Bond dissociation energy of O2 measured by fully state-to-state resolved threshold fragment yield spectra.

We have determined the bond dissociation energy of O2 by measuring fully state-to-state resolved threshold fragment yield spectra in the XUV energy region, O2X3Σg-,N″,J″→O(PJ3)+O(S1o3)/O(S2o5). Our results have yielded a bond dissociation energy value of 41 269.19 ± 0.10 cm-1, which is consistent with previous measurements but exhibits a significantly lower uncertainty, approximately five times smaller. It is noteworthy that this study is the first to simultaneously achieve fine structure state resolution for the parent O2 molecule and spin-orbit state resolution for the O(3PJ) fragments in the measurement of O2 bond dissociation energy. As a result, our findings have established a solid foundation for the obtained data.

Bond dissociation energy of N2 measured by state-to-state resolved threshold fragment yield spectra.

The precise determination of the bond dissociation energy of N2 is crucial for thermochemistry database and theoretical calculations. However, there has been ongoing debate regarding its exact value. In this study, we used the velocity map imaging method combined with an extreme ultraviolet laser to measure the threshold fragment yield (TFY) spectra of N2 in the N(2D) + N(2D) photodissociation channels. By integrating the signals within a small circular area on the fragment velocity map images, we were able to obtain TFY spectra at nine different dissociation thresholds. These spectra are rotational state-resolved for the N2(J″) molecules and spin-orbit state-resolved for the dissociation channels involving N(2D) fragments. By employing the Wigner threshold law to simulate the TFY spectra and conducting statistical analysis on the comprehensive dataset, we determined the N2 bond dissociation energy to be 78 691.09 ± 0.15 cm-1. This work now places N2 among the few diatomic molecules with bond dissociation energies measured at sub-wavenumber precision.

Predissociation dynamics of D2 revealed by variation in fragment anisotropy parameters along the Fano profile.

We conducted a study on the variations of the fragment anisotropy parameters (ß) along the Fano profiles for the predissociation of the D2 molecule. These variations, known as ß profiles, were measured for the D(2l) fragments from the predissociation of the 4pπD'Πu1υ'=1 and 4pσB″Σu+1υ'=2 states. The measured ß profiles show significant asymmetry and broader linewidths compared to the corresponding Fano profiles. By fitting the ß profiles, we were able to determine the fragment anisotropy parameters associated with the resonance state, continuum state, and the interference effect between them. Additionally, we determined the ratios of the absorption cross sections between the unperturbed and perturbed continuum states interacting with the resonance states although these ratios were found to be very small. Furthermore, we derived approximate formulas to calculate the parameters characterizing the ß profile. Despite the linewidths of the four Fano profiles being narrower than our instrumental resolution, we were still able to determine the product of the linewidth with the Fano q parameters. These findings demonstrate the utility of the ß profile as an effective tool for studying the predissociation dynamics in diatomic molecules.

Spectra and Predissociation Dynamics of the 4pσB''1∑u+ and 4pπD' 1Πu States of D2.

The spectra and predissociation dynamics of the Rydberg states 4pσB″1∑u+(υ'=2) and 4pπD'1Πu±(υ'=1) of the D2 molecule have been studied by a combination of XUV laser pump, UV laser probe, and velocity map imaging methods. The D(2l) fragment yield spectrum and the D2 excitation spectrum have been measured. Although the predissociation rates were found to be small and comparable to the radiative decay rates, the D(2l) fragments were observed for all the Rydberg states. The measured D(2l) fragment angular distributions are in good agreement with a reported model that has an analytical formula for anisotropy parameters. The results demonstrated that the 4pσB″1∑u+ and 4pπD'1Π u+ states dissociate via direct and indirect coupling with an 1∑u+ state, respectively. For predissociation of the 4pσB″1∑u+ state, the experimental data showed that D(1s) + D(2s) is the main channel, supporting previous theoretical predictions that the predissociation occurs by coupling to the vibrational continuum of the 3pσB'1∑u+ state.

Photodissociation of HCl in the photon energy range 14.6-15.0 eV: Channel-resolved branching ratios and fragment angular distributions.

For the HCl molecule, four photodissociation channels are open in the excitation energy region 14.6-15.0 eV: H(2s) + Cl(2P3/2), H(2p) + Cl(2P3/2), H(2s) + Cl(2P1/2), and H(2p) + Cl(2P1/2). We measured the fragment angular distributions and the branching ratios of the four dissociation channels by using the extreme ultraviolet laser pump and UV laser probe, delay-time-curve, and velocity map imaging methods. The channel-resolved fragment angular distributions and fragment yield spectra show that various Rydberg states (superexcited states) contribute to the absorption cross sections, including the [A2Σ+]4pσ, [A2Σ+]4pπ, [A2Σ+]3dσ, [A2Σ+]3dπ, and [A2Σ+]5sσ states. Most of the H(2s) + Cl(2P1/2) channels correlate with the 1Σ+ states, while the other channels correlate with mixing excitations of the 1Σ+ and 1,3Π states. The channel branching ratios are dependent on the excitation energies. When the four channels are open, the channel branching ratios of H(2s) + Cl(2P3/2) and H(2p) + Cl(2P1/2) are small. Based on the recent ab initio potential energy curves, the Rydberg states converging to the ion-core A2Σ+ are proposed to be predissociated by the nuclear vibrational continua of the Rydberg states converging to the ion-core X2Π.

Influence of fullerenol on hIAPP aggregation: amyloid inhibition and mechanistic aspects.

Fullerenols have garnered significant scientific interest in nano-technology and biomedicine. A detailed understanding of their interactions with proteins is fundamentally important for their biomedical applications. Human islet amyloid polypeptide (hIAPP) is an intrinsically disordered protein and its aggregation is associated with type 2 diabetes. Here, we investigated the nano-bio-interactions of fullerenol with hIAPP and focused on the effect of C60(OH)24 on hIAPP aggregation by replica-exchange molecular dynamic simulations. Our simulations show that isolated hIAPP dimers transiently populated amyloid-precursor (ß-hairpin) containing ß-sheet structure, whereas C60(OH)24 completely suppressed this fibril-prone structure, thus inhibiting hIAPP aggregation. The simulation-predicted inhibitory effect of fullerenols was validated by atom force microscopy and thioflavin T fluorescence experiments. We find C60(OH)24 binds to hIAPP via hydrogen bonding interactions with polar residues T9, Q10, N14, N21, N22, N31, N35 and T36 as well as the collective van der Waals and hydrogen-bonding interaction with Y37. Molecular dynamic simulations show that C60(OH)24 destabilized the hIAPP protofibril by mostly binding to the 20SNNFGAILSS29 amyloid core region. This study not only helps to understand the mechanisms involved in hIAPP aggregation and amyloid inhibition, but also provides new clues for the development of therapeutic candidates against type 2 diabetes.

Predissociation dynamics of D2 + hv→ D(1s1/2) + D(2p1/2,3/2, 2s1/2) revealed by the spin-orbit state resolved fragment branching ratios and angular distributions.

For molecular photodissociations, the spin-orbit state resolved fragment branching ratios and angular distributions provide deep insight into the dynamics. For the first excited state of the H(2p1/2,3/2) atom, a branching ratio measurement is a challenge because of small energy spacing between them. For the D(2p1/2,3/2) fragments from the predissociation of D2 + 14.76 eV → D(1s) + D(2s, 2p1/2,3/2) in the 2pπC1Πu (υ = 19) state, we made such measurements by pumping the D(2s, 2p1/2,3/2) fragments to high-lying Rydberg states that are subsequently ionized by a delayed-pulse electric field. In the 2pπC1Πu (υ = 19) state, the D2 molecule dissociates via both shape and Feshbach resonances correlating to the channels D(1s) + D(2p3/2) and D(1s) + D(2p1/2), respectively. The measured spin-orbit branching ratios, 2p3/2/(2p1/2 + 2p3/2), correspond to the diabatic limit, 2/3, which indicates strong spin-orbit state mixings near the dissociation limits. The spin-orbit state resolved fragment angular distributions also support the diabatic dissociation mechanism and illustrate simultaneous shape and Feshbach resonances for the R(0) transition.

Vacuum Ultraviolet Laser Desorption/Ionization Mass Spectrometry Imaging of Single Cells with Submicron Craters.

Mass spectrometry imaging (MSI) is a crucial label-free method to distinguish the localization patterns in single cells. MALDI-TOF MS and ToF-SIMS are now bearing the responsibility. However, MALDI-TOF MS is limited to micron spatial resolution and ToF-SIMS suffers from severe molecular fragmentation. Here, we proposed a new MSI methodology of vacuum ultraviolet laser desorption/ionization (VUVDI) with high spatial resolution, achieving higher ion yields and less fragmentation compared with ToF-SIMS at submicron level. The fluorescence image and mass spectrum of VUVDI were obtained simultaneously. In addition, the adjustable laser fluence acquired selective detection for different molecular and fragmental ions, thus realizing molecular identification. Furthermore, MSIs of single cells with submicron craters were presented. These results suggest VUVDI is a potential mass spectrometry method that provides a soft ionization source and submicron spatial resolution for molecular analysis in life science.

Oscillation of Branching Ratios Between the D(2s)+D(1s) and the D(2p)+D(1s) Channels in Direct Photodissociation of D_{2}.

The direct photodissociation of D_{2} at excitation energies above 14.76 eV occurs via two channels, D(2s)+D(1s) and D(2p)+D(1s). The branching ratios between the two have been measured from the dissociation threshold to 3200 cm^{-1} above it, and it is found that they show cosine oscillations as a function of the fragment wave vector magnitudes. The oscillation is due to an interference effect and can be simulated using the phase difference between the wave functions of the two channels, analogous to Young's double-slit experiment. By fitting the measured branching ratios, we have determined the depths and widths of the effective spherical potential wells related to the two channels, which are in agreement with the effective depths and widths of the ab initio interaction potentials. The results of this Letter illustrate the importance of the relative phase between the fragments in controlling the branching ratios of the photodissociation channels.

Performance of a nonempirical exchange functional from density matrix expansion: comparative study with different correlations.

Recently, Tao and Mo proposed a meta-generalized gradient approximation for the exchange-correlation energy with remarkable accuracy for molecules, solids, and surfaces. To better understand this functional, in this work, we make a comparative study of the TM and TMTPSS functionals, the latter of which is a combination of the TM exchange with the original TPSS correlation functional, on atoms, molecules, and hydrogen-bonded complexes by the use of eight well-known databases. Our calculations show that, while the TMTPSS functional achieves better accuracy for atomization energies or enthalpies of formation, harmonic vibrational frequencies, and atomic excitation energies than the TM functional, it is less accurate for proton affinities, molecular bond lengths, and in particular hydrogen-bonded complexes.

Electronic and Tunneling Predissociations in the 2pπC1Πu±(υ = 19) and 3pπD1Πu±(υ = 4, 5) States of D2 Studied by a Combination of XUV Laser and Velocity Map Imaging.

The predissociation mechanism of D2 near the threshold for the production of the D(2s, 2p) fragments has been studied by measuring the fragment yield spectra, fragment velocity map images, and fragment branching ratios D(2s)/(D(2s) + D(2p)) using a combination of XUV laser and velocity map imaging. The predissociation dynamics of the 2pπC1Πu±(υ = 19) and 3pπD1Πu±(υ = 4,5) states were studied. The 2pπC1Πu±(υ = 19) state is a bound state due to a shallow barrier. For the R(0) transition to the 2pπC1Πu+(υ = 19) state, the experimental results suggest that the predissociation occurs via three channels with decreasing importance: l-uncoupling with the 2pσB1Σu+ state, tunneling, and l-uncoupling with the 3pσB'1Σu+ state. For the R(1) transition to the 2pπC1Πu+(υ = 19) state, the first channel plays the dominant role. For the Q(1) transition to the 2pπC1Πu-(υ = 19) state, the predissociation occurs via tunneling as required by symmetry. For the predissociation of the 3pπD1Πu+(υ = 4,5) states, the experimental data confirm the earlier results indicating that the main perturbing state is 3pσB'1Σu+. The Beutler-Fano profiles and the associated spectroscopic parameters for the various predissociations have also been obtained. The measured Fano-parameters q for the P- and R-branches of the 3pπD1Πu+ state are found to have opposite signs, and their relationships are in agreement with a formula derived from the Fano equation. Rotationally resolved Beutler-Fano profiles were measured for the P(2) and P(3) lines.

Accurate excitation energies of molecules and oligomers from a semilocal density functional.

Excitation energy plays an important role in energy conversion, biological processes, and optical devices. In this work, we apply the Tao-Mo (TM) nonempirical meta-generalized gradient approximation and the combination TMTPSS (TMx + TPSSc), with TPSSc being the correlation part of the original TPSS (Tao-Perdew-Staroverov-Scuseria) to study excitation energies of small molecules and oligomers. Our test set consists of 17 molecules with 134 total excited states, including singlet, triplet, valence, and Rydberg excited states. Our calculation shows that both the TMTPSS and TM functionals yield good overall performance, with mean absolute errors (MAEs) of 0.37 eV and 0.42 eV, respectively, outperforming commonly used semilocal functionals LSDA (MAE = 0.55 eV), PBE (MAE = 0.58 eV), and TPSS (MAE = 0.47 eV). In particular, TMTPSS can yield nearly the same accuracy of B3LYP (MAE = 0.36 eV), with lower computational cost. The accuracy for semilocal density functional theory continues to hold for conjugated oligomers, but they become less accurate than hybrid functionals, due to the insufficient nonlocality.

Accurate Semilocal Density Functional for Condensed-Matter Physics and Quantum Chemistry.

Most density functionals have been developed by imposing the known exact constraints on the exchange-correlation energy, or by a fit to a set of properties of selected systems, or by both. However, accurate modeling of the conventional exchange hole presents a great challenge, due to the delocalization of the hole. Making use of the property that the hole can be made localized under a general coordinate transformation, here we derive an exchange hole from the density matrix expansion, while the correlation part is obtained by imposing the low-density limit constraint. From the hole, a semilocal exchange-correlation functional is calculated. Our comprehensive test shows that this functional can achieve remarkable accuracy for diverse properties of molecules, solids, and solid surfaces, substantially improving upon the nonempirical functionals proposed in recent years. Accurate semilocal functionals based on their associated holes are physically appealing and practically useful for developing nonlocal functionals.

Predissociation dynamics in the 3pπD(1)Πu (±)υ=3 and 4pσB(″) (1)Σu (+)υ=1 states of H2 revealed by product branching ratios and fragment angular distributions.

The predissociation dynamics of H2+XUV→H2 (*)→H(1s)+H(2s,2p) has been studied by measuring the fragment branching ratios between the H(2s) and H(2p) states and the fragment angular distributions using the XUV (extreme ultraviolet) laser pump and UV(ultraviolet) laser probe method. The fragment angular distributions for the predissociation of the 3pπD(1)Πu (+)υ=3 state show parallel transitions, demonstrating that the main components of the dissociating state have (1)Σu (+) symmetry. The measured fragment branching ratios, H(2s)/(H(2s) + H(2p)), for the transitions R(0), R(1), and P(2) in 3pπD(1)Πu (+)υ=3←X(1)Σg (+)υ(″)=0 are in good agreement with one of the previous theoretical predictions. The predissociations of the 3pπD(1)Πu (-)(υ=3) state arising from the Q(1), Q(2), and Q(3) lines have also been observed. The angular distributions and the state distributions of the excited fragments (all found from the H(2p) state) illustrate that the dissociating states for the Q lines have the expected Πu (-) symmetry. The predissociation dynamics of the transition 4pσB(″1)Σu (+)υ=1←X(1)Σg (+)υ(″)=0 was also studied. Their fragment angular distributions show the expected parallel transitions, and most of the fragments are in the H(2s) states. The Beutler-Fano profiles and the associated spectroscopic parameters for the predissociations have also been obtained by measuring the fragment yield of H(2s, 2p) as a function of excitation photon energies.

Rotationally resolved vibrational spectra of AsH3 (+)X̃(2)A2 (″): Tunneling splittings studied by zero-kinetic-energy photoelectron spectroscopy.

The rotationally resolved vibrational spectra of AsH3 (+)X̃(2)A2 (″) have been measured for the first time with vibrational energies up to 6000 cm(-1) above the ground state using the zero-kinetic-energy photoelectron method. The symmetric inversion vibrational energy levels (v2 (+)) and the corresponding rotational constants for v2 (+)=0-15 have been determined. The tunneling splittings of the inversion vibration energy levels have been observed and are 0.8 and 37.7 (±0.5) cm(-1) for the ground and the first excited vibrational states, respectively. The first adiabatic ionization energy for AsH3 was determined as 79 243.3 ± 1 cm(-1). The geometric parameters of AsH3 (+)X̃(2)A2 (″) as a function of inversion vibrational numbers have been determined, indicating that the geometric structure of the cation changes from near-planar to pyramidal with increasing inversion vibrational excitation. In addition to the experimental measurements, a two-dimensional theoretical calculation considering the two symmetric vibrational modes was performed to determine the energy levels of the symmetric inversion, which are in good agreement with the experimental results. The inversion vibrational energy levels of SbH3 (+)X̃(2)A2 (″) have also been calculated and are found to have much smaller energy splittings than those of AsH3 (+)X̃(2)A2 (″).

Performance of a nonempirical density functional on molecules and hydrogen-bonded complexes.

Recently, Tao and Mo derived a meta-generalized gradient approximation functional based on a model exchange-correlation hole. In this work, the performance of this functional is assessed on standard test sets, using the 6-311++G(3df,3pd) basis set. These test sets include 223 G3/99 enthalpies of formation, 99 atomization energies, 76 barrier heights, 58 electron affinities, 8 proton affinities, 96 bond lengths, 82 harmonic vibrational frequencies, 10 hydrogen-bonded molecular complexes, and 22 atomic excitation energies. Our calculations show that the Tao-Mo functional can achieve high accuracy for most properties considered, relative to the local spin-density approximation, Perdew-Burke-Ernzerhof, and Tao-Perdew-Staroverov-Scuseria functionals. In particular, it yields the best accuracy for proton affinities, harmonic vibrational frequencies, hydrogen-bond dissociation energies and bond lengths, and atomic excitation energies.

The Renner-Teller effect in HCCCl(+)(X̃(2)Π) studied by zero-kinetic energy photoelectron spectroscopy and ab initio calculations.

The spin-vibronic energy levels of the chloroacetylene cation up to 4000 cm(-1) above the ground state have been measured using the one-photon zero-kinetic energy photoelectron spectroscopic method. The spin-vibronic energy levels have also been calculated using a diabatic model, in which the potential energy surfaces are expressed by expansions of internal coordinates, and the Hamiltonian matrix equation is solved using a variational method with harmonic basis functions. The calculated spin-vibronic energy levels are in good agreement with the experimental data. The Renner-Teller (RT) parameters describing the vibronic coupling for the H-C≡C bending mode (ε4), Cl-C≡C bending mode (ε5), the cross-mode vibronic coupling (ε45) of the two bending vibrations, and their vibrational frequencies (ω4 and ω5) have also been determined using an effective Hamiltonian matrix treatment. In comparison with the spin-orbit interaction, the RT effect in the H-C≡C bending (ε4) mode is strong, while the RT effect in the Cl-C≡C bending mode is weak. There is a strong cross-mode vibronic coupling of the two bending vibrations, which may be due to a vibronic resonance between the two bending vibrations. The spin-orbit energy splitting of the ground state has been determined for the first time and is found to be 209 ± 2 cm(-1).

The Renner-Teller effect in HCCCN(+)(X̃(2)Π) studied by zero-kinetic energy photoelectron spectroscopy and theoretical calculations.

The spin-vibronic energy levels of the cyanoacetylene cation have been measured using the one-photon zero-kinetic energy (ZEKE) photoelectron spectroscopic method. All three degenerate vibrational modes showing vibronic coupling, i.e., Renner-Teller (RT) effect, have been observed. All the splitting spin-vibronic energy levels of the fundamental H-C≡C bending vibration (v5) have been determined. The spin-vibronic energy levels of the degenerate vibrational modes have also been calculated using a diabatic model in which the harmonic terms as well as all the second-order vibronic coupling terms are used. The theoretical predictions are in good agreement with the experimental data and are used to assign the ZEKE spectrum. It is found that the RT effects for the H-(CC)-CN bending (v7) and the C-C≡N bending (v6) vibrations are weak, whereas they are strong for the H-C≡C bending (v5) vibration. The cross-mode RT couplings between any of the two degenerate vibrations are strong. The spin-orbit resolved fundamental vibrational energy levels of the C≡N stretching (v2) and C-H stretching (v1) vibrations have also been observed. The spin-orbit energy splitting of the ground state has been determined for the first time as 43 ± 2 cm(-1), and the ionization energy of HCCCN is found to be 93 903.5 ± 2 cm(-1).

Photodissociation dynamics of superexcited O2: dissociation channels O(5S) vs. O(³S).

The photodissociation dynamics of O2, O2 + hυ → O((3)P) + O(2p(3)((4)S)3s, (3)S/(5)S), has been studied by combining the XUV laser pump / UV laser probe and velocity map imaging methods in the photon energy range 14.64-15.20 eV. The fragment yield spectra of O((3)S) and O((5)S) and their velocity map images have been recorded using the state-selective (1+1) REMPI method to detect the fragments. The fragment yield spectra show resolved fine structure that arises from the predissociated Rydberg states I, I' and I″ ((3)Π(Ω = 0,1,2)). The branching ratios between the two decay channels have been measured by one-photon ionization of the fragments O((3)S) and O((5)S) simultaneously. It is surprising to find that the dissociation cross sections for the production of O((5)S) are larger than, or comparable to, those of O((3)S) for the I and I' states, while the cross sections for the production of O((5)S) are smaller than those of O((3)S) for the I″ state. All fragments O((5)S) arise from perpendicular transitions, which provides direct experimental evidence about the symmetry assignments of the states I, I' and I″ excited in this energy region. Although most of the fragments O((3)S) arise from perpendicular transitions, some of them are from parallel transitions. Based on the calculated ab initio potential energy curves, we propose that the neutral dissociation into O((3)P) + O((3)S) occurs mainly via the interaction of the Rydberg states I, I', and I″ with the vibrational continuum of the diabatic 8(3)Π(u) state (1π(u)⁻¹(a4Π(u))3sσ(g), ³Π(u)), while the neutral dissociation into O((3)P) + O((5)S) occurs mainly via the interaction of Rydberg states I, I', and I″ with the diabatic 7(3)Π(u) (1π(g)⁻¹(X²Π(g))3pσ(u), ³Π(u)).