Lord Lecture Day 2 of 2 | Dr. Michael McCarthy (Harvard-Smithsonian Center for Astrophysics)

Title: Unveiling the Chemical Complexity of Space: The Laboratory as the Engine of Astronomical Discovery ABSTRACT: Nearly all of the roughly 350 molecules identified in interstellar and circumstellar space were found by matching astronomical radio spectra with laboratory rotational spectra. Yet for the first thirty years of molecular radio astronomy, beginning with OH in 1963, laboratory measurements largely played a supporting role, often confirming or refuting tentative astronomical detections. In the late 1990s, work in our laboratory was largely responsible for reversing that order, establishing an approach that continues to this day. Sensitive Fourier-transform microwave spectroscopy of discharge products in supersonic jets allowed laboratory discoveries to leapfrog astronomical observations, enabling efficient, targeted searches for carbon chains, radicals, silicon-carbon clusters, high-energy isomers, molecular ions, and metal-bearing species. This lecture traces that laboratory-led partnership over three decades. This effort played a major role in discovering two long-predicted classes of interstellar molecules: negatively charged molecular ions and aromatic molecules, from benzonitrile in a starless dark cloud in 2018 to polycyclic aromatic hydrocarbons with as many as seven fused rings. Their abundances challenge our understanding of carbon chemistry at ten kelvin, while recent detections of metal dicarbides reveal further unexpected chemical diversity. The lecture will also illustrate the complementary strengths of broadband chirped-pulse and sensitive cavity-enhanced spectroscopy for characterizing complex chemical mixtures with exquisite isomeric resolution. It will conclude with prospects for a next-generation spectrometer with a sensitivity approaching the fundamental detection limit and possible connections to the carriers of the enigmatic diffuse interstellar bands. BIO: Michael McCarthy is Deputy Director of the Center for Astrophysics | Harvard & Smithsonian and a Senior Physicist at the Smithsonian Astrophysical Observatory. He received a B.S. in Chemistry from the University of Alaska in 1986 and a Ph.D. in Physical Chemistry from MIT in 1992, working with Robert Field, and came to the CfA as a CfA postdoctoral fellow with Patrick Thaddeus. Over the past three decades, his group has developed sensitive microwave and millimeter-wave instrumentation that has enabled the discovery and characterization of hundreds of reactive molecules in the laboratory, including long carbon chains, organic and aromatic rings, energetic isomers, molecular ions, and metal-containing species. Many of these have since been identified in space, and his laboratory work underpins the recent detections of aromatic molecules in molecular clouds. He is the author of nearly 300 peer-reviewed publications, a Fellow of the American Physical Society and of the American Astronomical Society, and the recipient of the 2016 Barbara Mez-Starck Prize and the 2025 Laboratory Astrophysics Prize of the American Astronomical Society. He serves as an Associate Editor of ACS Earth and Space Chemistry. Group Website: https://lweb.cfa.harvard.edu/amp/mccarthygroup/index.html Refreshments will be served outside of 6-120 at 3:30PM.