Lecture 01; atomic spectroscopy by Dr. Salma Amir
- 1. Lecture No. 01 Course title: Atomic Spectroscopy Topic: Introduction to Atomic Spectroscopy Course instructor: Dr. Salma Amir GFCW Peshawar
- 2. Atomic spectroscopy Atomic spectroscopy refers to the absorption and emission of ultraviolet–visible (UV-VIS) light by atoms and monoatomic ions and is conceptually similar to the absorption and emission of UV-VIS light by molecules Broadly classified as Flame Emission Spectroscopy (FES) Atomic Absorption Spectroscopy (AAS) Atomic Florescence Spectroscopy (AFS)
- 3. Emission, absorption, and fluorescence by atoms in a flame. In atomic absorption, atoms absorb part of the light from the source and the remainder of the light reaches the detector. Atomic emission comes from atoms that are in an excited state because of the high thermal energy of the flame. To observe atomic fluorescence, atoms are excited by an external lamp or laser. An excited atom can fall to a lower state and emit radiation.
- 4. Production of gaseous-phase atoms The procedure by which gaseous metal atoms are produced in the flame may be summarized as follows. When a solution containing a suitable compound of the metal to be investigated is aspirated into a flame, the following events occur in rapid succession: 1. Evaporation of solvent leaving a solid residue; 2. Vaporization of the solid with dissociation into its constituent atoms, which initially, will be in the ground state; and 3. Some atoms may be excited by the thermal energy of the flame to higher energy levels, and attain a condition in which they radiate energy.
- 5. The resulting emission spectrum thus consists of lines originating from excited atoms or ions. These processes are conveniently represented diagrammatically
- 6. Absorption by atoms Atoms in which no electrons are in the higher levels are said to be in the ground state. This state is designated in energy level diagrams as E0 Atoms in which there is an electron in the higher level are said to be in an excited state. Excited states are designated in energy level diagrams as E1, E2, E3, etc. An energy level diagram consists of short horizontal lines representing the levels or states, with each line labeled as E0, E1, etc. Often, an energy level diagram shows the movement of electrons between levels with longer vertical arrows. The movement of an electron between electron energy levels is called an electronic energy transition.
- 7. Where can an electron obtain the required energy to be promoted to a higher level? One way is for it to come into contact with light of the same energy. When the light of this energy “strikes” an atom and causes an electron to be promoted to a higher energy level, the energy of the light becomes part of the electron’s energy and therefore “disappears.” It is absorbed. It is important to keep in mind that the light coming in must be exactly the same energy as the energy difference between the two electronic levels; otherwise, it will not be absorbed at all. If it is not absorbed and is in the visible region, we see it. It becomes part of the light that is reflected and therefore detected by our eyes. The absorption of light by atoms consists of the absorption of only a few very specific wavelengths because the energy difference between two levels is very specific.
- 8. Absorption spectra (origin of spectral lines) Absorption spectra for atoms are due to atoms in the gas phase absorbing UV-VIS light from a light source and are characterized by very narrow wavelength absorption bands called spectral lines. These very narrow bands result from energy transitions between electronic energy levels in which there are no vibrational levels superimposed. An absorption spectrum is a plot of the amount of light absorbed by a sample vs. the wavelength of the light. The amount of light absorbed is called the absorbance. It is symbolized as A . An absorption spectrum is obtained by using a spectrometer to scan a particular wavelength region and to observe the amount of light absorbed by the sample along the way.
- 9. Emission by atoms Under certain conditions, analytes present in samples of matter will emit light, and this light can be useful for qualitative and quantitative analysis. For example, most processes used to obtain free ground state atoms in the gas phase from liquid phase solutions of their ions result in a small percentage of the atoms being elevated to the excited state, even if no light beam is used. Whether a light beam is used or not, excited atoms return to the ground state because the ground state is the lowest energy state. The energy loss that occurs when the atoms return to the ground state may be dissipated as heat, but it may also involve the emission of light because the difference in energy between the ground state and the excited state is equivalent to light energy. Energy level diagrams can be used to depict such a process, and an emission spectrum, the plot of emission intensity vs. wavelength, may be plotted.
- 10. Emission spectra Emission spectra for atoms and monoatomic ions are due to gas phase atoms or monoatomic ions in excited states emitting UV- VIS light because they are returning to the ground state. Emission spectra are also characterized by very narrow wavelength bands (also called spectral lines) because they too involve electronic energy levels in which there are no vibrational levels superimposed. In fact, for a given kind of atom, the emission lines occur at the same wavelengths as the absorption lines because they involve the same energy levels.
- 11. Consider the simplified energy-level diagram, where Eo represents the ground state in which the electrons of a given atom are at their lowest energy level and E1,E2 , E3 , etc., represent higher or excited energy levels
- 12. Absorption vs Emission The process of emission is the reverse of that of light absorption by atoms, so downward-pointing arrows are used to indicate the return to the ground state; the wavelengths emitted are the same as those that are absorbed. Thus the lines in an emission spectrum of a metal are often at the same wavelengths as the lines in the absorption spectrum for the same metal.