# Rotation Spectra - Basic transition question

1. Jun 14, 2014

### unscientific

1. The problem statement, all variables and given/known data

In rotations of diatomic molecules such as HCL, the hamiltonian is found to be:

$$\hat H = \frac{\hat L^2}{2\mu a^2}$$

where $\mu$ is the reduced mass, a is the separation.

(a) Find the energy levels and separation.

(b) Explain why rotational spectra of HCl due to emission or absorption of electric dipole radiation consists of equally spaced lines. Given spacing $20.57cm^{-1}$, calculate the separation $a$.

2. Relevant equations

3. The attempt at a solution

Part(a)

$$E = \frac{l(l+1)\hbar^2}{2\mu a^2}$$

$$\Delta E = E_l - E_{l-1} = \frac{l(l+1) - l(l-1)}{2\mu a^2}\hbar^2 = \frac{l\hbar^2}{\mu a^2}$$

Part (b)

By dipole transition selection rules, $\Delta l = \pm 1$.

m not sure why it would be equally spaced, since spacing depends on $l$. I tried letting $l \rightarrow \infty$, which implies spacing goes to zero as $l(l+1) − l(l−1) \approx l^2−l^2 = 0$.

2. Jun 14, 2014

### Staff: Mentor

Imagine that $\Delta E$ was independent of $l$, how many spectral lines would be present in the rotational spectrum?

Start plotting a rotational spectrum for the $0 \rightarrow 1$, $1 \rightarrow 2$, $2 \rightarrow 3$, etc. You will see clearly the pattern.

3. Jun 15, 2014

### unscientific

The problem is, I can't show why $\Delta E$ is independent of $l$. If it is, then of course for each spontaneous emission it would be of equal energy.

4. Jun 15, 2014

### king vitamin

This statement is false.

I think you need to keep thinking about your result. For example, what does the energy spectrum of your system look like? As a function of l? Maybe try plotting this (as suggested by DrClaude)?

5. Jun 15, 2014

### Staff: Mentor

Your equation is correct, $\Delta E$ is dependent on $l$.

My guess is that you are stumbling here because you are not making the right connection between the energy levels and the spectrum.

6. Jun 15, 2014

### unscientific

From 1 to 0, I get $\frac{\hbar^2}{\mu a^2} = E_0$.

From 2 to 1, I get $2E_0$

From 3 to 2, I get $3E_0$

and so on..

As you can see here, the energy emitted gets bigger and bigger.

But the difference between one emission and the another is constant, $E_0$.

Does this mean that the spacing is constant?

Using $\frac{\hbar^2}{\mu a^2} = 20.57cm^{-1} = 2057 m^{-1}$, where $\mu \approx m_e$

I get $a = 4 \times 10^{-21} m$, which is ridulous.

Last edited: Jun 15, 2014
7. Jun 15, 2014

### Staff: Mentor

That indeed means that the spacing is constant. Each line in the spectrum is separated from the next by $E_0$ (think photon energy).

You have to use the reduced mass of HCl. Also, you will have to convert 20.57 cm-1, which is wavenumbers, into energy.

8. Jun 15, 2014

### unscientific

Energy of emission is $\hbar k_l c$.

Thus, $\frac{\hbar^2}{\mu a^2} = \hbar c (\Delta k)$. Solving, I get $a = 3.24 \times 10^{-10} m$.

9. Jun 15, 2014

### Staff: Mentor

In spectroscopy, "wavenumbers" mean $\bar{\nu} = 1 / \lambda$, so the energy of emission is actually $h c \bar{\nu}$.

Apart from having used $\hbar$ instead of $h$, as noted above, your answer is two orders of magnitude too small.

10. Jun 15, 2014

### unscientific

Ok, so it should be:

$$\frac{\hbar^2}{\mu a^2} = h c (\Delta \bar \nu)$$

Using that, and $\Delta \bar \nu = 2057$, I get: $1.28 \times 10^{-10} m$

Why is it missing two orders of magnitude? I used $\mu = \frac{35m_p^2}{35m_p + m_p} \approx m_p$

Last edited: Jun 15, 2014
11. Jun 16, 2014

### Staff: Mentor

For your information, $\bar\nu$ corresponds to the wavenumber of the photon. Therefore, you have $| \Delta E_{\mathrm{atom}} | = h c \bar{\nu}$, so there is no $\Delta \bar \nu$. (I put absolute values on $\Delta E_{\mathrm{atom}}$ because it can be negative or positive for emission or absorption.)

That's not good enough for spectroscopic work. Normally, the problem should specify which isotope of Cl it is, as you can easily distinguish 35Cl from 37Cl in a rotational spectrum. In the absence of this information, I think you should either choose one isotope or use the average atomic weigth, but you can't use something like $35 m_p$ or the approximation of neglecting the presence of H (you are not comparing a proton with an electron). Look up the exact values (Wolfram Alpha is a good resource).

I don't know. Maybe if you list all the constants you are using I can spot the error.

12. Jun 16, 2014

### unscientific

I was asked to use the mass of the isotope 35Cl. Now with $35m_p$ in 35Cl, $m_p$ in H, the reduced mass becomes $\frac{35m_p^2}{36m_p} \approx m_p$.

I use $\Delta \bar \nu = \nu_l - \nu_{l-1}$ to represent the spacing between emissions.