Development of a cavity ringdown spectrometer for measuring electronic states of Be clusters JACOB STEWART, MICHAEL SULLIVAN, MICHAEL HEAVEN DEPARTMENT 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

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).

Be 3 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

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 2 S ← 2 P transitions Ca + signal also seen

First cluster signal He carrier gas at ~ 10 atm Cavity axis ~ 3 mm from nozzle Optimization still needed Region of 0-0 band of 2 3 Π g ← X 3 Π u transition of Al 2

Next steps Use Al 2 signal to optimize cluster production Optimize timing of ablation and ringdown Reduce noise in ringdown signal (mode matching) Spectroscopy of Be 3 (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