Coulomb Explosion Imaging and Ionization of - PowerPoint Presentation

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Coulomb Explosion Imaging and Ionization of Diiodomethane in Strong Laser Pulses

Kyle JensenMentors: Dr. Artem Rudenko and Dr. Daniel RollesKansas State University

Motivation

Halomethanes are a hot topic within atmospheric chemistryHalomethanes provide a source of reactive halides leading to ozone destructionS. R. Nichols, “Strong field dynamics and control of molecular dissociation

”, PhD Thesis, Stony Brook University, 2008.In particular, diiodomethane (CH

2

I

2) represents a good test system for studies of atomic motion in molecules triggered by light: dissociation, vibrations, …D. Geißler, B.J. Pearson and T. Weinacht, „Wave packet driven dissociation and concerted elimination in CH2I2”, J. Chem Phys. 127 204305 (2007).The main goal of this project: create a “movie” of the molecule either vibrating or breaking apart (dissociating).

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Overview

BackgroundA Concept of the Pump-Probe ExperimentExperimental SetupResultsSummary and Outlook

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A “snapshot” of molecular motion

Pump pulse excites vibrations (rotations, dissociation, etc…

Distance between the fragments is reconstructed from the measured fragment energies:

Probe pulse removes two electrons from the two nuclei, and the molecule “explodes” due to the Coulomb repulsion: “

Coulomb explosion imaging”

molecule

Pump-Probe Concept

Experimental Setup

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Interferometer

COLTRIMS:

A “Reaction Microscope”

for ions

Laser

Mach-Zehnder Interferometer

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COLTRIMS

Cold target recoil ion momentum

spectroscopyDetects resulting ions and electrons after ion/photon interactions with a gas target (measures mass to charge ratios, kinetic energies and emission angles

)

Provides insight for a target’s potential dissociation channels

C. M. Maharjan, “Momentum imaging studies of electron and ion dynamics in a strong laser field”, PhD Thesis, Kansas State University, 2007.710-6

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Laser

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Gas JetE,B

COLTRIMS

Main componentsGas jetUltrahigh vacuum chamberPressure: 10-11 torr

Differential pumping stagesSpectrometer

Time- and

Position Sensitive

Detector (MCP + Delay line)Focusing mirror (not shown)Laser directed into vacuum from the back810-6

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-4

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Laser

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Gas Jet

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Experimental Specifications

Pulsar Laser Specs25 fs pulseWavelength: 800 nmRepetition Rate: 10 kHzIntensity:

Molecule: CH

2

I

2Ionization Potential: 9.34 eVVapor Pressure: 141 torrVibrational period (bending): ~300 fs 9

Diiodomethane (CH2I

2

)

~300 fs

Data AnalysisCalibrate Time of Flight (

ToF) spectra(define mass to charge ratios)Classify potential dissociation channels and measure fragment energiesAnalyze the ion yields and kinetic energy release (KER) as a function of pump-probe delay

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ToF Ion Calibration

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ToF Spectrum

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CH2I

2

++

I2+

CH

2

+

CH

2

I

+

I

+CH

2I2+H2O+I++

Classification of Dissociation Channels: Ion-Ion Coincidence Spectrum

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CH2

I

+

+I+

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~300 fs

- I

+

CH

2

I+

I

2

+

CH

2

I

2

+

Delay / fsNormalized ion yieldDelay-dependent yields of different fragments300 fs: bending vibrations!

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~300 fs

~300 fs

Enlarged view of the CH

2

I

+

channel

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Delay

(

fs)

KER

(eV)Delay-dependent KER of CH2I+ + I+ coincident channel

Bound molecule

Dissociating molecule

Conclusions and OutlookWe have measured the ionic fragments after ionization, dissociation and Coulomb explosion of CH

2I2 molecule in a pump-probe experimentWe observe bending vibrations of the molecule and mapped one of the dissociation pathwaysIn the future, this technique will be applied to study different light-induced reactions in similar molecules

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AcknowledgementsThis material is based upon work supported by National

ScienceFoundation Grant: NSF grant number PHYS-1461251Dr. Artem RudenkoDr. Daniel Rolles

Lee PearsonBalram KaderiyaFarzaneh

Ziaee

Raju

Pandiri18