Selected Publications

For a complete publication list, see my CV.
See also my Google Scholar Profile.

	An axis-specific rotational rainbow in the direct scatter of formaldehyde from Au(111) and its influence on trapping probability* G. Barratt Park, Bastian C. Krüger, Sven Meyer, Alexander Kandratsenka, Alec M. Wodtke, and Tim Schäfer Physical Chemistry Chemical Physics 19, 19904, (2017) *Selected for the 2017 PCCP "Hot Topics" Collection
In this work, we employ a new rotationally resolved 1 + 1′ resonance-enhanced multiphoton ionization (REMPI) scheme to measure the rotational distribution in formaldehyde molecules directly scattered from the Au(111) surface at incidence kinetic energies in the range 0.3–1.2 eV. The results indicate a pronounced propensity to excite a-axis rotation (twirling) rather than b- or c-axis rotation (tumbling or cartwheeling), and are consistent with a rotational rainbow scattering model. Analysis suggests that the high degree of rotational excitation has a substantial influence on the trapping probability of formaldehyde at incidence translational energies above 0.5 eV.
	A 1+1′ resonance-enhanced multiphoton ionization scheme for rotationally state-selective detection of formaldehyde via the à 1A2 ←X̃ 1A1 transition G. Barratt Park, Bastian C. Krüger, Sven Meyer, Alec M. Wodtke, Tim Schäfer Physical Chemistry Chemical Physics 18, 22355, (2016)
The formaldehyde molecule is an important model system for understanding dynamical processes in small polyatomic molecules. However, prior to this work, there have been no reports of a resonance-enhanced multiphoton ionization (REMPI) detection scheme for formaldehyde suitable for rovibrationally state-selective detection in molecular beam scattering experiments. In the current work, we present a new rotationally-resolved 1 + 1′ REMPI scheme via the à 1A2 ← X̃ 1A1 transition. The results indicate that the REMPI intensities are directly proportional to the à ← X̃ absorption intensities and can be used for quantitative measurement of X̃-state population distributions.
	The ν6 fundamental frequency of the à state of formaldehyde and Coriolis perturbations in the 3ν4 level G. Barratt Park, Bastian C. Krüger, Sven Meyer, Dirk Schwarzer, Tim Schäfer The Journal of Chemical Physics 144, 194308, (2016)
The rovibrational interaction between the near-degenerate ν4 and ν6 (out-of-plane and in-plane wagging modes, respectively) of the ground electronic state of formaldehyde is a textbook example for the Coriolis effect in polyatomic molecules. However, in the first excited singlet Ã1A2 electronic state, the analogous Coriolis interaction does not occur, because the à state suffers from a pseudo-Jahn-Teller distortion, which causes a double-well potential energy structure in the q′4 out-of-plane coordinate, and which dramatically reduces the effective ν′4 frequency. The ν′4 frequency is reduced by such a great extent in the à state that it is the 3ν′4 overtone which is near degenerate with ν′6. In the current work, we report the precise ν′6 fundamental frequency in the à state, and we determine the strength of the a-axis Coriolis interaction between 3ν′4 and ν′6. Knowledge of the Coriolis interaction strengths among the lowest-lying levels in the à state will aid the interpretation of the spectroscopy and dynamics of many higher-lying band structures, which are perturbed by analogous interactions.
	Observation of b2 symmetry vibrational levels of the SO2 C̃ 1B2 state: Vibrational level staggering, Coriolis interactions, and rotation-vibration constants G. Barratt Park, Jun Jiang, Catherine A. Saladrigas, Robert W. Field The Journal of Chemical Physics 144, 144311, (2016)
The C̃1B2 state of SO2 has a double-minimum potential in the antisymmetric stretch coordinate, such that the minimum energy geometry has nonequivalent SO bond lengths. However, low-lying levels with odd quanta of antisymmetric stretch (b2 vibrational symmetry) have not previously been observed because transitions into these levels from the zero-point level of the X̃ state are vibronically forbidden. We use IR-UV double resonance to observe the b2 vibrational levels of the C̃ state below 1600 cm−1 of vibrational excitation. This enables a direct characterization of the vibrational level staggering that results from the double-minimum potential. In addition, it allows us to deperturb the strong c-axis Coriolis interactions between levels of a1 and b2 vibrational symmetry and to determine accurately the vibrational dependence of the rotational constants in the distorted C̃ electronic state.
	The origin of unequal bond lengths in the C̃ 1B2 state of SO2: Signatures of high-lying potential energy surface crossings in the low-lying vibrational structure G. Barratt Park, Jun Jiang, Robert W. Field The Journal of Chemical Physics 144, 144313, (2016)
The vibrational level pattern in the C̃ state of SO2 is analyzed and the origins of the vibronically distorted potential surface are characterized. The measured staggering pattern is consistent with a vibronic coupling model for the double-minimum, which involves direct coupling to the quasi-bound 2 1A1 state. The degree of staggering in the ν′3 levels increases with quanta of bending excitation, which is consistent with the approach along the C̃ state potential energy surface to a conical intersection with the 2 1A1 surface at a bond angle of ~145°.
	Editor's Choice: Millimeter-wave optical double resonance schemes for rapid assignment of perturbed spectra, with applications to the C̃ 1B2 state of SO2 G. Barratt Park, Caroline C. Womack, Andrew R. Whitehill, Jun Jiang, Shuhei Ono, Robert W. Field The Journal of Chemical Physics 142, 144201, (2015)
Millimeter-wave detected, millimeter-wave optical double resonance (mmODR) spectroscopy is a powerful tool for the analysis of dense, complicated regions in the optical spectra of small molecules. The availability of cavity-free microwave and millimeter wave spectrometers with frequency-agile generation and detection of radiation (required for chirped-pulse Fourier-transform spectroscopy) opens up new schemes for double resonance experiments. We demonstrate a multiplexed population labeling scheme for rapid acquisition of double resonance spectra, probing multiple rotational transitions simultaneously. We also demonstrate a millimeter-wave implementation of the coherence-converted population transfer scheme for background-free mmODR, which provides a ~10-fold sensitivity improvement over the population labeling scheme.
	Communication: Observation of local-bender eigenstates in acetylene Adam H. Steeves, G. Barratt Park, Hans A. Bechtel, Joshua H. Baraban, Robert W. Field The Journal of Chemical Physics 143, 071101, (2015)
The eigenstates that embody large-amplitude, local-bending vibrational motion in acetylene were directly observed by stimulated emission pumping spectroscopy for the first time. The Nb = 14 level, lying at 8971.69 cm−1 (J = 0), is assigned on the basis of degeneracy due to dynamical symmetry breaking in the local-mode limit. The level pattern for the Nb = 16 level, lying at 10 218.9 cm−1, is consistent with expectations for increased separation of ℓ = 0 and 2 vibrational angular momentum components. Increasingly poor agreement between our observations and the predicted positions of these levels highlights the failure of currently available normal mode effective Hamiltonian models to extrapolate to regions of the potential energy surface involving large-amplitude displacement along the acetylene ⇌ vinylidene isomerization coordinate.
	Simplified Cartesian basis model for intrapolyad emission intensities in the bent-to-linear electronic transition of acetylene G. Barratt Park, Joshua H. Baraban, Adam H. Steeves, Robert W. Field The Journal of Physical Chemistry A 119, 857, (2015)
The acetylene emission spectrum from the trans-bent electronically excited à state to the linear ground electronic X̃ state has attracted considerable attention because it grants Franck–Condon access to local bending vibrational levels of the X̃ state with large-amplitude motion along the acetylene ⇌ vinylidene isomerization coordinate. In this article, we show that, when the degenerate bending levels of the X̃ state are treated in the Cartesian two-dimensional harmonic oscillator basis (instead of the usual polar basis), there is a simplifying propensity for only one zero-order bright state within each polyad. It is straightforward to model the intrapolyad intensities, as long as corrections are made for anharmonic interactions. The emission patterns are not only relevant to the ground-state acetylene ⇌ vinylidene isomerization, they also reveal mixed upper-state characters that arise directly from the trans ⇌ cis isomerization in the electronically excited à state.
	Full dimensional Franck-Condon factors for the acetylene à 1Au—X̃ 1Σg+ transition. I. Method for calculating polyatomic linear—bent vibrational intensity factors and evaluation of calculated intensities for the gerade vibrational modes in acetylene G. Barratt Park The Journal of Chemical Physics 141, 134304, (2014)
Franck-Condon vibrational overlap integrals for the à 1Au—X̃ 1Σg+ transition in acetylene have been calculated in full dimension in the normal mode basis. Because the transition involves a large change in the equilibrium geometry of the electronic states, two different types of corrections to the coordinate transformation are considered to first order: corrections for axis-switching between the Cartesian molecular frames and corrections for the curvilinear nature of the normal modes at large amplitude. The angular factor in the wavefunction for the out-of-plane component of the trans bending mode, ν4″, is treated as a rotation, which results in an Eckart constraint on the polar coordinates of the bending modes. The results are in qualitative agreement with experiment.
	Perspective: The first ten years of broadband chirped pulse microwave spectroscopy G. Barratt Park, Robert W. Field The Journal of Chemical Physics 144, 200901, (2016)
Since its invention in 2006, the broadband chirped pulse Fourier transform spectrometer has transformed the field of microwave spectroscopy. The technique enables the collection of a ≥10 GHz bandwidth spectrum in a single shot of the spectrometer, which allows broadband, high-resolution microwave spectra to be acquired several orders of magnitude faster than what was previously possible. I had the opportunity to be involved in this technique since its early days and was thrilled to be invited to write this review article. I focus on the entirely new classes of experiments that are made possible with intense, frequency-flexible, broadband microwave and millimeter-wave pulses and comment on current trends and future directions.
	A chirped-pulse Fourier-transform microwave pulsed uniform flow spectrometer: II. Performance and applications for reaction dynamics Chamara Abeysekera, Lindsay N. Zack, G. Barratt Park, Baptiste Joalland, James M. Oldham, Kirill Prozument, Nuwandi M. Ariyasingha, Ian R. Sims, Robert W. Field, Arthur G. Suits The Journal of Chemical Physics 141, 214203, (2014)
As part of a collaboration between the Field and Suits groups, we have developed chirped-pulse Fourier-transform microwave (CP-FTMW) spectroscopy as a kinetic probe for bimolecular reaction products that are thermalized in pulsed uniform flows. The appearance of reaction products are probed on the μs timescale. Two benchmark reactive systems are used to illustrate and characterize the performance of this new apparatus: the photodissociation of SO2 at 193 nm, for which the vibrational populations of the SO product are monitored, and the reaction between CN and C2H2, for which the HCCCN product is detected in its vibrational ground state. Future directions are discussed, with an emphasis on exploring the low temperature chemistry of complex polyatomic systems.
	Chirped-Pulse Millimeter-Wave Spectroscopy of Rydberg-Rydberg Transitions Kirill Prozument, Anthony P. Colombo, Yan Zhou, G. Barratt Park, Vladimir S. Petrovic, Stephen L. Coy, Robert W. Field Physical Review Letters 107, 143001, (2011)
Transitions between Rydberg states of Ca atoms, in a pulsed, supersonic atomic beam, are directly detected by chirped-pulse millimeter-wave spectroscopy. Broadband, high-resolution spectra with accurate relative intensities are recorded instantly. Free induction decay (FID) of atoms, polarized by the chirped pulse, at their Rydberg-Rydberg transition frequencies, is heterodyne detected, averaged in the time domain, and Fourier transformed into the frequency domain. Millimeter-wave transient nutations are observed, and the possibility of FID evolving to superradiance is discussed.
	Design and evaluation of a pulsed-jet chirped-pulse millimeter-wave spectrometer for the 70-102 GHz region G. Barratt Park, Adam H. Steeves, Kirill Prozument, Justin L. Neill, Robert W. Field The Journal of Chemical Physics 135, 024202, (2011)
Chirped-pulse millimeter-wave (CPmmW) spectroscopy is the first broadband (multi-GHz in each shot) Fourier-transform technique for high-resolution survey spectroscopy in the millimeter-wave region. The design is based on chirped-pulse Fourier-transform microwave (CP-FTMW) spectroscopy, developed in the Brooks Pate group. Initial applications of the new spectrometer are demonstrated and challenges of implementing the chirped-pulse technique at higher frequencies are discussed.