Volume 90, Issue 4, 15 February 1989
Index of content:

Infrared–infrared double resonance investigation of the N=2 and N=3 stretching manifolds of silane and the local mode model
View Description Hide DescriptionWe report on our IR–IR double resonanceexperiment study of the excited stretching state of silane involving two or three vibrational quanta, and on the interpretation of our results by using the local mode model. The DR spectra were recorded by means of two separately tunable IR beams, one of which was used to pump selected lines inside the υ_{3} or 2υ_{3} IR bands, and the other (the probe) to observe transitions from the pumped vibration–rotation states. We characterized 7 out of the 14 vibrational states belonging to these manifolds for the first time, and the A _{1}+F _{2} diad localized previously at 6362 cm^{−} ^{1} has been studied with much higher accuracy than before. Only the two highest states of the N=3 manifold are still unknown. The vibrational energies are very well reproduced by the local mode model (≂1 cm^{−} ^{1} rms deviation), and the harmonic interbond couplings have been determined more precisely than before. The rotational structures of the excited states are also well accounted for by Champion’s theory of vibration–rotation coupling in T _{ d }, adapted to the description of stretching vibrations in terms of local modes. More high resolution (0.01 cm^{−} ^{1} or better) data are needed however to fully assess this point.

A complete set of elastic constants of crystalline anthracene by Brillouin scattering
View Description Hide DescriptionAll 13 elastic constants of a vapor grown, uncut, anthracenesingle crystal were determined from acoustic phononvelocities obtained by the Brillouin scattering method. The phononvelocities are plotted for three crystallographic planes containing the crystal axis. The relationships between phononvelocities and lattice dynamics are discussed. A minimizing procedure is introduced for converting phononvelocities of low symmetry systems into elasticity coefficients. This is shown to have several advantages over previous methods used. The results are compared with those obtained by previous studies of anthraceneelastic constants.

Vibrational exchange upon interconversion tunneling in (HF)_{2} and (HCCH)_{2}
View Description Hide DescriptionModel calculations are presented to interpret the large H–F and C–H stretching vibrational dependencies of the interconversion tunneling splittings and the corresponding infrared vibrational‐tunneling state selection rules in (HF)_{2} and (HCCH)_{2}. The model consists of two potential curves in the tunneling coordinate, coupled by an interaction term that allows the vibrational excitation to be exchanged between the two monomer units, permitting tunneling to occur. The interaction term is approximated by resonant infrared transition–dipole coupling. The magnitudes of the calculated vibrational dependencies, their isotopic shifts, and the predicted selection rules are in agreement with previous experimental observations.

Infrared‐microwave double resonance and diode laser spectroscopy of the ν_{1}/ν_{3} dyad of SnH_{4}
View Description Hide DescriptionThe interacting ν_{1} and ν_{3} bands of SnH_{4} have been studied by infrared absorption and infrared‐microwave double resonance spectroscopy. The infrared spectrum has been taken with Doppler‐limited resolution using a tunable diode laser. Pure rotational transitions within the ν_{3} state and rovibrational transitions between the ν_{1} and ν_{3} state have been observed by infrared‐microwave double resonance. The infrared and microwave data have been fitted simultaneously to the ν_{1} and ν_{3} Hamiltonian coupled by one vibration–rotation interaction term. Seventeen spectroscopic constants have been determined for each of the five most abundant isotopic species.

Broadening and shifting of the Raman Q branch of HD
View Description Hide DescriptionThe line broadening and shifting of the vibrational Q branch in pure HD has been measured for transitions J=0 to 3 at room temperature over the density range 0.8 to 10.6 amagat. The shifting and broadening coefficients have been determined with an uncertainty of ±0.2×10^{−3} cm^{−1} /amagat, which now provides a discriminating test for various semiclassical and quantal theoretical calculations. The line broadening coefficients are compared with linewidth data from other spectroscopic branches and with measurements of the rates of state‐to‐state rotational energy transfer. Use of an exponential gap law for the rates of rotational energy transfer allows estimates to be made of the contributions to the linewidths from rotationally inelastic, elastic vibrational dephasing, and elastic reorientation processes. This analysis suggests that rotational energy transfer occurs approximately 30% faster in v=1 than in v=0.

Extended measurements of the ν_{2} band of CD_{3} and the determination of the vibrational potential function for methyl
View Description Hide DescriptionMeasurements in the ν_{2} (umbrella) vibrational spectrum of CD_{3} are reported. The data include transitions up to and including the v _{2}=4–3 bands and cover a wider range of rotational excitation than has been previously reported [Frye, Sears, and Leitner, J. Chem. Phys. 8 8, 5300 (1988)]. The spectroscopic data have been combined with all previously published data pertaining to the methyl radical and an estimate of the vibrational potential function for the species has been made. From this, we predict the positions of so far unobserved vibrational bands for all the various symmetric isotopic modifications and also extract an equilibrium structure. Preliminary results on the population distribution of v _{2} levels produced on the photofragmentation of acetone‐d _{6} at 193 nm are reported and it is suggested that the methyl v _{3}=1 and v _{3}=0 levels may be produced in this reaction with a population inversion.

Charge transfer states in phenothiazine crystal
View Description Hide DescriptionObservation of charge transfer states in phenothiazine crystal which have lower energies than Frenkel excitons is reported. Two weak absorption peaks are observed at 15 K around 420 nm (2.9 eV) in the absorption edge, the absorption coefficient of which can be modulated up to a few percent by an external electric field. The modulation is so large, because the transitions are not overlapped with strong exciton absorption bands and susceptible to direct observation. The final states of the transitions have large dipole moments (∼50 D). The conductivity band gap is estimated to be 3.35 eV.

A laser spectroscopic study of the X ^{1}Σ^{+} _{0}−C ^{ 1}Π_{1} transition of MgAr: Evidence for Λ‐type doubling
View Description Hide DescriptionThe X ^{1}Σ^{+} _{0} and the C ^{1}Π_{1} 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 ^{1}Σ^{+} _{0}) electronic state.

Single crystal electron paramagnetic resonance spectra of triplet excitons in [(Cp*Ru)_{2}(η^{6},η^{6}‐[2_{2}](1,4)cyclophane)^{2} ^{+}][(TCNQ)^{2−} _{4} ]
View Description Hide DescriptionSingle crystals of [(Cp*Ru)_{2}(η^{6},η^{6}‐[2_{2}](1,4)cyclophane)^{2} ^{+}][(TCNQ)^{2−} _{4}] (Cp*=η^{5}‐C_{5}Me_{5}, TCNQ=tetracyanoquinodimethane) have been examined by EPRspectroscopy at temperatures down to 4 K. The anisotropic spectrum measured in the 77–150 K range is characteristic of an excitonic triplet species (I). This behavior is attributed to interactions between electrons located in one of the TCNQ acceptor stacks present in the title compound. The direction of maximum electronic interaction coincides with the stacking axis of the crystals (the crystallographic a axis). Intensity measurements establish that I is a thermally excited triplet with a J=560±30 cm^{−} ^{1}, exhibiting zero field splittings of ‖D‖=187 MHz (66G) and ‖E‖=32 MHz (10 G). A second although weaker feature in the spectrum is tentatively assigned to a second triplet (species II) having very much smaller values of ‖D‖ and J than those of I. Species II is attributed to an impurity present in very low concentrations.

An improved algorithm for reaction path following
View Description Hide DescriptionA new algorithm is presented for obtaining points on a steepest descent path from the transition state of the reactants and products. In mass‐weighted coordinates, this path corresponds to the intrinsic reaction coordinate. Points on the reaction path are found by constrained optimizations involving all internal degrees of freedom of the molecule. The points are optimized so that the segment of the reaction path between any two adjacent points is given by an arc of a circle, and so that the gradient at each point is tangent to the path. Only the transition vector and the energy gradients are needed to construct the path. The resulting path is continuous, differentiable and piecewise quadratic. In the limit of small step size, the present algorithm is shown to take a step with the correct tangent vector and curvature vector; hence, it is a second order algorithm. The method has been tested on the following reactions: HCN→CNH, SiH_{2}+H_{2}→SiH_{4}, CH_{4}+H→CH_{3}+H_{2}, F^{−}+CH_{3}F→FCH_{3}+F^{−}, and C_{2}H_{5}F→C_{2}H_{4}+HF. Reaction paths calculated with a step size of 0.4 a.u. are almost identical to those computed with a step size of 0.1 a.u. or smaller.

Evidence for valence hole localization in the Auger decay and fragmentation of carbon and silicon tetrafluorides
View Description Hide DescriptionMonochromatic synchrotron radiation was used to excite selectively core electrons of the carbon and fluorine atoms in carbon tetrafluoride and silicon and fluorine in silicon tetrafluoride. The fragmentation processes were examined using time‐of‐flight mass spectroscopy. The mass spectra show the distribution of ions collected in coincidence with low and high energyelectrons. Distinct changes in the mass spectra with atomic site of excitation and photonenergy are observed. The observation of F^{2} ^{+} ions following fluorine 1s excitation in SiF_{4} provides significant evidence for a ‘‘valence bond depopulation’’ mechanism involving the formation of a localized, two‐hole final state that persists on the time scale of fragmentation. In contrast, no F^{2} ^{+} was observed for CF_{4}, which indicates that fragmentation for this molecule is more characteristic of a delocalized two‐hole state.

Quenching of N_{2}(a ^{1}Π_{ g }, v’=0) by N_{2}, O_{2}, CO, CO_{2}, CH_{4}, H_{2}, and Ar
View Description Hide DescriptionWe have determined quenching rate coefficients for N_{2}(a ^{1}Π_{ g }, v’=0) by N_{2}, O_{2}, H_{2}, CO_{2}, Ar, CH_{4}, and CO using two‐photon laser excitation. Quenching by N_{2} appears to proceed via collisional coupling to N_{2}(a’ ^{1}Σ^{−} _{ u }, v=0) with a rate coefficient of 2.2±0.2×10^{−} ^{1} ^{1} cm^{3} molecule^{−} ^{1} s^{−} ^{1}. Quenching by Ar is nearly as efficient. Gas‐kinetic rate coefficients are obtained for quenching by CO, O_{2}, H_{2}, CO_{2}, and CH_{4}. Collisional energy transfer from N_{2}(a,0) to CO(A) is observed in these experiments. However, quenching by O_{2}, H_{2}, CO_{2}, and CH_{4} is thought to be reactive.

Vibrational relaxation of OH (X ^{2}Π_{ i }, v=2)
View Description Hide DescriptionVibrational relaxation rates for the v=2 level of the X ^{2}Π_{ i } state of the OH radical have been measured in a low pressure flow system, using a novel two‐laser pump‐and‐probe technique. The OH is prepared in the v=2 level by overtone pumping (2←0) and monitored by ultraviolet laser‐induced fluorescence in the (1,2) band of the A–X system. Scanning the time delay between the lasers at a given collider pressure produces exponential decay whose rate as a function of collider pressure yields the rate constant. We determine values (all cm^{3} s^{−} ^{1} units) for NH_{3}: (1.20±0.15)×10^{−} ^{1} ^{0}; CH_{4}: (2.3±0.2)×10^{−} ^{1} ^{2}; CO_{2}: (6.7±1.1)×10^{−} ^{1} ^{3}; N_{2}O: (4.6±0.6)×10^{−} ^{1} ^{3}; O_{2}: (2.6±0.54)×10^{−} ^{1} ^{3}; N_{2} and H_{2}: ≤10^{−} ^{1} ^{4}. Except for ammonia, these are two to three orders of magnitude smaller than those measured for relaxation of v=1 in the A ^{2}Σ^{+}excited state of OH, where attractive forces appear to play a role.

The argon hydrogen–fluoride differential scattering cross section
View Description Hide DescriptionThe total differential cross section for Ar–HF was measured at a collision energy of 1637 K (141 meV). Although diffraction oscillations were not resolved, a broad primary rainbow peak was observed. Scattering calculations were carried out for the Ar–HF interaction potentials developed by Douketis e t a l. and Hutson and Howard. The spherical potential and infinite order sudden approximations do not give a good description of the total differential scattering. The final‐l labeled coupled states approximation, on the other hand, is in good agreement with the more accurate close coupled approximation. Neither potential predicts cross sections which are in good agreement with the measuredscattering intensities. Contributions to the total scattering from elastic and inelastic processes were investigated using the coupled states approximation. The contributions to the total scattering from different initial HF rotational states were also studied. The total scattering for j initial=0 differs significantly from that for other values of j initial. It is therefore important to know the distribution of rotational states in the HF beam if one is to compare calculated total differential cross sections with measured ones. The largest inelastic cross section is for the j=0 to j’=1 transition. Collisions in which the Ar atom interacts strongly with the anisotropic potential well, rather than near head‐on collisions off the repulsive wall of the potential, are responsible for the large j=0 to j’=1 cross section. The results of accurate SCF calculations for Ar–HF are also reported in this paper.

Photodissociation of gas‐phase halogenated benzene radical cations
View Description Hide DescriptionPhotodissociation was observed and studied for a series of gas‐phase cations of fluoro‐ and chlorobenzenes, using trapped‐ion cyclotron resonance techniques. All of these dissociations were interpreted as involving absorption of two or more photons.Photodissociation(PDS)spectra of trichlorobenzene and trichloro‐trifluorobenzene ions were compared with literature fluorescenceemission spectra. In both cases, the sharp fluorescence peak at the 0‐0 transition energy was not reflected in the PDSspectra; this was interpreted as showing high fluorescence quantum yield, and correspondingly low internal conversion quantum yield, for ions in the B̃ state vibrational ground state. Collisionless (infrared radiative) relaxation rates for several of the ions were determined by chopped‐laser two‐photon dissociation, showing that the presence of several chlorines, and particularly several fluorines, enhances the rate of infrared emission. The highest radiative cooling rate constant observed was 31 s^{−1} for trifluorobenzene ion. For those ions whose visible–UV fluorescence quantum yields have been reported as near unity, the observation of photodissociation was accounted for by a combined mechanism involving fluorescence‐pumping heating of the ions, followed by two‐photon photodissociation.

Optical–optical double resonance studies of rotational autoionization of NO
View Description Hide DescriptionOptical–optical double resonance spectroscopy is used to probe Rydberg series converging to the first ten rotational levels of NO^{+} X ^{1}Σ^{+}, v ^{+}=0. Above the lowest ionization threshold, rotational autoionization of Rydberg series converging to higher thresholds is observed. Predissociation of these Rydberg states is found to compete with rotational autoionization in much the same manner as predissociation competes with vibrational autoionization in the region of the first few vibrational limits of NO^{+}. The presence of this competing decay process, which has a decay rate similar to that of rotational autoionization, permits the comparison of rotational autoionization rates for different changes in rotational quantum number (ΔN ^{+}). Rotational autoionization by ΔN ^{+}=2 is found to be faster than by ΔN ^{+}=1 or 3. This results from the requirement that ΔN ^{+}=even processes require interactions between levels that both have even or both have odd values of orbital angular momentuml, while ΔN ^{+}=odd processes require interactions between levels of which one has even l and the other has odd l. In NO, the latter interactions are known to be quite weak. The electric field dependence and pressure dependence of the ionization threshold are also discussed.

Reactions of Ar^{+}, Ne^{+}, and He^{+} with SiF_{4} from thermal energy to 50 eV c.m.
View Description Hide DescriptionGuided ion‐beam techniques are used to measure the cross sections for reaction of SiF_{4} with Ar^{+}, Ne^{+}, and He^{+} from thermal to 50 eV. Charge transfer followed by loss of F atoms are the sole processes observed. All SiF^{+} _{ x } (x=0–4) products are observed, except for SiF^{+} _{4} from reaction with Ne^{+} and He^{+}, and Si^{+} from reaction with Ar^{+}. At high energies, the dominant products are SiF^{+} _{3} in the Ar system, and SiF^{+} in both the Ne and He systems. There is some evidence in the Ne system for an excited state of SiF^{+} _{3} at 5.7 eV. In the Ar^{+} and Ne^{+}reactions, the observed energetics are consistent with literature thermochemistry, but with He^{+}, reaction barriers are observed. A value of ΔH ^{0} _{ f,298} (SiF^{+} _{3})=−30.1±0.9 kcal/mol is derived, which is in agreement with previous values but is much more precise. The observed product distributions and energetics are explained by consideration of the potential energy surfaces and the difference in ionization potentials of the rare gases. Finally, the relationships of these reactions to plasma deposition and etching are discussed.

Scattering from a classically chaotic repellor
View Description Hide DescriptionWe report a study of the classical scattering of a point particle from three hard circular discs in a plane, which we propose as a model of an idealized unimolecular fragmentation. The system possesses a fractal and chaotic metastable classical state. On the basis of a coding of the system dynamics, we develop a method to construct the invariant probability measure and to calculate the particle escape rate, the Hausdorff dimension, the Kolmogorov–Sinai entropy per unit time and the mean largest Lyapunov exponent of the repellor. The relations between these characteristics of the system dynamics are discussed. In particular, we show that, in general, chaos inhibits escaping from the metastable state. The theory is compared with numerical simulations. We also introduce the classical tools necessary for the semiclassical quantization of the dynamics; the latter is discussed in the following paper.

Semiclassical quantization of the scattering from a classically chaotic repellor
View Description Hide DescriptionThe scattering of a point particle from three hard discs in a plane is studied in the semiclassical approximation, using the Gutzwiller trace formula. Using a previously introduced coding of the classical dynamics, the needed summation over the classical periodic orbits is performed. The trace function is then given in terms of Ruelle zeta functions. A semiclassical limit upper bound is obtained on the lifetimes of the scattering resonances. This bound is larger than the classical lifetime when the classical repellor is chaotic but coincides with it when the repellor is periodic. We conclude that classical chaos dramatically influences the lifetimes of the scattering resonances. Our upper bound for the resonance lifetime is compared with the results of numerical calculation of the full quantum dynamics. The distribution of the imaginary parts of the complex wave numbers of the resonances is also calculated.

Exact quantization of the scattering from a classically chaotic repellor
View Description Hide DescriptionThe S matrix for the scattering of a point particle from three hard discs fixed on a plane is calculated exactly using Green’s theorem. The S matrix is obtained explicitly from S=I−i C M ^{−} ^{1} D, where the matrix M describes the multiple scattering between the three discs and the matrices C and D describe the free propagation from the edges of the three discs to large distances. The scattering resonances are located in the complex wave number plane as the zeros of the determinant of the matrix M, and their symmetry representations are determined. The results of this calculation are compared with the results obtained from the semiclassical approximation described in the preceding paper.