CYCLOPROPYLACETYLENE STUDIED IN COLD FREE JET EXPANSION, ROOM TEMPERATURE GAS, AND DILUTE SOLUTION: TIER MODEL IVR PAM L. CRUM, GORDON G. BROWN, KEVIN O. DOUGLASS, BRIAN C. DIAN JAMES E. JOHNS, PRADEEP M. NAIR, HYUN S. YOO, AND BROOKS H. PATE Department of Chemistry, University of Virginia, McCormick Rd., P.O. Box , Charlottesville, VA 22904

Spectroscopic Techniques for Range of Molecular Environments Frequency Domain Isolated molecule MW-IR Double Resonance Time Domain Isolated molecule FS Transient Absorption Spectroscopy Solvated molecule FS Transient Absorption Spectroscopy

Vibrational Dynamics of Isolated and Solvated Molecules Intramolecular Vibrational Energy Redistribution (IVR) and Solvent-Induced Vibrational Energy Relaxation (VER) IVR –Total amount of vibrational energy constant –Threshold density of states ~10 states/cm -1 –Only pathway for energy flow in isolated molecule –Rate is molecule dependent VER –Energy removed from molecule (solute) –Rate is solvent dependent

E k IVR k VER v=1 v=0 Acetylenic C-H Stretch Fundamental Measurements: k TOT = k IVR + k VER Acetylenic C-H Stretch Fundamental Measurements: k TOT = k IVR + k VER Yoo, H.S. et al. J. Phys. Chem A (8) Yoo, H.S. et al. J. Phys. Chem A (8) Yoo, H.S. et al. J. Phys. Chem A (8) Vibrational Dynamics in Solution [≡C―H]

Cyclopropylacetylene (CPA) Exceptionally fast CCl 4 relaxation time (sub ps) What made this molecule different from the other Terminal Acetylenes studied? 2 TOOLS TO GET AT THE DYNAMICS MW-IR DR : FS TAS : t

Pulsed-IR FTMW Detector Advantages –Scan rate = 18 cm -1 /hr –No hot bands –Sensitive Limitations –Linewidth ~ 0.03 cm -1 –Can’t tell where energy goes OPO/OPA 4 mJ/pulse of 3  m light 0.03 cm -1 bandwidth Pulsed IR Nd:YAG Laser

CPA Ground State Depletion

Slow Scans of CPA R(1) P(2) P(1) R(2) FTMW Monitor MHz

Timescales for IVR in CPA Frequency Domain FTMW-IR  1 ~ 1 ps  2 = 7 ps  3 = 70 ps (Single Rotational Level Analysis)

Tier Model of IVR State Specific Dynamics of Ultracold CPA Bright State First Tier Bath States  1  1 ps  2 = 7 ps Second Tier  3 = 70 ps Direct information on the identity of the interacting states is unavailable from IR spectrum

Two Color Transient Absorption Spectroscopy v=2 v=0 v=1 X-H – 100cm -1 X-H ν pump1 ~ 3320cm -1 ν probe ~ 3220cm -1 ~ 10μJ ~ 12μJ v=0 v=1 X-H – 40cm -1 X-H ν pump1 ~ 3320cm -1 ν probe ~ 3280cm -1 ~ 10μJ

300K Gas Phase Dynamics by Femtosecond Transient Absorption Spectroscopy CPA (g) v = 2-1 probe  1 = 0.6 ps  2 = 6.0 ps  3 = 38 ps

Initial Decay, Recurrences, and Oscillations: Data Agreement

Gas Phase Measurement of Population Relaxation into CH Bend Overtone Levels  rise ~ 1 ps (laser limited)  fall1 ~ 7 ps  fall2 ~ 40 ps

Normal Vibrational Mode Shifts due to 13 C C5 C4 C1 C2

GSD IR Spectra of CPA: Normal Species and 13 C (Natural Abundance)

13 C Isotopomers: Effect on Dynamics time (ps) Intensity NS 13 C_2 13 C_5

Vibrational Dynamics of CPA in 300K Gas Acetylenic CH Stretch Bright State Acetylenic CH Bend Overtone Excitation (1200 cm -1 ) Third Tier  1 ~ 1 ps  2 = 7 ps Second Tier  3 = 40 ps

Comparison of 300K Gas and Dilute Solution Dynamics (0.05 M CCl 4 ) Initial rates are the same in gas and solution but solution relaxation is more efficient Solution relaxation is not single exponential and “breaks off” at about 1/5 of the full amplitude

Vibrational Dynamics of CPA in Dilute Solution Acetylenic CH Stretch Bright State Acetylenic CH Bend Overtone Excitation (1200 cm -1 ) Third Tier  1 ~ 1 ps  2 = 7 ps Second Tier  3 = 40 ps

Solution Phase Bleach Recovery and 300K Gas Dynamics CCl 4 :  1 = 0.6 ps  2 = 7 ps  3 = 41 ps Gas:  1 = 0.6 ps  2 = 6 ps  3 = 38 ps

Summary of CPA Dynamics Free jet expansion (FTMW):  1 ~ 1 ps  2 = 7 ps  3 = 70 ps 300K Gas phase fs data:  1 ~ 0.6 ps  2 = 7 ps  3 = 38 ps Dilute Solution fs data:  1 ~ 0.6 ps  2 = 7 ps  3 = 40 ps Thermal rate enhancement in the final IVR time scale is typically observed (coupled modes are thermally populated).

Conclusions Dynamics in all cases dominated by intramolecular processes CPA IVR dynamics follow Tier Model Strong coupling to first tier –States with two quanta in C-H bend

Acknowledgements Pate Lab Group Members Funding: NSF Chemistry SELIM Program University of Virginia John D. and Catherine T. Macarthur Foundation The Jeffress Trust

Quantum Decoherence Model Calculation: Coupled Degenerate States H = H mol (Q 1,Q 2 ) + H bath ({q i }) + H mol-bath H mol = H 1 (Q 1 ) + H 2 (Q 2 ) + W 12 : E 1 = E 2 H mol-bath  kQ n 2 q i : Only pure dephasing terms included The dynamics are modeled using random matrix methods: W 12 E 1 + E bath H = W 12 E 2 + E bath The dephasing interactions for the two coupled intramolecular states are assumed to be uncorrelated.

N = 24 N 1 = 9N 2 = 40 Extending the Quantum Decoherence Model to Other Common IVR Models I. IVR in a Sparse II. IVR DominatedIII. Tier Model Intramolecular Bathby a Single, Strong Resonance

Solvent Dephasing Effects for Tier Model IVR

GSD Slow Scans of CPA

GSD of 13 C Species

Dynamics from frequency- domain spectrum: Free jet expansion timescales:  1 ~ 1.5 ps  2 ~ 6 ps

Comparing ultracold free jet expansion to gas-phase room-temperature Gas-Phase Ultrafast Time- Domain Isolated molecule Room-Temp Dynamics of all thermally populated levels (vibrational hot bands and rotational) fs transient absorption spectroscopy Free Jet Expansion Frequency- Domain Isolated molecule Ultracold Dynamics of a single rotational level (State- Resolved) MW-IR Double- Resonance ~ 300 fs pulse ~ 100 cm -1 bandwidth

Timescales for IVR in CPA Frequency Domain FTMW-IR  1 = 1.5 ps  2 = 7 ps  3 = 70 ps Time Domain Gas Phase  1 = 0.1 (0.02) ps  2 = 7 (0.9) ps  3 = 50 (15) ps

Timescales for IVR in CPA Frequency Domain FTMW-IR  1 = 1.5 ps  2 = 7 ps  3 = 70 ps Time Domain Gas Phase  1 = 0.6 ps  2 = 7 ps  3 = 42 ps Time Domain Solution Phase  1 = 0.6 ps  2 = 7 ps  3 = 60 ps

Comparison of Solution Rate and Initial Gas-Phase IVR Rate k TOT = k IVR + k VER

Dynamics of the Acetylenic C-H stretch for Gas-Phase CPA Femtosecond ResolutionPicosecond Resolution

Dynamics of the Acetylenic C-H stretch for Solution-Phase CPA

13 C Isotopomers: Effect on Dynamics

Data Agreement Average Survival Probability (all species) FS Gas Dynamics

13 C Isotopomers: Normal Vibrational Mode Shift & Effect on Dynamics The survival probability calculation for the normal species of CPA is compared to the same calculation for each of the 13 C isotopomers.

Solution Phase Measurement of Population Relaxation into CH Bend Overtone Levels  rise = 1.2 ps (laser limited)  fall1 = 6.4 ps  fall2 = 45 ps

Transient Absorption Spectroscopy for Terminal Acetylenes v=2 v=0 v=1 0 0 – 108cm -1 11 22 Excited State Absorption SEP GSD 0 – 40cm -1 v CHs = 0, v CHb = cm -1 in other modes 00 v CHs = 0, v CHb = cm -1 in other modes 11 11 11 22 22 Excited State Absorption Bend Overtone Absorption Bleach Recovery