Population inversion transfer

A third pumping method (Fig. Ic) uses an electrical discharge in a mixture of gases. It relies on electronic excitation of the first component of the gas mixture, so that those atoms are raised to an upper energy level. The two components are chosen so that there can be a resonant transfer of energy by collisions from the upper level of the first component to level 3 of the second component. Because there are no atoms in level 2, this produces a population inversion between level 3 and level 2. After laser emission, the atoms in the second component return to the ground state by collisions. 

The earliest of the magnetization transfer experiments is the spin population inversion (SPI) experiment [27]. By selectively irradiating and inverting one of the 13C satellites of a proton resonance, the recorded proton spectrum is correspondingly perturbed and enhanced. Experiments of this type have been successfully utilized to solve complex structural assignments. They also form the basis for 2D-heteronuclear chemical shift correlation experiments that are discussed in more detail later in this chapter. 

In this chapter Prof. Rice has advocated two techniques that should be useful for evaluations of optimal fields for laser control of chemical reactions (i) reduced space of eigenstates for representations of nuclear wavepackets and (ii) the use of effective reaction coordinates. Both techniques have already been used for efficient evaluations of reaction probabilities in model reactions. See, for example, Ref. 1 for the prediction of population inversion and Ref. 2 for the demonstration of rather strong deviations of chemical reactions from the reaction path, specifically in the case of hydrogen transfer reactions. 

An example of the use of Eq. (2) is provided by the wavepacket interferometry experiments of Scherer, Fleming et al. [11]. These workers have demonstrated that the phase of the light can be used to control constructive versus destructive interference of wavepackets in the excited electronic state. An alternative way of interpreting their experiment is that the phase of the second pulse relative to the first determines the direction of population transfer between the two electronic states. In the spirit of the present discussion, absorption versus stimulated emission is being controlled by the choice of phase of the light relative to the instantaneous pge peg Since the direction of population transfer is not determined in this case by population inversion... 

Consider the 13C— H bond as a two-spin system. CH coupling occurs between one nucleus with small population difference (13C) and another one with large polarization (1H). Fig. 2.43(a) illustrates this situation by the number of dots on the energy levels. Population inversion of the proton levels 1 and 3 connected by the transition 1H1 is achieved by an appropriate 180° pulse, which turns the double cone of precession shown in Fig. 2.1 upside down. Thereafter, the inverted proton population difference controls both carbon-13 transitions (Fig. 2.43(b)). This is the polarization or population transfer making up an enhanced absorption signal for one transition (e.g. 13Ci in Fig. 2.43 (b)) and an enhanced emission on the other (e.g. 13C2 in Fig. 2.43(b)). 

In general, E-V-R transfer leads to a population inversion among vibrational levels and thus is of great importance for laser applications, and new types of laser such as those demonstrated recently4,5 may be built. 

Considering the four types of energy, translational, rotational, vibrational and electronic, it is convenient to classify ten types of molecular energy transfer. In some, the type of energy is preserved, for example the E-E process which produces population inversion in the He/Ne laser8... 

In the adiabatic limit, t is considered to be a parameter, and is called an adiabatic state. One of the interesting properties of this limit is that a population can be inverted by evolving the system adiabatically. This process is called adiabatic passage. Population transfer induced by a laser is generally called coherent population transfer. For a two-level system, the complete population inversion is produced by a n -pulse or by adiabatic rapid passage. 

The net result of the phenomenon just described is the possibility of having lasini without the need for population inversion [303-306]. We can envision a pumpi process that transfers some population to state [ ,). This state can emit to the 1... 

Continuous wave operation of COIL is facilitated by the hyperfine structure of the atom. Iodine has a nuclear spin of, so the P /2 and Pz/2 levels are split by hyperfine interactions. Figure 8 shows the allowed transitions between the hyperfine sublevels and a high resolution emission spectrum. The F = 3 — F" = 4 transition is most intense, and this is the laser line under normal conditions. Collisional relaxation between the hyperfine sub-levels of Pz 2 maintains the population inversion, while transfer between the Fi/2 levels extracts energy stored in the F = 2 level. Hence, if it is not sufficiently rapid, hyperfine relaxation can limit power extraction. 

Using NX (a) to replace 02(a) in chemical iodine lasers is a topic of current interest. It has been established that transfer from NCl(a) can excite I efficiently, at rates that are high enough to sustain a population inversion. There is also evidence that NF(a) can excite I.i34,i35 is probable that iodine lasers driven by NX (a) will operate at much higher temperatures than COIL. If combustors are used to generate F or H atoms, the primary gas flow will be hot. In addition, as NF(a) or NCI (a) carry more energy than is needed to excite I, the pump process will also liberate heat. 

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