Frequency modulation Doppler spectroscopy

Chapter 3 is devoted to pressure transformation of the unresolved isotropic Raman scattering spectrum which consists of a single Q-branch much narrower than other branches (shaded in Fig. 0.2(a)). Therefore rotational collapse of the Q-branch is accomplished much earlier than that of the IR spectrum as a whole (e.g. in the gas phase). Attention is concentrated on the isotropic Q-branch of N2, which is significantly narrowed before the broadening produced by weak vibrational dephasing becomes dominant. It is remarkable that isotropic Q-branch collapse is indifferent to orientational relaxation. It is affected solely by rotational energy relaxation. This is an exceptional case of pure frequency modulation similar to the Dicke effect in atomic spectroscopy [13]. The only difference is that the frequency in the Q-branch is quadratic in J whereas in the Doppler contour it is linear in translational velocity v. Consequently the rotational frequency modulation is not Gaussian but is still Markovian and therefore subject to the impact theory. The Keilson-... 

Ye J, Ma L-S and Hall J L 1996 Sub-Doppler optical frequency reference at 1,064 mm by means of ultrasensitive cavity-enhanced frequency modulation spectroscopy of a C2HD overtone transition . Opt. Lett., 21, 1000 - 2... 

The main part of the book presents various applications of lasers in spectroscopy and discusses the different methods that have been developed recently. Chapter 6 starts with Doppler-limited laser absorption spectroscopy with its various high-sensitivity detection techniques such as frequency modulation and intracavity spectroscopy, cavity ring-down techniques, excitation-fluorescence detection, ionization and optogalvanic spectroscopy, optoacoustic and optothermal spectroscopy, or laser-induced fluorescence. A comparison between the different techniques helps to critically judge their merits and limitations. 

Using the dispersion profiles of Doppler-free molecular lines in polarization spectroscopy (Sect. 7.4), it is possible to stabilize a laser to the line center without frequency modulation. An interesting alternative for stabilizing a dye laser on atomic or molecular transitions is based on Doppler-free two-photon transitions (Sect. 7.5) [5.77]. This method has the additional advantage that the lifetime of the upper state can be very long, and the natural linewidth may become extremely small. The narrow Is —2s two-photon transition in the hydrogen atom with a natural linewidth of 1.3 Hz provides the best known optical frequency reference to date [5.76]. 

This shows that a modulation of the exponential decay is observed if both matrix elements for the transitions 1 - f and 2 - f are nonzero (Fig.11.23b). The measurement of the modulation frequency allows determination of the energy separation of the two levels, even if their splitting is less than the Doppler width. Quantum beat spectroscopy therefore allows Doppler-free resolution. 

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