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The fundamental problem in harnessing nuclear fusion is getting the two particles (say, two deuterons) close enough together for the attractive (but short-range) nuclear force to overcome the Coulomb repulsion. The 鈥渂ulldozer鈥 method is to heat the particles up to fantastic temperatures and allow the random collisions to bring them together. A more exotic proposal is muon catalysis, in which we construct a 鈥渉ydrogen molecule ion,鈥 only with deuterons in place of protons, and a muon in place of the electron. Predict the equilibrium separation distance between the deuterons in such a structure, and explain why muons are superior to electrons for this purpose.

Short Answer

Expert verified

The equilibrium separation distance between deuterons isR=6.7310-13m.

Step by step solution

01

The equilibrium separation distance between the deuterons

The equilibrium separation distance between the deuterons butmemrthe reduced mass of themuon:

role="math" localid="1658383790270" mr=mmdm+md=m2mpm+2mp=m1+m/2mp.m=207me,

so

1+mmp=1+20729.1110-311.6710-27=1.056;mr=207me1.056=196me

02

Explaining why muons are superior to electrons

This shrinks the muonic \Bohr radius鈥 down by a factor of nearly 200. but nowis the ground state energy of the muonic atom. The potential energy associated with the deuteron-deuteron repulsion is the same as, and since the productis independent of mass, it doesn鈥檛 matter whether we write it using the electron values or the muon values. the entire molecule shrinks by that same factor of 196. The equilibrium separation for the electron case was 2.493 a (Problem 7.10),Therefore, The general equation of

R=2.4931960.52910-10m=6.7310-10m

Thus the equilibrium separation distance between deuterons is R=6.7310-10m.

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Most popular questions from this chapter

Using Egs=-79.0eV for the ground state energy of helium, calculate the ionization energy (the energy required to remove just one electron). Hint: First calculate the ground state energy of the helium ion, He+, with a single electron orbiting the nucleus; then subtract the two energies.

Apply the techniques of this Section to the H-and Li+ions (each has two electrons, like helium, but nuclear charges Z=1and Z=3, respectively). Find the effective (partially shielded) nuclear charge, and determine the best upper bound on Egs, for each case. Comment: In the case of H- you should find that (H)>-13.6eV, which would appear to indicate that there is no bound state at all, since it would be energetically favourable for one electron to fly off, leaving behind a neutral hydrogen atom. This is not entirely surprising, since the electrons are less strongly attracted to the nucleus than they are in helium, and the electron repulsion tends to break the atom apart. However, it turns out to be incorrect. With a more sophisticated trial wave function (see Problem 7.18) it can be shown that Egs<-13.6eVand hence that a bound state does exist. It's only barely bound however, and there are no excited bound states, soH- has no discrete spectrum (all transitions are to and from the continuum). As a result, it is difficult to study in the laboratory, although it exists in great abundance on the surface of the sun.

Quantum dots. Consider a particle constrained to move in two dimensions in the cross-shaped region.The 鈥渁rms鈥 of the cross continue out to infinity. The potential is zero within the cross, and infinite in the shaded areas outside. Surprisingly, this configuration admits a positive-energy bound state

(a) Show that the lowest energy that can propagate off to infinity is

Ethreshold=2h28ma2

any solution with energy less than that has to be a bound state. Hint: Go way out one arm (say xa), and solve the Schr枚dinger equation by separation of variables; if the wave function propagates out to infinity, the dependence on x must take the formexp(ikxx)withkx>0

(b) Now use the variation principle to show that the ground state has energy less than Ethreshold. Use the following trial wave function (suggested by Jim Mc Tavish):

(x,y)=A{cos(蟺虫/2a)+cos(蟺测/2a)e-xaandyacos(x/2a)e-y/axaandy>acos(y/2a)e-y/ax.aandya0elsewhere

Normalize it to determine A, and calculate the expectation value of H.
Answer:

<H>=h2ma2[28-1-(/4)1+(8/2)+(1/2)]

Now minimize with respect to 伪, and show that the result is less thanEthreshold. Hint: Take full advantage of the symmetry of the problem鈥 you only need to integrate over 1/8 of the open region, since the other seven integrals will be the same. Note however that whereas the trial wave function is continuous, its derivatives are not鈥攖here are 鈥渞oof-lines鈥 at the joins, and you will need to exploit the technique of Example 8.3.

(a) Prove the following corollary to the variational principle: If gs=0then谩贬帽Efe whereEfe is the energy of the first excited state. Comment: If we can find a trial function that is orthogonal to the exact ground state, we can get an upper bound on the first excited state. In general, it's difficult to be sure that is orthogonal togsi since (presumably) we don't know the latter. However, if the potentialV(x) is an even function of x, then the ground state is likewise even, and hence any odd trial function will automatically meet the condition for the corollary.

(b) Find the best bound on the first excited state of the one-dimensional harmonic oscillator using the trial function(x)=Axe-bx2

Use a gaussian trial function (Equation 7.2) to obtain the lowest upper bound you can on the ground state energy of (a) the linear potential V(x)=|x| (b) the quartic potential:V(x)=x4

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