Amorphous Solid Water (ASW): Pulsed Laser Ablation of ASW/CO2 Thin Films

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• 作者：Oscar Rebolledo-Mayoral ; Jaimie Stomberg ; Stephanie McKean ; H. Reisler ; C. Wittig
• 刊名：The Journal of Physical Chemistry C
• 出版年：2012
• 出版时间：January 12, 2012
• 年：2012
• 卷：116
• 期：1
• 页码：563-569
• 全文大小：918K
• 年卷期：v.116,no.1(January 12, 2012)
• ISSN：1932-7455

文摘

Thin films composed of 400鈥?00 monolayers (ML) of either amorphous solid water (ASW) or ASW/CO₂ mixtures are grown atop a MgO(100) substrate under ultrahigh vacuum conditions. Samples are irradiated at an infrared frequency of 3424 cm^鈥?, which lies within the broad OH stretch band of condensed water. Ablation is achieved using 10 ns pulses whose energy (<2.7 mJ) is focused to a beam waist of approximately 0.5 mm. By using a time-of-flight mass spectrometer to monitor ablated material, excellent single-shot detection is demonstrated. This capability is essential because, in general, the first infrared pulse can induce irreversible changes throughout the irradiated volume. With ASW/CO₂ samples, CO₂ is released preferentially. This is not surprising in light of the metastability of the samples. Indeed, repeated irradiation of the same spot can rid the sample of essentially all of the CO₂ in as little as a few pulses, whereas only 10鈥?0 ML of H₂O are removed per pulse. The influence of the substrate is profound. It cools the sample efficiently because the characteristic time for heat transfer to the substrate is much less than the infrared pulse duration. This creates temperature gradients, thereby quenching processes such as explosive boiling (phase explosion) and the heterogeneous nucleation of cavities that take place at lower depths in significantly thicker samples, i.e., with sufficient inertial confinement. This efficient quenching accounts for the fact that only 10鈥?0 ML of H₂O are removed per pulse. The presence of small protonated water cluster ions in the mass spectra is interpreted as evidence for the trivial fragmentation mechanism examined assiduously by Lewis and co-workers. Mixed samples such as ASW/CO₂, where species segregation plays a pivotal role, add interesting and potentially useful dimensions to the ablation phenomenon.