CO2 molecule is a linear symmetric molecule, with two oxygen atoms on both sides of the carbon atom, indicating the equilibrium position of the atom. The atoms in the molecule are always in motion and must vibrate around their equilibrium positions. According to the molecular vibration theory, CO2 has three different vibration modes:

1. two oxygen atoms vibrate perpendicular to the molecular axis in the same direction, while carbon atoms vibrate perpendicular to the molecular axis in the opposite direction. Because the vibration of the three atoms is synchronous, it is also called deformation vibration

2. the two oxygen atoms vibrate in opposite directions along the molecular axis, that is, the two oxygen atoms reach the maximum value and equilibrium value of vibration at the same time, while the carbon atom in the molecule is still, so its vibration is called symmetric vibration.

3. three atoms vibrate along the axis of symmetry, in which the vibration direction of carbon atom is opposite to that of two oxygen atoms, which is also called antisymmetric vibration kinetic energy. In these three different modes of vibration, different groups of energy levels are determined.

Carbon dioxide laser is a kind of molecular laser. The main substance is carbon dioxide molecule. It can represent a variety of energy states, depending on its vibration and rotation.

The mixed gas in carbon dioxide is the plasma (plasma) formed by the low-pressure gas (usually 30-50 Torr) caused by the release of electrons. As the Maxwell Boltzmann distribution law says, in the plasma, molecules show a variety of excited states. Some will present high-energy states (00o1), which show asymmetric oscillation.

Through natural emission, this high-energy state will fall to the symmetrical swing state (10o0) and emit photons that may propagate in any direction (a wavelength of 10.6 μ M).

Excitation process:

In the CO2 laser tube, the main working substance is composed of CO2, nitrogen and helium. CO2 is the gas generating laser radiation, and nitrogen and helium are auxiliary gases.

With helium, the thermal relaxation process of 010 level can be accelerated, which is more conducive to the evacuation of 100 and 020 laser levels. The addition of nitrogen mainly plays an energy transfer role in the CO2 laser tube, and plays a strong role in the accumulation of upper level particles of CO2 laser and the high-power and high-efficiency laser output.

Excitation condition of CO2 laser tube: DC current of tens of Ma or hundreds of Ma is usually input into the discharge tube. During discharge, the nitrogen molecules in the mixed gas in the discharge tube are excited by the impact of electrons. At this time, the excited nitrogen molecule collides with CO2 molecule. N2 molecule transfers its energy to CO2 molecule. CO2 molecule transitions from low energy level to high energy level to form particle number inversion and emit laser.