Laser mixtures

Figure 7.3 Computed electron energy distributions for a 1-1-8 laser mixture (41).

Figure 7.8 Dependence of stimulated emission cross section for the P(20) line of the (001)-(100) laser transition in COa on gas temperature and pressure for 1-1-8 laser mixture [8].

Figure 7.9 Variation of small signal gain, a0 with gas temperature and electron density for a 1-1-8 laser mixture and reduced average electron energy of 1.5 eV [8].

Figure 7.16 Calculated temporal evolution of neutral minority species in 20 Torr 1-1-8 laser mixture [52],...

Figure 7.19 Calculated ionization stability boundary for 1-7-12 laser mixture [53].

Fig. 10. Diagram of the apparatus used to demonstrate NCl(o) pumping of an 1 laser. Mixtures of CIN3 and CH2I2 in He were photodissociated by pulses from an excimer laser operating at 193 nm. Reproduced with permission from Ref. 9.

All the intracavity laser flux cannot be extracted because the RGH laser mixture contains many absorbers at the laser wavelength. The percent contribution of the absorption channel is shown in Fig. 8. Main absorbers seem to be... 

Figure 2b shows a laser pulse at about 1.8-/xm wavelength which is obtained with the apparatus shown in Fig. 1 when the CO laser mixture is replaced by a He/Ar mixture. The fact that lasing occurs over most of the excitation pulse indicates that laser pulse termination is not a property of the laser apparatus itself but results from differences between the two laser gas mixtures. 

Laser mixtures. Mixtures of rare gases with nitrogen, and carbon dioxide with helium are used for lasers. Excimer lasers use the rare gases mixed with fluorine or hydrogen chloride. 

The dominant chemistry for an electrically excited laser mixture is ion chemistry rather than neutral chemistry. Both relativistic... 

The electron density in a typical rare eas halide laser is calculated to be on the order of lO " to 10 electrons per cm and is dependent on the piunp power density, the type of plasma (i.e., e-beam excited or discharge), and laser mixture. 

One very striking result of the HQ based laser mixtures is the initial overshoot in electron density followed by a relaxation down to densities roughly predicted by steady state model predictions where attachment and electron production are in local equilibrium. The initial overshoot is due to the fact that ground vibrational state HQ does not attach rapidly. A degree of vibrational excitation is needed to turn on the attachment process. In contrast, the F2 based lasers do not show this initial transient. F2 effectively attaches when it is in v=0. 

We see for both the fluorine and chlorine based laser mixtures that densities of order 2-3 x 10 " electrons per cm are observed in steady state. This was at some variance with the commonly accepted computer models at the time. In the course of readjusting electron rate constants it was found that laser energy output was not terribly sensitive to (or a very good measure of) the electron density. Towards the end of the pulse (or sooner for dilute halogen mixtures) we see that the electron density increases rapidly. This is due to... 

By measuring a large number of density differences actual densities can be inferred. This has not been done yet for these measurements, but estimates of the upper Xe level densities suggest that the values of density difference are probably within roughly 30% of the actual lower level density. Densities of the two lowest xenon excited states are of order 3 x 10 and 1.7 x 10 per cm for canonical 0.16% HQ laser mixtures. For mixtures lean in HQ, the xenon excited state density "runs away" as the HQ bums out (see Fig. 6). Preliminary modelling efforts show that the steady state densities are about what is expected, but the magnitude and time dependence of the Xe run away are not as expected. 

The preceding are examples of some of the rich chemistry and kinetics that are still unknown about the rare gas halide lasers. Many other examples abound, but they are either not yet the subject of current research or there is still a significant amount of additional research needed. These include the kinetics of specific (v, J) states in XeF (a bound-bound laser transition), any quantitative understanding of ArF (one of the more potentially useful lasers), measurements of halogen bumup phenomena in KrF lasers, electron temperatures or distributions, the energy to produce an ion pair in actual laser mixtures, and a host of other challenging puzzles. On top of these "chemical physics" challenges there still remain numerous... 

Metastable Density Measurements in Electron Beam Pumped XeO Laser Mixtures , to be published. 

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