electron spin resonance investigations of matrix isolated metal clusters at 4 k bradford michael...

Electron Spin Resonance Investigations of Matrix Isolated Metal

Clusters at 4 KBradford Michael Mess, John J. Banisaukas III, and Lon B. Knight, Jr.

Department of Chemistry, Furman University, Greenville, SC

Introduction

• Studying metal clusters is necessary to understand the transition from atomic and molecular properties to metallic bulk properties

• Metal clusters also play an important role in developing new catalysts

Formation of Metal Clusters

hν

metal atoms

Laser Vaporization

metal sample

Thermal Vaporization

metal atoms

copper electrodes

tantalum tube

Cluster size depends on: • Concentration ratio of rare gas to metal vapor

• Deposition temperature

Deposition Site

Metal atom

Neon atom

Copper target cooled to 4 K

Cluster formation

Matrix Isolation Apparatus

Gases used:

neon

argon

methane

Metals used:

magnesium

calcium

zinc

8 cm

Background pressure:

8 x 10-7 Torr

Liquid helium cryostat

copper electrodes

Magnesium and Calcium Experiments

Deposited in:

neon

argon

methane

Flow rates:

2.4 – 5.0 sccm

Goal: Maximize high spin species

Mg deposits

Dilute: grayish-red

Medium: dark red

Heavy; dark gray

Ca deposits

All: gray black

Conditions

Deposit length:

1 hour

Deposit temperatures:

4 – 40 K

ESR spectra of high spin Mg neutral clusters in neon at 4 K

no hyperfine – due to π-bonding

g┴ = 1.996(1) g║ = 2.000(1) D (zfs) = 2645(5) MHz

ESR spectra of high spin Ca neutral clusters in neon at 4 K

MIXTURE OF HIGH SPINCALCIUM CLUSTERS

SIMULATED

Cax : Cay :: NEON 4K

MIXTURE OF HIGH SPINCALCIUM CLUSTERS

SIMULATED

Cax : Cay :: NEON 4KCax : Cay :: NEON 4K

Signals only seen in neon

Zinc Experiments

Deposited in:

neon

Average flow rate:

4.4 sccm Goal: control the size of clusters

Zn deposits

Dilute: colorless/light orange

Medium: dark orange

Heavy; black

Conditions

Deposit length:

1 hour

Deposit temperatures:

4 – 40 K

X – irradiation:

30 minutes at 60 keV

ESR spectra of zinc dimer and trimer cations in neon at 2.6 K

Zn+ Zn+Zn+ Zn+Zn+ Zn+

67Zn+ (4.11 % abundance)

I = 5/2

Deposit temperature: 2.6 K

Higher Zn concentration

Lower Zn concentration

Magnetic parameters (MHz) of zinc monomer, dimer, and trimer, cation

radicals

Relative line positions of zinc cations

AcknowledgmentsThank you to:

Dr. Knight

TJ

Grant Hendrickson