Resonance broadening ionization

The two-photon ionization spectrum of the Na2o cluster is broadened by many possible vibrational transitions. So the width is not related to the decay time of the resonance. 

Resonance line broadening due to chemical exchange and quadrupole-induced relaxation in the H and nB n.m.r. spectra of some boron-nitrogen adducts ArNMe2,BY3 (Y = halogen) has been observed and used to determine the mechanism of amine scrambling in these adducts.165 This is thought to occur via a unimolecular ionization rather than a B—N bond-rupture process. 

The powerful technique of two-photon absorption, which permits limitations of Doppler broadening to be overcome (see Chapter 1), has been used in a study on sodium atoms in which the Stark effect in the 5s 2Si and 4d D and D levels was observed.188 The radiative lifetimes of the S and D Rydberg levels of Na,189 the use of laser-induced resonance fluorescence for the measurement of small concentrations of Na vapour,170 the quenching of Na(32P) and K(42P) by N2, 02, H2, and HaO,171 the chemi-ionization reactions of photoexcited atoms,172 and excitation of the K(42P ) level in collisions with rare-gas atoms173 have been the subjects of recent reports. 

The calculated photoabsorption spectrum of this cluster shows a collective excitation with a peak at 2.12 eV. The tail of the resonance extends up to 3 eV, and concentrates a sizable amount of strength, due to particle-hole transitions that interact with the collective excitation and lead to its broadening. One of the most important particle-hole transitions is that from the HOMO level to the continuum the energy of this ionization threshold is indicated by the arrow at 2.6 eV. Similar TDLDA calculations have been performed for pure Na [104] and pure K [127] clusters. Comparing the positions of the collective resonances, it can be concluded that the position of the resonance in K2oNa2o is closer to that in pure K clusters thus, the surface, made of K atoms, controls the frequency of the collective resonance. 

The radiationless decay of a quasidiscrete excited state of an atom or molecule into an ion and electron of the same total energy is called autoionization. The quasidiscrete state must, of course, lie above the first ionization potential of the atom or molecule. The occurrence of autoionization may be inferred from the appearance of absorption spectra or ionization cross-section curves which exhibit line or band structure similar to that expected for transitions between discrete states. However, in the case of autoionization the lines or bands are broadened in inverse proportion to the lifetime of the autoionizing state, as required by the uncertainty principle. In the simple case of one quasidiscrete state embedded in one continuum, the line profile has a characteristic asymmetry which has been shown to be due to wave-mechanical interference between the two channels, i.e., between autoionization and direct ionization. In an extreme case the line profile may appear as a window resonance, i.e., as a minimum in the absorption cross section. 

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