Dense interstellar clouds, and in particular pre-stellar cores, are the most extremely cold naturally occurring environments, with temperatures as low as 5 K. Observations reveal a rich inventory of molecules, evidence of a surprisingly active chemistry occurring under these extreme conditions. Using carefully controlled expansions through Laval nozzles, we can recreate gas phase environments at such low temperatures (the CRESU technique: Cinétique de Réaction en Ecoulement Supersonique Uniforme). Using ultraviolet photolysis or infrared pumping combined with probes such as laser-induced fluorescence or chirped pulse Fourier transform millimeter wave spectroscopy, we can measure absolute cross sections or rate constants for a variety of reactive and inelastic molecular collisions to provide essential data for astrochemical models and astronomical observations. I will describe our latest measurements, including work on the measurement of absolute rate constants for the formation of weakly bound complexes, the first stage in homogeneous nucleation.

Molecular spectroscopy plays a critical role in the characterization of planets beyond our solar system. The more than 5500 confirmed exoplanets span a broad range in size, temperature, and atmospheric composition from hot Jupiters to temperate terrestrial planets. A new window into exoplanets and their atmospheres was opened with the launch of JWST on December 25th, 2021. I will discuss what we thought we knew about atoms, molecules, and aerosols in the atmospheres of exoplanets before the launch of JWST and what we are learning in this new era. Our understanding and interpretation of exoplanet observations hinges on fundamental molecular and atomic opacities and I will highlight areas of critical need. The diversity of exoplanets presents both challenges and opportunities in the exploration of the physics and chemistry shaping these distant worlds.

We present a new technique for the detection of molecules desorbed from an ice surface using broad-band millimeter-wave rotational spectroscopy. The approach permits interrogation of molecules that have undergone the slow warmup process of temperature-programmed desorption (TPD), analogous to the warmup phase of icy grains in the interstellar medium as they approach the central protostar. The detection is conformer- and isomer-specific and quantitative, as afforded by chirped-pulse rotational spectroscopy. In our initial investigations, we highlight unique capabilities of the instrument to gain insight into ice composition, sublimation dynamics, and characterization of the vibrational temperature of molecules that have thermally desorbed. To achieve this, we combine ice TPD with buffer gas cooling, followed by detection in the millimeter-wave regime. Here we report our findings from studies of the desorption of n- propanol and n-propyl cyanide ices which yield distinct conformer-specific sublimation profiles. In addition, we have measured the vibrational temperature of iso-propyl cyanide following desorption. We find that the two low-frequency modes observed (v30=1 and v29=1) yield unique vibrational temperatures, both of which are colder than the temperature at which iso-propyl cyanide desorbs. Finally, we demonstrate the detection of molecules that have been formed in energetically processed ices with both UV irradiation and high-energy electrons.

The topology of multidimensional potential energy surfaces defines the bimolecular collision outcomes of open-shell radicals with molecular partners. To model the interaction potential between colliding molecules, it is necessary to characterize the monomer collision orientations and their impact on the mechanistic pathways. Here, we report on the isomer-specific scattering mechanisms between NO and small alkanes, encoded in the spectroscopic and dynamical signatures of the incipient collision complexes. IR action spectroscopy with 1+1 resonance-enhanced multiphoton ionization of NO products was employed to characterize the fundamental CH stretch transitions of the target complexes, as well as to initiate the nonreactive decay mechanisms. Furthermore, velocity map imaging (VMI) was utilized to explore the dynamics before and after IR activation of NO-alkane complexes, imprinted on the NO photoproducts. Taken together, these results indicate the sensitivity of bimolecular collisions to the subtle features on coupled potential energy surfaces, leading to substantial differences in the energy-exchange mechanisms during the evolution to products.

Ultrafast X-ray spectroscopic investigations use transitions from localized inner shells of specific atomic sites in molecules or solids to empty or partially filled valence orbitals or bands. With femtosecond pulses, the X-ray spectroscopic approach obtains new information about coherent electronic and vibrational dynamics. The interpretation of such spectra involves a new regime of core-to-valence X-ray probing that depends on energy shifts due to surrounding electron densities, bond elongation with vibrational excitation, and electronic coherences due to charge migration. Observations of the passage through conical intersections reveal the precise timescales of curve crossing dynamics. Coherent vibrational superpositions obtain the slopes of inner shell potentials with bond extension in small molecules. In solids, the measurement of rapid carrier relaxation and the excitation of solid-state coherent phonon motion reveal mechanistic details of these dynamics. Jahn-Teller distortion is observed on few-femtosecond timescales as a molecule abruptly changes symmetry, with a few periods of ensuing coherent vibrational motion. Progress in revealing the full power of time-resolved X-ray spectroscopy for the investigation of novel features in molecular and solid-state coherent dynamics is described. Y. Kobayashi and S. R. Leone, "Characterizing coherences in chemical dynamics with attosecond time-resolved X-ray absorption spectroscopy," J. Chem. Phys. 157, 180901 (2022). E. Ridente, D. Hait, E. A. Haugen, A. D. Ross, D. M. Neumark, M. Head-Gordon, and S. R. Leone, "Femtosecond symmetry breaking and coherent relaxation of methane cations via X-ray spectroscopy," Science 380, 713 (2023). L. Barreau, A. D. Ross, V. Kimberg, P. Krasnov, S. Blinov, D. M. Neumark, and S. R. Leone, "Core-excited states of SF6 probed with soft X-ray femtosecond transient absorption of vibrational wavepackets," Phys. Rev. A 108, 012805 (2023). Y. Kobayashi, D. M. Neumark, and S. R. Leone, "Theoretical analysis of the role of complex transition dipole phase in XUV transient-absorption probing of charge migration," Opt. Express 30, 5673 (2022).