I have been a card-carrying spectroscopist for 52 years. I began my career studying spectroscopic perturbations in CS and CO. I eventually graduated to vibrational polyads in acetylene and Multichannel Quantum Defect Theory (MQDT) models for Rydberg states of CaF. My experimental arsenal evolved from atomic resonance lamps to finicky cw dye lasers to user-friendly Nd:YAG pumped dye lasers, ending up with Chirped Pulse Millimeter Waves, non-finicky solid state cw lasers, and death-defying dreams about Stimulated Raman Adiabatic Passage (STIRAP). It has become possible to record an enormous quantity of unimaginably high quality spectra quickly. Increases by factors of 10⁶ in spectral velocity have been claimed. Yet everything rests on assigning the spectrum. But the assignment game has changed. Instead of looking for patterns, we deal with meta-patterns. Our goal is to build a complex model that represents all of the energy levels and associates a multi-component eigenvector with each observed eigenstate. Eigenvectors can reveal what a molecule is thinking about doing when it grows up. Spectroscopy becomes a form of molecular psychoanalysis. A spectroscopist can observe the emergence and describe the mechanistic origin of new classes of large-amplitude intramolecular motions. This makes it possible to directly characterize things, such as transition states, which dogma has labeled “spectroscopically unobservable.” Where is 21st century spectroscopy headed? I will discuss examples that include: spectroscopic perturbations of the S₂ B³Σ⁻_u state, the SO₂ C state with its unequal SO bond-lengths, and the transition state for trans-cis isomerization in the S₁ state of acetylene.

The excited-state population of the C₂ ¹AΠ_u state produced in photolysis of S₁ acetylene was investigated. The pulsed UV laser (216.5 nm) excites acetylene into J=8 e-symmetry level of the S₁ 3⁴ level, and subsequently dissociates the S₁ acetylene into C₂ fragments. A frequency-modulated near-infrared probe laser beam is used to detect the C₂ population in the ¹AΠ_u state. The sensitivity and the fast response of the experimental setup has been verified by I₂ excited state measurements. The setup will be used to record the C₂ A−X transitions, which are fitted with a Voigt function. The derived lineshape and line intensities will be analyzed, and we will use the information to calculate the A state populations of C₂ and map the populations with time-resolution following the photolysis.

He₂⁺ is a three-electron system for which highly accurate ab initio calculations are possible. The latest calculations of the rovibrational energies of He₂⁺ by Tung et al. W.-C. Tung, M. Pavanello and L. Adamowicz, J. Chem. Phys., 136, 104309, 2012.ave a reported accuracy of 120 MHz, although they do not include relativistic and quantum electrodynamics (QED) effects. We determined the rovibrational structure of ⁴He⁺₂ from measurements of the Rydberg spectrum of metastable a ³Σ_u⁺ He₂ (He^*₂ hereafter) and Rydberg-series extrapolation using multichannel quantum-defect-theory C. Jungen, Elements of Quantum Defect Theory, in : Handbook of High-resolution Spectroscopy, 2001.D. Sprecher, J. Liu, T. Krähenmann, M. Schäfer, and F. Merkt, J. Chem. Phys., 140, 064304, 2014.. He^*₂ molecules are produced in supersonic beams with velocities tunable down to about 100 m/s by combining a cryogenic supersonic-beam source with a multistage Zeeman decelerator A. W. Wiederkehr, S. D. Hogan, M. Andrist, H. Schmutz, B. Lambillotte, J. A. Agner, and F. Merkt., J. Chem. Phys., 135, 214202, 2011.M. Motsch, P. Jansen, J. A. Agner, H. Schmutz, and F. Merkt, Phys. Rev. A, 89, 043420, 2014.. They are then excited to high-np Rydberg states by single-photon excitation. In the experiments, we use a pulsed uv laser system, with a near Fourier-transform-limited bandwidth of 150 MHz. The Zeeman deceleration reduces the systematic uncertainty arising from a possible Doppler shift and greatly simplifies the spectral assignment because of its spin-rotational state selectivity P. Jansen, L. Semeria, L. E. Hofer, S. Scheidegger, J. A. Agner, H. Schmutz, and F. Merkt. Phys. Rev. Lett., 115, 133202, 2015. Results will be presented on the rotational structure of the lowest three vibrational levels of He⁺₂. The unprecedented accuracy that we have obtained for the v⁺=0 rotational intervals of He₂⁺ L. Semeria, P. Jansen and F. Merkt, J. Chem. Phys., 145, 204301, 2016.nables the quantification of the relativistic and QED corrections by comparison with the results of Tung et al.^a

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Footnotes:

W.-C. Tung, M. Pavanello and L. Adamowicz, J. Chem. Phys., 136, 104309, 2012.h C. Jungen, Elements of Quantum Defect Theory, in : Handbook of High-resolution Spectroscopy, 2001.

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A. W. Wiederkehr, S. D. Hogan, M. Andrist, H. Schmutz, B. Lambillotte, J. A. Agner, and F. Merkt., J. Chem. Phys., 135, 214202, 2011.

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P. Jansen, L. Semeria, L. E. Hofer, S. Scheidegger, J. A. Agner, H. Schmutz, and F. Merkt. Phys. Rev. Lett., 115, 133202, 2015.. L. Semeria, P. Jansen and F. Merkt, J. Chem. Phys., 145, 204301, 2016.e

The recent observations [1,2] of shape and Feshbach resonances in the high-resolution photoelectron spectra of H₂, HD and D₂ in the vicinity of the dissociation thresholds of H₂⁺, HD⁺ and D₂⁺ raise questions concerning their potential role in the formation of H₂⁺ and its isotopomers in the early universe by radiative association, a topic of astrophysical interest [3]. Close-coupling calculations for the cross sections of the reactions

H+ + H -> H2+ + hv

H+ + D -> HD+ + hv

D+ + H -> HD+ + hv

D+ + D -> D2+ + hv

will be presented which take into account nonadiabatic couplings involving rovibronic and hyperfine interactions, as well as relativistic and radiative corrections. The calculated energies and widths will be compared with the experimental results of Ref. [1,2] for H₂⁺ and new data for HD⁺ and D₂⁺. The effect of the resonances on the radiative association rate coefficients will be discussed, also in comparison with earlier studies [4]. [1] M. Beyer and F. Merkt, Phys. Rev. Lett. 116, 093001 (2016). [2] M. Beyer and F. Merkt, J. Mol. Spectrosc. 330, 147 (2016). [3] Molecule formation in dust-poor environments, J. F. Babb and K. P. Kirby, in "The molecular astrophysics of stars and galaxies", T. W. Hartquist and D. A. Williams, eds., Oxford University Press, Oxford, 1998, pp. 11-34. [4] D. E. Ramaker and J. M. Peek, Phys. Rev. A 13, 58 (1976).

Ethynyl, C₂H, is an important intermediate in combustion processes and has been widely observed in interstellar space. Spectroscopically, it is of particular interest because it possesses three low-lying electronic surfaces: a ground ²Σ⁺state, and a low-lying ²Π excited electronic state, which splits due to the Renner-Teller effect. Vibronic coupling among these states leads to a complicated, mixed-character, energy level structure. We have previously reported workA. T. Le, G. E. Hall, T. J. Sears, J. Chem. Phys. 145 074306, 2016 n three bands originating from the X̃(0,0,0) ²Σ ground state to excited vibronic states: two ²Σ − ² Σ transitions at 6696 and 7088 cm⁻¹and a ²Π − ²Σ transition at 7108 cm⁻¹. In this work, the radicals were formed in a hot, non-thermal, population distribution by u.v. pulsed laser photolysis of a precursor. Kinetic measurements of the time-evolution of the ground state populations following collisional relaxation and reactive loss were also made, using some of the stronger rotational lines observed. Time-dependent signals in mixtures containing a variable concentration of precursor in argon suggested that vibronically hot C₂H radicals were less reactive than the relaxed, thermalized, radical. Two additional hot bands originating in states X̃(0,1¹,0) ²Π and X̃(0,2⁰,0) ²Σ, have now been identified in the same spectral region. In a new series of experiments, we have measured the kinetics of formation and decay of representative levels involving all the assigned transitions, i.e. originating in X̃(0,v₂,0), with v₂ = 0 ,1, and 2, in various concentrations of mixtures of precursor, inert gas and hydrogen. The new spectra also show greatly improved signal-to-noise ratio in comparison to our previous work, due to the use of a transient FM detection scheme, and additional spectral assignments seem likely. Both kinetics and spectroscopic results will be described in the talk. Acknowledgments: Work at Brookhaven National Laboratory was carried out under Contract No. DE-SC0012704 with the U.S. Department of Energy, Office of Science, and supported by its Division of Chemical Sciences, Geosciences and Biosciences within the Office of Basic Energy Sciences. A. T. Le, G. E. Hall, T. J. Sears, J. Chem. Phys. 145 074306, 2016 o

We report high-resolution photoelectron imaging of B₁₁⁻ and B₁₂⁻ at 354.7 and 532.0 nm, respectively, resolving several low-frequency vibrational modes for neutral B₁₁ and B₁₂. The vibrational information is highly valuable to verify the structures of the neutral clusters. Several isomers are considered, and vibrational frequencies are calculated for B₁₁ and B₁₂ using density functional theory. Comparisons between the experimental and calculated vibrational frequencies prove that the B₁₁ neutral and anion both possess a perfectly planar C_2v structure. The B₁₂⁻ anion is quasi-planar with C_s symmetry, while the experiment confirms that neutral B₁₂ possesses C_3v symmetry. The high-resolution photoelectron spectra also allow the electron affinities of B₁₁ and B₁₂ to be measured more accurately as 3.401 and 2.221 eV, respectively. It is shown that high-resolution photoelectron imaging can be an effective method to determine structures of small neutral boron clusters, complementary to infrared spectroscopy.

r0pt Figure NO₂ a key component of photochemical smog and an important species in the Earth's atmosphere, is an example of a molecule which exhibits significant mixed orbital character in the HOMO. In photoelectron experiments the geometric properties of the parent anion orbital are reflected in the photoelectron angular distribution (PAD), an area of research that has benefited largely from the ability of velocity-map imaging (VMI) to simultaneously record both the energetic and angular information, with 100% collection efficiency. Photoelectron spectra of NO₂⁻, taken over a range of wavelengths (355nm-520nm) with the ANU's VMI spectrometer, reveal an anomalous jump in the anisotropy parameter near threshold. Consequently, the orbital behavior of NO₂⁻ appears to be quite different near threshold compared to detachment at higher photon energies. This surprising effect is due to the Wigner Threshold law, which causes p orbital character to dominate the photodetachment cross-section near threshold, before the mixed s/d orbital character becomes significant at higher electron kinetic energies.K. J. Reed, A. H. Zimmerman, H. C. Andersen, and J. I. Brauman, J. Chem. Phys. 64, 1368, (1976). doi:10.1063/1.432404y extending recent work on binary character modelsD. Khuseynov, C. C. Blackstone, L. M. Culberson, and A. Sanov, J. Chem. Phys. 141, 124312, (2014). doi:10.1063/1.4896241o form a more general expression, the variable mixed orbital character of NO₂⁻ is able to be described. This study provides the first multi-wavelength NO₂ anisotropy data, which is shown to be in decent agreement with much earlier zero-core model predictionsW. B. Clodius, R. M. Stehman, and S. B. Woo, Phys. Rev. A. 28, 760, (1983). doi:10.1103/PhysRevA.28.760f the anisotropy parameter.

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K. J. Reed, A. H. Zimmerman, H. C. Andersen, and J. I. Brauman, J. Chem. Phys. 64, 1368, (1976). doi:10.1063/1.432404B D. Khuseynov, C. C. Blackstone, L. M. Culberson, and A. Sanov, J. Chem. Phys. 141, 124312, (2014). doi:10.1063/1.4896241t W. B. Clodius, R. M. Stehman, and S. B. Woo, Phys. Rev. A. 28, 760, (1983). doi:10.1103/PhysRevA.28.760o

r0pt Figure The nature of the isomeration process that turns vinylidene H₂CC to acetylene HCCH, requiring a 1,2-hydrogen atom shift across the molecule, is a long standing puzzle that has its origin in a 1989 photoelectron measurement of vinylidide (H₂CC⁻)K. M. Ervin et al. J. Chem. Phys.91 5974 (1991). In recent years the photoelectron spectrum of vinylidide has been revisited, using improved experimental techniques, including velocity-map imaging for the detection of photoelectrons, low-temperature near-threshold methods (cryo-SEVI)J. A. De Vine et al. J. Am. Chem. Soc. 138 16417 (2016). and sophisticated ab inito calculationsL. Guo et al. J. Phys. Chem. 119 8488 (2015). The simple normal-mode structure, 1064 nm velocity-map image illustrated, is proving a challenge to decipher. However, the dramatic change in the photoelectron angular distribution of the inner-ring structure is characteristic of vibronic couplingA. Weaver et al. J. Chem. Phys. 94 1740 (1991). The lowest electronic state with the correct symmetry, B̃  ¹B₂, is 4eV higher in energy.

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Footnotes:

K. M. Ervin et al. J. Chem. Phys.91 5974 (1991).. J. A. De Vine et al. J. Am. Chem. Soc. 138 16417 (2016)., L. Guo et al. J. Phys. Chem. 119 8488 (2015).. A. Weaver et al. J. Chem. Phys. 94 1740 (1991)..

Understanding the initial stages of polycyclic aromatic hydrocarbon (PAH) aggregation, the onset of soot formation, is an important goal on the pathway to cleaner combustion processes. PAHs with short alkyl chains, present in fuel-rich combustion environments, can undergo reactions that will chemically link aromatic rings together. One such example of a linked diaryl compound is dibenzyl ether, C₆H₅-CH₂-O-CH₂-C₆H₅. The –CH₂-O-CH₂- linkage has a length and flexibility well-suited to forming a π-stacked conformation between the two phenyl rings. In this talk, we will explore the single-conformation spectroscopy of dibenzyl ether under jet-cooled conditions in the gas phase. Laser-induced fluorescence, chirped pulse Fourier transform microwave (8-18 GHz region), and single-conformation infrared spectroscopy in the alkyl CH stretch region were all carried out on the molecule, thereby interrogating its full array of electronic, vibrational and rotational degrees of freedom. This work is the first step in a broader study to determine the extent of π-stacking in linked aryl compounds as a function of linkage and PAH size.

3-phenyl-2-propynenitrile (Ph-C ≡ C-C ≡ N) is of interest to the study of Titan’s atmosphere as it is a likely product of the photochemical reaction between two known species in that environment: benzene and cyanoacetylene. The gas phase jet-cooled resonant two-photon ionization, laser induced fluorescence, and preliminary dispersed fluorescence spectra were previously reported without firm assignments due to the scarcity of totally symmetric vibrations and the prevalence of strong bands of b2 and b1 symmetry vibrations. These had called into question the identity and geometry of the excited state(s) involved in the transitions. We will here present the completed set of dispersed fluorescence data along with an analysis of the potential energy surfaces and vibronic coupling characteristic of the close-lying excited states in this intriguing molecule.

Whereas benzyl radical, a prototypic aromatic free radical, has been the subject of numerous spectroscopic studies, chloro-substituted benzyl radicals have received less attention, due to the difficulties associated with production of radicals from precursors. The weak C-Cl bond can be easily dissociated in corona discharge of high voltage, leading to the formation of other benzyl-type radicals. During past years, we have concentrated the spectroscopy of chloro-substituted methylbenzyl radicals produced from corona discharge of precursor seeded in a large amount of helium carrier gas using a pinhole-type glass nozzle in a technique of corona excited supersonic expansion. From the analysis of the spectrum observed, we can easily distinguish the origin bands in the D₁ → D₀ transition of the isomeric chloro-substituted methylbenzyl radicals with the additivity rule, Y. W. Yoon, C. S. Huh, and S. K. Lee, Chem. Phys. Lett. 525-526, 44-48 (2012). Y. W. Yoon and S. K. Lee, J. Phys. Chem. A 117, 2485-2491 (2013).discovered from the analysis of a series of benzyl-type radicals. Also, the displacement of phenylic Cl by benzylic H was confirmed to be dependent on the distance between Cl and H atoms. The benzyl-type radicals produced in corona discharge from precursor were determined based on the bond dissociation energies and molecular structure of precursor molecules as well as the agreement of the observed with the calculated ones from Gaussian program, from which the 2-methyl-3-chlorobenzyl, 2-methyl-4-chlorobenzyl, 2-methyl-5-chlorobenzyl, and 2-methyl-6-chlorobenzyl radicals were newly identified.

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Footnotes:

Y. W. Yoon, C. S. Huh, and S. K. Lee, Chem. Phys. Lett. 525-526, 44-48 (2012).

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