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Photodetachment microscopy
Permanents: Christophe Blondel, Christian Delsart, Cyril Drag
Thésitif : Walid Chaibi
In the usual photodetachment experiments carried out in the presence of an electric field, one measures only the total probability of removing the extra electron, as a function of the excitation energy. Photodetachment microscopy, by the use of an electron detector with a high spatial resolution, makes it possible also to observe the spatial distribution of the ejected electron. This is a direct view of the spatial structure of the wave function of an atomic electron, with its node and antinode structure. The axial symmetry of the wave function around the direction of the field makes this structure look like a ring pattern. The photodetachment rings were observed for the first time in 1996 [1], from negative ion Br-.
The phenomenon under study can be considered, from a semi-classical point of view, as interference between two electron waves. The pattern is the figure of interference produced by an electronic dual-wave interferometer, with the negative ion as the electron pointlike source, and the electric field the element that makes trajectories recombine. In the external field, the classical trajectories are those of a motion of constant acceleration, similar to free-fall (cf. fig. 1). Of the emitted electronic wave, a half "falls" directly towards the detector and the other half, initially emitted in the opposite direction, is reflected by the field. The quantitative analysis of the electronic interferograms, essentially by counting the fringes, gives a very precise measurement of the initial kinetic energy of the photodetached electron. This makes it possible, provided one knows the wavelength of the exciting laser, to calculate the detachment threshold of the negative ion under study. This quantity is important, as it is the electron affinity of the neutral atom on which the negative ion was formed. Ref. [2] describes a measurement of the electron affinity of oxygen performed in this way.
- Figure 2
- Photodetachment interferogram of OH- in an electric field. The external rings are rings of classical accumulation corresponding to various thresholds. The structure of the central spot is the electronic interference pattern relating to the true detachment threshold of OH-.
Photodetachment microscopy was recently extended to molecular negative ions [3]. The interferograms of various thresholds corresponding to the transitions from the rotational states of OH- towards the rotational states of OH were observed (cf. fig. 2). These recordings made it possible to precisely determine the first detachment energies (v=0, J<5).
A project under development aims at further exploring the process of photodetachment, by using pulsed lasers. Such an excitation means would make it possible to change much more easily the studied threshold or ion (by frequency mixing) and would offer the possibility of non-linear excitation in the ion itself, in a multiphoton detachment regime.
Further reading:
[1] The Photodetachment Microscope
C. Blondel, C. Delsart and F. Dulieu
Phys. Rev. Lett. 77 3755 (1996)
[2] Electron affinities of 16O, 17O, 18O, the fine structure of 16O-, and the hyperfine structure of 17O-
C. Blondel, C. Delsart, C. Valli, S. Yiou, M.R. Godefroid and S. Van Eck,
Phys. Rev. A 64 052504 (2001)
[3] Photodetachment Microscopy of the P,Q, and R branches of the OH-(v=0) to OH(v=0) detachment threshold
F. Goldfarb, C. Drag, W. Chaibi, S. Kröger, C. Blondel, and C. Delsart,
J. Chem. Phys. 122 014308 (2005)
Photodetachment microscopy
In the usual photodetachment experiments carried out in the presence of an electric field, one measures only the total probability of removing the extra electron, as a function of the excitation energy. Photodetachment microscopy, by the use of an electron detector with a high spatial resolution, makes it possible also to observe the spatial distribution of the ejected electron. This is a direct view of the spatial structure of the wave function of an atomic electron, with its node and (...)
