Pressure Broadening and Spectral Overlap in the Millimeter Wave Spectrum of Ozone International Symposium on Molecular Spectroscopy 65 th Meeting — June 21–25, /22/20101 Corey Casto and Frank C. De Lucia The Ohio State University TE02

Introduction 6/22/20102  Millimeter region:  GHz  Atmospherically broadened lines  Van Vleck-Weisskopf line profile  P = 100 – 760 torr  FWHM = 0.5 – 4.6 GHz  Significant overlap  Isolated lines? TE02

Experiment 6/22/20103  Cavity characteristics  3 second scan  ~6000 resonances in the 172 – 262 GHz range  FSR = 14.5 MHz  Q ~ 7  10 6  Ringdown   = 1/c    ~ 5  s Quasi-Optical Isolator InSb Detector DAQ Pulse Generator BWO HV Power 10.3 m TE02

Low Pressure 6/22/20104  Good agreement between model and experiment  Intensities and linewidths are consistent  Residual is not significantly greater at the peaks  Lines are isolated and less prone to mixing effects TE02

High Pressure 6/22/20105  Similar noise to low pressure  Discrepancies near line peaks  Causes? TE02

Causes of error near line peaks 6/22/20106  Intensity  Frequency  Experiment  Linewidth  Line Mixing TE02

Residuals at various pressures 6/22/20107  Ozone concentration is constant ~ 150 ppm  Narrow features are consistent  Broad features increase by an order of magnitude TE02

Causes of error near line peaks 6/22/20108  Intensity  Frequency  Experiment  Linewidth  Line Mixing TE02

Global linewidth adjustment fit 6/22/20109   a (T 0 /T) n a   a Air-broadened linewidth in MHz/torr  n a Air-broadened coefficient of temperature dependence  Overlap makes measurement of linewidths difficult  Global fit prevents pressure dependent parameters TE02

Global Fit Linewidth Adjustments 6/22/  Disagreement is <5% for most lines  Many lines are known to within 2-5%  How does this affect residuals? TE02

Residuals with global fit parameters 6/22/  Low pressure residuals don’t increase  High pressure residuals improve near line centers  Linewidth error is not sufficient to explain residual  Published parameters accurately describe our data TE02

Causes of error near line peaks 6/22/  Intensity  Frequency  Experiment  Linewidth  Line Mixing TE02

Line mixing characteristics 6/22/  Lines not isolated  Inelastic collisions allow coupling of lines  First order effects can be expressed in a modified line profile  Wing skewing  Small center shift  Linear in pressure Y = Y 0 P TE02

Line mixing 6/22/  Residuals largest at the wings of strong lines  Conditions for line mixing  Lower state energy gap is less than thermal energy  Line separation is less than width E 1 - E 2 < kT <  1,2 E 1 - E 2 < kT <  1,2 TE02

Future Work 6/22/  Model line mixing  Which lines mix?  What are reasonable parameters?  Explore other dense spectra  Formaldehyde (H 2 CO)  Nitric acid (HNO 3 )  Nitrogen dioxide (NO 2 )  Acetonitrile (CH 3 CN)  Methanol (CH 3 OH) TE02