Ground-state Neutral Molecules Research Articles

Nonadiabatic nuclear dynamics of the ammonia cation studied by surface hopping classical trajectory calculations.

The Landau-Zener (LZ) type classical-trajectory surface-hopping algorithm is applied to the nonadiabatic nuclear dynamics of the ammonia cation after photoionization of the ground-state neutral molecule to the excited states of the cation. The algorithm employs a recently proposed formula for nonadiabatic LZ transition probabilities derived from the adiabatic potential energy surfaces. The evolution of the populations of the ground state and the two lowest excited adiabatic states is calculated up to 200 fs. The results agree well with quantum simulations available for the first 100 fs based on the same potential energy surfaces. Three different time scales are detected for the nuclear dynamics: Ultrafast Jahn-Teller dynamics between the excited states on a 5 fs time scale; fast transitions between the excited state and the ground state within a time scale of 20 fs; and relatively slow partial conversion of a first-excited-state population to the ground state within a time scale of 100 fs. Beyond 100 fs, the adiabatic electronic populations are nearly constant due to a dynamic equilibrium between the three states. The ultrafast nonradiative decay of the excited-state populations provides a qualitative explanation of the experimental evidence that the ammonia cation is nonfluorescent.

Assigning Photoelectron Spectra of Transition Metal Organometallic Complexes on the Basis of Kohn−Sham Orbital Energies

Kohn−Sham orbital energies may be interpreted as approximate vertical ionization potentials. A new method is proposed to correct the 2−3 eV difference in the measured and predicted vertical ionization potentials produced by standard DFT functionals and basis sets. Negatives of Kohn−Sham orbital energies are shifted so that the orbital energy of the HOMO matches the negative of the first vertical ionization energy obtained as a difference in the energy of the ground-state ion and ground-state neutral molecule at the equilibrium geometry of the latter. This way, photoelectron spectra can be predicted without prior experimental information. Standard DFT functionals (B3LYP, B3PW91, BLYP, B3P86) and basis sets (LanL2DZ, 6-311G**, TZVP) were utilized to probe this method for transition metal carbonyl complexes. At the B3LYP/TZVP level, the first vertical ionization energies were computed with an average difference of 0.00 ± 0.22 eV. A good accordance between the predicted and the experimental ionization energies was achieved, and the orderings of MOs were in line with the most reliable assignments available in the literature.

Fitting atomic correlation parameters for RECEP (rapid estimation of correlation energy from partial charges) method to estimate molecular correlation energies within chemical accuracy

The accuracy of the RECEP method (Chem Phys 1997, 224, 33 and Chem Phys Lett 1999, 307, 469) has been increased considerably by the use of fitted atomic correlation parameters. This method allows an extremely rapid, practically prompt calculation of the correlation energy of molecules after an HF-SCF calculation. The G2 level correlation energy and HF-SCF charge distribution of 41 closed-shell neutral molecules (composed of H, C, N, O, and F atoms) of the G2 thermochemistry database were used to obtain the fitted RECEP atomic correlation parameters. Four different mathematical definitions of partial charges, as a multiple choice, were used to calculate the molecular correlation energies. The best results were obtained using the natural population analysis, although the other three are also recommended for use. For the 41 molecules, the G2 results were approached within a 1.8 kcal/mol standard deviation (the mean absolute difference was 1.5 kcal/mol). The RECEP atomic correlation parameters were also tested on a different, nonoverlapping set of other 24 molecules from the G2 thermochemistry database. The G2 results of these 24 molecules were approached within a 2.3 kcal/mol standard deviation (the mean absolute difference was 1.9 kcal/mol). This method is recommended to estimate total correlation energies of closed shell ground-state neutral molecules at stationary (minimums and transition states) points on the potential surface. Extension of the work for charged molecules, radicals, and molecules containing

Quantum-resolved angular distributions of neutral products in electron-stimulated processes: NO desorption from and NO 2 dissociation on Pt(111)

We present the first quantum-resolved angular distributions of ground-state neutral molecules which are products of electron stimulated desorption (ESD) and electron stimulated dissociation. Laser resonance-enhanced multiphoton ionization (REMPI) and two-dimensional imaging have been used to obtain angular distributions of NO desorbed by 350 eV electrons from O-precovered Pt(111). In a similar fashion, we have measured angular distributions for the NO product of NO 2 dissociation on clean and O-precovered Pt(111). In all cases, we observe narrow widths which are roughly the same as ion distributions determined by ESDIAD (ESD ion angular distributions). The angular distribution for NO ESD is sharply peaked (7° half-width at half maximum) along the surface normal for an O coverage (θ o) of 0.25 monolayer (ML). The angular distribution of the NO product from dissociation of side-bonded NO 2 on clean Pt(111) is unexpectedly peaked about the surface normal, and thus does not reflect dissociative forces parallel to the surface or the ∼ 25° off-normal ground-state bond direction. On O-precovered Pt(111), where NO 2 is N-bonded, ∼ 6° off-normal beams are observed. When the substrate is precovered with θ o > 0.5 ML, local disorder creates asymmetric site geometries which result in multiple peaked angular distributions with both normal and off-normal (∼9–10°) components; similar effects for NO ESD are observed. In all these studies, the NO angular distributions are invariant to rotational or vibrational state. This implies that the lateral translational degrees of freedom are essentially de-coupled from the internal modes of the molecule. The results are discussed in terms of desorption mechanisms, dissociative forces, site geometries, and disordered coadsorbate layers.

Multiphoton ionization of N_2 by the third harmonic of a Nd:YAG laser: a new avenue for air diagnostics

Ionization of N(2) by the third harmonic of a Nd:YAG laser is demonstrated and confirmed experimentally from both spectral and ion-collection measurements. Six photons at 355 nm excite the ground-state neutral molecule to the B(2) summation operator(+)(u) (upsilon' = 8) ionized state. Intense radiation at 391.4 nm associated with the B(2) summation operator(+)(u) (upsilon = 0) ? X(2) summation operator(+)(g) (upsilon'' = 0) transition was detected. The intensity of this line scales quadratically with the laser power. An ion concentration of 3.25 x 10(13) ions/cm(3) was detected after irradiation by a 45-mJ laser pulse focused to a diameter of 17 microm. The potential application of this ionization scheme for air velocity and temperature measurements is presented.

Molecular charge transfer: Experimental and theoretical investigation of the role of incident-ion vibrational states in N2+–N2 and CO+–CO collisions

The effect of reactant-ion vibrational energy on total charge-transfer cross sections has been examined for 0.03–2.20 keV N2+–N2 and CO+–CO collisions. A multistate impact parameter treatment was applied to the examination of charge transfer between the above ions occupying various vibrational levels and ground-state neutral molecules. The resulting first-order coupled equations were solved numerically, and convergence of the calculated cross sections was achieved by systematic introduction of additional states to the wavefunction expansion for the total system. The calculated cross sections were weighted according to the vibrational distribution present in the laboratory reactant-ion beam formed by electron impact and the results compared with experiment in which we employed time-of-flight techniques to measure the forward-scattered neutral N2 and CO products. Reactant-ion vibrational state distributions were varied by changing the ionizing electron beam energy in a controlled electron-impact mass spectrometer ion source. The vibrational state population of the reactant-ion beam was estimated from absolute excitation cross sections spectroscopically measured for electron-impact ionization of the above molecules, the squares of the overlap integrals of the respective ground and excited ionized states (where needed), and total electron-impact cross sections for molecular ion formation. The major contribution to the charge-transfer cross sections arises from those reaction channels with large vibrational overlaps and small energy defects with respect to the initial channel. Multistate cross sections for those channels involving near-reasonant states with favorable vibrational overlaps are found to be closely approximated by the low-velocity limit proposed by Bates and Reid. Measured cross sections and their dependence on reactant-ion vibrational state distributions are correctly predicted by the multistate model below approximately 1.5 keV ion kinetic energy. There are, however, indications that competitive, inelastic, electron-transfer processes tend to occur at higher kinetic energies.