Four beam paths

Fig. 2. Geometry of degenerate four-wave mixing (BOXCARS geometry) for short-pulse, time-resolved measurements of the nonlinear response. Beams 1, 2, and 3 are derived from a single laser beam by the use of beam splitters and the beam paths are adjusted for the pulses to arrive simultaneously at the sample. By delaying one of the beams with respect to the others, the time-resolved measurements can be performed.

Avoiding inadvertent exposure to the laser beam is an essential part of laser safety. Wherever possible, the laser optical path should be horizontal and well below eye level and ideally the beam path should be fully or at least partially enclosed. A beam dump should be inserted to terminate the beam path at some appropriate point and should be made from a material capable of absorbing the full intensity of the laser beam. AU work with class 3B and four lasers should be carried out in a designated laser room, which should be clearly identified with a suitable warning notice and separate from the main laboratory. Lasers may also be required to have beam shutters or key-controlled interlocks to prevent operation if the laser casing or room door is open. 

Powerful concentrated laser beams are useful for cutting and drilling and have to be guarded to prevent anyone getting into the beam path. Less powerful lasers, used for surveying for example, are not usually hazardous except perhaps to the eyes so precautions are advisable to prevent eye exposure to the beam. Lasers products are classified into four classes according to the risk of injury that they pose, and consequently the precautions that must be taken to protect people from their harmful effects detailed information on the classes and the safety precautions is published in BS EN 60825-1 1994, lEC 60825-1 1993 Safety of laser products. Equipment classification, requirements and user s guide. The classes are ... 

Figure 8.24 Beam paths in the four-wave mixing experiment. The diffracted light beam (J4) is made to fall on the photodiode using a semi silvered mirror.

The quadrupole analyzer uses oscillating electrical fields to separate ions based on their stable trajectories. The analyzer consists of four parallel poles placed between the ion source and the detector in such a manner that the path of the ion beam travels through the middle. The rods have DC voltages applied to opposite rods to carry the same charge, yielding one set of positive rods and one set of negative rods. All four rods have an oscillating radio frequency (RP) applied to them (Honour, 2003). If the ion mass is too... 

When charge-transfer bonds are obscured by those of the original donors and acceptors, one may find of value a difference method [1] (e.g., Forster s tandem method [70]). Four cuvets of equal path length are used, two containing the charge-transfer complex solutions in series in the indicator beam of a double-beam spectrophotometer, and two cuvets, one with the unreacted donor and the other with unreacted acceptor solution, also in series in the reference beam. A difference spectrum is thus obtained which, however, needs special care in its interpretation. 

The experimental details of data collection are too many to discuss in depth here. The crystal specimen should be small enough that the longest path of an incident and reflected ray does not exceed three or four times the reciprocal of the linear absorption coefficient. The specimen should be of roughly uniform proportions if possible. As already stated, it should be of such a shape that the dimensions are easy to measure accurately for computing absorption corrections. The crystal must be mounted on a fiber sufficiently stiff that it does not move out of the center of the beam due to gravity or air currents and with a cement that does not turn the apparatus into a very expensive hygrometer. 

The quadrapole mass spectrometer also employs an electron beam to ionize the particles using the quadrapole instead of a magnet to deflect the path of the particles and filter out all but the specific component to be analyzed. The quadrapole is a set of four similar and parallel rods (see Fig. 

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