Sodium atom light emission from

For a typical sodium atom, the initial velocity in the atomic beam is about 1000 m s1 and the velocity change per photon absorbed is 3 crn-s. This means that the sodium atom must absorb and spontaneously emit over 3 x 104 photons to be stopped. It can be shown that the maximum rate of velocity change for an atom of mass m with a photon of frequency u is equal to hu/lmcr where h and c are Planck s constant and the speed of light, and r is the lifetime for spontaneous emission from the excited state. For sodium, this corresponds to a deceleration of about 106 m s"2. This should be sufficient to stop the motion of 1000 m-s 1 sodium atoms in a time of approximately 1 ms over a distance of 0.5 m, a condition that can be realized in the laboratory. 

Eor example, street lamps use the emissions from excited sodium atoms, the dazzling colors of a fireworks display come from photons emitted by metal ions in excited states, and the red light in highway flares often comes from excited Sr ions. 

Spectroscopy is the study of the absorption and emission of radiation by matter. The most easily appreciated aspect of the absorption of radiation is the colour shown by substances that absorb radiation from the visible region of the spectrum. If radiation is absorbed from the red region of the spectrum, the transmitted or unabsorbed radiation will be from the blue region and the substance will show a blue colour. Similarly substances that emit radiation show a particular colour if the radiation is in the visible region of the spectrum. Sodium lamps, for instance, owe their characteristic orange-yellow light to the specific emission of sodium atoms at a wavelength of 589 nm. 

The ions in the sample solution are converted to neutral atoms in an air-acetylene flame. Light from a hollow cathode or an electrodeless discharge lamp (EDL) is passed through the flame. The light absorption of the atoms in the flame, which is proportional to the ion concentration in the sample, is measured by a detector following a monochromator set at the appropriate wavelength. This principle holds for measurements performed in the AAS mode. In the AES mode, the light emitted from the atoms excited in the flame is measured. Most commercial instruments can be run in both modes. Sodium may be measured more favorably in the emission mode. 

Since an electron that has been promoted to the conduction band will have a greater energy than those left in the valence band, there is a possibility for the electron to lose this excess energy. The spontaneous return of electrons in the conduction band to the valance band is known as recombination, and is usually accompanied by light emission and heat (Figure 4.4). This phenomenon happens all the time for excited-state molecules. For instance, consider what happens when one supplies sodium atoms with sufficient energy to promote an electron from the 3s energy level... 

The most common type of emission spectrometer in use today (inductively coupled plasma-optical emission spectroscopy, or ICP-OES) atomizes a sample by passing an electric current into a gas plasma that contains the sample. In these optical emission methods, the sample is heated to high temperature. At this temperature the individual elements glow with their representative colors, e.g., red for potassium, yellow for sodium. The light from the sample is focused on a monochrometer to select a wavelength appropriate for the element of interest. That light at the correct wavelength is focused on a detector that measures its intensity (Fig. 4.8). 

Figure 7.23 Scxlium vapor lamps, which are used for commercial and highway lighting, have a yellow glow due to the emission from excited sodium atoms. 

The lines in the emission and absorption spectra of the atoms correspond to the electronic transitions as electrons move from one orbital to another. The energy of the photon emitted, or absorbed, corresponds to the difference in energies of the electron in the two orbitals involved. The yellow/orange glow of sodium lamps or street lights corresponds to the photons emitted as excited sodium atoms with the outermost electron in the Ap orbital fall down to the ground atomic state in which it is in the 3s orbital. 

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