JSpecific Dynamics in an Optical Centrifuge Matthew J

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 68 th Molecular Spectroscopy Symposium at the Ohio State University Funding: University of Maryland National Science Foundation

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.

Operating Principles of the Optical Centrifuge Interaction energy • 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. Karczmarek, J. ; Wright, J. ; Corkum, P. ; Ivanov, M. , Optical centrifuge for molecules. Phys. Rev. Lett. 1999, 82 (17), 3420 -3423.

Creating an Optical Centrifuge Two oppositely-chirped 800 nm pulses, each with opposite circular polarization Create a linear electric field which angularly accelerates 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.

Previous Optical Centrifuge Studies of CO 2 Transient IR absorption: appearance of J=76 followed by relaxation (10 Torr) “prompt” rise is pressure-dependent: collision-induced transient signals Detector Response Yuan, L. W. ; Teitelbaum, S. W. ; Robinson, A. ; Mullin, A. S. , Proc. Natl. Acad. Sci. U. S. A. 2011, 108 (17),

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.

Quantum-resolved Transient IR Absorption of CO 2 + Optical Centrifuge → CO 2 (0000, J≈220) CO 2(0000, J≈220) + CO 2(300 K) → CO 2(0000, J) + CO 2(0000, J’) High-J Probing CO 2(0000, J) + IR → CO 2(0001, J± 1) Low-J Probing CO 2(0000, J) + IR → CO 2(1001, J± 1)

Optical Centrifuge and High Resolution Transient IR Spectrometer Energy: 50 m. J/pulse Pulsewidth: 100 ps Beam waist: 26 µm Rep. Rate: 10 Hz OPO* λ~2. 7 µm Diode Laser λ~4. 3 µm *Optical Parametric Oscillator

Assessment of Strong Field Phenomena Compare transient absorption for CO 2 J=76 with same total power (~35 m. J/pulse)

Transient Absorption Measurements of CO 2 J=54 and 76 J=76 t’=290 ns J=54 t’=2. 0 ms t”=21 ms t”=4. 5 ms • Transient appearance then decay is seen for both states • J=76 appearance is ~10 x faster than J=54 • Collision-induced decay of J=76 is ~5 x faster than J=54

Doppler Broadened Transient Absorption Line Profile Early Time Translational Temperatures of J=76 Long Time Translational Temperatures τ1=170 ns τ2=7. 2 µs 10 ns between collisions at 10 Torr

Doppler Broadened Transient Absorption Line Profile of J=54 Early Time Translational Temperatures Long Time Translational Temperatures

Time Dependent Temperatures and Populations for J=76 and J=54 τA=1. 3 µs J=54 τR=31 µs J=76 J=54 τA=240 ns J=76 Both J=76 and J=54 show molecules appear into these states with large translational energies. τR=1. 8 µs

Transient Absorption Measurements of CO 2 J=36 Depletion at line center Raw Transient Smoothed Transient Appearance in wings

Doppler Broadened Transient Absorption Line Profile of J=36 Appearance Depletion 20 ns between collisions at 5 Torr

Time Dependent Temperature and Population for CO 2 J=36 τA=2. 5 µs τD=1. 2 µs Molecules appear into J=36 with high translational energy and those that leave the state have low translational energy. The rates at which population enters and leaves J=36 are only ~2 x different.

Transient Absorption of CO 2 J=0

Doppler Broadened Transient Line Profile of CO 2 J=0 Early Time Translational Temperatures Long Time Translational Temperatures τ=1. 9 µs

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.

3 -State Rotational Distribution Trot Decay ~32 Collisions Use appearance population from J=76, 54, and 36.

Quasi-Equilibrium at 550 K J=54 J=0 Conservation of energy indicates that ~2% of CO 2 molecules are initially excited by the optical centrifuge to J ~220

Summary Ø We have used high resolution transient IR absorption to investigate the J-dependent behavior in an optical centrifuge. Ø We see evidence for fast translational energy gain followed by relaxation due to collisions in the optical centrifuge. Ø Results show evidence for long-lived energy content in molecules. Ø J-dependent profiles show the rotation to rotationtranslation energy transfer process through a collisional cascade. The CO 2 molecules reach a quasi-equilibrium temperature of ~550 K.

Quasi-Equilibrium at 550 K Erot = Centrifuge-Induced Rotational Energy NJ = Number Density of Centrifuged Molecules J=54 J=0 Ntot = Total number density in cell Ti = 300 K Tf ≈ 550 K

Depletion Transient Absorption from Low J