Transitional hollow cathode

Fig. 3. Determination of rotational temperatures in a transitional hollow cathode. Rotational lines used are of the R-branch of 2S-2S (0,0) CN 388.3 nm and 2S-2S (0,0) N+ 391.4 nm band (positive m —J values belong...

Fig. 3. Determination of rotational temperatures in a transitional hollow cathode. Rotational lines used are of the R-branch of...

In general, only atoms in the flame that are the same as in the hollow cathode material can absorb the specific lines emitted by this material. The only requirement of the monochromator, then, is to isolate the desired line from other lines of the cathode material and the lines of the filler gas. One line of the element is usually absorbed more strongly than others (it has a higher oscillator strength ). This often, but not necessarily, corresponds to the electronic transition from the ground state to the lowest excited state. This line is selected for maximum sensitivity measurements. For high concentrations, a line with a lower oscillator strength may be selected. 

Relative Intensities of Elemental Transitions from Hollow Cathode Lamps Inert Gases... 

RELATIVE INTENSITIES OF ELEMENTAL TRANSITIONS FROM HOLLOW CATHODE LAMPS... 

In AAS, the excitation source inert gas emission offers a potential background spectral interference. The most common inert gases used in hollow cathode lamps are Ne and Ar. The data taken for this table and the other tables in this book on lamp spectra are from HCLs however, electrodeless discharge lamps emit very similar spectra. The emission spectra for Ne and Ar HCLs and close lines that must be resolved for accurate analytical results are provided in the following four tables. This information was obtained for HCLs and flame atom cells and should not be considered with respect to plasma sources. In the Type column, I indicates that the transition originates from an atomic species and II indicates a singly ionized species. 

In Chapter 1, section 7, it was explained that very precise overlap of atomic absorption and emission profiles is required to obtain sensitive absorbance measurements. Absorption spectra of atoms at flame temperatures are much simpler than the emission spectra emitted by hollow cathode lamps. The possible transitions corresponding to electronic excitation of an atom may be shown as vertical lines on an energy level diagram, in which the vertical displacement... 

The switching device is a commercial Pseudospark, model FS 2000 (Alstom). The rated maximum anode voltage and current are 32 kV and 30 kA. In the present application, the anode voltage is 30 kV, but the peak anode current is only 4 kA. The switch operates at the transition between the hollow cathode and superemissive modes. The switch-current rise-time is 15 ns, limited by the current channel and connection inductances. 

Fig. 17. Line shape of (a) the 3s 3d -> 3s 3p transition X = 3092.7 A) in neutral A1 and (b) the 3d ->3p transition (X = 4529 A) in ionic Al following 300-keV Ar bombardment of clean ( ) and an oxidized (A) aluminum target. The solid line represents a reference line measured with a hollow-cathode lamp. In (a), dashed lines are corresponding calculations based on the resonance tunnel model (after Reinke et at., 1991).

Figure 1 is a sketch of the atomic absorption process. In lA, the emission spectrum of a hollow-cathode lamp is shown, with emission lines whose half-width is typically about 0.02 A. For most practical purposes, the desired element in the sample can be considered as being able to absorb only the "resonance lines, whose wavelengths correspond to transitions from the minimum energy state to some higher level. In IB, the sample is shown to absorb an amount "x which corresponds to the concentration of the element of interest. As seen in Figure 1C, after the flame, the resonance line is reduced while the others are unaflFected. In order to screen out the undesired emission, the radiation is now passed through a filter or monochromator (ID) which is tuned to pass the line... 

The A 2A- X 211 Emission Spectrum. The first time, the A X emission spectrum was excited in a hollow-cathode discharge through helium that contained small amounts of hydrogen and phosphorus vapor. A system of three red-degraded bands at 422.8, 385.4, and 356.7 nm was Identified with the v = 0- 1, 0 0, and 1 0 bands. Their rotational and fine structures are those expected for a transition, where the upper state approaches... 

The long absorption cells naturally lend themselves to a hollow-cathode discharge configuration for the study of molecular ions. A preliminary experiment revealed the HCO line at 1 THz with a signal-to-noise ratio (100 1 with a 1 s time constant) equivalent to that obtained using the laser sideband technique (16). Possible transitions in H2D and OH have also been observed however they are weak and only tentatively identified, and further work is underway. 

Fig. 1.44 Optogalvanic spectrum of a uranium hollow-cathode lamp filled with argon buffer gas. In the upper spectrum (a) taken at 7 mA discharge current, most of the lines are argon transitions, while in the lower spectrum (b) at 20 mA many more uranium lines appear, because of sputtering of uranium from the hollow cathode walls [125]...

In atomic physics the selective excitation of single atomic levels was achieved with atomic resonance lines from hollow cathode lamps even before the invention of lasers. However, in molecular spectroscopy only fortuitous coincidences between atomic resonance lines and molecular transitions could be used in some cases. 

Level-crossing spectroscopy was used in atomic physics even before the invention of lasers [831, 842-844]. These investigations were, however, restricted to atomic resonance transitions that could be excited with intense hollow-cathode or microwave atomic-resonance lamps. Only a very few molecules have been studied, where accidental coincidences between atomic resonance lines and molecular transitions were utilized [836]. 

Figure 5.9 Time evolution of an OG signal in a hollow-cathode discharge lamp, associated with an atomic transition (Ar line at 811.369 nm), after excitation with a 10 ns pulse from a Ti sapphire laser...

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