Dynamics of phase transitions in the UV laser irradiation of molecular solids

Abstract

The main objective of this thesis has been to examine the phase transformations induced in molecular systems (solids) upon irradiation with nanosecond laser pulses. To this end we choose toluene (C6H5CH3), which upon vapor condensation at low temperatures, forms a glass of high optical quality. Thus, its structural changes upon UV (248 nm) laser irradiation can be probed via optical transmission/reflection and imaging. We demonstrate that distinctly different structural processes occur in different fluence ranges. Irradiation in the ~15-30 mJ/cm2 range results in ‘devitrification’, whereas irradiation at higher fluences results in annealed glass re-formation. Devitrification-vitrification can be alternately effected by appropriately tuning the laser fluence. The capability of pulsed laser irradiation for inducing phase transformations in molecular systems provides the potential for high time-resolved studies of the dynamics of molecular glasses. In particular, the dynamics of the laser-induced vitrification/devitrification shows differences from that of the phase changes induced in conventional thermogravimetric studies. We ascribe these differences in the orders-of-magnitude higher rates of heating and cooling involved in laser heating. Most importantly, at higher laser fluences, but still well below the ablation threshold, (homogeneous) bubble formation at 60-100ns is demonstrated. The density and size of the bubbles increases with increasing laser fluence. However, below the ablation threshold, the bubbles eventually decay (at ∼100 -200 ns). In contrast, above the ablation threshold, material ejection (plume formation) is observed following bubble formation. The observation of bubble formation at fluences well below the ablation threshold, in combination with the previous mass spectrometric study of the desorption dynamics, demonstrates that ablation is due to explosive boiling. This is the first demonstration of melting and subsequent superheating of molecular systems in UV laser irradiation which shows that, in the nanosecond case, ablation is due to ‘explosive boiling’ due to overheating of the film. Most importantly, nucleation is observed at fluences/ temperatures well below those indicated by classical nucleation theory. This discrepancy cannot be ascribed to the factors/ limitations invoked in corresponding studies on liquids (e.g. heterogeneous bubble formation). Thus, bubble nucleation/ growth is not quantitatively well described in laser irradiation of solids. Most importantly, a pronounced dependence of bubble nucleation/ growth on the structure of the as-deposited solid is observed. This is most surprising given that melting should be much faster and, thus, bubble formation should not keep any ‘memory’ of the solid structure. This result demonstrates that the simple ‘solid to-liquid-to bubble nucleation-to gas’ model may not be applicable. We suggest that nucleation occurs competitively with melting and, thus, provides the means for the observed dependence. Tentatively, we ascribe these effects to the sensitivity/ dependence of bubble nucleation on the extent of free volume, presence of defects, variations in the crystallinity etc. in the solid. These studies have been extended to the examination of the phase transformations induced in the irradiation of dopant/toluene bi-component cryogenic systems, where dopants include (CH3)2O, C6H12, C10H22. Qualitatively, similar dynamics as for the neat C6H5CH3 films/solids are observed, however, in their case, segregation effects dominate the processes. In previous studies of our group on laser ablation of molecular cryogenic solids it was found that there is qualitative agreement between the experimental results and Molecular Dynamics simulations on these systems. However, this agreement has often been questioned, due to various simplifications introduced by the simulations. Quantitatively, we find that below the ablation threshold, the activation energies of the desorbates are consistent with the energies specified by thermal desorption spectroscopy, while above the ablation threshold there is no such correlation. However, the temperatures estimated at the ablation thresholds are much lower than the temperatures assumed by theoretical models and MD simulations (spinodal temperatures). Both results suggest that ablation is due to ‘phase explosion’ (i.e., by a process which entails long-scale coherent motions of the atoms /particles of the system). We indicate that explosive boiling is the responsible process, entailing localized bubble formation at temperatures between the boiling temperature (at the external pressure) and well below those predicted by theoretical models. Matrix-Assisted-Pulsed-Laser-Evaporation (MAPLE) has emerged as a highly promising technique for the deposition of polymers and biopolymers in intact and functional form. However, mechanistic understanding of the procedure is still limited. Herein, we examine laser (248 nm) induced desorption from condensed CHCl3 solid, which has been employed as potential matrix in MAPLE. In view of the result of the pronounced increase in solid absorptivity, which is attributed to the formation and accumulation of absorbing photoproducts, the mechanistic difficulties indicated in studies employing multipulse irradiation protocols are resolved. A number of additional implications are also discussed. Finally, a preliminary study of explosive boiling in the UV (248 nm) laser irradiation of solutions of polystyrene and metallic nanoparticles of different sizes is presented. In this case, bubble formation at 40-100 ns is observed. Bubble formation is facilitated in the case of the polystyrene nanoparticles, evidently due to the formation of gaseous species by the photolysis of the nanoparticles.