B Why do metastable states in atoms exist?

[Zahid Iftikhar]

Hi
My question relates to existence of metastable states in atoms which help out laser production. Is there any physical reason why some orbits allow electrons to stay for comparatively longer time 10^-3 s than others which allow only 10^-8s?
Is this stay time same for all materials?
Please guide.

 

[Drakkith]

I believe the reason is related to the underlying physics governing the transition between energy states, known as selection rules.
It's quite complicated and I don't know if there is an easy way to explain it. Perhaps someone more knowledgeable in this area can expand on this.

 

[DrDu]

Drakkith is right. The most common reason for metastable states in atoms (or molecules) is due to the metastable state having different spin than the ground state. Often, the ground state is a singlet (i.e. the spins of the electrons are all paired up) while some long-living excited state is a triplet state (i.e. the spin of two electrons is oriented parallel). As the coupling of the spin to orbital momentum is a relativistic effect, it is quite small, at least in lighter atoms.

 

[Zahid Iftikhar]

Thanks indeed. That sound interesting. I need to study it more.

 

[Marisa5]

In a triplet, the unpaired excited electron has the same spin orientation as the unpaired ground electron, so it has to spin flip to get back down. This has a very low probability of occurring which makes the triplet state of let's say neon, for instance, metastable.

 

[Zahid Iftikhar]

In a triplet, the unpaired excited electron has the same spin orientation as the unpaired ground electron, so it has to spin flip to get back down. This has a very low probability of occurring which makes the triplet state of let's say neon, for instance, metastable.

Marisa5 Thanks indeed for this further help. This all helped me a lot. Great forum, great people.

 

[BPHH85]

As a light addition to the already good explaind facts...for OLEDs (organic LED) there are some materials that are mostly triplet emitters through intersystem crossing process that lead to metastable states that are inefficient for light emission. Thus one will dope this materials with heavy atoms or molecules that lead to high spin orbit coupling to reduce the lifetime and increase the decay propability of this state to get a more efficient light generation. So you can have an influence on the lifetime of this states. At least in this case.