Determination of velocity distribution, angular distribution, and vector correlation of photofragments using sub-Doppler fluorescence-imaging method

Abstract

With sub-Doppler resolution, the fluorescence-imaging techniques can be modified to determine velocity distribution, angular distribution, and vector correlation of state-selected photofragments, even in an uncollimated molecular beam. This new method is proposed as “sub-Doppler fluorescence-imaging” in which two experimental schemes are developed. The dependence of fluorescence intensities, at any selected velocity and recoil angle in the scattering plane, with respect to the variation of polarization vectors of the probe laser and emitted fluorescence is derived using density matrix formalism. The intensity patterns of photofragments with v–J and μ–v–J correlations are simulated. The laser ablation of B atoms at 248 nm demonstrates the feasibility of this method. Two-dimensional velocity distribution of the laser-ablated B(2P1/2,3/20) atoms is measured and the ablation mechanism is discussed.

A New Sub-Doppler Fluorescence Imaging Method in Studying Laser Ablation of B Atoms at 248 nm

“…Figure shows three images of the B( 2 ) atoms probed through the B( 2 S) ← B( 2 ) transitions at (a) ν 0 − 1.12 cm -1 , (b) ν 0 (40024.4 cm -1 ), and (c) ν 0 + 1.12 cm -1 , with a fixed delay time of 1.26 μ s, which is slightly longer than that of the duration (∼1.2 μ s) of the luminous ablation plume. Preliminary and similar images of the B( 2 ) atoms have been reported previously but without detailed analysis . In this article, the extraction of 2D distributions of the ablated species from the full 3D space is analyzed as follows.…”

“…In contrast to the photodissociation experiment using an uncollimated molecular beam, the bandwidth of the probe laser is in fact not necessary to be sub-Doppler for the experiment with pulsed laser ablation, where a focused laser spot has clearly defined the spatial origin for all of the ablated products. Generally, it is more efficient to record and average the image while scanning laser frequency through the whole Doppler profile, provided that the quantum state of the product of interest is well selected.…”

“…Recently, we have developed a new “sub-Doppler fluorescence imaging” experimental technique to determine both velocity and angular distributions as well as vector correlations of state-selected photofragments . This method combines Doppler-selection of velocities , and two-dimensional (2D) fluorescence imaging techniques, − thus being able to measure the 3D distributions of photofragments directly in the center-of-mass frame even with an uncollimated molecular beam.…”