Review for Exam 2 The Schrodinger Eqn. What is important? The Schrödinger Wave Equation Operators, expectation values The simple harmonic oscillator Quantum mechanics applied to the Hydrogen atom: quantum number, energy and angular momentum Study Chapters 5 and 7 hard! http://nobelprize.org/physics/laureates/1933/schrodinger-bio.html Erwin Schrödinger (1887-1961)

The Schrödinger Wave Equation The Schrödinger Wave Equation for the wave function Y(x,t) for a particle in a potential V(x,t) in one dimension is: where The Schrodinger Equation is the fundamental equation of Quantum Mechanics. Note that it’s very different from the classical wave equation. But, except for its inherent complexity (the i), it will have similar solutions.

Time-Independent Schrödinger Wave Equation The potential in many cases will not depend explicitly on time: V = V(x). The Schrödinger equation’s dependence on time and position can then be separated. Let: And substitute into: which yields: Now divide by y(x) f(t): The left side depends only on t, and the right side depends only on x. So each side must be equal to a constant. The time-dependent side is:

Time-Independent Schrödinger Wave Equation Multiply both sides by f /iħ: which is an easy differential equation to solve: But recall our solution for the free particle: in which f(t) = exp(-iwt), so: w = B / ħ or B = ħw, which means that: B = E ! So multiplying by y(x), the spatial Schrödinger equation becomes:

Stationary States The wave function can now be written as: The probability density becomes: The probability distribution is constant in time. This is a standing-wave phenomenon and is called a stationary state. Most important quantum-mechanical problems will have stationary-state solutions. Always look for them first.

Normalization and Probability The probability P(x) dx of a particle being between x and x + dx is given in the equation The probability of the particle being between x1 and x2 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.

Expectation Values In quantum mechanics, we’ll compute expectation values. The expectation value, , is the weighted average of a given quantity. In general, the expected value of x is: If there are an infinite number of possibilities, and x is continuous: Quantum-mechanically: And the expectation of some function of x, g(x):

Bra-Ket Notation This expression is so important that physicists have a special notation for it. The entire expression is called a bracket. And is called the bra with the ket. The normalization condition is then:

General Solution of the Schrödinger Wave Equation when V = 0 In free space (with V = 0), the wave function is: which is a sine wave moving in the x direction. Notice that, unlike classical waves, we are not taking the real part of this function. Y is, in fact, complex. In general, the wave function is complex. But the physically measurable quantities must be real. These include the probability, position, momentum, and energy.

Momentum Operator To find the operator for p, consider the derivative of the wave function of a free particle with respect to x: With k = p / ħ we have: This yields: This suggests we define the momentum operator as: . The expectation value of the momentum is:

Position and Energy Operators The position x is its own operator. Done. Energy operator: Note that the time derivative of the free-particle wave function is: Substituting w = E / ħ yields: This suggests defining the energy operator as: The expectation value of the energy is:

Simple Harmonic Oscillator Simple harmonic oscillators describe many physical situations: springs, diatomic molecules and atomic lattices. Consider the Taylor expansion of an arbitrary potential function: Near a minimum, V1[x-x0] ≈ 0.

Simple Harmonic Oscillator Consider the second-order term of the Taylor expansion of a potential function: Letting x0 = 0. Substituting this into Schrödinger’s equation: We have: Let and , which yields:

The Parabolic Potential Well The wave function solutions are: where Hn(x) are Hermite polynomials of order n.

The Parabolic Potential Well yn |yn |2 The Parabolic Potential Well Classically, the probability of finding the mass is greatest at the ends of motion (because its speed there is the slowest) and smallest at the center. Contrary to the classical one, the largest probability for the lowest energy states is for the particle to be at (or near) the center. Classical result

Correspondence Principle for the Parabolic Potential Well As the quantum number (and the size scale of the motion) increase, however, the solution approaches the classical result. This confirms the Correspondence Principle for the quantum-mechanical simple harmonic oscillator. Classical result

The Parabolic Potential Well The energy levels are given by: The zero point energy is called the Heisenberg limit: