Pulsed infrared desorption of carbon dioxide guest molecules in amorphous solid water

The trapping and release of CO₂ from thin films of amorphous solid water (ASW), < 500 monolayers (ML), were investigated by irradiating vapor-deposited ASW/CO₂ mixtures with pulsed 3424 cm⁻¹ infrared radiation and detecting the ejected molecules using time-of-flight mass spectrometry (TOF-MS). The studies were concerned with the preferential removal of CO2 over H₂O after consecutive low energy radiation pulses (< 1.2 mJ/pulse), and the ejection of water aggregates with high energy pulses (> 1.2 mJ/pulse) which were ionized to protonated water clusters of the form (H₂O)nH⁺ with n = 1 – 6. ❧ Amorphous solid mixtures of ASW/CO₂ with typical composition ratios of 3:1 or 4:1 (H₂O:CO₂) were prepared by co-depositing CO₂ during ASW formation on a MgO(100) substrate. Porous ASW was formed by dosing the MgO substrate with gaseous H₂O at 90 K under ultra-high vacuum conditions. CO₂ multilayers that formed atop the ASW/CO2 film were removed by annealing the film to ~ 112 K and returning it to 90 K. Ablation was achieved through vibrational excitation of the asymmetric stretch of H2O molecules in the ice film. The 3424 cm⁻¹ light was generated by optimizing the second Stokes component in the stimulated Raman scattering (SRS) of 1064 nm radiation from a Nd:YAG laser (10 ns pulses) in D2 (ν = 2987 cm⁻¹). The overall quantum Raman conversion efficiency of ~ 4% was obtained. ❧ Single-pulse irradiation of ASW and H₂O/CO₂ amorphous mixtures resulted in the ejection of both species with high translational energies. The rapid temperature increase after irradiation facilitates CO2 removal via evaporation prior to segregation of CO₂. The high thermal conductivity of the crystalline substrate quenches the heterogeneous nucleation of cavities that take place near the substrate. The presence of small protonated water clusters is interpreted as evidence for trivial fragmentation occurring near the surface of the film. ❧ Despite the significant energy transfer to the substrate, near complete removal of CO2 has been measured through consecutive pulses incident on the same location at the film’s surface. CO₂ removal occurred in as few as 6 pulses, whereas only 10 – 20 ML of H₂O were removed per pulse. The ability of the ice to trap CO₂ changes after each pulse. Whereas the first pulse irradiates an amorphous film, consecutive irradiation of film can induce irreversible changes to the morphology of the irradiated volume thereby inducing partial crystallization. ❧ Future experiments are aimed at exploring guest-host interactions after irradiation, where energy deposition occurs in less time than the characteristic time for thermal transfer to the substrate. Studies where transport and segregation of dopants play a pivotal role in the enhancement of phase explosions and where the energetic release of aggregates through morphology changes is exploited to achieve lift-off of large surface-bound species are also outlined.