Width of Atomic Lines

The widths of atomic lines are quite important in atomic spectroscopy. For example, narrow lines are highly desirable for both absorption and emission spectra because they reduce the possibility of interference due to overlapping lines. Furthermore, as will be shown later, line widths are extremely important in the design of instruments tor atomic emission spectroscopy. For these reasons, we now consider some of the variables that influence the width of atomic spectral lines. 

Measuring Doppler widths of rotational lines by laser-probe techniques gives velocity distributions in just the same way as measuring Doppler widths of atomic lines by conventional means. In this method a laser beam with a very narrow band width is tuned over the spectral line to determine the profile of the Doppler broadened line. The line shape can be interpreted to give the average velocity of the product. As yet, this method has been applied only to rotational energy transfer studies however, with the availability of mode-locked lasers providing narrow band widths, this procedure may become more widely used. 

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. 

We now consider in somewhat more detail a simplified approach based on the curve of growth . For this, we ignore fine details of the observed line profile and use the equivalent width (EW) defined in Fig. 3.4, WA = f RdX or Wv = f Rdv, where R(AX) or R(Av) is the relative depression below the continuum at some part of the line. The curve of growth is a relationship between the equivalent width of a line and some measure of the effective number of absorbing atoms. Equivalent... 

The lines in the spectrum of Fig. 1 are, even under optimum experimental conditions, quite broad. This is due to small unresolved hyperfine interaction with the eighteen equivalent hydrogen atoms. Either the width of the lines, or nmr measurements (La Mar et al., 1973) can reveal the magnitude of this y-hydrogen splitting (ca. 0.15 G—dependent on temperature and solvent). 

Figure 1. Relation of line width, transition energy, and recoil energy, (a) Overlap (schematic) of emission and absorption lines in optical transitions, (b) Absence of overlap (schematic) of emission and absorption lines in nuclear transitions involving atoms free to recoil. Drawn to scale, separation between two lines would be about 4 X 10 the width of each line at half rriaximum...

Hydrogen is the most abundant chemical element in the universe, and in its various atomic and molecular forms furnishes a sensitive test of all of experimental, theoretical and computational methods. Vibration-rotational spectra of dihydrogen in six isotopic variants constituting all binary combinations of H, D and T have nevertheless been recorded in Raman scattering, in either spontaneous or coherent processes, and spectra of HD have been recorded in absorption. Despite the widely variable precision of these measurements, the quality of some data for small values of vibrational quantum number is still superior to that of data from electronic spectra [106], almost necessarily measured in the ultraviolet region with its concomitant large widths of spectral lines. After collecting 420... 

Profile of an atomic line (a) the half width Av is the width of the line when k = l/2k (b) the effect of self-absorption as the concentration of atoms increases from 1 to 5. 

The second factor involves the theory that defines the natural width of the lines. Radiations emitted by atoms are not totally monochromatic. With plasmas in particular, where the collision frequency is high (this greatly reduces the lifetime of the excited states), Heisenberg s uncertainty principle is fully operational (see Fig. 15.4). Moreover, elevated temperatures increase the speed of the atoms, enlarging line widths by the Doppler effect. The natural width of spectral lines at 6000 K is in the order of several picometres. 

The natural broadening which results from the finite lifetime of excited states. The energy of a state and its lifetime are related by the principle of uncertainty (section 2.2) which implies a minimal spread of the actual energy of any excited state of finite lifetime this gives an absolute limit to the width of atomic spectral lines. 

The last two mechanisms of the broadening of atomic spectral lines are in most cases the real experimental limitations in atomic spectroscopy. The half-widths of such lines are usually of the order of 10-3 nm. 

Fig. 48. The surface patter of Si02 stripes on a silicon wafer was prepared electrochemically by applying a bias pulse to locally remove the terminal hydrogen atoms. The aspect ratio (height/width) of oxide lines improves significantly when the relative humidity is lowered from 61% to 14%. Reproduced from [445]...

We will concentrate here on correction using a continuous emission lamp. The method consists of measuring, alternatively, the atomic absorption from the line of the element and the non specific absorption from a continuous spectrum lamp, over an range centred on the line and defined by the monochromator bandwidth. As this is much greater than the width of the line being analysed, we can consider that the second measurement corresponds solely to continuous (non specific) absorption. Continuous spectrum lamps used to correct the background arc ... 

A low-pressure atomic emission lamp such as neon or argon emits atomic lines of sufficiently narrow linewidth to be considered infinitely narrow for most Raman spectrometers (an example is shown in Fig. 10.1). The width of atomic emission lines depends on temperature and pressure, but is generally... 

The line widths of atoms in a medium such as a flame or plasma are about 0.1 to 0.01 A. The wavelengths of atomic lines are unique for each element and are often used for qualitative analysis. 

Atomic spectral lines have finite widths. With ordinary measuring spectrometers, the observed line widths are determined not by the atomic system but by the spee-trometer properties. With very-high-resolution spectrometers or with interferometers, the actual widths of spectral lines can be measured. Several factors contribute to atomic spectral line widths. 

The width of atomic emission lines in flames is on the order of 10" nm. The width can be measured with an interferometer. 

Flame atomic absorption spectroscopy (AAS) is currently the most widely used of all the atomic methods listed in Table 28-1 because of its simplicity, effectiveness, and relatively low cost. The technique was introduced in 1955 by Walsh in Australia and by Alkemade and Milatz in Holland. The first commercial atomic absorption (AA) spectrometer was introduced in 1959, and use of the technique grew explosively after that. Atomic absorption methods were not widely used until that time because of problems created by the very narrow widths of atomic absorption lines, as discussed in Section 28A-1. 

The widths of atomic absorption lines are much less than the effective bandwidths of most monochromators. 

Owing to the line broadening mechanisms, the physical widths of spectral lines in most radiation sources used in optical atomic spectrometry are between 1 and 20 pm. This applies both for atomic emission and atomic absorption line profiles. In reality the spectral bandwidth of dispersive spectrometers is much larger than the physical widths of the atomic spectral lines. 

A primary source is used which emits the element-specific radiation. Originally continuous sources were used and the primary radiation required was isolated with a high-resolution spectrometer. However, owing to the low radiant densities of these sources, detector noise limitations were encounterd or the spectral bandwidth was too large to obtain a sufficiently high sensitivity. Indeed, as the width of atomic spectral lines at atmospheric pressure is of the order of 2 pm, one would need for a spectral line with 7. = 400 nm a practical resolving power of 200 000 in order to obtain primary radiation that was as narrow as the absorption profile. This is absolutely necessary to realize the full sensitivity and power of detection of AAS. Therefore, it is generally more attractive to use a source which emits possibly only a few and usually narrow atomic spectral lines. Then low-cost monochromators can be used to isolate the radiation. 

On the width of atomic resonance lines from hollow cathode lamps, Spectrochim Acta, Part B 26 207-235. 

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