Savas Georgiou received his B.Sc. in chemistry and mathematics from Knox College, Illinois (1983), and his Ph.D. in physical chemistry (on the study of photodissociation dynamics of organometallic compounds) from the University of Utah (1988). He subsequently performed postdoctoral work on Raman spectroscopy of biomolecules at Princeton University.

A combined experimental and molecular dynamics simulation study of laser ablation of a model bicomponent system with solutes of different volatility provides a consistent picture of the mechanisms of material ejection. The comparison of the ejection yields shows that there are two distinct regimes of molecular ejection, desorption at low laser fluences, and a collective ejection of a volume of material or ablation at higher fluences. Ejection of volatile solutes dominates in the desorption regime, whereas nonvolatile solutes are ejected only in the ablation regime.

Time-resolved examination of the surface temperature was performed for various MALDI matrices, irradiated by UV (λ ) 355 nm, τ ) 5 ns) laser pulses. The temperature was measured by detecting the emitted blackbody radiation from the sample surface. The maximum surface temperature was observed to occur during the falling edge of the laser pulse, with a delay of 2 ns with respect to the laser pulse peak. Its value was found to depend on the kind of the matrix, the thickness of the sample and the laser fluence. For 2,5-dihydroxybenzoic acid, the most popular matrix, the peak temperature was found to be ∼850 and 1100 K at the lower (10 J/m 2 ) and the higher (100 J/m 2 ) fluences, respectively, used in this work.

The ablation of thick C6H5Cl films at 248 nm is studied with emphasis on the formation efficiency and desorption dynamics of the observed photoproducts. To this end, the desorbates are probed as a function of the laser fluence via time-of-flight quadrupole mass spectrometry. At low fluences, indicated to represent the subablation regime, we observe a strong induction effect for the parent molecule and desorption of only one new species, namely of HCl. Phenyl products are indicated to be formed, but they start desorbing only as parent peak induction becomes significant. In contrast, at higher fluences, desorption of the parent molecule from freshly deposited films is intense from the very first laser pulse, and a number of additional photoproducts are observed. Three main species, namely (C6H5)2, C6H4Cl2, and C12H9Cl, are observed, while Cl and C12H8Cl2 are detected mainly at higher fluences. All products are compatible with the known gas phase and solution chemistry of C6H5Cl, indicating that no new reaction channels open up above the ablation threshold. Over the full fluence range, the phenyl photoproducts differ significantly from HCl in their desorption observables. Specifically, the phenyl photoproducts are described by nearly the same velocity distribution as the parent molecule and their desorption efficiency closely correlates to that of C6H5Cl. In contrast, HCl exhibits a much more complicated translational behavior, but generally it is nearly equilibrated with the parent molecule. We argue that these differences derive mainly from the much higher volatility of HCl as compared with that of the phenyl photoproducts. Thus, for the phenyl derivatives, the “driving force” for their ejection is evidently their entrainment in the C6H5Cl jet, whereas for HCl, being quite volatile, its desorption appears to be largely independent of that of the parent molecule.

It has been proposed recently that hyperfine depolarization of selected molecular rotational states can be used to produce molecules with highly polarized nuclear spins, and formulas for two distinct nuclei have been given in the limit of hierarchical approximation. Here we present the general, nonhierarchical coupling formalism for the derivation of the H ͓i͔ ͑k͒ ͑I , t͒ factors that govern the time dependence of the nuclear polarizations.The described technique, especially when combined with methods that polarize the electronic angular momentum, can lead to the production of highly polarized atoms from molecular photodissociation, at densities close to that of the parent molecule. In addition, we calculate the time dependence of the H and F nuclear spin polarizations, following the pulsed-laser preparation of the HF ͑v =1,J =1,m =1͒ state. It is shown that the polarization of the F and H nuclear spins attain values of about 85% and 70% at time delays of about 1 s and 4 s, respectively. Similar results are shown for the pulsed preparation of the DF ͑v =0,J =1,m =1͒ state, demonstrating the D atoms can also be significantly polarized.