Spectrophotometry Basics

Irradiance

This is the power emitted by the body per unit area and wavelength (W · nm^-1 • cm^-2). Furthermore, the irradiance of a lamp that is showing is similar to a black body.

Emissivity

It is the ratio of the irradiance of the lamp and a black body at the same temperature:

Emissivity (e, t) = I (lamp) (E, T) / I (black body) (E, T).

Gray Body

This occurs when the emissivity is independent of the wavelength of the lamp in question.

Deuterium Lamp

It is a low-pressure lamp that has a smooth continuum between 200 and 400 nm, and from this last value, the spectrum is more irregular with a large number of peaks. It also has a life of a few hundred hours. The QTH lamp (quartz-tungsten-halogen) issues more visible light than the Deuterium lamp in the visible spectrum, whereas in the ultraviolet, it is reversed. For this reason, spectrophotometers use Deuterium lamps between 190 and 320 nm, while from 320 to 3000 nm, they use QTH lamps.

Mercury Lamp

It is a low-pressure lamp whose bulb contains a gas, usually containing atoms of Mercury. What it does is pass a current through the gas to ionize and excite both molecules it contains. The light exiting the lamp is mainly due to the intrinsic emission of these molecules. One particular example of such lamps are those of Mercury-Argon (Hg-Ag) or the Mercury-Neon (Hg-Ne). The process leading to emission of radiation can be expressed as: e^- + Hg + Ec - à Hg⁺ + e^- + Ec'. As the excitation of mercury occurs only for certain energy levels, the spectrum will be composed of discrete and monochromatic lines due to transitions out voltage of mercury.

Xenon Arc Lamp

The gas that these lamps have within them is Xe. In their spectra, emission lines are observed between 750 and 1000 nm. They are used to mimic the solar spectrum since the spectrum of a lamp of this type can be approximated by a black body at a temperature between 5000 and 6000 K.

Xenon Arc Lamp with Hg

Usually, Xe arc lamps have mercury added, so that you have a high-pressure Xe-Hg lamp. Overlapping emission spectra with those of Xe and Hg means that the spectrum of a lamp of this type will be more or less smooth because of the continuous emission of Xe, except in a certain frequency range in which peaks are appreciated due to the emission of Hg. These peaks of Hg widen due to the interaction of the molecules of mercury with the surroundings.

Detectivity (D)

It is the inverse of the NEP; therefore, a good detector must have high detectivity. For most infrared detectors, D is inversely proportional to the area of the detector and the frequency bandwidth D f (½ raised to a power). Therefore, to compare different detectors, specific detectivity (D*) is defined as D* = D (root) Area * ?f. The larger the sensor area, the greater the thermal noise, and therefore it is not very appropriate for the sensor area to be too large.

NEP (Noise Equivalent Power)

Radiant power is needed (in W) so that the detector gives a signal equal to the noise of the detector. Ideally, it should be as small as possible, which is achieved by cooling the detector. Many times liquid nitrogen is used to cool the detector.

Time Response of a Detector

It is not that a switch gives the signal as it comes, but has some delay due to the time it takes to convert and amplify the light signal into a current signal. Then, we define the time constant of a detector as the time it takes to give 63% of the total signal it will give us at the output. The smaller the time constant of a detector, the faster it will be.

Range of Linearity of a Detector

This is the range in which the detector response is proportional to the intensity of light that is coming. It is interesting to know the range of linearity of the detector because this will be its working range.

Photoconductive Detector

Photoconductors are materials whose conductivity changes when illuminated. Thus, candidates are being photoconductive semiconductors. What happens is that when the photon arrives, it excites the electron from the valence band to the conductive band, making the material more conductive.

The most commonly used semiconductors to build photoconductors (used in optical spectroscopy) are: silicon, germanium, and lead sulfide. The schematic of a photoconductor detector consists of an electrical circuit connected to a battery and a resistance plus the resistance of the photoconductor. What is done is to measure the current intensity, which will vary according to the conductivity of the photoconductor.

Fig. Schematic of a silicon diode detector in photoconductive mode. When light is incident on the silicon, it will be more conductive and increase the current in the ammeter.

Photodiode or Photovoltaic Detector

They are constructed by the union of a p-type semiconductor with an n-type. The potential difference that exists between the p region and the n region is proportional to the amount of light that comes in because the more light that reaches it, the larger the number of electron-hole pairs that are created. It is desirable that the detector has just the sensitive area that we will use because if we had more, the noise would be greater, which would worsen the detector. When photons with energies greater than the band gap excite electrons, electron-hole pairs are created. If these pairs are close to the transition region, the electrons in the p side go to the n side, and the holes in the n side go to the p side. The electron-hole pairs created far from the transition zone will recombine before arriving. These detectors can be used by measuring the current through them or by measuring the voltage appearing between the p and n regions when light impacts them.

Dynode

A component of the photomultiplier. The multiplier is the same as the dynode, i.e., where the number of electrons increases. Among the dynodes is a voltage divider so that the tension between them melts away and thus ensures that each liberated electron is accelerated towards the next dynode.

Photocathode

It is the very material on which the photoelectric effect occurs. To measure the electron at the output, what is done is to create a very high voltage difference between the photocathode and the first dynode. Then, as the electrons begin to collide with the dynode on their way to the anode, more electrons are released, producing a current (electron stream) that is measurable. The photocathode and dynode have a certain rate of thermionic emission that is responsible for the dark current.

Dispersion in a Monochromator

Is defined as width at half height = x width Slit Scattering. If we have a monochromator with a dispersion of D = 1.4 nm/mm, and we know that the width of the entrance and exit slit is 1 mm, then the width of the peak at half height would be 1.4 nm.

Thermopile

A type of thermal detector, which operates by heating. That is, when a light signal reaches the detector, it is heated, and its temperature changes, causing some physical property to change. The thermopile is based on the "Seebeck Effect", which is the effect of thermocouples, where the heating of the detector produces a voltage that can be measured.

Pyroelectric

In this type of thermal detector, what happens is that the temperature increase in the detector makes the polarization within the material (ferroelectric) vary, which can be measured.

Optical Density

It is a practical measurement of the absorption and is defined as: DO (E) = log (Io (E) / I (ë)). It is directly proportional to material thickness (d) and therefore to the concentration of absorbing centers. It would, therefore, be an absolute measure of the concentration, assuming a known absorption cross-section s, or its determination if an alternative procedure is used to measure the concentration of absorbing centers.

Absorption Coefficient and its Relation to the Optical Density

It will be some function of the wavelength of the incident radiation, a = a (l).

Absorption coefficients are related to fundamental properties of the medium. Let's see how to obtain experimental information about them and their dependence on wavelength. The light produced by a suitable source for studying the spectral range is dispersed and selected by a monochromator element (prism, diffraction grating), then being sent through the sample to a detector that eventually transforms the information into an electrical signal that allows storage and registration. The relationship between optical density and the absorption coefficient is:

alpha = 2303 DO / d

Work Function (f)

It is the energy invested by the photon to remove an electron, which, together with the kinetic energy absorbed by the electron, adds up to the total energy of the incident photon. So that: hv = Ø + 1/2me^-Ve². This is called the photoelectric effect. The minimum energy to free an electron is when the electron has no kinetic energy once released; therefore: hvo = Ø.