Modulation velocity

Under low-frequency excitation, the flame front is wrinkled by velocity modulations (Fig. 5.2.5). The number of undulations is directly linked to frequency. This is true as far as the frequency remains low (in this experiment, between 30 and 400 Hz). The flame deformation is created by hydrodynamic perturbations initiated at the base of the flame and convected along the front. When the velocity modulation amplitude is low, the undulations are sinusoidal and weakly damped as they proceed to the top of the flame. When the modulation amplitude is augmented, a toroidal vortex is generated at the burner outlet and the flame front rolls over the vortex near the burner base. Consumption is fast enough to suppress further winding by the structure as it is convected away from the outlet. This yields a cusp formed toward burnt gases. This process requires some duration and it is obtained when the flame extends over a sufficient axial distance. If the acoustic modulation level remain low (typically v /v < 20%),... 

Depth profiling of a solid sample may be performed by varying the interferometer moving-mirror velocity (modulated IR radiation). By increasing the mirror velocity, the sampling depth varies, and surface studies may be performed. Limitations do exist, but the technique has proven to be quite effective for solid samples [21]. In addition, unlike diffuse reflectance sampling techniques, particle size has a minimal effect upon the photoacoustic measurement. 

Velocity-modulated allosteric regulation, V, ,-TYPE ALLOSTERIC SYSTEM VENOM EXONUCLEASE Venom phosphodiesterase, PHOSPHODIESTERASES VESICLE TRANSPORT IN CELLS Vibrational bond stretching mode,... 

A typical electromechanical device for velocity modulation consists of two coils (30,32, 86-88) a drive coil across which a voltage related to the... 

Since the frequency is large, the distance over which a concentration wave proceeds is small, then exp (- / 3) can be considered as equal to one and the velocity modulation being rapidly damped close to the wall, the convective term can be disregarded in the homogeneous part of Eq. (3-2) which becomes ... 

Wang, S., Morris, M.D., Plastic microchip electrophoresis with analyte velocity modulation. Application to fluorescence background rejection. Anal. Chem. 2000, 72, 1448-1452. 

We conclude this section on modulation spectrometers by describing a particularly novel and important method, known as velocity modulation, which was originally developed by Gudeman, Begemann, Pfaff and Saykally [10]. It applies specifically to ionic species, and has been used primarily to study the infrared vibration-rotation spectra of molecular ions. If we need an excuse to include it in this book, however, it is provided by Matsushima, Oka and Takagi [11] who used the method to study the far-infrared rotational spectrum of the HeH+ ion. 

Figure 10.14. Velocity modulation spectrometer introduced by Gudeman, Begemann, Pfaff and Saykally [10] to study the infrared vibration-rotation spectra of molecular ions.

Polak M, Gruebele M, Saykally RJ. Velocity modulation laser spectroscopy of negative ions the v3 band of azide (N3 ). J Am Chem Soc 1987 109 2884-2887. 

The majority of Mdssbauer experiments that are conceived to elucidate problems of chemical structure make use of velocity modulated spectrometers. Most frequently, these are of the constant acceleration type, in which equal time intervals are spent in equal velocity increments. The primary information storage device for such spectrometers is commonly a multichannel analyzer operated in the multiscaler mode. Since the minimum observable line widths (2rnat= hf ) for S7Fe and 1,9 Sn are 0.1946 and 0.6193 mm/s, respectively, and since on the order of at least 5-10 data points on each side of the resonance maximum are... 

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.)...

Gudeman C S and Saykally R J 1984 Velocity modulation infrared laser spectroscopy of molecular ions Annual Review of Physical Chemistry ed B S Rabinovitch, J M Schurr and H L Strauss (Palo Alto, CA Annual Reviews)... 

Rosenbaum N H, Owrutsky J C, Tack L M and Saykally R J 1986 Velocity modulation laser spectroscopy of negative ions the infrared spectrum of hydroxide (OH ) J. Chem. Phys. 84 5308-13... 

The method of velocity modulation of the y-ray energy by means of the Doppler effect was described by Mossbauer in 1958 [2] and provides the basis for all modern spectrometers. 

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