Absorption lines under pressure

The apparatus used to perform vibrational relaxation experiments in supercritical fluids consists of a picosecond mid-infrared laser system and a variable-temperature, high-pressure optical cell (68,73). Because the vibrational absorption lines under study are quite narrow (<10 cm-1), a source of IR pulses is required that produces narrow bandwidths. To this end, an output-coupled, acousto-optically Q-switched and mode-locked Nd YAG laser is used to synchronously pump a Rhodamine 610 dye laser. The Nd YAG laser is also cavity-dumped, and the resulting 1.06 pm pulse is doubled to give an 600 u.l pulse at 532 nm with a pulse duration of "-75 ps. The output pulse from the amplified dye laser ("-35 uJ at 595 nm, 40 ps FWHM) and the cavity-dumped, frequency-doubled pulse at 532 nm... 

The sample must be in the gas phase. Microwave absorption lines are considerably broadened by molecular collisions at intermediate and high pressures at atmospheric pressure, microwave absorption lines are tens of thousands of megacycles wide. Hence the gas is kept at low pressure, typically 0.01 to 0.1 torr under these conditions, line widths run about MHz. The compound studied need not be a gas at room temperature, but it must have sufficient vapor pressure to give detectable absorption. To study involatile compounds such as the alkali halides, the waveguide must be heated to 500-1000°C high-temperature microwave spectroscopy presents great experimental difficulties, but it has been used to study most of the alkali halides. 

The conducting phase of TMQ /16/. Microwave conductivity experiments, performed at low temperature on the samples used for the pressure experiment, have succeeded in showing an increase by a factor 10 from 300 K down to loo K, /41/. The possibility of susceptibility measurement via low field method is limited by the EPR line broadening occurring at low temperature and under pressure. The spin susceptibility was derived from a fit of the EPR absorption line shape with a Lorentzian curve. The proton relaxation time was measured under pressure at low field lOe- with a pulse spectrometer. Figures... 

In figure 2 the solid lines represent a plot of the differences in ordinate for the curve with evacuated cell and the other curves in succession of figure 1 in other words a plot of A, of the preceding discussion, against s. Since, however, we have here three absorption lines which are incompletely resolved, it is necessary to divide the area under the solid curves into such parts as would be expected if it were possible to examine each line individually in the absence of others. The dotted lines indicate how this was done on the basis of assumptions which appear reasonable. In the upper corner of the figure are plotted the areas under the dotted curves for the line at 80.4 m as a function of pressure. This line is the one due to an increase in quantum number from 5 to 6. One may note several indications that the absorption lines broaden very appreciably with increase in pressure, among them being the shape of the last mentioned... 

The shape and width of each absorption line are functions of temperature and pressure of the medium. The Lorenz profile is usually used to define the shape of these lines at moderate temperatures under local thermodynamic equilibrium conditions. Although other profiles, such as Doppler or Stark profiles, can also be used to define the line shapes, the Lorenz profile, which adequately describes collision-broadened lines, is more appropriate for most applications, including those in combustion systems. For more details, the reader is referred to Siegel and Howell [1] or Modest [3]. 

The half width of elemental lines is of the order of 0.002 nm when observed by emission spectroscopy with flame or electrothermal atomisation. A number of reasons can cause broadening of the linewidth, of which the most important and best understood are natural, pressure, resonance, and Doppler broadening. If a stable and sensitive detection is to be achieved, the linewidth of the excitation radiation must be narrower than the full width at half maximum (FWHM) of the analyte line. Under these conditions, the entire radiant energy produced by the excitation source will be available for absorption by the analyte. The typical line sources used for atomic absorption are element specific excitation sources such as the hollow cathode lamp or the electrodeless discharge lamp. But even continuum sources can be used with appropriate instrumental designs. 

Colunui LC involves the elution under pressure of sequential samples in a closed, on-line" system, with dynamic detection of solutes, usually by UV absorption. The predominant mode of HPLC is reversed phase on bonded silica columns, while normal phase TLC on silica gel is most widely used. Reasons for this difference include the change in propenies of silica gel columns caused by continuous mobile phase flow (which is unimportant for silica gel layers because they are used only once), and the impractical ity of using solvents with high percentages of water to develop many brands of bonded reversed phase layers. 

Fig. 6. UV-Vis absorption spectral change of trans-25 (0.126 mM) in acetonitrile under a nitrogen atmosphere upon photoirradiation with three bright lines (Amaj[ = 365, 436, and 546, nm) of a super-high-pressure Hg lamp. The spectra are depicted at 10 min intervals of photoirradiation. The irradiation with each bright line was continued for 30 min in ascending order of wavelength. (Reprinted with permission from Ref. 153.)...

Few, yet large, absorption bands are obtained when compounds are studied in the condensed phase, whether pure or in solution. However, when the same compound is studied under reduced pressure in the gas phase, the spectrum is seen to have fine structure. Only very simple molecules give a line spectrum in the gas phase. 

In the framework of the impact approximation of pressure broadening, the shape of an ordinary, allowed line is a Lorentzian. At low gas densities the profile would be sharp. With increasing pressure, the peak decreases linearly with density and the Lorentzian broadens in such a way that the area under the curve remains constant. This is more or less what we see in Fig. 3.36 at low enough density. Above a certain density, the l i(0) line shows an anomalous dispersion shape and finally turns upside down. The asymmetry of the profile increases with increasing density [258, 264, 345]. Besides the Ri(j) lines, we see of course also a purely collision-induced background, which arises from the other induced dipole components which do not interfere with the allowed lines its intensity varies as density squared in the low-density limit. In the Qi(j) lines, the intercollisional dip of absorption is clearly seen at low densities, it may be thought to arise from three-body collisional processes. The spectral moments and the integrated absorption coefficient thus show terms of a linear, quadratic and cubic density dependence,... 

The theoretical relation, Ve — hv, has been abundantly checked in measurements of spectroscopy and physics but its direct application to complex molecules in chemical reactions has not been established. Bombardment of mercury atoms or other simple atoms at low pressures by electrons under controlled voltages causes the emission of monochromatic light at the wave lengths predicted by this formula. Moreover, the ionization potential, at which the electron is completely separated from its atom, corresponds directly to the wave length at which the discrete lines of the spectrum merge into a continuous spectrum. This continuous spectrum is due to the fact that the kinetic energy of the expelled electron and ion is not quantized. The close agreement between the ionization potential and the lowest frequency of continuous absorption, where ionization first starts, constitutes another proof of the relation Ve — hv. 

The on-line detection of TNT and some of its breakdown products during their SC-CO2 extraction out of contaminated soils was simulated under different pressures and temperatures. The nitroaromatic compounds dissolved in sc-C02 were identified by their characteristic UV-VIS absorption bands. A quantification of the analytes was possible for concentrations lower than 1 ppm. 

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