Absorption Properties of SF₆ near 10.6 µm

Abstract

<p>The work described in this thesis concerns the investigation of the absorption properties of SF₆ in the 10.4 µm band of CO₂ lasers. Although SF₆ has been used frequently as a saturable absorber in CO₂ laser systems, the complex dynamical behaviour of its absorption not understood prior to our work. A model is developed which allows for the multilevel nature of the absorption processes. This model treats all the SF₆ vibrational and rotational levels as belonging to a bath of levels characterized by single vibrational temperature and a single rotational temperature. Energy absorbed from a laser pulse is rapidly distributed throughout this bath, establishing a new vibrational population distribution characteristic of the higher vibrational temperature.</p> <p>The vibrational equilibrium is readily maintained at high SF₆ pressures. It is demonstrated that in this case the model successfully predicts the transmission and pulse shaping of high power CO₂ laser pulses. In particular, the rapid decrease in absorption at high incident intensities is shown to be due to vibrational heating and not to any intensity saturation process. Dissociation which occurs as a result of sufficient vibrational heating is discussed. The effect of dissociation on single pulse transmission is determined through simple modifications of the model.</p> <p>The model is also shown to have application to low SF₆ pressures. Previous investigations concerning vibrational relaxation rates in SF₆ would indicate that vibrational equilibrium cannot be maintained in such a situation. More recent results show that in fact sufficiently rapid vibrational energy exchange can occur. The final portion of this thesis discusses the dependence of these rates on the vibrational energy of the molecules. An approximate, but conceptually simple, method of incorporating these effects into the model is described. In conclusion, it is shown that the model is capable of determining the SF₆ absorption behavior for a wide range of SF₆ pressures and CO₂ laser wavelengths when both intensity saturation and vibrational heating effects are included.</p>