Atoms 2012 update -- start with single electron: H-atom

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1 Atoms 2012 update -- start with single electron: H-atom x z φ θ e -1 y 3-D problem - free move in x, y, z - easier if change coord. systems: Cartesian Spherical Coordinate (x, y, z) (r, θ, φ) Reason: V(r) = -Ze 2 / r depend - separation not orientation electrostatic potential basis of chemistry Note: (if proton is fixed at 0,0,0 then r =[x 2 +y 2 +z 2 ] 1/2 ) r = (x e x p )i + (y e y p )j + (z e z p )k --vector, p e r = [(x e x p ) 2 + (y e y p ) 2 + (z e z p ) 2 ] 1/2 --length (scalar) Goal separate variables V(r) x,y,z mixed no problem for K.E. T already separated First step formal: reduce from 6-coord: x e,y e,z e & x p,y p,z p to 3-internal coordinates. Eliminate center of mass -- see notes (Center of Mass) on the Web site whole atom: R = X+Y+Z X = (m e x e +m p x p )/(m e +m p ).. etc. this normalizes the position correct for mass for H-atom these are almost equal to: x p, y p, z p but process is general move equal mass issue other 3 coord: relative x = x e x p etc. ideal for V(r) Problem separates, V = V(r) only depend on internal coord. Hψ = [-h 2 /2M R 2 + -h 2 /2μ r 2 + V(r)] Ψ(R,r) = EΨ H - sum independent coord. M = m e + m p μ = m e. m p /( m e + m p ) 33 VII 33

2 Ψ(R,r) = Ξ(R) ψ(r) product wave function separates summed Η like before (eg.3-d p.i.b.) i.e. Operate H on Ψ(R,r) and operators pass through R and r dependent terms, Ξ(R) and ψ(r), to give: First term: + second two terms: (= E [Ξ(R) ψ(r)]) (-h 2 /2M)ψ(r) 2 R Ξ(R) and Ξ(R)[(-h 2 /2μ) 2 r +V(r)]ψ(r) so divide through by Ψ(R,r) = Ξ(R) ψ(r) and results are independent (R and r) sum equal constant, E VII 34 R-dependent equation: (-h 2 /2M)(1/Ξ(R)) R 2 Ξ(R) = E T Motion (T) of whole atom free particle -not quantized -ignore r-dependent equation: (1/ψ(r))[(-h 2 /2μ) r 2 +V(r)]ψ(r) = E int relative (internal) coord. - formal: we let E = E T +E int internal equation simplified by conversion: x,y,z r,θ,φ result internal: H(r)ψ(r) = [(-h 2 /2μ) r 2 Ze 2 /r]ψ(r) = E int ψ(r) (idea potential only depends on r, so other two coordinates, φ,θ only contribute K.E.) r,θ,φ 2 = 1/r 2 { / r(r 2 / r)+[1/sinθ] / θ(sinθ / θ) + [1/sin 2 θ] 2 / φ 2 } Separation, one coord. at time. Easy to separate φ depend. to get φ only in 1 term, multiply r 2 sin 2 θ through Hψ = Eψ Aside, just for information, Step by step separation, start: [(-h 2 /2μ) r,θ,φ 2 Ze 2 /r] ψ(r,θ,φ) = (-h 2 /2μr 2 ){ / r(r 2 / r)+ [1/sinθ] / θ(sinθ / θ) + [1/sin 2 θ] 2 / φ 2 }ψ(r,θ,φ) Ze 2 /r ψ(r,θ,φ) = E int ψ(r,θ,φ) 34

3 multiply by r 2 sin 2 θ : 1. (-h 2 /2μ) sin 2 θ{ / r(r 2 / r)+ [1/sinθ] / θ(sinθ / θ) + 2 / φ 2 [r 2 sin 2 θ Ζe 2 /r]}ψ(r,θ,φ) = r 2 sin 2 θe int ψ(r,θ,φ) VII 35 isolate the φ dependent terms: 2. (-h 2 /2μ) {sin 2 θ / r(r 2 / r)+ sinθ / θ(sinθ / θ) (2μ/h 2 ) r 2 sin 2 θ(e int + Ze 2 /r)}ψ(r,θ,φ) = (h 2 /2μ) 2 / φ 2 ψ(r,θ,φ) see all φ operators one side Use ψ(r, θ, φ) = R(r) Θ(θ) Φ(φ) and cancel out h 2 /2μ 3. {sin 2 θ / r(r 2 / r)+ sinθ / θ(sinθ / θ)} R(r)Θ(θ)Φ(φ) r 2 sin 2 θ(2μ/h 2 ) (E int + Ze 2 /r) R(r)Θ(θ)Φ(φ) = - 2 / φ 2 R(r)Θ(θ)Φ(φ) Recall 2 / φ 2 only operate on Φ(φ) part, rest pass through Same the other side, Φ(φ) pass through / r and / θ 4. Φ(φ) {sin 2 θ / r(r 2 / r)+ sinθ / θ(sinθ / θ)} R(r)Θ(θ) Φ(φ)r 2 sin 2 θ(2μ/h 2 ) (E int + Ze 2 /r)r(r)θ(θ) = - R(r)Θ(θ) 2 / φ 2 Φ(φ) divide through as before by Ψ(r,θ,φ) 5. [R(r)Θ(θ)] 1 {sin 2 θ / r(r 2 / r)+sinθ / θ(sinθ / θ)} R(r)Θ(θ) r 2 sin 2 θ(2μ/h 2 )(E int +Ze 2 /r)r(r)θ(θ) = - [Φ(φ)] 1 2 / φ 2 Φ(φ) = const. right side (red) independent of left side, so must be constant a) Let Φ part equal constant, m 2 : 2 / φ 2 Φ(φ) = -m 2 Φ(φ) y x +m -m rotation about z-axis Φ(φ) = e imφ m = 0,±1,±2... note: equivalent to particle on a ring problem -m 2 important, requires complex exponential 35

4 b) Can similarly separate Θ(θ) but arithmetic messier 1 st divide through by sin 2 θ -- separate r and θ terms 1. [R(r)Θ(θ)] 1 { / r(r 2 / r)+(sinθ) 1 / θ(sinθ / θ)} R(r)Θ(θ) r 2 (2μ/h 2 ) (E int + Ze 2 /r) R(r)Θ(θ) = m 2 / sin 2 θ Note 1 st & 3 rd terms have r but middle term only θ, separate 2. [Θ(θ)] 1 { (sinθ) 1 / θ(sinθ / θ) - m 2 /sin 2 θ} Θ(θ) = [R(r) ] 1 { / r(r 2 / r) + (2μ/h 2 ) r 2 (E int + Ze 2 /r)} R(r) = const. 36 VII 36 Result- only has θ on left and r on right, let const. = -l(l+1): 3. { (sinθ) 1 / θ(sinθ / θ) - m 2 /sin 2 θ + l(l +1)} Θ(θ) = 0 LeGendre polynomial: Θ lm (θ) = P l m (cos θ) l = 0,1,2, l = 0 2 /2 l = 1 m = 0 (3/2) 1/2 cos θ l = 1 m = ±1 (3/4) 1/2 sin θ l = 2 m = 0 (5/8) 1/2 (3 cos 2 θ 1) l = 2 m = ±1 (15/4) 1/2 (sin θ cos θ) l = 2 m = ±1 (15/16) 1/2 (sin 2 θ).... polynom: P(x) but x = cosθ, l = 3 m = 0 ~ (5 cos 3 θ 3 cos θ) as m inc. cosθ --> sinθ c) Radial function messier, has V(r), yet but must fit B.C. r R nl (r) 0 (must be integrable) exponential decay, penetrate potential ~ e -αr damp (works, r always +) Must be orthogonal this works when fct. oscillate (wave-like) power series will do

5 Associated LaGuerre Polynomial solves radial equation { r -2 / r(r 2 / r) + (2μ/h 2 ) (E int + Ze 2 /r) - l(l +1)/r 2 } R(r) = 0 VII 37 divide by r 2, multiply by R(r) & move l(l+1) term to left side R nl = [const] (2σ/n) l 2l 1 L + n + l (2σ/n) e -σ/n σ = Zr/a 0 Quantum #: n = 1, 2, 3, l = 0, 1, 2, n 1, l m n = 1, l = 0 ~ e -σ n = 2 l = 0 ~ (2 σ)e -σ/2 n = 2 l = 1 ~ σe -σ/2 n = 3 l = 0 ~ (1 2σ/3 + 2σ 2 /27)e -σ/3 n = 3 l = 1 ~ (σ σ 2 /6)e -σ/3 n = 3 l = 2 ~ σ 2 e -σ/3.... Note: n restricts l ( n 1) and l restricts m ( l) Comparison of potentials when potential not infinite, levels collapse, when sides not infinite and vertical w/f penetrates potential wall Particle in box; Stubby box; infinite, steep potential, get expanding Energylevels, ψ(wall)=0 37 Finite potential get collapsing energy levels continuous at top dissociation ψ(x) penetrate wall

6 Harmonic oscillator; V(x) Anharmonic oscillator VII 38 sloped potential wavefct penetrate wall, finite pot. levels collapse, H-atom Solutions for l = 0 higher l, less nodes If rotate this around r = 0, get a symmetric well and shapes look like υ = even harmonic oscillator shapes finite potential bends over, V = 0 at r =, get collapse of E-levels as n Energy vary with nodes curvature as before Yellow (Engel) penetrate classic forbidden region 38

7 These functions (3 independent solutions) can be combined ψ(r,θ,φ) = R nl (r) Θ l m (θ) Φ m (φ) VII 39 Note: only R nl depend on r as does V(r) Energy will not depend on θ,φ for H-atom, differ for He etc. Often write: Y l m (θ,φ) = Θ m l (θ) Φ m (φ) spherical harmonics Angular parts give shapes but not size these are eigenfunctions of Angular Momentum recall : L = r x p, L 2 ~ -h 2 2 r, L z = xp y -yp x, L z = -ih / φ L 2 Y l m (θ,φ) = l(l + 1) h 2 Y l m [H,L 2 ] = 0 commute: L z Y l m (θ φ) = mhy l m [H,L z ] = 0 simul.sol n This is source of familiar orbitals, l = 0,1,2,3.. or s,p,d,f Solving R nl equation E n = -Z 2 e 4 μ/2h 2 n 2 = -Z 2 R/n 2 exactly Bohr solution, R Rydberg, (must be, since works) Familiar: n = 1 l = 0 m = 0 1s n = 2 l = 0 m = 0 2s l = 1 m = 0,±1 2p (2p 0 + 2p ±1 ) n = 3 l = 0 m = 0 3s l = 1 m = 0,±1 3p (3p 0 + 3p ±1 ) l = 2 m=0,±1,±2 3d (3d 0, 3d ±1, 3d ±2 ) (lt) compare 1s and 2s (rt) probability (ψ ψ) and radial distribution (4πr 2 ψ ψ) Note: 1s decays, 2s has node (2-σ) term, 2p starts at 0 39

8 Compare probability distributions, see expansion in size with n, l VII 40 Note: 2s node makes dip Note: # radial nodes (in R nl ) in probability (e- density) decrease with l, i.e. # = n-l-1 Similarly mix Θ ±2 to get d xy, d x 2-y 2 and Θ ±2 for d xz, d yz 40

9 Angular functions have no radial value --> just surface, combine with radial function to get magnitude or e - density, Best represented as a contour map or probability surface VII 41 Real orbitals, take linear combinations of ±m values, eliminate i dependent terms, get x,y,z functions Cartesian form: e imφ + e -imφ = cosφ + i sinφ + cosφ i sinφ = 2 cos φ ~ x i (e imφ - e -imφ ) = 2 sin φ ~ y recall: x = r cos φ and y = r sin φ 41

10 42 VII 42

11 H-atom solutions, complex orbitals, eigen values of angular momentum: VII 43 Linear combinations of H-atom solutions Real orbitals 43

12 Contour plots of orbitals VII 44 Radial function effect represented by contours, each line represents lower e- density sign change makes node between lobes or radially (e.g. see top, 3p vs. 2p) 44

13 Linear comb. of degenerate orbitals also solutions H-atom show effect of mixing s and p orbitals: VII 45 Hybrids linear comb. of s and p orient for bonding sp 2 orbitals / opposite direction, (s±p) are 180 sp 2 3 orbitals / in plane / 120 apart sp 3 4 orbitals / 4 vertices of tetrahedron (109 ) 45

14 Energy level diagram H-atom E n VII 46 Allowed any n change: Δn 0 l, m l as before: Δl = ±1, Δm l = 0, ±1 no energy dependence on l, but spectral transitions do depend on l Spectral transitions match Balmer series but also must account for Θ,Φ functions Allowed selection rules (see box) n n' = 1 Lyman must start p orbital --> end 1s n n' = 2 Balmer must start d or s orbital end 2p or start in p orbital end in 2s etc. Test with Zeeman effect m l βh = E' add E from field More complex than Bohr, but same energies and angular momenta, similar restriction on solution to line spectra 46

15 47 VII 47