Far-wing absorption

Finally, we should mention the effect of far-wing absorption, an important but poorly understood aspect of line shape. Collision-induced opacity is observed in spectral regions well-separated by as much as 10 to 100 cm from the line center. This very weak absorption is not described by the wings of a simple Lorentz line shape discussed above it shows an exponentially decreasing dependence on the separation from the line center (see, e.g., Bimbaum, 1979). The anomalous far-wing absorption is due to inadequacies in the hard-sphere collision model used... 

Although long-time Debye relaxation proceeds exponentially, short-time deviations are detectable which represent inertial effects (free rotation between collisions) as well as interparticle interaction during collisions. In Debye s limit the spectra have already collapsed and their Lorentzian centre has a width proportional to the rotational diffusion coefficient. In fact this result is model-independent. Only shape analysis of the far wings can discriminate between different models of molecular reorientation and explain the high-frequency pecularities of IR and FIR spectra (like Poley absorption). In the conclusion of Chapter 2 we attract the readers attention to the solution of the inverse problem which is the extraction of the angular momentum correlation function from optical spectra of liquids. 

Collisional redistribution of radiation. A system A + B of two atoms /molecules may be excited by absorption of an off-resonant photon, in the far wing of the (collisionally) broadened resonance line of species A. One may then study the radiation that has been redistributed into the resonance line - a process that may be considered the inverse of pressure-broadened emission. Interesting polarization studies provide additional insights into the intermolecular interactions [118, 388]. 

Collision-induced lines , and the induced far wings of allowed lines, are usually very diffuse, typically of a width of Aco 1/Af 1012 Hz reflecting the short duration (At p/vtms 10-12 s) of intermolecular interactions, just as was seen in the case of collision-induced absorption above. In the low-density limit, induced intensities vary as the square of the gas density, which reflects the supermolecular origin. With increasing density, cubic and higher components have been identified. 

Information concerning the interaction potentials of Cs (7S, 5D)-rare gas pairs is obtained by interpreting the temperature dependence of the 6S-7S,5D far wing and satellite profiles. A sensitive laser fluorescence technique is used to obtain the absorption coefficient of the mixture. The collision induced oscillator strength, a rapidly varying function of the interatomic distance in the case of such forbidden transitions, is also deduced. Experimental potentials and oscillator strengths are compared with available calculated values. 

P. A. Lund, O. F. Nielsen, and E. Praestgaard. Comparison of depolarized Rayleigh-wing scattering and far-infrared absorption in molecular liquids. Chem. Phys., 25 167-173 (1978). 

For measurements of cluster-size distributions in cold molecular beams (Sect. 4.3), or for monitoring the mass distribution of laser-desorbed molecules from surfaces, these combined techniques of laser ionization and mass spectrometry are very useful [105, 106]. For the detection of rare isotopes in the presence of other much more abundant isotopes, the double discrimination of isotope-selective excitation by the first laser LI and the subsequent mass separation by the mass spectrometer is essential to completely separate the isotopes, even if the far wings of their absorption lines overlap [107]. The combination of resonant multiphoton ionization (REMPI) with mass spectrometry for the investigation of molecular dynamics and fragmentation is discussed in Chap. 5. 

If we want to observe single collision dynamics we need to operate near to where the collisional and spontaneous emission damping rates are comparable. Measurements are made over a range of pressures and theory developed by Burnett and Cooper used to extrapolate to zero pressure and thus extract the single collision quantities of interest (see the theory section below). The other condition we need is that multiple scattering should be absent. This is easy to achieve for the incoming path since absorption in the far wings will be very small. The fluorescence, around line center, will, of course be heavily trapped at any reasonable densities say cm — even for rather small... 

Resonance broadening in the alkali metals. The technique described in section 8.7.2 has been applied to a study of resonance broadening in potassium by Lewis et al. (1971). In principle the shape of the resonance lines of the alkali metals can also be studied in absorption as described by Chen and Phelps (1968). However, most absorption experiments have been performed with only moderate spectroscopic resolution and the observations are confined to the far wings of the lines. The widths of the absorption lines are almost entirely determined by the effect of collision broadening, but unfortunately it is not possible to apply the results of the impact theories since these apply only to that part of the line profile satisfying c explained in... 

For resonance lines, self-absorption broadening may be very important, because it is applied to the sum of all the factors described above. As the maximum absorption occurs at the centre of the line, proportionally more intensity is lost on self-absorption here than at the wings. Thus, as the concentration of atoms in the atom cell increases, not only the intensity of the line but also its profile changes (Fig. 4.2b) High levels of self-absorption can actually result in self-reversal, i.e. a minimum at the centre of the line. This can be very significant for emission lines in flames but is far less pronounced in sources such as the inductively coupled plasma, which is a major advantage of this source. 

Up to now OH vibrational state populations have been measured for only two wavelengths. In accordance with the above considerations, absorption in the far red wing of the absorption spectrum, A =193 nm,... 

We now examine more carefully the lineshape in the low-energy part far from the 0-0 transition value.52 53 From a perturbational point of view, this absorption may be visualized as a successive absorption of many phonon energy quanta up to the excitonic band, where a density of excitonic final states exists. In this scheme, the low-energy absorption wing exists only at finite temperature. If v phonons are necessary to reach the excitonic band, the dominant term in originates from the consecutive absorption of v phonons following the diagram... 

The far quasistatically broadened wings of the Na D transition give important contributions to the emission and absorption spectrum of a high-pressure sodium discharge. We report experimental and theoretical spectra and discuss in particular the Na2 triplet satellite at 551.5 nm and the Na2 singlet satellite at 805 nm. Extension of this type of spectroscopy to other alkali metal vapors is discussed. 

When the quasistatic contribution to the absorption coefficient, as described by eq. (1), is incorporated in the LTE model (5.) a more realistic emission spectrum of the HPS discharge results (de Groot and Woerdman, to be published). This is already evident when the quasistatic absorption spectrum is compared with the extrapolated dispersive, resonantly broadened Na D absorption profile (see Figure 3). In the far red wing (X > 650 nm) the contribution of the A Sj, - X Zg transition is much larger than the dispersive contribution in a LTE model the same holds for the emission spectrum. In the far blue wing (X< 560 nm) the contribution of the Ilg - transition likewise dominates the dispersive contribution. 

Furthermore, as it is seen from numerous experiments, the far red wing of the absorption profile of chromophores in solutions does not appear to follow a Lorentzian decay, as implemented in the conventional expression for the frequency dependent linear absorption cross section, but rather some kind of fast exponential, Urbach-like, decay [22, 23]. 

The computed absorption and loss factor are shown, respectively, in Figs. 31a and 31b Fig. 31c represents the low-frequency wing of the /r(v) curve characteristic for the non-resonance loss spectrum. Fig. 32 represents the frequency dependence of the dielectric constant s (v) and the Cole-Cole plot s"[s (v)]. The latter two graphs also agree with experiment. We see that our theory agrees reasonably well with the spectra observed by Bertie et al. [51]. Note the empirical formulas (72)-(74) by Hufford [20] could be applied for describing of the far IR ice loss spectrum (viz., at v < 100 cm-1), if the constant cfit in Eq. (72) is fitted properly (see Fig. 33). For T = 100 K cfit 17. 

---