Unravelling He-Ne Laser: Questions & Answers

[ananthu]

I have some serious doubts regarding He-Ne laser:

1. In He, the ground state energy value is taken is 0 eV and the meta stable state is taken as 20.61 eV. similarly in Ne the meta stable levels are taken as 20.66 and 18.7 eV respectively. How these values are arrived at? In Bohr's expression the ground energy value of hydrogen atom is taken as -13.6 eV. Next one is -3.4 eV and so on. How these negative values help in calculating the energy values of He and Ne? Because all the values expressed in the said laser are positive and none of them bear negative sign.

2.Why He is preferred along with Ne? Is it not possible to directly excite the Ne atoms to the meta stable state to produce laser, because it is only the neon atoms that are responsible for laser action? Is there any special reason?

3.After emitting the photons of wavelength 6328 Angstrom units, it is said that the electrons jump from the meta stable state of value 20.66 eV to another lower level 18.7 eV where it emits incoherent beam by spontaneous emission. Then, does it mean that the resultant beam from He-Ne laser consist of a mixture of both coherent and incoherent beams? Does it not violate one of the basic characteristics of laser which should be necessarily coherent?

4. Finally, has anyone found the reason for why only certain substances contain the so called meta stable levels but others do not, in nature?

 

[JeffKoch]

I'd suggest picking up a laser physics textbook and starting there, you have some fairly basic misunderstandings here - Milonni and Eberly is a good one, but there are many others with more or less technical background required. You might start with the Wikipedia article on "Helium-neon laser" and see if that makes sense.

 

[ananthu]

Thank you for your reply. I already went through the Wikipedia article on He Ne laser. But the answers to the specific doubts I have raised above are not found in the article. The other articles are too lengthy and complex to follow and understand. If anybody who has understood the topic very well could give simple briefing about what I have asked will be very useful to me.

 

[JeffKoch]

1) Energy is always defined relative to something else, higher or lower. Bohr's formula has negative signs because by convention the zero is the energy of an electron and proton "fully" separated and non-interacting. The bound states are therefore negative relative to this reference energy because an electron and proton would prefer to become bound, in fact as tightly bound as possible. The other values you quoted come from a reference that defines the ground, most-tightly-bound states of the neutral atoms as zero - so excited states are increasingly positive, ionized atoms are more positive still, and the limit of "fully" separated electrons and nucleii are much higher still. Relative to this fully ionized level, all the bound levels are therefore negative. Calculating these energy levels is an entire line of research that people spend professional careers working on - if you want to know the answer, you rely on these people and look it up in a book or in a journal article. This is pretty basic stuff, which is why I suggested you dig into this and learn more about it by reading.

2) This is explained in the wiki article. Electron collisions excite the He to a metastable state, not the Ne, and excitation transfer from He to Ne excites the Ne preferentially to a particular excited state, building up a population inversion in Ne.

3) Laser emit all kinds of lines that are not laser lines. If you look at any laser with a spectrometer, you find it is not monochromatic - the line itself has some spectral width that is not zero, there are other very weak emission lines, and generally there is a continuum baseline.

4) Metastable just means relatively stable, i.e. decay rates are low enough that these states are stable relative to other levels you care about. Often this means that there is not an "allowed" radiative decay directly to a significantly lower state, but otherwise there is nothing magic or special about certain elements or ionization stages.