Resolution, Doppler limited

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. 

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).

While the early optical measurements suffered from limited resolution, the development of atomic beam methods provided a useful tool in the study of atomic and nuclear magnetic moments [ 12,13] (for a review see [ 14]) and it became possible to measure the nuclear magnetic moments (and nuclear spins) in a direct way for both stable and radioactive isotopes, by using a variety of methods ] 15]. The study of optical IS was, however, limited to Doppler-limited optical spectroscopy until the invention of the laser and the development of suitable high-resolution optical methods (a review can be found in [16]). It is also possible to obtain information on the nuclear charge distribution by electron scattering experiments and from muonic X-ray transitions and electron K X-ray IS [17], perhaps even with a higher accuracy than with optical spectroscopy. 

Tunable diode laser spectroscopy has been employed in order to observe the Zeeman effect in the i.r. absorption of molecules with no electromagnetic moment, due to differences between the excited- and ground-state g-factors. Doppler-limited resolution was obtained for and CHjDI in the region 820—... 

The spectrum of the out-of-plane band of thiophene centered at 712 cm has been measured in Doppler-limited resolution with a diode-laser spectrometer and with a high-resolution FT spectrometer <93ZN(A)1193>. The band at 712 cm which is the Ui, band <65SA689>, is of symmetry Bi. 

Almost all of the spectroscopy described in our book involves techniques which have been developed since the publication of Herzberg s book. Rotational energy levels were very well understood in 1950, and the analysis of rotational structure in electronic spectra was a major part of the subject. The major disadvantage of the experimental methods used was, however, the fact that the resolution was limited by Doppler broadening. The Doppler line width depends upon the spectroscopic wave-lengtii, the molecular mass, the effective translational temperature, and other factors. However, a ballpark figure for the Doppler hue width of 0.1 cm would not be far out in most cases. Concealed wifliin tiiat 0.1 cm are many subtle and fascinating details of molecular structure which are major parts of the subject of this book. 

In order to estimate the size of the acceptable solid angle of the radiation with which to illuminate an interferometer operating in the visible/UV spectral region, it is necessary to make a few assumptions. We will assume that Doppler-limited resolution is desired over the spectral range from about 7000 A to 3000 A, or in frequency, from about 14,300 cm"l to 33,300 cm"l. It is not likely that this entire bandwidth of 19,000 cm"l would be covered in one spectrum, but it is important to know both the high frequency and low frequency limits when evaluating system performance. 

I will now discuss the work which has been carried out by classical spectroscopy on the electronic spectra of diatomic and polyatomic molecules including unstable chemical species which can be classified as free radicals or molecular ions. More than 1000 diatomic (1) and 100 polyatomic species (2) have been found and studied with Doppler-limited resolution,... 

The instrumental resolution of the spectrometers is limited by the combined frequency fluctuation from each CO2 laser (about 15 kilohertz). This, of course, is less than any Doppler limited linewidth and, therefore, does not limit our resolution except for possible sub-Doppler work. This high resolution provides an excellent way of studying pressure shifts and line shape studies of spectral lines. The measurement of OH concentrations in our atmosphere as a function of altitude using absorption and emission measurements requires an accurate knowledge of its linewidth in the atmosphere. 

The spectral resolution is generally limited by the resolving power of the dispersing spectrometer. Only with large and expensive instruments (e.g., Fourier spectrometers) may the Doppler limit be reached [1]. 

The sensitivity of the optothermal technique is illustrated by the comparison of the same sections of the overtone spectrum of C2H4 molecules, measured with Fourier, optoacoustic, and optothermal spectroscopy, respectively (Fig. 1.34). Note the increase of spectral resolution and signal-to-noise ratio in the optothermal spectrum compared to the two other Doppler-limited techniques [87]. More examples can be found in [88]. 

The demands of experimental equipment are much less stringent than those necessary for complete resolution and analysis of absorption spectra of the same molecule. This advantage still remains if a few upper levels are simultaneously populated under Doppler-limited excitation [160],... 

Particularly for polyatomic molecules with their complex visible absorption spectra, the reduction of the Doppler width is essential for the resolution of single lines [392]. This is illustrated by a section from the excitation spectrum of the SO2 molecule, excited with a single-mode frequency-doubled dye laser tunable around X = 304 nm (Fig. 4.4b). For comparison the same section of the spectrum as obtained with Doppler-limited laser spectroscopy in an SO2 cell is shown in Fig. 4.4a [391]. 

Fig. 4.4 Section of the excitation spectrum of SO2 (a) taken in a SO2 cell at 0.1 mbar with Doppler-limited resolution (b) taken in a collimated SO2 beam [391]...

Fig. 5.56a,b. Discontinuous tuning of lasers (a) part of the neon spectrum excited by a single-mode dye laser in a gas discharge with Doppler-limited resolution, which conceals the cavity mode hops of the laser (b) excitation of Na2 lines in a weakly collimated beam by a single-mode argon laser. In both cases the intracavity etalon was continuously tilted but the cavity length was kept constant... 

The linewidth of the unstabilized single-mode laser has been measured to be smaller than 260 kHz, which was the resolution limit of the measuring system [5.144]. An estimated value for the overall linewidth Av is 25 kHz [5.146]. This extremely small linewidth is ideally suited to perform high-resolution Doppler-free spectroscopy (Chaps. 7-10). 

Besides these three examples, a large number of atoms and molecules have been studied in molecular beams with high spectral resolution. For atoms mainly hyperfine structure splittings, isotope shifts, and Zeeman splittings have been investigated by this technique, because these splittings are generally so small that they may be completely masked in Doppler-limited spectroscopy [9.6,9.7]. An impressive illustration of the sensitivity of this technique is the... 

By using tunable diode lasers in the 9 and 11 im regions, the [8] and vg bands [9] of gaseous NF2 (N2F4 heated to ca. 100 to 150°C) were measured at Doppler limited resolution (preliminary studies [10,11]). 

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