Optical pumping relaxation processes

The first optical laser, the ruby laser, was built in 1960 by Theodore Maiman. Since that time lasers have had a profound impact on many areas of science and indeed on our everyday lives. The monochromaticity, coherence, high-intensity, and widely variable pulse-duration properties of lasers have led to dramatic improvements in optical measurements of all kinds and have proven especially valuable in spectroscopic studies in chemistry and physics. Because of their robustness and high power outputs, solid-state lasers are the workhorse devices in most of these applications, either as primary sources or, via nonlinear crystals or dye media, as frequency-shifted sources. In this experiment the 1064-mn near-infrared output from a solid-state Nd YAG laser will be frequency doubled to 532 nm to serve as a fast optical pump of a raby crystal. Ruby consists of a dilute solution of chromium 3 ions in a sapphire (AI2O3) lattice and is representative of many metal ion-doped solids that are useful as solid-state lasers, phosphors, and other luminescing materials. The radiative and nonradiative relaxation processes in such systems are important in determining their emission efficiencies, and these decay paths for the electronically excited Cr ion will be examined in this experiment. 

If the symmetry increases or decreases drastically, the spin-lattice relaxation process is fast, or the optical pumping is not successful in producing large spin alignment, the above rules would not be as definite. 

A well-known and important phenomenon in the area of nuclear-spin resonance (NMR) in gases, liquids, or solid samples is dynamic nuclear-spin polarisation (DNP) (see e.g. [M6]). This term refers to deviations of the nuclear magnetisation from its thermal-equilibrium value, thus a deviation from the Boltzmann distribution of the populations of the nuclear Zeeman terms, which is produced by optical pumping (Kastler [31]), by the Overhauser effect [32], or by the effet solide or solid-state effect [33]. In all these cases, the primary effect is a disturbance of the Boltzmann distribution in the electronic-spin system. In the Overhauser effect and the effet solide, this disturbance is caused for example by saturation of an ESR transition. Owing to the hyperfine coupling, a nuclear polarisation then results from coupled nuclear-electronic spin relaxation processes, whereby the polarisation of the electronic spins is transferred to the nuclear spins. 

Included are the optical pumping (induced absorption and emission) and all relaxation processes that refill level 1) from other levels ) or that deplete 1). The optical pumping probability... 

The investigation of fast processes, such as electron motions in atoms or molecules, radiative or collision-induced decays of excited levels, isomerization of excited molecules, or the relaxation of an optically pumped system toward thermal equilibrium, opens the way to study in detail the dynamic properties of excited atoms and molecules. A thorough knowledge of dynamical processes is of fundamental importance for many branches of physics, chemistry, or biology. Examples are predissociation rates of excited molecules, femtosecond chemistry, or the understanding of the visual process and its different steps from the photoexcitation ofrhodopsin molecules in the retina cells to the arrival of electrical nerve pulses in the brain. 

In the electronic ground states of molecules collision-induced transitions represent, for most experimental situations, the dominant mechanism for the redistribution of energy, because the radiative processes are generally too slow. In cases where a nonequilibrium distribution has been produced (for example, by chemical reactions or by optical pumping), these collisions try to restore thermal equilibrium. The relaxation time of the system is determined by the absolute values of collision cross sections. 

In a conventional optical pumping experiment in solids, an intense pumping beam perturbs the net populations of various atomic or molecular energy levels in a sample, which are then monitored to provide information about the relaxation processes that cause the system to return to its equilibrium state. While this technique is sensitive to the net level populations of the atoms, it is relatively insensitive to processes which preserve net populations while redistributing them spatially through the sample volume, for example by mutual spin flip interactions or reabsorption... 

The effect of optical pumping on the saturation of population densities is illustrated by a two-level system with population densities N and N2. The two levels are coupled to each other by absorption or emission and by relaxation processes, but have no transitions to other levels (Fig. 3.22). Such a true two-level system is realized by many atomic resonance transitions without hy-perfine structure. 

Mollenauer and co-workers used the MCD effect in the absorption band of an F center in order to detect, for the first time, changes in the ground state spin polarization induced by ENDOR in the relaxed excited state. A He-Ne laser served for optical pumping as well as for monitoring the magnetic circular dichroism, while several preceding experiments still had used conventional lamps. The laser beam, which passed a quarter wave plate with a stress modulation frequency of SO kHz, was irradiated onto a KI crystal that contained the F centers under study. The transmitted laser light was monitored with a photomultiplier and processed with lock-in technique. 

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