Types of Lasers and CO2 Molecules

Types of Lasers

• Carbon Dioxide Lasers belong to the class of molecular gas lasers.
• Electrons in molecules can be excited to higher energy levels.
• The distributions of electrons define the electronic state of the molecule.
• Molecules also have other energy levels besides electronic levels.

Carbon Dioxide (CO2) Molecules

• Carbon dioxide (CO2) is a symmetric molecule (O=C=O).
• It has three modes of vibration.
  • Symmetric vibration (0.172 eV)
  • Bending vibration (0.083 eV)
  • Asymmetric vibration (0.291 eV)

Vibrational Modes

• Symmetric Stretching Mode: The carbon atom remains stationary while both oxygen atoms move simultaneously along the molecular axis.
• Bending Mode: Oxygen and carbon atoms vibrate perpendicular to the molecular axis.
• Asymmetric Stretching Mode: Oxygen and carbon atoms oscillate asymmetrically, with oxygen atoms moving in one direction and carbon atoms in the opposite direction.

Active Medium

• The active medium consists of a mixture of CO2, N2, and He or water vapor.

Optical Resonators

• A pair of concave mirrors, one completely polished and the other partially polished, are placed on either side of the discharge tube.

Population Inversion

• Population inversion is achieved through the application of an electric discharge to the gas mixture.
• When an electric discharge is applied to the CO2, N2, and He/water vapor mixture in a tube, electron collisions excite the molecules to higher electronic, vibrational, and rotational energy levels.
• These levels are populated further via radiationless transitions from higher excited states.
• The presence of other molecules, like N2, helps resonant energy transfer, improving pumping efficiency. Nitrogen's role is analogous to helium in a He-Ne laser.

Diagram of Carbon Dioxide Laser

• Shows the components including mirrors, gas flow, electrodes, and vacuum pump.

Carbon Dioxide Laser Characteristics

• Capable of generating continuous laser beams with several kilowatts of power while maintaining spectral and spatial coherence.
• Molecules have a more complex energy level structure than atoms/ions due to electronic, vibrational, and rotational motions.

Energy-Level Diagram

• The energy-level diagram illustrates the main physical processes taking place in the laser.
• An electric discharge applies energy to a CO2, N2, and He mixture causing electrons to excite nitrogen molecules, increasing vibrational rotational energy levels.
• The excited state aligns with the 4 (001) vibrational-rotational level in CO2 molecules.

Initial Energy Absorption

• Initially, most electrical discharge energy is absorbed by nitrogen molecules.
• A small portion directly promotes CO2 molecules from their ground state (000) to the excited vibrational state (001).

Subsequent Energy Transfer

• Energy subsequently transfers through collisions between N2 and CO2 molecules.
• N2 transfers its excitation energy to CO2, promoting more CO2 molecules to excited vibrational states.

Laser Light Emission

• Excitation causes CO2 molecules in the vibrational (001) state to release energy and transition to lower vibrational levels like (100) or (020).
• Released energy emits laser light in 10.6 µm or 9.6 µm wavelengths.

Residual Transitions

• Residual decay transitions from higher levels to lower levels primarily dissipate energy as heat, not light.

Population of Fourth Energy Level

• The population of the fourth energy level in the CO2 laser is increased by the presence of He molecules, which helps depopulate the lower laser levels.

Output Power Dependence

• The CO2 laser's output power directly correlates to the length of the active medium.

Continuous-Wave (CW) CO2 Lasers

• Low-power CW CO2 lasers (up to 50 W) are usually available in sealed tube configurations.

Advantages of CO2 Lasers

• Simple construction
• Continuous output, beneficial for steady laser beam applications
• High efficiency
• High output power, useful in industrial and medical applications that require powerful beams
• Scalable power output by adjusting the gas discharge tube length improving control over power levels.

Disadvantages of CO2 Lasers

• Contamination effects (oxygen forming CO, impacting efficiency)
• Temperature sensitivity (operating temperature significantly affects output power, consistent performance needs optimal conditions.)
• Corrosion of reflecting plates (mirrors, especially high-reflectivity ones, susceptible to corrosion over time)
• Risk of eye damage (infrared radiation emission, invisible to the naked eye; accidental exposure poses serious eye damage risk without visual warning).

Vibrational Energy Levels in CO2

• V1, V2, and V3 relate to vibrational energy modes of the CO2 molecule.
  • V1 (symmetric stretching)
  • V2 (bending)
  • V3 (antisymmetric stretching)
• These correlate to specific atomic movements.
• Energy levels are labeled according to the vibrational mode excitation.

Explore the fascinating world of lasers, focusing on carbon dioxide (CO2) molecules and their vibrational modes. This quiz delves into the molecular energy levels and the different types of vibration that CO2 undergoes. Test your knowledge of laser technology and the properties of CO2 as an active medium.