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

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

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.

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

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.

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.

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?

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

Dissociation of Cl2 in an Optical Centrifuge Cl2 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 85 542 (2000).

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)

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=0.0003 cm-1 =10 MHz 300 K Doppler Broadening > 0.004 cm-1

IR Probe Schemes Direct appearance Direct depletion Collisional cascade

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

Criteria for Effective Optical Centrifugation 1 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)

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

New Spectroscopy

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

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

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?

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

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, 101-111 (2011)

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

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

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 2271.1752 2271.18 0.005 R94 2271.3408 2271.35 0.009 R95 2271.4991 2271.50 0.001 R96 2271.6503 2271.65 0.000 R97 2271.7942 2271.80 0.006 R98 2271.9308 2271.93 -0.001 R99 2272.0602 2272.07 0.010 Yuan, Toro, Mack, Mullin, Faraday Discussions 2011

(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

in an optical centrifuge Collisional cascade in an optical centrifuge

Collisions in the Optical Centrifuge: CO2 L. Yuan, S. Teitelbaum, A. Robinson and A. S. Mullin, PNAS 108, 6872-6877 (2011) Appearance of mid-J states Rise and decay times are pressure dependent (10 ns collision time) Signal is independent of IR polarization

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

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

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

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

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

OCS emission data Circular-linear Linear-circular

Observed OCS emission and tabulated S-atom emission lines (NIST) CO emission 5 (800 nm) photons Also, CO+ visible emission l=480-600 nm from A2P1-X2S+ transitions Observed OCS emission and tabulated S-atom emission lines (NIST)

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

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

Role of bending in accessing dissociative states? OCS Potential Energy Curves 1 photon: 1.5-1.58 eV 2 photon: 3.0-3.17 eV 3 photon: 4.5-4.8 eV Role of bending in accessing dissociative states? Minimize multiphoton absorption to see centrifuged molecules Direct detection of CO products

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

First job: Improve signal to noise ratio (CO2 J=76) 2011 First Signals: 2009 I(n0) I(n0)-I(n0+0.01 cm-1) focused IR 1 mm IR S/N~80 2010 S/N~5

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

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

Polarization dependence of Etrans kOC fast relaxation slow relaxation

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

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

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) 10-46 kg m2, c) 1015 m/kg Estimate of centrifuged J states

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

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

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?

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.