The chemi-ionization reactions of atomic lanthanides M + O → MO⁺ + e⁻ are currently being investigated as a method to artificially increase the localized electron density in the ionosphere for uniform radio wave propagation. Recent experiments involving the release of atomic samarium (Sm) into the upper atmosphere have resulted in the production of a cloud with blue and red emissions[1]. Spectroscopic characterization of both SmO and its cation SmO⁺ is required to accurately determine the fraction of SmO⁺ present in the release cloud. While the low-lying states of SmO have been previously spectroscopically characterized, the analysis was hindered due to the production of SmO under high temperature conditions[2,3]. In this experiment, SmO was jet-cooled to 70K and laser-induced fluorescence (LIF) spectra were obtained over the range from 15,000-16,000 cm⁻¹. Dispersed laser-induced fluorescence (DLIF) spectra were also obtained for vibrational characterization of the ground and low-lying states. Fluorescence lifetime measurements have been used to determine Einstein A coefficients. Data and analysis of ground and low-lying excited states of SmO will be presented. [1] Ard, S.G. et al. J. Chem. Phys.2015, 143, 204303. [2] Hannigan, M. C. J. Mol. Spec. 1983, 99, 235-238. [3] Linton, C. et al. J. Mol. Spec. 1987, 126, 370-392.

The chemi-ionization reactions of atomic lanthanides M + O → MO⁺ + e⁻ are currently being investigated as a method to artificially increase the ion density in the ionosphere for uniform radio wave propagation. Recent experiments involving the release of atomic neodymium (Nd) into the upper atmosphere have resulted in the production of a cloud with green emission[1]. Based on the cloud emission, it is believed that NdO was the primary product, but spectroscopic characterization of NdO and NdO⁺ is needed to properly identify the emitting species. While NdO is well characterized above 590 nm, little spectroscopic data exits at emission wavelengths below 590 nm[2,3]. Recently, the ionization energy of NdO was determined by REMPI and PIE methods[4], as well as the ion ground state vibrational separation ∆G_1/2⁺ but there exists no experimental rotational characterization of the low-lying states of NdO⁺. In this experiment, NdO was supersonically expanded and then ionized at 193 nm. Laser-induced fluorescence (LIF) spectra of both NdO and NdO⁺ was obtained at 16,650-20,000 cm⁻¹. Electronic states were vibrationally characterized using dispersed laser induced fluorescence (DLIF) techniques. Data and analysis of the ground and low-lying states of NdO and NdO⁺ will be presented. [1] Ard, S.G. et al. J. Chem. Phys.2015, 143, 204303. [2] Kaledin, L.A. et al. Acta Physica Hungarica 1983, 54, 189-212. [3] Linton, C. et al. J. Mol. Spec. 2004, 225, 132-144. [4] VanGundy, R.A. et al. J. Chem. Phys. 2019, 114302

In this project, we have studied the electronic structure of RhO²⁺ using high level quantum mechanical calculations. Multireference method (MRCI) has been employed in combination with large basis sets to construct the potential energy profiles of the ground and excited states of RhO²⁺ with different spin multiplicities. Spectroscopic constants have been tabulated for 20 states along with the spin-orbit calculation for a few low-lying states. The ground state of RhO²⁺ is ²Π followed by ⁴∆ and ²∆. The equilibrium bond lengths for these states are between 1.594-1.752 Å and the electronic structure is in situ Rh⁴⁺O²⁻ that corresponds to an oxo moiety. The next state is ⁶Σ⁺ with an equilibrium bond length of 2.161 Å and possesses an in situ electronic structure of Rh³⁺O⁻ that corresponds to oxyl character with one unpaired electron on oxygen. Further investigation reveals that ⁶Σ⁺, which is 19.6 kcal/mol higher in energy than the ²Π (ground state), has the potency to activate C-H bond of methane with higher efficiency. Finally, from the spin-orbit calculations, the ground state of RhO²⁺ is assigned ²Π_1/2 followed by the state ⁴∆_7/2. In future, this project is going to probe the effect of ligands in the electronic structure of RhO²⁺.

Production of H₂ gas via water splitting reactions facilitated by solar light harvesting, can be a solution to the constant increase of energy demand. Water splitting is an endothermic process which requires about 2.69 eV per water molecule. High endothermicity of this process can be overcome by exploiting the sun light and using a suitable catalyst. Transition metal oxide (MO) catalysts are promising candidates for this process because of their multiple low-lying electronic states. The rate determining step in the water splitting reactions is often the activation of the O-H bonds. Here we have tested water activation strengths of several neutral and charged MOs computationally. The complicated electronic structures of these MOs forced us to perform multi-reference calculations. For these MOs the water activation process can be partitioned into three major processes. 1) Formation of the H₂O…OM interacting complex. 2) Surpass the transition state barrier which involve H-O bond cleavage of metal bound water. 3) HO-M-OH product formation. Due to the multiple low-lying excited states of these species, spin crossovers and electron excitation from ground to higher states are expected. Among the considered H₂O+[MoO]^−/+/2+ reactions, the anionic system is the best candidate for the water activation, which has a lower activation energy barrier and create a more stabilized product than interacting complex. On the contrary, MoO^2+/+ stabilizes the MoO^2+/+…H₂O interacting complex over products. A similar pattern was observed for the H₂O+[RuO]^−/0/+/2+ reactions. The suitability of RuO species for the water activation vary in RuO²⁺ < RuO⁺ < RuO < RuO⁻ order.

l0pt Figure The ab initio molecular dynamics (AIMD) method provides a computational route for the real-time simulation of reactive chemistry and spectroscopy. An often-overlooked capability of this approach is the opportunity to examine the electronic evolution of a chemical system, including chemical bond formation and electrical responses to radiation. For AIMD trajectories based on Hartree-Fock or density functional theory (DFT) methods, the real-time evolution of orbitals can provide detailed insights into the time-dependent electronic structure of a complex. However, the molecular orbital character, ordering, and associated phase are not preserved throughout the trajectory, due to the presence of different electronic Hamiltonians at each time step. By exploiting the similarity in neighboring timesteps’ electronic structure, an algorithm has been developed that allows for reliably tracking the character of molecular orbitals throughout an AIMD trajectory, including cases of orbital crossing and degeneracy. In this presentation, examples of the results of this tool will be shown, including reactive trajectories. These examples will also highlight the adiabatic character of the evolution of molecular orbitals. The utility of this approach for both educational purposes and analysis of trajectories will be demonstrated. Possibilities for extension to diabatic analyses will also be discussed.

Diarylethene (DAE) derivatives are an important class of photochromic molecular switches that undergo ring-opening and ring-closing reactions following excitation with visible or UV light, respectively. Here, we use resonance Raman spectroscopy to examine the wavelength-dependence of the cycloreversion (ring-opening) reaction following excitation into the two lowest electronic absorption bands of a common DAE derivative. Resonance Raman spectroscopy reveals the initial dynamics on the excited-state potential energy surface based on mode-specific enhancements of the vibrational spectrum that depend on the electronic resonance condition. Although the vibrational frequencies report on the ground-state structure of the molecule, the relative intensities of the Raman transitions reflect the initial motion immediately following excitation to the upper electronic state. Specifically, we report stimulated resonance Raman spectra and Raman gain profiles for excitation into two separate absorption bands centered near 560 nm and 370 nm. Although excitation into either band results in the cycloreversion reaction with similar  ∼ 2% quantum yield, the resonance Raman spectra indicate that the initial dynamics are different for the two excited states. Excitation into the lower energy absorption band reveals resonance enhancement of a 986 cm⁻¹ mode that corresponds to a ring breathing mode of the central cyclohexadiene ring, and likely represents motion directly along the ring-opening reaction coordinate. In contrast, excitation into the higher energy absorption band results in resonance enhancement for 1400 cm⁻¹and 1600 cm⁻¹ modes that represent ethylenic stretching along the conjugated backbone of the molecule and of the peripheral phenyl rings, respectively, and probably do not map directly onto the reaction coordinate. These observations provide key information for understanding the reactivity of DAE derivatives following excitation in the visible and near-UV.

The structure of Naphthazarin (NZ) has been considered in spectroscopic for a long time. However, there are some ambiguity in spectroscopic data up to now. In this work, an easy and feasible method has been introduced for studying the proton tunneling in NZ to assess symmetry. For this purpose, a two-dimensional potential energy surface which has been capable to explain the movement of hydrogen atom with high accuracy has been used. This potential energy function, which couples OH stretching with in-plane bending modes has been applied on a fixed skeleton geometry of NZ calculates the tunneling splitting, OH stretching, and in-plane bending frequencies. To study the effect of the saddle point and the bending force constant of OH on transition frequencies of hydrogen bond, a differentiation was applied on two dimensional function towards X and Y, respectively. The tunneling splitting of 15.3 and 0.8cm⁻¹and the barrier height of 65.3 and 127.3 kJ/mol for stepwise and concerted proton transfer, respectively, indicate that both proton transfers are probable to happen, but NZ of D−2h symmetry will last for the longer time. In addition, as the saddle point position and the bending force constant increase, the tunneling frequency increases. It seems that the interaction between these two modes (the bending force constant) is responsible for higher OH stretching and lower OH bending frequencies in the bent hydrogen bonded systems compared to those in linear hydrogen bonded systems. This coupling, also, predicts a strong fermi resonance between OH stretching and the second overtone of OH bending modes in NZ and explains the cause of broading for OH stretching frequency in the bent H-bonded systems such as those in the enol form of β-diketones.

The global electronic structures of C₂ ¹Π_g and ³Π_g states, up to the energy of the first symmetry-allowed C(2s2p³)+C(2s²2p²) dissociation channel, are modeled using a diabatic state interaction picture. The experimental observations (v=0-8 of C¹Π_g; v=0-12 of d³Π_g and e³Π_g, and five v levels of the coupled 3,4³Π_g states) are reproduced with an average residual of 0.3 cm⁻¹. The key concept behind the diabatization is the valence-hole electron configurations, for which an electron in the anti-bonding 2σ_u orbital is excited to a higher energy orbital. For C₂, when this electron is excited to a bonding orbital (i.e. 3σ_g/2π_u←2σ_u), the valence-hole configuration is the lowest energy configuration within a given symmetry manifold. These valence-hole configurations have a nominal bond-order of three, and correlate to the excited C(2s2p³)+C(2s²2p²) channel with 2p←2s electron promotion in one of the carbon fragments. A diabatic valence-hole state with bond-order of three is expected to have a high dissociation energy, and in our model, it dissociates into the lowest symmetry-allowed C(2s2p³)+C(2s²2p²) channel. As R increases, this deeply-bound valence-hole state crosses several other low-lying electronic states (derived from the 2σ_u²2σ_g² configuration), all of which dissociate into the low-energy C(2s²2p²)+C(2s²2p²) channels without 2p←2s promotion. The curve-crossings between the valence-hole and the 2σ_u²2σ_g²-type states in C₂ are analogous to the ionic (A⁺B⁻)/covalent (AB) curve crossings in more ionic species. In both cases, the electronic structure landscape of the low-lying valence states is systematically disrupted due to their curve crossings with a single diabatic state (valence-hole or A⁺B⁻). We believe that the valence-hole state is an important but hitherto neglected feature in the electronic structure model. The C₂ molecule offers a perfect platform to study the role of valence-hole states in causing fundamental changes of the global electronic structure landscape. The valence-hole-induced curve crossings in C₂ occur at much lower excitation energy than the analogous ones in N₂, CO, NO, and O₂. The effects from these curve crossings are gloriously and uniquely sampled in the molecular constants of low-lying states of C₂.

Despite the fact that diatomic alkaline earth monoxides are deeply bound, closed shell molecules, hypermetallic species of the form MOM are also found to be stable. Formally these molecules have ionic bonds of the form M⁺O²⁻M⁺. Each M⁺ ion hosts an unpaired electron, and the interactions between these well separated electrons are weak. As a consequence the singlet-triplet energy interval is small, and the lowest energy singlet state is strongly multi-reference in character. In the present study we have characterized MgOMg by means of laser induced fluorescence and resonance enhanced multi-photon ionization (REMPI) spectroscopy. Rotationally resolved electronic spectra have been recorded for multiple bands of the A¹A₁ – X¹Σ⁺_g transition in the range of 21500 – 23000 cm⁻¹. The spectra were consistent with bent and linear equilibrium structures for the excited and ground states. Fluorescence decay lifetimes were found to be 39±1 ns. The attribution of the observed excitation bands to MgOMg was confirmed by recording REMPI spectra with mass-resolved ion detection. Two-color ionization measurements defined an ionization energy of 53071(20) cm⁻¹, close to a computational prediction of 53330 cm⁻¹.

Heterodimers consisting of an alkaline and alkaline-earth metal, such as LiMg have the potential for being laser cooled to ultra-cold temperatures. The X1²Σ⁺ ground state of LiMg allows manipulation by both magnetic and electric fields, and its non-zero dipole moment offers the possibility that it could be used in quantum computing devices. Current experimental data on LiMg and LiMg⁺ are sparse^1,2. In the present study we used resonance enhanced two-photon ionization spectroscopy (RE2PI) and laser induced fluorescence (LIF) to record rotationally resolved spectra of the X1²Σ⁺, 2²Π and 3²Σ⁺ states. Near-UV spectra previously assigned as E-X and F-X transitions were also reevaluated. Vibrationally resolved spectra of the X1²Σ⁺ and 1²Π state were observed using dispersed LIF. Pulsed-field ionization - zero kinetic energy photoelectron spectroscopy (PFI-ZEKE) was used to determine the ionization energy (IE) and the low-energy states of LiMg⁺. An IE of 4.7695(4) eV for LiMg was found, and vibrationally resolved spectra yielded molecular constants for LiMg⁺ that were consistent with a substantial strengthening of the bond upon ionization. 1. Berry, K.R. and Duncan, M.A., (1997) Photoionization spectroscopy of LiMg. Chem. Phys. Lett., 279, 44-49 2. Pichler, G., Lyyra, A.M., Kleiber, P.D., Stwalley, W.C., Hammer, R., Sando, K.M., and Michels, H.H., (1989) Laser-induced chemiluminescence of the lithium-magnesium (LiMg) excimer. Chem. Phys. Lett., 156, 467-71

Atomically-precise gold nanoclusters (AuNCs) in the tens to hundreds of Au atoms size range can exhibit superatomic characteristics where the geometric and electronic stability of the cluster is driven by "shell-closing" electronic configurations. The superatomic model predicts that Jahn-Teller-like distortions stabilize clusters near the closing of a shell by rearranging the molecular orbitals. We have collected a highly-resolved electronic absorption spectrum of an ellipsoidal phosphine-protected AuNC that supports the qualitative model for non-spherical superatomic clusters. The lower energy transitions reasonably match the metal-metal transitions predicted by density functional theory (DFT) calculations. In addition, we confirm these results by a qualitative "particle-in-a-box" numerical calculation that treats the core electrons as particles in an ellipsoidal potential. The results of this qualitative model are consistent with both the results of the DFT calculation as well as the electronic structure predicted for superatomic clusters with non-spherical cores.

At high vibrational quantum numbers, ion-pair states (A⁺-B⁻) behave like high Rydberg states. Their level structure can be approximately described by Rydberg's formula after replacing the Rydberg constant R_∞ by the appropriate mass-scaled value R_IP=R_∞ \fracμ_A⁺−B⁻m_e.E. Reinhold and W. Ubachs, Mol. Phys., 103:10, 1329-1352, (2005)e present the results of spectroscopic investigations of high-lying excited ion-pair states of small polyatomic molecules in the vicinity of ion-pair dissociation thresholds. Such states have been extensively characterized in diatomic molecules (see ref. ^a,S. Mollet, F. Merkt, Phys. Rev. A., 82, 032510, (2010)nd references therein) and are at the heart of the technique of threshold ion-pair production spectroscopy.J.D.D. Martin, J.W. Hepburn. Phys. Rev. Lett., 79, 3154, (1997)nly few studies have been carried out on polyatomic molecules.R. C. Shiell, X. K. Hu, Q. J. Hu, J. W. Hepburn, J. Phys. Chem., 104, 19 (2000)^,Q. J. Hu, Q. Zhang, J.W. Hepburn. J. Chem. Phys., 123, 074310, (2006)olyatomic molecules exhibit denser series of ion−pair dissociation thresholds corresponding to the different rovibrational levels of the positively and negatively charged fragments. We will present the results of our investigations of OCS and H_2 Q. J. Hu, Q. Zhang, J.W. Hepburn. J. Chem. Phys., 123, 074310, (2006)P

Gold is a versatile material with unique properties that underpin its importance in medicine, catalysis, electronics and basic research. The theoretical description of even very small Au clusters remains challenging due to relativistic effects, d electrons participating in the bonding, spin-orbit coupling and charge-transfer effects. Using photodissociation spectroscopy, we have obtained the first optical spectrum of Au₂⁺. The quality of the spectrum is high enough to allow a detailed analysis of the ground and excited state surfaces of this important model system.^{[1, 2]} The results presented show that standard TD-DFT methods completely fail to describe this seemingly simple, H₂⁺ like system. Only with a relativistic multireference treatment including spin-orbit coupling were we able to obtain a qualitative agreement between experimental results and theory.
[1] M. Förstel et al., Angew. Chem. Int. Ed., 2020, 123, 21587-21592. [2] M. Förstel et al., Rev. Sci. Instrum., 2017, 88, 123110.