Spectroscopy Presentation Transcript:
2.What is Spectroscopy?
Spectroscopy pertains to the dispersion of an object's light into its component colors (i.e. energies). By performing this dissection and analysis of an object's light, astronomers can infer the physical properties of that object (such as temperature, mass, luminosity and composition).
3.The Nature of Light
4.Notice that radio, TV, and microwave signals are all light waves, they simply lie at wavelengths (energies) that your eye doesn't respond to. On the other end of the scale, beware the high energy UV, x-ray, and gamma-ray photons! Each one carries a lot of energy compared to their visible- and radio-wave brethren. They're the reasons you should wear sunblock, for example.
5.General Types of Spectra
Typically one can observe two distinctive classes of spectra: continous and discrete. For a continuous spectrum, the light is composed of a wide, continuous range of colors (energies). With discrete spectra, one sees only bright or dark lines at very distinct and sharply-defined colors (energies). As we'll discover shortly, discrete spectra with bright lines are called emission spectra, those with dark lines are termed absorption spectra.
6.Continuous spectra arise from dense gases or solid objects which radiate their heat away through the production of light. Such objects emit light over a broad range of wavelengths, thus the apparent spectrum seems smooth and continuous. Stars emit light in a predominantly (but not completely!) continuous spectrum. Other examples of such objects are incandescent light bulbs, electric cooking stove burners, flames, cooling fire embers and... you. Yes, you, right this minute, are emitting a continuous spectrum -- but the light waves you're emitting are not visible -- they lie at infrared wavelengths (i.e. lower energies, and longer wavelengths than even red light). If you had infrared-sensitive eyes, you could see people by the continuous radiation they emit!
Discrete spectra are the observable result of the physics of atoms. There are two types of discrete spectra, emission (bright line spectra) and absorption (dark line spectra). Let's try to understand where these two types of discrete spectra.
8.Emission Line Spectra
In the diagram below, a hydrogen atom drops from the 2nd energy level to the 1st, giving off a wave of light with an energy equal to the difference of energy between levels 2 and 1. This energy corresponds to a specific color, or wavelength of light -- and thus we see a bright line at that exact wavelength! ...an emission spectrum is born, as shown below:
9.A hydrogen atom in the ground state is excited by a photon of exactly the `right' energy needed to send it to level 2, absorbing the photon in the process. This results in a dark absorption line.
Spectroscopy is the use of the absorption, emission, or scattering of electromagnetic radiation by matter to qualitatively or quantitatively study the matter or to study physical processes. The matter can be atoms, molecules, atomic or molecular ions, or solids. The interaction of radiation with matter can cause redirection of the radiation and/or transitions between the energy levels of the atoms or molecules.
11.The Light of Knowledge is an often used phrase, but it is particularly appropriate in reference to spectroscopy. Most of what we know about the structure of atoms and molecules comes from studying their interaction with light (electromagnetic radiation). Different regions of the electromagnetic spectrum provide different kinds of information as a result of such interactions. Realizing that light may be considered to have both wave-like and particle-like characteristics, it is useful to consider that a given frequency or wavelength of light is associated with a "light quanta" of energy we now call a photon. As noted in the following equations, frequency and energy change proportionally, but wavelength has an inverse relationship to these quantities.
12.Mass Spectrometry: Sample molecules are ionized by high energy electrons. The mass to charge ratio of these ions is measured very accurately by electrostatic acceleration and magnetic field perturbation, providing a precise molecular weight. Ion fragmentation patterns may be related to the structure of the molecular ion. • Ultraviolet-Visible Spectroscopy: Absorption of this relatively high-energy light causes electronic excitation. The easily accessible part of this region (wavelengths of 200 to 800 nm) shows absorption only if conjugated pi-electron systems are present. •
Absorption of this lower energy radiation causes vibrational and rotational excitation of groups of atoms. within the molecule. Because of their characteristic absorptions identification of functional groups is easily accomplished. •
Nuclear Magnetic Resonance Spectroscopy: Absorption in the low-energy radio-frequency part of the spectrum causes excitation of nuclear spin states. NMR spectrometers are tuned to certain nuclei (e.g. 1H, 13C, 19F & 31P). For a given type of nucleus, high-resolution spectroscopy distinguishes and counts atoms in different locations in the molecule.