Buffer gas cooling and mid-infrared spectroscopy of polyatomic molecules

Abstract

This thesis reports buffer gas cooling, beam formation, and state-selective absorption spectroscopyof polyatomic molecules, focusing on the trioxane molecule, C3H6O3. The molecules are injectedfrom room temperature into a cryogenically-cooled cell containing a buffer gas of helium or neon.They are detected inside the cell by state-selective mid-infrared wavelength modulation spectroscopyusing a quantum cascade laser. We recorded trioxane spectral features from both the Q- and R-branches of thev5 vibrational band, between 977.1 and 978.6 cm−1. We resolved spectral features asnarrow as 6 MHz, and observed molecules with temperatures as low as 7.4±0.3 K. We estimate thedensity of cold trioxane in the cell to be 5×10^21m−3. After characterising the molecules inside thecell, we extracted a beam of trioxane from the cell and detected it using a mass spectrometer. Westudied in detail the parameters that affect the beam and learned how to produce beams with highflux. We attempted to detect molecules in a single state in the beam using laser absorption, andhave tentatively observed a signal by this method. The work shows that quite complex moleculescan be cooled to low temperature using this method, either inside a cell or in a beam, and showsthe enormous simplification of the spectrum that occurs at these low temperatures, enabling highresolution spectroscopy of complex species. An interesting application of the work is to produce coldbeams of chiral molecules and use these to measure the parity-violating energy difference betweenchiral enantiomers arising from the weak interaction. The progress made in this thesis are the firstimportant steps in this direction.