F-center lasers

Subsequent to the advent of the dye laser, tunable lasers based upon other lasing media were developed which operate over various wavelength ranges. Nobable among these are the f-center lasers and diode lasers which are tunable in the infrared. 

Figure 2-1. A schematic diagram of the apparatus used to record photofragment angular distributions of complexes. An F-center laser is used to pump transitions in the parent complex which leads to dissociation. A second F-center laser is used as a probe to state selectively detect the fragments. The electrodes are used to orient the parent molecules prior to excitation.

Sources. The ultimate source for spectroscopic studies is one that is intense and monochromatic but tunable, so that no dispersion device is needed. Microwave sonrces such as klystrons and Gnnn diodes meet these requirements for rotational spectroscopy, and lasers can be similarly nsed for selected regions in the infrared and for much of the visible-ultraviolet regions. In the 500 to 4000 cm infrared region, solid-state diode and F-center lasers allow scans over 50 to 300 cm regions at very high resolution (<0.001 cm ), but these sources are still quite expensive and nontrival to operate. This is less trne... 

Finally, electrons have been generated with nanosecond pulses from the KrF exciplex laser at 248 nm via photodetachment of OH and Cl in aqueous and methanol solutions, and could be via one-photon photoionization in liquids with the ArF laser at 193 nm, and via one and two-photon processes in aqueous naphthol and naphtholate with nanosecond pulsed nitrogen lasers at 337 nm. With the improved fluxes and short pulses now available from gain modulation techniques applied to the TEA exciplex lasers, semiconductor lasers and F-center lasers, they may prove to be convenient sources for future studies of electron relaxation and transfer processes from the ultraviolet to infrared region. 

The operational lifetime of an laser depends on how long the crystal has been pumped. Under the best conditions, a single crystal can be made to operate for only several days. This decay, coupled with the awkwardness of creating and storing the active crystals, and the discovery of more stable color center lasers, has virtually eliminated the use of Fj lasers. However, the F center laser was the first powerful color center laser, and in the hands of skilled experimenters it has been used to generate tunable near infrared radiation. 

In order to illustrate the difference between APTE and cooperative up-conversion, we shall discuss an example of a line-narrowing effect in n-photon summation as a means to distinguish between both processes (Auzel 1984a, 1985). Irradiating Er +-doped samples with IR photons at 1.5 pm leads to various visible emissions. Room-temperature IR F-center laser excitation between 1.4 and 1.6 pm of 10% Er -doped vitroceramic and of Er +iYPs leads to emission bands from near IR to UV. Such emissions may be ascribed to multiphoton excitation of the order of 1 to 5, either by APTE or by the cooperative type, with energy levels of single ions (APTE), or with pair levels, respectively, as illustrated by fig. 23. Successive absorptions in fig. 23 a involve a combination of several J states. The APTE effect, because of self matching by multiphonon processes, involves only and J = Y states. 

The acetylene-hydrogen cyanide complex is of particular interest in view of the fact that the C-H stretches of both monomers can be probed using the F-center laser. Earlier microwave studies of this system showed the existence of a T-shaped isomer S shown in Figure 3, in which the acetylene acts as the proton acceptor. During our infrared investigation of this species, a linear isomer was also found in which the acetylene now acts as the acid. By studying the two C-H stretch vibrational modes of these two isomers it is possible to learn a great deal about the anisotropy of the vibrational... 

Image plates use stimulated luminescence from storage phosphor materials. The commercially available plates are composed of extremely fine crystals of BaFBrEu2+. X-rays excite an electron of Eu2+ into the conduction band, where it is trapped in an F-center of the barium halide with a subsequent oxidation of Eu2+ to Eu3+. By exposing the BaFBrEu" complex to light from a HeNe laser the electrons are liberated with the emission of a photon at 390 nm [38]. 

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... 

Chapter 6 is devoted to discussing the main optical properties of transition metal ions (3d" outer electronic configuration), trivalent rare earth ions (4f 5s 5p outer electronic configuration), and color centers, based on the concepts introduced in Chapter 5. These are the usual centers in solid state lasers and in various phosphors. In addition, these centers are very interesting from a didactic viewpoint. We introduce the Tanabe-Sugano and Dieke diagrams and their application to the interpretation of the main spectral features of transition metal ion and trivalent rare earth ion spectra, respectively. Color centers are also introduced in this chapter, special attention being devoted to the spectra of the simplest F centers in alkali halides. 

The frequency chain works as follows to the second harmonic of the He-Ne laser at 3.39 jum a NaCl OH color center laser at 1.70 pm is phase locked. To the second harmonic of the color center laser a laser diode at 848 nm is then phase locked. This is accomplished by first locking the laser diode to a selected mode of the frequency comb of a Kerr-lens mode-locked Ti sapphire femtosecond laser (Coherent model Mira 900), frequency-broadened in a standard single-mode silica fiber (Newport FS-F), and then controlling the position of the comb in frequency space [21,11]. At the same time the combs mode separation of 76 MHz is controlled by a local cesium atomic clock [22]. With one mode locked to the 4th harmonic of the CH4 standard and at the same time the pulse repetition rate (i.e. the mode separation) fixed [22], the femtosecond frequency comb provides a dense grid of reference frequencies known with the same fractional precision as the He-Ne S tandard [23,21,11]. With this tool a frequency interval of about 37 THz is bridged to lock a laser diode at 946 nm to the frequency comb, positioned n = 482 285 modes to lower frequencies from the initial mode at 848 nm. 

Fig. 2. Spectroscopy of electrons in fluids. Ensemble shows absorption maxima and coefficients in n-alcohols at picosecond (A ) and nanosecond (A) times, diols (B), amines (C), ethers (D), alkanes ( ), and color center (F). The absorption (F ) and stimulated (Fg) emission in KBr color center laser are also shown.

The absorption spectrum of e is indeed a fingerprint of its molecular environment, as Fig. 2 shows in ensemble form. For interest, the F center in KBr and its color center FjiA) laser are shown too. More quantitative spectra are shown in Fig. 3, which displays for the first time the full visible and near-IR nanosecond absorption spectra of ej in 1-octanol and... 

B. Henderson and G.F. Imbusch, Optical Spectroscopy of Inorganic Solids, Oxford Science Publication, Clarendon Press, Oxford, 1989. An extcn.sivc treatment of luminescent centers in solids. Chapters on color centers, lasers and experimental techniques are included. 

Figure 4. Sketch of the experimental set-up for single-molecule magnetic resonance spectroscopy. In part (a) the sample, S, is mounted between a cover, C, and a LiF substrate, in the joint focus of a parabolic mirror, P, and a lens, L. The microwaves are provided by a loop, I, and the whole sample holder is positioned in the central bore of a superconducting magnet, M. Residual light of the incident laser is blocked by a beam block, B. In part (b) of the figure the sample is mounted at the tip of a single mode fibre, F, centered in the focus of a parabolic mirror, P. Microwaves are provided by a loop, L.

The F-center is the most fundamental color center defect in the alkali halide lattice. Although it is not laser-active, the optical properties of the F-center are important in understanding the laser physics of other color center lasers. The fundamental absorption band of the F-center, called the F band, corresponds to a transition fi om the Is-like ground state to the 2p-like first excited state of the square-well potential. The F-band transition is very strong, and dominates the optical spectrum of the alkali-halide crystal. In fact, the term F-center comes fi om the German word Farbe, meaning color, and refers to the strong color imparted to the otherwise transparent alkali-halide crystals. 

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