Broadening pressure

The relatively high concentration of foreign molecules in flame AAS results in a Lorentzian broadening line value similar to that of the Doppler broadening value. The frequency shift may be considerable and lead to a 

If the interaction involves charged particles the pressure broadening is called the Stark effect, while collisions with uncharged particles is due to the van der Waals effect, and collisions between the same kind of atoms gives rise to the Holtzmark or resonance broadening effect. 

The spectral line width is mainly determined by the Doppler and pressure line broadening effects, and the total line broadening (Avj-) may be written 

When collisions occur between gas phase atoms or molecules there is an exchange of energy, which leads effectively to a broadening of energy levels. If t is the mean time between collisions and each collision results in a transition between two states there is a line broadening Av of the transition, where 

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The effeets of sueh eollisionally indueed kieks are treated within the so-ealled pressure broadening (sometimes ealled eollisional broadening) model by modifying the free-rotation eorrelation funetion through the introduetion of an exponential damping faetor exp( - t /x) ... 

As with pressure broadening, this exponential time dependence, when subjected to Fourier transformation, yields ... 

Except at very low frequencies, pressure broadening may be removed simply by working at a sufficiently low pressure. Doppler broadening may be reduced or removed by two general methods, which will be discussed briefly below. 

Such beams have many uses, including some imporfanf applications in specfroscopy. In particular, pressure broadening of specfral lines is removed in an effusive beam and, if observations are made perpendicular to fhe direction of fhe beam, Doppler broadening is considerably reduced because fhe velocify componenf in fhe direction of observation is very small. 

The choice of phase of the sample is important. Generally, in high-resolution spectroscopy (see Section 3.3.1 for a discussion of resolution), the sample is in the gas phase at a pressure which is sufficiently low to avoid pressure broadening (see Section 2.3.3). In the liquid... 

C y = correction fac tor for pressure broadening of radiation from water vapor. d = particle diameter. 

Green S., Monchik L., Goldflam R., Kouri D. J. Computational tests of angular momentum decoupling approximations for pressure broadening cross sections, J. Chem. Phys. 66, 1409-12 (1977). 

De Pristo A. E., Rabitz H. Scaling theoretic deconvolution of bulk relaxation data state-to-state rates from pressure-broadening linewidths,... 

Baranger M. Problem of overlapping lines in the theory of pressure broadening, Phys. Rev. Ill, 494—504 (1958). 

Pressure broadening allows for the presence of nearby molecules to perturb the positions of the energy levels, leading to a spreading of the transition frequency as a function of collisions and hence pressure. 

Linewidth The spread in wavelengths or frequencies associated with a transition in an atom or molecule. There are three contributions natural linewidth associated with the lifetime of the transition pressure broadening associated with the presence with the other molecules nearby Doppler broadening associated with relative motion of the molecule and light source. 

Collisional or pressure broadening and resonance broadening. These are caused by collisions between unlike and like atoms respectively in the sample vapour. Only the former is significant in flames. 

Although 2.45 MHz does not coincide with the maximum micro-wave absorption for water, there is a huge pressure broadening effect at atmospheric pressure, so there is a sufficient overlap for the absorption process to be fairly efficient. 

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