Competition between intramolecular vibrational energy redistribution (IVR) and intermolecular vibrational energy transfer (VET) of excited methylene iodide (CH2I2) in solution has been measured in real time. After excitation of the C−H− stretch overtone and C−H− stretch containing combination bands of CH2I2 between 1.7 and 2.4 μm an increase followed by a decrease in the transient electronic absorption at 400 nm has been monitored. The transient absorption has been attributed to vibrational energy flow from the initially excited degrees of freedom to vibrational states with larger Franck-Condon (FC) factors for the electronic transition (long wavelength wing) and energy loss due to energy transfer to the solvent. A model based upon the dependence of the electronic absorption on the internal energy 〈E〉 of CH2I2 has been used to determine the times for intramolecular vibrational energy redistribution and intermolecular energy transfer to the solvent. In the simplest version of our model the internal energy of the molecule probed by the population of the FC-active modes rises and decays exponentially on a picosecond (ps) time scale, which reflects the initial intramolecular vibrational energy redistribution and the subsequent energy transfer to the solvent. This simple approach was able to accurately describe the measured transient absorption for all solvents and excitation wavelengths. Overall time constants for IVR have been found to be on the order of 9−10 ps, almost independent of the excitation wavelength, the excited modes, and the solvent. In contrast, energy transfer to the solvent takes significantly longer. Overall time constants for VET have been determined in the range between 60 and 120 ps depending on the solvent, the excitation energy, but not on the mode which was initially excited.

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Applications, features, and mechanistic details of laser assisted liquid phase dispersion mass spectrometry are highlighted and discussed. It has been used in the past to directly isolate charged molecular aggregates from the liquid phase and to determine their molecular weight employing sensitive time-of-flight mass spectrometry. The liquid matrix in this MALDI (matrix assisted laser desorption and ionization) type approach consists of a 10 microm diameter free liquid filament in vacuum (or a free droplet) which is excited with a focused infrared laser pulse tuned to match the absorption frequency of the OH-stretch vibration of bulk water near 2.8 microm. Due to these features we will refer to the approach as free liquid matrix assisted laser dispersion of ions or ionic aggregates (IR-FL-MALDI), although also LILBID ("laser induced liquid beam (bead) desorption and ionization") has been proposed early as a descriptive acronym for the technique and may be used alternatively. Low-charge-state macromolecular adducts are isolated in the gas phase from solution via a yet poorly characterized mechanism which sensitively depends upon the laser intensity and wavelength, and after the gentle liquid-to-vacuum transfer the aggregates are analyzed via time-of-flight (TOF) mass spectrometry (MS). Possible mechanisms for the isolation and charging of biomolecules directly from liquid solution are discussed in the present contribution. Recent technical advances such as minimizing the sample consumption, strategies for high throughput mass spectrometry, and coupling of liquid beam MS with HPLC will be highlighted as well. An interesting feature of IR-FL-MALDI is what we call the linear response, i.e., a surprising linearity of the gas phase mass signal on the solution concentration over many orders of magnitude for a large number of biomolecular systems as well as ions. Due to these features the approach may be regarded as a true solution probing spectroscopy, which enables elegant biokinetic studies. Several experiments in which time resolved IR-FL-MALDI-MS has recently been employed successfully are given. A particular highlight is the possibility to quantitatively detect oxidation states in solution, which clearly distinguishes the present approach from other established MS source concepts. Due to the good matrix tolerance also proteins in complex mixtures can be monitored quantitatively.

Transient femtosecond IR-pump-UV-probe spectroscopy is employed to investigate the intramolecular vibrational energy redistribution (IVR) and the intermolecular vibrational energy transfer (VET) of benzene, toluene (CH 3 -C 6 H 5 ), and R,R,R,-trifluorotoluene (CF 3 -C 6 H 5 ) selectively excited in overtones or combination bands of C-H stretch vibrations in solution. Global IVR and VET rate coefficients are derived from the measured transient absorption profiles using a simple kinetic model. The study reveals the effect of a methyl rotor and the effect of methyl rotor fluorination on the mechanisms and time scales of IVR and VET in aromatic model systems. For the present case, it turned out that the methyl rotor in toluene is not simply an enhancer for IVR; however, its fluorination accelerates IVR significantly. These results suggest that the methyl rotor effect on an aromatic ring in solution is more subtle than expected from previous gas-phase studies. In particular, the corresponding relaxation rates in this case are not simply governed by the number of lowest order resonances, such as found for aliphatic molecules. Instead, in aromatic molecules also, the very large number of higher order anharmonic resonances may play a pronounced role. Because the IVR rates are not at all correlated with the total density of states, we conclude that intramolecular vibrational energy relaxation of a zeroth order C-H stretch overtone or combination vibration in these molecules is not in its statistical limit and that hierarchical IVR, such as known for isolated molecules, still survives to some extent in solution.Our results further suggest that VET rates are not always simply correlated with the lowest frequency modes of the molecules.

The hydration of gas phase Na+ ions produced by electrospray ionization was investigated via high resolution time-of-flight mass spectrometry. Water clusters with up to 80 H2O units have been observed with attention to special peculiarities of mass peaks and structures in the envelope of the mass spectra. For solvated Na+-ions no particularly prominent peaks corresponding to "magic numbers" have been observed in the spectra. In addition, the mass spectra of Na+(H2O)(n) aggregates are compared with those of the H3O+(H2O)(n) system. As a very first step towards a theoretical understanding of the intensities of the experimental mass spectra we have calculated global minimum structures for the Na+(H2O)(n) system with n=4-25, using global geometry optimization methods and a simple model potential for this system. Structural and energetic trends and the systematic build-up of solvent shells of the clusters were studied. Within our simple theoretical model we have not observed (symmetric) clathratelike structures with a central Na+-ion inside as global minimum structures. Instead, a structural transition for n between 17 and 18 water units was found, in agreement with the experimental observations. The unimportance of clathratelike structures and the competition between the two structural principles discovered in this work are proposed as an explanation for the notorious lack of "magic" numbers in the mass spectrum of Na+(H2O)(n) clusters. (C) 2002 American Institute of Physics