Emission line narrowing

In this section experimental results are described, which are obtained by applying the conventional pump-probe technique to m-LPPP films kept in vacuum at the temperature of liquid nitrogen [25], These results allow the identification of the primary excitations of m-LPPP and the main relaxation channels. In particular, the low and high excitation density regimes are investigated in order to get an insight into the physical processes associated with the emission line-narrowing phenomenon. 

The spectrum is dominated by strong C emission lines. He II 4686 is a strong emission line, narrow in Of(C) and broad in the Of-WR(C). He II 4200, 4541 are in absorption. The two established Of-WR(C) stars are Abell 30 and Abell 78, both of which also show H-deficient material in the inner parts of their nebulae. 

The emission spectrum from a hollow cathode lamp includes, besides emission lines for the analyte, additional emission lines for impurities present in the metallic cathode and the filler gas. These additional lines serve as a potential source of stray radiation that may lead to an instrumental deviation from Beer s law. Normally the monochromator s slit width is set as wide as possible, improving the throughput of radiation, while being narrow enough to eliminate this source of stray radiation. 

In principle all the X-ray emission methods can give chemical state information from small shifts and line shape changes (cf, XPS and AES in Chapter 5). Though done for molecular studies to derive electronic structure information, this type of work is rarely done for materials analysis. The reasons are the instrumental resolution of commercial systems is not adequate and the emission lines routinely used for elemental analysis are often not those most useftil for chemical shift meas-ure-ments. The latter generally involve shallower levels (narrower natural line widths), meaning longer wavelength (softer) X-ray emission. 

At high excitation fluence (2.4 mJ/cm2) the emission spectrum collapses into a narrow line located at 2.53 eV (see Fig. 8-2 a, dashed line), slightly blue-shifted with respect to the low intensity emission. The process strongly depends on the shape of the excitation area. For a rectangular spot, line narrowing occurs at 55 pj/cm2, i.e. 45 times a lower fluence than for a small circular spot, while the... 

In the previous section we have shown that for an excitation density above the PL line-narrowing threshold, the shape of the SE spectrum remain unchanged. This demonstrates that the spontaneous emission spike is not due to a new transition existing in the phoioexcited material, as would be the case for excitonic mol-... 

Atoms, ions and molecules present in the stars provide additional opacity at wavelengths corresponding to specific atomic transitions these give rise to comparatively narrow absorption lines (see Fig. 3.2) with intensities related to the abundances of the relevant elements (and much else). Despite the name, processes other than pure absorption (e.g. scattering and fluorescence) are involved in the production of these lines and, while they are often treated in LTE, this is now only a simplifying approximation which often works fairly well, but needs to be checked by more detailed calculations for each particular case. (In some cases, there are even emission lines or emission components, e.g. the solar Ca+ H and K lines in the near UV, which are so strong that the chromosphere affects their central parts.)... 

The experiments on the iodine separation were conducted as follows. A tubular vessel of pyrex glass, having at one end a plane window and at the other end a conical light-trap, was evacuated and then filled with iodine at about 0.17 mm. pressure, and then with hexene at about 6 mm. partial pressure. The tube was then subjected to the intense light from two Cooper-Hewitt glass mercury arcs, using a filter of 0.05 molal potassium dichromate 2 cm. in thickness to cut off all radiations on the violet side of the green mercury line. The lamps were rim at considerably below the rated capacity, and were cooled by a blast of air to keep the emission lines as narrow as possible. 

An important observation was that the emission lines are not confined to the narrow visible region of the electromagnetic spectrum. Instrumental detection showed that discrete lines are also present in the infrared and ultraviolet wavelengths, and eventually it showed this in the X-ray region also. It became clear that the wavelength of the line simply corresponded to the energy... 

This type of interference normally takes place when the absorption of an interfering species either overlaps or lies veiy near to the analyte absorption, with the result that resolution by the monochromator almost becomes impossible, Hollow-cathode-source invariably give rise to extremely narrow emission-lines, hence interference caused due to overlap of atomic spectral lines is rather rare. 

The higher branched derivatives of the spiro compounds, 43 and 44 also exhibit spectral narrowing under pulsed excitation, with emission lines at 428 and 443 nm, and line widths of 3.2 and 3.9 nm, respectively (Fig. 3.24). For the same film thickness, the threshold is lower for 4-Spiro2 than for spiro-sexiphenyl despite the lower absorption, which is attributed to the higher luminescence quantum yield. 

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