Time-Resolved Recombination Dynamics of Large IBr-(CO2)n (n=11-14) Clusters Joshua P. Martin, Joshua P. Darr, Jack Barbera, Matt A. Thompson, Robert Parson, and W. Carl Lineberger JILA, University of Colorado Ohio State Molecular Spectroscopy Conference June, 2008

Dihalide Solvation Studies Problem Complexity of liquid environment hinders detailed study of recombination dynamics Motivation “Caging” first observed with photodissociating dihalides in liquids and considered a mechanical effect Cluster studies show “caging” can occur with only a few solvent molecules due to electronic perturbation Partially solvated IBr− can act as a model to study the fundamentals of “caging” Method Time-of-Flight mass spectrometry simplification of cluster environment enables mass selection of IBr-(CO2)n clusters in gas-phase Ultrafast pump-probe spectroscopy Start with a brief review of solvated IBr- photodissociation

Solvated IBr− Chromophore Dynamics R(I−Br) (Å) Energy (eV) X 2Σ+1/2 A′ 2Π1/2 IBr--based products (recombined) 795 nm 795 nm _ − I- based products (single surface) I Br _ Br--based products (multi-surface) I− + Br Br− + I Initial solvation of the chromophore begins on the smaller Br end Charge localized on IBr- in cluster Addition of one solvent molecule causes charge transfer to occur Addition of second solvent molecule initiates caging IBr−(CO2)5 and larger show ~100% recombined products Upon recombination, a 795 nm probe pulse excites caged product to A′ state Detection of photoproducts as function of pump-probe delay Sanford, et al, J. Chem Phys. 122 054307 (2005)

Experimental Apparatus Ion Optics 0 kV -1 kV Pump-Probe Off-Axis MCP Reflectron On-Axis Channeltron Deflected Anions -3.5 kV Acceleration Stack Supersonic Expansion Electron Gun Potential Switch Mass Gate First compare I2-(CO2)n and IBr-(CO2)n dynamics Wiley-McLaren TOF mass spectrometer allows for mass selection of clusters Interaction with femtosecond pump-probe produces photoproducts Photoproducts are mass separated in secondary reflectron mass spectrometer and detected off-axis as a function of pump-probe delay time

I2-(CO2)n vs. IBr−(CO2)n A′ 2P Absorption Recovery Summarize these results I2-(CO2)n Fast absorption recovery times n = 8: t ~ 25 ps n = 14: t ~ 11 ps n = 17: t ~ 8 ps As solvation number increases the absorption recovery time decreases This trend generally true for a variety of solvents Papanikolas, et al, J. Chem. Phys., 99, 8733 (1993) IBr-(CO2)n Long absorption recovery times n = 5: t ~ 12 ps n = 7: t ~ 140 ps n = 8: t ~ 900 ps n = 10: t ~ 900 ps As solvation number increases the absorption recovery time increases Dribinski, et al, J. Chem Phys. 125 133405 (2006)

I2-(CO2)n vs. IBr−(CO2)n Summary Nanosecond recovery times were unexpected Extended study to larger clusters (n > 10) to further investigate unexpected dynamics

IBr−(CO2)11-14 A′ 2P Absorption Recovery Fast absorption recovery times n = 11: t ~ 19 ps n = 12: t ~ 10 ps n = 13: t ~ 5 ps n = 14: t ~ 3 ps n = 13,14 show non-exponential behavior What causes the recovery times to decrease as the clusters increase in size?

Does theory predict similar recovery times? Solvation Effects n = 5 n = 6 n = 7 n = 8 n = 10 n = 14 R(I−Br) (Å) Energy (eV) X 2Σ+1/2 A′ 2Π1/2 Br− + I I− + Br IBr−(CO2)8 Potentials Does theory predict similar recovery times? IBr−(CO2)n clusters reach a maximum solvent asymmetry at n = 8 Dynamic well on A′ state deepened by electronic perturbation from asymmetric solvation (106 V cm-1 solvent field) The A′ well depth for n=8 is ~300 meV Well causes A′ trapping Thompson, et al, in preparation

MD Simulation: IBr−(CO2)n Dynamics A′ state trapping seen for n = 8 - 11 Minimal A′ state trapping seen for n = 5 - 7 and n = 12 - 14 Why are data for n = 13 and 14 non-exponential? Thompson, et al, in preparation

IBr−(CO2) 14 Non-Exponential Behavior Pump and probe are both 795 nm The cross section for IBr- peaks at 740 nm The maximum cross section for our excitation wavelength will occur from vibrationally excited ground state levels Population transverses intermediate vibrational levels Coherence Peak n = 14 Look at the summary of experimental and theoretical results

Experimental vs. Simulated Recovery Times

Conclusions Maximum solvation asymmetry around the chromophore reached at n = 8, mid-point in first solvation shell Solvent-induced electronic perturbations cause trapping and resulting long recovery times Good qualitative agreement between experiment and theory for all cluster sizes

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