Xiaoguo Zhou

The Microscopic Structure of Liquid Methanol from Raman Spectroscopy

The microscopic structure of liquid methanol has been systematically investigated with Raman spectroscopy in a temperature range of 15-55 degrees C. The unbonded free -OH stretching vibrational band has been observed at approximately 3660 cm(-1) in pure liquid. With the aid of depolarization measurements and theoretical calculations, four featured spectral components have been unambiguously identified and assigned to four well-defined vibrational modes of clusters in chain or ring forms. Furthermore, the cluster size distribution and its temperature dependence have been derived from the spectral fittings for the first time, which lead to the conclusion that the trimer, tetramer, and pentamer are the dominant clusters in liquid methanol, taking up more than 50% of total clusters.

A threshold photoelectron-photoion coincidence spectrometer with double velocity imaging using synchrotron radiation

A novel threshold photoelectron-photoion coincidence (TPEPICO) imaging spectrometer at the U14-A beamline of the Hefei National Synchrotron Radiation Laboratory is presented. A set of open electron and ion lenses are utilized to map velocity imaging of photoelectrons and photoions simultaneously, in which a repelling electric field using an extra lens is applied to magnify images of photoelectrons instead of traditional accelerating electric field in order to suppress the contribution of energetic electrons in the threshold photoelectron spectroscopy (TPES) and the mass-selected TPEPICO spectroscopy. The typical energy resolution of TPES is measured to be 9 meV (full width at half maximum), as shown on the (2)P(1/2) ionization of argon. The measured mass resolving power for the present TPEPICO imaging spectrometer is above 900 of M/DeltaM. Subsequently as a benchmark, oxygen molecule is photoionized by monochromatic synchrotron radiation at 20.298 eV and dissociates to an oxygen atomic ion and a neutral oxygen atom, and the translation energy distribution of oxygen atomic ion is measured by the time-sliced imaging based on mass-selected TPEPICO experiment. The kinetic energy resolution of the present ion velocity imaging is better than 3% of DeltaE/E.

Complete Raman Spectral Assignment of Methanol in the C–H Stretching Region

In this work, the Raman spectrum of gaseous methanol in the C-H stretching region was investigated by polarized Photoacoustic Raman spectroscopy (PARS). On the basis of the depolarization ratio measurement and density functional theory (DFT) calculations, a complete spectral assignment has been presented. The band at ~2845 cm(-1) was assigned to CH3 symmetric stretching, the bands at ~2925 and ~2955 cm(-1) were assigned to two Fermi resonance modes of CH3 bending overtones, and the bands at ~2961 and ~3000 cm(-1) were assigned to out-of-plane and in-plane vibrations of splitting CH3 antisymmetric stretching. Such assignments can clarify the confusions among the previous spectral studies from the different experimental methods and be confirmed by the Raman spectrum of liquid methanol. Furthermore, the large splitting of 39 cm(-1) between two antisymmetric stretching in gaseous methanol was ascribed to the strong coupling between CH3 and OH groups within methanol molecule because it decreased rapidly in other long-chain alcohol, such as CH3CD2OH.

Identification of alcohol conformers by Raman spectra in the C-H stretching region.

The spontaneous polarized Raman spectra of normal and deuterated alcohols (C2-C5) have been recorded in the C-H stretching region. In the isotropic Raman spectra, a doublet of -CαH stretching vibration is found for all alcohols at below 2900 cm(-1) and above 2950 cm(-1). By comparing the experimental and calculated spectra of various deuterated alcohols, the doublets are attributed to the -CαH stretching vibration of different conformers. For ethanol, the band observed at 2970 cm(-1) is assigned as the stretching vibration of -CαH in the Cα-O-H plane of the gauche-conformer, while the band at 2895 cm(-1) is contributed from both the -CαH2 symmetrical stretching vibration of the trans-conformer and the -CαH stretching vibration out of the Cα-O-H plane of the gauche-conformer. The population of gauche-conformer is estimated to be 54% in liquid ethanol. For the larger alcohols, the same assignments for the doublet are obtained, and the populations of gauche-conformers with plane carbon skeleton are found to be slightly larger than that of ethanol, which is consistent with results from molecular dynamics simulations.

Dissociation of Vibrational State-Selected O2+Ions in the B2Σg¯State Using Threshold Photoelectron–Photoion Coincidence Velocity Imaging

Using the recently developed threshold photoelectron-photoion coincidence (TPEPICO) velocity imaging mass spectrometer (Tang et al. Rev. Sci. Instrum.2009, 80, 113101), dissociation of vibrational state-selected O(2)(+)(B(2)Σ(g)(¯), v(+) = 0-6) ions was investigated. Both the speed and angular distributions of the O(+) fragments dissociated from individually vibronic levels of the B(2)Σ(g)(¯) state were obtained directly from the three-dimensional time-sliced TPEPICO velocity images. Two dissociation channels, O(+)((4)S) + O((3)P) and O(+)((4)S) + O((1)D), were respectively observed, and their branching ratios were found to be heavily dependent on the vibrational states. A new intersection mechanism was suggested for the predissociation of O(2)(+)(B(2)Σ(g)(¯)) ions, especially for dissociation at the energy of the v(+) = 4 level. In addition, the anisotropic parameters for O(+) fragments from different dissociative pathways were determined to be close to zero, indicating that the v(+) = 0-6 levels of B(2)Σ(g)(¯) predissociate on a time scale that is much slower than that of molecular rotation.

NO+ formation pathways in dissociation of N2O+ ions at the C2Σ+ state revealed from threshold photoelectron-photoion coincidence velocity imaging.

Using the novel threshold photoelectron-photoion coincidence (TPEPICO) velocity imaging technique, the dissociative photoionization of N(2)O molecule via the C(2)Σ(+) ionic state has been investigated. Four fragment ions, NO(+), N(2)(+), O(+), and N(+), are observed, respectively, and the NO(+) and N(+) ions are always dominant in the whole excitation energy range of the C(2)Σ(+) ionic state. Subsequently, the TPEPICO three-dimensional time-sliced velocity images of NO(+) dissociated from the vibrational state-selected N(2)O(+)(C(2)Σ(+)) ions have been recorded. Thus the kinetic and internal energy distributions of the NO(+) fragments have been obtained directly as the bimodal distributions, suggesting that the NO(+) fragments are formed via both NO(+)(X(1)Σ(+)) + N((2)P) and NO(+)(X(1)Σ(+)) + N((2)D) dissociation channels. Almost the same vibrational population reversions are identified for both dissociation pathways. Interestingly, the obtained branching ratios of the two channels exhibit some dependence on the excited vibrational mode for N(2)O(+)(C(2)Σ(+)), in which the excited asymmetrical stretching potentially promotes dissociation possibility along the NO(+)(X(1)Σ(+)) + N((2)D) pathway. In addition, the measured anisotropic parameters of NO(+) are close to 0.5, indicating that the C(2)Σ(+) state of N(2)O(+) is fully predissociative, indeed, with a tendency of parallel dissociation, and therefore, the corresponding predissociation mechanisms for the N(2)O(+)(C(2)Σ(+)) ions are depicted.

Dissociation limit and dissociation dynamic of CF4+: Application of threshold photoelectron-photoion coincidence velocity imaging

Dissociation of internal energy selected CF4(+) ions in an excitation energy range of 15.40-19.60 eV has been investigated using threshold photoelectron-photoion coincidence (TPEPICO) velocity imaging. Only CF3(+) fragment ions are observed in coincident mass spectra, indicating all the X(2)T1, A(2)T2, and B(2)E ionic states of CF4(+) are fully dissociative. Both kinetic energy released distribution (KERD) and angular distribution in dissociation of CF4(+) ions have been derived from three-dimensional TPEPICO time-sliced images. A parallel distribution of CF3(+) fragments along the polarization vector of photon is observed for dissociation of CF4(+) ions in all the low-lying electronic states. With the aid of F-loss potential energy curves, dissociation mechanisms of CF4(+) ions in these electronic states have been proposed. CF4(+) ions in both X(2)T1 and A(2)T2 states directly dissociate to CF3(+) and F fragments along the repulsive C-F coordinate, while a two-step dissociation mechanism is suggested for B(2)E state: CF4(+)(B(2)E) ion first converts to the lower A(2)T2 state via internal conversion, then dissociates to CF3(+) and F fragments along the steep A(2)T2 potential energy surface. In addition, an adiabatic appearance potential of AP0(CF3(+)∕CF4) has also been established to be 14.71 ± 0.02 eV, which is very consistent with the recent calculated values.

Predissociation dynamics of N(2)O(+) at the A (2)Sigma(+) state: Three pathways to form NO(+)((1)Sigma(+)) revealed from ion velocity imaging.

The predissociation dynamics of nitrous oxide ion (N(2)O(+)) at its first excited state A (2)Sigma(+) has been investigated with ion velocity imaging by probing the NO(+) fragments. The parent ion N(2)O(+), prepared at the ground state X (2)Pi(000) through (3+1) resonance-enhanced multiphoton ionization (REMPI) of jet-cooled N(2)O molecules at 360.55 nm, was excited to different vibrational levels of the A (2)Sigma(+) state in a wavelength range of 280-320 nm, and then predissociated to form NO(+) and N fragments. The internal energy distribution of the NO(+) fragment was determined from its ion velocity images. With the help of potential energy surfaces (PESs) of N(2)O(+), three dissociation pathways have been proposed to interpret the three kinds of NO(+) fragments with different internal state distributions: (1) the A (2)Sigma(+) state couples to a dissociative 1 (4)Sigma(-) state via a bound 1 (4)Pi state to form the NO(+)+N((4)S) channel, where NO(+) fragment is rotationally hot; (2) the A (2)Sigma(+) state couples to dissociative states (2)Sigma(-)/(2)Delta via the 1 (4)Pi state to form the NO(+)+N((2)D) channel, where NO(+) fragment is also rotationally hot; (3) the A (2)Sigma(+) state couples to the high energy region of the ground state X (2)Pi and then dissociates to form the NO(+)+N((2)D) channel, where NO(+) is rotationally cold.

Investigation of the Mechanism of the Reaction between Atomic Oxygen Radical Anion and Benzene

Abstract The reaction mechanism between atomic oxygen radical anion and benzene has been investigated using the density functional theory (DFT). Geometries of the reactants, products, complexes, and transition states involved have been optimized at the B3LYP/6-31+G(d, p) level, and their vibrational frequencies and zero-point energies (ZPEs) have been calculated subsequently at the same level. The multichannel pathways, e.g., the H atom abstraction, oxide ion formation, H2+ transfer, and proton transfer, are confirmed by the calculated potential energy surface of this reaction. Based on the G2MP2 energies, a reasonable description has been proposed qualitatively to explain the inconsistency of previous experimental conclusions.

Identification of Free OH and its Implication on Structural Changes of Liquid Water

The molecular structure of liquid water has been an outstanding issue for many years. The identification of free −OH holds the key in differentiating structure models for liquid water. By analyzing the relative changes of the intensity and depolarization ratio in temperature dependent Raman spectra, the occurrence of free −OH in liquid water is unambiguously determined. Furthermore, upon the increase of temperature from 5 °C to 85 °C, the structure of liquid water undergoes significant change, but the relative proportion of free −OH is considerably small and remains almost unchanged. This implies that the breaking of hydrogen bond from the tetrahedral structure prefers to occur at the site of the hydrogen acceptor. The energetic favoring of the structural change for liquid water is thus clearly revealed from experiments.