Development of a cavity ringdown spectrometer for measuring electronic states of Be clustersJACOB STEWART, MICHAEL SULLIVAN, MICHAEL HEAVENDEPARTMENT OF CHEMISTRY, EMORY UNIVERSITY

Studying metal clusters Information about bonding in metallic systems

How do properties change as cluster size changes?

When does a transition from “cluster” to bulk behavior occur?

Image from http://commons.wikimedia.org/wiki/File:Be-140g.jpg

Providing experimental benchmarks

Theoretical methods are important tools for studying clusters

Need experimental data to ensure the results are accurate

What level of theory is necessary for accurate results?

Why study beryllium? Only 4 electrons

Importance of many-body effects

Able to obtain geometries of clusters

Theoretical calculations of beryllium dimer

Heaven et al., Annu. Rev. Phys. Chem., 62, 375 (2011).

Experimental data shows the way

Stimulated emission pumping spectrum

Observed vibrational states up to dissociation

Multiconfiguration methods needed

Merritt et al., Science, 324, 1548 (2009).

Be3 excited state potential energy surface from Heaven et al., Annu. Rev. Phys. Chem., 62, 375 (2011).

What about larger clusters?

No experimental data yet for larger clusters

Fundamental interest in observing Jahn-Teller effect for closed shell systems

Can cheaper methods (like DFT) be accurate for larger clusters?

How to measure small Be clusters?

Fluorescence measurements?

Action spectroscopy?

Attempted, but little to no signal seen

Dissociation from larger clusters poses a problem

Need to use direct absorption spectroscopy

Cavity ringdown spectroscopy

Use of a high-finesse cavity increases path length

Absorption signal related to decay time

Not affected by laser intensity fluctuations Image from Yunjie Xu’s web page at

http://www.chem.ualberta.ca/~xu/

Producing metal clusters

Laser ablation to generate metal vapor

Vapor plume cooled by supersonic expansion

Metal cluster spectrometer

Excimer-pumped tunable dye laser

KrF excimer for ablating metal rod

Delay generator to control timing

PC records and fits ringdowns

Testing the spectrometer

Scherer et al., Chem. Phys. Lett., 242, 395 (1995).

Need to optimize timing, cluster production conditions

Use atomic Al and Al clusters as a first test of spectrometer

Timing the ringdown and ablation

Need to determine when clusters cross cavity axis

Preliminary data using atomic aluminum

2S ← 2P transitions

Ca+ signal also seen

First cluster signalHe carrier gas at ~10 atm

Cavity axis ~3 mm from nozzle

Optimization still needed

Region of 0-0 band of 2 3Πg ← X 3Πu transition of Al2

Next steps Use Al2 signal to optimize cluster production Optimize timing of ablation and ringdown Reduce noise in ringdown signal (mode matching) Spectroscopy of Be3 (guided by configuration interaction calculations)

Junquera-Hernández et al., J. Chem. Phys., 121, 7103 (2004).

Conclusions We have built a laser ablation source for producing metal clusters

Al clusters have been observed by cavity ringdown spectroscopy

Spectrometer will soon be used for observing Be clusters

Acknowledgments Heaven group Michael Sullivan