The confined variational method is used to study the elastic scattering of the positron from the ground-state
helium with the scattering energy in the range from 0.05 eV to 11.02 eV. Describing the correlation effect with
explicitly correlated Gaussians, we obtain accurate phase shifts, S-wave scattering length, elastic scattering cross
sections, and annihilation parameters for different incident momenta. Specifically, by a least-squares fit of the
data to the effective-range theory, we determine the room temperature annihilation parameter Zeff = 3.955, which
is in perfect agreement with the measured result of 3.94 ± 0.02 [J. Phys. B 8, 1734 (1975)].
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Pump-probe microscopy is an emerging nonlinear imaging technique based on high repetition rate lasers and fast intensity modulation. Here, we present new methods for pump-probe microscopy that keep the beam intensity constant and instead modulate the inter-pulse time delay or the relative polarization. These techniques can improve image quality for samples that have poor heat dissipation or long-lived radiative states and can selectively address nonlinear interactions in the sample. We experimentally demonstrate this approach and point out the advantages over conventional intensity modulation.
