Third Order Spectrometer

In a self-heterodyne experiment, however, there is no independent control over the phase of the local oscillator field, so that the complete information on the complex third-order polarization of Equation (32) cannot be obtained. It is necessary to analyze in more detail the measurement process in order to determine the accessible information. In the actual experiment the spectrometer performs the Fourier transform of the generated third-order field of Equation (31) with respect to time coordinate t2, generating the field components of El3,(ti oy, ) given by ... 

There was a further extention to higher temperatures by Myers and Bartle who shock-heated mixtures of O2, O3, CO, and CO2 in Ar at 0.5-1.5 atm and 2680 "" or 2940 °K. They found that the emission intensity between 2500 and 7200 A was proportional to [CO][0] with a rate coefficient of 3.8 x 10 l.mole . sec at 2943 "K. This result is consistent with that of Hartunian et al Wong et used a mass spectrometer connected to a stirred reactor to measure the rate of the CO + 0 reaction at 400-500 °K at pressures of 0.75-3.5 torr. Although they did not report the pressure dependence of the rate of reaction, they felt that the reaction had to be third-order and, on that basis, calculated an upper limit to the rate coefficient28 A 4 x 10 l. mole" sec" at 500 °K. They suggested... 

In the research described in this note the authors have measured the critical absorption wave-lengths in the K series of most of the available chemical elements from tungsten to uranium, both inclusive. They used an ionization spectrometer, and examined spectra of the first, second and third orders. In 1918 Duane and Shimizu3 measured four of these wave-lengths in spectra of the first order by the ionization method. Measurements with the same spectrometer of the K critical absorption had previously been made for most of the chemical elements down to man-... 

Amazingly, the third-order kinetics of this type of reaction were first measured for li" in 1928 this was the first study of anion reaction chemistry performed using a mass spectrometer. Some electrons are thermalized rather than captured by the precursor. This can lead to electron attachment to form odd-electron ions, while halide ion association forms even-electron ions. 

Table 3.6. Shim gradients found on high-resolution spectrometers. Lower field instruments (<500 MHz) may utihse only 20 or so gradients (such as those in the left panel), whilst higher field spectrometers may employ in excess of 30. Shims up to third order are those most likely to need periodic optimisation as part of longterm spectrometer maintenance for which the most significant interacting shims are Usted. Those shown in square brackets interact less strongly with the listed gradient whilst those that interact with Z° (the main field) may cause momentary disruption of the lock signal when adjusted...

As described in Chapter 3, grating spectrometers will respond to harmonically related wavelengths that is, if a grating spectrometer is adjusted to record spectral emission at 6000 A in the first order, spectral emission at 3000 A in the second order also will be recorded at 6000 A and emission at 2000 A will be recorded as third-order emission at the same 6000 A position. Overlapping spectral orders therefore may unnecessarily complicate the observed spectrum and, in some cases, interfere with spectral lines used for analytical purposes. 

Figure 2 The Third Order Tunable Far Infrared Spectrometer.

Figure 2.22 The contribution of components to the total background in a wavelength-dispersive spectrometer, as a function of wavelength A = first order, B = second order, C = third order, and D = fourth order.

Whereas the resolution in linear Raman spectroscopy is limited in principle by the slit width of the spectrometer, a considerable improvement in the instrumental resolution was attained through the development of the techniques of nonlinear or coherent Raman spectroscopy, where the interaction of two laser beams with the third-order susceptibility of the sample creates the spectrum. In this case, the resolution is determined by the convoluted linewidth of the two lasers, the Doppler effect, and pressure broadening of the spectral lines. 

We have at present a maximum of 25 spherically bent quartz crystals (c )=30 mm) mounted for simultaneous focusing of the AlKa radiation onto the sample in front of the spectrometer slit. The result of the monochromatization is shown in Fig.40. It is fortuitous that at an almost identical Bragg angle ( > 78 ) as first order focusing of AlKa 2 occurs, second order focusing of AgLa, third order ScKg ... 

Semi-quantitative analysis facilitates fast and simple multi-element measurements with limited precision. ICP-MS offers excellent semi-quantitative capabilities, as a result of the high ionisation efficiencies achieved for the majority of elements and the simplicity of the resulting mass spectra. Semi-quantitative determinations are mostly based on a comparison of response tables and the actual count rates of the sample. The response (intensity I in counts/s) of an analyte ion depends on the concentration of the analyte element, the isotopic abundance of the observed isotope, the ionisation efficiency, the atomic mass and the efficiencies of nebuUsation, ion transmission and ion detection in the mass spectrometer. In most ICP-MS instraments a plot of the atomic response, Ra, versus atomic mass yields a smooth response curve, which is fitted best by a third order polynomial (Figure 4.3). The atomic response is defined by equation (4.7) and is equivalent to the molar response divided by the Avogadro constant, IVa-... 

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