Recent advances in high-resolution photoionization, photoelectron, and photodissociation studies based on single-photon vacuum ultraviolet (VUV) and two-color infrared (IR)-VUV, visible (VIS)-ultraviolet (UV), and VUV-VUV laser excitations are illustrated with selected examples. We show that VUV laser photoionization coupled with velocity-map-imaging (VMI)-threshold photoelectron (VMI-TPE) detection can achieve comparable energy resolutions, but higher detection sensitivities than those observed in VUV laser pulsed field ionization-photoelectron (PFI-PE) measurements. For molecules with known intermediate states, IR-VUV and VIS-UV excitation schemes are highly sensitive for rovibronically selected and resolved PFI-PE studies. The successful applications of the VUV-PFI-PE, VUV-VMI-TPE and VIS-UV-PFI-PE methods to state-resolved and state-to-state photoelectron studies of transient radicals and transitional metal-containing molecules are highlighted. The most recently established VUV-VUV pump-probe time-slice VMI-photoion method is shown to be promising for state-to-state photodissociation studies of small molecules relevant to planetary atmospheres and for the fundamental understanding of photodissociation dynamics.

Transition metal (M) carbides, nitrides, and oxides (MX, X = C, N, and O) are important molecules in astrophysics, catalysis, and organometallic chemistry. The measurements of the ionization energies (IEs), bond energies, and spectroscopic constants for MX/MX⁺ in the gas phase by high-resolution photoelectron methods represent challenging but profitable approaches to gain fundamental understandings of the electronic structures and bonding properties of these compounds and their cations. We have developed a two-color laser excitation scheme for high-resolution pulse field ionization photoelectron (PFI-PE) measurements of MX species. By exciting the neutral MX species to a single rovibronic state using a visible laser prior to photoionization by a UV laser, we have obtained fully rotational resolved PFI-PE spectra for TiC⁺, TiO⁺, VCH⁺, VN⁺, CoC⁺, ZrO⁺, and NbC⁺. The unambiguous rotational assignments of these spectra have provided highly accurate IE values for TiC, TiO, VCH, VN, CoC, ZrO, and NbC, and spectroscopic constants for their cations.

La(C₄H₆) was observed from the reactions of laser-vaporized La atom with propene (CH₂CHCH₃),isobutene [CH₂C(CH₃)2],1-butyne (CHCC₂H₅), and 2-butyne (CH₃CCCH₃) in a metal cluster beam source and investigated by mass-analyzed threshold ionization (MATI) spectroscopy in combination with quantum chemical calculations and spectral simulations. La(cyclobutene) [La(CH₂CHCHCH₂)] and La(trimethylenemethane) [LaC(CH₂)₃] were identified from the La + propene reaction. The formation of the two isomers involves two steps: the first step is the reaction of La+ CH₂CHCH₃ with the products of La(CH₂) + C₂H₄, and the second step is the reaction of LaCH₂ + CH₂CHCH₃ which preduces La(C₄H₆) + H₂. For the La + isobutene reaction, La(trimethylenemethane) was formed by dehydrogenation. For the reaction of La + 1- and 2-butynes, preliminary data analysis suggested that La(C₄H₆) was formed by simple association. All these structural isomers have similar geometries in the neutral and singly charged ion states, as evidenced by the very strong origin bands and short Franck-Condon profiles in the MATI spectra.

Activation of small hydrocarbons by transition metal centers has gained much attention because of its applications in chemical catalysis. A series of La(C_nH_m) complexes (n= 2, 4, 5 or 6 and m= 0, 2, 6, 5 or 8) were produced by La atom-mediated activation of 1,3-butadiene (CH₂=CH−CH=CH₂) in a supersonic molecular beam. Molecular structures and electronic states of the reaction products were investigated by mass-analyzed threshold ionization spectroscopy and theoretical calculations. In this work we will discuss the formation, binding, and structures of La(C₂H₂), La(C₄H₆), and La(C₆H₆). La(C₂H₂) is formed by the C-C bond activation and H migration in a primary reaction; La(C₄H₆), a metallacycle, is formed through an association reaction; and La(C₆H₆) is produced by the C-C bond coupling and cyclization in a secondary reaction. The organic moiety in La(C₆H₆) is a distorted benzene molecule. All three complexes have a two-fold metal binding mode and a doublet ground electronic state, with La(C₂H₂) in C_2v point group and La(C₄H₆) and La(C₆H₆) in C_s.

The wavelength dependence for the photoisomerization of the isolated retinal protonated n-butylamine Schiff base (RPSB) is explored in the gas phase using a new technique that combines laser spectroscopy and ion mobility spectrometry. The technique involves exposing electrosprayed ions to tunable laser radiation as they are introduced into the drift region of an ion mobility spectrometer. Ions that absorb laser radiation photoisomerize, resulting in a detectable change in their drift speed through N₂ buffer gas. Without laser irradiation, 4 peaks are observed in the arrival time distribution of RPSB. The most intense and slowest peak is assigned as the all-trans isomer by comparison with the calculated collision cross sections. With laser radiation, there is a clear depletion of the all-trans isomer peak between 440 to 660 nm, corresponding to the S₁←S₀ transition, with a maximum effect at 615±5 nm. There is also evidence of photoisomerization below 450 nm associated with the onset of the S₂←S₀ transition. The photoisomerization action spectrum of RPSB is expected to mimic its absorption spectrum and should prove useful for calibrating theoretical descriptions of the isolated chromophore.

High-resolution vibrational spectroscopy of transient species is important for determining their molecular structures and understanding their chemical reactivity. However, the low abundance and high reactivity of molecular radicals pose major challenges to conventional absorption spectroscopic methods. The observation of dipole-bound states (DBS) in anions extend autodetachment spectroscopy to molecular anions whose corresponding neutral radicals possess a large enough dipole moment ( > 2.5 D).^1,2 However, due to the difficulty of assigning the congested spectra at room temperature, there have been only a limited number of autodetachment spectra via DBS reported. Recently, we have built an improved version of a cold trap³ coupled with high-resolution photoelectron imaging.⁴ The first observation of mode-specific auotodetachment of DBS of cold phenoxide have shown that not only vibrational hot bands were completely suppressed, but also rotational profile was observed.⁵ The vibrational frequencies of the DBS were found to be the same as those of the neutral radical, suggesting that vibrational structures of dipolar radicals can be probed via DBS.⁵ More significantly, the DBS resonances allowed a number of vibrational modes with very weak Frank-Condon factors to be “lightened” up via vibrational autodetachment.⁵ Recently, our first high-resolution vibrational spectroscopy of the dehydrogenated uracil radical, with partial rotational resolution, via autodetachment from DBS of cold deprotonated uracil anions have been reported.⁶ Rich vibrational information is obtained for this important radical species. The resolved rotational profiles also allow us to characterize the rotational temperature of the trapped anions for the first time.⁶ Reference

Lignin is a heteroaromatic biopolymer that is an essential component in the cell wall of plants. The structural and chemical properties of lignin provide plants with macroscopic structural rigidity, and protection against microbial invasion leading to subsequent cell wall degradation. For this reason, lignin presents a major inhibition to the efficient harvesting of biomass. Given the variability of lignin composition and structure among species, environment, etc., the field of “lignomics” seeks to “sequence” lignin oligomers into constituent unit types (H, G, S) and linkages. This is predominantly done by means of tandem mass-spectrometry, by first generating a library of characteristic fragmentation pathways built from collision-induced dissociation of model dilignol ions, and applying them to the interpretation of fragmentation in larger ions. While these methods have proven powerful, UV photofragmentation spectroscopy of lignin ions cooled in a 22-pole cold ion trap provides an alternative approach to lignomics based on fragmentation following resonant UV excitation. This approach serves as a complimentary method to pure MSⁿ-based methods with the potential for unveiling dissociation pathways only accessed by UV excitation. Further, the multichromophoric nature of lignin enables site-selectivity for the energy imparted into the molecule/ion when differentiation of the site absorptions may be possible. IR spectroscopy of the cold ions can be used for detailed analysis of the preferred conformations and binding sites of metal cations. UV spectroscopy and photofragmentation mass spectrometry has been carried out on the model (G-type) β-O-4 and β-β dilignol linkages complexed with Li⁺ and Na⁺. The UV spectral signatures were found to vary between dilignols and metal complexes, and unique photofragmentation pathways were observed among the four complexes. IR spectroscopy in the OH stretch region was used as a probe of the conformation and binding preferences. In all cases, the dominating factor driving these structures was metal⁺-oxygen electrostatic interactions with the alkali metal ion bound at the linkage between the lignol sub-units.

Low kinetic energy (KE) electrons can induce various chemical events when allowed to interact with molecules of different sizes and complexities. We would report in this talk the fragmentation behavior of two isomeric molecules, crotonaldehyde (CA) and methacrolein (MA) upon impinging with low-energy electrons of kinetic energy in the range of 10-70 eV and the fragmentations have been probed by quadrupole mass spectrometry. At 10 eV, the mass spectra of both CA and MA show intense molecular ion peak at m/z 70, and also the peaks for H and CO loss channels at m/z 69 and m/z 42, respectively. A distinct peak for MA at m/z 41 (HCO loss) is observed at 10 eV whose intensity increases more rapidly than the case of CA. For MA, the CO loss channel is more preferred, while for CA it is the H loss channel. Electronic structure calculations reveal that these preferences are associated with relative energetic positions of the transition states, and the details will be presented in the talk.

Figure The non-classical structure of the 2-norborny cation (C₇H₁₁⁺) which was at the center of "the most heated chemical controversy of our time"¹ has been observed in the condensed phase and recently using X-ray crystallography. However, no gas phase vibrational spectrum has been collected. The C₇H₁₁⁺ cation is produced via H₃⁺ protonation of norbornene by pulsed discharge in a supersonic expansion of H₂/Ar. Ions are mass-selected and probed using infrared photodissociation spectroscopy. Due to high exothermicity, protonation via H₃⁺ leads to a structural isomerization to the global minimum structure 1,3-dimethylcyclopentenyl (DMCP⁺). Experiments are currently being conducted to find softer protonation techniques that could lead to the authentic 2-norbornyl cation.
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¹Schleyer,P.v.R. et. al.; Stable Carbocation Chemistry, John Wiley & Sons,Inc.; New York, 1997, Cahpter 2

The conformational preferences of peptide backbones and the resulting hydrogen bonding patterns provide critical biochemical information regarding the structure-function relationship of peptides and proteins. The spectroscopic study of cryogenically-cooled peptide ions in a mass spectrometer probes these H-bonding arrangements and provides information regarding the influence of a charge site. Leucine enkephalin, a biologically active endogenous opiod peptide, has been extensively studied as a model peptide in mass spectrometry. This talk will present a study of the UV and IR spectroscopy of protonated leucine enkephalin [YGGFL+H]⁺ and two of its analogues: the sodiated [YGGFL+Na]⁺ and C-terminally methyl esterified [YGGFL-OMe+H]⁺ forms. All experiments were performed in a recently completed multi-stage mass spectrometer outfitted with a cryocooled ion trap. Ions are generated via nano-electrospray ionization and the analyte of interest is isolated in a linear ion trap. The analyte ions are trapped in a 22-pole ion trap held at 5 K by a closed cycle helium cryostat and interrogated via UV and IR lasers. Photofragments are trapped and isolated in a second LIT and mass analyzed. Double-resonance UV and IR methods were used to assign the conformation of [YGGFL+H]⁺, using the NH/OH stretch, Amide I, and Amide II regions of the infrared spectrum. The assigned structure contains a single backbone conformation at vibrational/rotational temperatures of  10 K held together with multiple H-bonds that self-solvate the NH₃⁺ site. A “proton wire” between the N and C termini reinforces the H-bonding activity of the COO-H group to the F-L peptide bond, whose cleavage results in formation of the b₄ ion, which is a prevalent, low-energy fragmentation pathway for [YGGFL+H]⁺. The reinforced H-bonding network in conjunction with the mobile proton theory may help explain the prevalence of the b₄ pathway. In order to elucidate structural changes caused by modifying this H-bonding activity, structural analogues were investigated. Determining the [YGGFL+Na]⁺ structure will lend insight as to the impact of the ammonium group and methyl esterification of the C-terminus eliminates the carboxy proton. The talk will also report on high resolution, cold UV spectra, non-conformation specific IR gain spectra and conformation specific IR dip spectra for the analogues.

We present the infrared spectra of deprotonated glycine chains, (Gly_n−H)⁻, where n=1-4, obtained by cryogenic ion vibrational spectroscopy. Comparisons between the experimental and calculated spectra reveal the dependence on chain length of the hydrogen bonding motifs within these species. First, an interaction between the terminal carboxylate and the neighboring amide N−H is present in all the peptide structures. In the longer (Gly₃−H)⁻ and (Gly₄−H)⁻ structures, the peptide chain folds such that the terminal NH₂ group also interacts with the carboxylate. Both of these also display an interaction between the terminal NH₂ and the neighboring amide N−H. Finally, (Gly₄−H)⁻ is the first among these species to contain an amide-amide interaction. Analysis of the N−H peak positions elucidates the interplay among these different hydrogen bonds, especially around the negatively charged carboxylate moiety.

The abundance of copper and copper compounds within the crust of the Earth and the well characterized coordination chemistry of copper species has led to investigation of copper as a catalyst for several different chemical reactions. Among the most notable of these has been the application of copper containing compounds to water oxidation. The chemistry used in this reaction has thus far been dominated by expensive materials containing the rare metals ruthenium or iridium. In this presentation we study several copper species which have attracted interest in this field including CuOH(H₂O)_n⁺ clusters and Cu(II) oligoglycine complexes. We find that in the case of CuOH(H₂O)_n⁺ clusters that the undercoordinated CuOH(H₂O)⁺ cluster has a strong affinity towards activating D₂ while the higher coordinated CuOH(H₂O)₂⁺ and CuOH(H₂O)₃⁺ clusters show no propensity for the activation of D₂. In the case of the Cu(II) oligoglycine complexes we find that the charge environment strongly affects the diagnostic CO stretch frequencies, creating a clear spectroscopic fingerprint for assessing the charge and interactions within these systems. Despite the small size (6-12 atoms) of the CuOH(H₂O)_n⁺ clusters electronic structure methods including DFT and MP2 give poor agreement with the experimental results while DFT calculations on the relatively large Cu(II) oligoglycine species show excellent agreement with experiment.

Glucose-6-Phosphate (G6P) is one member of a class of simple phosphorylated sugars that are relevant in biological processes. We have acquired a gas phase infrared spectrum of G6P⁻ using cryogenic ion vibrational spectroscopy (CIVS) in a home-built spectrometer. The experimental spectrum was compared with calculated vibrational spectra from a systematic conformer search. For both of the α and β anomers, results show that only the lowest energy conformers are present in the gas phase. If spectral signatures for similar sugars could be cataloged, it would allow for conformer-specific determination of mixture composition, for example, for glycolyzation processes.

The metastable ³∆₁ state in trapped HfF⁺ is being used for an ongoing measurement of the electron electric dipole moment (eEDM)¹. ThF⁺, which has a larger effective electric field and a longer-lived ³∆₁ state, offers increased sensitivity for an eEDM measurement. Recently, the Heaven group has spectroscopically studied the low-lying states of ThF^{+ 2}. However, to date there is no detailed information available about technically-accessible laser transitions in the near-infrared region of the spectrum, which are necessary for state preparation and detection in an eEDM experiment. By applying the technique of frequency comb velocity modulation spectroscopy (VMS)³ to ThF⁺ we can acquire 150 cm⁻¹ of continuous, ion-sensitive spectra with 150 MHz resolution in 25 minutes. Here, we report on extensive broadband, high-resolution survey spectroscopy of ThF⁺ in the near-IR where we have observed and accurately fit several rovibronic transitions. In addition, we have observed and characterized numerous rovibronic transitions from an unknown thoriated species of molecular ions.
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¹H. Loh, K. C. Cossel, M. C. Grau, K.-K. Ni, E. R. Meyer, J. L. Bohn, J. Ye, E. A. Cornell, Science 342, 1220 (2013). ²B. J. Barker, I. O. Antonov, M. C. Heaven, K. A. Peterson, J. Chem. Phys. 136, 104305 (2012). ³L. C. Sinclair, K. C. Cossel, T. Coffey, J. Ye, E. A. Cornell, PRL 107, 093002 (2011).

Bands corresponding to structural isomers of matrix-isolated O₄⁺ are observed upon deposition of ions into argon matrices doped with moderate (0.1-1%) concentrations of O₂. These bands have been assigned based on previous matrix isolation spectroscopy,¹ as well as high-level computational studies.² In the current work, these bands are observed upon co-deposition of Cu⁻ and Ar⁺ ions at low-energies. The Cu⁻ is present only as a non-interacting counter-ion, as is verified by studies using exclusively high-energy Ar⁺ beams; in this case, the spectroscopy of the O₄⁺ species is completely equivalent, however there is now also an intense peak corresponding to O₄⁻ counter-ion species. Following deposition at 20 K, the matrices are cooled to 10 K, where the FTIR spectra show a band at 1119 cm⁻¹for the trans-O₄⁺ isomer, and a doublet at 1329/1331 cm⁻¹, corresponding to the cyclic-O₄⁺ isomer, based on earlier work. There is also a band at 1186 cm⁻¹that was previously assigned to a larger O₆⁺ complex. A temperature series taken in 1 K increments between 10 and 20 K reveal two reversible interconversion processes: the 1119 cm⁻¹band decreases between 10 and 14 K while a new band grows in at 1242 cm⁻¹, and the 1186 band shows a similar interconversion between 11 and 16 K with the 1331 cm⁻¹peak of the cyclic-O₄⁺ doublet, while the 1329 cm⁻¹peak diminishes and broadens over the same temperature range. The interconverting peak pairs can be converted into equilibrium constants based on relative changes in integrated intensities, and the associated van't Hoff plots show linear trends with ∆H values in the range expected based on computational work. Finally, the 1186 cm⁻¹and 1331 cm⁻¹peak pair exhibit strong photochromism at 10 K: irradiation with red light converts 1186 to 1331, while irradiation with blue light shifts the equilibrium in the other direction. In both cases the phenomena is completely reversible and reproducible, with the original intensity ratio being restored after a few minutes once the light has been switched off. Irradiation has no effect on the other spectra bands, indicating that the 1329 and 1331 cm⁻¹bands cannot arise from the same species, as previously thought.
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¹Jacox, M.E.; Thompson, W.E.; J. Chem. Phys. 100, 750 (1994); Zhou, M.-F., et al.; J. Chem. Phys. 110, 9450 (1999). ²Lindh, R.; Barnes, L.; J. Chem. Phys. 100, 224 (1994).