Measuring differential cross sections using the Doppler shift

Abstract

We discuss two methods, one of them new, for recovering level-specific differential cross sections in crossed molecular beams experiments from the Doppler profiles of line shapes observed by laser induced fluorescence. The angular resolutions of the two methods are compared and shown to be complementary. An experiment using both methods can have moderately good angular resolution at all scattering angles. In the first method, which has previously been demonstrated experimentally, the Dopper profile is taken with the laser beam parallel to the relative velocity of the collision system. Good angular resolution is obtained between π/4 and 3π/4. In the second method, which is proposed here, the Doppler profile is taken with the laser beam perpendicular to this relative velocity, and the best angular resolution is obtained in the regions 0 to π/4 and 3π/4 to π. This method requires an integral transform to recover the cross section from the Doppler profile. A practical implementation of this transform is presented along with a numerical example showing its relative insensitivity to noise in the profile.

The Sc+NO→ScO+N reaction: Rotational state distribution in ScOX 2Σ+(v″=0)

“…In the case where w is well defined, i.e., w is unique, the distribution of scattering angles P() can be derived by application of the so-called ''Doppler technique'' [20][21][22][23] in which one measures the frequency shift associated with w:…”

Angular Selection of Products by Use of Saturated‐Absorption Techniques in the Cs(7P)+H2→CsH+H​ Reaction

“…In the case where it is not possible to adjust the analysis laser beam along the collision axis (the most general case), at least two recordings of fluorescence profiles with two different orientations of the laser beam are necessary to obtain the angular scattering probability without ambiguity (even if the product velocity is unique). [2][3][4][5][6] In saturated-absorption experiments using this bent arrangement, one obtains directly, at a given frequency, the contribution of only two scattering directions which reduce to one for fl 90. Another application of this saturation technique is offered for experiments where it is essential to select one particular direction of scattering: for example, for experiments on alignment/ orientation of reagents where it was shown that the scattering may not be symmetrical with respect to the collision axis.…”

“…lasers are used, for the spectral analysis of fluorescence profiles, hence high resolution informations on the velocity and the angular scattering probability of reaction products, directly in the center of mass (Doppler technique). [2][3][4][5][6] In cases where the laser intensity is large enough to saturate the absorption by product molecules, the use of classical saturated-absorption techniques increases the spectral resolution, allowing for example the identification of hyperfine components in fluorescence signals. 7'8 Here, we present another application of saturated-absorption techniques to crossed-beam experiments: the selection of reaction products which scatter in definite planes or in definite directions.…”

11 State-to-state rotational inelastic scattering of free radicals

“…Based on the fact that the Doppler shift (∆v D ) from a resonant absorption frequency (v R ) is proportional to the velocity component (v | | ) parallel to the direction of the laser propagation, appropriate analysis of a Doppler profile of the scattered products yields the SR-DCS. 28 Kinsey and Pritchard applied this method (ADDS: angular distributions using the Doppler shift) to measure the SR-DCS for the inelastic scattering of Na 2 ϩ Ar. 29 ADDS has been also applied to reactive scattering.…”