Excitation of the Sample

The excitation source must provide the energy required to (1) vaporize the sample, (2) dissociate any molecular species present, and (3) excite the atoms. The excitation source may be a dc arc, an ac spark, a flame, or a 

The choice of type of excitation is determined by several factors. Some of these are type of sample, form of the sample (solid, liquid), sensitivity required, and time requirements of the analysis. 

Since the dc arc is the most sensitive excitation source, it will be the excitation source of choice if extremely low detection limits are required. It also is the most difficult source to use in terms of reproducibility. The arc column wanders in the arc gap, the gap length changes during arcing, the background usually is high, and selective vaporization of the sample elements into the arc occurs. However, the dc arc is applicable to solid or liquid samples, sample, preparation is a minimum, and the power source is inexpensive. 

An ac spark source provides much better reproducibility than does the dc arc. It is ideal for metallurgical analytical problems, using self-electrodes or the Petry point-to-plane technique. Detection limits using the ac spark are not as low as with the dc arc, but the ac spark can be used satisfactorily to higher concentration levels. The ac spark is very noisy and more ion lines appear with ac spark discharge than appear with dc arc excitation. Fractional distillation is not a problem with the ac spark. 

Flame sources usually are used for easily excited elements, with liquid samples, to produce simple spectra. Although they have been used with large spectrographs, flame sources are most commonly used with small spectrometers. A discussion of flame emission spectrometers is presented in Chapter 9. 

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The instrument has two monochromators one to select the wavelength to be used for excitation of the sample, the other to scan the wavelength range of the light emitted by the sample. 

A series of papers has recently appeared in the literature concerning the dynamics of photo-induced formation of electron and hole centres at the surface of MgO powders.12 14 Monochromatic excitation of the sample with 282 nm photons leads to the creation of well-separated electron and hole centres at the surface which were monitored via EPR in term of a trapped hole (O ion) and a trapped electron according to the following equation ... 

Routine inorganic elemental analysis is carried out nowadays mainly by atomic spectrometric techniques based on the measurement of the energy of photons. The most frequently used photons for analytical atomic spectrometry extend from the ultraviolet (UV 190-390 nm) to the visible (Vis 390-750 nm) regions. Here the analyte must be in the form of atoms in the gas phase so that the photons interact easily with valence electrons. It is worth noting that techniques based on the measurement of X-rays emitted after excitation of the sample with X-rays i.e. X-ray fluorescence, XRF) or with energetic electrons (electron-probe X-ray micro-analysis, EPXMA) yield elemental information directly from solid samples, but they will not be explained here instead, they will be briefly treated in Section 1.5. 

Measurements of dynamics in the subnanosecond regime are possible using pump-SH probe experiments where an initial pulse causes either a photo- or thermal excitation of the sample and the SH probe beam monitors the transient surface properties [69, 72, 73, 118-120]. Although experiments of this type have yet to be reported for an electrochemical system, experiments on Si samples excited under ambient and vacuum conditions have been published [69, 72, 73, 120]. 

The prism at the outlet of the laser serves to separate the laser emission of the gas fluorescence and allows for a clean excitation of the sample. For excitation using solid-state lasers, this element is dispensable. The lens (element 5) collects the fluorescent signal and focuses on the aperture of the monochromator. The filter is used to eliminate excitation that is spread over the surface of the sample. The optical chopper serves to modulate the light at a defined frequency, which serves as reference for the lock-in amplifier. A data acquisition system controls the pace of the monochromator and reads the signal of the lock-in, generating the sample s emission spectrum. 

Light-induced ESR (LESR) can only detect those spins that are generated by optical excitation of the sample investigated. The LESR experimental procedure consists of a comparison between two measurements ... 

As an example needed for further discussions we give Fig. 2. Optical transitions (3) and (4) during excitation of the sample form paramagnetic states of lithium at T = 30 K. The capture of free electrons by the traps keeps... 

Among the earlier experimental XPS valence band studies of Li2S04 [1, 11, 22, 45] we refer to the analysis done by Calabrese and Hayes [21]. In their study Mg Ka radiation was used for the excitation of the sample, thus obtaining a somewhat better resolution than that of the earlier studies. Comparing these data to the theoretical results [8] they found few minor problems (there are differences in the relative energies and in the intensity of the 4ai + 3t2 group). These problems stimulated further studies of this field. 

It was found that excitation of the samples during the growth process with a second narrow-band light source corresponding to the kn, of either the in-plane or out-of-plane quadrupole SPR halted the nanoprism fusion so that a unimodal growth occurred. As can be seen in Figure 11.55 [109], this resulted in the formation of a single... 

The spectroscopy described thus far is based on the measurement of the intensity of fluorescence produced under steady-state conditions of excitation. Steady-state fluorimetry is derived from the excitation of the sample with a continuous beam of exciting radiation. The lamps and the power supplies used in conventional fluorimeters are sources of continuous radiation. After a short period of initial excitation of the sample, a steady state is established in which the rate of excitation of the analyte is equal to the sum of the rates of all processed, deactivating the lowest excited singlet state including fluorescence. When fhe sfeady state... 

The possibility of 1.5 pm ultrafast all-optical modulator realization based on GaAs/(AlGa)jiOy heterostructures is discussed. The excitation of the samples by 150fs laser pulses leads to about 25 nm shift of the reflection spectrum. The mean decay time for nonlinear reflection in heterostmctures ranges from 1.0 to 2.5 ps. 

Decrease the duration of excitation of the sample by measuring the fluorescence intensity immediately after excitation. 

The appearance of different lines of the same element and of lines of different elements is a function of the particular source employed for the excitation of the sample (Nachtrieb, 1950). Two essentially independent processes condition this phenomenon (1) volatilization of the inorganic salts which constitute the sample ( ) excitation of the atom to a higher energy state. 

Limited periods of illumination or excitation of the sample represent still another cause of photon starvation. Brief periods of illumination are often necessary in a number of spectroscopic applications to avoid deleterious effects. For example, prolonged irradiation of various fluorescing compounds can often result in bleaching. Similarly, microsample analysis by Raman spectrometry can easily cause structural and compositional damage to the sample upon long exposure to the intense excitation radiation. 

The apparatus employs a passively mode locked Nd/YAG laser oscillator, a Pockel cell pulse extractor, and Nd/YAG laser amplifier to produce laser pulses at 1064 nm. Non-linear crystals convert 30% of this light to 355 nm, which is used for excitation of the sample. The optical path length of the 355 nm light is varied by a computer-controlled time delay stage. 

We employ method B to study effects of this type. In this mode, our apparatus yields relative high-resolution fluorescence spectra at different time windows after excitation of the sample by the 355 nm pulse. The spectra are acquired by the upconversion method. The upconverted fluorescence spectrum is recorded simultaneously at all monitored wavelengths by an optical multichannel analyzer. It is constructed from a poly-chromator (HR320 Instruments SA) and an intensified silicon photodiode array detector (Princeton Applied Research Model 1412). The detector is interfaced to our Cromemco computer. 

Figure 7. Two-color fluorescence spectra of the I-naphthylmethyi and 2-naphthyl-methyl radicals. Spectra were produced by excitation of the sample with a 25-ps, 266-nm laser pulse followed by a 25-ps, 355-nm pulse delayed by 60 ps. Key to (halo-methyljnaphthalenes a, I-(chIoromethyI)naphthaIene b, 2-(chloromethyl)naphthalene c, I-(bromomethyI)naphthalene and d, 2-(bromomethyl)naphthalene. Note the fluorescence in the blue region of Spectrum c is due to impurities that contaminated the sample of l-(bromomethyl)naphthalene (28).

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