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