1. 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

2. 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

3. 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 (2005)

4. 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

5. 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 (2006)

6. 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

7. 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?

8. 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

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

10. 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

11. Experimental vs. Simulated Recovery Times

12. 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

13. Thank you for your attention