What is Spectroscopy?

General Overview of Spectroscopy

Collection and analysis of a spectrum usually involves: (1) a source of light (or other electromagnetic radiation), (2) an element to separate the light into its component wavelengths, and (3) a detector to sense the presence of light after separation of wavelengths. The device employed to receive light, divide it into its component wavelengths, and detect the spectrum is called a spectrometer. The term spectrometry is usually used instead of spectroscopy when the intensities of the signals at different wavelengths are measured electronically. Spectra can be obtained either in the form of emission spectra, which show one or more bright lines or bands against a dark background, or absorbance spectra, which have a continually bright background and depict the spectral information as one or more dark lines.

Spectrometers and spectrometric methods of analysis have many of the characteristics of the ideal analytical method and instrument.

Absorbance spectroscopy measures the loss of electromagnetic energy after the energy interacts the sample under study. For example, if a light beam containing a broad mixture of wavelengths is directed at a vapor of atoms, ions, or molecules, the particles will absorb those wavelengths that can excite them from one quantum state to another. Consequently, the absorbed wavelengths will be missing from the original light mixture (spectrum) after it has passed through the sample. Because most atoms and many molecules have unique and identifiable energy levels, a measurement of the missing (absorbance) lines enables identification of the absorbing species. Absorbance within a continuous band of wavelengths is also possible. This type of absorbance is particularly common when there is a large population of absorbance lines that have been broadened by strong perturbations from surrounding atoms (e.g., collisions in a high-pressure gas, or interactions with nearby neighbors in a solid or liquid).

In the laboratory, transparent containers or chambers with windows at both ends serve as absorbance cells for the production of absorbance spectra. Light with a continuous distribution of wavelength is transmitted through the cell. When a gas or liquid is introduced, the change in the transmitted spectrum produces the absorbance spectrum of the gas or liquid. In other cases, the sample to be studied need not be contained at all. For example, interstellar molecules can be analyzed by studying the absorbance of the electromagnetic waves from a background star.

The transmission properties of the Earth's atmosphere determine which parts of the electromagnetic spectrum of the Sun and other astronomical sources of electromagnetic waves are able to pass through the atmosphere. The absorbance of ultraviolet and X-ray radiation by the upper atmosphere prevents this portion of the electromagnetic spectrum from reaching the Earth. The fact that water vapor, carbon dioxide, and other gases reflect infrared light is important in determining how much heat from the Earth is lost through radiation into space. This phenomenon, often called the greenhouse effect, works in much the same way as the glass panes of a greenhouse. In a greenhouse, electromagnetic energy in the form of visible light is able to pass through the glass and enter the building. The visible light strikes objects inside the greenhouse, and these objects at least partially absorb the light. Some of this absorbed energy is re-radiated as infrared light, which is reflected back into the building by the glass, thus keeping the greenhouse warm. In a similar way, the transmission spectrum of the atmosphere is an important factor in determining the global temperature of the Earth.

The second principal form of spectroscopy, emission spectroscopy, uses some means to excite the sample of interest to a higher energy level. After the atoms or molecules are excited, they relax spontaneously to lower energy levels, emitting radiation corresponding to the energy differences, delta E = h nu = hc / lambda, between the various energy levels of the quantum system. In analytical use, this emitted radiation is the complement of the missing wavelengths in absorbance spectroscopy. Thus, the emission bands also serve as a characteristic "fingerprint" that can be associated with a unique atom, ion, or molecule. Early excitation methods often involved placing the sample in a flame or an electric-arc discharge. Plasma emission spectroscopy is now becoming increasingly popular. The atoms or molecules are excited by collisions with electrons, the broadband light in the excitation source, or collisions with energetic atoms. The analysis of the emission lines is done with the same types of spectrometers as used in absorbance spectroscopy.

Types of Electromagnetic-Radiation Sources

Continuum (or Broadband) Light Sources

Although flames and discharges provide a convenient method of excitation, the high-temperature, relatively low pressure environment can strongly perturb the sample being studied (in fact, flames and discharges usually destroy the sample being studied,as well as the rest of the sample matrix). Excitation using broadband-light sources, in which the generation of the light is separated from the sample under study, can provide a less destructive means of sample excitation. Higher energy excitation corresponds to shorter wavelengths, but unfortunately, there are not many intense sources of ultraviolet and vacuum-ultraviolet radiation. Tunable laser sources based on optical parametric oscillators still do not completely cover this region, and such lasers are relatively expensive. In consequence, excitation in an electron discharge remains a common method in this portion of the electromagnetic spectrum. (The term vacuum ultraviolet refers to the short-wavelength portion of the electromagnetic spectrum where the photons are energetic enough to excite a typical atom from the ground state to ionization in one transition. Under these conditions, the light is highly absorbed by air and most other substances, so the light-beam path must be maintained under vacuum.)

A broadband-light source that is often used for both emission and absorbance spectroscopy is a metal filament heated electrically to a high temperature. A typical example is a tungsten (wolfram) light bulb. Because the atoms in the metal are packed closely together, their individual energy levels merge together; and the emission lines overlap and produce a continuous (nondiscrete) spectrum. Similar phenomena occur in high-pressure arc lamps, in which broadening of spectral emission lines also occurs due to high collision rates.

An arc lamp is made from a transparent tube containing gases that are excited by an electric discharge. High-energy electrons bombard the atoms, exciting them to high-energy atomic states or to an ionized state in which the outermost electron is removed from the atom. The electromagnetic radiation that is emitted in the arc lamp is usually a mixture of discrete atomic lines that come from the relaxation of the atoms to lower energy states, and continuum radiation resulting from closely spaced lines that have been broadened by collisions with other atoms and the electrons. If the gas pressure in the arc lamp is high enough, a large portion of the light is emitted in the form of continuum radiation.

Light sources that emit primarily radiation with discrete, well-defined frequencies are also widely used in spectroscopy. The first sources of spectral emission lines were simply arc lamps, or another form of electrical discharge in a sealed tube containing gas in which the pressure was kept so low that a significant portion of the radiation was emitted in the form of discrete lines. The Geissler discharge tube, of which the neon lamp used in advertising signs provides an example, is such a source. Other examples of low-pressure sources are hollow cathode lamps and electrodeless lamps driven by microwave radiation. When specific atomic lines are desired, a small amount of the desired element is added to the discharge.

Lasers are line sources that emit high-intensity electromagnetic radiation over a very narrow frequency range. (The word LASER is an abbreviation of Light Amplification by Stimulated Emission of Radiation.) The invention of the laser by the American physicists Arthur Schawlow and Charles Townes in 1958, the demonstration of the first practical laser by the American physicist Theodore Maiman in 1960, and the subsequent development of laser spectroscopy techniques by many researchers revolutionized a field that had previously seen most of its conceptual developments before 1900. Intense, tunable (adjustable-wavelength) light sources now cover most of the visible, near-infrared, and near-ultraviolet portions of the spectrum. Lasers have been used for selected wavelength regions in the infrared to submillimeter microwave range, and on the opposite end of the spectrum, for wavelengths as short as the soft X-ray region (the term "soft" denotes the lower end of energies).

Typically, light from a tunable laser (such as a dye laser, semiconductor diode laser, or free-electron laser) is directed into the sample under study just as the more traditional light sources are used in absorbance or emission spectroscopy. For example, in fluorescence emission spectroscopy, the frequency and amount of light scattered by the sample is measured as the frequency of the laser light is varied. There are many advantages to using a laser as a light source: (1) The light from lasers can be made highly monochromatic (meaning light of essentially one "color"--i.e., composed of a very narrow range of frequencies). As the light is tuned across the frequency range of interest and the absorbance or fluorescence is digitized, extremely narrow spectral features can be measured. Modern tunable lasers can easily resolve spectral features less than 1 million hertz wide, while the highest-resolution grating spectrometers have resolutions that are hundreds of times worse. Atomic lines as narrow as 30 hertz out of a transition frequency of 6 E14 hertz have been observed with laser spectroscopy. (2) Because the laser light in a given narrow frequency band is much more intense than virtually all broadband sources of light used in spectroscopy, the amount of fluorescent light emitted by the sample can be greatly increased. Laser spectroscopy is sufficiently sensitive to observe fluorescence from a single atom in the presence of 1E20 different atoms.

One potential limitation to resolution in the spectroscopy of gases arises from the motion of the atoms or molecules relative to the observer. The Doppler shifts that result from the movement of the atoms broadens any sharp spectral features that the sample gas might have. A cell containing a gas of atoms, like helium,will have atoms moving both toward and away from the light source, so that the absorbing frequencies of some of the atoms will be shifted up (those moving toward the light source) while others will be shifted down (those moving away from the light source) .

A spectrometer, as stated earlier, is an instrument used to analyze light transmitted through a sample (in the case of absorbance spectroscopy) or emitted light (in the case of emission spectroscopy). The spectrometer contains an element that isolates the light into its component wavelengths and a detection system for recording the relative intensities of each of the component wavelengths. The main methods for selecting wavelengths or frequencies of electromagnetic radiation are discussed below.

Glass prisms were the first devices used to break up or disperse light into its component colors. The path of a light ray bends (refracts) when it passes from one transparent medium to another--e.g., from air to glass. Different colors (wavelengths) of light are refracted through different angles; therefore a ray leaves a prism in a direction that depends on its wavelength. The degree to which a ray bends at each interface can be calculated using Snell's law. Snell's Law states that, if n1 and n2 are the refractive indices of the medium outside the prism and of the prism itself, respectively, and the angles i and r are the angles that the light ray of a given wavelength makes with a line at right angles to the prism face, then the equation n1 sin i = n2 sin r holds for all rays. The refractive index of a medium, indicated by the symbol n, is defined as the ratio of the speed of light in a vacuum to the speed of light in the medium. Typical values for n range from 1.0003 for air at 0 degree C and atmospheric pressure, to 1.5-1.6 for typical glasses, to 4 for germanium in the infrared region of the electromagnetic spectrum.

Because the index of refraction of optical glasses varies by only a few percent across the visible spectrum, different wavelengths are separated by small angles. Prism instruments are therefore usually used only when low spectral resolution is required.

Modeling electromagnetic wave propagation. At all points along a given wavefront (usually denoted by the crest of the wave), the advancing light wave can be thought of as being generated by a set of spherical radiators, according to a principle first enunciated by the Dutch scientist Christiaan Huygens and later made quantitative by Fraunhofer. The new wavefront is defined by the line that is tangent to all the wavelets (secondary waves) emitting from the previous wavefront. If the emitting regions are in a plane of infinite extent, the light will propagate along a straight line normal to the plane of the wavefronts. However, if the region of the emitters is bounded or restricted in some other way, the light will spread out by a phenomenon named diffraction.

Diffraction gratings are formed from closely spaced transmitting slits on a flat surface (transmission gratings) or alternate reflecting grooves on a flat or curved surface (reflection gratings).

If collimated light (light in which all the rays are parallel) is directed toward a transmission grating, the wavefronts pass through and spread out as secondary waves from the transparent parts of the grating. Most of these secondary waves, when they meet along a common path, interfere with each other destructively, so that light does not leave the grating at all angles. At some exit angles, however, secondary waves from adjacent slits of the grating are delayed by exactly one wavelength, and in this case waves reinforce one another when they meet--that is, the crests of one wave fall on top of the crests of the other. In this case, constructive interference takes place, and light is emitted in directions where the spacing between the adjacent radiators is delayed by one wavelength. Constructive interference is also observed for delays of integral numbers of wavelengths. The light diffracts according to the formula m lambda = d(sin i - sin r), where i is the incident angle, r is the reflected or transmitted angle, d is the spacing between grating slits, lambda is the wavelength of the light, and m is an integer (usually called the order of interference). If collimated light having several constituent wavelengths is directed upon a grating at a fixed angle i, different wavelengths will be diffracted in slightly different directions and can be observed and recorded separately. Each wavelength is also diffracted into several orders (or groupings). Gratings are usually blazed (engraved) so that a particular order will be the most intense. A lens or concave mirror can then be used to produce images of the spectral lines.

As the grating in a spectrometer is rotated about an axis parallel to the slit axis, the spectral lines of a sample are transmitted successively through the instrument. An electronic photodetector placed behind the slit can then be used to measure the intensity of light at each wavelength in the spectrum. The advantage of such an arrangement is that electronic photodetectors (like silicon CCDs or even photomultiplier tubes) are extremely sensitive, have a fast time response, and respond linearly to the energy of the light over a wide range of light intensities.

A third class of devices for isolating frequencies or wavelengths in spectra are known as interferometers. These instruments divide the light with semitransparent surfaces, producing two or more beams that travel different paths and then recombine. In spectroscopy, the principal interferometers are those developed by the American physicist A.A. Michelson (1881) in an attempt to find the luminiferous ether--a hypothetical medium thought at that time to pervade all space--and by two French physicists, Charles Fabry and Alfred Perot (1896), specifically for high-resolution spectroscopy.

In the Michelson interferometer, an incident beam of light strikes an angled semitransparent mirror (called the beam splitter) and divides the light into a reflected and transmitted wave. These waves continue to their respective mirrors, are reflected, and return to the semitransparent mirror. If the total number of oscillations of the two waves during their separate paths add up to be an integral number just after recombining on the partially reflecting surface of the beam splitter, the light from the two beams will add constructively and be directed toward a detector. This device then acts as a filter that transmits preferentially certain wavelengths and reflects others back to the light source, resulting in a visible interference pattern. A common implementation of the Michelson interferometer has one movable mirror mounted upon a carriage so that length of the light path in that path can be varied. A spectrum is obtained by recording photoelectrically the light intensity of the interference pattern as the mirror is moved when an absorbance cell is placed in one of the arms of the interferometer. The resulting signals contain information about many wavelengths simultaneously. A mathematical operation, called a Fourier transform, converts the recorded modulation in the light intensity at the detector into the usual frequency domain of the absorbance spectrum. There are two advantages of the interferometer: (1) the frequency resolution of the interferometer increases with increasing length of travel of the movable mirror, and is not a function of slit width (the throughput advantage), (2) the entire spectrum is recorded simultaneously with one detector (the multiplex advantage).

The Fabry-Perot interferometer consists of two reflecting mirrors that can be either curved or flat. Only certain wavelengths of light will resonate in the cavity: the light is in resonance with the interferometer if m(lambda /2) = L, where L is the distance between the two mirrors, m is an integer, and lambda is the wavelength of the light inside the cavity. When this condition is fulfilled, light at these specific wavelengths will build up inside the cavity and be transmitted out the back end for specific wavelengths. By adjusting the spacing between the two mirrors, the instrument can be scanned over the spectral range of interest.

The principal detection methods used in optical spectroscopy are photoconductive (semiconductor), photoemissive (photomultipliers), and photographic (e.g., film). Prior to about 1940, most spectra were recorded with photographic plates or film, in which the film is placed at the image point of a grating or prism spectrometer. An advantage of this technique is that the entire spectrum of interest can be obtained simultaneously, and low-intensity spectra can be easily taken with sensitive film and integration of signal over time..

Photoemissive detectors quickly replaced photographic plates in most analytical applications. When a photon with sufficient energy strikes a surface, it can cause the ejection of an electron from the surface into a vacuum. A photoemissive diode consists of a surface (photocathode) appropriately treated to permit the ejection of electrons by low-energy photons and a separate electrode (the anode) on which electrons are collected, both sealed within an evacuated glass envelope. A photomultiplier tube has a cathode, a series of electrodes (dynodes), and an anode sealed within a common evacuated envelope. Appropriate voltages applied to the cathode, dynodes, and anode cause electrons ejected from the cathode to collide with the dynodes in succession. Each electron collision produces several more electrons; after a dozen or more dynodes, a single electron ejected by one photon can be converted into a fast pulse (with a duration of less than 1E-8 second) of as many as 1E7 electrons at the anode. In this way, individual photons can be counted with good temporal resolution (separation in time).

Other photodetectors include imaging tubes (e.g., television cameras), which can measure a spatial variation of the light across the surface of the photocathode, and microchannel plates, which combine the spatial resolution of an imaging tube with the light sensitivity of a photomultiplier. A night vision device consists of a microchannel plate multiplier in which the electrons at the output are directed onto a phosphor screen and can then be read out with an imaging tube.

Solid-state detectors such as semiconductor photodiodes detect light by using photons to excite electrons from immobile, bound states of the semiconductor (the valence band) to a state where the electrons are mobile (the conduction band). The mobile electrons in the conduction band and the vacancies, or "holes," in the valence band can be moved through the solid with externally applied electric fields, collected onto a metal electrode, and sensed as a photoinduced current. Microfabrication techniques developed for the integrated-circuit semiconductor industry are used to construct large arrays of individual photodiodes closely spaced together. The device, called a charge-coupled device (CCD), permits the charges that are collected by the individual diodes to be read out separately and displayed as an image.

References

Encyclopedia Brittanica

Peters, Hayes, and Hieftje, "Chemical Separations and Measurements", Saunders, 1974.