States—Laser Action

F+Ha HF- +. H AH =-139.9 kj is also exothermic and can produce energy rich HF molecules. The heat of chemical reaction is distributed in various vibrational-rotational modes to give vibrationally excited HF or HC1 in large numbers. Emission from these hot molecules can be observed in the infrared region at h 3.7 (j-m. The reaction system in which partial liberation of the heat of reaction can generate excited atoms or molecules is capable of laser action (Section 3.2.1). They are known as chemical lasers. The laser is chemically pumped, without any external source of radiation. The active molecule is born in the excited state. Laser action in these systems was first observed by Pimental and Kasper in 1965. They had termed such a system as photoexplosion laser. 

It is well known that rapid exothermic bimolecular elementary reactions can lead to product molecules in which the initial rotational and vibrational energy distributions are dissimilar to those of equilibrium at laboratory temperatures, Tq. A nonrigorous but useful description of such distributions is that the rotational and vibrational temperatures , and Ty (obtained by fitting the observed distributions to Boltzmann distributions for and Ty) are such that Tr> To <. Ty. It is possible for complete vibrational population inversion to occur in the initial products of reactions. This corresponds to the case when the population of an excited vibrational level 6(v) exceeds that of the vibrational ground state, 6(v) > 6(0), and leads to the unrealistic description Ty < 0. An intermediate case— partial inversion— is also observed when ly > 0 as rotational relaxation is more rapid than vibrational relaxation, Tq, and thus inversion exists over a limited range of rotational quantum numbers in respect of P(J) or R(J) transitions to the vibrational ground state. Laser action... 

Before proceeding further, it is well to define the meaning of the term chemical laser as it is commonly used by workers in this field. ° A chemical laser is a laser that depends for its excitation on an exothermic chemical reaction in which the energy release of reaction is converted into the specific nonthermal) excitation of reaction product energy states. Laser action can occur in the product energy states directly, or, as in the case of transfer chemical lasers, in a second atomic or molecular species... 

Laser action involves mainly the 3/2 hi/i transition at about 1.06 pm. Since is not the ground state, the laser operates on a four-level system (see Figure 9.2c) and consequently is much more efficient than the ruby laser. 

The ground configuration of Ar is KL3s 3p, giving an inverted P /2 multiplet. The excited states involved in laser action involve promotion of an electron from the 3p orbital into excited As,5s,Ap,5p,3d,Ad,... orbitals. Similarly, excited states of Kr involved arise from promotion of an electron from the Ap orbital. In Ar the KL3s 3p configuration gives rise to 5, V, terms (see Section 7.1.2.3). Most laser transitions involve the core in one of the states and the promoted electron in the Ap orbital. 

Such a situation suggests the possibility of creating a population inversion and laser action between two such states, since any molecules in the repulsive ground state have an extremely short lifetime, typically a few picoseconds. A laser operating by this mechanism is a... 

Lasers are devices for producing coherent light by way of stimulated emission. (Laser is an acronym for light amplification by stimulated emission of radiation.) In order to impose stimulated emission upon the system, it is necessary to bypass the equilibrium state, characterized by the Boltzmann law (Section 9.6.2), and arrange for more atoms to be in the excited-state E than there are in the ground-state E0. This state of affairs is called a population inversion and it is a necessary precursor to laser action. In addition, it must be possible to overcome the limitation upon the relative rate of spontaneous emission to stimulated emission, given above. Ways in which this can be achieved are described below, using the ruby laser and the neodymium laser as examples. 

The fabrication of lasers based upon color centers adds a further dimension to the laser wavelengths available. Ordinary F centers do not exhibit laser action, but F centers that have a dopant cation next to the anion vacancy are satisfactory. These are typified by FLi centers, which consist of an F center with a lithium ion neighbor (Fig. 9.26a). Crystals of KC1 or RbCl doped with LiCl, containing FLi centers have been found to be good laser materials yielding emission lines with wavelengths between 2.45 and 3.45 p,m. A unique property of these crystals is that in the excited state an anion adjacent to the FLi center moves into an interstitial position... 

The determining feature by which laser action can be efficiently obtained from this type of active medium is the fact that the atoms that form the dimmer are only bound in the excited state. Figure 2.9 shows a schematic diagram of the laser energy levels in a molecule of excimer. The laser transition is produced between two molecular electronic levels in which the potential energy curve for the fundamental state is repulsive. This ensures the population inversion. 

When designing a new solid state laser system, an appropriate choice of the matrix - active center combination is needed. On the one hand, the active center should display optical transitions in the transparency region of the solid, which consequently requires the use of wide-gap materials. Additionally, the transitions involved in the laser action should show large cross sections in order to produce efficient laser systems. This aspect, which is directly related to the transition probability, is treated in depth in Chapters 5 and 6, where the physical basis of the behavior of an optically active center in a solid is studied. 

The upward arrows in the figure indicate the pumping channels to various high energy levels by flashlamp (0.5 /tim) or semiconductor lasers (0.8 /rm), where Nd + ions display strong absorption transitions. The downward arrow indicates the widely used laser emission at 1.06 /xm, associated with the -> " ln/2 transition. In addition, laser action is also generally possible from the same " F3/2 level to the " 19/2 state at around 0.9 /xm and to the 113/2 state at around 1.3 /xm. 

Compact and stable devices are available that take advantage of the improved quality of the crystal lasers, as well as increased pump efficiencies. Hundreds of different models of Nd + based lasers have demonstrated laser action (Kaminskii, 1981). It is possible to operate these Nd + solid state lasers in the continuous regime, with output powers ranging from 1 W to 1000 W. Pulsed operation is also possible, with a pulse length from the picosecond range, via mode-locking, to tens of nanoseconds by Q-switch operation. 

Figure 2.18 shows the range covered by different tunable solid state laser systems based on transition metal ions. As observed, a good variety of matrices have shown tunable laser action on the basis of Cr + as an active ion. The fundamental aspects determining the tunability of those Cr + based systems will be the subject of Section 6.4 in Chapter 6. 

Hoskins and Soffer (117) measured the fluorescent lifetime of the neodymium 4Fy2 state in yttrium oxide. They found a value of approximately 260 /zsec both at room temperature and at liquid-nitrogen temperature. They also observed a weaker long-lived component in the decay. They were unable to say whether this was evidence for a low-transition-probability ion site, or an effect of trapping of the resonance radiation near 0.9 /x. They report laser action, with a threshold of 260joules. This is a fairly high value for most crystalline materials. 

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