Electron – hydrocarbon molecular ion reactions

Mark Bannister, Randy Vane, Herb Krause, Eric Bahati, Mike Fogle, DRS

Oak Ridge National Laboratory

and collaborators

Nada Djuric, Duska Popovic, Momir Stepanovic, Gordon Dunn, Yang-Soo Chung, Tony Smith, Barry Wallbank, Rich Thomas, Vitali Zhaunerchyk

• Provide experimental benchmarks for a portion of the full database of electron – hydrocarbon reactions needed to model edge/divertor physics of ITER and other devices

• Ultimately explore and similarly provide experimental benchmark results for “state-selective” reactions

• Develop models capable of representing a broad range of electron – hydrocarbon data

Objectives

• Reactions studied to this point in time and those planned for study

• Laboratory facilities used to make the measurements

• Presentation of the measured data and comparison to other experimental and theoretical results (Bannister)

• Preliminary molecular dynamics – energy deposition model results

• Summary/outlook

Outline

CHx+ Electron-Impact Dissociation

• CH+ → C+

• CH2+ → CH+, C+ (CH2

2+)

• CH3+ → CH+, C+

CD3+ → CD2

+

Reactions studied to this point in time

• Crossed-beams (DE, DI, ionization)

– CH4+, C2H2

+, C2H3+,…

• Merged-beams (DR, DE)

– CH+, CH2+,…

– Energy-loss technique to measure direct

excitation of CH2+ in the 5-15 eV range

Future CHx+ Dissociation Experiments

Caprice ECR

ECR, cold molecular ion source, HV platform

Ion-neutral Merged-beams

Electron-ion Merged-beams

Grazing ion-surface, ion-solid

Ion trap

Electron-ion crossed-beams

Normal incidence ion-surface

Floating beamline

The ORNL Multicharged & Molecular Ion Research

Facility (MIRF)

Recoil ion spectrometer COLTRIMS

Caprice ECR Ion Source• Produces a broad range of charge states and species

• Gas feed, mini-oven, biased sputter probe

• 50 kW coil power, RF, analyzer, pumps, controls

• 300 lb water coil cooling

All Permanent Magnet ECR Ion Source

• Five element optic developed for floating ion-surface scattering experiment will be used on the new Caprice ECR floating beamline

Deceleration Optics for Floating Beamline

• Performance better than Caprice source

• All permanent magnet design, no axial field power supplies (50 kW savings)

• No separate cooling loop for hexapole

• Optimum for placement on high voltage platform

Cold molecular ion source/trap – developing cold molecular ion sources, place on high voltage platform for acceleration towards endstations - MEIBEL, ion-atom, COLTRIMS

– building an electrostatic reflecting beam ion trap, use it to further cool molecular ions, feed cooled ions to diagnostic experiments

– develop local expertise and capabilities in state-prepared molecular ion production and science, extension of collaboration at CRYRING

CrossedElectronBeam

Electrostatic Mirror

Cryo-Cooler (4 K)

Electrostatic Mirror

Ion

Sou

rce

CCDCamera

Injection offusion relevant

molecules,biomolecules,atmospheric molecules

Injection offusion relevant

molecules,biomolecules,atmospheric molecules

Trapping of molecules to cool and interact with

electrons, photons, and neutrals

Trapping of molecules to cool and interact with

electrons, photons, and neutrals

Reaction microscope – analyze fragments to

determine reaction rates, chemical branching

fractions, distributions of kinetic energy release

Reaction microscope – analyze fragments to

determine reaction rates, chemical branching

fractions, distributions of kinetic energy release

Cold molecular ion source/trap

Electron-ion collisions, crossed-beams

• Ions from ECR source interact at 90° with magnetically confined electron beam

• Product ions are magnetically analyzed and detected by CEM or fast discrete dynode detector

• Parent ions collected in one of 3 Faraday cups

• Electrons chopped to separate signal from background due to ionization on residual gas

Example: CH2+ Dissociation

• In the 1-5 eV range, DR (black) is the dominant channel

• For E=5-15 eV, DE leading to CH+ (red) and C+ (blue) fragments is largest

• For E>20 eV, DE/DI producing H+ (purple) fragments is dominant

• Surprisingly, ionization yielding CH2

2+ (green) ions is only a factor of 10 less than DE/DI of CH+ and C+ fragments at 100 eV

Merged electron-ion beams apparatus

• ECR source on 250-kV platform enables detection of neutral fragments from DR

• Measurements of DR rate coefficients using energy-sensitive particle counting detector

• Imaging of neutral fragments – study dynamics of dissociation

• Segmented SBD being developed will be energy- and position-sensitive down to 10 keV protons

particle counting detector

fragment imaging detector

DR on MEIBEL: Rate Coefficients

DR of 120 keV H2+ ions by Ecm = 0 – 1 eV electrons

MEIBEL

CRYRINGLarsson et al.1995(v=0,1)

Single-pass expts

ro-v

ibra

tional

tem

pera

ture

DR rate for H2+ is

strongly dependenton ro-vibrational distribution

Auerbach

Peart &Dolder

e- + H2+ (v) → H(1s) + H(nl) + KER(n,v)

• Absolute cross sections for production of CHx+ (x=0,1,2) ion

fragments that are sum of channels:

– CH+ → C+ + H Dissociative excitation (DE)

– CH+ → C+ + H+ Dissociative ionization (DI)

– CH+ → C+ + H- Resonant ion pair formation (RIP) this should be very

small

Total expanded uncertainties are at a level equivalent to 90%-confidence for statistics

• Experimental data are compared to data of Janev and Reiter from Report Jülich-3966 and from the HYDKIN online database, including all channels where available:

– Direct DE

– Capture-autoionization dissociation (CAD) – also known as resonant DE

– DI

Description of data presented

CH+ → C+

CH+ → C+, H+

CH2+ → CH+

CH2+ → CH+

Two possible mechanisms for the DE enhancement in the 5-15 eV range:

(1)Allowed excitations to 2A, 2B electronic states followed by pre-dissociation

(2) CAD(RDE) through Rydberg states of CH2 that converge to

the electronic states of CH2

+

CH2+ → C+

CD3+ → CD2

+

CH3+ → CH+

CH3+ → C+

Molecular dynamics energy deposition models

• Goal: Develop relatively simply computational models which can predict electron – molecular ion fragmentation cross sections for a wide range of systems and impact energies

• Motivation: Experience has shown that most electron – molecular ion reactions require very detailed quantum structure and quantum scattering calculations

• Approach: Use a molecular dynamics approach, building in more and more levels of complexity as needed, coupled with an energy deposition model

• nuclear motion treated to varying degrees of completeness – fixed at equilibrium distances, moving on model curves, full quantum chemical potentials

• electronic state binned given computational quantum chemistry values of dissociation energies, molecular orbital energies

• further elaborations possible, e.g., Fermion molecular dynamics for electronic motion to approximate dynamic correlation

MD energy deposition model results: e + CH+

MD energy deposition model results: e + CH+

MD energy deposition model results: e + CH+

Dissociative recombination

Summary/outlook

• Data for dissociative channels measured at ORNL for several hydrocarbon molecular ions, setting key experimental benchmarks for the overall database needed in fusion

• Further “hot” ion source measurements planned for dissociative excitation, ionization, and recombination of hydrocarbon molecular ions to provide similar benchmarks for other species

• New “cool” source, trapped and cooled, molecular ion measurements planned to begin determination of state controlled benchmarks for DE and DI

• Continued development and exercise of the molecular dynamics energy deposition model in order to provide data over the widest range of species and impact energies