Electronic State Potential Energy Surface Research Articles

A crossed laser-molecular beam study of the one and two photon dissociation dynamics of ferrocene at 193 and 248 nm

A crossed laser-molecular beam study of the one and two photon dissociation mechanism of bis (cyclopentadienyl) iron (ferrocene, FeCp2) has been performed at 193 and 248 nm. By combining electron bombardment mass spectroscopy with time-of-flight (TOF) measurements, the photodissociation mechanism at 193 nm is shown to have two distinct mechanisms. (1) FeCp2+hν→FeCp*+Cp; (2) FeCp+2hν→FeCp+Cp, FeCp→Fe+Cp. For the first mechanism, which accounts for less than 5% of the photodissociation events, the FeCp* velocity distribution is quantitatively consistent with a statistical dissociation producing FeCp in an excited, ligand field electronic state. The velocity distributions of the Cp and Fe fragments produced by the second mechanism (FeCp is an unstable intermediate) are also in excellent agreement with microcanonical calculations for both Cp elimination steps using the known metal–ligand bond energies of ferrocene. For the second mechanism, dissociation occurs on the lowest potential energy surface for each Cp elimination. Although one photon is energetically sufficient to remove one Cp ligand from ferrocene, RRKM calculations of the lifetime indicate that Cp elimination is extremely slow for dissociation along the ground electronic state potential energy surface. Hence, after internal conversion to the ground electronic state, the large photon absorption cross section (∼4 Å2) for the experimental irradiation conditions allows additional photons to be absorbed until the dissociation rate exceeds the up pumping rate. The large photon energy causes the dissociation rate to increase by many orders of magnitude for each additional photon absorbed. Consequently, there is strong selectivity for the total number of photons absorbed. Both mechanisms, occurring on two different electronic potential energy surfaces, suggest that dissociation induced by excitation of the ligand-to-metal charge transfer states accessed at 193 nm can be quantitatively described as a statistical, unimolecular decomposition. At 248 nm, the measured product velocity distributions are qualitatively consistent with the mechanism deduced from the 193 nm results, but the energy available for translation at this wavelength is too small to extract quantitative product translational energy distributions which are required to independently test the applicability of the statistical dissociation model.

Bond breaking without barriers: Photofragmentation of ketene at the singlet threshold

Ketene (CH2CO) in a supersonic free jet was photodissociated by a tunable pulsed laser in the frequency range just above the threshold for production of singlet methylene, CH2 (ã 1A1). CH2 was detected by laser-induced fluorescence (LIF). The appearance threshold and yield curve of individual 1CH2 rotational states were obtained by scanning the photolysis laser frequency with a fixed LIF probe laser frequency. The dissociation occurs on the ground electronic state potential energy surface. The threshold for CH2CO→1CH2+CO is found to be 30 116.2±0.4 cm−1. By varying the delay between the photolysis and probe pulses, a lower bound of 7×107 s−1 was set for the dissociation rate on the triplet surface at the singlet energy threshold. The yield curves, or photofragment excitation (PHOFEX) spectra, exhibit sharp steps spaced by the CO rotational term values. The experimental data provide a rigorous test of theoretical models of photofragment dynamics. The data clearly show that nuclear spin is conserved through the photodissociation. PHOFEX curves calculated from phase space theory (PST) are in excellent agreement with the experiment and show that there is no barrier along the reaction coordinate. The singlet/triplet branching ratio as a function of photolysis laser frequency is inferred from PST fits to the PHOFEX data. Comparisons with the statistical adiabatic channel model (SACM) are also presented. The data show that the only dynamical constraints on product state rotational energy distributions are conservation of energy, angular momentum, and nuclear spin.

Analysis of the temperature dependence of the 1.DELTA. .rarw. X1.SIGMA.+ absorption spectra of carbonyl sulfide, nitrous oxide and carbon disulfide

The sum rules that apply for electronic/vibrational transitions in molecules are used to derive expressions for the mean frequency of an absorption spectrum for the case of a forbidden electronic transition that becomes allowed due to Herzberg-Teller coupling of the electronic motion with the bending vibration. The results are used to analyze data on the 225-nm system of OCS, the 182-nm system of N/sub 2/O, and the 320-nm system of CS/sub 2/, all of which are transitions of the type /sup 1/..delta.. reverse arrow X/sup 1/..sigma../sup +/. For OCS, previous results are reexamined to determine the effect of vibrational excitation of the bending mode on the mean frequency of the absorption spectrum. For N/sub 2/O, the component absorption spectra, intensities, and mean frequencies are derived from experimental data for the (00/sup 0/0) and (01/sup +/-1/0) vibrational states. Values for the curvature of the upper electronic state potential energy surface in the Franck-Condon region for the OCS and N/sub 2/O are used to construct bending mode potential energy curves which are compared with previous results for analogous transitions in isoelectronic molecules. 42 references, 4 figures, 2 tables.

ChemInform Abstract: STRUCTURE AND PREDISSOCIATION DYNAMICS OF ELECTRONICALLY EXCITED NITROGEN DIOXIDE: A RESONANCE RAMAN STUDY

Resonance Raman spectra of NO2 have been recorded as a function of excitation frequency in the range 0–8000 cm−1 above the predissociation threshold. These spectra are interpreted using the time‐dependent formulation of resonant Raman scattering, to elucidate the shape of the excited electronic state potential energy surface, and the dynamics which occur on it immediately following photoexcitation. Our results show that the dominant optical transition in this spectral range is 2B2–2A1. Our Raman spectra reveal a marked decrease in the dynamical time scale of the predissociation of the 2B2 state as excitation frequency increases. At excitation frequencies approximately 4850 cm−1 above the dissociation threshold, the 2B2 predissociative lifetime becomes comparable to a vibrational period. We have analyzed the resonance Raman spectra, along with the absorption spectrum, to determine the magnitudes of the slopes of the potential surface along the symmetric stretch and bending normal coordinates for the 2B2 ex...

Structure and predissociation dynamics of electronically excited nitrogen dioxide: A resonance Raman study

Resonance Raman spectra of NO2 have been recorded as a function of excitation frequency in the range 0–8000 cm−1 above the predissociation threshold. These spectra are interpreted using the time-dependent formulation of resonant Raman scattering, to elucidate the shape of the excited electronic state potential energy surface, and the dynamics which occur on it immediately following photoexcitation. Our results show that the dominant optical transition in this spectral range is 2B2–2A1. Our Raman spectra reveal a marked decrease in the dynamical time scale of the predissociation of the 2B2 state as excitation frequency increases. At excitation frequencies approximately 4850 cm−1 above the dissociation threshold, the 2B2 predissociative lifetime becomes comparable to a vibrational period. We have analyzed the resonance Raman spectra, along with the absorption spectrum, to determine the magnitudes of the slopes of the potential surface along the symmetric stretch and bending normal coordinates for the 2B2 excited state in the Frank–Condon region.

An ab initio calculation of the potential surface and rotation—vibration energies of the silyl radical

We have calculated 64 points on the ground electronic state potential energy surface of the silyl radical (SiH 3) using the MRD CI technique. This potential surface gives an inversion barrier of 1951 cm −1 and an equilibrium geometry of r e = 1.480 Å and α e(HSiH) = 111.2°. Using the non-rigid invertor Hamiltonian with this potential we determine for SiH 3 that ν 1 = 2424 cm −1, ν 2 = 778 cm −1, ν 3 = 2106 cm −1, and ν 4 = 976 cm −1; the inversion splitting is calculated to be 0.11 cm −1. Rotational constants and centrifugal distortion constants have also been calculated.

Ab initio rotation-vibration energies of H 3O +

For the H 3O + molecular ion 47 points on the ground electronic state potential energy surface have been computed by configuration interaction calculations including all possible single and double excitations that can be generated from a canonical set of rather accurate Hartree-Fock SCF molecular orbitals. Combining these points with a set of 14 previously published points that were calculated in exactly the same way gives a good coverage of the surface. An analytic potential function has been fitted that involves all the vibrational coordinates to these points, and this involves the least squares optimization of the values of 13 parameters in the potential function. Using the nonrigid invertor Hamiltonian with this surface the low lying rotation-vibration energies have been calculated; it is hoped that these results will be of use in the search for the spectrum of H 3O +. The equilibrium bond length is found to be 0.978 Å, the equilibrium HOH bond angle is 111.6°, and the inversion barrier is 712 cm −1.