Maximum-allowed divergence

Since 0/ < d, the throughput for 1-cm" resolution measurements is limited by the maximum-allowed divergence of the beam through the interferometer rather than by the detector foreoptics. For measurements at 2-cm resolution (on this particular spectrometer), 0/ = 0d = 0.06 cm sr, and this value is used in the following paragraph. 

An approximate calculation for the differences in optical retardation for the central and the extreme ray of a beam-path which is diverging as it travels through the FTIR modulator, separated by a half-angle (in radians) of a, shows that the maximum allowable value of a to achieve a required resolution Av at a wavenumber of is ... 

Uij = Rij/Rij. Similarly, Ga[(ljp u /) + 1] attains its maximum value whenever the direction of the acceptor arm Ij is in the direction -u//. Thus, the product of these three functions attains a value close to unity only if, simultaneously, Rij is about Rh, the direction of Hia is about that of Xij, and the direction of jp is about that of -u /. Such a configuration is said to be favorable for HB formation. Clearly, if all of the above three conditions are fulfilled, then the interaction energy is about sub- The sum of the various terms in the curly brackets of (2.7.8) arises from the total of eight possible favorable directions for HB formation (four when molecule i is a donor and four when molecule i is an acceptor). The variances a and a are considered as adjustable parameters. Note that of the eight terms in the curly brackets, only one may be appreciably different from zero at any given configuration X Xy. Clearly, the HB part of the potential in (2.7.8) does not suffer from a possible divergence as in the Bjerrum model, and clearly does not allow two bonds to be formed by a pair of water molecules (Fig. 2.48b). 

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