Signal atoms

The primary goal of the theory of AAS is to establish the relationship between the measured analytical signal, atomic absorbance. A, and the analyte concentration, c, in the sample. Theoretical description of the two stages involves entirely different sciences theory of atomization is based on thermodynamics, kinetics and molecular physics, while description of absorbances is based on optics and spectroscopy. Below, the two stages will be considered separately. 

Spray valves benefit from three-stage microprocessor control, which manages prespray, spray and post-spray. Continuous spray can be initiated and terminated with a continuous signal. Atomizing air pressure also must be tightly controlled to ensure even fluid dispersion. 

Madey and co-workers followed the reduction of titanium with XPS during the deposition of metal overlayers on TiOi [87]. This shows the reduction of surface TiOj molecules on adsorption of reactive metals. Film growth is readily monitored by the disappearance of the XPS signal from the underlying surface [88, 89]. This approach can be applied to polymer surfaces [90] and to determine the thickness of polymer layers on metals [91]. Because it is often used for chemical analysis, the method is sometimes referred to as electron spectroscopy for chemical analysis (ESCA). Since x-rays are very penetrating, a grazing incidence angle is often used to emphasize the contribution from the surface atoms. 

In order to achieve a reasonable signal strength from the nonlinear response of approximately one atomic monolayer at an interface, a laser source with high peak power is generally required. Conuuon sources include Q-switched ( 10 ns pulsewidth) and mode-locked ( 100 ps) Nd YAG lasers, and mode-locked ( 10 fs-1 ps) Ti sapphire lasers. Broadly tunable sources have traditionally been based on dye lasers. More recently, optical parametric oscillator/amplifier (OPO/OPA) systems are coming into widespread use for tunable sources of both visible and infrared radiation. 

This represents a large signal for a small number of atoms on the surface. With care, 10 monolayers of gold on silicon can be detected. 

If the molecules could be detected with 100% efficiency, the fluxes quoted above would lead to impressive detected signal levels. The first generation of reactive scattering experiments concentrated on reactions of alkali atoms, since surface ionization on a hot-wire detector is extremely efficient. Such detectors have been superseded by the universal mass spectrometer detector. For electron-bombardment ionization, the rate of fonnation of the molecular ions can be written as... 

Figure B2.3.17. REMPI spectra of the HCl and DCl products from the reaction of Cl atoms with (CH3)3CD [63], The mass 36 and 2 ion signals are plotted as a fiinction of the 2-photon wavenumber. Assigmnents of the...

Figure B2.5.10. LIF signal of free Na atoms produced in the photodissociation of Nal. t - q is the delay between the photolysis pulse (at L) and the probe pulse. Adapted from [111].

Figure B2.5.17. (a) Time-dependent intensity / and redueed fluenee F/Fq for a single-mode CO2 laser pulse used in the IR laser photolysis of CF I. Fq is the total fluenee of the laser pulse, (b) VIS-REMPI iodine atom signals obtained with CO2 laser pulses of different fluenee (after [113]).

The velocity distribution/(v) depends on the conditions of the experiment. In cell and trap experiments it is usually a Maxwell-Boltzmann distribution at some well defined temperature, but /(v) in atomic beam experiments, arising from optical excitation velocity selection, deviates radically from the nonnal thennal distribution [471. The actual signal count rate, relates to the rate coefficient through... 

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