ALBERT

Notizen: The formation of the C–H bond insertion product methylmagnesium hydride (CH3MgH) and the simultaneous emission of atomic triplet magnesium are observed following photoexcitation of the first allowed singlet resonance transition of atomic magnesium isolated in solid methane matrices at 12 K. Isotopic variation of the solid methane hosts produces observable differences in the relative branching ratios into the photophysical (atomic triplet emission) and photochemical (insertion product formation) channels. In solid perdeuteromethane (CD4), the intensity of the atomic emission is approximately five times that in solid methane (CH4) while the rate of formation of the insertion product shows the opposite behavior in the two solids. No singlet atomic magnesium emission is observed in the solid Mg/methane systems and the rise time of the atomic triplet emission is deduced to occur on a time scale of less than 10 ns. A simple model derived from spin and orbital correlations between reactants and products is presented which considers the effect of the low symmetry of an insertive reaction coordinate in the approach geometry of atomic magnesium to methane yielding the linear product CH3MgH. Using this model, the absence of the singlet atomic emission is explained in terms of the attractive nature of the singlet surface with respect to the formation of a bent, inserted intermediate. The observation of an enhanced rate of atomic magnesium intersystem crossing is thought to occur as a result of the symmetry-induced participation of the repulsive triplet surface in the process leading to the linear insertion product. The model also suggests an origin for the observed kinetic isotope effects. Differences in the observed behavior of the 1P state of atomic magnesium in gas-phase and solid-phase quenching experiments (explicitly the formation of fragmented products only with no unreacted atomic triplet in the former case and the formation of the insertion product with intense atomic triplet emission in the latter) are discussed in relation to the presence of efficient relaxation pathways in the solid phase and the absence of such pathways in the single-collision conditions of the gas-phase experiments.

Notizen: The X 1Σ+0 and the C 1Π1 states of MgAr have been characterized spectroscopically. The MgAr van der Waals molecules, created in a new laser-vaporization, pulsed supersonic jet apparatus, were studied using laser-induced fluorescence. High-resolution spectra revealed lambda-type doubling consistent with the presence of the nearby higher-lying repulsive MgAr (D 1Σ+0) electronic state.

Notizen: The metastable a˜(3Π0−) and A(3Π0+) states of the Mg(3s3p)Ar molecule were synthesized in a free-jet expansion of metastable Mg(3PJ) atoms, produced by the laser vaporization of a magnesium rod, in pure Ar. The a˜(3Π0−), A(3Π0+), and E(3∑+) states were characterized spectroscopically by laser-induced fluorescence. The 3Π(v=0) states were found to be very near the Hund's case (a) limit, with only 0.31 cm−1 separating the 3Π0+ and 3Π0− levels. The internuclear separations, re, were found to be 3.63 A(ring) and the dissociation energies 316 and 296 cm−1, respectively. The E(3∑+) state is quite deeply bound (1128 cm−1) with a much smaller re, 2.83 A(ring), and it is postulated that the Ar atom has substantially penetrated the outer lobe of the 4s Rydberg orbital of the Mg(3s4s 3S1) atomic state to which this state correlates. Despite large populations of both Mg(3P1) and Mg(3P2), neither the 3Π1 nor 3Π2 states of MgAr were observed, apparently because the molecular 3Π1,2 states are collisionally quenched to the lower-energy 3Π0 state much faster than the corresponding atomic 3P1,2 multiplets are deactivated to the 3P0 level.

Notizen: Fluorescence excitation spectra recorded for the A–X system of jet-cooled Cu2 show conclusive evidence of a ΔΛ=0 transition, and the A state is thereby definitively assigned as 1Σ+u. A previous assignment of the B state as 1Σ+u is confirmed, but the vibrational levels of this state are complicated by the presence of a perturbation at v'=0. The perturbing state does not, however, appear to be either of the two optically accessible electronic states in this spectral region. Anomalously large electronic isotope shifts are observed for the A and B states, and this behavior is discussed in terms of the correspondingly large "specific mass shifts'' observed in the optical spectra of atomic copper for transitions that couple states differing in the number of d electrons. Due to the large spin-orbit coupling constants in the "d-hole'' configurations, it is proposed that the low-energy-excited molecular states of Cu2 derived from these configurations should be described by Hund's case (c) coupling. Dynamical effects observed in the gas phase and in solid matrices are briefly discussed in terms of this bonding scheme.

Notizen: Optical absorption spectra recorded at 12 K for the 1 P–1 S transition of atomic magnesium isolated in solid methane and perdeuteromethane exhibit significant differences in their spectral positions and bandwidths. The greater blue shift in CD4 relative to CH4 was found to be unchanged upon sample annealing. Shifts in the spectral position of the resonance transition in the two solids are discussed in the context of mass related differences in the lattice parameters of these light solids which is a manifestation of their behavior as quantum solids. From the blue shift observed in CD4 , the isotope having the smaller cage size, it is proposed that repulsive forces dominate in the interaction of the atomic magnesium guest species with its host. The greater bandwidth of the absorption profile in CH4 , relative to CD4 is discussed in terms of a Jahn–Teller model where the effective frequency of the phonon modes coupling with the electronic transition of the optically active impurity are higher in the former solid.

Notizen: The visible emission resulting from photoexcitation of the first allowed singlet resonance transition of atomic magnesium isolated in the solid methanes is analyzed with spectral and time resolution. Two emission bands are observed in both solid methane (CH4) and solid perdeuteromethane (CD4) at 516 and 560 nm. The measured radiative lifetimes of the two bands are 12 and 19 ms, respectively, and independent of isotopic variation of the host. On the basis of their spectral position and radiative lifetimes, the emission features are assigned to the spin-forbidden 3P–1S intercombination band of atomic magnesium. Emission spectra recorded at various temperatures in the range 12–33 K were found to show reversible temperature dependence whereby the intensity of the higher energy 516 nm band decreased monotonically with a concomitant increase in that of the lower energy 560 nm band. Time-resolved measurements of the two bands in Mg/CH4 and Mg/CD4 show that the radiative lifetime of the 516 nm band decreases with increasing temperature while that ofthe 560 nm band remains constant but showing a rise time component. The magnitude of this rise time is correlated with that of the decreasing lifetime of the 516 nm band. With a combination of the temperature dependence observed in the radiative lifetimes and spectral intensities of the 516 and 560 nm emission bands in the Mg/CH4 and Mg/CD4 systems, dynamical information is extracted which allows for the creation of a simple kinetic model and an identification of the mechanism giving rise to this temperature dependent phenomenon. The experimental data is found to exhibit linear behavior on an Arrhenius plot indicating an activated process of population interconversion between the emitting levels. In solid methane the barrier in this activated process is 177 and 125 cm−1 in solid perdeuteromethane. The origin of this process is discussed in terms of (a) structural phase transitions of the solid methanes and (b) the effect of strong Jahn–Teller coupling between phonon modes of the solid methane host and the spin triplet electronic state of the atomic magnesium guest yielding symmetry-distinct minima on this surface. The lengthened values of the measured radiative lifetimes in the solid methanes, indicate population equilibration amongst the total degeneracy of the triplet state J sublevels of atomic magnesium in these solids. Because of the extremely low temperatures at which the experiment is conducted, extensive orbital reduction factors are implied to achieve such equilibration conditions. This is a manifestation of very strong Jahn–Teller coupling, a necessary condition for the stabilization of symmetry-distinct minima on the triplet spin state surface and the mechanism proposed to explain the presence of the temperature dependent emission features.

Notizen: Laser induced fluorescence (LIF) excitation spectra recorded for the vibrational bands in the Mg(3s3p 1P1)⋅Xe(1Π1)←Mg(3s3s 1S0)⋅Xe (X 1Σ+) system have been analyzed, yielding absolute vibrational assignments and values of ωexe=1.585±0.02 and ωe=97.5±1.0 cm−1 for the 1Π1 state of 24Mg132Xe. From a Birge–Sponer extrapolation, the well depth of this state is estimated to be 1500 cm−1. Simulations of rotationally structured spectra of three of the most intense vibrational bands are consistent with R‘e=4.56±0.12 A(ring) for the X 1Σ+ state. From Morse function extrapolation of the excited state rotational constants from the simulations, and Franck–Condon intensity simulations of the 1Π1←X 1Σ+ vibrational progressions, R'e for the 1Π1 state is estimated to be 3.07±0.10 A(ring). The 1Π1 state of MgXe fluoresces strongly. The corresponding 1Π1 states of ZnXe and CdXe do not fluoresce, but "action'' spectra from the production (via predissociation) of metal atom 3PJ states are observed. Possible reasons for these differences are discussed in terms of spin–orbit induced predissociation. It is concluded that predissociation of the MgXe(1Π1) state is not observed because the crossing between the repulsive 3∑+1 and the attractive 1Π1 potential curves does not occur until energies higher than those accessible experimentally. Possible reasons for the behavior of the diatomic MgXe(1Π1) state vs that of Mg(3s3p 1P1) isolated in solid Xe, where production of Mg(3s3p 3PJ) states competes with Mg(3s3p 1P1) fluorescence, are also discussed. Finally, the attractive "bonding'' interactions in the MgXe(1Π1) state are analyzed in terms of electrostatic interactions and compared with those for other Π-type states of metal/rare-gas van der Waals diatomic molecules.