Doppler-limited rotational spectrum

Figure 6.2 provides a representative example for a Doppler-limited rotational spectrum, while Figure 6.3 shows such a comparison for resolved hyperfine structure (Lamb-dip technique in conjunction with frequency modulation has been employed for recording the spectrum [88]). 

An example for high-resolution IRS is given in Fig. 6.1-26, where the uj 0-branch of CH3D is displayed. This spectrum has been recorded with the quasi-cw inverse Raman spectrometer developed by Bermejo et al. (1990) whose. schematic arrangement is shown in Fig. 3.6-15 and described in Sec. 3.6.2.3. It represents a Doppler-limited spectrum of the C-D stretching band. The authors were able to assign the observed transitions by performing a theoretical fit to the observed data which allowed them to refine some of the rotational-vibrational constants. 

This is, first of all, due to the manifold of rotational and vibrational levels within each electronic state and furthermore to a larger variety of angular momentum coupling, such as spin-rotation interaction, A-type doubling, fine and hyperfine structure. In addition different kinds of perturbations may further increase the line density and the complexity of the spectrum. Even for small molecules, such as diatomic or triatomic molecules, the spacings between rotational lines of an electronic transition may become much smaller than the Doppler-width. This implies that single rotational lines often cannot be resolved with "classical" Doppler-limited techniques. 

Figure 1.18. Part of the two-photon spectrum of benzene at different resolutions a) vibrational structure of the S - S transition, b) Q branch of the most intense vibrational line (I4i) with a resolution 5v limited by the Doppler broadening, and c) elimination of the Doppler broadening which yields individual rotational lines (by permission from Neusser and Schlag, 1992).

The power is usually insufficient to saturate transitions, so that the resolution is then limited by the Doppler effect. In some molecules the line density in a vibration-rotation band is sufficiently high that the average spacing is less than the Doppler width of each line so that only a band envelope is obtained. Even where this is not the case, it may prove impossible to give an unambiguous analysis from a simple absorptior spectrum. 

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