An orbital hopping model for the calculation of intensity distributions of ionization spectra. The rotational linestrength distribution of NO ZEKE spectra
✍ Scribed by V.M. Akulin; G. Reiser; E.W. Schlag
- Book ID
- 103030106
- Publisher
- Elsevier Science
- Year
- 1992
- Tongue
- English
- Weight
- 437 KB
- Volume
- 195
- Category
- Article
- ISSN
- 0009-2614
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✦ Synopsis
We present a new approach for understanding rotational linestrength distributions in ZEKE spectra of NO. A model is described in which orbital hopping of an electron is induced by scattering at the ion core. For diatomic molecules the intensity distribution only depends on an effective interatomic distance and charge displacement. In the case of NO, these parameters agree extremely well with independent experimental data.
Very high resolution spectroscopy based on the detection of photoelectrons with zero kinetic energy (ZEKE) allows detailed information to be obtained about ionic species [ 1,2 1. In the case of NO rotationally resolved ZEKE spectra have been published [ 2,3] and good agreement has been obtained using ab initio calculations including angular momentum theory [ 41. Another approach for theoretical understanding is by multi-channel quantum defect theory (MQDT), as has recently been performed for the ZEKE-PFI spectra of water [ 51. Nevertheless, a simple qualitative theory of the process that could establish a correspondence between the parameters of the system and the main features of ZEKE spectra would be of definite help.
In this Letter, we present a simple model applicable to diatomic molecules, that gives good agreement with experimental data for ZEKE measurements of rotational linestrength distributions of NO, achieved out of different rotational states of the A '2 state. The model is as follows. After photon absorption the molecule is described in terms of the motion of a free (or a Rydberg) electron in a Coulomb potential and the motion of the molecular ion core. The interaction between these motions is described by the scattering of the electron at the ionic core which can take place when the electron approaches the ion. It results in nonadiabatic horizontal transitions, in which energy and angular momentum are transferred from the electron to the ionic core and vice versa. We treat the scattering process quantum mechanically, whereas the rest of the motion of the electron can be considered in classical mechanics. The reasoning behind this is as follows: The electron that has absorbed a quantum of radiation transfers to a Kepler orbit corresponding to a given energy and angular momentum. Near the edge of the ionization continuum, as is the case for ZEKE-PFI measurements, the Kepler period strongly depends on energy, i.e. the change of electron energy induced by the change of only one rotational quantum of the molecular ion dramatically changes the time of the next visitation of the core. This implies that any two electron paths with different history of scattering have different arrival schedules and