OTHER SPECTROSCOPY - PHYSICAL AND THEORETICAL CHEMISTRY - The Handy Chemistry Answer Book (2014)

The Handy Chemistry Answer Book (2014)

PHYSICAL AND THEORETICAL CHEMISTRY

OTHER SPECTROSCOPY

What is microwave spectroscopy?

Microwave spectroscopy, as the name implies, is spectroscopy carried out using electromagnetic radiation in the microwave region of the spectrum (0.3 to 300 GHz). The energies associated with microwaves are relatively low, and these energies are typically a good match for energy-level spacing between the different rotational levels of molecules. Thus, microwave spectroscopy is typically used to study the rotational energy levels of molecules. The rotational energy levels of molecules are typically studied in the gas phase.

What is infrared spectroscopy?

Infrared spectroscopy is carried out using somewhat higher energy electromagnetic radiation (300 GHz to 400 THz) than microwave spectroscopy. The infrared region of the spectrum is usually a good match for the vibrational energy-level spacing in molecules, so infrared spectroscopy is typically used to study the vibrational energy levels of molecules. Vibrational spectroscopy can be used to study molecules in the gas, liquid, and solid phases, as well as molecules on surfaces.

How does RADAR work?

RADAR, which stands for RAdio Detection And Ranging, works by sending out electromagnetic radiation, allowing it to bounce off of objects, and receiving it again after it has been reflected. The RADAR system measures things like how long it took for the signal to make it back, how the frequency of the signal has changed, and how the strength of the signal has changed. From this information, the RADAR system can “see” where the objects that reflected the light are located. This can also be used to determine the speed of an object, such as when the police use a RADAR gun to track the speed of a vehicle.

What is UV/Vis spectroscopy?

Electromagnetic radiation in the ultraviolet and visible region of the spectrum (40 to 1000 THz) is higher in frequency (and thus energy) than that in either the microwave or infrared. This makes it a good match for the larger energy-level spacing associated with transitions between electronic energy levels. UV/Vis spectroscopy can be used to study molecules in any phase; however, it is most commonly used for liquid samples.

What is Beer’s law?

Beer’s law tells us how the amount of electromagnetic radiation absorbed by a sample is related to the concentration of the absorbing species. Beer’s law tells us that the absorbance is equal to the length of the sample, l, times the concentration, c, of the absorbing species in the sample, times the molar absorptivity coefficient, ε, of the species.

A = εlc

In this equation A is absorbance, which is defined as the negative of the logarithm of the ratio of the intensity of light passing through a sample to that incident on it. Basically, this gives us a measure of how much light a sample is absorbing and how much light is passing through it.

What is fluorescence?

Fluorescence is a process by which molecules that have absorbed light can re-emit light to release some of the energy they absorbed. For fluorescence to take place, a molecule must first absorb a photon of light, which causes an electron to be excited to a higher energy level. At the same time, this process will typically also cause some vibrational excitations to take place. Some of the energy associated with this absorption will be given off through relaxation of the excited vibrational energy levels. For fluorescence to occur, the electronic excitation relaxes by emitting a photon of light. Some of the energy was dissipated as the vibrational energy levels relaxed, so the photon that is emitted has less energy than the photon that was initially absorbed. Remember that less energy means a lower frequency, so the photons that are emitted have a lower frequency than those that were absorbed.

Why do “black” lights make white materials appear to glow?

“Black” lights are lights that emit ultraviolet light at frequencies on the upperedge, or higher, relative to what our eyes can see. Many objects can absorb these frequencies and then undergo fluorescence, giving off light at lower frequencies that our eyes can see. This is the reason “black” lights cause these materials to appear to glow.

What is mass spectrometry?

Mass spectrometry is a method of chemical analysis that involves determining the molecular mass of charged particles by measuring the mass-to-charge ratio of an ionized molecule or molecular fragment. There are several ways of performing mass spectrometry, but the general procedure involves making the sample into a vapor, ionizing the sample, and then detecting the ions that form in a way that separates them according to their mass-to-charge ratio. After being ionized, the molecules in the sample will often fragment into smaller ions, and these too are detected according to their mass-to-charge ratio.

This technique can be useful for carrying out an accurate determination of the mass of a molecule as well as for obtaining structural information about molecules via their fragmentation patterns. It also allows the elemental composition of a sample to be determined.

How does a microscope work?

A microscope is all about lenses. The lens near the sample you’re looking at is called the objective lens, and this lens is responsible for collecting the light from the sample and focusing it. Typically there will be a light under or behind the sample that provides the light used to view the sample. At the other end is another lens called the ocular lens, and the total magnification of the microscope is determined by multiplying the magnification of the objective lens by that provided by the ocular lens. The apparatus we typically think of when we think of a microscope is essentially just a big framework used to hold the lenses, the sample, and perhaps other optical devices used to improve the image of the sample.

What is electron microscopy?

An electron microscope is a microscope that uses a beam of electrons to produce an image of a sample (rather than using light like in a standard microscope). There are several ways of obtaining an image, but the original was the transmission electron microscope (TEM), which produces an image by passing an electron beam directly through the sample. Electron microscopes offer a significant advantage in resolution over traditional light microscopes. This is due to the fact that the wavelengths associated with electrons are much shorter than those associated with visible light. It possible to achieve resolutions of up to roughly 10,000,000 times magnification using electron microscopes, as compared to about 2,000 times magnification in the best light microscopes.

Image

Simple optical microscopes work by magnifying images using lenses.

What is electrical resistance?

Electrical resistance describes how a material opposes an electric current through the material. The resistance is related to the applied voltage through the relationship:

R = V/I

where V is the applied voltage and I is the current through the material. Typically the resistance is a constant, and thus the current will increase linearly with an increase in the applied voltage. This relationship is Ohm’s law. As you can see, a material with a higher resistance (R) will have a lower current (I) for a given applied voltage (V).

What is a voltage?

A voltage, or potential difference, is the difference in electric potential energy between two points. The voltage describes the amount of work that would need to be done, per unit charge, to move a charged object between the two positions. A voltage may be present due to a static electric field, to electric current flowing through a magnetic field, or due to magnetic fields that change over time.