# Presentation: 685042

## ELECTRON- AND PHOTON-INDUCED CHEMISTRY: RELEVANCE TO ENVIRONMENTAL REMEDIATION NEEDS

- Thom Orlando
- Environmental Molecular Sciences Laboratory
- K8-88, P.O. Box 999
- Pacific Northwest National Laboratory
- Richland, WA 99352.

## ELECTRON- AND PHOTON-INDUCED CHEMISTRY: RELEVANCE TO ENVIRONMENTAL REMEDIATION NEEDS

- Stability of Materials and Hydrogen/Oxgen Production
- During Storage of Spent Nuclear Fuels  (Explosion Hazards?)
- Gas Production in Mixed (chemical/radioactive) Waste
- Storage Environments (Flamabilty/Explosion Hazards?)
- Low-temperature Plasma Destruction of Toxic Solvents
- Low-temperature Plasma Remediation of NOx - Surface
- and Materials Chemistry Effects.

## Slide 3

## Where theory can help in understanding electron-interactions with molecular solids...

- Example:Water
- 	Feshbach resonances at deep valence levels?
- 	Are these shape resonances?
- 	What effects DEA yields in the condensed phase?
- 	Are cross sections bigger at the surface or in the bulk?
- 	Do two hole states persist in the condensed phase?
- 	Do holes hop in the a1 bands?
- 	Ionization cross sections in the condensed phase?
- 	Do most excited states autoionize in the condensed phase?
- 	What are the excited states which lead to
- 	molecular elimination?

## ULTRAHIGH VACUUM SYSTEM FOR

**ULTRAHIGH VACUUM SYSTEM FOR**
**ELECTRON/PHOTON**
**STIMULATED INTERFACE**
**REACTIONS**

## RESONACE ENHANCED MULTIPHOTON IONIZATION (REMPI) AND TIME-OF-FLIGHT (TOF) TECHNIQUES

- Phosphor
- Screen

- Signal

- E-Beam

- Laser

- Gated Integrator

- O atom

- 

- Ion Extraction Pulse

- Field Free
- Region

- Chevron CEMA

- -800 V	+500V +1500 V

- Electron or Excimer Beam

- REMPI
- Beam

- IP

- (3P)

- (3P)

- O+ Ion

- REMPI

- O+

- O

- TOF

- D2O + e -> D2 + O-

- 106 molecules/cm3 q.s.

- D2

## Temperature Dependence of DEA ResonancesW. C. Simpson et. al. J. Chem. Phys. 107, 8668 (1997)

## THRESHOLDS FOR THE ELECTRON STIMULATED DESORPTION (ESD) OF O+, H+  AND OH+  IONS FROM THE SURFACE OF CUBIC  ZrO2 FOLLOWING EXPOSURE TO WATER VAPOR

**THRESHOLDS FOR THE ELECTRON STIMULATED DESORPTION (ESD) OF ****O****+****,**** H****+****  ****AND**** OH****+****  ****IONS FROM THE SURFACE OF CUBIC  ****ZrO****2**** ****FOLLOWING EXPOSURE TO WATER VAPOR**

- The data are offset vertically for display  and the size of the symbols represents the estimated uncertainty in the measurements

## ELECTRONIC STRUCTURE OF ZIRCONIUM OXIDE

**(30 ± 2 eV) **

- CB

- O(2p) / VB

- O(2s)

- Zr(4p)

- Zr(4s)

- Y(4p)

- Y(4s)

- Zr(4d) / CB

- O(2p) / VB

- O(2s)

- Zr(4s)

**O**

**2**

** ****/ Zr :**

**ZrO**

**2**

**(100):**

- Zr(4p) metal

- Zr(4p) oxide

- 10

- 20

- 30

- 40

- 50

**Binding Energy (eV)**

**(30 ± 1 eV) **

## 4+

**4+**

**2-**

- Stimulated desorption of O+ from ZrO2 involves a
- multi-electron Auger decay process. (See *M. Knotek *
*and P. Feibelman, Phys. Rev. Lett. 40, (1978) 964).*
- In this model, a Zr(4p) core hole, formed by the impact
- of an incident electron or photon, is filled by a valence
- electron residing on a neighboring oxygen atom.  To
- conserve energy, one or two additional electrons are
- ejected from the same atom.  Once formed, the O+
- desorbs due to the reversed Madelung potential.
- We are presently examining the neutral oxygen yields.

_**  **__**ZrO**__**2**__** DAMAGE MECHANISM:**_

## EXPERIMENTAL

**EXPERIMENTAL**

- N(H2O)
- DOSAGE

- N(H2)
- ANALYSIS

- 60Co
- GAMMA SOURCE
- 1.8Gy/s @ 300K

-   MexOy OXIDE

- GAS
- CHROMATOGRAPH

## • HYDROGEN YIELDS FROM WATER ADSORBED ON DIFFERENT OXIDE SURFACE PLOTTED VS. OXIDE BAND GAP SHOW  MAXIMUM  NEAR  5 eV • 5.1 eV IS AN ENERGY OF H-OH BOND BREAKING IN WATER MOLECULE

**ACCORDING TO EFFECT ON THE ****H****2**** YIELD OXIDES CAN BE CLASSIFIED INTO THREE GROUPS:**
**INHIBITORS : G(H****2****)=10****-3****-10****-1 ****(100eV)****-1 **
**Oxides which lower the H****2**** yield  as compared with the radiolysis of pure gas-phase water:**
- MnO2 , Co3O4 ,CuO and Fe2O3
**INDIFFERENT : G(H****2****)=0.1-3 (100eV)****-1 **
**Oxides with H****2****  yields which are similar to or slightly greater than radiolysis of pure gas-phase water: **
- MgO, CaO, SrO, BaO, ZnO, CdO, Cu2O, NiO, Cr2O3 , Cr(OH)3 , Al2O3 , CeO2 , SiO2 , TiO2 , Nb2O5  and WO3
**PROMOTERS : G(H****2****) > 3 (100eV)****-1  ****(up to 100-130) **
**Oxides which increase the H****2**** yield as compared with the radiolysis of pure gas-phase water:**
- Ga2O3 , Y2O3 , La2O3 , Nd2O3 , Sm2O3 , Eu2O3 , Gd2O3 , Yb2O3 , Er2O3 , HfO2 and ZrO2
**NOTE:   **the G-values quoted above are defined in terms of the radiant energy adsorbed directly by the water molecules.

## •THE HIGHER  G-VALUES (>3) ARE  DUE TO  EFFECTIVE  ENERGY TRANSFER AT THE OXIDE-WATER INTERFACE•THE STANDARD G-VALUE DEFINITION  HAS  TO BE MODIFIED WHEN DISCUSSING RADIOLYSIS AT INTERFACES

-  (1MeV)

*d**0** =?*

*?*

*?*

**QUESTIONS:**
- WHAT  KIND  OF
- PARTICLES OR EXCITATIONS  TRANSFER THE ENERGY TO THE SURFACE?
-  DISTANCE OF
- THE ENERGY MIGRATION?
- REACTION OF THE ADSORBED MOLECULE  WITH  MIGRATING  PARTICLES OR EXCITATIONS?

## MECHANISM OF THE ENERGY TRANSFER TO WATER MOLECULES:

****(1MeV)+ ZrO2    ex* (~5eV)
- ex* (~5eV)************H2OADS  H2O*(A3,1B1)ADS
-  H2O*(A3,1B1)ADS H + OH  or H2 + O

**PARAMETERS CONTROLLING THE ENERGY TRANSFER:**
* *THE OXIDE BAND GAP AND WATER ADSORPTION FORM
- THE DISSOCIATION THRESHOLD OF THE H2OADS COMPLEX  SHOULD OVERLAP THE EXCITON  ENERGY SPECTRUM
-  ENERGY MIGRATION DISTANCE
- THE ENERGY (EXCITON) MIGRATION DISTANCE  SHOULD BE MUCH LONGER THAN CRYSTAL LATTICE PARAMETER TO PROVIDE HIGH  EFFICIENCY OF THE  WATER RADIOLYSIS  IN COMPARISON TO  HOMOGENEOUS  PROCESS

## Sodium nitrate (NaNO3) is a major solid component of the radioactive/ chemical mixed wastes, and is present both as salt cake and in colloidal suspension. Experiments to date have shown that exposure of NaNO3 single crystals to low-energy (5 - 100 eV) electrons results in highly efficient destruction of the NO3- and  the release of ionic and neutral species, predominantly the persistent radical NO 

**Primary Radiation Promotes Radiolysis **
**via Creation of Secondary Electrons**

- High energy primary
- electrons react with
- matter to create
-  thousands of low
- energy secondary electrons.
- Secondary electrons react with solid and liquid phases to create electron-hole pairs and radicals.
- Reactive  ions and radicals undergo redox and free radical reactions with organics to promote decomposition.

**Sodium nitrate (NaNO****3****) is a major solid component of the radioactive/ chemical mixed wastes, and is present both as salt cake and in colloidal suspension. Experiments to date have shown that exposure of NaNO****3**** single crystals to low-energy (5 - 100 eV) electrons results in highly efficient destruction of the NO****3****-**** and  the release of ionic and neutral species, predominantly the persistent radical NO **

## RADIATION-INDUCED REACTIONS AT ORGANIC/NaNO3  INTERFACES

- Radiolysis of NaNO3
- interfaces produces
- primarily O - , O, NO,
- and organic radicals.
- These species react
- and contribute to the
- “aging” of wastes.
- These radicals can
- also lead to the
- production of RNO
- compounds which
- can form N2O.

- N-O Na+

- O
- O

- O

- R-H

- R

- RH

- NO

- R

- RNO

- e

**6.4 eV PHOTON-STIMULATED DESORPTION (PSD)**
**OF SODIUM NITRATE SINGLE CRYSTALS:**
**----FLUENCE DEPENDENCE ----**

- 1-Photon Fluence
- Dependence
- No induction time
- for desorption
- NOT DEFECT
- mediated
**EXCITON** mediated

## VELOCITY DISTRIBUTIONS FOR O (3P) AND NO (2) FROM BOTH 100 eV ESD (OPEN CIRCLES) AND 6.4 eV PSD (FILLED CIRCLES)

**VELOCITY DISTRIBUTIONS FOR ****O (****3****P) ****AND ****NO (****2********) ****FROM BOTH 100 eV ESD (OPEN CIRCLES) AND 6.4 eV PSD (FILLED CIRCLES)**

- The solid lines are the calculated 423K (surface temperature) Maxwell - Boltzmann distributions.
- The **O (****3****P)** and **NO (****2********)** velocity distributions from ESD and PSD  are **similar to each other, and have non-thermal components.**
**K. Knutsen and T.M. Orlando, Applied Surface Science 127-129 (1998) 1-6.**

## SCHEMATIC OF THE PHOTON-  AND ELECTRON- STIMULATED DESORPTION OF NO AND O  FROM NaNO3 CRYSTALS

- (A) Exciton production
- via excitation of the
**NO****3****-** band
- (B) Surface exciton
- ( **NO****3*********-** ) trapping via
- stabilization with
- respect to the bulk.
- The surface exciton
- decays to form
**NO (g) + O (g) + O****-**** (sf**)

- K. Knutsen and
- T. M. Orlando,
- Phys. Rev. B55,
- 13246, 1997.

**8 eV**

## PROPOSED REACTION  SCHEME FOR THE NaNO3 / CH3OH OVERLAYER EXPERIMENTS

**PROPOSED REACTION  SCHEME FOR ****THE NaNO****3**** / CH****3****OH ****OVERLAYER EXPERIMENTS**

**CH****3****OH**

**H************+********CH****3****OH ******** H****2**** ****+**** CH****2****OH**
**CH****3****O********+**** ****NO********   ********CH****2****O ****+ ****HNO + NO******** ********N****2****O **(44amu) **+**** ********OH **
- 
******CH****2****OH********+**** ****NO******** ********CH****2****O ****+ ****HNO + NO******** ********N****2****O **(44amu) **+**** ********OH**
- **+ ****NO******** ******** CH****3****NO **(45 amu)
******CH****3**
-                  **+ ****NO****2 ******** CH****3****NO****2**** **(61 amu)
******OH     ****+********CH****3****OH ******** CH****2****OH ****+**** ****H****2****O**** **(18 amu)

**e****-**

## Where’s the NOx?

- Photo credits:  Barbara Gaitley, JPL image P-48863A

- Where’s the NO*x*?

- Where is L.A. ???

## Where theory can help in understanding electron-and photon-interactions with interfaces:

- 	How are excited states modified on surfaces?
- 	(Oxides are relevant to the environment.)
- 	What are the mechanisms of energy transfer?
- 	Density of states change at boundary layers.
- 	How does this effect energy loss processes?
- 	How do we deal with secondary scattering?