Example of an experimental photodetachment interferogram recorded in an electric field of 364.1 V m−1 (left) and the best-fitting theoretical electron interferogram (right), calculated for an initial electron kinetic energy of 59.4(4) m−1
Figure 2. Example of an experimental photodetachment interferogram recorded in an electric field of 364.1 V m−1 (left) and the best-fitting theoretical electron interferogram (right), calculated for an initial electron kinetic energy of 59.4(4) m−1. Subtracting this energy from the laser wavenumber 1239 762.1 m−1 would directly yield the detachment threshold (here threshold 3P2), were it not for the Doppler shift of the laser frequency in the ion frame. Comparison of the raw difference of 1239 702.7 with the result of the final adjustment of the 3P2 threshold at 1239 711.8 m−1 reveals that in this very case, the photon energy, as seen by the ions, was positive-shifted by 9.2 m−1. This corresponds exactly to the 3° deviation from orthogonality that we have set between the laser and ion beam, in order not to illuminate the electron detector with the laser. The other spot produced by the reflected laser beam in the same experiment, 3 mm downstream on the ion beam, is energy-lowered symmetrically. Alternative photodetachment to the lower fine-structure 3P0 and 3P1 levels produces a photoelectron background, but the higher energy of the corresponding electrons causes their interference patterns to be completely smoothed out, the only signature of these lower photodetachment channel being a uniform background.
A beam of Sn− ions produced by a caesium sputtering ion source is photodetached in the presence of an electric field, with a single-mode ring Ti:Sa laser. The laser wavelength, about 806 nm, is set just above the excitation threshold of the 3P2, highest fine-structure sublevel of the 3P ground-term of Sn I. The photoelectron energy is measured by photodetachment microscopy. The measured photodetachment threshold is 1239 711.8 (11) m−1, from which an improved value of the electron affinity of tin can be deduced: 896 944.7 (13) m−1 or 1.112 070 (2) eV.