Highest resolution spectra

It should be noted with respect to method 1 that in most Rydberg excitation experiments to date pulsed lasers have been used even to obtain the highest resolution spectra, and this naturally precludes the measurement of lifetimes longer than -10 ns. 

Two-photon spectroscopy may also be used to obtain highest-resolution spectra. Doppler broadening, which originates in the random motion of molecules in the gas phase, prevents individual rotational lines of a vibronic transition from being resolved in conventional spectroscopy. However, if two photons of extremely monochromatic light coming from exactly opposite directions are absorbed simultaneously, the Doppler shifts of the two photons just cancel each other and the Doppler broadening is eliminated. As... 

Figure 16.13—Resolution. The figure on the left defines the parameters used to calculate resolution. To the right a low-resolution spectrum of a sample of lead is shown. The highest mass resolutions are achieved with cyclotron resonance instruments (see Fig. 16.8). The resolution greatly depends on the compound chosen for the calculation. For instruments in which Am is a constant, the upper mass limit theoretically corresponds to the maximum resolution, i.e. this the value at which masses m and m + 1 can no longer be distinguished from one another.

A Beckman DK-2A spectrophotometer was calibrated with a standard benzene vapor spectrum. The spectrophotometer was adjusted for the highest resolution with a tolerable signal-to-noise ratio. Since high resolution demands a narrow slit width, instrument sensitivity was set as high as possible. As a maximum signal response was needed, a low time constant was set, and a relatively slow scanning speed was used. 

The latter can be of either spectral or spatial type, the former relying on overlap of the relevant bands in the spectrum. In this context, it becomes important to ensure the highest resolution situation in the F spectrum during the time allowed for spin diffusion. This, in turn, suggests that proton decoupling should be employed during that time. The WISE (two-dimen-... 

Figure 5.18. The four quadrants of a phase-sensitive data set. Only the RR quadrant is presented as the 2D spectrum and this is phased to contain absorption-mode lineshapes in both dimensions to provide the highest resolution (R = real, I = imaginary). Positive contours are in black and negative in red.

The same arguments can apply to other forms of spectroscopy. However, transition energies are mueh larger than those involved in NMR a typical vibrational spectrum, for example, could cover hundreds or thousands of cm and 1 cm corresponds to a frequency of about 10 °Hz. In this case, even with the highest resolution, ca. 10 " cm only exchange rates of more than 10 Hz can be studied. Therefore exchange studies using techniques other than NMR are relatively rare. 

Thus, the spectrum generated by the Fourier transform contains wavenumbers separated by the inverse of the maximum path difference in centimetres. Instruments for routine analytical work typically have highest resolution of 1 or 0.5 cm, requiring a maximum path difference of 1 or 2 cm. This resolution is constant across the spectrum. Research instruments have been built with resolutions as high as 0.0009 cm, which requires a path difference in excess of 10 m. 

UV spectral data given in nm (loge) IR spectral data in cm H- and C-NMR spectra from first reference given, in most instances this is the most recent one or the spectrum recorded at highest resolution... 

The width of the lines in an n.m.r. spectrum is sensitive to molecular motion within the sample. The highest resolution is normally obtained with narrow-line spectra obtained from polymer molecules in solution where molecular motion is relatively easy. The lines are normally broader in solid samples and the line width can be used to study internal molecular motion or determine the degree of crystallinity in a semi-crystalline sample. 

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