Chen Lab

Ricardo Metz

Physical Chemistry
Spectroscopy and Dynamics of Gas-Phase Ions

BA 1986, Johns Hopkins Univ.; PhD 1991, Univ. of California, Berkeley; NSF Postdoctoral Fellow, 1991-1995, Univ. of Wisconsin, Madison

165 Goessmann


Principal Research Interests

Transition metal centers are ubiquitous in nature. They are abundant in interstellar space and are vital reaction sites in systems ranging from industrial catalysts to enzymes. We use a combination of mass spectrometry and laser photofragment spectroscopy to study the electronic and vibrational spectroscopy of gas-phase ions containing a metal (M) center, determining their bonding, structure and thermochemistry. We study two classes of molecules: organometallic ions such as PtCH2+ that are models for transition metal catalysis, and cluster ions such as Co(H2O)62+ that probe solvation of M2+.

Our experiments involve producing, selecting and photoexciting an ion and identifying its photodissociation products. We produce singly charged ions by laser ablation of a metal rod (to form M+), followed by ion-molecule reactions and cooling in a supersonic expansion to form a molecular beam. Multiply charged ions are generated by electrospray. Ions are injected into a reflectron time-of-flight mass spectrometer. A pulsed, tunable laser irradiates the mass-selected ion of interest in the reflectron and the masses of charged dissociation fragments are determined by their subsequent flight times to a detector. Monitoring fragment ion yield as a function of laser wavelength gives the photodissociation spectrum - the absorption spectrum of those ions that dissociate to give the product being monitored.

A practical scheme to convert methane to an easily transportable compound would greatly increase the utility of natural gas as an energy source. We study activation of methane and larger hydrocarbons by M+ and MO+ in the gas phase, where we can learn about the detailed reaction mechanism from the spectroscopy of the reaction intermediates, reactants and products. We also study the effect of the number and nature of solvent molecules on the absorption spectra of solvated M2+ and the interesting dissociation dynamics that occur when a multiply charged ion dissociates to two singly charged fragments. Electronic spectroscopy mostly gives information on excited electronic states, while vibrational spectroscopy is an ideal way to study the metal-ligand bond and how interactions with the metal affect bonds in the ligand, and we are developing new methods for measuring vibrational spectra of ions. Photoionization studies (at the Advanced Light Source) and calculations of the ground and excited electronic states complement the lab experiments.

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