This introductory talk will briefly overview the contributions of Marilyn Jacox to the technique of matrix isolation spectroscopy and her use of it to investigate the vibrational spectroscopy of a multitude of interesting molecular species. This work as well as her cataloging of the results of others on many more molecules is well known in the spectroscopic community. Much less well known is her life outside spectroscopy. The talk will conclude with some glimpses into the varied interests of her extraordinary life.

The methoxy radical, CH₃O, has attracted much attention because of its important molecular structure and also as a reaction intermediate in combustion and atmospheric chemistry. Previous investigations include laser-induced fluorescence, laser magnetic resonance, and stimulated emission pumping. High-resolution infrared spectrum of jet-cooled CH₃O, produced by laser photolysis of CH₃ONO, in the C-H stretching region 2850-2940 cm⁻¹ has been reported.¹ However, direct infrared absorption spectrum of CH₃O other than the C–H stretching region remains unreported. Irradiation of a p-H₂ matrix containing CH₃ONO at 3.2 K with UV light produced main features at 1365.4, 1427.5 (2₁⁻, 2₁⁺), 1041.8 (3₁⁻), 1346.8, 1427.5, 1520.9, 1520.9 (5₁⁻, 5₁⁺, 5₁⁻, 5₁⁺), and 689.3/694.9, 945.9/951.7, 1233.5, 1235.9 cm⁻¹ (6₁⁻, 6₁⁺, 6₁⁻, 6₁⁺); labels 2-6 in parentheses indicate transitions to vibrational states attributable to the umbrella, C–O stretching, CH₂ scissoring, and HCO deformation modes of CH₃O, respectively. These features appeared upon photolysis and diminished after five minutes; formation of CH₂OH was observed as CH₃O decayed. The assignments were based on comparison of observed vibrational wavenumbers with those predicted with the quadratic potential energy force field and quadratic dipole moment expansion calculated with the CCSD(T)/cc-pVTZ method.² Jahn-Teller and anharmonic vibrational contributions were included in the full Hamiltonian to estimate the correlation diagram connecting the harmonic eigenvalues to those of the fully coupled problem. Similarly, lines of CD₃O were observed upon UV photolysis of CD₃ONO, but became diminished within five minutes. These observations demonstrates the advantage of diminished cage effect of solid p-H₂; CH₃O and CD₃O are produced via in situ UV photodissociation of CH₃ONO isolated in p-H₂, but not in Ar or Ne.
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¹ J.-X. Han, Y. G. Utkin, H.-B. Chen, L. A. Burns and R. F. Curl, J. Chem. Phys. 117, 6538 (2009). ² J. Nagesh and E. L. Sibert III, J. Phys. Chem. A 116, 3846 (2012).

Three open chain isomers of C₇ H₃⁺ including a three member carbon ring in the structure were identified in a neon matrix held at 6 K using a mass-selected ion beam. Both indene and acyclic precursors were used in an ion source to produce the C₇ H₃⁺ cations which were codeposited with excess of neon to produce the matrix. The electronic absorption spectra were measured by the sensitive waveguide technique sampling around 10¹⁶ cm⁻³ species. Some of the cations show well-resolved fluorescence. Some neutral C₇H₃ isomers are also detected in absorption produced in the matrix by photo-bleaching of the cations. The charge of the mass-selected species is experimentally established using presence of electron scavengers, release of electrons, changing the kinetic energy during deposition and precursor dependence, whereas the structure of the isomers is inferred by comparison of the energies of the transitions with CASPT2 calculations. The assignment is supported by comparison of the vibrational frequencies inferred from the spectra and theory. The first time identification of the electronic transitions of these structurally interesting isomers provides their means for their in situ monitoring in hydrocarbon combustion processes and formation of aromatics therein.

        The ground ~X²E vibrations of the methoxy radical have intrigued both experimentalists and theorists alike due to the presence of a conical intersection at the C_3v molecular geometry. This conical intersection causes methoxy's vibrational spectrum to be strongly influenced by Jahn-Teller coupling, this leading to large amplitude vibrations and extensive mixing of the two lowest electronic states. This coupling combined with spin-orbit and Fermi couplings greatly complicates the assignments of states. In this talk we describe our efforts to assign the states of both CH₃O and CD₃O.         Using the potential energy force field and calculated spectra of Nagesh and Sibert¹ as a starting point, vibrational mixing is considered using various zero-order representations. When the zero-order states are the diabatic normal mode states, there is sufficient mode mixing that the normal mode quantum numbers are no longer good labels. The mixing of the zero-order states can be reduced by including additional terms in the zero-order Hamiltonian, H^o. We consider the choice of including the first order Jahn-Teller coupling between one of the three degenerate normal modes. As the rocking motion has the largest Jahn-Teller coupling, this is the coupling that is included in H^o. Although the normal mode quantum numbers of the rocking basis functions are no longer good quantum numbers, due to the Jahn-Teller induced vibronic mixing, the zero-order states can be labeled with the linear Jahn-Teller quantum numbers.² This work extends these ideas by considering an H^o that includes linear Jahn-Teller coupling between two sets of degenerate vibrations. Plots of the resulting zero-order states are presented, and the spectral transitions recently observed³ for both CH₃O and CD₃O in a p-H₂ matrix are assigned using these basis functions. The extent of state-mixing found for the full Hamiltonian H for various choices of H^o is illustrated via the use of correlation diagrams obtained by plotting the eigenvalues of H^o + δ(H−H^o) as a function of δ where δ varies from zero to one.

¹

Nagesh, J.; Sibert, E. L. J. Phys. Chem. A 2012, 116, 3846-3855.

²

Barckholtz, T. A.; Miller, T. A. Int. Revs. in Phys. Chem. 1998, 17, 435-524.

³

Yu-Fang Lee, Wei-Te Chou, and Yuan-Pern Lee (private communication).

We report new FTIR studies of the 193 nm photochemistry of deuterated formic acid (DCOOD) trapped in parahydrogen matrices. In our 2011 preliminary report,¹ we showed the 193 nm in situ photolysis of formic acid (HCOOH) produces small amounts of HCO and HOCO and that after the laser is turned off, we observe continued slow growth in the HOCO radical for up to 10 hours after photolysis. At that time we were unsure of the detailed chemical mechanism by which the HOCO continues to grow after photolysis, but we suspected it had to do with reactions of mobile H-atoms with the HCOOH precursor that remains at significant concentrations after photolysis under these conditions. The present deuterated formic acid photolysis studies provide strong circumstantial evidence that H-atom reactions with formic acid are the source of the continued HOCO growth. Further, variable-temperature kinetic studies conducted with the deuterated formic acid sample show a strong inverse temperature dependence to the reaction kinetics. Essentially, the reaction that leads to HOCO growth only occurs at temperatures below 2.4 K. We are currently trying to model the kinetics using standard methods and the most recent analysis will be presented at the meeting.
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¹David T. Anderson, Leif O. Paulson, 66th Ohio State University International Symposium on Molecular Spectroscopy, talk FE02 (2011).

Ultraviolet irradiation of solid parahydrogen (pH₂) matrices doped with suitable H-atom precursor molecules generates H atoms in situ through a series of photoinitiated chemical reactions; these H atoms move through the pH₂ matrix through a quantum diffusion process that involves the tunneling-mediated process H + H₂ → H₂ + H. The mobile H atoms may react chemically with other species that are also embedded in the pH₂ matrix; an investigation of the kinetics of these H-atom chemical reactions provides us with information about reaction dynamics in the pH₂ matrix environment. A recent study of the H + N₂O → HNNO reaction in solid pH₂ [Mutunga, Follett, and Anderson, J. Chem. Phys. 139, 151104 (2013)] demonstrates that this reaction exhibits strongly non-Arrhenius behavior, proceeding at measurable rates only when the temperature of the system drops below T ≈ 2.4 K. A molecular-level understanding of these findings requires information about how the solid pH₂ matrix environment affects the long-range entrance channel of the H + N₂O reaction. Here, we carry out quantum Monte Carlo simulations of a simple model system (Ar-doped solid pH₂) to investigate matrix-induced changes to long-range interactions between H atoms and other impurities embedded in solid pH₂ matrices. Our results suggest that the pH₂ matrix creates an effective long-range repulsion between the H atom and the Ar impurity, which we explain in terms of differential solvation energies of Ar and H atoms in solid pH₂.

Infrared spectra for matrix-isolated γ-butyrolactone and γ-butyrolactone-d₆ were obtained. The carbonyl stretching mode occurs at 1803 cm⁻¹ for the monomer species, 1786 cm⁻¹ for the dimer species, and 1774 cm⁻¹ for the trimer species (1797, 1789 and 1770 cm⁻¹ for the deuterated isotopomer.) Vibrational frequencies calculated using density functional theory are in agreement with the experimental values. Density functional theory was used to calculate the structures and binding energies of γ-butyrolactone dimers and trimers. Binding energies of 55-58 kJ mol⁻¹ are predicted for the dimer structures. Optimized geometries for stacked and ring trimer structures have been calculated, with predicted binding energies of up to 68 kJ mol⁻¹.

Interfacial matrix stabilization spectroscopy is a new technique based on matrix isolation spectroscopy, but where a cryogenic matrix is deposited over the top of a film sample, in order to characterize interactions between the substrate and molecular dopants. The IMSS technique harnesses the well-established ability of cryogenic matrices to trap and stabilize transient species, although in this case it is applied to intermediates relevant to heterogeneous catalysis. In this proof-of-concept study, we present data for CO and O₂ reactants binding to TiO₂ and Au/TiO₂ nanoparticle films, where in the latter case the Au nanoparticles were created by de-wetting of a 22.5 nm overlayer at  450 K. The films are first pre-saturated with CO at 40 K, then cooled to 20 K, at which point an argon matrix is deposited over the top of them. The spectra are then annealed in stages over a range of temperatures between 20 and 40 K. In all cases, the presence of the Ar matrix alters the appearance of the CO bands, revealing additional structure, such as a broad feature at 2150 cm⁻¹, which is typically attributed to CO interacting with OH groups on the TiO₂ surface, but is not observed at 40 K for these samples in the absence of the matrix. The interpretation is that the matrix induces a caging-effect that prevents molecules from desorbing from weak binding sites from which they would be "pumped away" in the vacuum chamber if the matrix were not present. Perhaps the most interesting feature of these spectra is a small but sharp band at 2112 cm⁻¹that appears ONLY when O₂ is added to the argon matrix as a dopant. This transient band grows in following annealing at 32 K, but then disappears upon annealing above 34 K, suggesting that it may correspond to a reactive intermediate. The band occurs for samples both with and without Au present on the TiO₂ surface, but shows a larger intensity in the latter case. Possible assignments for the observed band in light of previous studies from the literature will be discussed in detail.

Rotational dynamics is one of the most fascinating features of molecular solids. This report presents EPR studies of methyl radical rotation in solid Ar, a matrix possessing no nuclear magnetic moments that facilitates recording magnetic resonance spectra of high resolution. As a result, we were able to make a thorough investigation of the lineshape temperature behavior of the A- and E-symmetry multiplets related to different rotational states of the radical. Methyl radicals in solid Ar matrix were recorderd at the liquid helium temperature 4.2 K and in the range from 9 through 31 K. Both EPR multiplets exhibited anisotropic features which were explained by a model accounting for fast rotation about the radical C₃ axis and relatively slow reorientation about the in-plane C₂ axes. We present the first observation of anisotropic splitting in the powder EPR of methyl radical E-states suggesting that the A- and E- rotational states which differ in their spatial characteristics should exhibit a varying degree of coupling to the surrounding cage in the matrix. The simulation model of the E-state doublet was tentatively based on the transition of radical dynamics from rotational jumps at low temperatures to rotational diffusion at high temperatures with a conditional border 19 K. For the molecule in A-state , temperature assisted tunneling was supposed to occur at temperatures below 16 K while temperature activated rotational diffusion was taking place at higher temperatures.¹
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¹Y. D. acknowledges a partial financial support from Russian Foundation for Basic Research under grant 13-02-00373-a.

Amino acids are the building blocks of biological molecules, and thus the investigation of their physical and chemical properties would allow for further understanding of their functions in biological systems. In addition, the existence of amino acids in interstellar space has been discussed for many years, but it is still under intense debate. The effect of UV radiation on amino acids is one of the keys for their search in interstellar space, where strong UV radiation exists. In this experiment, conformational compositions of alpha and beta alanine and their UV photolysis were investigated via matrix-isolation FTIR spectroscopy and quantum chemical calculations. Solid parahydrogen was used as the matrix, which provides higher resolution spectra than other noble gas matrices. We have identified several stable conformers for both alpha and beta alanine in solid parahydrogen. A clear correlation between conformational ratio and sublimation temperature was found for beta alanine. Furthermore, it was found that UV photolysis of alanine yields not only its conformational changes, but also photodissociation into a CO₂ molecule and fragments. Observed spectra and their analysis will be discussed in relation to interstellar chemistry.

Highly unsaturated carbon chains of the HC_nH family are important to the studies of combustion chemistry and the interstellar medium (ISM). Several members of this family (n=2,4,6) have been detected in the ISM by infrared spectroscopy. We have successfully matrix-isolated HC₇H and MeC₇H species and studied them using electronic absorption, FTIR, and EPR spectroscopy. The ground state potential energy surface was explored using ab initio (CCSD(T)/cc-pVTZ (fc)) methods to discover triplet minima for both species. Equation of motion coupled cluster calculations (EOM-CCSD/ANO1) of low-lying excited states allowed for Franck-Condon simulations. The comparison of the simulated spectra to the vibronic progression observed experimentally in the UV/Vis spectra permits assignment of a linear ground state triplet structure for both HC₇H and MeC₇H.

Para-hydrogen (pH₂) soft quantum crystal is an ideal isolation matrix due to its impressive intrinsic properties, i.e. its significant lattice constant, large zero-point vibration as well as its ability to repair itself of crystal defects. To investigate molecular conformation of a chiral ketone, IR spectra of R-(+)-3-methylcyclopentanone (R3MCP), hosted in pH₂ crystal, were recorded as a function of sample concentration and host pH₂ crystal temperature over the low deposition range {3.5-6.0K}. IR spectra of R3MCP in pH₂ crystal will be presented and compared against corresponding spectra in Ar matrix as well as IR spectra of the neat crystalline R3MCP at low deposition temperatures. Furthermore, density functional theory calculations of simulated IR spectra for the optimized geometries of R3MCP, equatorial-methyl and axial-methyl conformers are compared against experimental spectra for the purpose of investigating molecular conformation. Upon comparison between theoretical and experimental IR spectra, vibrational modes arising from equatorial and axial conformers have been successfully assigned and related to the individual conformer’s structure.

Matrix isolation infrared spectroscopy was used to characterize the fundamental infrared peaks of the germanium hydride species GeH₂ and GeH in low temperature argon matrices that result from the vacuum-ultraviolet (VUV) photolysis of germane (GeH₄). Experiments were performed by depositing mixtures of GeH₄ with argon onto a CsI window cooled to 12 K while simultaneously photolyzing the mixture with 121 nm VUV radiation from a hydrogen resonance lamp. For GeH₂, the fundamental infrared peaks are observed at 1839.1 cm⁻¹(ν₃, antisymmetric stretch), 1816.6 cm⁻¹(ν₁, symmetric stretch) and 913.4 cm⁻¹(ν₂, bend). For GeH, the fundamental infrared stretching peak is observed at 1813.4 cm⁻¹. The assignment of the observed peaks is established by performing experiments with isotopic germane (GeD₄), by performing matrix annealing experiments (warming to 25 - 35 K and refreezing to 12 K), by performing mercury-xenon lamp matrix photolysis experiments (200 - 900 nm), and by comparison to quantum chemical calculations performed at the B3LYP and MP2 levels of theory. This work corrects what appears to be incorrect assignments made in the earlier report of Smith and Guillory¹.
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¹G. R. Smith and W. A. Guillory, J. Chem. Phys. 56, 1423 (1972)

Spectroscopy of doped superfluid helium nanodroplets (He_N) gives information about the influence of this cold, chemically inert, and least interacting matrix environment on the excitation and relaxation dynamics of dopant atoms and molecules ¹. We present the results from laser induced fluorescence (LIF), photoionization (PI), and mass spectroscopy of Cr ² and Cu ³ doped He_N. From these results, we can draw a comprehensive picture of the complex behavior of such transition metal atoms in He_N upon photo-excitation. The strong Cr and Cu ground state transitions show an excitation blueshift and broadening with respect to the bare atom transitions which can be taken as indication for the solvation inside the droplet. From the originally excited states the atoms relax to energetically lower states and are ejected from the He_N. The relaxation processes include bare atom spin-forbidden transitions, which clearly bears the signature of the He_N influence. Two-color resonant two-photon ionization (2CR2PI) also shows the formation of bare atoms and small Cr-He_n and Cu-He_n clusters in their ground and metastable states ^{c 4}. Currently, Cr dimer excitation studies are in progress and a brief outlook on the available results will be given.
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¹C. Callegari and W. E. Ernst, Helium Droplets as Nanocryostats for Molecular Spectroscopy - from the Vacuum Ultraviolet to the Microwave Regime, in Handbook of High-Resolution Spectroscopy, eds. M. Quack and F. Merkt, John Wiley & Sons, Chichester, 2011. ²A. Kautsch, M. Koch, and W. E. Ernst, J. Phys. Chem. A, 117 (2013) 9621-9625, DOI: 10.1021/jp312336m ³F. Lindebner, A. Kautsch, M. Koch, and W. E. Ernst, Int. J. Mass Spectrom. (2014) in press, DOI: 10.1016/j.ijms.2013.12.022 ⁴M. Koch, A. Kautsch, F. Lackner, and W. E. Ernst, submitted to J. Phys. Chem. A

We report on the first spectroscopic investigation of the ground and excited states of RbSr. The molecules are prepared in their vibronic ground state (X²Σ⁺_1/2, ν" = 0) in a sequential pickup process¹ on the surface of helium nanodroplets, confined in a cold (0.38 K) and weakly perturbing superfluid environment². Utilizing resonance-enhanced multi-photon ionization time-of-flight (REMPI-TOF) spectroscopy and laser induced fluorescence (LIF) spectroscopy our investigations cover the spectral regime of 11500 cm⁻¹ - 23000 cm⁻¹. The weak interaction between molecules and helium droplets causes a broadening of the observed transitions. For spectrally resolved band systems the helium droplet isolation approach facilitates the determination of molecular constants. Our assignment is assisted by theoretical calculations of potential energy curves based on a multireference configuration interaction (MRCI) approach. Several strong transitions could be identified; the most prominent spectral feature is a vibrational resolved band system at 14000 cm⁻¹. In contrast to the excitation spectra, dispersed fluorescence (DF) spectra are not influenced by the helium environment, because the molecules leave the droplets upon photoexcitation, revealing detailed insights into the electronic structure of the free RbSr molecule.
Our experiments will aid the ongoing search for optimal pathways for the preparation of ultracold ground state RbSr molecules^3,4.
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¹G. Krois, J.V. Pototschnig, F. Lackner and W.E. Ernst, J. Phys. Chem. A, 117 (50), 13719-13731 (2013) ²C. Callegari and W. E. Ernst, Helium Droplets as Nanocryostats for Molecular Spectroscopy - from the Vacuum Ultraviolet to the Microwave Regime, in: Handbook of High-Resolution Spectroscopy, eds. M. Quack and F. Merkt, John Wiley & Sons, Chichester, (2011) ³P.S. \.Zuchowski, R. Guerout, and O. Dulieu, arXiv preprint arXiv:1402.0702 (2014) ⁴B. Pasquiou, A. Bayerle, S.M. Tzanova, S. Stellmer, J. Szczepkowski, M. Parigger, R. Grimm, and F. Schreck, Phys. Rev. A, 88 (2), 023601 (2013)

Hydroxymethylene (HCOH) and the d₁-isotopologue (HCOD) are isolated in low temperature helium nanodroplets through pyrolysis of glyoxylic acid. Transitions measured in the infrared spectra are assigned exclusively to the trans-conformation based on previously reported anharmonic frequency calculations (P. R. Schreiner, et. al. Nature 453, 906-909 and L. Kuziol, et. al. J. Chem. Phys. 128, 204310). For the OH(D) and CH stretches, a- and b-type lines are observed, and when taken in conjunction with CCSD(T)/cc-pVTZ computations, lower limits to the vibrational band origins are determined. The relative intensities of the a- and b-type transitions provide the orientation of the transition dipole moment in the inertial frame. Overall, the He nanodroplet data is in excellent agreement with the anharmonic frequency computations, confirming the appreciable Ar-matrix shift of the OH and OD stretches and a Fermi resonance interaction in HCOH between the ν₂ CH stretch and the ν₃+ν₄ combination band.