l0pt Figure Bimetallic materials comprised of gold and silver have useful optical and electronic properties, which are complicated by quantum mechanical, relativistic, and isotopic effects. To provide a bottom-up perspective on these larger systems, the smallest monocation comprised of gold and silver-diatomic AuAg+-is spectroscopically probed using resonance enhanced photodissociation (REPD). The ¹⁹⁷Au¹⁰⁷Ag⁺ and ¹⁹⁷Au¹⁰⁹Ag⁺ isotopologues are confined in a cryogenically cooled (ca. 5 K) quadrupole ion trap and are exposed to tunable light while detecting Au+ photofragment ions using a time-of-flight mass spectrometer. Electronic spectra in the UV exhibit a transition from the X²Σ⁺_1/2 ground state to an excited state that is yet to be assigned. Vibronic progressions for this transition extend over more than 30 quanta for both isotopologues, but with striking differences in band intensities (see Figure). This difference in photodissociation yield between the two isotopologues arises because the vibronic energies and associated wavefunctions depend on the reduced mass, leading to a difference in the coupling of the excited state levels and the repulsive electronic state that leads to dissociation. The observed photodissociation intensities for ¹⁹⁷Au¹⁰⁷Ag⁺ and ¹⁹⁷Au¹⁰⁹Ag⁺ are successfully modelled by calculating respective vibronic energies and wavefunctions of their bound and dissociative electronic states.

r0pt Figure The binding motif of nitrogen on transition metals is an interesting issue. Here we present the electronic spectrum of the òΣ⁺ → X̃²Σ⁺ transition of , which was measured via photodissociation spectroscopy. The spectrum contains a long progression, caused by symmectric and asymmetric strech vibrations. We extract harmonic frequencies, anharmonicities and cross-anharmonicities of the excited state via a Dunham expansion and harmonic frequencies of the ground state via Franck-Condon simulations. In comparison to density functional theory calculations, the observed frequencies agree well with the theory. We also discuss the binding motif of in the ground and excited state.

[1] M. Förstel, K. Pollow, T. Studemund, O. Dopfer, Chem. Eur. J. 2021, 27, 15075-15080.
[2] M. Förstel, K. M. Pollow, K. Saroukh, E. A. Najib, R. Mitric, O. Dopfer, Angew. Chem. Int. Ed. 2020, 123, 21587-21592.

Electronic spectrum measurements are reported for mass selected C₆⁺ in the gas phase using photodissociation spectroscopy. Carbon cluster cations are produced by laser vaporization and mass selected using a time-of-flight mass spectrometer. Photodissociation of C₆⁺ measured a strong absorption at 417.1 nm. Experimental results are accompanied with calculations at the B3LYP/Def2TZVP level that explores predicted isomeric structures, their energetics, and vibrational and electronic spectra. Electronic excitations and vibrational hot bands in the spectrum agree more with frequencies predicted for the linear structure than those of the cyclic structure.

Interstellar dust consists mainly of μm-sized silicate particles. Their origin and evolutionary development processes are still poorly understood. So far, only molecular SiO as a possible precursor has been observed and identified in a circumstellar disk [1]. We present experimental data and quantum chemical calculations of absorption and dissociation properties of Si₂O₂⁺ clusters. These cations represent possible intermediates between the circumstellar diatomic SiO molecule and the silicate grains observed in the interstellar medium. These optical spectra provide the first spectroscopic information for any Si_nO_m⁺ cation larger than SiO⁺. These spectra are the first optical absorption spectra of Si₂O₂⁺ cations. We were able to obtain those by photodissociation spectroscopy of mass-selected ionsin a tandem mass spectrometer coupled to a laser vaporization source [2]. Here, the experimental results will be compared with TD-DFT calculations and discussed in an astrophysical context.
Literature:
[1] R. Wilson et al., Astrophys. J., 1971, 167, L97. \ [2] M. Förstel et al., Rev. Sci. Instrum., 2017, 88, 123110.

The second period p-block elements reign as mainstays in a plethora of scientific disciplines due to their varied chemical bonding and significant natural abundance. Indeed, the range of chemistries of these p-block elements is profoundly expanded when these atoms, particularly electron deficient boron, bond with transition metals and lanthanides. Exotic metallaboron compounds have been demonstrated to participate in a wide array of bonding schemes, mechanisms, and geometries. Here, resonant two-photon ionization spectroscopy and ab initio quantum chemical calculations are used to elucidate the chemical bonding and electronic structure of triatomic metal diboride complexes. These previously unstudied species are demonstrated to require an extension of the Dewar-Chatt-Duncanson bonding model of organometallic chemistry. Instead of weakening the bond in the diboron ligand via backdonation into the pi* antibonding orbital, the metal-boron and boron-boron bonds are strengthened by backdonation into the pi bonding orbital of diboron. Moreover, it is shown that the lanthanide atoms in these species exhibit a rare +1 oxidation state, further defining this family of molecules as a special class of monovalent lanthanide compounds.

Resonant two-photon ionization spectroscopy has been used to determine the bond dissociation energies (BDEs) and ionization energies (IEs) of rhenium containing small molecules. The ultraviolet spectra of these molecules display a highly congested collection of indeterminate vibronic states. Couplings among these states allow the molecule to find a path to dissociation as soon as the ground separated atom limit is exceeded in energy, allowing a precise measurement of the bond energy from the observation of a sharp predissociation threshold. Measurements provide BDE values of 5.731(3) eV (ReC), 5.359(3) eV (ReC₂), 5.635(3) eV (ReN), 5.510(3) eV (ReO), and 3.947(3) eV (ReS). The ionization energy of ReC, 8.425(15) eV, was determined from the observed onset of one-color two-photon ionization. By combining our ReC results with the ionization energy of Re (7.83352(11) eV) in a thermochemical cycle, the BDE of cationic ReC+ was determined as 5.140(15) eV.Kramida, A.; Ralchenko, Y.; Reader, J.; NIST ASD Team, NIST Atomic Spectra Database (version 5.9). National Institute of Standards and Technology, Gaithersburg, MD: 2019.his is in excellent agreement with that measured using guided ion beam mass spectrometry, 5.13(12) eV.Kim, J.; Cox, R. M.; Armentrout, P. B., Guided ion beam and theoretical studies of the reactions of Re+, Os+, and Ir+ with CO. J. Chem. Phys. 2016, 145 (19), 194305/1-194305/13.html:<hr /><h3>Footnotes: Kramida, A.; Ralchenko, Y.; Reader, J.; NIST ASD Team, NIST Atomic Spectra Database (version 5.9). National Institute of Standards and Technology, Gaithersburg, MD: 2019.T Kim, J.; Cox, R. M.; Armentrout, P. B., Guided ion beam and theoretical studies of the reactions of Re+, Os+, and Ir+ with CO. J. Chem. Phys. 2016, 145 (19), 194305/1-194305/13.

Quantum dot sensitized solar cells have been a rising star in in the field of photovoltaics and materials science. Here, UV probe transient spectroscopy is employed to directly investigate the metal oxide dynamics in CdSe sensitized ZnO. The excitonic transition in ZnO lays at 365 nm, and charge injection results in a bleaching of this transition due to phase space filling. Combining UV and visible probe transient spectroscopy allows direct comparison between the spectrally separated CdSe and ZnO signals. The two regions show a difference in kinetics, with the ZnO showing a delayed signal from charge injection contrary to the abrupt decay of the quantum dot signal, which has been attributed to the formation of an interfacial exciton at the boundary of the ZnO and CdSe. High fluence measurements with band edge excitation gives evidence that charge injection follows a second mechanism that does not show delayed charge separation, indicating the interfacial exciton state is not forming. Understanding this effect could pave the way for more effective photovoltaics.

We are interested in the photochemistry and spectroscopy of interstellar molecules, and recently focused on isomers of pyridine. We examined the photochemistry of cyanomethylene cyclopropane in low temperature conditions relevant to the interstellar medium. Cyanomethylene cyclopropane was mixed with argon prior to deposition onto a CsI window at temperatures below 30 K. We collected IR spectra in the range of 400-4,000 cm⁻¹. We irradiated the molecule at λ 200 nm using a Xe/Hg arc lamp, and observed IR bands indicative of a new organic nitrile develop over time; no change to the IR spectrum of cyanomethylene cyclopropane was observed when a λ 295 nm UV cut-off filter was used. The new IR bands produced from this process were compared to other experimental and predicted IR spectra of C₅H₅N isomers to interrogate the C₅H₅N potential energy surface .

Para-aminobenzoic acid (PABA) is one of the original sunscreen chemical agents. As these agents often undergo photodissociation during the process of UV absorption, understanding the photochemical behaviour of sunscreen agents is highly important. In this study, the photolysis of PABA was studied at three different UV ranges (UVA: 355 nm, UVB: > 280 nm, and UVC: 266 nm and 213 nm) using parahydrogen (pH₂) matrix isolation Fourier-Transform infrared (FTIR) spectroscopy. Parahydrogen has weak cage effects that allow radicals to escape the lattice site and therefore prevent further radical recombination reactions. PABA was found to be stable under UVA irradiation. However, PABA dissociated into 4-aminylbenzoic acid (the PABA radical) through amino hydrogen atom loss under UVB and UVC irradiation.¹ The production of the PABA radical supports a previously proposed mechanism of the formation of the carcinogenic PABA-thymine adduct. The infrared spectrum of the PABA radical was analyzed with quantum chemical calculations. Two conformers of this radical were observed in the pH₂ matrix. Both conformers of the PABA radical were stable in solid pH₂ for hours after irradiation. This work displays that pH₂ matrix isolation spectroscopy is effective for sunscreen agent photochemical studies. 1. McKinnon, A.; Moore, B.; Djuricanin, P.; Momose, T. UV Photolysis Study of Para-Aminobenzoic Acid Using Parahydrogen Matrix Isolated Spectroscopy. Photochem. 2022, 2, 88 – 101.

Matrix isolation has recently proven successful for the spectroscopic characterization of amino acids in their neutral form. Here, we utilize solid parahydrogen, a cage-free matrix host, to study the photochemistry of a number of amino acids. The photochemistry of alanine, glycine, leucine, proline, and serine will be presented. Irradiation by 213 nm light resulted in α-carbonyl C-C bond cleavage and hydrocarboxyl (HOCO) radical production from all five amino acids. The temporal behavior of the Fourier-transform infrared spectra revealed that HOCO radicals rapidly reach a steady state, which occurs predominantly due to photodissociation of HOCO into CO + OH or + H. In alanine, glycine, and leucine, the amine radicals generated by the α-carbonyl C-C bond cleavage rapidly undergo hydrogen elimination to yield ethanimine, methanimine and 3-methylbutane-1-imine, respectively. As an analogue to gas phase photochemistry, the photodissociation pathways identified here provide new insights into the behavior of amino acids in interstellar space.