VUV Spectroscopy of Astrophysical Ices Michael Davis

Introduction  Experimental background  Apparatus and techniques  Results and spectra  Discussion

Background Accretion of gas phase atoms and molecules A mantle of simple volatile molecules is formed, processed by UV/charged pratical irradiation An outer volatile mantle protects an inner organic mantle from irradiation Chemical explosions and other processes release molecules into the gas phase

Background  Ion irradiation caused by Jupiter’s magnetosphere  Ices exist in bulk  Higher temperatures (~100K) than ISM (~10K)

Apparatus

Apparatus  Designed to travel to irradiation sources  CaF/MgF 2 substrate for IR/UV transmission  Helium/nitrogen cryogens, achieves temperatures <20K  Reaches base pressure mbar Ion gauge Rotary feed- through Sample mount Cryogen inlet

Apparatus Sample deposition Irradiation Detection methods FTIR SOURCE RF DEUTERIUM DISCHARGE LAMP (UV) DETECTOR FTIR Spectroscopy VUV Spectroscopy PMT SYNCHROTRON SOURCE ELECTRON GUN ION SOURCE SYNCHROTRON

Experiments  VUV spectroscopy undertaken at:  ISA Storage Ring, University of Aarhus  SRS, Daresbury Laboratory  Most recent beam-times September 2004 (ISA) and December 2004 (SRS)  Looking at simple ices (NH 3, CO, CO 2 ) and mixtures with water  Depositions at different speeds and temperatures

Experiments  Why use VUV spectroscopy?  Complements FTIR studies  Highlight differences between gas and solid phase VUV data  Knowing electronic transitions allows to know the effects of irradiation by UV discharge lamp

Experiments  Presenting NH 3 and NH 3 /H 2 O results  Depositing at 25K, 75K, 85K and 95K  Annealing up to 120K  Depositing ~0.2µm in <2 minutes and ~30 minutes  Mixtures at 9:1, 1:1 and 1:3 ratios

Deposition speed  In ammonia, no difference in curve features  Absorbance is increased  Other molecules show drastic changes

Temperature Effects  Annealing from 20K to >75K shifts the main peak and adds another feature  Depositing at 95K has a similar effect, but more pronounced 194nm

Temperature Effects  Depositing at ~75K-85K causes major changes  New features seen, existing features modified 194nm

Temperature Effects  Some features enhanced or decreased  No major changes to curve shape  Completely different result than annealing the 20K sample

NH 3 :H 2 O Mixtures  Adding small amounts of water to the ammonia does not change the spectra significantly  No water features visible

NH 3 :H 2 O Mixtures  Additional H 2 O suppresses the crystal structure  Some H 2 O remains after NH 3 has been desorbed  No significant annealing effects

Summary  Outline of experimental techniques and apparatus  Deposition speed has little effect in NH 3  Deposition temperature has a major effect in NH 3  Annealing has some effect on spectra  Sample is not strongly affected by water “impurities”

Acknowledgements  Prof. Nigel Mason  Dr Anita Dawes  Dr Robin Mukerji  Philip Holtom  Bhalamurugan Sivaraman  Sarah Webb  David Shaw (SRS)  The Open University  EPSRC  PPARC