Intermodulation method

To selectively record Doppler-free signals in this way, it has become general practice to use an intermodulation method first introduced by M. S. Sorem and A. L. Schawlow.(24) In this method, the sample is irradiated with two counterpropagating laser beams which are chopped at two different audio frequencies f and 2 signal is observed as a modulation of the total excitation rate at the sum or difference frequency fj[ + f2 Such an intermodulation occurs at the center of a Doppler-broadened lines, where both beams are interacting with the same atoms so that they can saturate each other s absorption. Care is required to avoid spurious signals caused by nonlinear mixing in the detector or amplifiers. 

A complete set of magnetic hf coupling constants and the P nuclear spin-rotation interaction constants (all in MHz) were obtained from an analysis of the hf structure of twelve lH(total H nuclear spin) = 0 and fourteen Ih=1 rovibronic lines of the A (0, 0, 0) X (0, 0, 0) band. An Intermodulated fluorescence (IMF) method was applied, which was first shown in [17] to be able to eliminate the Doppler width. A dye laser was employed as a source. Ground state constants were taken from a preliminary publication of [3] (T c was obtained with the sum rule) [6] ... 

Tables in the present chapter contain in their upper part data on the spin-rotation interaction constants e q (q = Inertial axes a, b, c) and In their lower part data on the rotational constants A, B, and C. Methods of measurement, references, and remarks are the same for either constant. The quartic (A, 8 and A, 5) and sextic (O, (p) centrifugal distortion constants listed In the first table (for the electronic ground state of PHg) are defined by the A-reduced form (asymmetric reduction) of the respective Hamiltonian for spin-rotation Interaction and rotation [1 to 3]. Abbreviations used in the tables are MW for microwave absorption, EL AB for electronic absorption, LMR for laser magnetic resonance, FIR for far IR, IMF for Intermodulated fluorescence.

Fig. 2.13 Intermodulated fluorescence method for saturation spectroscopy at small densities of the sample molecules (a) experimental arrangement (b) hyperfine spectrum of the v" = 1,...

Distortion is a measure of signal impurity. It is usually expressed as a percentage or decibel ratio of the undesired components to the desired components of a signal. There are several methods of measuring distortion, the most common being harmonic distortion and several types of intermodulation distortion. 

SMPTEIM A method of measuring intermodulation distortion in an audio system standardized by the Society of Motion Picture and Television Engineers. 

Both harmonic and electrochemical frequency modulation (EFM) methods take advantage of nonlinearity in the E-I response of electrochemiced interfaces to determine corrosion rate [47-50]. A special application of harmonic methods involves harmonic impedance spectroscopy [5i]. The EFM method uses one or more a-c voltage perturbations in order to extract corrosion rate. The electrochemical frequency modulation method has been described in the literature [47-50] and has recently been reviewed [52]. In the most often used EFM method, a potential perturbation by two sine waves of different frequencies is applied across a corroding metal interface. The E-I behavior of corroding interfaces is typically nonlinear, so that such a potential perturbation in the form of a sine wave at one or more frequencies can result in a current response at the same and at other frequencies. The result of such a potential perturbation is various AC current responses at various frequencies such as zero, harmonic, and intermodulation. The magnitude of these current responses can be used to extract information on the corrosion rate of the electrochemical interface or conversely the reduction-oxidation rate of an interface dominated by redox reactions as well as the Tafel parameters. This is an advantage over LPR and EIS methods, which can provide the Z( ) and, at = 0, the polarization resistance of the corroding interface, but do not uniquely determine Tafel parameters in the same set of data. Separate erqreriments must be used to define Tafel parameters. A special extension of the method involves... 

Harmonic current responses occur at cOi, 2(o, 3(0i, as well as at CO2, 20)2, and 3(B2, etc. Additionally, a current response can be seen at various intermodulation frequencies, such as 2(0i c )2 and 2k)2 c0i. Consider the application of this method to a charge transfer controlled corrosion process with an E-I response that behaves according to Eq 19. Under the assumption that (O2 > cOi and Pa < Po> the corrosion current density, icorr> and Tafel parameters, (where = Jln 10) and b, can be determined from the equations summarized in Table 2. The current components at the angular frequency coi or 032 can be measured at o), or C02. The intermodulation components C0i C02 can be determined from the signal response at C0i+a)2 or 03i-corrosion rate and Tafel parameters directly from a single measurement (see Refs 29 and 30). Currently, there are no ASTM standards for this technique. 

---