Time-resolved dynamics with partially solvated anions W. Carl Lineberger S pectroscopy for Dynamics Minisymposium Molecular Spectroscopy 2007 Thanks to Rich Loomis

Solvation Dynamics in Cluster Ions Size-selected clusters as a “bridge” or a unique entity? –Achieve asymmetric environments more extreme than in bulk Affords access to solvent electric fields that enhance effects Can clarify understanding of condensed phase theories Can carry out more sophisticated simulations than in bulk Not intended to be a mimic of bulk –Close interplay between experiment and theory is crucial Structure of solvation shell Effect of chromophore: I 2 -, IBr - –Charge localization upon dissociation –Breaking the symmetry Caging viewed in the time domain Todd Sanford Jack Barbera Robert Parson Annette Svendsen Josh Martin Matt Thompson Vladimir Dribinski Josh Darr

Schematic IBr - (CO 2 ) n Photodissociation h + IBr - → I - + Br or I + Br - h 10  sec Uncaged Caged

Methodology  Ionic product distributions ~10  s after dissociation caged IBr – (CO 2 ) k uncaged I – (CO 2 ) k, Br - (CO 2 ) m  Caging probability  Solvent evaporation energetics  Ultrafast pump - probe recombination dynamics  Time-resolved picture of recombination  Theory (Parson and Thompson)  ab initio electronic structure for solute  Rigid solvent, polarizations included; cluster structure  Nonadiabatic molecular dynamics w/solvent allowed to polarize solute  First, a brief review of I 2 - electronic structure and I 2 - (CO 2 ) n cluster dynamics as a reference

I 2 - Valence Shell Potentials J. Faeder, N. Delaney, P, Maslen and R. Parson M. Zanni and D.M. Neumark I-I bond length, Angstroms Energy, eV   g,1/2   g,3/2   u,3/2   u,1/2   u,1/2 +   g,1/2 + I( 2 P 1/2 ) + I  I( 2 P 3/2 ) + I  1.0 eV 0.6 eV

I 2 - A 2  caging probability Caged fragment fraction 82% I 2 – (OCS) 4,5 18% I – (OCS) 8,9 What is the time required for the dissociated I 2 - to recombine?

I 2 - (CO 2 ) 8-17 A’ 2  Recombination ps recombination time, decreasing with solvation What is the physical picture of the caging?

N. Delaney, J. Faeder, P. E. Maslen, R. Parson, J. Phys. Chem. A 101, 8147 (1997). Electric field (solvent) perturbation of I 2 - states I-I bond length (Å)   g,1/2   u,3/2 2  u,1/2   u,1/2 +   g,1/2 + I * ( 2 P 1/2 ) + I  I( 2 P 3/2 ) + I    g,3/2 Isolated I 2 – I-I bond length (Å)   g,1/2   u,3/2 2  u,1/2   u,1/2 +   g,1/2 + I * ( 2 P 1/2 ) + I  I( 2 P 3/2 ) + I    g,3/2 I 2 – in solvent E field

 Asymmetric solvation driving recombination: Solvent induced modification of solute electronic structure.  Asymmetry (solvent E solute) from 3-4 CO 2 solvents produces curve crossings for levels (I and I*) separated by 1 eV.  Recombination time ~10 ps, decreasing with additional solvation. Also true for excitations leading to I*.  Similar, rapid recombination seen for OCS, N 2 O, CO 2 and Ar solvating I 2 - What happens when IBr - or ICl - replaces I 2 - ? Recombination mechanism

ICl - (CO 2 ) 4 IBr - (CO 2 ) 8 Initial solvation occurs about the smaller ion! We can now test the electron transfer picture. Breaking the symmetry

R I-Br, Å Br+ I - Br*+ I - Br - + I Br - +I* I - +I* I - +I IBr - and I 2 - Valence Shell Potentials Robert Parson and Matt Thompson R I-I, Å What do the calculated cluster structures look like?

IBr - (CO 2 ) n Minimum Energy Structures R. Parson and M. Thompson R I-Br, Å Br+ I - Br*+ I - Br - + I Br - +I* First, look at the dissociation products following excitation to the A’ 2  state.

IBr - based (caged) IBr - (CO 2 ) n A´ 2  state ionic photoproducts Next, look at the uncaged I - -based products.

I - based (uncaged) IBr - based (caged) IBr - (CO 2 ) n A´ 2  state ionic photoproducts And the uncaged Br - -based products.

I - based (uncaged) Br - based (charge transfer) IBr - based (caged) IBr - (CO 2 ) n A´ 2  state ionic photoproducts Br - -based products confirm electron transfer in recombination. And the uncaged Br - -based products.

n Caging Percentage A´ 2  State Caging Efficiency I 2 - (CO 2 ) n n IBr - (CO 2 ) n In the A´ state, IBr - caging behaves like I 2 -. By analogy, ps recombination is expected.

IBr - (CO 2 ) 8 A´ 2  State Recombination Recombination in 1000 ps, even for 100 % caging!! 797 nm pump and probe

 100 % recombination to X 2   1000 ps recombination time!!  Every IBr - ion reaches the ground state, despite the very long recombination time.  Very different from I 2 -, and might involve a solvent-induced excited state trap  Try IBr - (CO 2 ) 10 IBr - (CO 2 ) 8 A´ 2  Summary

IBr - (CO 2 ) 10 A´ 2  State Recombination So we next remove solvent: IBr - (CO 2 ) 5. Recombination still takes 900 ps. Presumably, IBr - is still trapped in the 2  state.

IBr - (CO 2 ) 5 A´ 2  State Recombination Coherence peak 12 ps recombination!! 1 ps delay

 Cage fraction 797 nm  12 ps recombination time  1 ps delay before any recombination seen  What happens when we add a 6 th solvent? With 8 solvents, we must reach 1000 ps IBr - (CO 2 ) 5 A´ 2  Summary

IBr - (CO 2 ) 6 A´ 2  State Recombination Cage fraction is ~95 %, recombination in 15 ps; 2 ps delay Coherence peak 15 ps recovery Summarizing the data,

X 2 - (CO 2 ) n A’ 2  Recombination Time Compare with the “understandable” I 2 - recombination IBr -

X 2 - (CO 2 ) n A’ 2  Recombination Time Huge difference for what was supposed to be a minor change! I2-I2- We need theory (and more experiments!)! IBr -

*Robert Parson and Matt Thompson Theoretical Methods* (1)  Solute ab initio  Eigenstates of bare anion  icMRCISD calculated via MOLPRO  S-O coupling, transition DMA and transition angular momentum calculated  Solute-solvent interactions  Distributed multipoles for solute charge density  Solvent polarizes solute wavefunctions  Cluster “structures”  Sample 200 configurations from 1 ns, 80 K molecular dynamics simulation on the ground electronic surface  Find “typical” structures  Calculate initial “solvent coordinate” Model Hamiltonian

*Robert Parson and Matt Thompson Theoretical Methods* (2)  Full nuclear dynamics w/rigid CO 2  Classical path surface hopping using least switches - Tully  Electronic deg of freedom quantum  New IBr - calculation at each time step Nonadiabatic molecular dynamics Need to reduce massive md trajectory data sets to a comprehensible packet of information

Solvent Coordinate,    is the c hange in energy when charge of –e is moved from one solute atom to the other  For a fixed solute configuration,  provides a measure of the solvent asymmetry  Plots of R(I-Br) vs.  provide a reduced dimensionality view of solvent and charge movement throughout a trajectory

IBr - (CO 2 ) n Initial Solvent Asymmetry (from 200 samples of 1 ns, 80 K trajectory) So, let’s look at a single IBr - (CO 2 ) 8 50 ps trajectory.

, eV A single IBr - (CO 2 ) 8 trajectory R I-Br, Å Br+I - Br*+I - Br - +I Br - +I* Now look at an ensemble of trajectories. R (I-Br), Å X

A set of IBr - (CO 2 ) 8 trajectories R I-Br, Å Br+I - Br*+I - Br - +I Br - +I* R(I – Br), Å 01-2 , eV At 50 ps, 89 % of the trajectories are still on the A’ surface! How well do experiment and theory compare?

 Each cluster size has an ensemble of a few hundred to a few thousand trajectories.  Construct a histogram of the time that each successful trajectory reaches some small energy in the ground state.  The resulting data fit as well as was appropriate to a [1 – exp(-t/  )] form, consistent with experimental results.  How do theory and experiment compare? Recombination Time Dependence

IBr - (CO 2 ) n Absorption Recovery Time Experiment and Theory Agreement is remarkable, but we need more experiments

 Solute asymmetry has unexpected, dramatic effect on caging dynamics  Excited state trapping arising from solvent  8 – 12 solvents  Simulations capture a remarkable amount of the important physics!  Essential need for both theory and experiment Summarizing And thank you for your attention!

A’ 2  1/2 B 2  1/2 X 2  1/2 Br - + I * Br - + I I - + Br I - + Br* Energy (eV) Calculations by M.Thompson and R. Parson IBr - (CO 2 ) 8 Potentials with CO 2 frozen at minimum energy configuration

Solvent-induced electronic relaxation

IBr - (CO 2 ) 8 2 ns trajectories R I-Br, Å Br+I - Br*+I - Br - +I Br - +I* R(I – Br), Å 01-2 , eV

IBr - (CO 2 ) 8 : A single trajectory

R I-Br, Å Br+I - Br*+I - Br - +I Br - +I* R(I – Br), Å 01-2 , eV IBr - (CO 2 ) 8 50 ps trajectories At 50 ps, 89 % of the trajectories are on the A’ surface!

IBr - (CO 2 ) 5 A´ 2  State Recombination Even minor features reproduce! Coherence peak

MD Simulation: IBr − (CO 2 ) n 790 nm 50 ps products What about the predicted recombination rate? Clear evidence for solvent induced trapping in A’ state over a small range of solvent sizes!

Fraction Caged 0.55 eV 674 nm 0.35 eV 760 nm 0.30 eV 790 nm Parent Cluster (n) IBr - (CO 2 ) n A´ 2  Caging efficiency depends on energy release One expects only a one solvent shift for the highest KE release. We see more!

IBr - (CO 2 ) n 790 nm A´ 2  Caging Now add 50 ps simulation results I - based (uncaged) Br - based (charge transfer) IBr - based (caged) Experiment

IBr - (CO 2 ) n 790 nm I - and Br - products I - based (uncaged) Br - based (charge transfer) Simulation is solid line - qualitatively OK

IBr - (CO 2 ) n 790 nm IBr - Products 50 ps simulation This modeling was done before the time-resolved experiment; no attention was paid to the IBr - electronic state.

A’ state trapping for 8   n  12. IBr - (CO 2 ) n X and A’ state distributions 50 ps into trajectory What about the predicted recombination rate?

IBr - (CO 2 ) n Minimum Energy Structures R. Parson and M. Thompson

IBr - (CO 2 ) n R. Parson and M. Thompson large average

R. Parson and M. Thompson large average

The cage effect: Time-resolved dynamics of partially solvated dihalides