Photofragmentation Spectroscopy of Molecular Ions

Besides excitation spectroscopy of bound-bound transitions, photofragmen-tation spectroscopy has gained increasing interest. Here predissociating upper levels of parent molecular ions M are excited, which decay into neutral and ionized fragments. The ionized fragments can be detected with a mass spectrometer while the neutral fragments need to be ionized by laser photons or by electron impact. 

For illustration, Fig.9.24 shows the number of O ions formed in the photodissociation reaction 

With a properly selected polarization of the laser the photofragments are ejected perpendicularly to the ion-beam direction. Their transverse energy distribution can be measured with a position-sensitive detector, because their impact position x,y at the ion detector centered around the position X = y = 0 of the parent ion beam is given by x = (v /v )z, z being the distance between excitation zone and detector [9.67,68]. 

A special version of ion spectroscopy is the Coulomb-explosion technique (Fig. 9.25). A collimated beam of molecular ions with several MeV kinetic energy pass through a thin foil where all valence electrons are stripped. By Coulomb explosion the fragments are ejected and are detected by a position sensitive detector. From the measured pattern the geometry and structure of the original parent ion M+ in its electronic ground state can be inferred. Excitation of the ions M+ by a laser just before they enter the foil allows the determination of the molecular structure in excited states [9.69]. 

The combination of fast ion-beam photofragmentation with field-dissociation spectroscopy opens interesting new possibilities for studying long-range ion-atom interactions. This has been demonstrated by Bjerre and Keiding [9.73] who measured the 0 -0 potential in the range 10-20 A from electric-field-induced dissociation of selectively laser-excited O ions in a fast beam. 

Dependance of the O photofragment signal on the absorption wavelength of 2, obtained by Doppler tuning at a fixed laser wavelength [9.76] 

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