THz Time-Domain Spectroscopy of Interstellar Ice Analogs

1. THz Time-Domain Spectroscopy of Interstellar Ice Analogs Sergio Ioppolo M. A. Allodi, B. A. McGuire, M. J. Kelley, G. A. Blake Division of Geological & Planetary Sciences, California Institute of Technology, 1200 E. California Blvd, Pasadena, California 91125, USA Email: ioppolo@caltech.edu

2. THz Spectroscopy • Definition of THz Region of the Electromagnetic Spectrum • 0.1 – 10 THz • 3.3 – 330 cm-1 • 3000 – 30 micron

3. THz Spectroscopy • THz frequencies are suitable for probing low energy light-matter interactions • This region of the spectrum is dominated by: • Large amplitude motions in solids • Phonon modes • Intermolecular vibrations • High-frequency torsional motions of individual molecules

4. THz Spectroscopy • THz frequencies are suitable for probing low energy light-matter interactions • This region of the spectrum is dominated by: • Large amplitude motions in solids • Phonon modes • Intermolecular vibrations • High-frequency torsional motions of individual molecules • Potential: • Detailed information on the structure of the ice. • More distinct, unambiguous spectra for molecular identification. • Characterization of the Interstellar Medium.

5. THz Spectroscopy • New telescopes allow observations in the THz: • Herschel, SOFIA, ALMA Photo Credit: NASA, USRA (Universities Space Research Association), and L-3 Communications Integrated Systems

6. THz Spectroscopy • New telescopes allow observations in the THz: • Herschel, SOFIA, ALMA • Interstellar ices can be detected in emission in the THz region through their thermal radiation. • To identify molecules in observational data we need lab spectra • Laboratory far-IR ice work (Moore & Hudson 1994) • Jena database interstellar dust (Posch et al. 2007; Mutschke et al. 2008)

7. ISO data - Herbig Ae stars HD142527 Need for an extensive gas/ice/dust database Cryst. H2O Malfait et al. 1999

8. THz Time-Domain Spectroscopy k • Collect a signal in the time domain • Must Fourier Transform numerically to view frequency content • Benefits to Time Domain Collection: • Molecular phase information is retained • Complex index of refraction measurements are simplified • ñ=n+iκ n k n

9. THz Time-Domain Spectroscopy • Collect a signal in the time domain • Must Fourier Transform numerically to view frequency content • Benefits to Time Domain Collection: • Molecular phase information is retained • Complex index of refraction measurements are simplified • ñ=n+iκ • Higher Resolution (up to ~ GHz) • More power in region of astronomical interest (0.1 - 3 THz)

10. THz Instrumentation • The Laser System • THz Pulse Generation via Two-Color Plasma • THz Detection via Electro-Optic Sampling (EOS)

11. THz Instrumentation - Laser system • Coherent Mantis: • Mode-locked Titanium:Sapphire (Ti:Sapph) oscillator • 100 fs, 6 nJ pulse with 80 nm of bandwidth @ 800 nm • 80 MHz repetition rate • Coherent Legend Elite: • Regenerative amplifier • 35 fs, 4 mJ pulse @ 800 nm • 1 kHz repetition rate

12. THz Instrumentation - THz Generation Beam Splitter 99% 1% Detection Generation

13. THz Instrumentation - THz Generation • BBO - frequency double amplified 800 nm output to 400 nm • Calcite - compensate for phase delay between 400 & 800 nm pulses • Dual-band waveplate - align the polarizations • Focus 400 nm & residual 800 nm light to form plasma in air • Mechanism: • 800 nm light generates plasma • 400 nm light accelerates electrons in plasma • Accelerated electrons emit THz pulses

14. THz Instrumentation - THz Detection Detection

15. THz Instrumentation - THz Detection • Detection beam & THz beam are focused down onto a crystal • Pockels Effect in crystal (ZnTe, GaP) • The THz pulse causes a rotation in the polarization of the probe beam in the ZnTe • Magnitude of polarization change is linear to applied THz electric field • Measure THz Electric Field not Intensity • A pair of balanced detectors sees a difference in signal

16. THz Instrumentation - THz Detection • Optomechanical delay line allows one to step through the entire THz waveform. • The electric field is measured as a function of delay time. • A fast Fourier transform (FFT) of the temporal waveform gives spectral distribution of the THz pulse in the frequency domain. Frequency Domain Time Domain

17. Caltech setup (side view) • P < 1x10-8 mbar • Tsurf = 10 - 300 K

18. Caltech setup (top view)

19. Caltech setup

20. First Laboratory Data Cryst. water ice @ 100 K

21. Preliminary FTIR results - Water ice Difference mid-IR spectra of water ice at different temperatures. crystalline ν1, v3 amorphous L2 v2 ν2 + L2

22. Preliminary THz results - Time Domain

23. Preliminary results - Frequency Domain BKG @ 250 K ~ 5 μm of cryst. H2O @ 150 K

24. Preliminary results - The THz spectrum Low resolution ~ 100 GHz / Range ~ 750 – 35 μm ~ 5 μm of cryst. H2O @ 150 K

25. Preliminary results - The THz spectrum Low resolution ~ 100 GHz / Range ~ 750 – 35 μm ~ 5 μm of cryst. H2O @ 150 K

26. Preliminary results - Molecular Complexity

27. Preliminary results - Molecular Complexity • Absorption feature in the THz are: • Distinct • Structure and Temperature dependent • What if we go towards higher complexity?

28. Sugars & Amino Acids (15% polyethylene pellets) • Glycolaldehyde: • Detected in the ISM • Sagittarius B2(N) • G31.41+0.31 • IRAS 16293-2422 (ALMA) • (Hollis et al. 2000; Beltran et al. 2009; Jørgensen et al. 2012) Gianni Strazzulla - Meudon meeting, Nov. 2012 Allodi et al. (2013) submitted to Space Sci. Rev.

29. Sugars & Amino Acids (15% polyethylene pellets) • Glutamic Acid: • Detected in the solar-system small bodies • Nakhla • (Glavin et al. 1999)

30. Improvements on Bandwidth October 2012 Cryst. H2O @ 150 K ZnTe crystal generation/detection

31. Improvements on Bandwidth December 2012 Cryst. H2O @ 150 K ZnTe crystal generation/detection Plasma generation/ZnTe detection

32. Improvements on Bandwidth February 2013 Next Few Months Cryst. H2O @ 150 K ZnTe crystal generation/detection Plasma generation/ZnTe detection Plasma generation/GaP detection • Ultimate Goal: • Plasma Detection • (Close the gap between Mid-IR and THz)

33. Future Laboratory work • Structure: • Deposition at different T • Annealing • Thickness • Deposition angle and rate

34. Future Observational work • Comparisons with archive data from the Herschel Space Telescope. • Submit a Cycle 2 proposal to SOFIA to conduct observations targeting ice species observed in the lab towards a variety of astronomical sources.

35. Summary • THz TD spectra provide a spectral fingerprint of intermolecular forces. • THz TDS can be used: • combined with the flagship facilities outlined above to identify the presence and temperature of complex species in the ISM; • as critical experimental input to the simulation of intermolecular interactions in solids. • Caltech database will provide insights into whether and to what extent interstellar ices offer a platform for the formation of complex molecules in space, long before the formation of planetesimal and planetar systems.

36. Acknowledgements … And You!!!