Robert W. Field’s research while affiliated with Massachusetts Institute of Technology and other places

A shared ambition of molecular spectroscopy and molecular dynamics is to establish the truths of chemical intuition on solid physical bedrock. Experimental characterization of the transition state was thought unattainable, but modern techniques have begun to achieve the incredible.

We utilize rotationally resolved Chirped-Pulse Fourier Transform millimeter-wave spectroscopy to study photodissociation dynamics of 1,3,5-Triazine (symmetric-Triazine) to form 3 HCN molecules. The state-specific vibrational population distribution (VPD) of the photofragments contains mechanistic details of the reaction. This photodissociation is performed using 266 nm radiation transverse to a seeded supersonic jet. The vibrational cooling inefficiency in the jet preserves the VPD of the photofragments, while rotational cooling enhances the signal of low-J pure-rotational transitions. The multiplexed nature of the spectrometer enables simultaneous sampling of several "vibrational satellites" of the J = 1 ← 0 transition of HCN. Excited state populations along the HCN bend (v2) and CN stretch (v3) modes are observed, which show ≥3.2% vibrational excitation of the photofragments. Observation of an at least bimodal VPD, along the even-v states of v2, implies an asymmetric partitioning of vibrational energy among the HCN photofragments. This suggests a sequential dissociation mechanism of symmetric-Triazine initiated by 266 nm radiation.

An extensive high-resolution visible wavelength emission spectrum of AlH, that covers eight bands of the A1Π−X1Σ+ system, has been observed with a Fourier-transform spectrometer for the first time. The 0−0,1,2 and 1−0,1,2,3,4 bands were recorded with an instrumental resolution of 0.03 cm−1. The best signal-to-noise ratio of 9000:1 was obtained for the strongest 0−0 band. In total, 259 ro-vibronic frequencies were measured with an absolute accuracy of about 0.0020 cm−1. The experimental data were fitted using the PGOPHER program written by Western (2017). Molecular constants were derived for the X1Σ+, ν=0−4 and A1Π, ν=0,1 levels. Also derived were rotational term values for the A1Π, ν=0,1 levels. The current data were combined with the previous ground state results of White et al. (1993) using the instruction called the constraints on parameters within PGOPHER. The Dunham parameters (Ykl) for the A1Π state of AlH were determined for the first time. A very small shifts of the A1Π, ν=0,J=17 and ν=1,J=5 levels have been observed and identified as result of the possible interaction between A1Π and 3Σ+ states.

Furan, a prototypical heteroaromatic cyclic ether, is an important molecule that has garnered attention from a wide range of fields, from food and health research to combustion and atmospheric chemistry. Chirped-pulse Fourier-transform rotational spectroscopy of jet-cooled furan in the W-band across three narrow regions 79.3-79.45 GHz, 88.6-88.8 GHz, and 97.95-98.15 GHz is reported here. A pyrolysis nozzle was used to efficiently populate vibrationally excited states of low-frequency modes. Apart from the v=0 state, vibrationally excited v=1 states of modes v11, v14, v10, and v13 were observed. Furan is a planar molecule and these observed excited state modes correspond to out-of-plane vibrations. The v=0 state and the vibrationally excited v11 and v14 were previously recorded and fit by Barnum et al (T. J. Barnum, K. L. K. Lee, B. A. McGuire, ACS Earth Space Chem. 5 (2021), 2986-2994). Rotational transitions in the J”=5 to 9 rotational states were fit for the previously undetected v10 mode and experimental linelists for v=1 states of modes v10 and v13 are provided, both of which are out-of-plane CH bending modes. Inertial defect was shown to be a useful quantity to extract structural information of vibrational modes of planar asymmetric tops. These rotational fits will assist astronomical searches for furan and its vibrational states.

Despite the long history of spectroscopic studies of the C2 molecule, fundamental questions about its chemical bonding are still being hotly debated. The complex electronic structure of C2 is a consequence of its dense manifold of near-degenerate, low-lying electronic states. A global multi-state diabatic model is proposed here to disentangle the numerous configuration interactions that occur within four symmetry manifolds of excited states of C2 (1Πg, 3Πg, 1Σu+ , and 3Σu+ ). The key concept of our model is the existence of two "valence-hole" configurations, 2σg22σu11πu33σg2 for 1,3Πg states and 2σg22σu11πu43σg1 for 1,3Σu+ states, that are derived from 3σg ← 2σu electron promotion. The lowest-energy state from each of the four C2 symmetry species is dominated by this type of valence-hole configuration at its equilibrium internuclear separation. As a result of their large binding energy (nominal bond order of 3) and correlation with the 2s22p2 + 2s2p3 separated-atom configurations, the presence of these valence-hole configurations has a profound impact on the global electronic structure and unimolecular dynamics of C2.