Direct Extraction of Alignment Moments from Inelastic Scattering Images

Abstract

We present a novel means of analyzing velocity-map images of angular momentum polarization in inelastic scattering. In this approach, linear combinations of angular distributions obtained by integrating select regions of images for two probe laser polarizations directly yield the alignment-free differential cross sections and the differential alignment moments. No fitting is needed in the analysis. The method relies on the fact that the angular distribution for out-of-plane scattering is encoded in the distribution along the relative velocity vector, and this may be recovered quantitatively owing to the redundancy of the in-plane and out-of-plane scattering for the horizontal polarization case.

Molecular square dancing in CO-CO collisions

“…Images were accumulated with the polarization direction of the VUV laser parallel and perpendicular to the scattering plane. Following the procedure given in Ref (21), partial integration of the outer radii or a stripe through the two images yielded sufficient information to directly determine parameters describing alignment of the product molecular rotation induced by the collision. Because these parameters were very similar to those measured for CO-Ar (17) and agreed well with predictions from a simple classical model ( 22) we presented alignment corrected images in Fig.…”

“…A set of procedures described in Refs. (17,21) were followed to fully simulate the CO-CO polarization dependent scattering images. Correction of the imaging signal from density to flux, and extraction of collision induced alignment information was carried out, but because the extraordinary channel described in the text did not show special alignment effects within the signal to noise ratio of the data, the alignment-free moment was extracted for analysis of the radiusdependent angular distribution of the images, i.e., the (j' = 15, j") pair-correlated scattering data using an onion-peeling inversion program similar to that in Ref.…”

A compact electrostatic lens for velocity map imaging experiments

“…As we show later in this paper, the degree of slicing obtained with our present design with 6 electrodes when using a typical 80 ns wide high voltage pulse to gate the channel-plate detector is slightly less than that from the conventional annular ring lens design. The tubular lens design can be used to obtain well-crushed images, however, and this is favourable, for example, when using the direct extraction method to obtain angular momentum polarisation information [18].…”

A simple resonance enhanced laser ionization scheme for CO via the A1Π state

“…Desirable experimental properties of a REMPI scheme are as follows: a minimum number of tunable wavelength lasers using pump lasers of normal operating intensity; the need to scan only a single wavelength; and efficient coupling of the laser radiation to the apparatus. The scheme should be fully state-selective, i.e., without overlapping probe transitions, and should be sensitive to alignment effects in the nascent products such as collision induced alignment in inelastic scattering 10 or a) Author to whom correspondence should be addressed. Electronic mail: parker@science.ru.nl from polarized products from photodissociation.…”

“…We have recently reported an application of REMPI of CO via the A 1 Π state using VUV radiation near 154 nm generated by four-wave mixing. 10 Strong CO + signals were observed using the most simple coupling of the laser radiations to the apparatus, where the resonant VUV radiation is not separated from the UV+ visible pump beams used in the four-wave difference-frequency mixing (FWDM) generation. This was applied to probe the rotational excitation of CO due to inelastic scattering with Ar or He using VMI technique and showed excellent sensitivity to collision induced alignment effects.…”