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

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

• 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.

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

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

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 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’)

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

Assessment of Strong Field Phenomena

Compare transient absorption for CO2 J=76 with same total power (~35 mJ/pulse)

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

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

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

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

Transient Absorption Measurements of CO2 J=36

Appearance in wingsDepletion at line center

Raw Transient

Smoothed Transient

Doppler Broadened Transient Absorption Line Profile of J=36

Appearance

Depletion

20 ns between collisions at 5 Torr

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.

Transient Absorption of CO2 J=0

Doppler Broadened Transient Line Profile of CO2 J=0Early 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

Use appearance population from J=76, 54, and 36.

Trot Decay ~32 Collisions

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

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.

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

Depletion Transient Absorption from Low J