课程名称︰物理化学二 课程性质︰必修 课程教师︰郑原忠 开课学院：理学院 开课系所︰化学系 考试日期（年月日）︰2010/3/27 考试时限（分钟）：120 是否需发放奖励金：是 （如未明确表示，则不予发放） 试题 : 1.For a particle with mass m in a one-dimensional box with lenth a, the ground-state wavefunction is φ=(2/a)^1/2sin(pix/a). Consider the particle in the ground state. (a)(5%)Show that φ is normalized. Hint:sin^2(x/2)={1-cos(x)}/2 (b)(5%)What is the probability that the particle is the middle third of the box? (c)(5%)What is the kinetic enerrgy of the particle? 2.(10%)A hermitian operator A satisfies ∫φ*Aψτ = ∫ψ(A φ )* dτ for arbitrary wavefunctions ψ and φ. Show that the momentum operator Px=-ihd/2pidx is hermitian. 3.For a particle in a two-dimensional square box, the total energy eigenfunctions are ψ(x,y)=Nsin(Nxpix/a)sin(Nypiy/a) (a)(5%)What is the pamiltonian of this system? Define additional parameters and also identify the boundary conditions for the eigenstates. (b)(5%)Obtain an expression for eigen-energies E(nxny) in terms of nx, ny, and a. (c)(5%)Contour plots of four eigenfunctions are shown on the right. The x and y directions of the box lie along the horizontal and vertical directions, respectively. Identify the quantum numbers Nx amd Ny for states a-d. (d)(5%)Give the degeneracies of the states a-d. Sort the four states in the order of increasing energy. (a)l-------- (b)l+++--- (c)l++--++ (d)l++-- l---+---- l---+++ l++--++ l++-- l--+++--- l+++--- l++--++ l++-- l---+---- l---+++ l++--++ l--++ l-------- l+++--- l++--++ l--++ l l l l--++ --------- ------- -------- ------- 4. A quantum particle with mass m in a harmonic potential is described by the hamiltonian H=p^2/2m+1mw^2x^2/2. Define the non-hermitian ladder operators: a=(mw2pi/2h)^1/2(x+ip/mw) a*=(mw2pi/2h)^1/2(x-ip/mw) We have showed that H can be re-written in terms of a and a* as H= hw(a*a+1/2)/2pi. On addition, a ψn(x)=n^1/2ψn-1(x) and a*ψn(x)= (n+1)^1/2ψn+1(x), whereψn(x) denotes the eigenfunctions of H with vibrational quantum number n=0,1,2.........Answer the following questions using the porperties of ladder operators: (a)(5%)Evaluate Hψn(x) using ladder operators to find the energy levels En. (b)(5%)The ground state state wavefunction is ψo(x)=(alfa/pi)^1/4* exp(-alfax^2/2) with alfa=mw2pi/h. What is the wavefunction of the first excited state ψ1(x)? Hint:use Px=-ihd/2pidx to evaluate a*ψo(x). (c)(10%)Consider the state ψ that is the equal superposition of ψo and ψ1:ψ=c{ψo(x)+ψ1(x)}.What is the value of the normalization constant c (assuming a real number)? What is the average ebergy and the standard deviation in energy? (d)(10%)Calculate the expectation values <x> ad<x^2> for the state ψ. (e)(5%)Sketch rough graphs of ψo(x), ψ1(x) in the harmonic potential. Label the energy levels. Use the inerference of ways to explain what you found in (d). 5.Answer true or false for the following statements(3 points each): (a)The zero point energy is lower for a He atom in a box than fot an electron. (b)Molecules with a longer pi-conjugated system tend to absorb photons with higher energyies. (c)If g(x) is an eigenfunction of the linear operator A, then cg(x) is also an eigenfunction of A, where c is an arbitrary constant. (d)According to the superposition principle, if g1(x) and g2(x) are both eigenfunctions of the linear operator A< then their linear combinations are also eigenfunctions of A. (e)The wavefunction of a system must satisfy the time-independent Schrodinge equation. (f)If we measure the observable A when the system's wavefunction is not an eigenfunction of A, then we can get an outcome that is not an eigenvalue of A. (g)For the n=25 harmonic oscillatoreigenfunction, the sign of ψ in the right=hand classical forbidden region is opposite the sign in the left- hand classical forbidden region. 6. Bonus questions(5 points each): (a)Explain why a harmonic osccillatorwhose energy expectation value equals to zero must violate the Heisenberg's uncertainty principle. Hint: you can take it for granted that if A is a hermitian operator, then the expectation value of A^2 for any wavefunctions must be greater or equal to zero. i.e.(A^2) >= 0 (b)Electron tunneling occurs in the scanning tunneling mucroscope, which makes possible atomic resolution of surfaces. Explain why? Hint:use the distance dependence of electronic tunneling probabilities. --

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