Special Topic 3.1 Lasers

Some lasers that are frequently used in the photochemical laboratory and their properties are listed in Table 8.4. 

Optical filters can modify the spectral output from spectrally broad light sources and essentially monochromatic light can be obtained from light sources with several sharp but widely separated emission lines, such as the low-pressure mercury arcs. Commercially available glass Liters and readily prepared filter solutions are inexpensive alternatives to 

Pure solvents may also act as optical filters for example, methanol and benzene cut off most of the radiation below 205 and 280 nm, respectively (Table 3.1). Their (internal) filter effect must be considered in planning photochemical experiments with solutions, because it may lower the reaction efficiencies. 

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Ultrashort fluorescence lifetimes are best determined by photon upconversion. An excellent description of the technique for non-specialists wishing to set up an experiment is in preparation as part of a current IUPAC project on ultrafast intense laser chemistry .181 The time resolution is basically limited by the temporal width of the laser pulses that are being used. An intense gate pulse at frequency vq is mixed with the fluorescence at a frequency vp in a nonlinear optical crystal (Special Topic 3.1), to create a short pulse at the sum frequency i j. The gate pulse thus represents a time window for the fluorescence... 

Experimental details 620 Sample solutions of previtamin D3 (2 x 10 3 m) in a 1 cm pathlength UV cell were purged with nitrogen and irradiated using a pulsed UV dye laser at 300 nm (Special Topic 3.1), containing Rhodamine 6G as a sensitizer in methanol or a methanol water mixture (1 1, v/v). After irradiation, an aliquot of the sample solution was analysed by HPLC. 

Experimental details.716 Grating of a hologram on a single crystal made of 119 was achieved using an He Ne laser (632.8 nm 50 mW) (Special Topic 3.1) for both writing and reading. Crystallographic data were obtained for monomer, dimer and mixed crystals that were isolated after different irradiation times. 

Stults, J. (1995). Matrix-assisted Laser-desorption Ionization Mass Spectrometry (MALDI-MS), Current Opinion in Structural Biology 5 691-698. The review contains a host of references for special topics, such as differentiation of sulfate and phosphate groups, MALDI with infrared lasers and succinic acid matrix, subfem-tomolar detection thresholds with a thin layer matrix, highly sensitive protein detection in the attomolar range, and high-resolution mass spectrometry. 

Dickey FM et al (eds) (2006) Laser beam shaping applications. Taylor and Francis, UK Dini JW (1993) Electrodeposition the materials science of coatings and substrates. Noyes, NJ Drobny JG (2001) Technology of fluoropolymers. CRC, Boca Raton Drobny JG (2003) Radiation technology for polymers. CRC, Boca Raton Esquivias L (ed) (2009) Progress in sol-gel production special topic volume with invited papers only. Trans Tech Publications, Switzerland... 

Over the past few years a better understanding of lasers has resulted in an evolution of classification systems for lasers. Before 2002 the older system of classification used Roman numerals with the most hazardous class being Class IV (see Special Topic 7.3.3.1 Old Laser Classification). As experience with lasers grew, the classification system was updated to include new defining specifications. Beginning in 2002 a newly revised classification system was phased in and was fully implemented in 2007. It is based on International Electrotechnical Commission (lEC) Standard 60825-1/ANSI Z136.1—2007 that separates lasers into four classes. Class 1 is the least hazardous, Class 4 is the most hazardous, and there are new subclasses." The classification of lasers is dependent on the dose of radiation that can be received from a laser. A brief description of these laser classes is presented in Table 7.3.3.2. 

You may also encounter another kind of laser in your classroom in the form of the laser pointer. Learn more about the hazards of these smaller lasers in Special Topic 7.3.32 Laser Pointers. 

The conventional flash photolysis setup to study photochemical reactions was drastically improved with the introduction of the pulsed laser in 1970 [17], Soon, nanosecond time resolution was achieved [13], However, the possibility to study processes faster than diffusion, happening in less than 10 10 s, was only attainable with picosecond spectroscopy. This technique has been applied since the 1980s as a routine method. There are reviews covering the special aspects of interest of their authors on this topic by Rentzepis [14a], Mataga [14b], Scaiano [18], and Peters [14c],... 

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