Atmospheric Remote Sensing Via Infrared-Submillimeter Double Resonance Sree H. Srikantaiah, Jennifer Holt, Christopher F. Neese Frank C. De Lucia Department of Physics, Ohio State University Dane J. Phillips IERUS Technologies Henry O. Everitt AMRDEC, Redstone Arsenal

Double resonance spectroscopy to transcend pressure broadening effects at Atmospheric pressure GHz 2T 1T 500mT 100mT Traditional smm /THz spectroscopy has absolute specificity at low pressures due to highly resolved rotational lines - great technique for sensing at low pressures At atmospheric pressure this specificity and sensitivity is lost due to pressure broadening (5GHz linewidths) Difficulty due to added problems in detection of molecular resonances due to atmospheric fluctuations But the linewidths at atmospheric pressure make this much harder At bottom, briefly note, but do not elaborate on the Specificity and sensitivity points (this is the introduction to the next chart)

Double resonance (DR) sensor : Methodology Example 12CH3F Pulsed/CW CO2 Laser THz Detector THz Source Gas Cell Choose suitable IR pump laser – absorption line coincidence Monitor modulation of the linked rotational transitions (at THz frequencies) initiated by the IR pump laser Establish a 3D specificity matrix (IR pump, THz probe, Time constant of the DR trace) with as many points as possible - to increase specificity for a particular molecule Spectral and temporal features

Optimum regime for atmospheric pressure DR sensor The collisional relaxation rate at atmospheric pressure is ~ 100ps Saturate (or come as close as you can) in 100 ps Probe the directly pumped rotational transitions before collisions equilibrate states. Linewidths of 10 GHz → large gain bandwidth for DR signal due to coalescing of closely spaced pressure broadened rotational lines getting pumped simultaneously

Experimental Setup 100 ps pump laser system TEA laser : 100ns Plasma switch : 25ns (10ps falling edge) Hot cell : < 100ps CO2 Hot Cell OFID ~ 100ps TEA CO2 laser 50ns 30 Hz 0.15 J Spectrometer Plasma shutter High speed IR detector THz Heterodyne Receiver THz source Sample cell Fast Data acquisition system Scope: 6GHz, 40 GS/s Mixer: 40 GHz Bandwidth Detector diode: < 1ns rise time DR signal 6 GHz , 40 GS/s Digitizing Oscilloscope Trigger

25 ns HWHM , ~100 ps falling edge 100 ps IR Laser system Transverely Excited Atmospheric pressure Laser (Source) OFID filter (Hot Cell at 400o C) Plasma shutter (Fast Switch) TEA Laser output 50 ns HWHM Plasma Switch output 25 ns HWHM , ~100 ps falling edge CO2 Hot cell output ~100 ps HWHM

P-type Ge Hot Hole detector: True pulse-width measurement relaxation time : ~ 2ps Expected rise time (1cm crystal) : 100ps Detector performance comparison 100 ps pulse !

Experimental Results Sample : 12CH3F (1 Torr of Gas) Pressure range : 1 - 760 T (air) Pump : 9P20 CO2 Laser excitation (Rabi Frequency = ~ 10 GHz) Probe: 654.582 GHz (V3=1 , J12,K2, Q branch) Cell : 2mm x 4mm cross section , 30 cm long rectangular waveguide cell Experimental Results few collisions 106 collisions 1 collision At 760 T, 1 collision = 100 ps Goal : pumping rate comparable to Collision rate (Rabi Frequency dependent) Phase 1: 100 ns pulse Phase 2: Plasma switch - 25ns pulse Phase 3: 100 ps pulse K relaxation Modify the energy level diagram depicting movement of population Rabi frequency is large enough to pump all the Ks but not adjacent Js J relaxation

Pump Laser pulse temporal shape: Nature of Double Resonance signature IR Pump Laser pulse (Plasma switch o/p) -25 ns pulse width -10ps cutoff transient 500ns pulse structure leak – remnant of the TEA laser pulse after plasma switch shuts off Corresponding Double resonance signal Pump Laser pulse temporal shape: Nature of Double Resonance signature 25 ns 500 ns Fast component Slow component

Phase 2: Double resonance with IR plasma switch pump DR signal at 700 T DR signal evolution with pressure 3.5 % modulation at 700 T S/N of 35:1

Pressure dependent Collision time vs DR Signal 81 % Pressure dependent Collision time vs DR Signal IR pump pulse 10 T 90 % 3.5 % 100 T 66 % 700 T

Illustration of Physics At low pressure Larger J non-equilibrium Pump on Pump off Pump on Pump off Vibrational State equilibrium Vibrational State equilibrium Pump off (vibrational level Equilibrium) Pressure broadening on probe Pump access to more J states Reduction of J non- equilibrium Pump on (J – non equilibrium) Approaches Vibrational state equilibrium Add Energy level diagram Equilibration with other excited vibrational states

Effects of Rabi Frequency and Pressure broadening Only Δνrabbi gain ∼ 10 GHz Δνrabbi gain ∼ 10 GHz Pressure broadening ∼ 5 GHz Pump on (J non Equilibrium) Pump off (Vibrational Equilibrium) Δν gain ~ 100 MHz (pressure broadening + K structure) Δν gain ~ 5 GHz Rabi Frequency: 13.5 GHz Doppler width = 36 MHz Pressure broadened linewidth = 4.6 GHz (FWHM) – Atmosphere

Phase 3: DR signal with 100 ps IR pump OFID Hot Cell output Double resonance signal @ 760 T No DR with 100 ps pulse !! 100 ps IR pump Power Comparison: Avg Power ( Energy) [Peak power] TEA Laser : 1500 mW (50 mJ) [0.5 MW] Plasma Switch : 270 mW (7 mJ) [0.35 MW] Hot Cell (100 ps pulse) : 60 µW (2µJ) [0.02 MW] Very low peak power !!

100 ps (one collision) Physics Pump population concentrated in ∼ 1 J Many collisions distribute population into many Js 1 Collision 260 collisions ~ equilibrates the vibrational pool 1 Collision doesn’t equilibrate the vibrational pool 260 Collisions Rabi Frequency does not scale with Pressure But must be big enough to pump on a 100 ps time scale 25 ns (1 collision) experiment is a surrogate for the 100ps (1 collision) experiment

Summary IR – submillimeter Double resonance studied with 25 ns IR pump. Rabi Frequency Effects Pressure Broadening Effects With a 25 ns pump , Data at 700 T indicates quick equilibration of pumped population into the vibrational manifold. Δα when pump is on (local J non-equilibrium) is only twice the value of Δα when pump is off (vibrational equilibrium) 25 ns (1 collision) experiment is a good surrogate for the 100ps (1 collision) experiment – very clearly depicts the signal enhancement achieved by staying in the 1 Collision limit

Defense Threat Reduction Agency This work was supported by grants from: Defense Threat Reduction Agency (DTRA)