Abstract
We report the first observation of single-shot soft x-ray laser induced desorption occurring below the ablation threshold in a thin layer of poly (methyl methacrylate)--PMMA. Irradiated by the focused beam from the Free-electron LASer in Hamburg (FLASH) at 21.7 nm, the samples have been investigated by atomic-force microscope (AFM) enabling the visualization of mild surface modifications caused by the desorption. A model describing non-thermal desorption and ablation has been developed and used to analyze single-shot imprints in PMMA. An intermediate regime of materials removal has been found, confirming model predictions. We also report below-threshold multiple-shot desorption of PMMA induced by high-order harmonics (HOH) at 32 nm. Short-time exposure imprints provide sufficient information about transverse beam profile in HOH's tight focus whereas long-time exposed PMMA exhibits radiation-initiated surface ardening making the beam profile measurement infeasible.
Highlights
In the last decades, rapid development of soft x-ray and XUV lasers opened a relatively new area of the laser-matter interaction
We suggest that the process of PMMA interchain cross-linking [15,28,29,30,31,32], competing with chain scissions, results in a reduction of desorption efficiency and an increase of ablation threshold
Considering the removal efficiency evolving in time, we cannot use PMMA for long-time exposure beam profile measurements unless we understand the mechanisms of surface hardening occurring there; for short-time exposures the PMMA imprint follows more or less reliably the real beam profile
Summary
Rapid development of soft x-ray and XUV lasers opened a relatively new area of the laser-matter interaction. These sources represented, for example, by free-electron lasers [1,2,3], laser-produced plasma-based lasers [4,5], high-order harmonics [6,7], and capillary discharge lasers [8,9], provide a large variety of interaction conditions. During the last several decades, both laser-induced ablation and desorption were extensively studied in IR-vis-UV spectral ranges [10,11]. These are relatively wellunderstood phenomena at longer wavelengths. In addition to material removal, material damage must be associated with changes of optical properties related to structural and chemical surface alteration
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