Fast beam kinematics

Before discussing the various features observable in three-level laser spectroscopy, with particular emphasis on fast beam experiments, the kinematics will be dealt with. 

The next important feature in fast beam laser spectroscopy concerns the velocity distribution. Due to kinematic velocity compression, the initial thermal distribution of velocities in the ion source is reduced by a factor R = 4cT/eV, where T is the ion source temperature and V the acceleration voltage. Including the voltage spread 6V, the Doppler width in our accelerator is 20-200 MHz, depending on mass and acceleration voltage. The transverse velocity distribution (particle wave front curvature) not affected during acceleration, contributes to the linewidth. 

Figure 10.12 Rotational alignment coefficient of the XeX product vs. the collision energy for the reaction of a fast Xe ( P2> beam with HX, X = Cl, Br, I. The dashed horizontal lines represent the maximum theoretical limit on the alignment [adapted from Simons (1987)]. The deviations of the experimental results from the spectator limit can be accounted for if there is a repulsive release of the exoergicity. Such an impulse can contribute significantly when the ejected atom is light and if the transition state is not collinear. With the available understanding of kinematic effects, experimental conditions can be chosen such that dynamical features arising from forces operating during the collision process can dominate.

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