Elimination of the Vacuum Pump Requirement for High Resolution Rotational Spectroscopy Jennifer A. Holt Ryan Daly Christopher F. Neese Frank C. De Lucia June 23, 2015

THz Chemical Sensors THz spectrometry is a powerful chemical analysis tool Spectrometers have historically been lab-scale machines Advances in THz source and detector technology enables smaller systems Spectroscopic cell and vacuum pump become the size limiters

Hewlett Packard Model 8400 (1974)

Mission Adaptable Chemical Sensor (MACS) 2010 Vacuum System Computer

Using Cell as Expansion Volume Cell SourceDetector Vacuum Pump Expansion from small volume Source Detector Piston acts as mirror Expansion requires ~1 Watt for 5 s Dead Space/Sample Inlet

Initial Cell Design Mostly constructed at Battelle, finished/tested at OSU 27” stroke x 1.125” bore Two valves built into end closure Original design assumes 40 torr starting pressure Modified using a syringe for pre- expansion and diffusion pump oil to fill dead volume Window Inlet Dead Space

Pressure Simulation

Initial Cell Setup Syringe for pre-expansion Sample N2N2 House Vacuum to purge manifold

Initial Trials Multiple trials alternating a dilute sample and N 2 Double expansion with 50cc syringe and cell Diffusion pump oil used to help fill dead volume Lines were fit using Voigt profile to calculate cell pressure Fractional absorption, cell length and pressure were uses to calculate mole fraction of the dilute sample.

Alternating OCS/N 2

Calculated Mole Fraction

Cell Contamination Alternating trials with ethylene oxide/nitrogen Residual lines from sample, as well as previous samples. Ethylene oxide Propionitrile contamination

Initial Results Measured lines are 1.7 times the Doppler limit. While there is some cell contamination, positive sample and null sample are clearly distinguishable. Calculated mole fraction of sample is reasonable.

Issues With the First System Diffusion pump oil used to reduce dead volume is difficult to control Inconsistent pressures trial to trial Contamination of cell persists until the cell is disassembled and cleaned. Incapable of reaching low pressures without an auxiliary pump (syringe)

Second Generation Cell Single stage from atmosphere requires ~10 6 expansion ratio Two ~10 3 expansion ratio design relaxes constraint on dead volume Easier valve actuation Does not require pre-evacuation or oil

Valve Porting Detail

New Cell Prototype

Remaining Work Troubleshoot and test new cell Investigate coatings to reduce contamination Improve sensitivity and reproducibility. Acknowledgements Semiconductor Research Corporation Battelle