Radiating Species
Despite the wide numbers of metal ion donors, they serve to form only a few atomic and molecular species that are useful as light emitters.
In many cases, chlorine donors have to be added in order to achieve sufficiently deep colors, as the desired emitting molecules have to be generated.
Some color emitters are of atomic nature (e.g. lithium, sodium). Presence of chlorine, and the reaction to monochlorides, may actually impair their color purity or intensity.
At high temperatures, the atoms will ionize. The emission spectra of ions are different than of neutral atoms; the ions may emit in undesired spectral ranges. E.g. Ba+ emits in blue wavelengths. Ionization can be suppressed by addition of an easier-to-ionize metal with weak visible emission of its own, e.g. potassium; the potassium atoms then act as electron donors, neutralizing the barium ions.
The color blue is notoriously difficult to produce in fireworks, as the copper compounds need to be heated at a specific temperature for the optimal shade of blue to be produced. Thus, a deep, rich blue is usually viewed as the mark of an experienced fireworks maker.
Care should be taken to avoid formation of solid particles in the flame zone, whether metal oxides or carbon; incandescent solid particles emit black body radiation that causes "washing out" of the colors. Addition of aluminium raises the flame temperature but also leads to formation of solid incandescent particles of aluminium oxide and molten aluminium. Magnesium has less such effect and is therefore more suitable for colored flames; it is more volatile than aluminium and more likely to be present as vapors than as particulates. Formation of solid particles of magnesium oxide can further be inhibited by presence of carbon monoxide, either by negative oxygen balance of the composition in presence of organic fuels, or by addition of the colorant in the form of an oxalate, which decomposes to carbon dioxide and carbon monoxide; the carbon monoxide reacts with the magnesium oxide particles to gaseous magnesium and gaseous carbon dioxide.
| Colour | Emitter | Wavelengths | Notes |
|---|---|---|---|
| Yellow | Sodium (D-line) | 589 nm | very strong, overpowers other colors, avoid contamination |
| Orange | CaCl (molecular bands) | most intense: 591–599 nm and 603–608 nm, and others | |
| Red | SrCl (molecular bands) | a: 617–623 nm b: 627–635 nm c: 640–646 nm |
The SrCl species tends to be oxidized to less desirable SrO; strontium-containing compositions are therefore usually formulated to be oxygen-deficient. |
| Red | SrOH(?) (molecular bands) | 600–613 nm | |
| Red | Li (atomic spectral lines) | ||
| Green | BaCl (molecular bands) | a: 511–515 nm b: 524–528 nm d: 530–533 nm |
Lines of BaOH and BaO are also present, emitting in yellow and yellowish-green (487, 512, 740, 828, and 867 nm for BaOH, 549, 564, 604 and 649 for BaO). The BaOH lines are much stronger than the BaO lines. In absence of chlorine, the BaCl lines are not present and only the BaOH and BaO lines are visible.
|
| Blue | CuCl (molecular bands) | several intense bands between 403–456 nm, less intense at 460–530 nm | Low dissociation energy of copper compounds causes presence of free copper atoms in the flame, weakly emitting in green (lines between 325–522 nm). In presence of chlorine, CuCl is formed, emitting strongly in blue. At higher temperatures CuCl dissociates and lines of atomic copper are present in the spectrum; CuO and CuOH are also formed, emitting molecular bands at green-yellow (535–555 nm) for CuOH and at orange-red (580–655 nm) for CuOH. Adequate control of temperature is therefore required for blue-burning compositions. |
| Infrared | Carbon particles | black body radiation | For good broadband infrared output, compositions producing lots of heat and carbon particles are required. The burning temperature should be lower than of visible-illuminating compounds. The intensity of the emitted radiation depends on the burn rate. Temperature can be increased by addition of magnesium. A magnesium/Teflon/Viton composition is common for missile decoy flares. |
| Infrared | CO2 (molecular bands) | mostly 4300 nm | Produced by carbon-containing fuels. |
| Infrared | Cs (atomic spectral lines) | two powerful spectral lines at 852.113 nm and 894.347 nm | Used in infrared illumination compositions. Metal is avoided in the compositions to prevent formation of bright, visible-radiating particles. |
| Infrared | Rb (atomic spectral lines) | spectral lines in near-infrared | Used in infrared illumination compositions, less commonly than cesium. |
Read more about this topic: Pyrotechnic Colorant
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