Excited electronic states, ultrafast cluster

In this book the real-time photodissociation dynamics of small sodium (Nan=3...io) and potassium (Kn=3...9) clusters are studied as a function of cluster size as well as excitation wavelength (Sects. 4.2-4.4). The ratio of dissociative to radiative decay is a measure of the predissociation of an electronic state [122, 133]. For the C state of Naa the electronic predissociation dynamics and especially the localization of its onset are analyzed in detail by ultrafast spectroscopy (Sect. 4.1). 

In Chap. 3, wave packet propagation could be observed for nearly all of the alkali dimer and trimer systems considered, over a rather long time compared to the wave packet oscillation period. The wave packet dynamics - a fingerprint of the excited molecule - definitely characterize the excited bound electronic state of these molecules. However, with the results on K3 (excited with A 800 nm), another phenomenon, which often governs ultrafast molecular and cluster dynamics, comes into the discussion photodissociation induced by the absorption of single photons. This photoinduced dissociation permits detailed study of molecular dynamics such as breaking of bonds, internal energy transfer, and radiationless transitions. The availability of laser sources with pulses of a few tens of femtoseconds today opens a direct, i.e. real-time, view on this phenomenon. 

Stationary spectroscopy on the C and D states of Na3 already indicated the onset of photoinduced fragmentation. Fragmentation becomes more important as the cluster size increases. As a result, nondissociative electronic excitation processes have not yet been observed for free metal clusters larger than trimers [20]. An alternative to conventional spectroscopy of such bound-free transitions was provided by depletion spectroscopy [2]. A deep insight into the dynamics of such photoinduced cluster fragmentation, however, is obtained with ultrafast observation schemes. The principle of such an... 

Following the above-mentioned spectroscopic study by Johnson and co-workers [55], Neumark and co-workers [56] explored the ultrafast real-time dynamics that occur after excitation into the CTTS precursor states of I (water) [n — 4-6) by applying a recently developed novel method with ultimate time resolution, i.e., femtosecond photoelectron spectroscopy (FPES). In anion FPES, a size-selected anion is electronically excited with a femtosecond laser pulse (the pump), and a second femtosecond laser pulse (the probe) induces photodetachment of the excess electron, the kinetic energy of which is determined. The time-ordered series of the resultant PE spectra represents the time evolution of the anion excited state projected on to the neutral ground state. In the study of 1 -(water), 263 nm (4.71 eV) and 790 nm (1.57 eV) pulses of 100 fs duration were used as pump and probe pulses, respectively. The pump pulse is resonant with the CTTS bands for all the clusters examined. 

---