A Dunham analysis of the A¹Π−X¹Σ⁺ band was carried out by Müller, and predictions of the pure rotational transition frequencies were made.H. S. P. Müller,A&A,514, L6(2010)ore recently, in submillimeter to THz region, several rotational lines were observed for ¹²CH⁺, ¹³CH⁺, and CD⁺.T. Amano, Ap.J.Lett., 716, L1 (2010)T. Amano, J. Chem. Phys., 133, 244305 (2010)S. Yu et al,The 70th International Symposium on Moecular Spectroscopy, RD06(2015)n this investigation, those newly obtained rotational lines are incorporated in the Dunham analysis. The Λ-doubling splittings in ¹Π electronic states have been expressed as (1/2)qJ(J+1) in most investigations. However, it should be noted that the e-levels of ¹Π state interact with ¹Σ⁺ states, while the f-levels with ¹Σ⁻ states. For CH⁺, the e-levels of A¹Π are pushed upward from the interaction with the ground X¹Σ⁺ state. The ¹Σ⁻ states are not known experimentally and they, if any, should lie high over the A¹Π state. In this analysis, only the f-levels are included in the least-squares analysis by neglecting the Λ-doubling. The mass independent parameters have been obtained, and the conventional spectroscopic parameters are derived for each isotopologue. These results should be useful for determining the potential energies of this fundamental ion.Y. S. Cho and R. J. LeRoy,J. Chem. Phys.,144, 024311(2016)html:<hr /><h3>Footnotes: H. S. P. Müller,A&A,514, L6(2010)M T. Amano, Ap.J.Lett., 716, L1 (2010)

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S. Yu et al,The 70th International Symposium on Moecular Spectroscopy, RD06(2015)I Y. S. Cho and R. J. LeRoy,J. Chem. Phys.,144, 024311(2016)

In our previous work on the Lamb dips of the ν₂ fundamental band of H₃⁺, the saturated absorption spectrum was obtained by the third-derivative spectroscopy using frequency modulation [1]. However, the frequency modulation also causes error in absolute frequency determination. To solve this problem, we have built an offset-locking system to lock the OPO pump frequency to an iodine-stabilized Nd:YAG laser. With this modification, we are able to scan the OPO idler frequency precisely and obtain the profile of the Lamb dips. Double modulation (amplitude modulation of the idler power and concentration modulation of the ion) is employed to subtract the interference fringes of the signal and increase the signal-to-noise ratio effectively. To Determine the absolute frequency of the idler wave, the pump wave is offset locked on the R(56) 32-0 a₁₀ hyperfine component of ¹²⁷I₂, and the signal wave is locked on a GPS disciplined fiber optical frequency comb (OFC). All references and lock systems have absolute frequency accuracy better than 10 kHz. Here, we demonstrate its performance by measuring one transition of methane and sixteen transitions of H₃⁺. This instrument could pave the way for the high-resolution spectroscopy of a variety of molecular ions. [1] H.-C. Chen, C.-Y. Hsiao, J.-L. Peng, T. Amano, and J.-T. Shy, Phys. Rev. Lett. 109, 263002 (2012).

Calculation of ab initio potential energy surfaces for molecules to high accuracy is only manageable for a handful of molecular systems. Among them is the simplest polyatomic molecule, the H₃⁺ cation. In order to achieve a high degree of accuracy ( < 1 cm⁻¹) corrections must be made to the to the traditional Born-Oppenheimer approximation that take into account not only adiabatic and non-adiabatic couplings, but quantum electrodynamic corrections as well. For the lowest rovibrational levels the agreement between theory and experiment is approaching 0.001 cm⁻¹, whereas the agreement is on the order of 0.01 – 0.1 cm⁻¹for higher levels which are closely rivaling the uncertainties on the experimental dataL.G. Diniz, J.R. Mohallem, A. Alijah, M. Pavanello, L. Adamowicz, O.L. Polyansky, J. Tennyson Phys. Rev. A (2013), 88, 032506.O.L. Polyansky, A. Alijah, N.F. Zobov, I.I. Mizus, R.I. Ovsyannikov, J. Tennyson, L. Lodi, T. Szidarovszky, A.G. Császár Phil. Trans. R. Soc. A (2012), 370, 5014.. As method development for calculating these various corrections progresses it becomes necessary for the uncertainties on the experimental data to be improved in order to properly benchmark the calculations. Previously we have measured 20 rovibrational transitions of H₃⁺ with MHz-level precision, all of which have arisen from low lying rotational levelsJ.N. Hodges, A.J. Perry, P.A. Jenkins II, B.M. Siller, B.J. McCall J. Chem. Phys. (2013), 139, 164201.A.J. Perry, J.N. Hodges, C.R. Markus, G.S. Kocheril, B.J. McCall J. Molec. Spectrosc. (2015), 317, 71-73.. Here we present new measurements of rovibrational transitions arising from higher rotational and vibrational levels. These transitions not only allow for probing higher energies on the potential energy surface, but through the use of combination differences, will ultimately lead to prediction of the “forbidden” rotational transitions with MHz-level accuracy.

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L.G. Diniz, J.R. Mohallem, A. Alijah, M. Pavanello, L. Adamowicz, O.L. Polyansky, J. Tennyson Phys. Rev. A (2013), 88, 032506.

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J.N. Hodges, A.J. Perry, P.A. Jenkins II, B.M. Siller, B.J. McCall J. Chem. Phys. (2013), 139, 164201.

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This talk will present the analysis of the rotationally resolved pulsed-field-ionization zero-kinetic-energy (PFI-ZEKE) photoelectron spectrum of H₂O and will be focussed on the Ã⁺←X̃ transitions. H₂O⁺ in the Ã⁺ state is predicted to be linear M. Brommer, B. Weis, B. Follmeg, P. Rosmus, S. Carter, N. C. Handy, H. J. Werner, and P. J. Knowles, J. Chem. Phys. 98, 5222 (1993) The sensitivity and the high resolution of PFI- ZEKE photoelectron spectroscopy allowed us to observe the rotational structure of low bending vibrational levels of the Ã⁺ state of H₂O⁺ from the X̃ ground electronic state of H₂O. The assignment of the rotational structure of ionic levels previously observed by optical spectroscopy of the Ã⁺ - X̃⁺ band system of H₂O⁺ T .Huet, I. H. Bachir, J. L. Destombes, and M. Vervloet, J. Chem. Phys. 107,5645 (1997).H. Lew, Can. J. Phys. 54, 2028 (1976). will be presented and the intensity distribution of the photoelectron spectrum will be discussed in terms of the even or odd nature of the orbital angular momentum quantum number l of the photoelectron. Tentative assignments will be presented for several low-lying vibrational levels of the Ã⁺ state and compared with theoretical predictions ^c. They will also be discussed in terms of the rotational structure of higher Ã⁺ vibrational levels of the same symmetry.

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M. Brommer, B. Weis, B. Follmeg, P. Rosmus, S. Carter, N. C. Handy, H. J. Werner, and P. J. Knowles, J. Chem. Phys. 98, 5222 (1993). T .Huet, I. H. Bachir, J. L. Destombes, and M. Vervloet, J. Chem. Phys. 107,5645 (1997).

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We applied the two-color resonant two-photon mass analyzed threshold ionization technique to record the cation spectra of 4-chloro-2-fluoroanisole by ionizing via five intermediate vibronic levels. The excitation and adiabatic ionization energies were determined to be 35 227, and 67 218 cm⁻¹, respectively. Spectral analysis and theoretical calculation suggest that the geometry of the aromatic ring of the neutral species in the S₁ state is non-planar, but that of the cation in the D₀ state is planar.

The vibronic, photoionization efficiency, and cation spectra of 3,5-difluorophenol have been recorded by using the resonant two-photon ionization and mass-analyzed threshold ionization techniques . The distinct spectral features mainly result from the in-plane substituent-sensitive bending and ring deformation vibrations of these species in the electronically excited S₁ and cationic ground D₀ states. Comparing these results with those of previously reported 2,4-difluorophenol, 2,5-difluorophenol, and 3,5-difluorophenol, we gain knowledge about the substituent effect on the transition energy and molecular vibration of these positional isomers.

Highly variable and potentially revolutionary, ionic liquids (IL) are a class of molecules with potential for numerous Air Force applications such as satellite propulsion, but the complex nature of IL structure and intermolecular interactions makes it difficult to adequately predict structure-property relationships in order to make new IL-based technology a reality. For example, methylation of imidazolium ionic liquids leads to a substantial increase in viscosity but the underlying physical mechanism is not understood. In addition the role of hydrogen bonding in ILs, especially its relationship to macroscopic properties, is a matter of ongoing research. Here, structural motifs are identified from jet-cooled infrared spectra of different imidazolium based ionic liquids, such as 1-ethyl-3-methylimidazolium bis(trifluoromethyl-sulfonyl)imide. Measurements of the C-H stretches indicate three structural families present in the gas phase.

The reaction of Ce + propene (CH₂=CH-CH₃) was carried out in a laser-ablation supersonic molecular beam source. CeC₂H₂, CeC₃H₄, CeC₃H₆, CeC₄H₆, CeC₆H₁₀, and CeC₆H₁₂ were identified by photoionization time-of-flight mass spectrometric measurements, and their structures and electronic states were investigated with mass-analyzed threshold ionization (MATI) spectroscopy and theoretical calculations. The metal complexes containing two or three carbon atoms were formed by the C-C bond breakage (CeC₂H₂), dehydrogenation (CeC₃H₄), or metal insertion into a C-H bond (CeC₃H₆) of a propene molecule. The larger complexes with four to six carbons are formed through secondary reactions involving C-C bond coupling and dehydrogenation. The ground electronic states of the neutral CeC₂H₂, CeC₃H₄, CeC₃H₆, and CeC₄H₆ complexes are triplets with a 4f¹6s¹ electron configuration on the Ce center, and those of the corresponding ions are doublet with a 4f¹ configuration. Their MATI spectra are much more complex than those of the corresponding La species formed in the La + propene reaction previously observed by our group. The spectral complexity arises from possibly multiple electronic transitions due to the existence of a 4f electron of the Ce atom which could be located in any one of the seven f-atomic orbitals or involved in considerable spin-orbit interactions.

Y(C₄H₆) was observed from the reactions of laser-vaporized Y atom with 1-butene (CH₂=CHCH₂CH₃) and iso-butene (CH₂=C(CH₃)₂) in a pulsed molecular beam source, and its structural isomers were investigated with mass-analyzed threshold ionization spectroscopy combined with electronic structure calculations and spectral simulations. Y(C₄H₆) was identified as a five-membered metallacycle [Y(CH₂-CH=CH-CH₂)] from the Y + 1-butene reaction and a tetrahedral structure [YC(CH₂)₃] from the Y + iso-butene reaction. The metallacycle has a C_s structure with Y binding to the two terminal carbon atoms, whereas the tetrahedron has C_3v symmetry with Y binding to the tertiary carbon atom of trimethylenemethane. Both isomers have a doublet ground state with the highest molecular orbital being largely a Y 5s character. Ionization removes the metal based electron, and the resultant singlet ion has a similar structure to the neutral complex. However, the adiabatic ionization energy [46309(5) cm⁻¹] of the tetrahedron is considerably higher than that [43473(5) cm⁻¹] of the cyclic structure.

The diffuse spectra arising from the excess proton in dilute acids suggests that its behavior is highly dependent on the local environment surrounding it. In this work, we report how the spectra of the H₃O⁺, NH₄⁺, and CH₃NH₃⁺ ions respond when docked to the rigid, tri-coordinated binding pocket of the 18-crown-6 ether using cryogenic ion vibrational predissociation (CIVP) spectroscopy with D₂ tagging at 10 K. The H₃O^+•18-crown-6 ether complex displays a broad (350 cm⁻¹ FWHM) unstructured band arising from the OH stretching fundamentals, which is significantly broader than the corresponding band (125 cm⁻¹ FWHM) in the Eigen cation (H₉O₄⁺) spectrum. Perdeuterated isotopologue studies for both systems yield sharper bands with clear multiplet structures, indicating that the broadening arises from nuclear quantum effects. The key displacements underlying this coupling were explored using the vibrationally adiabatic scheme introduced by McCoy in the context of similar broadening in the Ca₂⁺OH⁻(H₂O)_n system.Christopher J. Johnson, Laura C. Dzugan, Arron B. Wolk, Christopher M. Leavitt, Joseph A. Fournier, Anne B. McCoy, Mark A. Johnson, J. Phys. Chem. A 118, 2014.html:<hr /><h3>Footnotes: Christopher J. Johnson, Laura C. Dzugan, Arron B. Wolk, Christopher M. Leavitt, Joseph A. Fournier, Anne B. McCoy, Mark A. Johnson, J. Phys. Chem. A 118, 2014.

The binding of alkaline earth dications to the biologically relevant carboxylate ligand has previously been studied using vibrational sum frequency generation (VSFG) spectroscopy of the air-water interface, infrared multiple photon dissociation (IRMPD) spectroscopy of clusters, and DFT methods. These results suggest the presence of both monodentate and bidentate binding motifs of the M²⁺ ions to the cayboxyl head groups depending on the extent of solvation. We revisit these systems using vibrational predissociation spectroscopy to measure the gas-phase vibrational spectra of the D₂-tagged microhydrated [MgOAc(H₂O)_n=1−5]⁺ and [CaOAc(H₂O)_n=1−6]⁺ clusters. The spectra show that [MgOAc(H₂O)_n]⁺ switches from bidentate to monodentate binding promptly at n = 5, while [CaOAc(H₂O)_n]⁺ retains its bidentate attachment such that the sixth water molecule initiates the second solvation shell. The difference in binding behavior between these two divalent metal ions is analyzed in the context of the local acidity of the solvent water molecules and the strength of the metal-carboxylate and metal-water interactions. This cluster study provides insight into the chemical physics underlying the unique and surprising impacts of Mg²⁺ and Ca²⁺ on the chemistry mediated by sea spray aerosols. Funding for this work was provided by the NSF’s Center for Aerosol Impacts on Climate and the Environment.

The local structures of DNA and RNA are influenced by protonation, deprotonation and noncovalent interactions with cations. In order to determine the effects of Na⁺ cationization on the gas-phase structures of 2^′-deoxycytidine, [dCyd+Na]⁺, and cytidine, [Cyd+Na]⁺, infrared multiple photon dissociation (IRMPD) action spectra of these sodium cationized nucleosides are measured over the range extending from  500 to  1850 cm⁻¹using the FELIX free electron laser. Complementary electronic structure calculations are performed to determine the stable low-energy conformations of these complexes. Geometry optimizations, frequency analyses, and IR spectra of these species are determined at the B3LYP/6-311+G(d,p) level of theory. Single-point energies are calculated at the B3LYP/6-311+G(2d,2p) level of theory to determine the relative stabilities of these conformations. Comparison of the measure IRMPD action spectra and computed linear IR spectra enable the conformations accessed in the experiments to be elucidated. For both cytosine nucleosides, tridentate binding of the Na⁺ cation to the O2, O4^′ and O5^′ atoms of the nucleobase and sugar is observed. Present results for the sodium cationized nucleosides are compared to results for the analogous protonated forms of these nucleosides to elucidate the effects of multiple chelating interactions with the sodium cation vs. hydrogen bonding interactions in the protonated systems on the structures and stabilities of these nucleosides.

The chemical and structural diversity and the extent of post-transcriptional modification of RNA is remarkable! Presently, there are 142 different naturally-occurring and many more synthetically modified nucleosides known. Uridine (Urd) is the most commonly modified nucleoside among those that occur naturally, but has also been an important target for synthesis and development of modified nucleosides for pharmaceutical applications. Indeed, modified nucleosides are of pharmaceutical interest due to their bioactivities. In particular, 5-bromouridine (br⁵Urd) has been shown to exhibit antiviral activity to human immunodeficiency virus and has been used in RNA labeling studies. Halogenation is a common modification employed in pharmaceutical studies that enables systematic variation is the electronic properties of the molecule of interest due to the availability of halogen substituents that vary in size, dipole moment, polarizability, and electron withdrawing properties. In order to elucidate the influence of 5-halogenation on the intrinsic gas-phase structure and stability on the protonated form of Urd, synergistic spectroscopic and theoretical studies of the protonated forms of the 5-halouridines are performed here, where x⁵Urd = 5-fluorouridine (f⁵Urd), 5-chlorouridine (cl⁵Urd), br⁵Urd, and 5-iodouridine (i⁵Urd). Infrared multiple photon dissociation (IRMPD) action spectra of the protonated forms of the 5-halouridines, [x⁵Urd+H]⁺, are measured over the IR fingerprint region using the FELIX free electron laser and the hydrogen stretching region using an OPO/OPA laser from 3300-3800 cm⁻¹. Complementary electronic structure calculations are performed to determine the stable low-energy conformations available to these species and to predict their IR spectra. Comparative analyses of the measured IRMPD spectra and predicted IR spectra are performed to elucidate the preferred sites of protonation, and the low-energy tautomeric conformations that are populated by electrospray ionization to be determined. Comparisons among these systems and to results previously reported for the protonated form of uridine, [Urd+H]⁺, provides insight into the impact of the 5-halogen substituent on the structures and IR signatures.