Classification and Production of Spectra through Excitation
1. Flame Spectra: The temperature of a Bunsen flame is high enough to vaporize certain solids. Thus if a piece of platinum wire is dipped into a sodium salt and then placed in the flame, a vivid yellow color is obtained which is characteristic of the element sodium. This method of excitation can only be used for a limited number of metals, the main class being the alkali and alkaline earth metals such as sodium, potassium, lithium, calcium and barium. The line spectra produced in each case consist of lines of different colors, but some lines have a greater intensity than others. Thus, sodium is characterized by two prominent yellow lines barely distinguishable in a small spectroscope, and lithium by a prominent green line.
2. Spark Spectra: If metal electrodes are connected to the secondary of an induction coil and placed a few millimeters apart, a spark can be obtained which bridges the gap. It was discovered that a much more intense and violent spark could be obtained by placing a capacitor in parallel with a gap. This spark is known as condensed spark. The solid under investigation forms one of the electrodes, and is vaporized at the high temperature obtained.
3. Arc Spectra: This is the method most used in industry. If two metal rods connected to a d.c. voltage supply are placed in contact with each other and then drawn a few millimeters apart, a continuous spark, known as an arc, is obtained across the gap. The arc is a source of very high temperature, and therefore vaporizes substances very readily. In practice the two rods are placed in a vertical position, and a small amount of the substance investigated is placed on the lower rod.
4. Discharge-tube Spectra: If a gas is contained at low pressure inside a tube having two aluminium electrodes and a high a.c. or d.c. voltage is applied to the gas, a ‘discharge’ occurs between the electrodes and the gas becomes luminous. This is the most convenient method of examining the spectra of gases. The luminous neon gas in a discharge tube has a reddish color, while mercury vapor is greenish-blue.
