High-resolution Electronic Spectra Research Articles

Spectroscopically resolved resonant interatomic Coulombic decay in photoexcited large He nanodroplets

Interatomic Coulombic decay (ICD) processes play a crucial role in weakly bound complexes exposed to intense or high-energy radiation. Using large helium nanodroplets, we demonstrate that ICD is efficient even when the droplets are irradiated by weak synchrotron radiation at relatively low photon energies. Below the ionization threshold, resonant excitation of multiple centers efficiently induces resonant ICD as previously observed for intense pulses [A. C. LaForge , ]. More surprisingly, we observe ICD even above the ionization threshold due to recombination of photoelectrons and ions into excited states which subsequently decay by ICD. This demonstrates the importance of secondary processes, in particular electron scattering and recombination, in inducing ICD in extended condensed phase systems. High-resolution ICD electron spectra in combination with coincidence imaging of electrons and ions reveal the relaxation dynamics of highly excited and ionized weakly bound nanosystems. Published by the American Physical Society 2024

Temperature Dependence of the Electronic Absorption Spectrum of NO2.

The nitrogen dioxide (NO2) radical is composed of the two most abundant elements in the atmosphere, where it can be formed in a variety of ways including combustion, detonation of energetic materials, and lightning. Relevant also to smog and ozone cycles, together these processes span a wide range of temperatures. Remarkably, high-resolution NO2 electronic absorption spectra have only been reported in a narrow range below about 300 K. Previously, we reported [ J. Phys. Chem. A 2021, 125, 5519-5533] the construction of quasi-diabatic potential energy surfaces (PESs) for the lowest four electronic states (X̃, Ã, B̃, and C̃) of NO2. In addition to three-dimensional PESs based on explicitly correlated MRCI(Q)-F12/VTZ-F12 ab initio data, the geometry dependence of each component of the dipoles and transition dipoles was also mapped into fitted surfaces. The multiconfigurational time-dependent Hartree (MCTDH) method was then used to compute the 0 K electronic absorption spectrum (from the ground rovibrational initial state) employing those energy and transition dipole surfaces. Here, in an extension of that work, we report an investigation into the effects of elevated temperature on the spectrum, considering the effects of the population of rotationally and vibrationally excited initial states. The calculations are complemented by new experimental measurements. Spectral contributions from hundreds of rotational states up to N = 20 and from 200 individually-characterized vibrational states were computed. A spectral simulation tool was developed that enables modeling the spectrum at various temperatures─by weighting individual spectral contributions via the partition function, or for pure excited initial states, which can be probed via transient absorption spectroscopy. We validate these results against experimental absorption spectroscopy data at high temperatures, as well as via a new measurement from the (1,0,1) initial vibrational state.

High-Resolution Electronic Spectrum of the 1,4,6-Heptatrienyl Radical in the Gas Phase.

We report the gas-phase identification of the 1,4,6-heptatrienyl (C7H9) radical via its - electronic transition spectrum. The optical absorption spectrum in the 590-630 nm region is recorded using cavity ring-down spectroscopy in combination with a supersonic plasma jet. An analysis of the rotationally resolved origin band spectrum has allowed an accurate determination of spectroscopic constants for both the electronic ground and excited states of this radical. Ab initio calculations at the CASPT2/cc-pVTZ level have been performed to predict the radical structure and molecular constants that are in good agreement with the experimental results. By combination with available experimental data for the allyl and 1,4-pentadienyl radicals, we extrapolate an excitation energy gap between the ground and first excited states of a long C2n-1H2n+1 polyenyl chain converging to a residual energy gap, suggesting the presence of residual nonuniform C-C bond lengths in a long polyenyl chain.

Electronic Structure and Solvation Effects from Core and Valence Photoelectron Spectroscopy of Serum Albumin.

X-ray photoelectron spectroscopy of bovine serum albumin (BSA) in a liquid jet is used to investigate the electronic structure of a solvated protein, yielding insight into charge transfer mechanisms in biological systems in their natural environment. No structural damage was observed in BSA following X-ray photoelectron spectroscopy in a liquid jet sample environment. Carbon and nitrogen atoms in different chemical environments were resolved in the X-ray photoelectron spectra of both solid and solvated BSA. The calculations of charge distributions demonstrate the difficulty of assigning chemical contributions in complex systems in an aqueous environment. The high-resolution X-ray core electron spectra recorded are unchanged upon solvation. A comparison of the valence bands of BSA in both phases is also presented. These bands display a higher sensitivity to solvation effects. The ionization energy of the solvated BSA is determined at 5.7 ± 0.3 eV. Experimental results are compared with theoretical calculations to distinguish the contributions of various molecular components to the electronic structure. This comparison points towards the role of water in hole delocalization in proteins.

Electron-rotation coupling in diatomics under strong-field excitation

The photoexcitation and photodissociation of diatomic molecules by intense pulse lasers has been the subject of extensive investigations over the past decades. However, the usually employed theoretical framework neglects the coupling between the molecular rotational angular momentum ($\mathbf{R}$) and the angular momentum of the electrons projected onto the molecular axis $\mathrm{\ensuremath{\Omega}}=\mathrm{\ensuremath{\Lambda}}+\mathrm{\ensuremath{\Sigma}}$, which results in the known $\mathrm{\ensuremath{\Lambda}}$-doubling phenomenon in high-resolution electronic spectra of diatomic molecules. While neglecting this coupling is an excellent approximation in the weak-field or perturbative regime owing to the large mass difference between the rotating atoms and the electrons, the approximation breaks down for intense laser pulses because of the repeated Rabi cycling of the electronic transitions, which can have a significant effect on the rotational degrees of freedom of the molecule. By correcting the transition dipole matrix elements and introducing angular basis sets based on Wigner D functions, the conventional theoretical treatment is generalized to a universal description valid for both the weak- and strong-field regimes. The theoretical treatment developed here is applied to the $|^{1}\mathrm{\ensuremath{\Sigma}}\ensuremath{\rangle}$ to $|^{1}\mathrm{\ensuremath{\Pi}}\ensuremath{\rangle}$ transitions in diatomic systems. Our results reveal that, for field intensities resulting in about one Rabi cycling for extreme ultraviolet or x-ray transitions, the theoretical predictions by the conventional theoretical frame need to be corrected when considering observables such as the molecular alignment and the angular distribution of the photofragments.

High Precision Electronic Spectroscopy of Ligand-Protected Gold Nanoclusters: Effects of Composition, Environment, and Ligand Chemistry.

Atomically precise gold nanoclusters (AuNCs) are a class of nanomaterials valued for their electronic properties and diverse structural features. While the advent of X-ray crystallography of AuNCs has revealed their geometric structures with high precision, detailed electronic structure analysis is challenged by environmental, compositional, and thermal averaging effects present in electronic spectra of typical samples. To circumvent these challenges, we have adapted mass spectrometer-based electronic absorption spectroscopy techniques to acquire high-resolution electronic spectra of atomically precisely defined nanoclusters separated from a synthetic mixture. Here we discuss recent results using this approach to link the surface chemistry of triphenylphosphine-protected AuNCs to their electronic structure and expand on key elements of the experiment and the link between these gas-phase measurements and solution-phase behavior of AuNCs. Chemically derivatized Au8(P(p-X-Ph)3)72+ and Au9(P(p-X-Ph)3)83+ clusters, where X = -H, -CH3, or -OCH3, are used to derive systematic trends in the response of the electronic spectrum to the electron-donating character of the ligand shell. We find a linear relationship between the substituent Hammett parameter σp and the transition energy between both sets of clusters' highest occupied and lowest unoccupied molecular orbitals, a transition that is localized in the metal core within the limits of the superatomic model. The similarity of the mass-selective and solution-phase UV/vis spectra of Au9(PPh3)83+ indicates that the interpretation of these experiments is transferable to the condensed phase. He and N2 environments are introduced to a series of isovalent clusters as a subtle probe of discrete environmental effects over electronic structure. Strikingly, select bands in the UV/vis spectrum respond strongly to the identity of the environment, which we interpret as a state-selective indicator of interfacially relevant electronic transitions. Physically predictable trends such as these will aid in building molecular design principles necessary for the development of novel materials based on nanoclusters.

High-Resolution Absorption and Electronic Circular Dichroism Spectra of (R)-(+)-1-Phenylethanol. Confident Interpretation Based on the Synergy between Experiments and Computations.

Using density functional theory and its time-dependent extension for excited states, the S0 →S1 high-resolution vibronic absorption and electronic circular dichroism spectra of (R)-(+)-1-phenylethanol are computed and compared to experimental spectra measured in jet-cooled conditions in the region within 1000 cm-1 of the 0-0 transition. The agreement between theory and computation is satisfactory and allows a confident assignment of several experimental bands in terms of fundamentals of different modes. Cases are documented for which the analysis of optical anisotropy factors, owing to their signed nature, remarkably enhances the possibility of a robust assignment of the experimental absorption bands. Computational analysis shows that the experimental spectra are dominated by Herzberg-Teller contributions and that the electronic circular dichroism spectrum and the anisotropy factors are also strongly modulated by the effect of Duschinsky mixings.

The Renner–Teller effect observed in the and electronic states of H2S+

In 1995, Balzer et al. recorded a well-resolved photoelectron spectrum of the outer valence region of H2S between 12.7 and 14.5 eV. The vibronic energies were identified with the aid of calculated ionisation potentials. Following on from this, vacuum ultraviolet pulsed-field ionisation spectroscopy was used by Hochlaf et al. in 2004 to record partially resolved rotational spectra of H2S+. Finally, in 2010, Han, Kang and Kim recorded rotationally resolved electronic spectra of the transitions of H2S+ above the barrier to linearity, by use of mass-analysed threshold ionisation photo-fragment excitation spectroscopy. By making use of new high-resolution jet-cooled electronic spectra recorded in Basel, and the rotational analyses carried out in 1972 and 1983, the different methods have been linked to provide a detailed picture of the relationship between the four different types of spectroscopy.

Using high resolution electronic spectroscopy to probe the effects of ring twist on charge transfer in 2-phenylindole and N-phenylcarbazole

High resolution electronic spectra of 2-phenylindole (PI) and N-phenylcarbazole (PC) have been recorded in the collision-free environment of a molecular beam. Inertial defects determined from fits of the spectra were used to determine the twist angles between the two chromophores and their attached benzene rings in the ground (S0) and excited (S1) electronic states. PI was found to be significantly more planar than PC, especially in the S1 state. Stark-effect measurements of the permanent electric dipole moments of both molecules in both states show that significantly more charge is transferred from the phenyl group to the chromophore in PI (0.13e) than in PC (0.076e) when the photon is absorbed. Thereby demonstrated for the first time is a direct connection between photo-induced geometry change and charge transfer on excitation of an isolated molecule by light.

High-spin electronic states of lanthanide-arene complexes: Nd(benzene) and Nd(naphthalene)

Neodymium (Nd) complexes of benzene and naphthalene were synthesized in a laser-ablation supersonic molecular beam source. High-resolution electron spectra of these complexes were obtained using pulsed-field ionization zero electron kinetic energy (ZEKE) spectroscopy. Second-order Møller-Plesset perturbation calculations were employed to aid spectral and electronic-state assignments. The adiabatic ionization energies were measured to be 38 081 (5) cm(-1) for Nd(benzene) and 37 815 (5) cm(-1) for Nd(naphthalene). For the Nd(benzene) complex, the observed frequencies of 831 and 286 cm(-1) were assigned to C-H out-of-plane bending and Nd(+)-C(6)H(6) stretching modes in the (6)A(1) ion state and 256 cm(-1) to the Nd-C(6)H(6) stretching mode in the (7)A(1) neutral state. To confirm these assignments, the ZEKE spectrum of the deuterated species was recorded, and the corresponding vibrational frequencies were measured to be 710 and 277 cm(-1) in the ion state and 236 cm(-1) in the neutral state. For the Nd(naphthalene) complex, the observed vibrational modes were C(10)H(8) bending (394 cm(-1)), Nd(+)-C(10)H(8) stretching (286 and 271 cm(-1)), Nd(+)-C(10)H(8) bending (80 cm(-1)), and C(10)H(8) twisting (105 cm(-1)) in the (6)A(') ion state and metal-ligand bending (60 cm(-1)) and ligand twisting (55 cm(-1)) in the (7)A(') neutral state. The formation of the ground state of the Nd(benzene) complex requires 4f → 5d and 6s → 5d electron excitation of the Nd atom, whereas the formation of the ground state of Nd(naphthalene) involves the 6s → 5d electron promotion.

Excited state electron transfer precedes proton transfer following irradiation of the hydrogen-bonded single water complex of 7-azaindole with UV light

High resolution electronic spectra of the single water complex of 7-azaindole (7AIW) and of a deuterated analog (7AIW-d(3)) have been recorded in a molecular beam, both in the absence and presence of an applied electric field. The obtained data include the rotational constants of both complexes in their ground (S(0)) and first excited (S(1)) electronic states, their S(1)-S(0) electronic transition moment and axis-tilting angles, and their permanent electric dipole moments (EDM's) in both electronic states. Analyses of these data show that the water molecule forms two hydrogen bonds with 7AI, a donor O-H···N(7) bond and an acceptor O···H-N(1) bond. The resulting structure has a small EDM in the S(0) state (μ = 0.54 D) but a greatly enhanced EDM in the S(1) state (μ = 3.97 D). We deduce from the EDM's of the component parts that 0.281 e(-) of charge is transferred from the acidic N(1)-H site to the basic N(7) site upon UV excitation of 7AIW, but that water-assisted proton transfer from N(1) to N(7) does not occur. A model of the resulting electrostatic interactions in the solute-solvent pair predicts a solvent-induced red-shift of 1260 cm(-1) which compares favorably to the experimental value of 1290 cm(-1).

On the Excited State Dynamics of Vibronic Transitions. High-Resolution Electronic Spectra of Acenaphthene and Its Argon van der Waals Complex in the Gas Phase

Rotationally resolved fluorescence excitation spectroscopy has been used to study the dynamics, electronic distribution, and the relative orientation of the transition moment vector in several vibronic transitions of acenaphthene (ACN) and in its Ar van der Waals (vdW) complex. The 0(0)(0) band of the S(1) ← S(0) transition of ACN exhibits a transition moment orientation parallel to its a-inertial axis. However, some of the vibronic bands exhibit a transition moment orientation parallel to the b-inertial axis, suggesting a Herzberg-Teller coupling with the S(2) state. Additionally, some other vibronic bands exhibit anomalous intensity patterns in several of their rotational transitions. A Fermi resonance involving two near degenerate vibrations has been proposed to explain this behavior. The high-resolution electronic spectrum of the ACN-Ar vdW complex has also been obtained and fully analyzed. The results indicate that the weakly attached argon atom is located on top of the plane of the bare molecule at ~3.48 Å away from its center of mass in the S(0) electronic state.

Higher-order resonant inter-shell electronic recombination for heavy highly charged ions

Using forced evaporative cooling on stored highly charged ions (HCIs) in an electron beam ion trap, high-resolution electronic recombination spectra were obtained. Inter-shell tri-electronic recombination is reported for HCIs, mainly for C-like ions of Ar, Fe and Kr, where simultaneously with the K-shell excitation an additional L electron is excited at the resonant recombination of a free electron. Clear indications of quadru-electronic recombination are found, where besides the K electron two additional L electrons are excited simultaneously. The experimental technique is described; then the results on first- and higher-order resonant recombination processes are discussed and compared with multi-configuration Dirac–Fock calculations.

High resolution fluorescence excitation spectra of both enantiomers of naproxen in the gas phase: Are they equivalent or not?

Two chiral conformers of the optically active molecule naproxen have been identified in its high resolution electronic spectrum, differing only in the orientation of the attached isopropionic acid group relative to the naphthalene ring. Both conformers exhibit b-type spectra, indicating that the S1 states of the isolated molecules are similar to those of other 2-substituted naphthalenes. The measured rotational constants of the left- and right-handed forms of both conformers are identical to within the experimental error, ±0.1MHz (∼1:109), indicating that they are enantiomers of each other, not diastereomers. The naphthyl ring with its attached methoxy group is insufficient to provide chiral discrimination.

Self-organized antireflecting nano-cone arrays on Si (100) induced by ion bombardment

Self-organized nano-cone arrays are fabricated on Si (100) by means of Ar+ ion bombardment at normal incidence with ion energy of 1.5 keV and current density of 1000 µA cm−2. The nano-structured Si surface appears black as seen by the naked eye. The measured reflectance of the surface is less than 11% over the wavelength range from 350 to 2000 nm as compared to that of >∼30% for the polished Si. An enhancement of more than 25% in absorption is observed in this region. The cones are densely distributed over the surface with an average height of ∼350 nm and base width of ∼250 nm. Incorporation of metal atoms such as Fe and Cr is found to be mandatory for the formation of the nano-structures during ion bombardment. High-resolution electron spectra show that for each cone, the apex is metal-enriched, and the rest is nearly free of metal atoms, showing good crystallinity with the same crystallographic orientation as the substrate.

Multielectron spectroscopy: Auger decays of the krypton3dhole

The emission of one or two Auger electrons, following Kr $3d$ inner-shell ionization by synchrotron light, has been investigated both experimentally and theoretically. All electrons emitted in the process are detected in coincidence and analyzed in energy thanks to a magnetic-bottle electron time-of-flight spectrometer. In addition, noncoincident high-resolution electron spectra have been measured to characterize the cascade double-Auger paths more fully. Combination of the two experimental approaches and of our calculations allows a full determination of the decay pathways and branching ratios in the case of Kr $3d$ single- and double-Auger decays. The ${\mathrm{Kr}}^{3+}$ threshold is found at $74.197\ifmmode\pm\else\textpm\fi{}0.020$ eV.

High-resolution Fourier transform emission spectroscopy of the A1Π– X1Σ + system of 74Ge 80Se

Natural germanium and selenium consist of, respectively, five and six stable isotopes. Several of these isotopes have considerable abundances and one should expect to observe the bands of at least six isotopic variants of germanium monoselenide (GeSe). In this paper, for the first time, the results of the high-resolution electronic spectrum of the main transition A 1Π– X 1Σ + of the specific isotopomer 74Ge 80Se, excited in a microwave discharge and recorded in the 33 500–26 000 cm −1 region using a Fourier transform spectrometer, is discussed. From the rotational analysis of 25 bands involving v″ = 0–12 and v′ = 0–7, accurate vibrational and rotational constants of the A 1Π state are determined. The present study has revealed perturbations in the v′ = 6 and 7 levels of the A 1Π state.

Vibronic coupling in indole: II. Investigation of the 1La–1Lb interaction using rotationally resolved electronic spectroscopy

High-resolution electronic spectra of indole (C(8)H(7)N) and their detailed analysis are reported. Thirteen low-lying vibronic bands--from the electronic origin transition at 35,231.4 cm(-1) up to 1000 cm(-1) above--are recorded with rotational resolution. Besides inertial parameters and inertial defects these spectra yield detailed information, for each individual band, on the transition-dipole-moment orientations in the molecular inertial frame as well as on the reorientation of that inertial frame upon electronic excitation. The natural lifetimes of the individual vibronic states have also been determined. Strongly varying orientations of the transition-dipole-moments, unexpected positive inertial defects, and decreasing lifetimes, which are only partly related to increased excitation energy, are observed. These results are clear indications of the interaction of the two lowest electronically excited singlet states ((1)L(b) and (1)L(a)). Our experimental findings are strongly supported by, and in excellent agreement with, the theoretical description of the interaction of the two electronic states described in the preceding paper. These results provide clear evidence for strong vibronic coupling of the two electronic states (1)L(b) and (1)L(a) and for the energetic location of the (1)L(a)-state more than 1000 cm(-1) above the (1)L(b) vibrationless state.

High Resolution Electronic Spectroscopy of o- and m-Toluidine in the Gas Phase. Barrier Height Determinations for the Methyl Group Torsional Motions

High resolution electronic spectra of o- and m-toluidine have each been recorded for the S(1)<--S(0) origin band transitions of the isolated molecules. Each spectrum is split into two sub-bands owing to tunneling motions along the methyl group torsional coordinate. Analyses of these data provide information about the preferred configurations of the methyl groups and the barriers opposing their motions in both the ground and excited electronic states. Despite their apparent similarities, the experiments reveal that these properties are quite different in the two molecules. Possible reasons for this behavior are discussed.

Synthesis, Crystal Structures, and Luminescence of Organic-Lanthanide Complexes with Nicotinate and Isonicotinate Ligands

Six lanthanide coordination compounds with two isomeric carboxylic acids, nicotinic acid (HL(1)) and isonicotinic acid (HL(2)), [(L(1))3Ln(H2O)2]2 (Ln = Eu, 1; Gd, 2; Tb, 3) and [( L(2))2Ln(H2O)4][NO3] (Ln = Eu, 4; Gd, 5; Tb, 6), have been synthesized and structurally characterized by single-crystal X-ray diffraction. Complexes 1-3 are dimeric whereas 4-6 are polymeric, all with 8-coordination of Ln(3+). The distinction between these lanthanide complexes is readily accomplished from the 10 K high resolution electronic emission spectra. Spectral interpretation is given for the Eu(3+) complexes 1, 4, whereas the spectra of 3 and 6 are more complex. The relationships between spectroscopic and crystallographic site symmetries are discussed. The calculated second rank crystal field strengths of Eu(3+) in 1 and 4 are intermediate in magnitude.