Determine electronic transitions by the emitted wavelength?

[FountainPen]

Homework Statement

What is the electronic transition of He II when it emits 468.6 nm photon.

Homework Equations

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

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.

 

[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

 

[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.