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Viewpoints on the 2017 Pacific Conference on Spectroscopy and Dynamics
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INTRODUCTION From January 19th to 22nd, 2017, physical chemists from the U.S. and around the world gathered at the Asilomar Conference Grounds in Pacific Grove, California, for the 64th meeting of the Pacific Conference on Spectroscopy and Dynamics (PCSD, www.pacificspectroscopy.org). The technical advancements presented spanned the spectrum from fundamental theory to applied spectroscopy, and the investigations covered a broad range of systems including: single molecules, gaseous clusters, aerosols, condensed-phase molecules, ionic liquids, thin films, nanomaterials, and biomolecules. With the common goal to more precisely understand the nature and dynamics of molecules and ensembles through spectroscopy, everyone in attendance contributed to a rich and insightful meeting this year at Asilomar.
student talks has been included. As participation broadened to the entire United States and beyond, coupled with the vast developments of spectroscopic instrumentation, techniques, and molecular insight since the 1950s, the Western Spectroscopy Conference was renamed in 2014 as the Pacif ic Conference on Spectroscopy and Dynamics.
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ATMOSPHERIC AND COMBUSTION CHEMISTRY The 2017 PCSD had a strong showing of speakers reporting on the gas-phase spectroscopy, kinetics, and dynamics of transient atmospheric and combustion-relevant species. Of particular focus were alkyl peroxy radicals and Criegee intermediates. Peroxy radicals (ROO) are critical reaction intermediates associated with oxidation chain reactions in the atmosphere and in low-temperature hydrocarbon combustion environments. Gary E. Douberly’s group (University of Georgia) has studied peroxy radicals formed through reaction of alkyl radicals with O2 inside helium droplets. A beam of helium droplets is doped with these reactants and irradiated with IR light. Rotationally resolved IR spectra of helium-solvated peroxy radicals are obtained via an action technique that measures photoinduced changes in the cross-section for droplet ionization in a mass spectrometer. The 0.4 K helium droplet environment stabilizes reactive species, and their IR spectra illuminate which isomers are formed and what reactive intermediates and barriers are involved. For the reaction of the allyl or ethyl radicals with O2, the helium droplet environment adiabatically funnels the reaction products into a single alkyl peroxy radical isomer [J. Chem. Phys. 2013, 139, 234301]. David L. Osborn’s group (Sandia National Laboratories) has been chasing hydroperoxyalkyl radicals (QOOH), a long sought-after target in combustion chemistry. The relative rates with which QOOH eliminates either HO2 or OH or reacts with O2 govern radical chain-branching and OH concentrations in low-temperature combustion environments. QOOH is formed slowly from ROO isomerization but reacts quickly in a barrierless reaction with O2, and it is therefore incredibly difficult to detect. Osborn and co-workers designed a resonance-stabilized QOOH radical formed in the oxidation of 1,3-cycloheptadiene [Science 2015, 347, 643]. Using photoionization mass spectrometry (PIMS), they identified QOOH via its mass, kinetic behavior, and photoionization threshold and measured the kinetics of its formation and loss, representing the first direct detection of any QOOH species. Krisztina Voronova (Sztáray Group, University of the Pacific) spoke about a similar technique, photoelectronphotoion coincidence (PEPICO) spectroscopy. The new CRF-PEPICO (Combustion Reactions Followed by PEPICO) design has the capabilities of PIMS for sensitive and specific kinetics measurements but also includes collection of a photoelectron spectrum for each mass. This yields an improved
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CONFERENCE HISTORY Formerly known as the Western Spectroscopy Conference, this annual meeting first convened in January 1954 to address the need for a regular scientific gathering in the western U.S. focused on the latest advances in molecular spectroscopy. The governing organizationthe Western Spectroscopy Association (WSA)was established one year prior by six leading spectroscopists from academia, industry, and national laboratories: W. C. Merrill, J. W. Otvos, G. C. Pimentel, S. S. Penner, S. R. Smith, and J. C. Guffy. In particular, a key organizer of the western spectroscopists was George Pimentel, Professor at U.C. Berkeley. A chemist and educator of the highest rank, Prof. Pimentel is remembered as an energetic, kind person with razor-sharp physical intuition. At least 20 invited talks from Pimentel students throughout the meeting’s history mark an impressive contribution. The meeting is distinguished by a warm, casual atmosphere that maintains the scientific rigor characteristic of premier spectroscopists. It is an annual highlight; some scientists, such as the late Prof. Rollie J. Myers, have attendance records spanning more than 50 years! By January of 1960 the meeting assumed its storied residence at Asilomar along the shores of the Monterey Peninsula in Pacific Grove, CA. For the many attendees throughout the meeting’s 64-year history, being at Asilomar has been part of the magic of the meeting. Academic and national laboratory researchers continue to contribute strongly to the annual program. While there were more invited speakers from industry than from national laboratories through the 1980s, a decline of presented industrial research has been observed since 1990. As the meeting has grown with time, so too has the support for students, postdoctoral scholars, and young faculty. The program expanded in the mid-1980s to include a student poster session, which has since increased to at least two poster sessions per meeting. By 1990, contributed talks reserved exclusively for selected postdocs and graduate students were included in the meeting agenda, and most recently an independent session of © 2017 American Chemical Society
Published: April 20, 2017 2863
DOI: 10.1021/acs.jpca.7b02628 J. Phys. Chem. A 2017, 121, 2863−2867
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The Journal of Physical Chemistry A structural fingerprint to distinguish between isomers in combustion and atmospheric reactions. In collaboration with the Swiss Light Source, Voronova and co-workers have also demonstrated a new method to suppress false coincidences, which increases the signal-to-noise ratio of PEPICO by 2−3 orders of magnitude [J. Chem. Phys. 2016, 145, 164202]. Several speakers reported on direct absorption measurements of atmospherically relevant species. Criegee intermediates (CIs) are carbonyl oxides formed during the ozonolysis of alkenes in the atmosphere; characterization of their unimolecular and bimolecular reaction rates is essential for climate modeling. Elizabeth Foreman (Murray Group, University of California, Irvine) gave a contributed talk on characterizing electronic transitions of the simplest CI, CH2OO, with broadband transient absorption spectroscopy. Foreman and co-workers also measured rate constants for reactions of CH2OO with the inorganic acids HCl and HNO3 [Angew. Chem. Int. Ed. 2016, 55, 10419]. They found these reactions to be fast and barrierless, making them competitive with the reaction of CH2OO with water in the atmosphere. Yuan-Pern Lee (National Chiao Tung University) discussed using step-scan Fourier transform infrared (FTIR) spectroscopy to study CIs as well as the dynamics of atom-molecule roaming reactions. Transient species are difficult to detect using conventional, slow-scanning FTIR spectrometers. In step-scan FTIR, spectra are measured as a function of time at individual, stepped interferometer positions. Using this method, the Lee group has reported the IR absorption spectrum of CH2OO, with 0.25 cm−1 resolution [J. Chem. Phys. 2015, 142, 214301] and larger CIs with 1 cm−1 resolution. They have also confirmed a roaming mechanism in the atomic chlorine + isobutene hydrogen abstraction reaction by measuring the formation kinetics, yields, and vibrational excitation of the HCl products via their IR emission [Sci. Rep. 2017, 7, 40105]. Bryce Bjork (Ye Group, University of Colorado, Boulder/ JILA) also presented direct absorption measurements of atmospherically important reactive intermediates. The reaction kinetics of the DOCO radical, the collisionally stabilized intermediate of the deuterated OH + CO reaction, were measured using mid-IR frequency comb spectroscopy [Science 2016, 354, 444]. This reaction is one of the dominant loss processes of OH in the atmosphere and is critical to atmospheric conversion of CO to CO2. The measurements of Bjork and co-workers represent the first detection of this intermediate under thermal reaction conditions and confirm the mechanism for HOCO formation.
Preliminary studies of the hypermetallic oxides CaOCa and MgOMg were reported. Evan Bieske (University of Melbourne) spoke about monitoring photoisomerization chemistry with ion mobility tandem mass spectrometry. Ions produced with an electrospray ionization (ESI) source fly through a buffer-gas-filled drift region, allowing separation of conformers with distinct collision cross sections. By illuminating the ESI syringe with UV light before ion production, one can study solution-phase equilibration kinetics subsequent to photoexcitation, as reported by the Bieske group for azoheteroarene molecular switches [Anal. Chem. 2016, 88, 11978]. Photoisomerization between conformers can also be studied in the gas phase as demonstrated for the chromophore retinal [Phys. Chem. Chem. Phys. 2015, 17, 22623], which may lend insight into the specific roles that retinal and its protein environment play in human vision. Timothy Minton (Montana State University) discussed scattering studies of hyperthermal O atoms on vitreous carbon and highly oriented pyrolytic graphite (HOPG) surfaces [J. Phys. Chem. C 2015, 119, 14780]. These experiments model the erosion and oxidation of carbon-based man-made materials in atmospheric entry environments. A molecular beam of O atoms is scattered off a carbon surface that can be heated to temperatures up to 2000 K. Scattered CO, O2, CO2, and O products are detected over a wide range of angles and arrival times with a rotatable mass spectrometer. Minton and coworkers see more reactive scattering from vitreous carbon than from HOPG, with increased production of the main reactive product, CO. On both surfaces, the increasing thermal desorption of O atoms with temperature limits the surface oxygen that is available for reaction at higher temperatures. With fewer reagent O atoms to react with carbon, the reactivity of the carbon surface decreases at high temperatures.
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ENERGY TRANSFER: MATERIALS AND TECHNIQUES PCSD 2017 covered a broad range of research in condensedphase energy transfer with a particular focus on improving the efficiency of solar photon absorption for sustainable energy production. Timothy W. Schmidt (University of New South Wales) discussed methods to harvest both high- and lowenergy portions of the solar spectrum: triplet−triplet annihilation (TTA), where two low-energy photons are upconverted into one high-energy photon via intermolecular energy transfer, and singlet fission (SF), where one highly excited singlet state splits into two correlated, lower-energy triplets with overall singlet character. Schmidt and co-workers have used TTA in a Pd-porphyrin sensitizer and rubrene emitter molecular system to increase the external quantum efficiency of an amorphous silicon photovoltaic device [RSC Adv. 2014, 4, 8059] and have studied SF dynamics in an aqueous TIPS−pentacene nanoparticle suspension [Phys. Chem. Chem. Phys. 2013, 15, 14797]. Stephen Bradforth’s group (University of Southern California) has also worked to understand TTA and SF mechanisms and dynamics for engineering and optimizing solar energy harvesting devices. The Bradforth group has recently focused on how structural orientation and electronic structure govern SF energy transfer in ethynyl−tetracene dimers in thin films and in solution. Tuning the relative orientation of tetracene monomers in the dimer yields a systematic study of triplet formation as a function of intertetracene overlap. A dimer geometry with twisted, poorly
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MOLECULAR BEAMS AND IONS Several speakers discussed studies of gas-phase systems using molecular beam and ion techniques. Michael C. Heaven (Emory University) described experiments to characterize alkaline earth metal-containing clusters and molecules. Alkaline earth metal species are widely used in magneto-optical trapping and laser-cooling schemes for ultracold precision experiments, and high-resolution spectroscopic characterization is critical for these applications. Using an arsenal of zero-electron kinetic energy, resonant multiphoton photoionization, and mass analyzed threshold ionization spectroscopies, the Heaven group has characterized the vibronic structure of BaO+ [J. Chem. Phys. 2015, 143, 044302], BaCl+, and BeOBe+. In complementary high-resolution anion photoelectron imaging experiments [J. Chem. Phys. 2017, 146, 054301], vibronic energy levels are being determined for the neutral species. 2864
DOI: 10.1021/acs.jpca.7b02628 J. Phys. Chem. A 2017, 121, 2863−2867
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The Journal of Physical Chemistry A
extent of hydrogen bonding and viscosity in different ILs [J. Phys. Chem. Lett. 2014, 5, 1541]. A study of CO2−IL complexes for atmospheric carbon capture applications revealed stretchbend anharmonic couplings and stochastic excitation and deexcitation of the CO2 bending mode [J. Chem. Phys. 2015, 142, 212425]. Both the SCN− and CO2 studies establish that disrupting the local ion cage encompassing the chromophore is a fundamental step in molecular diffusion and vibrational relaxation in IL-solvated complexes. Amber Krummel’s group (Colorado State University) also uses cyanate-based anions as sensitive experimental reporters of local solvent structure and dynamics in 2D IR experiments. The Krummel group built a high repetition rate 2D IR spectrometer and an IR-compatible microfluidics sample flow cell that facilitate high-throughput characterization of vibrational chromophores in various solvents. The spectrometer rapidly collects a 2D IR data set at a 100 kHz repetition rate using 220 fs mid-IR pulses generated through optical parametric chirpedpulse amplification [Opt. Exp. 2016, 24, 4117]. The microfluidics flow cell provides high sample precision via mixed solvent gradient tunability, and it assists the high-throughput capability of the 2D IR spectrometer [J. Phys. Chem. Lett. 2016, 7, 4865]. Eventually, the high repetition mid-IR source will find exciting new applications in 2D IR microscopy. Vibronic transitions are fundamental to molecular photochemistry. James D. Gaynor (Khalil Group, University of Washington) spoke about using 2D Electronic-Vibrational (2D EV) spectroscopy to investigate correlations between electronic and vibrational motion and identify specific vibrations coupled to electronic transitions. Spectral simulations indicate that vibronic coupling is required to observe 2D EV signal, rendering it sensitive to molecular vibronic coupling specifically. Gaynor and co-workers report an experimental setup using a tunable, octave-spanning (>1600−3200 cm−1) broadband mid-IR probe adaptively compressed to
