Photofragmentation dynamics and dissociation energies of MoO and CrO

Tuning the photodynamics of sub-nanometer neutral chromium oxide clusters through sequential oxidation

Sub-nanometer neutral chromium oxide clusters were produced in the gas phase through laser ablation and their low-lying excited state lifetimes were measured using femtosecond pump-probe spectroscopy. Time-dependent density functional theory calculations relate the trends in experimental lifetimes to the cluster's electronic structure. The photoexcited (CrO₂)_n (n < 5) cluster transients with the absence of up to four O atoms (Cr_nO_2n-x, x < 5) exhibit a ∼30 fs and sub-ps lifetime, attributed to instantaneous metallic e-e scattering and vibrationally mediated charge carrier relaxation, respectively. A long-lived (>2 ps) response is found in both small and clusters with low O content, indicating that terminal CrO bonds facilitate efficient excited state relaxation. The ∼30 fs transient signal fraction grows nearly linearly with oxidation, matching the amount of O-2p to Cr-3d charge transfer character of the photoexcitation and suggesting a gradual transition between semiconducting and metallic behavior in chromium oxide clusters at the molecular level. The results presented herein suggest that the photocatalytic properties of chromium oxides can be tunable based on size and oxidation.

Bonding, Thermodynamics, and Dissociation Dynamics of NiO+ and NiS+ Determined by Photofragment Imaging and Theory

We use photofragment ion imaging and ab initio calculations to determine the bond strength and photodissociation dynamics of the nickel oxide (NiO⁺) and nickel sulfide (NiS⁺) cations. NiO⁺ photodissociates broadly from 20350 to 32000 cm^-1, forming ground state products Ni⁺(²D) + O(³P) below ∼29000 cm^-1. Above this energy, Ni⁺(⁴F) + O(³P) products become accessible and dominate over the ground state channel. In certain images, product spin-orbit levels are resolved, and spin-orbit propensities are determined. Image anisotropy and the results of MRCI calculations suggest NiO⁺ photodissociates via a 3 ⁴Σ^- ← X ⁴Σ^- transition above the Ni⁺(⁴F) threshold and via 3 ⁴Σ^-, 2 ⁴Σ^-, and/or 2 ⁴Π and 3 ⁴Π excited states below the ⁴F threshold. The photodissociation spectrum of NiS⁺ from 19900 to 23200 cm^-1 is highly structured, with ∼12 distinct vibronic peaks, each containing underlying substructure. Above 21600 cm^-1, the Ni⁺(²D_5/2) + S(³P) and Ni⁺(²D_3/2) + S(³P) product spin-orbit channels compete, with a branching ratio of ∼2:1. At lower energy, Ni⁺(²D_5/2) is formed exclusively, and S(³P₂) and S(³P₁) spin-orbit channels are resolved. MRCI calculations predict the ground state of NiS⁺ to be one of two nearly degenerate states, the 1 ⁴Σ^- and 1 ⁴Δ. Based on images and spectra, the ground state of NiS⁺ is assigned as ⁴Δ_7/2, with the 1 ⁴Σ_3/2^- and 1 ⁴Σ_1/2^- states 81 ± 30 and 166 ± 50 cm^-1 higher in energy, respectively. The majority of the photodissociation spectrum is assigned to transitions from the 1 ⁴Δ state to two overlapping, predissociative excited ⁴Δ states. Our D₀ measurements for NiO⁺ (D₀ = 244.6 ± 2.4 kJ/mol) and NiS⁺ (D₀ = 240.3 ± 1.4 kJ/mol) are more precise and closer to each other than previously reported values. Finally, using a recent measurement of D₀(NiS), we derive a more precise value for IE (NiS): 8.80 ± 0.02 eV (849 ± 1.7 kJ/mol).

Molybdenum Oxide Clusters: Structure, Stability, and Electronic Properties

Ab initio calculations were carried out to understand the structural, electronic, and energetic properties of molybdenum oxide clusters, Mo_mO_n (m = 1-6; n = 1-3m), to understand the relationships between size, composition, and reactivity. In clusters with a low oxygen-to-molybdenum ratio, there are bridge-bonded and linearly bonded oxygen atoms on a molybdenum core, while at higher ratios, Mo atoms are separated from each other and oxygen atoms located between the molybdenum atoms. The energy gap between the highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) widens with n, i.e., at a high oxygen-to-molybdenum ratio. Stoichiometric Mo_mO_3m clusters (m > 1) have a HOMO-LUMO gap that ranges from 2.6 to 3.4 eV in neutral conditions and less than 0.6 eV in ionic states. The ionization potential of Mo_mO_3m clusters is higher than 10 eV. Mo_mO_n clusters qualitatively and quantitatively exhibit a similar electronic structure to the bulk. The energy of the reduction reaction, Mo_mO_n → Mo_mO_n-1 + 1/2O₂, is on average lower in clusters with high oxygen content; for example, the reduction energies of Mo₆O₁₈ and Mo₆O₉ are 2.23 and 5.19 eV, respectively. In the fragmentation of Mo_mO_n clusters, the general trend for clusters with a low oxygen-to-molybdenum ratio is the separation of a Mo atom or a Mo₂ dimer from the cluster, while clusters with higher oxygen content mostly form stoichiometric MoO₃, Mo₂O₆, and Mo₃O₉ clusters.

Bond dissociation energies of lanthanide sulfides and selenides

Resonant two-photon ionization spectroscopy has been employed to observe sharp predissociation thresholds in the spectra of the lanthanide sulfides and selenides for the 4f metals Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Lu. As these molecules possess a large density of electronic states near the ground separated atom limit, these predissociation thresholds are argued to coincide with the true 0 K bond dissociation energies (BDEs). This is because spin-orbit and nonadiabatic couplings among these states allow the molecules to predissociate rapidly when the BDE is reached or exceeded. The measured BDEs, in eV, are as follows: 5.230(3) (PrS), 4.820(3) (NdS), 4.011(17) (SmS), 3.811(8) (EuS), 5.282(5) (GdS), 5.292(3) (TbS), 4.298(3) (DyS), 4.251(3) (HoS), 4.262(3) (ErS), 5.189(3) (LuS), 4.496(3) (PrSe), 4.099(3) (NdSe), 3.495(17) (SmSe), 3.319(3) (EuSe), 4.606(3) (GdSe), 4.600(6) (TbSe), 3.602(3) (DySe), 3.562(3) (HoSe), 3.587(3) (ErSe), and 4.599(6) (LuSe). Through the use of thermochemical cycles, the 0 K gaseous heat of formation, Δ_fH_0K ^○, is reported for each molecule. A threshold corresponding to the onset of two-photon ionization in EuSe was also observed, providing the ionization energy of EuSe as 6.483(10) eV. Through a thermochemical cycle and the above reported BDE of the neutral EuSe molecule, the BDE for the Eu⁺-Se cation was also determined as D₀(Eu⁺-Se) = 2.506(10) eV. Bonding trends of the lanthanide sulfides and selenides are discussed. Our previous observation that the transition metal sulfides are 15.6% more strongly bound than the corresponding selenides continues to hold true for the lanthanides as well.

Bond dissociation energies of transition metal oxides: CrO, MoO, RuO, and RhO

Through the use of resonant two-photon ionization spectroscopy, sharp predissociation thresholds have been identified in the spectra of CrO, MoO, RuO, and RhO. Similar thresholds have previously been used to measure the bond dissociation energies (BDEs) of many molecules that have a high density of vibronic states at the ground separated atom limit. A high density of states allows precise measurement of the BDE by facilitating prompt dissociation to ground state atoms when the BDE is exceeded. However, the number of states required for prompt predissociation at the thermochemical threshold is not well defined and undoubtedly varies from molecule to molecule. The ground separated atom limit generates 315 states for RuO, 252 states for RhO, and 63 states for CrO and MoO. Although comparatively few states derive from this limit for CrO and MoO, the observation of sharp predissociation thresholds for all four molecules nevertheless allows BDEs to be assigned as 4.863(3) eV (RuO), 4.121(3) eV (RhO), 4.649(5) eV (CrO), and 5.414(19) eV (MoO). Thermochemical cycles are used to derive the enthalpies of formation of the gaseous metal oxides and to obtain IE(RuO) = 8.41(5) eV, IE(RhO) = 8.56(6) eV, D₀(Ru-O^-) = 4.24(2) eV, D₀(Cr-O^-) = 4.409(8) eV, and D₀(Mo-O^-) = 5.243(20) eV. The mechanisms leading to prompt predissociation at threshold in the cases of CrO and MoO are discussed. Also presented is a discussion of the bonding trends for the transition metal oxides, which are compared to the previously measured transition metal sulfides.

Photodissociation dynamics and the dissociation energy of vanadium monoxide, VO, investigated using velocity map imaging

Velocity map imaging has been employed to study multi-photon fragmentation of vanadium monoxide (VO) via the C 4Σ- state. The fragmentation dynamics are interpreted in terms of dissociation at the three-photon level, with the first photon weakly resonant with transitions to vibrational energy levels of the C 4Σ- state. The dissociation channels accessed are shown to depend strongly on the vibrational level via which excitation takes place. Analysis of the evolution of the kinetic energy release spectrum with photon energy leads to a refined value for the dissociation energy of ground state VO of D0(VO) = 53 126 ± 263 cm-1.

MELEXIR: maximum entropy Legendre expanded image reconstruction. A fast and efficient method for the analysis of velocity map imaging or photoelectron imaging data

The MELEXIR program obtains a Legendre expansion of the 3D velocity distribution from 2D images of ions or photoelectrons. The maximum entropy algorithm avoids inverse Abel transforms, is fast and applicable to low-intensity images.

Photofragment Imaging, Spectroscopy, and Theory of MnO. J Phys Chem A 2018;122:8047-8053. [PMID: 30226771 DOI: 10.1021/acs.jpca.8b07849]

Density functional and ab initio calculations, along with photodissociation spectroscopy and ion imaging of MnO⁺ from 21,300 to 33,900 cm^-1, are used to probe the photodissociation dynamics and bond strength of the manganese oxide cation (MnO⁺). These studies confirm the theoretical ground state (⁵Π) and determine the spin-orbit constant ( A' = 14 cm^-1) of the dominant optically accessible excited state (⁵Π) in the region. Photodissociation via this excited ⁵Π state results in ground state Mn⁺ (⁷S) + O (³P) products. At energies above 30,000 cm^-1, the Mn⁺ (⁵S) + O (³P) channel is energetically accessible and becomes the preferred dissociation pathway. The bond dissociation energy ( D₀ = 242 ± 5 kJ/mol) of MnO⁺ is measured from several images of each photofragmentation channel and compared to theory, resolving a disagreement in previous measurements. MRCI+Q calculations are much more successful in predicting the observed spectrum than TD-DFT or EOM-CCSD calculations.