Light conventional sources

The emitted beam of coherent radiation is narrow and can be focused into a very small area. This means that the density of radiation that can be delivered for any one pulse over a small area is very high, much higher than can be delivered by conventional light sources operating with similar power inputs. 

Most appHcations for which lasers are used were originally demonstrated using conventional light sources. In many cases, the appHcation was only marginally successful using conventional sources and requited the development of laser light sources to be practical. 

Because of the narrow line width, absorption of laser energy can excite one specific state in an atom or molecule. The laser is tuned so that its wavelength matches an absorption corresponding to the desired state, which may be an electronic state or vibrational state. Absorption of laser energy can lead to excitation of specified states much more effectively than absorption of light from conventional light sources. 

The apparatus and techniques of ion cyclotron resonance spectroscopy have been described in detail elsewhere. Ions are formed, either by electron impact from a volatile precursor, or by laser evaporation and ionization of a solid metal target (14), and allowed to interact with neutral reactants. Freiser and co-workers have refined this experimental methodology with the use of elegant collision induced dissociation experiments for reactant preparation and the selective introduction of neutral reactants using pulsed gas valves (15). Irradiation of the ions with either lasers or conventional light sources during selected portions of the trapped ion cycle makes it possible to study ion photochemical processes... 

In our laboratory we have utilized multiphoton infrared laser activation of metal ion-hydrocarbon adducts to probe the lowest energy pathways of complex reaction systems (6). Freiser and co-workers have utilized dispersed visible and uv radiation from conventional light sources to examine photochemical processes involving organometallic fragments... 

The mercury lamp has been the conventional light source used in photochemistry. The ground-state mercury atom, Hg, has two electrons in its highest occupied orbital, the 6s atomic orbital. Excited mercury... 

Pulsed lasers emit pulses so short that even the fastest photoprocesses can be monitored, which would be impossible using conventional light sources. 

When compared to fluorescence detectors for HPLC, the design of a fluorescence detector for CE presents some technical problems. In order to obtain acceptable sensitivity, it is necessary to focus sufficient excitation light on the capillary lumen. This is difficult to achieve with a conventional light source but is easily accomplished using a laser. The most popular source for laser-induced fluorescence (LIF) detection is the argon ion laser, which is stable and relatively inexpensive. The 488-nm argon ion laser line is close to the desired excitation wavelength for several common fluorophores. The CLOD for a laser-based fluorescence detector can be as low as 10 12 M. 

This method is equally applicable to atoms 26) and to molecules 22). In molecules the Zeeman splitting depends on the quantum number / of the total angular momentum and therefore the fluorescence from a single rotational level (v, f) need be observed. Because of this necessarily selective excitation, these molecular level-crossing experiments can be performed much more easily with lasers than with conventional light sources and have been sucessfully performed with Naj 2 > and NaK 29). 

These examples should have illustrated the importance of lasers as powerful tools in elucidating many details of photochemical processes which are not accessible with conventional light sources. 

In second harmonic generation, light of angular frequency oo pass through a crystal and generates a beam of angular frequency 2 . With conventional light sources the electric polarization induced in the medium depends linearly on the electric held ... 

Here <( t ) f(t")> is the autocorrelation function of the electromagnetic field. For the case of excitation by a conventional light source, where the amplitudes and the phases of the field are subject to random fluctuations, the field autocorrelation function differs from zero for time intervals shorter than the reciprocal width of the exciting source. In the limit 8v A, that is when the spectral width, 8v, of the source exceeds the inhomogenously broadened line width, the field autocorrelation function can be represented as a delta function... 

Although the preparation of the excited state has been described in terms of a delta function excitation, the same results should be obtained for the case of excitation by a broad-band, random, conventional light source. We have pointed out, in Section VI, that in the case of the non-radiative decay of an excited state, the same behavior is predicted to follow excitation by a light source characterized by a second-order autocorrelation function which describes random phases and excitation by a delta function pulse. A similar situation prevails when the radiative decay channel is also taken into account. 

Lasers are used as an excitation source for three reasons. Because the laser output is coherent it offers special advantages in directionality and focusing. Tunable lasers allow the possibility of examining several species. Finally, lasers provide significantly higher power levels than conventional light sources. 

To be effective, the light sources used for chiroptical detection systems must have an intensity that is much greater than those ordinarily used in polarimetry. This comes about because the angular rotations observed for the very low analyte concentrations and very short sample pathlengths typical of an analytical liquid chromatograph, are extremely small (mdeg. and less). Conventional light sources have been replaced with laser illumination but these are not without problems, a major one of which is the instability in the emission [23],... 

Electronically excited species are generally produced by direct excitation or as the products of photolysis. In addition to conventional light sources, fixed frequency or tuneable visible and ultraviolet lasers are regularly used for single photon excitation or photolysis, as well as... 

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