Raman and Scanning Electron Microscopy Snow Research - March Update

            Exciting renovations are in process at the Ault Lab, which are detailed in the previous February blog post. As Raman instrumentation is sensitive to dust and other particles, it is currently being held in storage. We will be switching focus to scanning electron microscopy for imaging and elemental analysis of snow until renovations are complete.

SEM differs significantly from Raman in relation to our project in two ways: the need for a vacuum and the tradeoffs required for proper imaging. SEM essentially is based around receiving backscatter from an electron beam focused on a sample which cannot be done at ambient pressures. This is particularly problematic for snow samples as they will sublimate at a quick rate in a vacuum. We hope to mitigate the issue through a variable-pressure chamber that will allow usage of only a partial vacuum. Furthermore, a coating such as gold is often required to offset charging of the sample; while coating allows crisp imaging of structures, it masks the sample which prevents elemental analysis. Our plan is to use a black carbon tape to reduce charging and liberal amounts of liquid nitrogen to replace the use of coating.

 Wergin, W.P.

Observations of snow crystals using low-temperature SEM

Journal: Scanning

1995

Along with sublimating, melting is a more significant problem in SEM compared to Raman, we expect to see deformed crystal structures similar to previous SEM snow image research 

We are continuing to read relevant literature on the subject of snow research, with specific emphasis on Dr. Sobron whose interests lie in the potential of Ramman in planetary exploration. In a paper entitled Low-Temperature Raman Spectroscopy of Materials Relevant for Planetary Exploration from the 42nd Lunar and Planetary Science Conference of 2011, he notes that Raman peaks are affected by temperature. Specifically as temperature decreases, peak widths become narrower, peak positions decrease, and intensities weaken in ice crystals. These changes are due to strengthened hydrogen bonding and weakened O-H bonds. As we will be working at temperatures well below freezing, we will need to account for these findings.

Preceding the renovations, we were able to continue gaining experience on Raman using silicon and quartz standards. The peak strengths were strangely muted in comparison to similar spectrum taken previously in the year. However, we were eventually able to successfully have the quartz background peaks to appear after adjusting the laser to a higher setting. Currently we are in the process of scheduling training on SEM and expect to start within one to two weeks.

In the meantime, we are pondering what additional factors must be considered as we continue creating a protocol for Raman analysis. Further research into the usage of the Linkam cooling stage revealed the maximum cooling rate is 30 degrees Celsius per minute. This suggests roughly 2-3 minutes will elapse from the time of placing the snow sample on the stage to final temperature stability at -30 Celsius. We are assuming the time to reach this maximum rate is short, however, we will confirm the speed of cooling once we are fully prepared to return to Raman. Tentatively, we expect to pre-chill the stage within a freezer in order to minimize the sample’s exposure to melting temperatures.

The University’s Undergraduate Research Opportunity Program will be hosting its annual symposium on April 23rd. We hope to present images and preliminary data from both Raman and SEM assuming no unexpected time constraints.