• Support PF! Buy your school textbooks, materials and every day products Here!

Question about quantum harmonic oscilator

Hi, I am preparing for a quantum mechanics exam, and I have this problem that I can`t solve:

I have to find the complete energy eigenvalue spectrum of a hamiltonean of the form:

H = H0 + c

and also another of the form

H = H0 + [tex]\lambda[/tex]x[tex]^{2}[/tex]

Where in both cases, H0 is the hamiltonean of an harmonic oscilator, c and lambda are constants. The variable is x.

I have to find the exact eigenvalues, cannot use perturbation theory, but I can use the fact that I already know the eigenvalues for the harmonic oscilator.

Can anybody help?

Thant you.
 

Answers and Replies

George Jones
Staff Emeritus
Science Advisor
Gold Member
7,231
785
H = H0 + c
Let's do this one first. After we get this, we'll do the second one. Might have to take a supper break in the middle

Suppose |psi> is such that

H0 |psi> = E |psi>.

What does H |psi> equal?
 
Thak you.

Well, I think that H|Psi> = (Ho+C)|Psi> = (E+C)|Psi>

The the eignevalues are just E+C, where E is the harmonic oscilator energy:

E = (n+1/2)[tex]\hbar[/tex][tex]\omega[/tex]

If it is just that it was so easy, but the second part looks more complicated.

I made a mistake when I copy the problems, the hamiltonean should read:

H = H0 + [tex]\lambda[/tex]x (instead of the x being squared).

Thank you!
 
George Jones
Staff Emeritus
Science Advisor
Gold Member
7,231
785
I made a mistake when I copy the problems, the hamiltonean should read:

H = H0 + [tex]\lambda[/tex]x (instead of the x being squared).

Thank you!
I think I liked it better with [itex]x^2[/itex]!

Maybe try and complete the square for the two terms

[tex]\frac{1}{2}m \omega^2 x^2 + \lambda x[/itex].
 
OK, I completed squares so the potencial energy part of the hamiltoneal reads:

(1/2) mw[tex]^{2}[/tex] [ x + ([tex]\lambda[/tex]/ (m w[tex]^{2}[/tex])) ][tex]^{2}[/tex] - ([tex]\lambda[/tex][tex]^{2}[/tex] / 2m).

I think that I should now define y as y= x + ([tex]\lambda[/tex]/ (m w[tex]^{2}[/tex]))

So that the term reads

(1/2) mw[tex]^{2}[/tex] y[tex]^{2}[/tex] - ([tex]\lambda[/tex][tex]^{2}[/tex] / 2m)

So, as a function of y, the hamiltonean is again an harmonic oscilator plus a constant, and I can solve it as the previous excercise.

My doubt is if the change of variable may alter the kinetic energy part of the hamiltonean, which depends on the second derivative of x. My guess is that not because the relation between x and y is linear.

Thank you for you help!
 
George Jones
Staff Emeritus
Science Advisor
Gold Member
7,231
785
[tex]\frac{d}{dx} = \frac{dy}{dx} \frac{d}{dy} = \frac{d}{dy},[/tex]

so, as you say, I don't think the kinetic energy term changes.
 

Related Threads for: Question about quantum harmonic oscilator

  • Last Post
Replies
3
Views
1K
  • Last Post
Replies
9
Views
4K
  • Last Post
Replies
15
Views
824
  • Last Post
Replies
4
Views
1K
  • Last Post
Replies
13
Views
2K
  • Last Post
Replies
8
Views
1K
  • Last Post
Replies
0
Views
2K
  • Last Post
Replies
0
Views
2K
Top