Femtosecond probing of sodium cluster ionNan+fragmentation

Abstract

We report on the first femtosecond time-resolved experiments in cluster physics. The photofragmentation dynamics of small sodium cluster ions Na" + have been studied with pump-probe techniques. Ultrashort laser pulses of 60-fs duration are employed to photoionize the sodium clusters and to probe the photofragments. We find that the ejection of neutral dimer Na2 and, observed for the first time, neutral trimer Na3 photofragments occur on ultrashort time scales of 2.5 and 0.4 ps, respectively. This and the absence of cluster heating reveals that direct photoinduced fragmentation processes are important at short times rather than the statistical unimolecular decay.PACS numbers: 36.40.+d, 33.80.-bThe stability and fragmentation dynamics of metal cluster ions formed through laser photoionization is a major issue in cluster physics and has been discussed in many experimental and theoretical publications [ll. Experiments with mass-selected cluster ion beams have shown evidence that fragmentation proceeds by evaporative processes in time regimes ranging from nanoseconds to microseconds [2]. In the discussion of the stability of metal cluster ions against fragmentation and their inherent time scales, it is often assumed that photoninduced electronic excitation is strongly coupled and relaxed to the internal cluster modes. In this model the excess energy from successive absorption of photons in photoionization with nanosecond lasers is quickly redistributed between the vibrational modes of the cluster ion, so that the resulting total energy is above the fragmentation threshold. Since the probability of localizing sufficient energy in a particular fragmentation coordinate is low, this leads to the nanosecond-microsecond fragmentation times observed in the unimolecular decay.In particular for sodium and potassium cluster ions, Brechignac et al. [3] have reported that the predominant fragmentation channels, the evaporation of neutral monomers and/or neutral dimers for even and odd numbered cluster ions, respectively, are associated with microsecond fragmentation times. They also found evidence for sequential evaporation of monomers or dimers and for a change of cluster size distribution toward lower masses with increasing laser power due to cluster heating and evaporative cooling.Time-resolved studies of cluster dynamics are rare. Lineberger and co-workers [4] reported on studies of the photodissociation and recombination dynamics of \i~ in mass selected l2~(C02) w clusters with picosecond pump-probe techniques. Studies of the proton transfer dynamics in finite-size molecular clusters as well as fragmentation dynamics of van der Waals clusters using picosecond pump-probe techniques have been reported by Zewail and co-workers [5]. Lifetimes and relaxation processes in electronically excited states of Na3 have been measured by Broyer et al.[6] using nanosecond-timedelayed two-photon ionization techniques.We report here on the first time-resolved studies of the dynamics of metal clusters during and immediately following the pho...

Picosecond photoelectron studies of cluster dynamics

“…Riley and coworkers recorded photoelectron spectra from electronically excited alkyl anilines that broadened as a function of time, which they attributed to IVR [16]. Other examples of TR/PES have been reported by the groups of Kimura [17], Gerber [18], Weber [19], and ourselves [6,14]. Applications of TR/PES to cluster dynamics include studies of IVR and vibrational predissociation in aniline-CH 4 by Knee and coworkers [20], coherent excitation in Na 2 and Na 3 by Gerber and coworkers [18], and solvent reorganization in phenol-(NH3) n by our group [6].…”

Collision induced dissociation of stored gold cluster ions

“…This leads to typical unimolecular decay times in the microsecond range. Gerber and coworkers found a much shorter time scale on the order of picoseconds for the emission ofNa 2 after irradiation of larger sodium clusters with femtosecond lasers [44].…”

Ultrafast photodissociation of Kn=3...9 clusters

“…The reason for this tremendous interest is, that small clusters are relatively easy to handle theoretically [5] as well as experimentally [6,7]. Compared to simple molecules, however, the complexity of the clusters is sufficient to receive new insight into phenomena like catalysis [8][9][10], solvation [11], reactivity [12][13][14][15], energy transfer [16] or fragmentation [17][18][19][20]. With the availability of ultrafast laser systems it then even became possible to investigate the real-time dynamics of these processes.…”