Frequency modulation Doppler

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-... 

A phase modulation can also be expressed as frequency modulation. The corresponding frequency deviation is the time derivative of the modulated phase angle (Pm t). According to the basic relationships Afrequency deviation Af(f) with respect to the carrier frequency fg, commonly known as the Doppler frequency shift... 

Micro-Doppler analyses can also be used for people identification. In that case, it is possible to detect Doppler frequency modulation caused by heartbeats, breathing and body motion (walk, run, head turns etc.). 

If instead of transmitting a continuous wave, the Doppler system transmits a sawtooth frequency-modulated (FM) wave, both the velocity and the instantaneous distance betw the missile and the ground station can be detd simultaneously. This ptinciple is explained in Ref 2, p 166 and illustrated in the graph. The FM radar system is capable of measuring accurately only short distances (Ref 2, p 166)... 

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... 

Figure 1.13 Velocity Modulation Spectra (VMS). An AC electric field applied along the laser propagation direction causes the velocity, hence the Doppler shift, of ionic but not neutral species to be modulated. Spectrum (a) shows 127 MHz = 0.0042 cm-1 Doppler shifts of a 2479.4113 cm-1 ArH+ transition. Also shown is an Ar line that does not exhibit an electric field dependent Doppler shift. This spectrum was actually recorded at a fixed (DC) electric field using counter-propagating (frequency modulated) laser beams (from Haese, et ai, 1983). Spectrum (b) shows a portion of the VMS Nj A2 Hu — X2E (7,3) band. Since the AC electric field causes the Nj line to be frequency modulated, phase sensitive detection at the modulation frequency (If) results in a first derivative lineshape. Spectra (b) and (c) were recorded simultaneously. The Nj lines are absent from spectrum (c), which was recorded in population modulation mode by phase sensitive detection at twice the modulation frequency (2f). Spectrum (c) contains a few lines belonging to a N2 B3I19 — A3eJ First Positive baud. The population of N2 A3Ej is modulated at 2f because each half cycle of the AC field results in a full cycle modulation of the population of electronically excited N2 molecules. (Spectra (b) and (c) axe from Radunsky and Saykally, 1987.)...

Figure 4. The upper panel shows frequency modulated lineshapes of nascent CN (N" = 24) in its ground electronic and vibrational state, which was produced from the photolysis of ICN with 248-nm light. The lower panel shows the resulting Doppler profile. [This figure was provided courtesy of S. W. North and G. E. Hall.]...

The drift tube has several uses. It may be held at a potential of up to 500V relative to earth, and this allows us to discriminate in favour of fragment ions produced at this potential (within the drift tube) by means of the ESA. A square wave potential of up to 10V may also be applied to the drift tube this velocity modulates the ion beam and hence, through the Doppler effect, is equivalent to frequency modulation, thus allowing phase sensitive detection to be employed. 

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]. 

Linear frequency modulation (LFM) pulse compression Doppler filtering SAR/ISAR imaging... 

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