# Resonance in electron impact on atoms

1. Nov 22, 2014

### vrinda mukund

Can someone please explain the significance of resonance levels that forms when electron collides with atoms. How this resonance levels are different from other levels ?

2. Nov 22, 2014

### Simon Bridge

You should be able to answer that from the meaning of "resonance".
Have a go, and then we' all be able to help you better.

3. Nov 23, 2014

### vrinda mukund

@Simon Bridge

Dear Sir,

When two frequencies match together so that the system oscillates at larger amplitudes, we say the system is in resonance. My doubt is how it is applicable to excited levels. Are they the levels with the same value for angular momentum J ? I that case, what makes it so special or different from other excited states.
To be more elaborate on my problem :

Ground state of Xenon is 5s2 5p6
First excited state for Xenon will be 5p6 6s1
If we write the term symbols the there will be five states :
1P1 3P0 3P1 and 3P2
Now, the states 1P1 and 3P1 are resonance levels.
So what i have guessed is levels with same value for J are said to be resonance levels . Is it really so ? How is it different from other levels 3P0 and 3P2

4. Nov 23, 2014

### Simon Bridge

... close:
If you make a plot of the amplitude vs driving frequency for a driven oscilator, you get a characteristic peak near the natural frequency of the oscillator. That peak is called "a resonance".

... in the case of electrons being shot at some atoms, what is the graph that would be plotted? What shape would it have?

5. Nov 23, 2014

### vrinda mukund

For electron-atom collisions, we can make energy Vs Cross section plots. We can plot how the cross section of a particular reaction, say excitation, changes with increasing energy of incident electron. The cross section will first increase with increase in energy and then will fall down.

6. Nov 23, 2014

### Simon Bridge

Well done - between the "increase" and "fall down" there is a peak; that peak is called a "resonance peak".
We say that the particular process that the cross-section is for has a resonance at the energy level of the peak.
That is basically the entire significance of "resonance" in this context.

7. Nov 23, 2014

### vrinda mukund

In that case, in the example of Xenon, which i explained before, is it like, the excitation cross section for excitation to the 5p5 6s1 levels will be maximum when electrons are excited to 1P1 and 3P1 states ?