Chemical reaction / photodissociation dynamics

Photoelectron spectroscopy, in all its forms, is a powerful tool for gleaning structural and dynamical information from atomic and molecular targets. However, many targets and processes have been out of reach due to the inefficiencies, or poor resolution, of the techniques used.

To reliably study difficult systems high resolution, high efficiency, high signal-to-noise and robustness is essential. We have recently developed a co-axial velocity-map imaging spectrometer which addresses all of these issues. By re-working the electrostatic electrode structure and aspects of its operation, we have achieve resolving powers of E/ΔE = 250 - 270 while using an effective interaction region size of 2mm3.

The technique of charged particle imaging for photodissociation, introduced by Chandler and Houston in 1987 [1] provides a method for the simultaneous determination of all particle speeds, including the complete angular distribution. Application of this technique has expanded rapidly following the seminal paper by Eppink and Parker [2], which described an electrostatic lens design capable of imaging a finite sized interaction region to yield images from which more detailed information could be derived.

Our implementation of a velocity-map imaging lens, located co-axial with the ion-beam of a negative-ion photofragment spectrometer, yields photodetachment spectra of gas-phase anions which demonstrate an energy resolution ΔE/E = 0.4% [3]. This is superior to any alternative photoelectron experimental technique and it represents a significant achievement for imaging methods.

Very high resolution velocity-map imaging of photoelectrons

Fast-beam negative ion spectrometer
Fast-beam negative ion spectrometer - located within RSPhysSE at the ANU

Experimental apparatus

experimental diagram
Anion photoelecton imaging apparatus, operation of which may be considered in three stages

Source region

  1. Pulsed supersonic jet
  2. Electron gun
  3. Molecular beam skimmer
  4. Extraction and acceleration of negative ion to a nominal energy
  5. Re-referencing, gating, and bunching of ion-packet

2m Time-of-flight mass separation stage

  1. & 8. Electrostatic ion beam deflectors
  2. four unipotential, or Einzel lenses
  1. beam apertures
  2. Potential re-referencing switch, beam deflectors & potential barrier: in combination with the ion gating, bunching, and potential re-referencing unit, mass resolving powers of m/Δm = 350-400 are achieved [1].

Imaging region

  1. Velocity-map imaging system - based on Eppink and Parker design [2], but modified and placed on an on-axis orientation (ΔE/E ~ 0.2-0.3 %
  2. (hν) 2nd harmonic (532nm) of Nd:YAG laser, Continuum Powerlite 9010
  3. matched pair of 75mm high spatial resolution micro-channel plates (MCPs) coupled to a phosphor detector
  4. high resolution CCD camera (1600 x 1200 pixels) centroided to 4800 x 3600 pixels, data analysis dome using a modified Abel inversion method [3a, 3b]

Experimental parameters

Ion beam energy: 500-535 eV
Ion beam divergence: ~ 1-2x103 rad
Ion beam size: 2.0 mm3
Laser: seeded Nd:YAG
Laser beam wavelength: 532 nm
Laser beam size: 2 mm diameter
Interaction region size: 2 mm3
ΔE/E: 0.38 %
Δv/v: 0.19 %

Photodetachment spectroscopy

Emperimental data for photodetachment of O- at 532 nm
Emperimental data for photodetachment of O- at 532 nm

O-(2P3/2,1/2)+hν→O(3P2,1,0)+e-

  1. Raw, unprocessed, photoelectron image (4800x3600 pixels)
  2. Inverse Abel transformed image
  3. A 10o slice through the raw PE image (horizontal and vertical)
  4. Integrated total PE intensity taken from transformed image
Total photoelectron spectrum
Total photoelectron spectrum

The spectrum (circles) is well represented by Gaussian functions (lines)

Chemical reaction dynamics

For some molecular species only the negative ion exists, with the neutral breaking apart if the negative charge is lost. The corresponding neutral species may represent an intermediate state of a chemical reaction (ABC), A + BC ↔ ABC ↔ AB + C, that is present for a short period of time, called a transition state. The transition state controls the rate of a reaction and is very difficult to study due to its short lifetime during the course of a chemical reaction. The photodetachment process provides a method to access the transition state directly

Photodetachment may yield a radical which is energetically unstable, that breaks apart. The spectra reveal details of the anion an neutral reaction path potential surfaces

References

[1] D. W. Chandler and P. L. Houston, Journal of Chemical Physics 87, 1445-1447 (1987).
[2] A. T. J. B. Eppink and D. H. Parker, Review of Scientific Instruments 68, 3477-3484 (1997).
[3] S. J. Cavanagh, S. T. Gibson, M. N. Gale, C. J. Dedman, E. H. Roberts, and B. R. Lewis, Physical Review A, submitted (July 2007).
[4] C. J. Dedman, E. H. Roberts, S. T. Gibson, and B. R. Lewis, Review of Scientific Instruments 72, 2915-2922 (2001).
[5] Research highlight (PDF 441k), RSPhysSE Annual Report (2003).

Contacts

Gibson, Stephen profile
Senior Fellow
53075
Cavanagh, Steven profile
Departmental Visitor

Updated:  15 June 2016/ Responsible Officer:  Head of Department/ Page Contact:  Physics Webmaster