1. Physical Chemistry 2nd Edition
Chapter 15
Using Quantum Mechanics on Simple Systems
Physical Chemistry 2nd Edition
Thomas Engel, Philip Reid

2. Objectives
Using the postulates to understand the particle in the box (1-D, 2-D and 3-D)

3. Outline The Free Particle The Particle in a One-Dimensional Box
Two- and Three-Dimensional Boxes
Using the Postulates to Understand the Particle in the Box and Vice Versa

4. 15.1 The Free Particle
For free particle in a one-dimensional space on which no forces are acting, the Schrödinger equation is
is a function that can be differentiated twice to return to the same function
where

5. 15.1 The Free Particle
If x is restricted to the interval then the probability of finding the particle in an interval of length dx can be calculated.

6. 15.2 The Particle in a One-Dimensional Box
15.1 The Classical Particle in a Box
When consider particle confined to a box in 1-D, the potential is

7. 15.2 The Particle in a One-Dimensional Box
Consider the boundary condition satisfying 1-D,
The acceptable wave functions must have the form of
Thus the normalized eigenfunctions are

8. 15.2 The Particle in a One-Dimensional Box
15.2 Energy Levels for the Particle in a Box
15.3 Probability of Finding the Particle in a Given Interval

9. Example 15.1
From the formula given for the energy levels for the particle in the box, for n = 1, 2, 3, 4… , we can see that the spacing between adjacent levels increases with n. This appears to indicate that the energy spectrum does not become continuous for large n, which must be the case for the quantum mechanical result to be identical to the classical result in the high-energy limit.

10. Example 15.1
A better way to look at the spacing between levels is to form the ratio By forming this ratio, we see that becomes a smaller fraction of the energy as
This shows that the energy spectrum becomes continuous for large n.

11. Solution
We have,
which approaches zero as Both the level spacing and the energy increase with n, but the energy increases faster (as n2), making the energy spectrum appear to be continuous as n→∞

12. 15.3 Two- and Three-Dimensional Boxes
1-D box is useful model system as it allows focus to be on quantum mechanics instead of mathematics.
For 3-D box, the potential energy is
Inside the box, the Schrödinger equation can be written as

13. 15.3 Two- and Three-Dimensional Boxes
The total energy eigenfunctions have the form
And the total energy has the form
15.4 Eigenfunctions for the Two- Dimensional Box

14. 15.4 Using the Postulates to Understand the Particle in the Box and Vice Versa
The state of a quantum mechanical system is completely specified by a wave function The probability that a particle will be found at time t in a spatial interval of width dx centered at x0 is given by
This postulate states that all information obtained about the system is contained in the wave function.

15. Example 15.2 Consider the function
a. Is an acceptable wave function for the particle in the box?
b. Is an eigenfunction of the total energy
operator, ?
c. Is normalized?

16. Solution
a. If is to be an acceptable wave function, it must satisfy the boundary conditions =0 at x=0 and x=a. The first and second derivatives of must also be well-behaved functions between x=0 and x=a. This is the case for We conclude that
is an acceptable wave function for the particle in the box.

17. Solution
b. Although may be an acceptable wave function, it need not be an eigenfunction of a given operator. To see if is an eigenfunction of the total energy operator, the operator is applied to the function:
The result of this operation is not multiplied by a constant. Therefore, is not an eigenfunction of the total energy operator.

18. Solution
c. To see if is normalized, the following integral is evaluated:

19. Solution
Using the standard integral and recognizing that the third

20. Solution Therefore, is not normalized, but the function
is normalized for the condition that
Note that a superposition wave function has a more
complicated dependence on time than does an
eigenfunction of the total energy operator.

21. Solution For instance, for the wave function under
consideration is given by
This wave function cannot be written as a product of a
function of x and a function of t. Therefore, it is not a
standing wave and does not describe a state
whose properties are, in general, independent of time.

22. 15.5 Acceptable Wave Functions for the Particle in a Box
15.4 Using the Postulates to Understand the Particle in the Box and Vice Versa
15.5 Acceptable Wave Functions for the Particle in a Box

23. Example 15.3
What is the probability, P, of finding the particle in the central third of the box if it is in its ground state?

24. Solution
For the ground state, From the postulate, P is the sum of all the probabilities of finding the particle in intervals of width dx within the central third of the box. This probability is given by the integral

25. Solution Solving this integral,
Although we cannot predict the outcome of a single
measurement, we can predict that for 60.9% of a large number of individual measurements, the particle is found in the central third of the box.

26. 15.4 Using the Postulates to Understand the Particle in the Box and Vice Versa
In any single measurement of the observable that corresponds to the operator , the only values that will ever be measured are the eigenvalues of that operator.

27. If the system is in a state described by the wave
15.4 Using the Postulates to Understand the Particle in the Box and Vice Versa
Postulate 4
If the system is in a state described by the wave
function , and the value of the observable a is
measured once each on many identically prepared
systems, the average value of all of these
measurements is given by

28. 15.4 Using the Postulates to Understand the Particle in the Box and Vice Versa
15.6 Expectation Values for E, p, and x for a Superposition Wave Function

29. Example 15.4
Assume that a particle is confined to a box of length a, and that the system wave function is
a. Is this state an eigenfunction of the position operator?
b. Calculate the average value of the position that would be obtained for a large number of measurements. Explain your result.

30. Example 15.4 a. The position operator . Because ,
where c is a constant, the wave function is not an
eigenfunction of the position operator.

31. Example 15.4
b. The expectation value is calculated using the fourth postulate:
Using the standard integral

32. Example 15.4
We have
The average position is midway in the box. This is
exactly what we would expect, because the particle is equally likely to be in each half of the box.