Understanding Metastable Matter: How Can Compounds Form?

[karmicthreat]

I understand that some atoms have somewhat stable states at states other than the ground state. What I don't understand is how are compounds able to be made with these metastable atoms? Wouldn't the energy released uppon forming the compound or the increased energy needed just immediately disrupt the metastable. If you had a metastable helium ion floating around it could never combine and form a diatomic helium molecule and still have its electrons in their metastable shells right?

Edit:
Well other than the fact that heluim isn't diatomic except at millikelvin temps.

 

[Astronuc]

Metastable states

A metastable state in an atom (i.e. excited electron) is not really stable in the sense that it has indefinite lifetime, but it simply means that the lifetime is longer than a normal transition to ground state. The life time can be on the order of microseconds or even milliseconds.

Much of the recent work regarding metastable states is in the area of noble gases (He, Ne, Ar, Kr, Xe) and has applications in lithography and etching of semiconductors. Noble gases do not necessarily form compounds. Other metastable states have been explored in alkali and alkali Earth metals, and these would form compounds. Certainly in compounds, the molecular orbitals would prevent atomic metastable states.

However, there are certainly solid state materials, like Ti-doped sapphire (Al₂O₃) in which meta-stable states are exploited for production of laser.

Here are some references. The Nobel Lecture by Willis Lamb is particularly noteworthy for its historical significance. Enjoy.

http://raptor.physics.wisc.edu/expts/fb.htm

Lithography with metastable rare gas atoms
http://physics.nist.gov/Divisions/Div841/Gp3/Projects/Atom/metasam_proj.html

Atoms join in the race for lithography in the next century
http://physicsweb.org/articles/world/11/8/3/1

http://www.iqo.uni-hannover.de/ertmer/nebec/
Towards Bose-Einstein Condensation of Neon

http://arxiv.org/abs/physics/0402108
A discrete time-dependent method for metastable atoms in intense fields

http://www.icpig.uni-greifswald.de/proceedings/data/Baguer_1 (pdf file)
Role of the fast atoms, ions and metastable Ar atoms in a HCD by ...

http://chemistry.anl.gov/ResearchHighlights-2001-2002/Young-Highlight-2001.pdf
Optical production of metastable rare gases (abstract)

"Metastable rare gas atoms have found wide use in fields spawned by the revolution
in laser manipulation of atoms, such as cold collision physics, optical lattices, atom
lithography, rare isotope detection, and, most recently, Bose-Einstein Condensation
(BEC). The high internal energy of the metastable atom enables both novel detection
schemes in BEC studies and localized surface damage of suitable resists, and thus
provides a niche beyond similar studies with ground state atoms."

http://nobelprize.org/physics/laureates/1955/lamb-lecture.pdf
Willis E. Lamb, Jr Nobel Lecture (1955) - impressive history
Fine structure of the hydrogen atom

http://ipstmail.umd.edu/Hill_Lab/pub/jphysb_19_359_86.pdf
Quenching of resonant laser-driven ionisation at high buffer gas pressures

http://vcs.abdn.ac.uk/ENGINEERING/lasers/amplification.html

For spontaneous emission of a photon, the probability of occurrence is inversely related to the average length of time that an atom can reside in the upper level of the transition before it relaxes
known as the SPONTANEOUS LIFETIME
- typically, the spontaneous lifetime is some tens of nanoseconds
- the shorter the spontaneous lifetime, the greater is the probability that spontaneous emission will occur

For some pairs of energy levels in certain materials
- the spontaneous lifetime can be of the order of microseconds to a few milliseconds,
which is a METASTABLE STATE
- the likelihood that a spontaneous transition will take place between these levels is relatively low

As the likelihood of spontaneous emission decreases the conditions which favour stimulated emission are enhanced
- if an atom is excited into a metastable state it can stay there long enough for a photon of the correct frequency to arrive
- this will stimulate the emission of a second photon
- input of one photon in yields two out

 

[Gokul43201]

Astronuc's covered this very nicely, but let me add one little thing here : a simplified picture to help understand the why and how of metastable systems.

When a system is at Potential Energy minimum, it's in a stable state. If the walls surrounding this local minimum are large compared to perturbations (eg :thermal fluctuations, fluctuations in chemical potential), the stability is high. If the walls are comparable to the size of thermal fluctuations, you have a metastable state.

The lifetime of this metastable state depends on the height of these walls (activation energy) compared to the magnitude of thermal fluctuations at the ambient temperature. So, you can have metastable states that live for nanoseconds (eg: excited electronic states in fluorescent materials) or for years (eg : high temperature phases in quenched steels).