No abstract

The dynamics of molecular multiphoton ionization and fragmentation of a diatomic molecule (Na2) have been studied in molecular beam experiments. Femtosecond laser pulses from an amplified colliding-pulse moddocked (CPM) ring dye laser are employed to induce and probe the molecular transitions. The final continuum states are analyzed by photoelectron spectroscopy, by ion mass spectrometry and by measuring the kinetic energy of the formed ionic fragments. Pumpprobe spectra employing 70-fs laser pulses have been measured to study the time dependence of molecular multiphoton ionization and fragmentation. The oscillatory structure of the transient spectra showing the dynamics on the femtosecond time scale can best be understood in terms of the motion of wave packets in bound molecular potentials. The transient Na2+ ionization and the transient Na+ fragmentation spectra show that contributions from direct photoionization of a singly excited electronic state and from excitation and autoionization of a bound doubly excited molecular state determine the time evolution of molecular multiphoton ionization.

Femtosecond laser pulses are applied to the study of the dynamics and the pathways of multiphotoninduced ionization, autoionization, and fragmentation of Na2 in molecular-beam experiments. In particular, we report on first results obtained studying electronic autoionization (leading to Na2 + +e~) and autoionization-induced fragmentation (leading to Na + +Na+e ~) of a bound doubly excited molecular state. The final continuum states are analyzed by photoelectron spectroscopy and by measuring the mass and the released kinetic energy of the corresponding ionic fragments with a time-of-flight arrangement.PACS numbers: 33.60.-q, 33.80.EhIn contrast to the well studied two-electron excited states of various atomic systems, the electronic autoionization of doubly excited molecular states is still a field where very little is known. Doubly excited states are crucial to the understanding of the dynamics of diatomic molecules at energies where electronic autoionization, ionic fragmentation, and neutral fragmentation compete. The study of the coupling of the different continua of fragmentation and ionization is also of great theoretical interest, because of the need for an extension of multichannel quantum-defect theory (MQDT) to treat doubly excited electronic states, dissociating states, and autoionizing Rydberg states together.Interesting studies of the dissociation and autoionization of unbound doubly excited states in H2 have recently been reported by several groups. 1 The first spectroscopic study of a bound doubly excited state, the l Lf (3

We report time-resolved experiments studying the dynamics of the Na3 B-X system. Femtosecond pump-probe techniques combined with ion time-of-flight (TOF) and zero kinetic energy (ZEKE) photoelectron spectroscopy allow us to observe the three-dimensional wavepacket motion in the excited Na3 B state and in the Na3 X state. The ground state wavepacket is induced by stimulated emission pumping during the pump pulse. The X-state dynamics is dominated by the three vibrational modes of the Na3. Furthermore we observed pseudorotational wavepacket motion in the B state. We do not observe a fragmentation of the Bstate within a time interval of 10 ps.

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...