Fast beam velocity distribution

In TOF-SARS [9], a low-keV, monoenergetic, mass-selected, pulsed noble gas ion beam is focused onto a sample surface. The velocity distributions of scattered and recoiled particles are measured by standard TOF methods. A chaimel electron multiplier is used to detect fast (>800 eV) neutrals and ions. This type of detector has a small acceptance solid angle. A fixed angle is used between the pulsed ion beam and detector directions with respect to the sample as shown in figure Bl.23.4. The sample has to be rotated to measure ion scattering... 

Fast positrons are created by bremsstrahlung pair-production in an electron microtron accelerator. These are moderated and bunched into 25 ns packets at 30 Hz, each comprised of 2 x 104 slow positrons. The positrons are guided by a 150-G magnetic field and implanted at 1-2 keV kinetic energy onto an Al(lll) crystal heated to 576 5 K as shown in Figure 7a. About 30% of the incident positrons come off the surface as thermal positronium with a velocity distribution that is a beam Maxwellian. 

If the first laser beam is interrupted at the time to for a time interval At that is short compared to the transit time T = z2 — Z )/v (this can be realized by a Pock-els cell or a fast mechanical chopper), a pulse of molecules in level /) can pass the pump region without being depleted. Because of their velocity distribution, the different molecules reach zi at different times t = to- -T. The time-resolved detection of the fluorescence intensity /fi(0 induced by the second, noninterrupted laser beam yields the distribution n T) = n(Az/v), which can be converted by a Fourier transformation into the velocity distribution n(v). Figure 4.13 shows as an example the velocity distribution of Na atoms and Na2 molecules in a sodium beam in the intermediate range between effusive and supersonic conditions. If the molecules Na2 had been formed in the reservoir before the expansion, one would expect the relation Up(Na) = V2up(Na2) because the mass m(Na2) = 2m(Na). The result of Fig. 4.13 proves that the Na2 molecules have a larger most probable velocity Up. This implies that most of the dimers are formed during the adiabatic expansion [410]. 

If the first laser beam is interrupted at the time to for a time interval Ar that is short compared to the transit time T = z2 z )/v (this can be realized by a Pockels cell or a fast mechanical chopper), a pulse of molecules in level /) can pass the pump region without being depleted. Because of their velocity distribution, the different molecules reach Z2 at different times t to + T. The time-resolved detection of the fluorescence intensity /fi(0 induced by the second, noninterrupted laser beam yields the distribution... 

Isotope shift and hyperfine structure studies on short-lived radioactive nuclides depend crucially on the sensitivity and the conditions of on-line production in nuclear reactions. Here, the search for a workable concept of experiments with fast isotope-separated beams was initiated by Otten. It was Kaufman who pointed out that the narrow velocity distribution along these beams corresponds to nearly Doppler-free conditions. The experimental scheme, worked out in early 1975, included cw-laser excitation with high resolution and efficiency of a neutralized fast atomic beam and detection of the resonance fluorescence. A detailed discussion of the basic... 

The two beams crossing at right angles are collimated to 4 and 6.5 F.W.H.M. respectively. Fast ionisation gauges (Beam Dynamics) are used to measure the time profiles, velocities and velocity distributions of the beams, as well as the attenuation of one beam by another. This latter factor remains lower than 10% in all experiments thus ensuring reasonably single collision conditions. 

The monochromatic laser beam is split into two beams 1 and 2, both of which cross the molecular beam perpendicularly but at different locations = A and 2 = 8 (see Fig.10.10). When the laser frequency is tuned to a molecular transition (vIj, JIj - V, J ), the molecules passing through the pump beam 1 are optically pumped and there are nearly no molecules left in the depleted level (vIjjJ . ). This means that the fluorescence excited by the probe beam 2 is very low. If the pump beam 1 is interrupted for a short time interval At (e.g., by a fast mechanical chopper) the molecules can pass during this interval At without being pumped. Because of their different velocities they reach the probe beam 2 at different times t = L/v, where L = 22 - The time-resolved fluorescence intensity induced by the cw probe beam therefore reflects the velocity distribution of molecules in the level (vV,JV). 

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. 

Initially the signals were displayed on a fast storage oscilloscope, and a typical particle scattering trace for 8°, 6°, and 12 is shown in Figure 4. A nearly Gaussian intensity distribution was always observed, and the particle velocity can be estimated from the known beam width divided by the measmed transit time. 

A fast atomic ion traveling through matter picks up and loses electrons as dictated by its velocity-dependent, charge-changing cross sections in that material. A beam of such ions displays a distribution of... 

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