Broadening of spectral lines

Anderson P. W., Talman J. D. In Proceedings of the Conference on Broadening of Spectral Lines, University of Pittsburg, Bell Telephone Syst. Monogr. p. 3117 (1955). 

Ben-Reuven A. Resonance broadening of spectral lines. Phys. Rev. A4, 2115-20 (1971). 

In our account here we neglect a third aspect of a spectral line, specifically its shape, beyond its characteristic frequency and strength. A natural line shape is almost impracticable to observe and would yield on analysis little or no additional information about intrinsic molecular properties. Another shape merely reflects components of molecular velocities in a direction parallel to the direction of propagation. Apart from these effects, further broadening of spectral lines due to finite durations, between collisions, of molecules in particular quantum states is attributed to interactions between colliding molecules rather than directly to... 

Lamb dip spectroscopy provides a very sensitive tool for studying small frequency shifts and broadening of spectral lines which normally would be undetectable because they may be small compared to the doppler width. These investigations yield information about collisions at low pressures, where the effect of far distant collisions is not suppressed by the more effective close collisions. This allows the potential between the collision partners at large intermolecular distances to be examined. 

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 ... 

To consider gas molecules as isolated from interactions with their neighbors is often a useless approximation. When the gas has finite pressure, the molecules do in fact collide. When natural and collision broadening effects are combined, the line shape that results is also a lorentzian, but with a modified half-width at half maximum (HWHM). Twice the reciprocal of the mean time between collisions must be added to the sum of the natural lifetime reciprocals to obtain the new half-width. We may summarize by writing the probability per unit frequency of a transition at a frequency v for the combined natural and collision broadening of spectral lines for a gas under pressure ... 

Of course our theory is significant only in the limit k-> 0. If k is not sufficiently small, the pseudo-eigenvalues etc. are no more well-defined quantities. This corresponds to the broadening of spectral lines for intense field. 

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... 

Such beams have many uses, including some important applications in spectroscopy. In particular, pressure broadening of spectral lines is removed in an effusive beam and, if observations are made perpendicular to the direction of the beam, Doppler broadening is considerably reduced because the velocity component in the direction of observation is very small. 

Ab initio calculations of systems M -X were performed recently for LiHe [114], NaHe [114, 115] and CsX [116]. Besides, there is indirect evidence on potentials from the broadening of spectral lines of Na [117,118], extreme-wing line broadening in Cs [119] and on oscillations in continuum spectra of RbX and CsX pairs [120]. All data are in line with the term pattern shown in Fig. 5.2. 

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

Following Van der Waals, we have leamt to think of the molecules as centres of forces and to consider these so-called Molecular Forces as the common cause for various phenomena The deviations of the gas equation from that of an ideal gas, which, as one knows, indicate the identity of the molecular forces in the liquid with those in the gaseous state the phenomena of capillarity and of adsorption the sublimation heat of molecular lattices certain effects of broadening of spectral lines, etc. It has already been possible roughly to determine these forces in a fairly consistent quantitative way, using their measurable effects as basis. 

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. 

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