The dynamics of vibrational wave packets excited in K2 dimers attached to superfluid helium nanodroplets is investigated by means of femtosecond pump–probe spectroscopy. The employed resonant three-photon-ionization scheme is studied in a wide wavelength range and different pathways leading to K2+ formation are identified. While the wave packet dynamics of the electronic ground state is not influenced by the helium environment, perturbations of the electronically excited states are observed. The latter reveal a strong time dependence on the timescale 3–8 ps which directly reflects the dynamics of desorption of K2 off the helium droplets.
Free sodium ammonia clusters Na (NH3) n up to n = 45 were generated in a pickup source by injecting a beam of neutral sodium atoms into the expansion zone of a piezo driven pulsed nozzle. The clusters thus formed are studied by one-photon ionisation in the region of 266 nm to 520 nm, time-of-flight mass spectrometry as well as photoelectron spectrometry. Ionisation thresholds for clusters up to n = 18 and dissociation energies for the neutral Na(NH3) n up to n = 6 are reported.
Helium nanodroplet isolation has been applied to agglomerate alkali clusters at temperatures of 380 mK. The very weak binding to the surface of the droplets allows a selection of only weakly bound, high-spin states. Here we show that larger clusters of alkali atoms in high-spin states can be formed. The lack of strong bonds from pairing electrons makes these systems nonmetallic, van der Waals-like complexes of metal atoms. We find that sodium and potassium readily form such clusters containing up to 25 atoms. In contrast, this process is suppressed for rubidium and cesium. Apparently, for these heavy alkalis, larger high-spin aggregates are not stable and depolarize spontaneously upon cluster formation.
Superfluid helium nanodroplets are doped with potassium atoms to form complexes with the alkali atom residing on the surface of the droplets. Dispersed laser-induced fluorescence spectra of such systems already revealed the formation of M(*)He ( M = Na,K) exciplexes upon electronic excitation [Reho et al., Faraday Discuss. 108, 161 (1997)]. By means of femtosecond pump-probe spectroscopy, this formation process now is followed in real time. We find K(*)He(n = 1) to be formed within 180 fs. Furthermore, the existence of exciplexes with n>1 is quantified suggesting that the first ring around the potassium atom contains four helium atoms.
Nam(H2O)n Clusters (n = 1 . . . 200, m = 1 . . . 50) are formed in a recently build pick-up arrangement. Preformed water clusters traverse a sodium oven, where sodium atoms are picked up. At low sodium vapour pressure (< 1 × 10 −4 mbar) pure Na(H2O)n clusters are observed in the mass spectra. At high sodium vapour pressure (> 1 × 10 −3 mbar) the water cluster pick up more than 50 Na atoms and reaction products Na(NaOH)n (n = 2, 4 . . . 50) dominate the mass spectra. The even number of NaOH units in the products indicate that also in a finite cluster the reaction occurs in pairs as in the macroscopic reaction.
PACS. 33.80.Eh Autoionization, photoionization, and photodetachment -36.40.Jn Reactivity of clusters -82.80.Rt Time of flight mass spectrometry
The photoinduced H-atom-transfer reaction in indole(NH3)
n
clusters has been analyzed by femtosecond time-resolved photoelectron−photoion coincidence spectroscopy. The different contributions to the measured time-dependent ion and electron signals resulting from ionization by one and two probe photons can be discriminated
and analyzed separately. In particular, the distinctively different dynamical behavior observed for clusters
with small (n = 1−3) and larger (n ≥ 4) numbers of ammonia molecules is elucidated. For the small clusters
an ultrafast process with a time constant of about 150 fs is identified and attributed to internal conversion
from the initially excited ππ* state to the πσ* state. In contrast, for the larger clusters (n ≥ 4) such an initial
ultrafast process is not observable probably for Franck−Condon reasons, while a structural rearrangement
mechanism after the H transfer on a time scale of 10 ps is clearly recognized.
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