Emission and Absorption Spectra

One can understand atomic spectra if one knows the concept of atomic energy levels. The movements of electrons from one level to another causes the spectra. The work of a spectoscopicst is to find the energy levels of an atom from the measured values of the wavelengths of the spectral lines emitted by the atoms. To analyze the spectra emitted by the lighter atoms is easy and that by heavier atoms is difficult.

Sodium atom emits the characteristic yellow light of wavelength 589.0 and 589.6 nm. This can be done in the following way. A strong beam of the yellow light from a sodium arc is concentrated on a glass bulb (highly evacuated) with a small amount of pure metallic sodium. The bulb is warmed to increase the sodium vapor pressure, resonance radiation takes place and it glows with the yellow light characteristic of sodium.

In contrast to emission a sodium atom in the ground state may absorb energy leading to absorption spectrum. For example if the continuous spectrum light from a carbon arc is sent through an absorption tube containing sodium vapor, the spectrum when examined shows a series of dark lines. This is known as absorption spectrum.

Sub Topics

X-Ray Emission
Properties and uses of X-rays

X-Ray Emission

That part of the electromagnetic spectrum with the wavelength range 10^-⁸ m to 10^-¹² m are called as X-rays W.C Roentgen, a German physicist discovered X-Rays in the year 1895. A photographic film wrapped in black color paper was found to be exposed when placed near a cathode-ray tube. So, Roentgen concluded that some invisible rays must have been emitted by the cathode ray tube and that it must have penetrated the black paper and hence could expose the photographic film. The invisible rays were called as X-Rays.

Coolidge tube is designed by Coolidge to produces these X-Rays. It was found that when highly energetic electrons are made to strike a metal target, electromagnetic radiation with the wavelength corresponding to X-rays are emitted. The diagram shows the arrangement to do that.

In a evacuated glass chamber a filament and a metallic target are fixed. The filament is made to emit electrons due to thermionic emission by heating it electrically. The target kept in front of the filament is given a higher potential using a d.c power supply. So the electrons are attracted by the target and get accelerated to very high speed. These high-speed electrons hit the target producing. Through a window X-rays are made to come out of the tube. Since the electrons stop, lot of heat is produced and the device is cooled by supplying cool water.

Depending on the accelerating potential applied between the filament and the target, either heard X-rays or soft X-rays are emitted. Higher potential to the target gives rise to hard X-rays, i.e. the energy of the photon emitted is more. If lesser soft X-rays (energy less, frequency and hence higher wavelength) are emitted. If the filament is heated to a higher value, the number of electrons emitted by the filament in a given time increases, thereby increasing the intensity of X-rays.

If the X-rays are analyzed for the intensity and the wavelength, one can draw a intensity V_r_m_s wavelength graph as show in the figure. In the graph show we find that X-rays are emitted only after l = 40 pm below which there is no emission. This is called as threshold wavelength.

If we look at the intensity we find that there is a continuous spectrum along with two peaks K_b and K_a. The continuous part is called as continuous X-rays and the peaks are called characteristic X-rays.

Why does this occur? To understand this one should know the energy changes that are happening to the electron. If electrons emitted due to thermionic emission has minimum energy, then by reaching the anode it may gain an energy due to electric field equal to eV. When this electron hits the target, it loses its kinetic energy due to collision. Sometimes a part of its energy comes out as a photon and rest is given to the colliding particle of the target. If the electron has a left over kinetic energy it may lose it in the next collision.

Sometimes the colling thermionic electron may knock out an inner electron of the atom. Depending on the type of collision, the energy of the photon emitted may range from 0 to eV.

The maximum energy of the photo = eV

So the cutoff wavelength 'l_m' depends only on the accelerating voltage 'V' applied between the target and the filament and does not depend on the material of the target.

In general,

And 'E' ranges from 0 to eV.

This explains the origin of continuous X-rays.

Properties and uses of X-rays

X-Rays follow a rectilinear propagation and travel with a velocity of 3 x 10⁸ms^-¹ in vacuum. They are not deflected by electric and magnetic field. Since they do not contain any charge but are diffracted by crystals, X-rays cause fluorescence when incident on certain materials such as barium platinocyanide. When X-rays pass through gas they ionize the gas. Due to their energy their penetrating power is more. They penetrate trough aluminum, wood, plastic etc and are stopped by only high-density materials. Due to the above properties it is used in the medical field extensively. Since it can pass through flesh and not through bones X-rays can photograph the bones of the human body. Such a photograph is called radiograph. Dentists also use X-rays to study tooth decay. But X-rays should not be used in excel or else it can damage the living cells and exposure of human body to X-rays for a longer period leads to cancer and genetic defects.

Other than the medical field X-rays find their own use in industry, to detect structural defects, fault of joints, welding etc. They also find use in customs and security counter to inspect suitcases without opening them.