Sum frequency mixing of fluorescence and ∼1300 nm gate pulses, in a thin β-barium borate crystal and non-collinear type II geometry, is quantified as part of a femtosecond fluorimeter [X.-X. Zhang et al., Rev. Sci. Instrum. 82, 063108 (2011)]. For a series of fixed phasematching angles, the upconversion efficiency is measured depending on fluorescence wavelength. Two useful orientations of the crystal are related by rotation around the surface normal. Orientation A has higher efficiency (factor ∼3) compared to B at the cost of some loss of spectral coverage for a given crystal angle. It should be used when subtle changes of an otherwise stationary emission band are to be monitored. With orientation B, the fluorescence range λ F > 420-750 nm is covered with a single setting of the crystal and less gate scatter around time zero. The accuracy of determining an instantaneous emission band shape is demonstrated by comparing results from two laboratories. Published by AIP Publishing. [http://dx

Vibrational predissociation of I, X, (X = Ne, Ar) van der Waals clusters are studied in realtime using picosecond pump-probe (LIF) and molecular beam techniques. The state-to-state rates of vibrational predissociation are measured for specific vibrational levels v] by monitoring the rise of nascent I,. Here, we report our study of k(uj.uj) 1,X(&u;) + I, (B,vj) + x.For I, Ne, the values of r = k -' (v],z$) decrease nonlinearly from 2 16 ps for v] = 13 to 53 ps for vi = 23. For 1,Ar (B,vj), which undergo electronic and vibrational predissociation, the risetime of the nascent I, is found to be 70 ps when 0; = 18 and 77 ps when v] = 21. A number of theoretical models for vibrational predissociation (the energy-gap law, the momentum-gap law, quantum and classical calculations) are compared with our experimental results in an attempt to understand the key features of the dynamics and the potential energy surface.

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In this paper (III) we report real-time studies of the picosecond dynamics of iodine in Ne clusters IzNe,( Iz = 2-4) + IT + nNe. The results are discussed in relation to vibrational predissociation (VP), basic to the 1,X systems, and to the onset of intramolecular vibrational-energy redistribution (IVR). The latter process, which is a precursor for the evaporation of the host atoms or for further fragmentation, is found to be increasingly effective as the cluster size increases; low-energy van der Waals modes act as the accepting (bath) modes. The reaction dynamics for I,Ne, are examined and quantitatively compared to a simple model which describes the dynamics as consecutive bond breaking. On this basis, it is concluded that the onset of energy redistribution is observed in 12Ne2. Comparison of 1,Ne and I,Ne2 to larger clusters (n=3,4) is accomplished by introducing an overall effective reaction rate. From measurements of the rates and their dependence on uf, the initial quantum number of the I3 stretch, we are able to examine the dynamics of direct fragmentation and evaporation, and compare with theory. I. INTRODUCTIONThe reaction dynamics of size-selected van der Waals (vdW) clusters form a basis for understanding the interplay between intramolecular vibrational-energy redistribution (IVR) and the direct fragmentation by vibrational predissociation (VP). For the case at hand, halogen-rare gas clusters, there are two interesting regimes for the dynamics which are determined by the effective size of the cluster. For small clusters, the "small-molecule" description of the dynamics may apply as in the case of I,He, I,Ne, and 1,Ar (n= 1 ), discussed in Papers (I) ' and (II).2 In this case, the dynamics of VP are governed only by the coupling between the initially prepared metastable bound state and the final dissociation continuum (see Fig. 1).For the larger clusters, the density of states plays an important role in the dynamics, perhaps reaching the statistical limit of the "large-molecule" case. The initially prepared state in this case couples not only to the mode(s) of the reactive coordinate(s), but also to nonreactive "bath modes" whose number increases with n. The transition from one regime to the other is interesting to explore, both theoretically and experimentally.An important question concerning the larger clusters (n > 1) relates to the mechanism: Do the rare gas atoms directly break one at a time (a sequential mechanism), or do they indirectly fragment following energy redistribution? And if indirect fragmentation is the pathway, does it occur by VP or evaporation? Consider, for example, 12Ne2. I,*Ne(vf--l)+It(uf-2)+Ne,where ui is the vibrational quantum number of the initial iodine stretch. These direct pathways in Eqs. ( 1) and (2) contrast a path in which energy is dissipated to the modes of the surrounding atoms/I, cluster. Because of the presence of the two neons, energy redistribution from the I2 stretch to nonreactive modes could compete with direct VP, leading to a new channel I~Ne2(uf)~I~Ne~(...

We report on first results of real-time studies on the photodissociation dynamics of electronically excited Cr(CO)6 and Cr(CO)6·(CH3OH) n heteroclusters generated in a molecular beam. The red part of the 1T1u metal to ligand charge-transfer state at 280 nm is excited, and the resulting photofragments are detected by multiphoton ionization. The decarbonylation dynamics is found to be much faster than the cross correlation width (230 fs) of our pump and probe pulses. The observed fragmentation pattern and its dynamics can tentatively be attributed to two competing dissociation channels, both starting from the primary photofragment Cr(CO)5, a simultaneous and a sequential channel. For the clusters we find that both these channels appear to be operative depending on pump laser intensity and that decarbonylation precedes evaporative loss of solvent molecules, which takes place on the picosecond time scale. Solvated cluster species as well as coordinatively unsaturated carbonyls are stabilized by evaporation.