Doppler profile

Velocity recoils are measured at short times after tire initial ultraviolet excitation pulse by probing tire nascent Doppler profiles for tire different spectral lines probed in tliese last steps. 

Figure C3.3.7. In the upper half of the figure are shown typical measured Doppler profiles for molecules scattered into the (OO O J= 72) or (OO l J = 17) states of CO2 by collisions with hot pyrazine having an energy of 40 640 cm In the lower half of the figure is shown a typical intennolecular potential identifying the hard and soft collision regimes and the kind of energy transfer they effect.

In practice the laser can operate only when n, in Equation (9.2), takes values such that the corresponding resonant frequency v lies within the line width of the transition between the two energy levels involved. If the active medium is a gas this line width may be the Doppler line width (see Section 2.3.2). Figure 9.3 shows a case where there are twelve axial modes within the Doppler profile. The number of modes in the actual laser beam depends on how much radiation is allowed to leak out of the cavity. In the example in Figure 9.3 the output level has been adjusted so that the so-called threshold condition allows six axial modes in the beam. The gain, or the degree of amplification, achieved in the laser is a measure of the intensity. 

In addition to CO(v = 0—2,7) populations, Houston and Kable recorded CO Doppler profiles to measure the translational energy release, and the vector correlation between the recoil velocity vector and the angular momentum vector of CO. Together, these data paint a compelling picture that two pathways to CH4 + CO are operative. The rotationally hot CO population (85% of total CO)... 

To analyze the data, first perform the vx- and ty-corrections and the time-to-speed transformation to make the velocity volume element the same for all data points, and then normalize each Doppler-selected TOF spectrum according to the averaged ID Doppler profile from several independent scans, I vz) = )T)(, vyS(vy vx 0,vz)dvy. Due to the large exothermicity of this reaction, the problematic density-to-flux transformation is not negligible (despite the large probe laser size used to minimize its effects) and needs to be accounted for (the wy-correction , see Sec. 3.3). By combining all the resulting TOF spectra, the product 3D velocity flux contour... 

Fig. 6. Doppler profile of H and D fragments from the photolysis of C2H2 and C2D2, respectively. The narrow doublet at the center is the Doppler profile of a supersonically-cooled H-atom beam.

Fig. 7. Raw TOF spectra for the photolysis of C2H2, obtained with two polarization configurations. The laser frequency was set at the center of the Doppler profile, i.e. u)q.

The molecular beam experiments of Liu and co-workers65,66 employed the Doppler profile time-of-fiight technique that allowed the ready observation of the excitation function (i.e. the total reactive ICS summed over... 

Thorn EMI 9235QB) near the phosphor screen to collect all of the ion signal as the probe laser was tuned over the 0(3Pj) Doppler profile at laser wavelengths of 226.23, 226.06 and 225.65 nm for j = 0, 1 and 2, respectively. The output of the photomultiplier was sent to a boxcar averager gated at the appropriate arrival time. The probe laser power was simultaneously measured with a photodiode in order to normalize the 0(3Pj) signal intensity for fluctuations in laser power. 

The branching ratios into the various 0(3Pj) states were surprisingly difficult to measure. The power dependence of the ionizing laser at the center of the Doppler profile was measured before each scan of the Doppler profile to provide proper normalization of the power dependence. It was found that the combination of our laser power and sample concentration put... 

Figure 7.9 Doppler profile for 51-Pegasi. (Reproduced by permission of Sylvain Korzennik, Harvard-Smithsonian Center for Astrophysics)...

Figure 21. Effect of crab angle on range-Doppler profile.

In Chapter 2, Jansson describes the determination of the system response function for a dispersive spectrometer system. We have made a number of such determinations using very-low-pressure samples of, for example, CO in the 5-fim region. As discussed by Jansson, one records the data and then removes the Doppler profile using deconvolution, yielding the system response function. 

It is of interest to compare the half-widths at half-intensity of the spectral functions of the three systems shown in Fig. 3.2. These amount to roughly 140, 80 and 50 cm-1 for He-Ar, Ne-Ar and Ar-Kr, respectively, which are enormous widths if compared to the widths of common Doppler profiles, etc. The observed widths reflect the short lifetimes of collisional complexes. From the theory of Fourier transforms we know that the product of lifetime, At, and bandwidth, A/, is of the order of unity, Eq. 1.5. The duration of the fly-by interaction is given roughly by the range of the induced dipole function, Eq. 4.30 (1/a = 0.73 a.u. for He-Ar), divided by the mean relative speed, Eq. 2.12. We obtain readily ... 

Photons from a tunable dye laser are absorbed by a metastable state of the ion. Spontaneous emission at several wavelengths follows. The ionizing electron energy is varied over the region of interest. At each setting the laser is frequency scanned across the Doppler profile of the io.i-beam... 

Both Welge (13), and Wittig and his colleagues (14), have been able to use high resolution tunable lasers to measure the Doppler profile of the individual rotational lines of NO and CN fragments produced by photodissociation. They are thus able to determine all of the energetics (translational, rotational, and vibrational) associated with a particular fragment. 

The rotation curve is calculated in two steps (1) by subtracting the global redshift component (i.e., cosmological redshift + Doppler effect arising from peculiar motion) from the Doppler profile measured directly across the galaxy s disk and (2) by determining the actual dynamical centre of the galaxy. 

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