Abstract

In highly focussed nanosecond laser pulses the yield of electrons must be kept below about one hundred per lasershot, otherwise the space charge of the ions in the laser-focus will seriously influence the electron energy spectrum. Especially for the study of electrons produced in multiphoton ionization a spectrometer has been designed which accepts an opening angle of 2π sr. The laser beam is focussed in a region where a magnetic field of 1 T is present. The strong field diverges along the axis of a 50 cm. time-of-flight tube to a weak field of 10⁻³ T, in which an electron detector is positioned. Photoelectrons with any velocity in the direction of the detector spiral around the magnetic field lines and reach the detector. In the region of the field divergence the spread in forward velocities is reduced to 5×10⁻⁴ by an inversed magnetic-mirror principle. The total velocity is unaffected. Experiments show the resolution to be 20 meV for electrons up to 10 eV energy. By applying a retarding potential in the high magnetic field region it is possible to select an opening angle different from 2π sr, thus permitting measurements of angular distributions as well.

© 1982 Optical Society of America