1. Classical Model of Rigid Rotor
A particle rotating around a fixed point, as shown below, has angular momentum and rotational kinetic energy (“rigid rotor”)
The classical kinetic energy is given by:
If the particle is rotating about a fixed point at radius r with a frequency ʋ (s−1 or Hz), the velocity of the particle is given by:
where ω is the angular frequency (rad s−1 or rad Hz). The rotational kinetic energy can be now expressed as:
Also
where

2. Note that there is no potential energy involved in free rotation.
Consider a classical rigid rotor corresponding to a diatomic molecule. Here we consider only rotation restricted to a 2-D plane where the two masses (i.e., the nuclei) rotate about their center of mass.
The rotational kinetic energy for diatomic molecule in terms of angular momentum
Note that there is no potential energy involved in free rotation.

3. Momentum Summary Classical QM Linear Momentum Energy Rotational
(Angular)
Momentum
Energy

4. Angular Momentum

5. Angular Momentum

6. Angular Momentum

7. Angular Momentum

8. Two-Dimensional Rotational Motion
Polar Coordinates y r f x

9. Two-Dimensional Rotational Motion

10. Two-Dimensional Rigid Rotor
Assume r is rigid, ie. it is constant

11. Two-Dimensional Rigid Rotor

12. Solution of equation

13. Energy and Momentum
As the system is rotating
about the z-axis

14. Two-Dimensional Rigid Rotor
18.0
12.5 E
8.0
4.5
2.0
0.5
Only 1 quantum number is require to determine the state of the system.

15. Spherical coordinates

16. Spherical polar coordinate

17. Hamiltonian in spherical polar coordinate

18. Rigid Rotor in Quantum Mechanics
Transition from the above classical expression to quantum mechanics can be carried out by replacing the total angular momentum by the corresponding operator:
Wave functions must contain both θ and Φ dependence:
are called spherical harmonics

19. Schrondinger equation

21. Two equations

22. Solution of second equation

23. Solution of First equation
Associated Legendre Polynomial

24. Associated Legendre Polynomial

25. For l=0, m=0

26. First spherical harmonics
Spherical Harmonic, Y0,0
                                              

27. l= 1, m=0

28. l= 1, m=0 θ
cos2θ 1 30
3/4 45
1/2 60
1/4 90

29. l=2, m=0 θ cos2θ 3cos2θ-1 1 2 30 3/4 (9/4-1)=5/4 45 1/2 (3/2-1)=1/2 601 2 30
3/4
(9/4-1)=5/4 45
1/2
(3/2-1)=1/2 60
1/4
(3/4-1)=-1/4 90 -1

30. l = 1, m=±1
Complex Value??
If Ф1 and Ф2 are degenerateeigenfunctions, their linear combinations are also an eigenfunction with the same eigenvalue.

31. l=1, m=±1
Along x-axis

35. Three-Dimensional Rigid Rotor States3 2 1
6.0 -1 -2 -3 E 2 1
3.0 -1 -2 1
1.0 -1
0.5
Only 2 quantum numbers are required to determine the state of the system.

36. Rotational Spectroscopy
J : Rotational quantum number
Rotational Constant

37. Rotational Spectroscopy
Wavenumber (cm-1)
Rotational Constant
Line spacing v Dv
Frequency (v)

38. Bond length
To a good approximation, the microwave spectrum of H35Cl consists of a series of equally spaced lines, separated by 6.26*1011 Hz. Calculate the bond length of H35Cl.