NeNePo

Figure 9. Principle of NeNePo spectroscopy. The probe pulse detaches the photoelectron from the negative ion, introducing a vibrational wavepacket into the ground state of the neutral particle. Its propagation is interrogated by the probe pulse, which ionizes it to a positive ion.

Figure 10. Experimental scheme for recording NeNePo spectra. The negative ions are mass analyzed in a quadrupole mass filter and introduced into an ion trap. There they are first neutralized (pump), then reionized to a positive ion (probe), so they can escape the trap and enter into another quadrupole mass filter for being detected.

Figure 11. NeNePo spectrum of neutral silver dimers. The progression exhibits the Ag2 vibration in its electronic ground state.

The first NeNePo experiments dealt with silver clusters, Ag3, Ags, Ag7, and Ag9, particularly with the first of these. The photodetachment and photoionization were done with a single titanium-sapphire laser producing pulses of approximately 60 fs duration. Doubled in frequency, these could be tuned over a wavelength span from above 420 to below 390 nm. As with the dimer, photodetachment was a one-photon process and photoionization a two-pho-ton process. (The clusters of odd numbers of atoms could be studied this way the even-numbered clusters require at least three photons in the available energy range for photoionization). The interval between pulses could be varied from zero (simultaneous pulses) to 100 ps the two pulses were made to differ in intensity by about a factor of 2, and either could be the leading pulse. 

Figure 12. NeNePo spectra of the silver trimer taken with wavelengths of a) X = 390 nm, (b) X = 400 nm, (c) X = 415 nm, and (d) X = 420 nm. Each curve has its own axis of zero signal. The time-independent background increases steadily with decreasing wavelength. The fine structure around Ar = 0 is due to the interference of pump and probe pulses [9).

Figure 13. Geometry changes of Ag3( ) during the NeNePo process.

The results of NeNePo with clusters of live, seven, and nine silver atoms are graphically presented, together with the relevant geometries that resulted from ab initio calculations [25], in Fig. 27. The temporal progressions have... 

Figure 27. NeNePo signals and relating geometry changes for Ag3, Ags, and Ag9.

L. Woste In stationary spectroscopy ZEKE certainly provides spectroscopic results at an impressive resolution. Using femtosecond pulses one can certainly not excite specific states as compared to ZEKE. The Fourier transform of the wavepacket evolution, however, exhibits also spectral resolution that easily reaches and even exceeds what we see in ZEKE spectra. For this reason, I do not see any disadvantage in using femtosecond NeNePo to probe states of a prepared molecule. 

A. Multistate Adiabatic Nuclear Dynamics and Simulations of NeNePo Signals... 

E. Fragmentation of Ag202 Interrogated by NeNePo Spectroscopy Reactivity Aspects... 

To accomplish this goals, the electronic and structural properties of noble metal clusters will be addressed first. Then the attention will be paid to MD on the fly and to the simulation of signals. Furthermore, the analysis of the signals and the comparison with the experimental findings will be presented allowing for the identification of processes and conditions under which they can be observed. Finally, cluster reactivity aspects and the scope of NeNePo spectroscopy will be addressed. 

For the simulation of NeNePo signals, a combination of the ab initio molecular dynamics on the fly with the vibronic density matrix approach in... 

However, expression (5) has to be modified if the emitted electrons carry away some amount of kinetic energy. Consequently, for the simulation of the transient photoionization NeNePo signal, the integrations of the populations of the anionic and cationic states over the entire range of possible excess energies Eq and E2 have to be carried out in order to provide an approximate treatment of continuum. This leads to the following expression of the NeNePo signals ... 

The importance of precise temperature control of the initial cluster ensemble in the NeNePo experiment has been emphasized [104]. Only through the experimental knowledge of the temperature parameter, a detailed comparison with theoretically obtained NeNePo signals becomes possible, and different contributions to the observed nuclear dynamics can be distinguished. Therefore, the original NeNePo experimental semp [90, 211] has been extended to enable the control of cluster temperature in the range between 20 and 300 K [212]. The... 

Figure 1. (a) Schematic representation of the setup for the temperature-controlled NeNePo... 

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