UV laser ablation of molecular cryogenic films

Abstract

In the irradiation of condensed phases at high laser irradiances, the ejection/removal of a high quantity of material is observed, and this is usually denoted by the term laser pulsed ablation (or explosive desorption). This phenomenon has found extensive and very important applications, but its mechanistic understanding is very poor. The present work addresses two questions: Can a physically significant criterion be established for defining the phenomenon? How physical and chemical processes in this regime differ from those at low light irradiances where conventional photophysics/chemistry holds? To address these questions, model systems composed of simple organic compounds frozen at low temperatures are studied. Time-of-flight mass spectrometry is employed to probe ejection efficiencies as a function of laser parameters and optical/imaging techniques are employed for probing morphological/structural changes induced in the film. Through the comparative examination of nonabsorbing dopants of different binding energies to the matrix (C6H5CH3), it is demonstrated that in the UV irradiation, two fluence ranges can be delineated where different material ejection mechanisms operate. At low fluences, ejection is consistent with surface vaporization/desorption, whereas at higher fluences, ejection is clearly different, entailing the unselective ejection of a volume of material. Therefore, the term “ablation” should be employed for denoting exclusively this higher fluence range. This differentiation has important implications for the applications of ablation. The ejection of dopants strongly-bound to the matrix is established as a direct experimental criterion for the identification of ablative regime. By the use of optical techniques, film melting and gas bubble formation is demonstrated below the threshold. Consequently, ablation of molecular solids withnanosecond pulses is due to “explosive boiling”. Besides the implications for ablation, this demonstration may provide new ways for studying organic compounds in metastable states. The translational distributions of the ejected desorbates in the UV irradiation are shown to exhibit distinctly different dependences above vs. below the threshold. In contrast to most models, in the ablation regime they are shown to be determined by the gas phase collisional dynamics and the ejection process, as well. A number of additional features are determined that can provide the basis for the development of more elaborate theoretical models on the flow dynamics of the ejected material. The photochemical processes are studied for prototypical organic compound (C6H5Cl). It is shown that UV ablation does not modify qualitatively the reactivity patterns. However, quantitatively, significant deviations from conventional kinetics are noted, suggesting that the description of reactivity in UV ablation requires consideration of the parallel influence of the thermal and physical processes. In particular, product formation is shown to occur mainly in the film and as a result, photolysis yields and photoproduct formation efficiencies depend critically on the time scale of material ejection. Finally, preliminary studies on the femtosecond irradiation of these systems show significant differentiations from the nanosecond case, suggesting that laser pulse width is crucial for the desorption and ablation ejection processes.