Collision-broadened linewidth

A further factor which affects the shape of a resonance line is the transport of resonance radiation through the. parent gas. Milne s early theory of self-absorption by the imprisonment of resonance radiation has been revised by Holstein (3) and Bichermau (4), taking into account the incoherent scattering of resonance photons. Furthermore, Walsh (" ) has extended the imprisonment lifetime calculation for cases where Doppler and collision broadening of the resonance line are simultaneously present, and in addition this author has examined the complications caused by the, hfs. The line shape, linewidth, and other properties of self-absorbed lines hare been discussed recently bv Tako (6), who summarizes the various effects of self-absorption as follows ... 

In flames, only Doppler and, to a lesser extent, collisions broadening contribute significantly to the overall linewidth. 

Values for AXj) are low for heavy elements (e.g. Au(II) at 200.08 nm has a value of 0.8 pm, whereas AXj) values are high for light elements such as Be(II) at 313.11 nm that has a value of 5.9 pm. Collisional broadening results due to collisions among analyte ions, atoms, and neutral Ar atoms and has also been called pressure broadening. Doppler broadening is dominant near the center of the band, whereas collision broadening dominates near the tails. AE linewidths dictate what resolution is needed to resolve one AE emission line from another. For each transition metal, there is a plethora of lines to consider. 

We have assumed that all hfs levels are equally populated apart from their statistical weights (2F + 1), because the collision broadening is larger than the natural linewidth and the hfs-splittings. The collisional refilling probability is /Jcoii = 2n y. 2.5 x 10 s 1.5 x 10 s ... 

The expanded beam from a HeNe laser at A = 3.39 p,m with lOmW power is sent through a methane cell T = 300 K, / = 0.1 mbar, beam diameter 1 cm). The absorbing CH4 transition is from the vibrational ground state (r 00) to an excited vibrational level with r 20 p,s. Give the ratios of Doppler width to transit-time width to natural width to pressure-broadened linewidth for a collision cross section CTj = 10 cm. ... 

Natural linewidths are broadened by several mechanisms. Those effective in the gas phase include collisional and Doppler broadening. Collisional broadening results when an optically active system experiences perturbations by other species. Collisions effectively reduce the natural lifetime, so the broadening depends on a characteristic impact time, that is typically 1 ps at atmospheric pressure ... 

The single-mode laser naturally gives less output power than a multimode laser with the same active volume since its induced emission is concentrated into a smaller frequency range. This loss in intensity, however, is much less than one would expect from the ratio of linewidths or from the reduction in oscillating mode number 3i. 32,41) jbis is due to the fact, that not only atoms with the exact transition frequency can contribute to the induced emission, but also those inside the homogeneous linewidth which is determined by collision processes in the case of gas lasers or by crystal line broadening in solid lasers... 

Linewidth is also affected by pressure broadening from collisions between atoms. Collisions shorten the lifetime of the excited state. The uncertainty in the frequency of atomic absorption and emission lines is roughly numerically equal to the collision frequency between atoms and is proportional to pressure. The Doppler effect and pressure broadening are similar in magnitude and yield linewidths of 10-3 to I0-2 nm in atomic spectroscopy. 

The collision halfwidth for a given transition is a function of temperature and the broadening species. In the present diode laser experiments, the temperature (and hence AVp) is known so that it is straightforward to infer values for the parameters a and AVq, and hence 2y, from the observed absorption linewidths. 

This applies most particularly to the linewidth broadening parameter, which can vary with the sample composition by up to an order of magnitude. The values for concomitant gases on the NH3 3,3 line range typically from 10 (He) to 205 (NH3) kHz Pa" (ref. 11, p. 364). This parameter reflects the different cross-sections experienced by the two molecules in collision. Its effect on the analytical... 

Lamb peaks (inverse Lamb dips) at the line centers of the absorbing transitions (Sect. 2.3). The line profiles of these peaks are determined by the pressure in the absorption cell, by saturation broadening, and by transit-time broadening (Vol. 1, Sect. 3.4). Center frequency coq, linewidth Aco, and line profile Pl(co) are measured as a function of the pressure p (Fig. 8.2). The slope of the straight line Aco p) yields the line-broadening coefficient [977], while the measurement of coo p) gives the collision-induced line shift. 

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