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QM  matrix elements of operators 
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#1
Apr2308, 11:32 AM

P: 58

1. The problem statement, all variables and given/known data
we shall describe a simple model for a linear molecule, say, CO2. the states L>, C>,R> are the eigenstates of D operator (corresponds to dipole moment) DL>=dL> , DC>=0 , DR>= +dR>. When the electron is localized exactly on the carbon atom, its energy is E1 and when it is localized on one of the oxygen atoms, it has energy E0 ( assuming E1 > E0). In addition, the electron can "jump" from one atom to another, and this jump is characterized by kinetic energy a. We can assume that the jumping occurs only between nearest neighbors. Write matrix representation of the Hamiltonian in the basis of L>, C>,R>. 2. The attempt at a solution we should build the matrix from the matrix elements  <LHL>, <LHC> etc. the matrix should look like that: E0 a 0 a E1 a 0 a E0 it seems very logical, but I still don't see the direct connection between the matrix elements and the given data. for example, why does the element <LHL> equals E0? what does the element <1H2> mean in general? 


#3
Apr2308, 11:50 AM

P: 58

well, the problem is that L> is not an eigenstate of H.
So all I know is that L> can be represented as a superposition of the Hamiltonian's eigenfunctions... What I understand from the data is that when I measure the energy in the state L> I get the result E0... so using H will change the state to one of the eigensates of H and yield the result E0...(?) 


#4
Apr2308, 01:11 PM

Mentor
P: 6,248

QM  matrix elements of operators
[tex]H\leftL\right> = E_0 \leftL\right> + a \leftC\right>.[/tex] Do you see why? I also don't find this to be very intuitive. For example, I get the energy eigenvalues of [itex]H[/itex] to be [tex]\frac{1}{2}\left( E_1 + E_0 + \sqrt{\left( E_1  E_0\right)^2 + 8a^2}\right)[/tex] [tex]E_0[/tex] [tex]\frac{1}{2}\left( E_1 + E_0  \sqrt{\left( E_1  E_0\right)^2 + 8a^2}\right)[/tex] with associated eigenvectors [tex]\left L \right> + \frac{1}{2a}\left( E_1  E_0 + \sqrt{\left( E_1  E_0\right)^2 + 8a^2}\right) \left C \right> + \left R \right>[/tex] [tex]\leftL\right>  \leftR\right>[/tex] [tex]\left L \right> + \frac{1}{2a}\left( E_1  E_0  \sqrt{\left( E_1  E_0\right)^2 + 8a^2}\right) \left C \right> + \left R \right>[/tex] 


#5
Apr2408, 05:20 AM

P: 58

first of all  thanks for your reply(:
secondly  yes, from the matrix I can see very clearly how H acts on any of the states. All I have to do is to multiply it (from the right) with the appropriate column vector; for instance, (1,0,0)T (T denotes "transpose") will represent L>. I also understand how to find the eigenvalues and the eigenstates of H  directly from the matrix. But these problems are to be solved after you find the matrix. And this is the part that is unclear to me how to create the matrix according to the given data. And to clarify my point: the problem is not technical. I know how to solve the exercise. My problem is that I don't understand why the following arguments are correct: 1) from the fact that the energy of the system in the state L> is E0 > we can conclude directly that <LHL> = E0. 2) in a similar way  from the fact that the electron can "jump" from L> to C> (and vice versa) and that this jump is characterized by kinetic energy a > we deduce that <CHL> = <LHC> = a. and so on... 


#6
Apr2408, 06:46 AM

Mentor
P: 6,248

For the given the matrix representation of H, this argument, however, is incorrect. I used Maple to solve for the energy eigenvalues and eigenstates, and the solution shows that L> is a complicated linear combination of three energy eigenstates. So, if the system is in state L>, then the given matrix implies that there are three possible energies of the system that each have a nonzero probability. This clashes with the statement of the question, which makes me wonder whether the question is even consistent. Did the question come from a book? If so, which one? From the prof? 


#7
Apr2408, 08:08 AM

P: 58

Hi again,
this is exactly what I thought  that L> should be an eigenstate of H, but it is not..!! the question comes from our tutorial, and we also had a homework assignment with a similar exercise. I believe the question was invented by our prof. or tutor. It does seem to be illogical and inconsistent. thanks a lot, anyway 


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