Overview of Research in the Drucker Group

    In the broadest terms, the aim of our research is to contribute to an understanding of photochemical reaction mechanisms. This is a complex problem that is being attacked from many different perspectives within the chemistry community. In the future, it is likely to be computational chemistry that provides the most detailed information about photochemical pathways. The experimental work in our molecular spectroscopy laboratory is designed to support the vigorous ongoing effort of the computational chemists to simulate photochemical events accurately and efficiently.

    What underlies the success (or failure) of any photochemical dynamics simulation is the accuracy of the computed excited-state potential-energy surface. This has steadily risen over the past decade, but still does not quite match the accuracy of ground-state calculations. Benchmark data from experiments are required for evaluating the excited-state calculations. This imperative has shaped my research program, which is focused on electronic spectroscopy. Our central experimental approach is laser absorption spectroscopy, conducted on low-pressure gas phase samples. Our work [1,2,3,4,5] has focused on small organic molecules that contain photochemically active functional groups or chromophores. Spectroscopic parameters available from our studies provide a rigorous test of computed excited-state properties. In broader terms, the spectra reveal changes in structure and dynamics that accompany photoexcitation and which can activate the molecule toward chemical transformation.

    The small, stable organic molecules we study have singlet ground-state configurations (S₀); the singlet excited states (S₁, S₂,¼) are most accessible spectroscopically. However we are particularly interested in the lowest-energy triplet states because of their importance in photochemistry. Several combined factors make the triplet species (designated as T_n, n=1,2, ¼) uniquely suited to act as photochemical intermediates. For one, the low-lying triplet states may be readily populated in a solution-phase photochemical environment. This occurs via rapid nonradiative relaxation (intersystem crossing) from an initially photoexcited S₁ state. The triplet yield can be significant, because S₁ ® T_n intersystem crossing is often the fastest decay process when the S₁ state is prepared with an appropriate amount of vibrational excitation [6,7,8,9,10]. Once the excited system reaches the triplet surface, it is especially prone to chemical reaction, due to a diradical electronic structure and slow radiative decay rate. Within this chain of events, the ultimate return to the ground state typically occurs via T₁ ® S₀ surface hopping. The shape of the T₁ potential surface can thus strongly influence ground-state product formation.

    We have exploited the highly sensitive cavity ringdown (CRD) method [4,11,12,13,14] to detect the nominally spin-forbidden T_n ¬ S₀ transitions in the gas phase. In the CRD technique, one monitors the leakage of a laser light pulse out of an optical cavity that is bounded by highly reflective mirrors. The decay function is I₀ exp(-kt), where I₀ is the initial pulse intensity, and k is the ringdown rate constant. In the presence of an absorbing sample, k increases when the laser is on resonance. The CRD absorption spectrum is recorded as the fitted value of k vs. wavelength. The very high sensitivity comes about because the measurement of k is almost completely immune to fluctuations in I₀, the laser source intensity. Moreover, the very high reflectivity available for CRD mirrors leads to long ringdown times, high precision of the fitted k values, and correspondingly low noise in the CRD spectrum.