Atomic spectral line width

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. 

It would appear that measurement of the integrated absorption coefficient should furnish an ideal method of quantitative analysis. In practice, however, the absolute measurement of the absorption coefficients of atomic spectral lines is extremely difficult. The natural line width of an atomic spectral line is about 10 5 nm, but owing to the influence of Doppler and pressure effects, the line is broadened to about 0.002 nm at flame temperatures of2000-3000 K. To measure the absorption coefficient of a line thus broadened would require a spectrometer with a resolving power of 500000. This difficulty was overcome by Walsh,41 who used a source of sharp emission lines with a much smaller half width than the absorption line, and the radiation frequency of which is centred on the absorption frequency. In this way, the absorption coefficient at the centre of the line, Kmax, may be measured. If the profile of the absorption line is assumed to be due only to Doppler broadening, then there is a relationship between Kmax and N0. Thus the only requirement of the spectrometer is that it shall be capable of isolating the required resonance line from all other lines emitted by the source. 

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. 

Spectral line width varies inversely with the excited-state lifetime according to Heisenbergs principle, AT X A H = hi 2n, where AT is the lifetime of the excited spin state, h is Planck s constant, and AH is the effective width of the absorption signal. Excited-state lifetimes are subject to environmental (including chemical) influences. The resulting line-shape changes yield information about the chemical environment of the Mn atoms. Both spin-lattice and spin-spin relaxation mechanisms can contribute to the overall lifetime. 

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. 

A major breakthrough came in Australia when Alan Walsh1,2 realized that light sources were available for many elements which emitted atomic spectral lines at the same wavelengths as those at which absorption occurred. By selecting appropriate sources, the emission line widths could be even narrower than the absorption line widths (Figure 2). Thus the sensitivity problem was solved more or less at a stroke, and the modern flame atomic absorption spectrometer was bom. 

The effect of atmospheric gas on the emission of an LIB plasma [146] exhibits selfabsorption in He compared to Ar as a result of increased free atom populations in the outer regions of the plasma. Spectral line widths do not correlate well with atmospheric gas. This rules out Doppler effects as a major source of broadening in the laser-induced plasma. The use of a low pressure (ca. 1 torr) to examine the influence of this variable on the shock wave or secondary plasma revealed an increased emission intensity, which confirmed the assumption that the secondary plasma was excited by the shock wave. 

Natural Broadening The natural line width of an atomic spectral line is determined by the lifetime of the excited state and Heisenberg s uncertainty principle. The shorter the lifetime, the broader the line, and vice versa. Typical radiative lifetimes of atoms are on the order of 10 s, which leads to natural line widths on the order of 10 nm. 

Pressure broadening An effect that increases the width of an atomic spectral line caused by collisions among atoms that result in slight variations in their energy states. 

Atomic spectral lines have a physical width as a result of several broadening mechanisms [12]. 

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. 

As the line widths of diode lasers are considerably narrower than those of atomic spectral lines excited in a thermal atomizer, spectra can be recorded at very high resolution. When performing the atomization at reduced pressure (e.g. at 100-500 Pa), pressure broadening is low as compared with the Doppler broadening. As the... 

Line Broadening. Atomic spectral lines have a physical width resulting from several broadening mechanisms [10], The natural width of a spectral line results from the finite lifetime of an excited state, T. The corresponding half-width in terms of frequency is ... 

Spectral Inleifereiice. Spectral interference of analyte lines with other atomic spectral lines is of minor importance compared with atomic emission work. It is unlikely that resonance lines emitted by the hollow cathode lamp coincide with an absorption line of another element present in the atomizer. However, it may be that several emission lines of the hollow cathode are within the spectral band width or that flame emission from bands or continuum occurs. Both contribute to the non-absorbed radiation, and decrease the linear dynamic range. Nonelement-specific absorption (Section 21.5.6) is a further source of spectral interference. 

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. 

---