# Homework Help: Determine electronic transitions by the emitted wavelength?

1. Sep 1, 2011

### FountainPen

1. The problem statement, all variables and given/known data
What is the electronic transition of He II when it emits 468.6 nm photon.

2. Relevant equations
$$\frac{1}{\lambda}=4R\left(\frac{1}{m^2}-\frac{1}{n^2}\right)$$

3. The attempt at a solution
I know it is a pashen-alpha line from googling but I don't know how to find that from this equation with two unknowns. I know the energy levels are discrete so there is probably only one transition that makes this wavelength. Don't know why He II and not He I so I might be missing something.

2. Sep 1, 2011

### ehild

HeI is the neutral He atom, HeII is the once-ionized helium, it has a single electron. The energy levels of HeII can be treated with the Bohr model, with Z=2.

You can find n and m by trial and error. They must be integer numbers in principle. m is the final state, it can be 1, 2, 3,... Find n for each of them. Because of the uncertainty of the wavelength, it will not be an integer number exactly, but it has to be very close to an integer.

ehild

3. Sep 1, 2011

### FountainPen

Thanks for explaining what He II is. I thought there would be some sort of mathematical way to determine the transition without brute force. Oh well.