Dissociation of Molecular Ions Studied by Coincidence Mass Spectrometry Pengqian Wang Department of Physics Western Illinois University Presented at Physics Class March 2, 2017

Macomb, ILLINOIS

In this talk Introduction to the Problems 1) Dissociation of molecular ions 2) Challenges: Pathway-specific measurement Experimental Methods 1) Coincidence mass spectrometry 2) Three-dimensional momentum imaging Some Experimental Results 1) Electron impact dissociative ionization 2) Molecules interact with intense lasers

Motivations for studying the dissociation of molecular ions Exploring the structure and energy states of molecules and molecular ions, which is a main goal in studying atomic and molecular physics. Understanding the dynamics of molecules when interacting with particle beams and intense lasers.

Introduction: mass spectrometry Mass spectrometry is the technique of measuring the spectrum of the masses of the constituent atoms or molecules in a material. It is usually performed by ionizing the sample using an electron beam and measuring the resultant ion fragments by a mass spectrometer. Mass spectrometers are also used to measure the initial momentum and kinetic energy distribution of the fragments, which are important in studying the molecular dissociation processes.

Dissociation of molecular ions: Molecular orbitals and potential energy curves Dissociative ionization Light or electron beams Ionization The measurable quantities are the fragment species, their momentum and the kinetic energy release. Dissociation Excitation

Dissociation channels of molecular ions Example: Dissociative ionization of CO _________________________________ Parent ion Dissociation channel CO  CO+  CO+  C+ + O  C + O+  CO2+  CO2+  C2+ + O  C + O2+  C+ + O+  CO3+  … _________________________________

Challenges in dissociation of molecular ions: Pathway-specific measurements Challenge #1: Challenge #2: Distinguishing different channels. Pathway-specific measurement: Identifying different pathways. CO+  C+ + O CO2+  C+ + O+ CO2+  C+ + O+ + Ek1 CO3+  C+ + O2+ CO2+  C+ + O+ + Ek2 Solution: Coincidence mass spectrometry Solution: Momentum imaging ~1 molecule dissociates per pulse. Measure the three-dimensional momentum of each fragment. Record the mass spectrum for each pulse.

Experimental Methods

Electron impact dissociative ionization of molecules: Experimental scheme Ion Detector Data Acquisition Mass Spectrometer Ions Pulsed Electron Gun Window Vacuum Chamber Vacuum Gauge Gas Valve

Electron impact dissociative ionization of molecules: apparatus Gas Sample in Turbo-molecular Pump Mass Spectrometer Electron Gun Control Vacuum Gauge Gas Valve Control

Time-of-flight mass spectrometer Acceleration Region 1 Acceleration Region 2 Flight Tube −V1 −V2 Backing Plate (Grounded)

Conventional non-coincidence time-of-flight mass spectrum: Example Electron impact ionization of carbonyl sulfide (OCS), 200 eV

Coincidence time-of-flight mass spectrum: Principle Single pulse mass spectra (~1 molecule/pulse) Corresponding raw coincidence mass spectra Accumulated raw coincidence mass spectra

Refining the data: Covariance mapping Raw HNCO Time-of-flight False True Covariance mapping: = – Raw False True

Three-dimensional momentum imaging: Time- and position-sensitive detectors Microchannel plate Resolution: 0.5 ns, 250 mm Delay-line anode

Some Experimental Results

Distinguishing the dissociation channels Example: Electron impact ionization of normal-butane, 200 eV n-C4H10 n-butane

Measuring the cross section data for different dissociation channels Example: Electron impact ionization of isocyanic acid (HNCO), 200 eV HNCO2+ → X+ +Y+ + n HNCO3+ → X+ +Y+ +Z+ + n

Determining the dissociation sequence Example: Tree-body sequential dissociation p - p A+ B +C B + C B+ q HNCO HNCO2+ HN+ + CO+ CO+ C+ + O -arctan(CO/C)= -66.8 (Exp. -66.33) HNCO2+  HNC+ + O+ HNC+  C+ + NH -arctan(HNC/C)= -66.0 (Exp. -66.73)

Three-dimensional momentum imaging q laser field p1 p2 x Example: H2+ + nhn (790 nm, 35 fs) H+ + H (x1, y1, t1, x2, y2, t2)  (p1x, p1y, p1z, p2x, p2y, p2z) Refined by p1 + p2 = 0 Laser field

Dissociation from different vibrational levels Example: H2+ + nhn  H+ + H Momentum distribution Kinetic energy distribution Laser field 1ssg

Summary Coincidence mass spectrometry distinguishes different dissociation channels and pathways. We measured the cross section data and studied the dynamics of the electron impact dissociative ionization of molecules. We performed three-dimensional momentum imaging for the dissociation of molecular ions in intense laser fields.

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