be out of phase or in phase with the original wave, and cancellations or resonances may occur. The reflection/cancellation effects can lead to almost pure transmission or pure reflection for certain wavelengths. For example, at the second boundary ( x = L ) for a wave passing to the right, the wave may reflect and be out of phase with the incident wave. The effect would be a cancellation inside the well. Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

Alpha-Particle Decay May nuclei heavier than Pb emits alpha particles (nucleus of He)! The phenomenon of tunneling explains the alpha-particle decay of heavy, radioactive nuclei. Inside the nucleus, an alpha particle feels the strong, short-range attractive nuclear force as well as the repulsive Coulomb force. The nuclear force dominates inside the nuclear radius where the potential is approximately a square well. The Coulomb force dominates outside the nuclear radius. The potential barrier at the nuclear radius is several times greater than the energy of an alpha particle (~5MeV). According to quantum mechanics, however, the alpha particle can “tunnel” through the barrier. Hence this is observed as radioactive decay. Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

Application of the Schrödinger Equation to the Hydrogen Atom The approximation of the potential energy of the electron- proton system is the Coulomb potential: To solve this problem, we use the three-dimensional time- independent Schrödinger Equation. For Hydrogen-like atoms with one electron (He + or Li ++ ) Replace e2 e2 with Ze 2 (Z (Z is the atomic number) Use appropriate reduced mass  Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

Application of the Schrödinger Equation The potential (central force) V ( r ) depends on the distance r between the proton and electron. Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu Transform to spherical polar coordinates to exploit the radial symmetry. Insert the Coulomb potential into the transformed Schrödinger equation.

Application of the Schrödinger Equation The wave function  is a function of r, θ and . The equation is separable into three equations of independent variables The solution may be a product of three functions. We can separate the Schrodinger equation in polar coordinate into three separate differential equations, each depending only on one coordinate: r, θ, or . Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

Solution of the Schrödinger Equation for Hydrogen Substitute  into the polar Schrodinger equation and separate the resulting equation into three equations: R ( r ), f ( θ ), and g (  ). Separation of Variables The derivatives in Schrodinger eq. can be written as Substituting them into the polar coord. Schrodinger Eq. Multiply both sides by r2 r2 sin 2 θ / Rfg Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu Reorganize

Solution of the Schrödinger Equation Only r and θ appear on the left-hand side and only  appears on the right-hand side of the equation The left-hand side of the equation cannot change as  changes. The right-hand side cannot change with either r or θ.θ. Each side needs to be equal to a constant for the equation to be true in all cases. Set the constant −mℓ2 −mℓ2 equal to the right- hand side of the reorganized equation –The sign in this equation must be negative for a valid solution It is convenient to choose a solution to be azimuthal equation Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu

Solution of the Schrödinger Equation satisfies the previous equation for any value of mℓ.mℓ. The solution be single valued in order to have a valid solution for any , which requires m ℓ must be zero or an integer (positive or negative) for this to work Now, set the remaining equation equal to −mℓ2 −mℓ2 and divide either side with sin 2  and rearrange them as Everything depends on r the left side and θ on the right side of the equation. Monday, Mar. 24, PHYS , Spring 2014 Dr. Jaehoon Yu