Fourier Transform Infrared Spectroscopy and Temperature Program Desorption(TPD) studies of Amorphous Solid Water(ASW)as a function of dosing temperature
EMSL Project ID
2415
Abstract
ASW is a metastable phase of water, which forms when depositing water vapor on a substrate below about 130K. ASW deposited at low temperatures (30-88K) on Au forms three-dimensional clusters at the bulk crystallization temperature, which matches TPD measurements done at EMSL. ASW deposited on Au(111) at 100K forms a uniform layer over the substrate. Clusters form when annealing the ASW to 120K. Initial work using a needle sensor Atomic Force Microscopy (AFM) at the EMSL showed these unexpected results in the kinetics of crystallization. This work was confirmed at UW using a non-contact AFM (ncAFM). Further experiments at UW showed that similar clustering happens on mica. Work done at EMSL using the needle sensor AFM showed that water deposited at 100K in the AFM at an incidence angle of 67 degrees on Au the ASW uniformly covers the plateaus of Au on mica. When the substrate was annealed to 120K, the ASW ceases to uniformly cover the substrate and forms three-dimensional clusters. Later, the UW?s ncAFM confirmed the observations done at EMSL and found similar results on mica. The water dosing conditions, (flux, angle, and substrate temperature) were almost identical between the two apparatuses. When the ASW is deposited at 88K or colder on Au, annealing to 120K doesn't change the surface morphology. For these depositions, AFM measurements show no change until 140K, when bulk diffusion becomes significant at laboratory time scales: then clusters form. Although ncAFM is capable of high spatial resolution, we are unable to determine the crystallinity of our initial films or the clusters that occur upon annealing. We believe that surface diffusion causes the transition from flat ASW to 3D clusters. I intend to determine the surface self-diffusion rate of ASW and investigate the crystallization kinetics of ASW on non-wetting substrates for my Ph.D. dissertation. This requires additional information on the kinetics of ASW crystallization on non-wetting substrates. The molecular beam scattering machines located in the EMSL can provide important data on these processes by combining carefully controlled water dosing conditions and high quality desorption and crystallization kinetics by TPD and FTIR. TPD and FTIR results from EMSL indicate that crystallization of thicker films of ASW from normal incidence vapor deposition on decane occurs at the same time as de-wetting of the H2O film from the decane substrate for depositions around 80K. Enquist followed the crystallization kinetics using FTIR for amorphous solid D2O deposited on self-assembled alkanethiols, but he didn?t carefully study the crystallization as a function of deposition temperature. To investigate these kinetics we plan to form ASW thin films by molecular beam deposition of water on decane (C10H22) deposited on Pt. The decane provides a reproducible non-wetting substrate that desorbs from the Pt at temperatures above which H2O desorbs from Pt. The crystallinity of the H2O can be monitored by changes in the infrared line shapes in the OH stretching region. We intend to vary the water dose flux, substrate temperature, and annealing temperature on thin (< 100 bilayer) ASW layers. TPD will be used to follow the activation energy required for a single water molecule to come off of the substrate. Previous studies have shown that there is a difference in the desorption rate of crystalline ice vs. ASW, this appears as a bump in the TPD spectrum. We plan on watching how this bump changes as a function of deposition temperature. This work will directly impact future work at the UW using AFM by determining where differences in the macroscopic kinetics are observed. This work will be carried out as a collaboration with the group of Bruce Kay in the EMSL as part of their ongoing BES Chemical Physics supported research on nanoscale ice films and will directly impact future work at the UW using AFM by determining where differences in the macroscopic kinetics are observed.
Project Details
Project type
Exploratory Research
Start Date
2002-06-01
End Date
2003-06-03
Status
Closed
Released Data Link
Team
Principal Investigator
Related Publications
Noncontact Atomic Force Microscopy Studies of Ultrathin Films of Amorphous Solid Water Deposited on Au(111)