Absorption line narrowing

Resonant two-step excitation by means of co- or counterpropagating cw dye laser beams results in Doppler-free spectra due to absorption line narrowing. Starting from the atomic ground state Ig), certain velocity ensembles, Doppler-tuned into resonance, are excited by one of the laser beams to the intermediate level i). When the second laser is scanned across... 

Pump-probe absorption experiments on the femtosecond time scale generally fall into two effective types, depending on the duration and spectral width of the pump pulse. If tlie pump spectrum is significantly narrower in width than the electronic absorption line shape, transient hole-burning spectroscopy [101. 102. 103. 104. 105. 106. 107. 108. 109. 110. 111. 112 and 113] can be perfomied. The second type of experiment, dynamic absorption spectroscopy [57, 114. 115. 116. 117. 118. 119. 120. 121 and 122], can be perfomied if the pump and probe pulses are short compared to tlie period of the vibrational modes that are coupled to the electronic transition. 

Another feature of the spectrum shown in Figure 10.19 is the narrow width of the absorption lines, which is a consequence of the fixed difference in energy between the ground and excited states. Natural line widths for atomic absorption, which are governed by the uncertainty principle, are approximately 10 nm. Other contributions to broadening increase this line width to approximately 10 nm. 

Equation 10.1 has an important consequence for atomic absorption. Because of the narrow line width for atomic absorption, a continuum source of radiation cannot be used. Even with a high-quality monochromator, the effective bandwidth for a continuum source is 100-1000 times greater than that for an atomic absorption line. As a result, little of the radiation from a continuum source is absorbed (Pq Pr), and the measured absorbance is effectively zero. Eor this reason, atomic absorption requires a line source. 

Selectivity Due to the narrow width of absorption lines, atomic absorption provides excellent selectivity. Atomic absorption can be used for the analysis of over 60 elements at concentrations at or below the level of parts per million. 

Figure 2.5 shows, for a sample in the gas phase, a typical absorption line with a HWHM (half-width at half-maximum) of Av and a characteristic line shape. The line is not infinitely narrow even if we assume that the instmment used for observation has not imposed any broadening of its own. We shall consider three important factors that may contribute to the line width and shape. 

When using the DIAL method to measure the concentration of a molecule with discrete absorption the wavelengths of the two laser beams, on and off a narrow absorption line, must be very similar (less than 1 nm separation) so that the background absorption and backscatter is the same for both. 

For quantitative analysis, the resolution of the spectral analyzer must be significantly narrower than the absorption lines, which are - 0.002 nm at 400 nm for Af = 50 amu at 2500°C (eq. 4). This is unachievable with most spectrophotometers. Instead, narrow-line sources specific for each element are employed. These are usually hoUow-cathode lamps, in which a cylindrical cathode composed of (or lined with) the element of interest is bombarded with inert gas cations produced in a discharge. Atoms sputtered from the cathode are excited by coUisions in the lamp atmosphere and then decay, emitting very narrow characteristic lines. More recendy semiconductor diode arrays have been used for AAS (168) (see Semiconductors). 

Figure 9. C02 Detection variation of modulation Index (m), with optical filter centre wavelength and bandwidth. The broad range of absorption lines causes a very complex variation of modulation indices when using narrow filters (not all peaks at narrow filter bandwidths are shown, as this would obscure the behaviour with wider filter bandwidths). Reference and measurement cells are assumed to he of 1 m length and contain 100% C02 gas at 1 Bar/20 °C.

The thermal stability of the centers responsible for the local modes of vibration has been investigated. In GaP, Sobotta etal. (1981) observed that annealing of the implanted samples at 240°C for one hour leads to a narrowing of the 2204 cm 1 absorption line. This is due to the annealing of the radiation damage. Annealing at 400°C for one hour decreases drasti-... 

Atoms, ions and molecules present in the stars provide additional opacity at wavelengths corresponding to specific atomic transitions these give rise to comparatively narrow absorption lines (see Fig. 3.2) with intensities related to the abundances of the relevant elements (and much else). Despite the name, processes other than pure absorption (e.g. scattering and fluorescence) are involved in the production of these lines and, while they are often treated in LTE, this is now only a simplifying approximation which often works fairly well, but needs to be checked by more detailed calculations for each particular case. (In some cases, there are even emission lines or emission components, e.g. the solar Ca+ H and K lines in the near UV, which are so strong that the chromosphere affects their central parts.)... 

Measurement of integrated absorption requires a knowledge of the absorption line profile. At 2000-3000 K, the overall line width is about 10-2 nm which is extremely narrow when compared to absorption bands observed for samples in solution. This is to be expected, since changes in molecular electronic energy are accompanied by rotational and vibrational changes, and in solution collisions with solvent molecules cause the individual bands to coalesce to form band-envelopes (p. 365). The overall width of an atomic absorption line is determined by ... 

To make accurate measurements of the integrated absorption associated with such narrow lines requires that the linewidth of the radiation source be appreciably smaller than that of the absorption line. In practice, this could be achieved with a continuum source only if expensive instrumentation of extremely high resolving power were used, and it is doubtful whether conventional photomultiplier detectors would be sufficiently sensitive at the resulting low radiation intensities. An alternative arrangement is to... 

The light passing through the flame must be of exactly the same frequency as the absorption line, in order to stimulate the analyte atoms in the flame to absorb. Because of the narrow absorption lines of the atomic plasma in the... 

Site-selection spectroscopy Maximum selectivity in frozen solutions or vapor-deposited matrices is achieved by using exciting light whose bandwidth (0.01-0.1 cm-1) is less than that of the inhomogeneously broadened absorption band. Lasers are optimal in this respect. The spectral bandwidths can then be minimized by selective excitation only of those fluorophores that are located in very similar matrix sites. The temperature should be very low (5 K or less). The techniques based on this principle are called in the literature site-selection spectroscopy, fluorescence line narrowing or energy-selection spectroscopy. The solvent (3-methylpentane, ethanol-methanol mixtures, EPA (mixture of ethanol, isopentane and diethyl ether)) should form a clear glass in order to avoid distortion of the spectrum by scatter from cracks. 

After X-ray irradiation of thermally annealed NaCl crystals, a small percentage of divalent europium ions are converted into trivalent europium ions (Aguilar et al, 1982). This is shown by the appearance of weak and narrow absorption lines at around 460 nm and 520 nm, related to the Fq D2 and Fq Di transitions of Eu + ions, respectively. For our purposes, this example allows us to compare the different band features between (RE) + and (RE) + ions Eu + ions show broad and intense optical bands (electric dipole allowed transitions), while Eu + ions present narrow and weak optical lines (forced electric dipole transitions). 

Of course, this exact overlap is no accident, as atomic absorption and atomic emission lines have the same wavelength. The very narrowness of atomic lines now becomes a positive advantage. The lines being so narrow, the chance of an accidental overlap of an atomic absorption line of one element with an atomic emission line of another is almost negligible. The uniqueness of overlaps in the Walsh method is often known as the lock and key effect and is responsible for the very high selectivity enjoyed by atomic absorption spectroscopy. 

An inherent difficulty in observing a narrow absorption line is that it does not absorb very much flux. The narrower the line, the less flux absorbed. In fact, the ideal -function line absorbs none at all. This statement may be verified by evaluating the integrated absorptance... 

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