Laser Metastable

Dye-Sensitized Photoisomerization. One technological appHcation of photoisomerization is in the synthesis of vitamin A. In a mixture of vitamin A acetate (all-trans stmcture) and the 11-cis isomer (23), sensitized photoisomerization of the 11-cis to the all-trans molecule occurs using zinc tetraphenylporphyrin, chlorophyU, hematoporphyrin, rose bengal, or erythrosin as sensitizers (73). Another photoisomerization is reported to be responsible for dye laser mode-locking (74). In this example, one metastable isomer of an oxadicarbocyanine dye was formed during flashlamp excitation, and it was the isomer that exhibited mode-locking characteristics. 

In addition to the thermal CVD reactions listed above, tungsten can be deposited by plasma CVD using Reaction(l)at350°C.[ ll P At this temperature, a metastable alpha structure (aW) is formed instead of the stable be.c. Tungsten is also deposited by an excimer laser by Reaction (1) at < 1 Torr to produce stripes on silicon substrate.P l... 

LDI Laser desorption/ionisation MAB Metastable atom bombardment... 

The F + H2 — HF + FI reaction is one of the most studied chemical reactions in science, and interest in this reaction dates back to the discovery of the chemical laser.79 In the early 1970s, a collinear quantum scattering treatment of the reaction predicted the existence of isolated resonances.80 Subsequent theoretical investigations, using various dynamical approximations on several different potential energy surfaces (PESs), essentially all confirmed this prediction. The term resonance in this context refers to a transient metastable species produced as the reaction occurs. Transient intermediates are well known in many kinds of atomic and molecular processes, as well as in nuclear and particle physics.81 What makes reactive resonances unique is that they are not necessarily associated with trapping... 

Tabet, J. C. Jablonski, M. Cotter, R. J. Hunt, J. E. Time-resolved laser desorption The metastable decomposition of Chlorpophyll-A and some derivatives. Int. J. Mass Spectrom. Ion Phys. 1985, 65,105-117. 

Spengler, B. Kirsch, D. Kaufmann, R. Metastable decay of peptides and proteins in matrix-assisted laser-desorption mass spectrometry. Rapid Comm. Mass Spectrom. 1991, 5,198-202. 

Time-resolved spectroscopy establishes that the 25-ps laser irradiation of the relatively persistent charge-transfer complex of p-bromoanisole with iodine monochloride generates the contact ion pair (see Fig. 15b) in which the metastable ICP undergoes mesolytic fragmentation to form the reactive triad, i.e.,... 

The explosive phenomena produced by contact of liquefied gases with water were studied. Chlorodifluoromethane produced explosions when the liquid-water temperature differential exceeded 92°C, and propene did so at differentials of 96-109°C. Liquid propane did, but ethylene did not, produce explosions under the conditions studied [1], The previous literature on superheated vapour explosions has been critically reviewed, and new experimental work shows the phenomenon to be more widespread than had been thought previously. The explosions may be quite violent, and mixtures of liquefied gases may produce overpressures above 7 bar [2], Alternative explanations involve detonation driven by phase changes [3,4] and do not involve chemical reactions. Explosive phase transitions from superheated liquid to vapour have also been induced in chlorodifluoromethane by 1.0 J pulsed ruby laser irradiation. Metastable superheated states (of 25°C) achieved lasted some 50 ms, the expected detonation pressure being 4-5 bar [5], See LIQUEFIED NATURAL GAS, SUPERHEATED LIQUIDS, VAPOUR EXPLOSIONS... 

EXAMPLE 1.7 The fluorescence lifetime measured from the metastable state Ej/2 ofNd + ions in the laser crystal yttrium aluminum borate (YAl3(B03)4) is 56 lus. If the quantum efficiency from this state is 0.26, determine the radiative lifetime and the radiative and nonradiative rates. 

In many real lasers, the upper laser level, 2 in Figure 2.6(a), is a metastable level that is, it has a long lifetime compared to the lower laser level (t > ti). If we can pump efficiently into such a longer-lived upper level, and provided that there is a lower energy level, E in Figure 2.6(a), with a short lifetime, then a population inversion is very likely to be established. 

The fluorescence lifetime of the /2 metastable state of Nd + ions in LaBGeOs (a solid state laser) is 280 /u.s and its quantum efficiency is 0.9. (a) Calculate the radiative and nonradiative rates from this excited state, (b) If the effective phonons responsible for the nonradiative rate have an energy of 1100 cm, use the Dieke diagram to determine the number of emitted effective phonons from the F3/2 excited state, (c) From which three excited states of the Nd + ions in LaBGeOs do you expect the most intense luminescence emissions to be generated ... 

HP-HT techniques have been used to synthesize and recover metastable forms of carbon nitrides. A mixmre of Ceo and N2, laser-heated up to 2000-2500 K at about 30 GPa, gives rise to a chemical reaction, visually indicated by... 

M. Fujiwara, P.S. Chow, D.L. Ma and R.D. Braatz, Paracetamol crystallization using laser backscatteiing and ATR-FTIR spectroscopy metastability, agglomeration, and control, Cryst. Growth Des., 2(5), 363-370 (2002). 

When molecules absorb a photon and produce an electronic excited state, the energy can be dissipated in several ways luminescence, radiationless decay to the ground state, and photochemistry. Luminescence dominated the older literature because it was easy to observe. A good review of luminescence is in Volume 3 of David Dolphin s seven-volume series The Porphyrins. Picosecond laser spectroscopy allowed for exploration of the radiationless decay pathways, particularly the initial steps that compete with luminescence and lead to photochemistry. Two principal forms of radiationless decay lead to long-term metastables ligand ejection and electron transfer. 

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