Principles of ionization spectroscopy

When an atom or molecule interacts with a photon of sufficient energy, ionization may occur through removal of an electron. Because the ionization potential of most small molecules is larger than 8 eV, highly energetic photons in the VUV are required to induce ionization through one-photon absorption. Tuneable radiation may be provided by synchrotron sources and by (normally very inefficient) non-linear conversion of radiation from visible or UV lasers. However, direct one-photon ionization exhibits little state selectivity, i.e. ionization of normally several vibrational and rotational levels of the electronic ground state occurs. Consequently, in the photo-ion one also encounters a superposition of several vibrational and rotational levels (determined mostly by the Franck-Condon factors between the initial and final vibrational states). 

In order to alleviate the problem of laser wavelengths and non-state selectivity, stepwise excitation/ ionization via a resonant intermediate state has been introduced, termed RIS, or more generally REMPP, the minimum number of photons required is two (see Section 9.2 for a more extensive discussion of REMPI schemes). For the process to work as desired. 

Broadly speaking, REMPI can be viewed as a multistep process. In the first step, photons with energy hvi populate an intermediate molecular level 

Out of AB, a second photon hi 2 initiates ionization of the molecule (step two). The main ionization processes out of the intermediate configuration AB are  

Laser Chemistry Spectroscopy, Dynamics and Applications Helmut H. Telle, Angel Gonzalez Urena Robert J. Donovan 2007 John Wiley Sons, Ltd ISBN 978-0-471-48570-4 (HB) ISBN 978-0-471-48571-1 (PB) 

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