Alexandrite

Alexandrite, like ruby, contains Cr ions but they are substituted in the lattice of chrysoberyl, BeAl204. The chromium ions occupy two symmetrically non-equivalent positions which would otherwise be occupied by aluminium ions. In this environment the 2 ground state of Cr is broadened, compared with that in ruby, by coupling to vibrations of the crystal lattice. 

A further advantage is the higher efficiency of the alexandrite laser because of its being a four-level laser. In the illustration in Figure 9.2(c), level 4 is a vibronic level and level 3 the zero-point level of the T2 state. Level 2 is a vibronic level of the 2 state and level 1 the zero-point level. Because of the excited nature of level 2 it is almost depopulated at room... 

Historically, the first type of laser to be tunable over an appreciable wavelength range was the dye laser, to be described in Section 9.2.10. The alexandrite laser (Section 9.2.1), a tunable solid state laser, was first demonstrated in 1978 and then, in 1982, the titanium-sapphire laser. This is also a solid state laser but tunable over a larger wavelength range, 670-1100 nm, than the alexandrite laser, which has a range of 720-800 nm. 

A further advantage, compared with the alexandrite laser, apart from a wider tuning range, is that it can operate in the CW as well as in the pulsed mode. In the CW mode the Ti -sapphire laser may be pumped by a CW argon ion laser (see Section 9.2.6) and is capable of producing an output power of 5 W. In the pulsed mode pumping is usually achieved by a pulsed Nd YAG laser (see Section 9.2.3) and a pulse energy of 100 mJ may be achieved. 

CH3I (methyl iodide) principal axes, 103 If rotation, 113 CH2NH (methanimine) interstellar, 120 Cr203 (chromium trioxide) in alexandrite laser, 347ff in ruby laser, 346ff HC3N (cyanoacetylene) interstellar, 120 HCOOH (formic acid) interstellar, 120 NH2CN (cyanamide) interstellar, 120... 

The development of lasers has continued in the past few years and 1 have included discussions of two more in this edition. These are the alexandrite and titanium-sapphire lasers. Both are solid state and, unusually, tunable over quite wide wavelength ranges. The titanium-sapphire laser is probably the most promising for general use because of its wider range of tunability and the fact that it can be operated in a CW or pulsed mode. 

The high performance of modem spectrographs means that low power lasers can be used as excitation sources. These are typically 10—100-mW devices which are air-cooled and can be operated from 117-V a-c lines. In the green, the Ar" (514.5-nm) laser remains the most popular but is being challenged by the smaller and more efficient frequency doubled Nd YAG (532-nm). In the nir, diode lasers (784-nm) and diode-pumped alexandrite... 

Manusciatti W, Fitzpatrick RE, Goldman MP (2000) Treatment of facial skin using combinations of CO Q-switched alexandrite, flashlamp-pumped pulsed dye, and Er YAG lasers in the same treatment session. Dermatol Surg 26 114-120 Jordan R, Cummins C, Burls A (2000) Laser resurfacing of the skin for the improvement of facial acne scarring a systematic review of the evidence. Br J Dermatol 142 413-423... 

Bellew SG, Alster TS (2004) Treatment of exogenous ochronosis with a Q-switched alexandrite (755 nm) laser. Dermatol Surg 30(4 Pt 1) 555-558... 

To summarize the state of technology for the chemist wishing to practice laser chemistry the laser devices exist with the capability one would like, but they are expensive. We may expect that cheaper pulsed laser systems based upon excimer, Nd YAG, N2, alexandrite, etc. may be in the offing in the near future. This has already begun to happen with a new generation of N2 pumped dye lasers from two manufacturers. No such prospects presently exist for c.w. lasers in the visible and ultraviolet, but one may hope that the ion laser will be radically improved or supplanted soon. For chemical applications which can use infrared excitation, satisfactory devices presently exist and the price is right. 

Aleuritic acid, physical properties of, 5 35t Aleurone layer, 3 567 Alexandrite, color of, 7 331 Ale yeast, 3 580... 

Other applications of the instrumentation and techniques here presented may include (i) temperature sensing employing alexandrite, ruby crystals, and Nd YAG crystals.(35 41) pH and partial pressure of carbon dioxide (pCCh) may be determined employing carboxy seminaphtorhodafluor (SNARF)-6.(42)... 

Figure 9.16. Performance of alexandrite based real time optical temperature sensors versus standard (Neslab RTE-J l IM) Equation 9.107 is used to obtain a working relation between Temperature and r. The fiber optic sensor monitored the bath temperature (—) in equilibrium with the standard (-).

Z. Zhang, K. T. V. Grattan, and A.W. Palmer, Fiber-optic high-temperature sensor based on the fluorescence lifetime of alexandrite, Rev. Sci. Instrum. 63, 3869-3873 (1992). 

In the system which uses crystalline alexandrite as the sensor material/381 a measurement reproducibility of 1 °C is achieved over a wide temperature region from 20 to 700°C. The same technique is applied to another fiber optic thermometer system which is designed for biomedical sensing applications and uses LiSrAlF6 Cr3+ as sensor material/391 The standard deviation of the measurement recorded by this system is better than 0.01°C within the 20 Cand 50°C region. 

It varies strongly with DqlB and may be positive or negative, as shown in Figure 11.11. The dashed lines indicate the energy level positions of a number of different crystals. In a high-strength crystal field, DqlB 2.3 as in ruby (Al203 Cr3+) and alexandrite(41) AE > 0 (2350 cm-1 and -800 cm-1 for ruby(42) and alexandrite,(41) respectively), and the Cr3+ emission is dominated by the sharp R lines (2E 4A2 transition). However,... 

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