Broadening, atomic spectral lines

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 ... 

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. 

Describe the factors which cause broadening of spectral lines. In atomic absorption spectrometry, why is it preferable for the source line-width to be narrower than the absorption profile How can this be achieved What are the differing requirements for resolution in monochromators for atomic emission and for atomic absorption spectrometry ... 

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. 

Taking advantage of advances in computational atomic and plasma physics and of the availability of powerful supercomputers, a collaborative effort - the international Opacity Project - has been made to compute accurate atomic data required for opacity calculations. The work includes computation of energy levels, oscillator strengths, photoionization cross-sections and parameters for pressure broadening of spectral lines. Several... 

Sobelman I., Vainshtein A., Yukov E., Excitation of Atoms and Broadening of Spectral Lines (Springer, Heidelberg 1998)... 

This aspect of the uncertainty principle, due originally to Bohr, refers to a measurement of the energy carried out during a time interval At. It implies that a short-lived excited state of an atom or molecule, for which A/ is small, will be associated with a relatively large uncertainty in energy E. This is one factor contributing to the broadening of spectral lines. 

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. 

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... 

Radiofrequency Spectroscopy of Stored Ions II Spectroscopy, FI. G. Dehmelt The Spectra of Molecular Solids, O. Schnepp The Meaning of Collision Broadening of Spectral Lines The Classical Oscillator Analog, A. Ben-Reuven The Calculation of Atomic Transition Probabilities, R. J. S. Crossley Tables of One- and Two-Particle Coefficients of Fractional Parentage for Configurations s s u Pq> U D. H. Chisholm, A. Dalgamo, and E. R. Innes... 

G. Peach, Collisional Broadening of Spectral Lines. Springer Handbook of Atomic, Molecular and Optical Physics (2006), pp. 875-888... 

The broadening of spectral lines can be due to Doppler effects, foreign gas, or resonance effects. Doppler broadening is due to the random motion of atoms. The atoms move in different directions and... 

According to the Bohr model of the atom, atomic absorption and emission linewidths should be infinitely narrow, because there is only one discrete value for the energy of a given transition. However, there are several factors that contribute to line broadening. The natural width of a spectral line is determined by the Heisenberg uncertainty principle and the lifetime of the excited state. Most excited states have lifetimes of 10 -10 ° s, so the uncertainty in the energy of the electron slightly broadens the spectral line. This is called the natural linewidth and is on the order of 10 A (1.0 A = 1.0 x lO" m). 

A. Ben Reuven The meaning of collisional broadening of spectral lines. The classical oscillation model. Adv. Atom. Mol. Phys. 5, 201 (1969)... 

W.R. Hindmarsh, J.M. Farr Collision broadening of spectral lines by neutral atoms . In Progr. Quantum Electronics, Vol. 2, Part 4, ed. by J.H. Sanders, S. Stenholm (Pergamon, Oxford 1973)... 

Sobelman, L.A. Vainstein, E.A. Yukov Excitation of Atoms and Broadening cf Spectral Lines, 2nd edn.. Springer Ser. Atoms Plasmas, Vol. 15 (Springer, Berlin, Heidelberg 1995)... 

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