Molecular Lasers

Molecular lasers are the most high powered and most efficient type of gas laser. They work by the transfer of vibrational energy from one type of molecule to another. The atoms making up the molecule, when it is excited, vibrate relative to one another, and the molecule has energy levels similar in form, although of different value, to the energy levels of isolated atoms. But the process does not involve the movement of orbiting electrons to more distant orbits.

The type of structure used in molecular lasers is very different from that used in neutral atom lasers. In one type of molecular laser, flowing nitrogen is excited by electrical discharge and then flows into the tube between the end mirrors which contains the active gas. This is usually carbon dioxide. The vibrational energy of the nitrogen molecules is transferred to the carbon dioxide molecules by collision, and the carbon dioxide atoms later return to the ground state giving up the energy to the laser beam.

An efficiency of about 15 per cent was obtained with the carbon dioxide type, which is far greater than the 0.1 per cent achieved with the neutral atom lasers. Apart from the fact that it is more efficient than the other molecular gases, carbon dioxide also has the advantage that it is chemically stable and can, if necessary, be excited directly by an electric discharge.

The path between the end mirrors, that is, the length of the carbon dioxide laser tube, must be as long as possible if large amounts of power are to be obtained. Tubes 20 m long have been used and if such lasers are to be conveniently mo­bile they must be folded in some way. Two, three and four tubes placed parallel to each other and optically coupled have been used in some carbon dioxide lasers.

Abbreviations

mc – megacycle – единица измерения частоты = 1млн герц = 1 Мгц

d.c. – direct current – постоянный ток

torr – тор - единица измерения давления

I тор = I мм ртутного столба

760 тор = I атмосфера

Check your understanding

Exercise 1. Answer the following questions.

1. Why do gases offer interesting possibilities as laser materials?

2. What is the most high powered type of gas laser?

3. Describe how helium-neon laser works. Why is the helium-neon type of laser of great value?

4. How do molecular lasers operate?

5. What are the advantages of carbon dioxide laser over neutral atom laser?

Exercise 2. Rearrange the sentences in a logical order to make a summary of the text.

1. If we want to obtain large amounts of power, the length of the carbon dioxide laser must be as long as possible.

2. The neon lasers were the first to be discovered and studied.

3. The vibrational energy of the nitrogen molecules transferred to the carbon dioxide molecules by collision.

4. Molecular lasers work by the transfer of vibrational energy from one type of molecules to another.

5. The energy from the internal discharge excites the helium atoms to a very high level. From which they normally would radiate energy.

6. The beam in the neon laser is reflected back and forth through the length of gas-filled tube by semi-reflecting end plates.

7. Later carbon dioxide atoms return to the ground state.

Exercise 3. Is it false or true?

1. Gas lasers, namely helium-neon lasers, have a very low level of monochromaticity.

2. The efficiency of the carbon dioxide laser is greater than that of the neutral atom lasers.

3. The first gas laser was built in 1960 at Bell Telephone Laboratories.

4. The vibrational energy of the nitrogen molecules is transferred to the carbon dioxide molecules by radiation.