The Hydrogen Atom. Thornton and Rex, Ch. 7

Transcription

1 The Hydrogen Atom Thornton and Rex, Ch. 7

2 Applying Schrodinger s Eqn to the Hydrogen Atom The potential: -1 e 2 V(r) = 4p e0 r Use spherical polar coordinates (with y(x,y,z) => y(r,q,f) ): 1 y 1 y ( r 2 ) + (sinq ) r 2 r r r 2 sinq q q 1 2 y 2m + + (E-V) y = 0 r 2 sin 2 q f 2 h 2 x = r sinq cosf z r = x 2 + y 2 + z 2 y = r sinq sinf z = r cosq x f q r y q= cos -1 (z/r) Polar Angle f= tan -1 (y/x) Azimuthal Angle

3 y(r,q,f) is separable: fi y(r,q,f) = R(r) f(q) g(f) Substitute this into S Eqn and apply appropriate boundary conditions to R, f, g. fi 3 separate equations and 3 quantum numbers. (For more, see section 7.2.) d 2 g = - ml 2 g d f 2 Azimuthal Eqn. 1 d d R 2m h l (l +1) ( r 2 ) + (E-V- ) R = 0 r 2 d r d r h 2 2m r 2 Radial Eqn. 1 d d f m l 2 (sinq ) + (l (l +1) - ) f = 0 sinq d q d q sin 2 q m l and l are quantum numbers. Angular Eqn.

4 The Radial Equation The Radial Equation is the Associated Laguerre Equation. We will find the ground-state solution. Require m l = 0, l = 0. 1 d d R 2m fi ( r 2 ) + (E-V ) R = 0 r 2 d r d r h 2 Substitute V = - e 2 /(4pe 0 r) and insert a trial solution: R = A e -r/a 0 This works if a 0 = 4pe 0 h 2 m e 2 (Bohr Radius) and E = - h 2 /(2ma 02 ) = - E 0 = ev

5 Higher order solutions can be found in terms of associated Laguerre functions. They are labeled by a quantum number n (called the principal quantum number). Energies are E = - E 0 / n 2 (just like the Bohr prediction.)

6 Angular and Azimuthal Equations The azimuthal equation is just a SHO equation with solution g = A e imf. Single-valuedness requires g(f) = g(f + 2p). fi m l is an integer. The angular equation is the Associated Legendre Equation. It is customary to combine the q and f solutions together as Spherical Harmonics Y(q,f) The quantum numbers satisfy l = 0, 1, 2, 3,... and m l = -l, -l+1, -l+2,..., 0,..., l-1, l (They also satisfy l < n. )

7 Y(q,f) 2 s (l=0) m l =0 p (l=1) m l =±1 0 d (l=2) m l =±2 ±1 0 f (l=3) m l =±3 ±2 ±1 0

8 R n l and P(r) = r 2 R 2 Larger n peaks at larger r

9 P(x,y,z) from r 2 R 2 Y*Y No φ (x-y plane) dependence, only θ (angle from z axis) in these basis functions since φ cancels in Y*Y

10 Atomic Quantum Numbers n l Principal Quantum Number Orbital Angular Momentum Quantum Number m l Magnetic Quantum Number n = 1, 2, 3,... l = 0, 1, 2, 3,... m l = -l, -l +1, -l +2,..., 0,..., l -1, l In summary: n > 0 l < n m l l

11 Angular Momentum Angular momentum of electron in the atom: L = mvr = l(l+1) h (Note that this disagrees with Bohr s original guess of L = n h.) For a given n, the energy is E n = -E 0 /n 2 independent of l. The different l states are degenerate. Historical Notation: l = s p d f g h States are usually labeled by the n number and the l letter. For example: n=3, l=1 3p state.

12 Magnetic Quantum Number l determines the total angular momentum: L = l(l+1) h m l gives the z component of L: L z = m l h Direction of L can never lie on z axis: m l < l(l+1) always. 2 h m l = +2 h +1 An example for l = 2 L z 0 0 fi L = 6 h -h -1-2 h -2 The choice of direction for the z axis is completely arbitrary. L x and L y are undetermined, except for L x 2 + L y 2 = L 2 - L z 2

13 Magnetic Effects An electron orbiting around a nucleus has magnetic moment m: -e -erv -e m = IA n = (pr 2 ) n = n = L (2pr/v) 2 2m The component in the z direction is: -e -e m z = L z = m l h = -m l m 2m 2m B where m B = e h/2m = 9.274x10-24 J/T = 5.788x10-5 ev/t is the Bohr magneton. In an external magnetic field, B, the magnetic dipole feels a torque: t = m x B m and has a potential energy: V B = -m B

14 If B is in the z direction then V B = -m z B = +m l m B B The energies for different m l, which were degenerate for B=0, are now separated into 2l +1 different levels. This is called the Normal Zeeman Effect. Energy of 2p - State m l = B

15 The Stern-Gerlach Experiment In the presence of an inhomogeneous magnetic field, there will be a net force on the atoms, depending on m l : F z = - d V B d z = m l m B d B d z i.e. In the +z direction for +m l No force for m l =0 In the -z direction for - m l Thus, one could split the atoms according to the quantum number m l : Beam of atoms B magnet S screen m l = +1 m l = 0 N magnet m l = -1

16 Electron Spin Stern and Gerlach did their experiment. The atoms split into two beams. But the number of m l values is always odd: (2l +1)! Goudsmit and Uhlenbeck proposed that the electron had an intrinsic spin and an intrinsic magnetic moment. In analogy with orbital angular momentum they proposed a magnetic spin quantum number: m s = ± 1/2 S

17 The electron s spin can either be oriented up or down : 1 + h h 2 The total spin quantum number is s = 1/2: S = s(s+1) h = 3/4 h -e m m B S = S = - [ 2 ] S m h Compare with: -e m m B L = L = - [ 1 ] L 2m h [ * ] are called gyromagnetic ratios: g s = 2 g l = 1

18 Selection Rules n l m l n l m l emitted photon Allowed transitions: lifetimes t ~ 10-9 sec D n = anything, Dl = ±1, Dm l = 0, ±1 Forbidden transitions: lifetimes much longer Ex. 2s Æ 1s, t ~ 1/7 sec

19 With no External B, E depends on n only Allowed transitions must change the l quantum number

20 Selection Rules and Normal Zeeman Effect B=0 B nonzero l = 2 Dm l =-1 Dm l = 0 Dm l =+1 m l = DE l = 1 m l = DE+m B B DE DE-m B B B field always splits spectral lines into 3 for normal Zeeman effect.

21

22