# I How can an atom change from ground to excited state?

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1. Mar 12, 2017

### ConfusedRookie

I am currently studying a Foundation Program, which includes chemistry.
-I can easily understand the distinguishable characteristics between ground and excited state via writing the electron configuration.
-But how can an tom changes from ground to excited state ? And can an atom changes from ground to excited state if it's at rest ?

2. Mar 12, 2017

### blue_leaf77

By receiving certain amount of energy which equals the energy gap between the ground and that excited state.
I don't understand what you mean there.

3. Mar 12, 2017

### bhobba

I think he means the phenomena of spontaneous emission.

The answer to that one is rather advanced, but I will give it anyway:
http://www.physics.usu.edu/torre/3700_Spring_2015/What_is_a_photon.pdf

Its because an electron is coupled to the quantum EM field that permeates all space so really isn't in a stationary state. At some unpredictable time it will spontaneous;y emit a photon by Fermi's Golden Rule for perturbations:
https://en.wikipedia.org/wiki/Fermi's_golden_rule

The fact it isn't strictly stationary can be considered a small perturbation from it actually being stationary.

Thanks
Bill

Last edited: Mar 13, 2017
4. Mar 12, 2017

### blue_leaf77

Actually the OP talked about excitation from the ground to excited state, it sounds more like he was asking if there is such a thing as "spontaneous excitation" which is what confuses me.

5. Mar 12, 2017

### bhobba

Indeed he did.

I was just reading between the lines - it was basically restating what he first said so I thought he really wanted to know the reverse - why it jumps down levels - I could be wrong of course.

Hopefully the OP will clarify.

Thanks
Bill

6. Mar 13, 2017

### ConfusedRookie

Hi everyone, sorry for the confusion. My problem is from this statement
"The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.

It follows that the hyperfine splitting in the ground state of the cesium 133 atom is exactly 9 192 631 770 hertz, ν( 133Cs)hfs = 9 192 631 770 Hz. At its 1997 meeting the CIPM affirmed that: This definition refers to a cesium atom at rest at a temperature of 0 K."

-> I get that when an atom receive energy it will become excited. And when it become excited, it moves (that why metals melt, that's also how liquids become gases)

-> What I am not sure is if an atom can become excited despite not moving ?

7. Mar 13, 2017

### Staff: Mentor

The motion you are discussing represents kinetic energy, which is only one form of energy. There can also be internal motion, such as vibration inside a molecule. There can also be other internal states, such as electronic states.

In the case at hand, the two levels involved are called hyperfine states. They are due to the interaction between the spin of the nucleus and the spin of the outermost electron in cesium. You can think of each spin as creating a little magnet, and you will get two different energy states, depending on whether the two magnets are aligned or anti-aligned, whether you get north with north and south with south, or north with south and south with north (the latter being of course lower energy).

8. Mar 13, 2017

### ConfusedRookie

Hi Dr Claude,
Could I sum up what I understand from you ?
=> The energy level in electron shells and the energy level in my case are basically the same, but not the same cause ?
In terms of the hyperfine states, I mean when an atom change from one hyperfine states to another. Does it mean it change from ground to excited states, too ? Or how energy level changes is different, too ?

9. Mar 13, 2017

### blue_leaf77

It could mean that the source tries to exclude the possibility of line shift due to Doppler effect when the atoms are in relative motion (as is the case if the temperature is different from 0 K) with respect to the detector.

10. Mar 13, 2017

### Staff: Mentor

Correct. One usually treats atoms in the following way. First, one considers only the Coulomb interaction, in a non-relativistic setting. This is the hydrogen atom as found in a first course on quantum mechanics. Then one has to include relativistic corrections, most notably the spin-orbit interaction (in H, it lifts the degeneracy between the different $l$ for a given $n$). Later, one also needs to take into account the spin of the nucleus, leading to hyperfine interaction.

It depends by what you mean with "ground." The usage I am most familiar with would call the hyperfine states we are talking about to be the hyperfine manifold of the ground state. The "ground electronic state" thus refers to the lowest energy state before hyperfine interaction is considered. But there is a difference in energy between the hyperfine states, so all but one hyperfine states are not the "true" ground state.

11. Mar 13, 2017

### ConfusedRookie

Thank you Blue_leaf77 and Dr Claude.
I am sure I need to research more on this topic as there are so many terms I have never seen before (such as hyperfine states,...).
Could I have one last question ? It's very close to the topic's question.

According to what I understand and learn after the forum and other explanations, there are many things about the ground-excited state that I am sure I am clear about but not sure if I understand the correct way:

A) Temperature
- Temperature = constant heat supplied (constant energy supplied).
- Temperature does not exist when temperature is 0 kelvin (absolute zero), which means no temperature
=> This explain why atoms still move at room temperature because there is always energy supplied from "temperature" or heat supplied. The more heat is supplied, the faster the atoms move. The only case that has no "temperature"/ "energy supply"/ "heat supply" is 0 kelvin (absolute zero) .

=> If we heat an atom to some point, it's excited. But afterwards, it is in its ground state again.

So from what I know, there are 3 things unclear to me.
1) Temperature = Constant source of energy supply = constant source of heat supply. This is the reason why we say that in the natural environment, atom will not stop moving ( As there is a constant source of energy supply).

2) "If we heat an atom to some point, it's excited. But afterwards, it is in its ground state again". In this case, we can fix the problem, we can make the atom always excited by constantly heating it or another way (this way I am not sure) is put in an environment with the suitable temperature that can make it "always" excited. According to what I see, the method of heating is temporary but the method of putting it in an environment with suitable temperature is a forever method (theoretically)

3) If the temperature is 0 Celsius there still exists temperature. But if the temperature is 0 K, theoretically, there will be no energy or heat supplied, making the atom stop moving (although electron still move as cloud of electrons). Am I correct ?

Many thanks.

12. Mar 13, 2017

### Comeback City

13. Mar 13, 2017

### Staff: Mentor

That's not correct. It's hard to know at what level to answer you, so please start by reviewing Wikipedia: https://en.wikipedia.org/wiki/Temperature

Temperature still exists, indeed, it has the value of 0 K or -273.15 °C.

Moving at a constant speed does not require a supply of energy.

As far as motion of gas particles is concerned, kinetic energy, hence speed, is directly proportional to temperature, hence gas particles are always moving except at absolute zero. Note that this is not true for internal motion or vibrations in solids, since even in the ground state there is some residual vibrational energy (called zero-point energy).

If it is in an internal excited state, then it will eventually relax to its internal ground state. Note that if we have a collection of atoms and heat it up to the point that a significant proportion can be found in an excited state, then, if the system is isolated (constant E) at any given time there will always be a significant proportion of atoms in excited states, since energy is conserved (what one atom loses, another one must gain).

Not correct, see above.

You have to distinguish two situations. A system can be at a constant temperature because it is isolated, or because it is in contact with a reservoir that is at a constant temperature.

See above. Also, note that the third law of thermodynamics forbids any system to actually reach 0 K.

14. Mar 13, 2017

### Staff: Mentor

15. Mar 13, 2017

### Comeback City

So how would you define the temperature that the scientists got the potassium atoms to in the experiment? It certainly wouldn't be an infinite positive temperature.

16. Mar 13, 2017

### Staff: Mentor

No, it is a negative temperature, but it represents a system hotter that any system with a positive absolute temperature. In other words, if you were to put in contact a system with a negative temperature in thermal contact with a system with a positive temperature, then heat will flow from the negative to the positive until equilibrium is reached.

Negative temperatures are only possible for systems with a finite set of possible states.

17. Mar 13, 2017

### Comeback City

I will check it out!

18. Mar 13, 2017

### ConfusedRookie

Thank you, DrClaude
From what you say,
"A system can be at a constant temperature because it is isolated, or because it is in contact with a reservoir that is at a constant temperature."

You mean that the case of constant temperature and the case of heating up an atom are not the same and should be classified into two different situations. Either of the two cases, as long as "the temperature is met in the case of constant temperature" or "we supply enough heat required "continuously"", the atom will being kept excited ?

19. Mar 13, 2017

### Staff: Mentor

I mean that constant temperature can be either due to a system being isolated or being in contact with a reservoir. For a big enough system, both situations lead to the same results. (For simplicity, let us avoid the complications of small systems. Temperature is easier to understand for an ensemble of particles.)

For a given temperature, a certain fraction of the atoms will be found in an excited state at any given time. No heat supply is necessary.