Multiphoton Detachment of Hydrogen Negative Ion

Abstract

Investigation of detachment of electrons from negative ions in intense laser fields has recently experienced a renaissance. The first multiphoton detachment experiment was performed by Hall, Robinson and Branscomb1 more than a quarter century ago. Theoretical investigation of the detachment process began at about the same period which lead to the first non-perturbative theory, the so called KFR-theory2–4, of laser atom interaction. The recent upsurge of theoretical interest5–13 in the problem is generated by the discovery of the above-threshold ionization (ATI) process14 and the progress made both in experimental techniques, measurement of detachment rates15–20 and theoretical methods for investigating laser-atom interaction in the non-perturbative domains of field strengths. The above threshold detachment (ATD) process in negative ions, which is analogous to the ATI process in neutral atoms, has been observed very recently 18–20 in several negative ions. The energy spectrum of both ATI and ATD show characteristic peaks separated by the constant energy of the incident photons. These spectra are universal in character in the sense that their overall structures are independent of the atom, or negative ion, involved. Much interests have arisen in this connection recently to get a better understanding of the similarities and differences between ATI and the ATD processes. Perhaps the most fundamental difference between the two systems lies in the asymptotic nature of the interaction potential seen by the electron. The atomic electron in the final state experiences a long range Coulomb potential (-r-1) whereas the active electron in the negative ion experiences short range (‘static’ and -r-4 polarization) potentials.