J-Specific Dynamics in an Optical Centrifuge Matthew J. Murray, Qingnan Liu, Carlos Toro, Amy S....
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Transcript of J-Specific Dynamics in an Optical Centrifuge Matthew J. Murray, Qingnan Liu, Carlos Toro, Amy S....
![Page 1: J-Specific Dynamics in an Optical Centrifuge Matthew J. Murray, Qingnan Liu, Carlos Toro, Amy S. Mullin* Department of Chemistry and Biochemistry, University.](https://reader030.fdocuments.in/reader030/viewer/2022032523/56649d8b5503460f94a72f0a/html5/thumbnails/1.jpg)
J-Specific Dynamics in an Optical Centrifuge
Matthew J. Murray, Qingnan Liu, Carlos Toro, Amy S. Mullin*Department of Chemistry and Biochemistry, University of Maryland, College Park, MD
68th Molecular Spectroscopy Symposium at the Ohio State University
Funding: University of Maryland and National Science Foundation
E⃗
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Extreme Orientation of Molecules
An optical centrifuge drives molecules to ultra-high rotational states with oriented angular momentum—a single MJ.
Compared to
Keller, A., Control of the Molecular Alignment or Orientation by Laser Pulses. In Mathematical Horizons for Quantum Physics, 2010.
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Operating Principles of the Optical Centrifuge
• A molecule with an anisotropic polarizability, Da, aligns with the electric field.
• During the optical centrifuge pulse, the electric field angularly accelerates from 0 to 1013 rad/sec.
Interaction energy
22 cos4
1)( EU
Karczmarek, J.; Wright, J.; Corkum, P.; Ivanov, M., Optical centrifuge for molecules. Phys. Rev. Lett. 1999, 82 (17), 3420-3423.
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Creating an Optical Centrifuge
Two oppositely-chirped 800 nm pulses, each with opposite circular polarization
(t)ω(t)ω2
1(t)Ω 21OC
2OCrot IΩ
2
1E • for CO2
• Energy of 19,000 cm-1
E⃗
Yuan, L. W.; Toro, C.; Bell, M.; Mullin, A. S., Spectroscopy of molecules in very high rotational states using an optical centrifuge. Faraday Discuss. 2011, 150, 101-111.
Create a linear electric field which angularly accelerates
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Previous Optical Centrifuge Studies of CO2
Transient IR absorption: appearance of J=76 followed by relaxation (10 Torr)
Yuan, L. W.; Teitelbaum, S. W.; Robinson, A.; Mullin, A. S., Proc. Natl. Acad. Sci. U. S. A. 2011, 108 (17),
“prompt” rise is pressure-dependent: collision-induced transient signals
Detector Response
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J-Specific Dynamics in the Optical Centrifuge
300 K distribution
Goal: Study the dynamics of a broad range of rotational states after the optical centrifuge pulse excites a sample
In this work we look at the dynamics of J=76, 54, 36, and 0.
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Quantum-resolved Transient IR Absorption of CO2
High-J ProbingCO2(0000, J) + IR → CO2(0001, J±1)
Low-J ProbingCO2(0000, J) + IR → CO2(1001, J±1)
CO2 + Optical Centrifuge → CO2 (0000, J≈220)
CO2(0000, J≈220) + CO2(300 K) → CO2(0000, J) + CO2(0000, J’)
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Optical Centrifuge and High Resolution Transient IR Spectrometer
*Optical Parametric Oscillator
Energy: 50 mJ/pulsePulsewidth: 100 psBeam waist: 26 µmRep. Rate: 10 Hz
OPO* λ~2.7 µm Diode Laser λ~4.3 µm
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Assessment of Strong Field Phenomena
Compare transient absorption for CO2 J=76 with same total power (~35 mJ/pulse)
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Transient Absorption Measurements of CO2 J=54 and 76
J=54J=76
t’=290 ns t’=2.0 ms t”=21 ms
t”=4.5 ms
• Transient appearance then decay is seen for both states
• J=76 appearance is ~10x faster than J=54
• Collision-induced decay of J=76 is ~5x faster than J=54
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Doppler Broadened Transient Absorption Line Profile of J=76
τ1=170 ns
τ2=7.2 µs
10 ns between collisions at 10 Torr
Early Time Translational Temperatures
Long Time Translational Temperatures
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Doppler Broadened Transient Absorption Line Profile of J=54
Early Time Translational Temperatures
Long Time Translational Temperatures
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Time Dependent Temperatures and Populations for J=76 and J=54
τA=1.3 µs
τR=31 µs
Both J=76 and J=54 show molecules appear into these states with large translational energies.
τA=240 ns
τR=1.8 µs
J=54
J=76
J=76
J=54
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Transient Absorption Measurements of CO2 J=36
Appearance in wingsDepletion at line center
Raw Transient
Smoothed Transient
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Doppler Broadened Transient Absorption Line Profile of J=36
Appearance
Depletion
20 ns between collisions at 5 Torr
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Time Dependent Temperature and Population for CO2 J=36
τA=2.5 µs
τD=1.2 µs
The rates at which population enters and leaves J=36 are only ~2x different.
Molecules appear into J=36 with high translational energy and those that leave the state have low translational energy.
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Transient Absorption of CO2 J=0
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Doppler Broadened Transient Line Profile of CO2 J=0Early Time Translational Temperatures
Long Time Translational Temperatures
τ=1.9 µs
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Time Dependent Temperature and Population for J=0
τD=1.25 µs
τR=110 µs
We see molecules being depleted from J=0 and J=36 are from a slower subset of molecules in the initial 300 K ensemble.
Population recovery of J=0 is relatively slow.
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3-State Rotational Distribution
Use appearance population from J=76, 54, and 36.
Trot Decay ~32 Collisions
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Quasi-Equilibrium at 550 K
J=54
J=0
Conservation of energy indicates that ~2% of CO2 molecules are
initially excited by the optical centrifuge to J ~220
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Summary We have used high resolution transient IR absorption to
investigate the J-dependent behavior in an optical centrifuge.
We see evidence for fast translational energy gain followed by relaxation due to collisions in the optical centrifuge.
Results show evidence for long-lived energy content in molecules.
J-dependent profiles show the rotation to rotation-translation energy transfer process through a collisional cascade. The CO2 molecules reach a quasi-equilibrium temperature of ~550 K.
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Quasi-Equilibrium at 550 K
J=54
J=0
E rotN J+kT i (N tot−NJ )+ 32k T i N tot =
52kT fN tot
Erot = Centrifuge-Induced Rotational Energy
NJ = Number Density of Centrifuged Molecules
Ntot = Total number density in cell
Ti = 300 K
Tf ≈ 550 K
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Depletion Transient Absorption from Low J