Vibrationally autoionizing Rydberg clusters: Spectroscopy and dynamics of pyrazine–Ar and –Xe clusters

Combined Surface-Hopping, Dyson Orbital, and B-Spline Approach for the Computation of Time-Resolved Photoelectron Spectroscopy Signals: The Internal Conversion in Pyrazine

A computational protocol for simulating time-resolved photoelectron signals of medium-sized molecules is presented. The procedure is based on a trajectory surface-hopping description of the excited-state dynamics and a combined Dyson orbital and multicenter B-spline approach for the computation of cross sections and asymmetry parameters. The accuracy of the procedure has been illustrated for the case of ultrafast internal conversion of gas-phase pyrazine excited to the ¹B_2u(ππ*) state. The simulated spectra and the asymmetry map are compared to the experimental data, and a very good agreement was obtained without applying any energy-dependent rescaling or broadening. An interesting side result of this work is the finding that the signature of the ¹A_u(nπ*) state is indistinguishable from that of the ¹B_3u(nπ*) state in the time-resolved photoelectron spectrum. By locating four symmetrically equivalent minima on the lowest-excited (S₁) adiabatic potential energy surface of pyrazine, we revealed the strong vibronic coupling of the ¹A_u(nπ*) and ¹B_3u(nπ*) states near the S₁ ← S₀ band origin.

Vibrational autoionization of state-selective jet-cooled methanethiol (CH3SH) investigated with infrared + vacuum-ultraviolet photoionization

Vibrational autoionization of Rydberg states provides key information about nonadiabatic processes above an ionization threshold. We employed time-of-flight mass detection of CH₃SH⁺ to record vibrational-state selective photo-ionization efficiency (PIE) spectra of jet-cooled methanethiol (CH₃SH) on exciting CH₃SH to a specific vibrationally excited state with an infrared (IR) laser, followed by excitation with a tunable laser in the vacuum-ultraviolet (VUV) region for ionization. Autoionizing Rydberg states assigned to the ns, np, nd and nf series are identified. When IR light at 2601 (ν₃, SH stretching mode) and 2948 cm^-1 (ν₂, CH₃ symmetric stretching mode) was employed, the Rydberg series converged to the respective vibrationally excited (ν₃ and ν₂) states of CH₃SH⁺. When IR light at 3014 cm^-1 (overlapped ν₁/ν₉, CH₃ antisymmetric stretching and CH₂ antisymmetric stretching modes) was employed, Rydberg series converging to two vibrationally excited states (ν₁ and ν₉) of CH₃SH⁺ were observed. When IR light at 2867 cm^-1 (2ν₁₀, overtone of CH₃ deformation mode) and 2892 cm^-1 (2ν₄, overtone of CH₂ scissoring mode) was employed, both Δν = -1 and Δν = -2 ionization transitions were observed; there is evidence for direct ionization from the initial state into the CH₃SH⁺ (ν₄⁺ = 1) continuum. In all observed IR-VUV-PIE spectra, the ns and nd series show intensity greater than the other Rydberg series, which is consistent with the fact that the highest-occupied molecular orbital of CH₃SH is a p-like lone pair orbital on the S atom. The quantum yields for autoionization of various vibrational excited states are discussed. Values of ν₁ = 3035, ν₂ = 2884, ν₃ = 2514, and ν₉ = 2936 cm^-1 for CH₃SH⁺ derived from the converged limits agree satisfactorily with values observed for Ar-tagged CH₃SH⁺ at 3026, 2879, 2502, and 2933 cm^-1.

Mass-Analyzed-Threshold-Ionization-Spectroscopy of Pyrazine and Pyrazine-Ar

Vibrational spectra of the pyrazine and the pyrazine-Ar cation in the electronic ground state (D 0) have been measured via several intermediate states of the first excited state (S 1) by mass-analyzed threshold ionization spectroscopy. Additionally, ab-initio calculations at the DFT, CIS and MP2 level of theory have been conducted to compare experimental findings with theory. For the pyrazine monomer a negative anharmonic behavior for combination bands containing the 16a or the 1 vibration could be observed for the first time. It was found that the 16a vibration has to be considered when discussing the nature of the vibronic coupling in the first excited state (S 1) as well as in the ionic ground state (D 0). For some vibrations a new assignment is given in the first excited state (S 1) and the ionic ground state (D 0). An earlier discussed appearance of internal vibrational redistribution (IVR) by activating the 516a1 vibrational state in the first electronically excited state (S 1) is rejected due to the results of the presented measurements.

For the pyrazine-Ar complex the ionization energy, as well as the binding energy in the ionic and the neutral ground state could be determined. These values are given by 74632 ± 5 cm-1, 630 cm-1 ± 20 cm-1 and 349 cm-1 ± 20 cm-1, respectively. The van-der-Waals vibrations in the first excited state could be reassigned and the frequencies of these vibrations have been found to be 12 cm-1, 27 cm-1 and 39 cm-1.

Pyrazine in supercritical xenon: local number density defined by experiment and calculation

Toward our goal of using supercritical fluids to study solvent effects in physical and chemical phenomena, we develop a method to spatially define the solvent local number density at the solute in the highly compressible regime of a supercritical fluid. Experimentally, the red shift of the pyrazine n-pi* electronic transition was measured at high dilution in supercritical xenon as a function of pressure from 0 to approximately 24 MPa at two temperatures: one (293.2 K) close to the critical temperature and the other (333.2 K) remote. Computationally, several representative stationary points were located on the potential surfaces for pyrazine and 1, 2, 3, and 4 xenons at the MP2/6-311++G(d,p)/aug-cc-pVTZ-PP level. The vertical n-pi* ((1)B(3u)) transition energies were computed for these geometries using a TDDFT/B3LYP/DGDZVP method. The combination of experiment and quantum chemical computation allows prediction of supercritical xenon bulk densities at which the pyrazine primary solvation shell contains an average of 1, 2, 3, and 4 xenon molecules. These density predictions were achieved by graphical superposition of calculated shifts on the experimental shift versus density curves for 293.2 and 333.2 K. Predicted bulk densities are 0.50, 0.91, 1.85, and 2.50 g cm(-3) for average pyrazine primary solvation shell occupancy by 1, 2, 3, and 4 xenons at 293.2 K. Predicted bulk densities are 0.65, 1.20, 1.85, and 2.50 g cm(-3) for average pyrazine primary solvation shell occupancy by 1, 2, 3, and 4 xenons at 333.2 K. These predictions were evaluated with classical Lennard-Jones molecular dynamics simulations designed to replicate experimental conditions at the two temperatures. The average xenon number within 5.0 A of the pyrazine center-of-mass at the predicted densities is 1.3, 2.1, 3.0, and 4.0 at both simulation temperatures. Our three-component method-absorbance measurement, quantum chemical prediction, and evaluation of prediction with classical molecular dynamics simulation-therefore has a high degree of internal consistency for a system in which the intermolecular interactions are dominated by dispersion forces.