chapter one

1.6:i
Energy Transfer in the Discharge:i

The most commonly observed laser transition in the CO2 molecule, barring the use of any frequency tuning mechanisms, are from the CO2 asymmetric stretch transitions, from the (00*1) to the (100*0)(10.6) micron and (02*0) (9.6) micron states, using the notation (v1v2*v3), where v1 refers to the symmetric stretch quantum number, v2 refers to the asymmetric stretch quantum number and v3 refers to the asymmetric stretch quantum number. There are literally dozens of other lasing transition. Which employing an interactivity grating .In a CO2 laser, depending on instantaneous gain medium and resonator conditions can easily choose.:i

Any single possible laser line can be forced through the use of an interactive grating. Rotational structure, having energies clustered very close to one another, may exist at any time. Nonradiative decay to short-lived lower lying states followed by Nonradiative decay to the ground state follows .N2 is added to the laser gas to more efficiently transfer energy from electron impact to the CO2 upper vibrations laser level. The glow discharge is a very effective mechanism for vibration excitation of nitrogen. Since N2 is ahomonuclear molecule, dipole radiate de-excited is forbidden. This allows for long-lived vibration states, which makes excited N2 molecules more readily available for collision excitation of CO2.:i

De-excitation is only accomplished collisionally with the wall or other gas constituents, the most beneficial of which is the CO2 molecule .The N2 (v=2) state is only 18cm-1 (2.2E-3 eV) from the upper laser level of the CO2 molecule .This makes resonant energy transfer between N2 and CO2 more likely.This energy is much smaller than the average kinetic energy of the molecules in the surrounding glow ,so vibration energy can easily be supplied to the CO2 molecules .:i

Energy transfer occurs from vibration levels up to v=4 in N2,because the ensuing anharmonicity of these states ,due to bond stretching ,is still well blow the average molecular kinetic energy .CO is isoelectronic with N2 and also has vibration levels easily excited in the glow discharge [24].:i

Thermal poisoning can occur, which is build up of lower lasing level populations in CO2.This results in a reduction in laser output power due to a clogging of the path from the upper lasing level to the ground state, where the CO2 upper lasing level is most efficiently populated through collisions with N2 these lower levels are cooled by the addition of He to the gas mix helium energy levels are much higher than the molecular energies of N2and CO2, above 20 eV.:i

For typical electron energies in the glow discharge of 1 to 3 eV, the discharge is not significantly affected by the addition of He other than to raise the electron temperature of the discharge [25].:i

Since the first ionization level of the He is higher than that of the other gas components, high energy impacts (higher “voltage”) is required to make it apart of the glow conducting path. Only a small amount of energy is lost from the discharge due to inelastic collisions with He and subsequent collisions with the walls. Thermal conductivity in gases is independent of pressure and since thermal conductivity of the He is roughly six times that of CO2 and N2, He makes an efficient transporter of waste heat to the walls of the discharge tube. The efficiency of heat transfer resulting from the addition of He to the mixture allows for higher discharge current before radiation saturation [23].:i

CO may also be added to the laser mix to improve efficiency, but it dose not transfer vibration energy as efficiently as N2, due to a difference between the CO v=1 level and the CO2 upper lasing level of 170 cm-1 . CO also has a dipole moment which creates a radiative decay channel to depopulate the electron impact excited CO, thus making CO less available for the job of CO2 excitation. CO is also a component in the dissociation equilibrium of CO2, so when using added CO with CO oxidation catalysis, larger concentrations of CO effect the CO2 concentration not always in a predictable manner. :i With these drawbacks, CO still adds to more efficient CO2 vibration excitation than electron impact alone. H2O can be added as a heat transfer enhance but is less efficient at cooling than He. H2O, in small concentrations, also has the beneficial side effect of homogeneous catalytic recombination of the dissociated CO2 products, CO and O.H2O in a larger concentration overwhelms the beneficial catalytic effects and effectively depopulates the upper lasing levels of CO2 .:i

Xe may also be added to a laser gas mix to effectively cool the electron temperature of the discharge for a given current, thereby reducing the amount of electron impact dissociation of CO2. The prohibitive cost of laboratory grade Xe prevented this investigator from utilizing it [25].:i