We report time-domain rotational spectroscopy of argon dimer and krypton dimer by implementing time-resolved Coulomb explosion imaging of rotational wave packets. The rotational wave packets are created in the dimers with a ultrashort laser pulse, and their spatiotemporal evolution is fully characterized by measuring angular distribution of the fragment ions. The pump-probe measurements have been carried out up to a delay time of 16 ns. The alignment parameters, derived from the observed images, exhibit periodic oscillation lasting for more than 15 ns. Pure rotational spectrum of Ar₂ is obtained by Fourier transformation of the time traces of the alignment parameters. The frequency resolution in the spectrum is about 90 MHz, the highest ever achieved for Ar₂. The rotational constant and the centrifugal distortion constant are determined with much improved presision than the previous experimental results: B₀ = 1.72713(9) GHz and D₀ = 0.0310(5) MHz. The present B₀ value does not match within the quoted experimental uncertainty with that from the VUV spectroscopy, so far accepted as an experimental reference to assess theories. Spectrum of the krypton dimer will be also reported.

Chirped-pulse and cavity microwave spectra are presented for the complex formed from trifluoroacetic acid (TFA) and trimethylamine (TMA). Both the parent complex and that formed from deuterated TFA have been observed. Based on measured ¹⁴N nuclear quadrupole coupling constants and supplemental computations at the MP2/6-311++G(df,pd) level of theory, the complex is shown to involve partial transfer of the TFA proton to the amine. Structural indicators of the degree of proton transfer are used to support this conclusion and comparisons with other related hydrogen bonded systems are presented. The relatively strong acidity of TFA as compared with other carboxylic acids, together with the relatively strong Brønsted basicity of TMA, likely underlie the ability of this system to undergo partial proton transfer in the gas phase without the aid of microsolvation.

The 1:1 adduct of 1-phenyl-2,2,2-trifluoroethanol (PhTFE) with water was investigated using chirped pulse Fourier transform microwave spectroscopy and computational methods. PhTFE itself was previously reported to have two stable conformations, I (gauche) and II (trans), however, only the most stable conformer, PhTFE I, was experimentally observed.¹ Rotational spectra of the two most stable PhTFE-H₂O conformers along with several deuterium and oxygen-18 isotopologues were assigned and their structures analyzed. The most stable complex exhibits PhTFE in the gauche configuration with water inserted into the existing intramolecular OH-F hydrogen bond. This conformer is stabilized by two intermolecular hydrogen bonds in addition to the intramolecular interactions present in the PhTFE monomer. Those being a strong interaction between the alcohol hydrogen on PhTFE and the oxygen on water OH-OH₂ and a weaker interaction between a fluorine on PhTFE and a water hydrogen F-HOH. Water tunnelling splitting was identified in the rotational spectrum showing the characteristic ortho versus para intensity ratio, which was attributed to the interchange of bonded and nonbonded hydrogen atoms of the water subunit. The second observed complex exhibits PhTFE in the trans configuration, indicating that complexation with water sufficiently stabilized PhTFE II such that it can survive the supersonic expansion. Stabilization is achieved by water interacting with both the alcohol hydrogen and the phenyl ring of PhTFE. The nonbonded hydrogen of the water subunit was shown to exhibit a large amplitude motion in both conformers. 1. Carlson, C. D.; Seifert, N. A.; Heger, M.; Xie, F.; Thomas, J; Xu, Y. J. Mol. Spectrosc., 2018, 351, 62–67.

The rotational spectra of m-anisaldehyde and its microsolvated complexes generated in a supersonic jet have been studied by chirped-pulse Fourier transform microwave spectroscopy (CP-FTMW) in the 2-8 GHz region. Four conformers have been detected for the monomer. The three most intense rotamers have line intensities high enough to allow the observation of the monosubstituted ¹³C isotopologues in natural abundance allowing the determination of their r_e, r_s and r_m structures. When allowing water vapor to expand in the supersonic jet, the spectra of ten new species assigned to microsolvated complexes appear. Seven monohydrated species have been identified reflecting the two possible interactions of water and the aldehyde group. The two dihydrated species observed are related to the most stable m-anisaldehyde conformer. The most abundant dihydrated complex presents a structure with water dimer closing a cycle with the aldehyde and methoxy groups which confers high stability. In the less abundant dihydrate, water dimer closes a cycle with the aldehyde group, a structure of great interest to better understand the solvation of aldehydes. Additionally, one complex of m-anisaldehyde with four molecules of water has been detected. In this species, the most stable conformer of m-anisaldehyde captures the tetramer of water adopting a stacked configuration. Work is in progress.

Low temperature photoluminescence spectroscopy (PL) revealed a change in vibrational coupling of mutated Water soluble chlorophyll binding protein complexes (WSCPs) with Chlorophyll a. Pigment-protein systems can adjust the range of absorbed wavelengths according to living conditions. However, the mechanism of spectral tuning is unclear. A study of point mutations in the Q57 site of the Lepidium virginicum (Lv) WSCP is expected to shed light on how hydrogen bonds and electrostatic interactions influence the emission spectrum of Chlorophyll a (Chl a) bound to WSCP. Steady state PL revealed the change of the electron-phonon coupling strength within the mutants at 7 K. Time-resolved (TR) PL detected the difference in the lifetimes of the WSCP mutants at 7 K. Both PL and TRPL results cannot be ascribed to the charge difference in the Q57 site of Lv WSCP alone. The influence of hydrogen bonding together with electrostatic interactions and geometry changes should be considered to correctly describe the mechanism of tuning of vibrational coupling in WSCP bound with Chl a complex.

Manganese oxides are among the most widely explored transition metal oxides for diverse biomedical applications and are also employed in a wide number of industrial processes. Its wide range of oxidation states provide manganese with extreme flexibility in electron occupancy that has also attracted increasing attention for use in photocatalytic processes. Neutral clusters are excellent mimics of the active sites of bulk materials, and can be employed to understand the local geometric and electronic structure properties, and oxidation states that provide the best charge carrier lifetimes and by extension optimal photochemical efficiency. Here, I will present our ongoing work on the ultrafast relaxation dynamics of neutral manganese oxide clusters, which are prepared through the laser ablation of a pure metal rod with a 532 nm Nd:YAG laser. A synchronized pulse of He seeded with 5% oxygen enables cluster formation through supersonic expansion. The neutral clusters are then studied through by combining two-color femtosecond spectroscopy with time-of-flight mass spectrometry. Our cluster distribution shows that manganese has a large range of oxidation states. The clusters are excited by the second harmonic of a Ti:Sapphire femtosecond (fs) laser system (400 nm = 3.1eV) and subsequently ionized through strong field ionization with the fundamental laser beam (800 nm = 1.55 eV). The subtle changes in the ultrafast dynamics upon the addition/subtraction of each atom are being evaluated to provide new understanding to the flow of energy through a material.

I will present our recent work¹⁻³ on the ultrafast dynamics of sub-nanometer neutral metal oxide clusters investigated with femtosecond pump-probe spectroscopy and supported by theoretical calculations. Absorption of a UV (400 nm) photon initiates several relaxation processes, with excited state lifetimes that are strongly dependent on the nature of the electronic transition. The atomic precision and tunability of gas phase clusters highlights how the simple picture of sequential oxidation of the metal atoms reveals a linear tunability to the contributions of each relaxation component to the total transient signal. In chromium oxides, a 30 fs transient signal fraction grows linearly with oxidation, matching the amount of O to Cr charge transfer character of the photoexcitation and highlighting a gradual transition between semiconducting and metallic behavior at the molecular level. The lifetimes of nickel oxide clusters exhibit a unique reliance on the nature of the atomic orbital contributions, providing new insights to the analogous band edge excitation dynamics of strongly correlated bulk material. Short lived dynamics in stoichiometric (NiO)n clusters are attributed to excitation between Ni-3d and Ni-4s orbitals, where their strong exchange coupling produces metallic-like electron-electron scattering. Oxygen vacancies introduce 3d to 4p transitions, which increases the lifetimes of the sub-picosecond component by 20-60 percent and enables the formation of long-lived (lifetimes greater than 2.5 ps) states. (1) Garcia, J. M.; Heald, L. F.; Shaffer, R. E.; Sayres, S. G. Oscillation in Excited State Lifetimes with Size of Sub-Nanometer Neutral (TiO₂)_n Clusters Observed with Ultrafast Pump–Probe Spectroscopy. J. Phys. Chem. Lett. 2021, 12, 4098–4103. (2) Garcia, J. M.; Sayres, S. G. Increased Excited State Metallicity in Neutral Cr₂O_n Clusters (n < 5) upon Sequential Oxidation. J. Am. Chem. Soc. 2021, 143 (38), 15572–15575. (3) Garcia, J. M.; Sayres, S. G. Orbital-Dependent Photodynamics of Strongly Correlated Clusters. Phys.Chem.Chem.Phys. 2022, 24, 5590-5597.

The study of the structure and internal motion of weakly bound gas-phase clusters is of considerable interest in understanding the nature of intermolecular bonding. Van der Waals clusters with rare gas atoms such as argon are particularly interesting due to their ability to freely internally rotate about the host molecule. Additionally, many experiments that utilize argon as the buffer gas for supersonic expansions suffer from subsequent clustering of the argon to the sample of interest. In high-resolution spectroscopy, this can cause difficulties with line identification and assignment. Therefore, it is useful to have a complete and accurate characterization of the rotational transitions for such complexes. In our research, nearly all of our supersonic expansions involve the use of argon gas, and methanol is often a molecular starting material for the chemistry that we wish to study making the target of this study the Ar-CH₃OH cluster. The spectrum from 140-335 GHz was collected via direct absorption spectroscopy using a supersonic expansion of argon seeded with vapor from a pure methanol sample. Numerous spectral lines were detected across this frequency range. Spectral analysis was conducted using the Effective Rotational Hamiltonian program (ERHAM) due to the presence of a low barrier methyl rotor, which ERHAM is well-suited to address. The spectral results and associated analysis for Ar-CH₃OH will be presented here.