1. Amy Mullin, University of Maryland
Spectroscopy of molecules in extreme rotational states using an optical centrifuge
Amy Mullin, University of Maryland
67th International Symposium on Molecular Spectroscopy The Ohio State University Columbus, Ohio June 18-22, 2012

2. Mullin Research Group Wendell Walters Matt Smarte Sam Teitelbaum
Qingnan Liu
Liwei Yuan
Jill Cleveland
Geraldine Echibiri
Carlos Toro
Thursday morning dynamics session
Supported by University of Maryland, NSF and the Beckman Foundation

3. Controlling energy in molecules
Preparing molecules with specific types and amounts of energy via optical excitation: control of l and time
Direct absorption
Multi-photon schemes with real or virtual states
Methods are well established for depositing well controlled amounts of energy into vibration, electronic states and translational motion.
No comparable methods exist for preparing molecules in high rotational states.

4. Traditional optical methods for rotational excitation are limited to small DJ values
Ro-vibration: IR
Pure rotation: mwave
J=0
J=1
J=2
J=3
J=4
J=5
Ro-vibronic: UV-vis
Rotational Selection Rules: DJ=0,±1 for absorption
DJ=0,±2 for Raman
Limited to small J through optical excitation or to broad distributions via heat, reactions or collisions

5. The high power optical centrifuge is a new tool for preparing and studying molecules in high rotational states
Based on Ivanov, Corkum and coworkers, PRL 85, 542 (2000)
Molecules with non-uniform polarizability respond to strong electric fields
(a)
Interaction energy
N2O
Anisotropic polarizability: Da
The E field starts at rest, then angularly accelerates during the optical centrifuge pulse
The most polarizable axis of the molecule tends to align with the polarization vector of the E field.

6. The optical centrifuge combines two oppositely chirped pulses with opposite circular polarization
Leg 1 Pulse Spectrum
Positive Chirp
Right-Polarized
Leg 2 Pulse Spectrum
Negative Chirp
Left-Polarized
Creates a linearly-polarized electric field that rotationally accelerates over the pulse duration.

7. Key molecular parameters: Da and I
An optical centrifuge traps molecules in an accelerating, rotating field
Angular acceleration of the field
Interaction energy
Rotational energy of trapped molecules
Key molecular parameters: Da and I
Control of molecular motion, orientation and energy
Behavior in high J? New chemistry?

8. An Optical Centrifuge works via Sequential Raman Excitation
t1=0.38 ps
t2=0.88 ps
Time
DE and N2O Raman Transitions during OC Pulse
108→110
218→220
DE, cm-1
0→2
Time, ps

9. Dissociation of Cl2 in an Optical Centrifuge
Cl Cl + Cl Edis= 20,030 cm-1 = 57 kcal/mol (J>420)
D.M. Villeneuve, S. A. Aseyev, P. Dietrich, M. Spanner, M. Y. Ivanov and P. B. Corkum, Phys. Rev. Lett (2000).

10. Progress and projects Demonstration of optical centrifuge action
CO2 and N2O transient signals
2. New spectroscopy of high-J states
Seeing new J states of N2O in an optical centrifuge
3. Collisional dynamics of centrifuged molecules
Transient IR probes of CO2: rotational, vibrational and translational energy profiles
4. When the optical centrifuge doesn’t just spin molecules
Troublesome OCS (Qingnan Liu)
5. Polarization dependent studies of centrifuged molecules
Where are the centrifuged molecules? (Carlos Toro)
6. Direct detection of centrifuged molecules?
When no signal can be a good thing. (Qingnan Liu)

11. Optical Centrifuge-Transient IR Absorption Spectrometer
Amplified Ti:sapphire laser with dual oppositely chirped output
800 nm, oppositely chirped pulses
Centrifuge gas cell
Centrifuge Laser
Low Pressure~20 mTorr, tcol~4 ms IR
High Pressure: 10 Torr, tcol~10 ns
Timing Box
mid-IR Diode Laser
IR Detector
Reference cell
Scanning Etalon
High resolution IR probing
Lock-in Amplifier
Single mode lead-salt diode laser
2100 – 2350 cm-1
State and Doppler-resolved transient detection
Dn= cm-1 =10 MHz
300 K Doppler Broadening > cm-1

12. IR Probe Schemes
Direct appearance
Direct depletion
Collisional cascade

13. time
l(0)=805 nm
53 ps pulse
time

14. Criteria for Effective Optical Centrifugation1 2 E
f=0
f=p
for a molecule to stay in the trap
in 2 dimensions
Pulse needs to be short enough to limit rotational excitation
Pulse needs to be long enough to trap ensemble
J. Karczmarek, J. Wright, P. Corkum, M. Ivanov, Phys. Rev. Lett. 82, 3420(1999)

15. Evidence for optical centrifuge action: transient IR absorption
N2O
Appearance signals depend on:
Timing of oc pulses
Both chirped beams
IR probe frequency
CO2

16. New Spectroscopy

17. CO2 in an Optical Centrifuge
J=220 appearance?
300 K
Low J depletion
J=14
Highest known state J=104

18. Estimate Erot for higher J states
For a separable Hamiltonian
low J O C
Rigid Rotor
Centrifugal Distortion
Higher-order terms
Estimate Erot for higher J states
Use known states
up to J=104

19. Direct appearance of high J states in the optical centrifuge
Looking from the top down
CO2, P164, calculated n
Scan range
Known OCS transitions
No appearance signal observed with 100 ns detector,
pressure = 8 Torr and collision time ~10 ns.
Possible reasons
Wrong spectral range?? Wrong detection configuration??
Extrapolation from low-J states?? Fast collisional cooling?

20. A stretched rotor has a larger moment of inertia.
Non-rigid rotors
low J
high J O C C O
higher J C O
A stretched rotor has a larger moment of inertia.
Trapped molecules will have higher Erot to compensate.
ab initio determination of high J states

21. IR Absorption of N2O: A bottom up approach
Hitran database
Highest reported J state of N2O is J=92
R. A. Toth, 1987
Known 13CO2 lines used as reference peaks
Predicted N2O lines near l=4.4 mm
Yuan, Toro, Mack, Mullin, Faraday Disc. 150, (2011)

22. Transients at early times
Appearance is prompt but not detector limited
Population persists for more than 2000 collisions
Yuan, Toro, Mack, Mullin, Faraday Discussions 2011

23. Spectroscopy of new N2O states in the optical centrifuge
Transient signals of high-J N2O states
Peak N2O signal
New transitions observed for J=93-99
Yuan, Toro, Mack, Mullin, Faraday Discussions 2011

24. Predicted frequency, cm-1 Observed frequency, cm-1
Observed frequencies for N2O high J transitions (J=93-99)
N2O transition
Predicted frequency, cm-1
Observed frequency, cm-1
Residuals, cm-1
R93
0.005
R94
0.009
R95
0.001
R96
0.000
R97
0.006
R98
-0.001
R99
0.010
Yuan, Toro, Mack, Mullin, Faraday Discussions 2011

25. (DI/I0 after ~27 collisions)
Energy profiles for N2O in the optical centrifuge
(DI/I0 after ~27 collisions)
Rotational distribution
Doppler broadening
Yuan, Toro, Mack, Mullin, Faraday Discussions 2011

26. in an optical centrifuge
Collisional cascade
in an optical centrifuge

27. Collisions in the Optical Centrifuge: CO2
L. Yuan, S. Teitelbaum, A. Robinson and A. S. Mullin, PNAS 108, (2011)
Appearance of mid-J states
Rise and decay times are pressure dependent
(10 ns collision time)
Signal is independent of IR polarization

28. Energy flow from the optical centrifuge
Translational Energy
Rotational Energy
at ~35 collisions
long time
profiles
450 K
600 K
L. Yuan, S. Teitelbaum, A. Robinson and A. S. Mullin, PNAS, 2011

29. Observation of a Vibrational Bottleneck
Vibrationally Excited CO2 (0330)
CO2 (0000)
after 50 collisions
after 35 collisions
Translational energy
t=3.3 ms
(0330) J=43 population
t=5.1 ms
Trot drops to minimum, then grows in

30. Other candidates for the optical centrifuge?
Molecule
Da a
Da/I|| b
CO2
14.25
2.9
OCS
29.2
3.1
CS2
63.87
3.7
CSe2 84
1.6
HCl
1.75
9.8
DCl
~1.75
4.9
H2O
1.0
HCCH
13.2
8.2
C6H6
39.0
3.9
a) atomic units, b) 1015 m/kg

31. Optical centrifuge “T-jump” estimate
NJ ~0.02 Ntot
Molecule
Da a
Da/I|| b
Erot, kcal/mol
Tf, K c
CO2
14.25
2.9 55
550 (observed)
OCS
29.2
3.1
105
782
CS2
63.87
3.7
192
1186
CSe2 84
1.6
566
2909
HCl
1.75
9.8
2.0
306
DCl
~1.75
4.9
4.0
316
H2O
1.0
2.3
308
HCCH
13.2
8.2 18
380
C6H6
39.0
3.9
112
812
a) atomic units, b) 1015 m/kg, c) based on our optical centrifuge

32. Observation of laser-induced plasma
OCS in the Optical Centrifuge
Observation of laser-induced plasma

33. OCS emission data
Circular-linear
Linear-circular

34. Observed OCS emission and tabulated S-atom emission lines (NIST)
CO emission
5 (800 nm) photons
Also, CO+ visible emission l=
nm from A2P1-X2S+ transitions
Observed OCS emission and tabulated S-atom emission lines (NIST)

35. Optical centrifuge harmonics
OCS absorption
F.Y.T. Leung Thesis
(M. Hoffman’s group, Caltech)
Optical centrifuge harmonics
267 nm
400 nm x3 x2 x1
800 nm
3 photon absorption
OCS has large polarizability x1 x2 x3

36. OCS Photodissociation at 248 nm (5.0 eV)
Science 303, 1852 (2004)

37. Role of bending in accessing dissociative states?
OCS Potential Energy Curves
1 photon: eV photon: eV photon: eV
Role of bending in accessing dissociative states?
Minimize multiphoton absorption to see centrifuged molecules
Direct detection of CO products

38. Polarization-dependent measurements of centrifuged molecules
crossed IR-OC configuration x z y
s-polarized IR
EIR
kOC
kIR
p-polarized IR

39. First job: Improve signal to noise ratio (CO2 J=76)
2011
First Signals: 2009
I(n0)
I(n0)-I(n cm-1)
focused IR
1 mm IR
S/N~80
2010
S/N~5

40. Polarization-dependent measurements
CO2 J=76
350 ns
Ttrans=1028±170 K
s-polarized
p-polarized
350 ns
Ttrans=1154±200 K

41. Evolution of Doppler-broadened line profiles
100 ns
150 ns
2180 K
1970 K
1760 K
200 ns
250 ns
1580 K
500 ns
918 K
350 ns

42. Polarization dependence of Etrans
kOC
fast relaxation
slow relaxation

43. Early time polarization dependence of Etrans
kOC
Early time polarization dependence of Etrans
14 measurements
Broader spread of Etrans seen in plane of OC rotation
Spatial inhomogeneity x vs y dimension

44. Polarization-dependent populations
kOC
Polarization-dependent populations
s-pol
p-pol
~50% more molecules with s-polarization
Net orientation of centrifuged molecules persists
for many collisions

45. Looking for direct appearance of centrifuged molecules
Da a
I b
Da/I c
CO2
14.25
7.0
2.9
OCS
29.2
13.9
3.1 CO
3.5
1.5
a) atomic units, b) kg m2, c) 1015 m/kg
Estimate of centrifuged J states

46. Transient appearance for CO(J) in the optical centrifuge
s-pol IR
J=29
Population(29) > Population(32)
Population(35)~0
Direct observation of J=32??
J=32
J=35

47. Transient absorption of CO(J) in the optical centrifuge
early time signals
s-pol IR
Same appearance time for
J=29 and 32
Slower than detector response
Collision-induced signal
No population in J=35 after “10” collisions
A collisional cascade down near Jinitial
detector response
J=29
J=32
J=35

48. Future directions
Short term directions
Polarization-dependent measurements: early time detection
Direct detection of centrifuged molecules
increase time response, reduce collisions, use mid-IR OPO
Competition between multiphoton absorption and optical centrifuge action
reduce power and extent of blue chirp to minimize absorption x
kIR z y
Long term goals
1) Characterize mJ distributions in the centrifuge (P vs R branch probing)
Spectroscopy and dynamics of molecules in extreme J states
New chemistry based on centrifugal forces?

49. Summary
A high power optical centrifuge drives sufficient population into high rotor states for transient IR probing
2. Centrifuged molecules rapidly spread energy among translation and rotational degrees of freedom
3. Observation of bending modes: collisions vs centrifuge?
4. Multiphoton absorption can compete with optical centrifuge action
5. Centrifuged molecules retain overall angular momentum alignment for long times.
6. Opens the door to a new realm in the area of molecular control, chemical dynamics, structure and reactivity.