High-resolution rotationally resolved

In this section, we will review previous studies of the CH overtone spectroscopy of CD3H. There exist high-resolution, rotationally resolved, experimental data for the first three CH overtones ( uv,) with n = 2, 3, 4) in the infrared and near infrared region (91,94). In the visible region, there also exists data from photoacoustic laser spectroscopy (96,97) and from the intracavity laser absorption spectroscopy (ICLAS) technique, which provides absolute intensities (85,86). Compared to methane, analysis of the spectra is much easier for CD3H, because of the relatively isolated CH chromophore. 

Vibrational transitions accompanying an electronic transition are referred to as vibronic transitions. These vibronic transitions, with their accompanying rotational or, strictly, rovibronic transitions, give rise to bands in the spectrum, and the set of bands associated with a single electronic transition is called an electronic band system. This terminology is usually adhered to in high-resolution electronic spectroscopy but, in low-resolution work, particularly in the liquid phase, vibrational structure may not be resolved and the whole band system is often referred to as an electronic band. 

Another difficulty with the infrared method is that of determining the band center with sufficient accuracy in the presence of the fine structure or band envelopes due to the overall rotation. Even when high resolution equipment is used so that the separate rotation lines are resolved, it is by no means always a simple problem to identify these lines with certainty so that the band center can be unambiguously determined. The final difficulty is one common to almost all methods and that is the effect of the shape of the potential barrier. The infrared method has the advantage that it is applicable to many molecules for which some of the other methods are not suitable. However, in some of these cases especially, barrier shapes are likely to be more complicated than the simple cosine form usually assumed, and, when this complication occurs, there is a corresponding uncertainty in the height of the potential barrier as determined from the infrared torsional frequencies. In especially favorable cases, it may be possible to observe so-called hot bands i.e., v = 1 to v = 2, 2 to 3, etc. This would add information about the shape of the barrier. 

Figure 0.2 Direct overtone spectroscopy of C2H2 using Fourier transform spectroscopy. Here, at high resolution, the entire band of rotational transitions, which accompany a given vibrational transition, can be resolved. Here the band, in the visible range, corresponding to the direct excitation of v = 5 of the v3 stretch mode is shown. (Adapted from Herman et al., 1991. See also Scherer, Lehmann, and Klemperer, 1983, and Figure 8.4.)...

Dimers. High-resolution spectra obtained by Fourier transform spectroscopy with long path lengths (up to 150 m) and temperatures down to 20 K have shown the bound state-to-bound state bands of a number of van der Waals molecules [267]. Spectra of complexes that contain H2 (e.g., H2Ar) can sometimes fully be resolved and rotationally assigned this provides valuable information concerning molecular interactions. Spectra of heavier complexes (e.g., N2Ar) may not be fully resolved but can still yield useful information. 

IR spectra of the fundamental vibrational band of small gaseous diatomic molecules, such as CO and NO, contain a large number of absorption lines that correspond to these vibrational-rotational energy transitions. Since many different rotational levels can be populated at ambient temperature, many different transitions at different energies may occur (Fig. 1). Vibrational-rotational lines are evident only in gas-phase spectra collected at sufficiently high resolution. These lines are not resolved in condensed-phase spectra because of frequent collisions between molecules hence, condensed-phase spectra are characterized by broad absorption bands occurring at the vibrational transition energies. 

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