Nonadiabatic MD Simulations of IBr-(CO2)n Photodissociation

IBr− Simulations Thompson, Parson Experiment Theory Model Hamiltonian Nonadiabatic MD Simulations of Nonadiabatic MD Minimal Structures IBr− (CO2 )n Photodissociation IR Results Branching Ratios Excited-State Trapping Long-time Simulations UV Results M. Thompson R. Parson Branching Ratios Summary JILA University of Colorado at Boulder 2006-01-24 / Lineberger Group Meeting

IBr− Simulations Outline Thompson, Parson Experiment Experiment Theory Model Hamiltonian Nonadiabatic MD Theory Minimal Structures IR Results Model Hamiltonian Branching Ratios Nonadiabatic MD Excited-State Trapping Long-time Simulations Minimal Structures UV Results Branching Ratios Summary IR Results Branching Ratios Excited-State Trapping Long-time Simulations UV Results Branching Ratios

IBr− (CO2 )n Photodissociation IBr− Simulations Thompson, Parson Lineberger Group Experiment Theory Model Hamiltonian Nonadiabatic MD Minimal Structures IR Results Branching Ratios Cluster ions generated in expansion Excited-State Trapping Long-time Simulations Ions size-selected via mass spectrometer UV Results Branching Ratios Laser pulse dissociates cluster Summary Product ratios detected by mass spectrometry Ground-state recombination studied via pump-probe

IBr− Simulations Model Hamiltonian Thompson, Parson Experiment Theory Model Hamiltonian Solute ab initio Nonadiabatic MD Minimal Structures Eigenstates of bare anion IR Results icMRCISD calculated via MOLPRO Branching Ratios Excited-State Trapping Spin-orbit coupling, transition DMA, and transition Long-time Simulations angular momentum calculated UV Results Branching Ratios Solute-solvent interactions Summary Distributed multipoles for solute charge density Solvent polarizes solute wavefunctions Dispersion-repulsion Pairwise atom-atom potentials Fit to replicate CR-CCSD(T) X− · · · CO2 calculations

IBr− Simulations Potential Energy Curves Thompson, Parson Experiment 2 Theory Model Hamiltonian 6-state icMRCI using Nonadiabatic MD Minimal Structures ECPnMDF ECPs with 1.5 2+ B2 Σ 1/2 IR Results CPP Branching Ratios Excited-State Trapping 2 a' 2 Π1/2 Augmented basis: 1 Long-time Simulations − I* + Br UV Results (7s7p3d2f)/[5s5p3d2f] − I + Br* Energy (eV) Branching Ratios 2 a 2 Π3/2 Spin-orbit effects via 0.5 Summary − I + Br SO-ECP 2 A' Π1/2 2 A Π3/2 − 0 Transition DMA, NACME, I + Br transition angular momentum -0.5 2+ XΣ 1/2 -1 2 3 4 5 6 7 8 R (Ang)

IBr− Simulations Potential Energy Curves Table of Energetics (in eV) Thompson, Parson Experiment Theory Model Hamiltonian Nonadiabatic MD Minimal Structures IR Results MDF Expt. ∆ Branching Ratios Asymptotes: 0.0000 0.0000 0.0000 Excited-State Trapping Long-time Simulations 0.3156 0.3045 -0.0111 UV Results Branching Ratios 0.7393 0.7614 0.0221 Summary 0.8932 0.9427 0.0495 Spin-Orbit: Br: 0.4237 0.4569 0.0332 I: 0.8932 0.9427 0.0495 0.3156 0.3045 -0.0111 ∆EA:

IBr− Simulations Model Hamiltonian Thompson, Parson Experiment Theory Model Hamiltonian Solute ab initio Nonadiabatic MD Minimal Structures Eigenstates of bare anion IR Results icMRCISD calculated via MOLPRO Branching Ratios Excited-State Trapping Spin-orbit coupling, transition DMA, and transition Long-time Simulations angular momentum calculated UV Results Branching Ratios Solute-solvent interactions Summary Distributed multipoles for solute charge density Solvent polarizes solute wavefunctions Dispersion-repulsion Pairwise atom-atom potentials Fit to replicate CR-CCSD(T) X− · · · CO2 calculations

IBr− Simulations Nonadiabatic Molecular Dynamics Thompson, Parson Experiment Theory Model Hamiltonian Nonadiabatic MD Classical path surface-hopping using least switches Minimal Structures (Tully, 1990) IR Results Branching Ratios Nuclear deg. of freedom, R(t) Excited-State Trapping Long-time Simulations Elec. deg. of freedom quantum, ci (t) UV Results Branching Ratios ˙ ˙ quantum: ı ci (t) = ci Ei − ı j cj R(t) · dij Summary ¨ classical: M R(t) = φn | R H|φn Hops preserve probabilities |ci (t)|2 in an ensemble of trajectories Requires only H(R) and its derivatives

Minimum Energy IBr− (CO2 )n Structures IBr− Simulations Thompson, Parson Experiment Theory Model Hamiltonian Nonadiabatic MD Minimal Structures IR Results Branching Ratios Excited-State Trapping Long-time Simulations UV Results Branching Ratios Summary

IBr− Simulations 50-ps Simulations at 790 nm Thompson, Parson Experiment 100 Theory 80 I− channel remains open Model Hamiltonian Experiment 60 Nonadiabatic MD − %I Theory at larger cluster size Minimal Structures 40 IR Results 20 Br− more prevalent in Branching Ratios 0 Excited-State Trapping 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 simulation usu. at cost of Long-time Simulations 100 IBr− in medium clusters UV Results 80 Branching Ratios 60 − At n > 8, IBr− product % Br Summary 40 dominates, but... 20 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 100 80 − 60 % IBr 40 20 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 No. of CO2

50-ps Simulations at 790 nm - GS IBr− Only IBr− Simulations Thompson, Parson Experiment 100 Theory 80 IBr− product in Model Hamiltonian Experiment 60 Nonadiabatic MD − %I Theory Minimal Structures medium-size clusters are 40 IR Results primarily trapped on 20 Branching Ratios 0 Excited-State Trapping excited-state 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Long-time Simulations 100 UV Results Can we visualize this 80 Branching Ratios 60 − trapping using % Br Summary 40 IBr− (CO2 )8 ? 20 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 100 80 − 60 % IBr 40 20 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 No. of CO2

IBr− Simulations Solvent Coordinate, ∆Φ Thompson, Parson Experiment Theory Model Hamiltonian Change in energy when Nonadiabatic MD Minimal Structures charge of −e is moved IR Results from one solute atom to Branching Ratios Excited-State Trapping another Long-time Simulations UV Results For a ﬁxed nuclear Branching Ratios conﬁguration, provides Summary measure of the solvent asymmetry Plots of R v. ∆Φ provide a picture of solvent and charge movement in a trajectory

Excited-State Trapping of IBr− (CO2 )8 IBr− Simulations Thompson, Parson Experiment Theory Model Hamiltonian Nonadiabatic MD 89% of trajectories Minimal Structures trapped in A state after IR Results Branching Ratios 50 ps Excited-State Trapping Long-time Simulations Only 5% relax to UV Results Branching Ratios ground-state Summary Expt. generally agrees with no g.s. recovery at early times in pump-probe

IBr− (CO2 )8 PE Surface IBr− Simulations Thompson, Parson Experiment 2 Theory Generated as a "pull" Model Hamiltonian Nonadiabatic MD surface from an Minimal Structures 1 IBr− (CO2 )8 minimal IR Results Branching Ratios energy structure Excited-State Trapping Long-time Simulations Surface shows a well 0 UV Results Branching Ratios generated due to solvent E (eV) Summary effects on A state -1 Increase in excitation energy (730 nm) does increase 50-ps IBr− GS -2 yield -3 2 3 4 5 6 7 8 R (Ang)

2-ns IR Simulations of IBr− (CO2 )8 IBr− Simulations Thompson, Parson Method and Results Experiment Theory Model Hamiltonian Nonadiabatic MD 100 2-ns trajectories Minimal Structures IR Results 75 relaxed to ground Branching Ratios Excited-State Trapping state Long-time Simulations UV Results Cluster needs to achieve Branching Ratios more symmetric Summary conﬁguration to allow transition to ground-state

2-ns IR Simulations of IBr− (CO2 )8 IBr− Simulations Thompson, Parson Ground-state Recovery Experiment Theory 1.5 80 Model Hamiltonian Nonadiabatic MD Minimal Structures IR Results No. of Recombined Trajectories 60 Branching Ratios Excited-State Trapping τ = 540 ps 1 Long-time Simulations Signal (Arb. Units) UV Results Branching Ratios 40 Summary 0.5 τ = 1293 ps 20 Pump-Probe Data Exponential Fit Simulation 0 0 0 500 1000 1500 2000 Recovery Time (ps)

IBr− Simulations 50-ps Simulations at 355 nm Thompson, Parson Experiment 100 Theory 80 Worse agreement with Model Hamiltonian 60 Nonadiabatic MD − %I experiment cf. IR Minimal Structures 40 IR Results simulations, but pattern 20 Branching Ratios 0 Excited-State Trapping is there 0 1 2 3 4 5 6 7 8 9 10 11 Long-time Simulations 100 Too small Br· · · CO2 UV Results 80 Branching Ratios 60 − attraction leads to % Br Summary 40 excess Br− product? 20 GS recombination due to 0 0 1 2 3 4 5 6 7 8 9 10 11 100 too facile spin-orbit 80 Experiment relaxation? Theory − 60 % IBr 40 20 0 0 1 2 3 4 5 6 7 8 9 10 11 No. of CO2

IBr− Simulations Summary Thompson, Parson Experiment Theory Model Hamiltonian Nonadiabatic MD Minimal Structures We have constructed an accurate potential energy IR Results surface for IBr− with associated properties. Branching Ratios Excited-State Trapping Long-time Simulations Simulations of IR photodissociation show good UV Results agreement with experimental product trends. Branching Ratios Summary Long-time IR sims provide conﬁrmation and explanation for long expt. GS recombination time UV simulation agreement generally there, but shows discrepancies

Nonadiabatic MD Simulations of IBr-(CO2)n Photodissociation

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