Clusters vibrational predissociation

U. Buck and I. Ettischer, Vibrational predissociation spectra of size selected methanol clusters New experimental results../. Chem. Phys. 108, 33 38 (1998). 

I.r. study of hetero-dimers formed from HCl, DCl, HF, and DF with (CH3)20, CH3OH, and (CH3)3COH Vibrational predissociation in HF.O(CH3)2 t I.r. absorption spectra of (H20)2 measured in the region of the O-H stretch by monitoring predissociation Rotational spectra and constants for HjO.HCN t Two-photon MPI/m.s. study of NHj clusters t VUV photoionization/m.s. study. Observation of unprotonated cluster ions (NHjlJ and (NHj) in addition to (NH3)2 and... 

I.r. laser spectroscopy and quadrupole mass spectrometry were used by Fischer et al. to study vibrational predissociation of clusters of C2H4, and CsHg, but-l-ene, cis- and trans-but-2-ene, and isobutene. They obtained spectra in the range 2900—3200 cm and for C2H4 clusters predissociation was observed to result from excitation near the v-i, and vg fundamentals and the i 2 + V12 combination band. The vibrational bands were observed to have Lorentzian lineshapes with IWHM of ca. 5 cm. A homogeneous broadening mechanism was assumed and the widths were used to calculate excited-state lifetimes. Valentini and co-workers studied the predissociation of C2H4 clusters at 950 cm in a crossed laser/molecular beam apparatus. 

C2H4), t Vibrational predissociation spectra of C2H4 clusters... 

I.r. predissociation spectra of Sip4 and CFjBr clusters 933 Vibrational predissociation of pyrimidine clusters. 934... 

The ethylene cluster experiments and interpretation are discussed below as follows. Section II summarizes important aspects of the experiment. Section III gives details on the spectral analysis with respect to inhomogeneous versus homogenous broadening as well as saturation effects. Section IV discusses structural information obtained from the spectra. Section V discusses the vibrational predissociation mechanism for these complexes and compares the results to other systems. 

The optothermal spectrum will faithfully represent the absorption spectrum provided that the molecules do not fluoresce prior to arrival at the detector. Fluorescence is not a problem because infrared fluorescence lifetimes are of the order of milliseconds. The transit time from the laser-molecular beam interaction region to the detector is tens of ps. Furthermore, it is possible to determine if the absorbing species is a stable monomer, or weakly bound cluster. When a stable monomer absorbs a photon, the amount of energy measured by the bolometer increases. When a weakly-bound species absorbs a photon greater than the dissociation energy, vibrational predissociation will take place and the dissociating fragments will not hit the bolometer element. 

Gough T E, Miller R E and Scoles G 1981 Infrared spectra and vibrational predissociation of (C02)n clusters using laser-molecular beam techniques J. Phys. Chem. 85 4041-6... 

Using photoelectron detection in a femtochemistry arrangement, we studied size-selected clusters of ionic systems, covering the transition from gas phase to condensed phase dynamics [6]. We investigated the solvent effect on Oj dissociation dynamics, and observed that the addition of one solvent (O2, N2, Xe or N2O) gives very different effects on the dynamics of the nuclear motion, whose wave packet bifurcates in two channels. These real time studies of the one-solvent dynamics provide the time scale of the distinctive processes of electron recombination and bond rupture, and vibrational predissociation— with both... 

Class A clusters are characterized by weak bonds (D = 200-1000 cm ) and low density of states. When the high-frequency monomer modes are excited with infrared radiation these dimers tend to decay via vibrational predissociation. IVR is inhibited... 

While the use of direct absorption methods has grown, indirect action spectroscopic methods continue to be widely and successfully used in the study of neutral molecular clusters. As mentioned earlier, there are two commonly used detection methods, mass spectrometers and bolometers. Because of the variety of mass-spectroscopic methods, there is an equally wide range of techniques used in neutral cluster spectroscopy. One of the oldest among these involves electron-impact mass spectrometry of a cw neutral beam combined with vibrational predissociation spectroscopy using a tunable cw or pulsed laser. The advent of continuously tunable infrared sources (such as color center lasers and LiNbOa optical parametric oscillators) allowed for detailed studies of size and composition variation in neutral clusters. However, fragmentation of the clusters within the ionizer of the mass spectrometer, severely limited the identification of particular clusters with specific masses. Isotopic methods were able to mitigate some of the limitations, but only in a few cases. 

Cavity ring down (CRD) spectroscopy, having proven to be a very sensitive method for detecting molecular species in a wide variety of environments, has also been applied to the mid infrared vibrational spectroscopy of hydrogen-bonded clusters of water " and alcohols.As a direct absorption method, it can be used to quantitatively measure important molecular properties, such as absorption cross sections and coefficients. Knowing these properties, as a function of cluster size and structure, is useful in making the connection to the condensed phase. The sensitive detection of methanol clusters, as shown in Fig. 13, is of considerable importance. These particular measurements nicely complement the action spectra of methanol clusters, detected by depletion of mass-detected signal via vibrational predissociation. 

Very few experiments providing direct information about the pathways of intramolecular energy flow in van der Waals complexes have been reported to date. In most cases studied so far, distinct channels for IVR and vibrational predissociation (VP) could not be detected separately. Among the polyatomic van der Waals complexes the cluster T Ar of s-tetrazine and argon is one of the few favourable exceptions. It exhibits several channels for... 

Photoinduced unimolecular decomposition reactions are among the simplest reactions which can be studied experimentally and theoretically. One such reaction which has received considerable attention is the vibrational predissociation of small isolated van der Waals (vdW) clusters for which one molecule is a chromophore and the other is a small "solvent" molecule. Two dynamical events may transpire in such a system following the initial photoexcitation to Si vibronic levels vibrational energy may be redistributed to modes other than the optically accessed zero order chromophore states and at sufficient energies the cluster may dissociate. The fundamental theoretical understanding of these two kinetic processes should be accessible in terms of Fermi s golden rulel and unimolecular reaction rate2 concepts. 

The linearly structure (HCN)2 cluster was studied by SCF and Cl calculations. Results available so far are within the framework of a collinear model only, given the fact that a full, reliable potential surface for this system is not available yet. We note that calculations of vibrational predissociation lifetimes that we carried out for this system in the framework of a preliminary collinear treatment have shown that to obtain lifetimes compatible with the experimental ones, the energy released from the excited C-H stretching mode must necessarily be mostly dumped into C-H bending and rotational excitation of the fragments. This means that the collinear treatment is unacceptable even qualitatively for describing the predissociation dynamics. However a collinear treatment may be a reasonable approximation for the spectroscopy of those modes that do not involve bending excitations. Table 1 shows the results of SCF and Cl calculations for several of the transition frequencies. Jacobi ("collision") coordinates seemed an intuitively reasonable choice for the SCF calculation, and were employed here. 

Van der Waals hetero-clusters are ideal model systems for the detailed study of a variety of photophysical and photochemical processes such as energy transfer, vibrational predissociation, fine-structure relaxation, and half-collision chemical reactions. In addition, the spectroscopy of these species provides central information on intermolecular forces and their additivity properties. 

R. Ahlrichs, S. Brode, U. Buck, M. DeKieviet, C. Lauenstein, A. Rudolph, B. Schmidt, The structure of C2H4 clusters from theoretical interaction potentials and vibrational predissociation data. Z. Phys. D 15(4), 341-351 (1990)... 

Hammer Ml, Diken EG, Roscioli JR, Johnson MA, Myshtddn EM, Jordan KD, McCoy AB, Bowman JM, Carter S (2005) The vibrational predissociation spectra of the HsO -RG (RG = Ar,Ne) clusters correlation of solvent perturbations in the free OH and shmed proton transitions of the Zundel ion. J Chem Phys 122 244301... 

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