Volume 90, Issue 9, 01 May 1989
Index of content:

Two‐photon absorption spectroscopy of ion beams: CO^{+} _{2} C̃ ^{2}Σ^{+} _{ g } state characterization
View Description Hide DescriptionTwo‐photon absorption spectroscopy with a mass‐selected beam of CO^{+} _{2} ions was used to study the predissociative C̃ ^{2}Σ^{+} _{ g } state of CO^{+} _{2}. The first photon pumped the Ã ^{2}Π_{ u }←X̃ ^{2}Π_{ g } transition and the second photon was used to scan through the C̃ ^{2}Σ^{+} _{ g }←Ã ^{2}Π_{ u } transition. A rotational analysis of two bands in this spectrum has been made. The C̃ ^{2}Σ^{+} _{ g } state is linear with a C–O bond length of 1.1552(2) Å in the v=0 level.

Analysis of dipole–dipole coupling in isotopic mixtures of N_{2} on Ni(110)
View Description Hide DescriptionWe have made a detailed comparison of experimental infrared spectra of isotopic mixtures of ^{1} ^{4}N_{2}/^{1} ^{5}N_{2} on Ni(110) with spectra calculated according to the dipole–dipole coupling theory of Persson and Ryberg [Phys. Rev. B 2 4, 6954 (1981)]. The values of the electronic and vibrational polarizabilities used to calculate the spectra are α_{ e } =4.4 Å^{3} and α_{ v } =0.26 Å^{3} . The frequency shift and the intensities of the calculated spectra agree with the corresponding experimental quantities. The full widths at half maximum (FWHM) of the calculated data are narrower than the experimental spectra. The results show that a 36 cm^{−1} vibrational coupling shift can be accounted for entirely by dipole–dipole coupling. The negative chemical shift of 42 cm^{−1} is attributed to increased backdonation into the 2π* orbital due to band formation at higher coverages.

Flame diagnostics and molecular constants of CaO by tunable diode laser spectroscopy
View Description Hide DescriptionThe infrared absorptionspectrum of the CaO molecule has been recorded with a tunable diode laser. Transitions with J‘=3 to J‘=54 of the v=1←0, v=2←1, and v=3←2 bands have been measured. The new data have been combined with previous millimeter wave measurements to yield a set of Dunham rovibrational constants. CaO monomers were produced by reacting hot calcium vapor with nitrous oxide. The distribution of CaO over the rotational and vibrational states has been determined by comparing the relative intensities of the rovibrational transitions. The vibrational temperature increased from 500 to about 1500 K with increasing N_{2}O pressure.

Two‐photon spectroscopy of the 5p ^{5}6p gerade states of Xe_{2}
View Description Hide DescriptionTwo‐photon resonant, three‐photon ionization spectra of jet cooled Xe_{2}, in the region of the Xe* 6p states between 70 000 and 80 000 cm^{−} ^{1}, are reported. A new progression, near Xe* 6p[3/2]_{2}, has been analyzed which is both vibrationally and isotopically resolved. Unambiguous assignment of upper state vibrational quantum numbers has resulted in precise molecular constants for the excited state. The transition has been assigned to 1_{ g }←0^{+} _{ g } using a b i n i t i o potential curves. Implications concerning the often used ΔΩ=0 ‘‘propensity rule’’ are discussed.

The infrared spectra of nitrous oxide–HF isomers
View Description Hide DescriptionTwo spectroscopically distinct isomers of a hydrogen bonded complex between nitrous oxide and hydrogen fluoride are observed by direct infrared laserabsorption detection in a slit supersonic expansion. The linear isomer FH–NNO contains a relatively rigid hydrogen bond to the nitrogen end of NNO. The bent isomer NNO–HF has a stronger hydrogen bond to the oxygen end of NNO, but this bond is characterized by a softer bending potential and thus the complex exhibits evidence of large amplitude bending motion. Rapid vibrational predissociation, as determined from the homogeneous broadening of the rovibrational absorption structure, is evidenced in both isomers. The linear isomer exhibits predissociation lifetimes which show structure as a function of the upper J’ rotational level, including narrow resonances which suggest excitation of NNO fragment vibrational modes.

Influence of the ac Stark effect on multiphoton transitions in molecules
View Description Hide DescriptionA multiphoton mechanism for molecular beam transitions is presented which relies on a large first‐order acStark effect to modulate the energy separation of the initial and final states of the multiphoton transition, but which does not require the presence of any intermediate level(s). The theoretical formalism uses ideas from the laser multiphoton literature for a two‐level system interacting with a monochromatic electromagnetic radiation field, together with a close analog of the rotating wave approximation. The diagonal matrix elements of the Hamiltonian operator corresponding to the large acStark effect are removed by a mathematical substitution which in effect transforms appropriate differences of these diagonal elements into transition moments involving higher harmonics of the frequency of the monochromatic radiation field. The electric field strength of the true monochromatic radiation field is ‘‘distributed’’ among the higher harmonics of the effective field according to an expression involving Bessel functions. Because these Bessel functions are bounded, there exists for a given time t of exposure to the radiation, a threshold for the magnitude of the transition dipole matrix element coupling the two levels: Below this threshold, the transition probability in a traditional one‐photon molecular beam electric resonance experiment cannot be made unity simply by increasing the amplitude of the radiation field. In fact, if the coupling matrix element is small enough, the molecular beam electric resonance signal cannot be detected within exposure time t. The algebraic formalism described above is checked by computer solution of an initial value problem involving four real coupled linear differential equations. It is then used to explain the multiphoton transitions previously observed in molecular beam electric resonance studies on the two symmetric top molecules OPF_{3} and CH_{3} CF_{3}, where the number of photons involved in a given transition varies from 1–40. Application of the analysis to other experiments is briefly discussed.

A photoion–photoelectron coincidence study of Kr and Xe dimers
View Description Hide DescriptionThe photoion–photoelectron coincidence (PIPECO) spectra for Kr^{+} _{2} and Xe^{+} _{2} in the wavelength regions of 825–970 and 900–1030 Å, respectively, have been measured at different nozzle temperatures and stagnation pressures (P _{0}). The ionization energies (IE) for Kr_{2} and Xe_{2} to Kr^{+} _{2}[I(1/2)_{ u }] and Xe^{+} _{2}[I(1/2)_{ u }] determined by the PIPECO spectra are in excellent agreement with the results of previous photoionization experiments. The PIPECO measurements for Kr^{+} _{2} and Xe^{+} _{2} also provide lower limits for the IEs of Kr_{2} and Xe_{2} to Kr^{+} _{2}[II(1/2)_{ u }] and Xe^{+} _{2}[II(1/2)_{ u }]. The PIPECO spectra for Kr^{+} _{2} and Xe^{+} _{2} display strong autoionization structures similar to those resolved in the corresponding photoionization efficiency spectra, indicating that a significant fraction of autoionizing electrons are slow electrons with near zero kinetic energies. The extreme weakness of the Kr^{+} _{2}[II(1/2)_{ u }] and
Xe^{+} _{2}[II(1/2)_{ u }] PIPECO bands observed at low P _{0} support the conclusion that excited Kr^{+} _{2}[II(1/2)_{ u }] and Xe^{+} _{2}[II(1/2)_{ u }] ions are dissociative with dissociation lifetimes shorter than 68 and 85 μs, respectively. These values are in accord with the calculated radiative lifetimes for the II(1/2)_{ u }→I(1/2)_{ g } transitions in Kr^{+} _{2} and Xe^{+} _{2}. The intensities for the II(1/2)_{ u } PIPECO bands relative to the I(1/2)_{ u } and I(3/2)_{ g } PIPECO bands for Kr^{+} _{2} and Xe^{+} _{2} are found to increase as P _{0} is increased, an observation attributed to the formation of Kr^{+} _{2} and Xe^{+} _{2} from fragmentation of excited Kr^{+} _{ n } and Xe^{+} _{ n } ions produced in the photoionization of Kr_{ n } and Xe_{ n }, n≥3. The fragmentation of excited Kr^{+} _{ n } and Xe^{+} _{ n } , n≥3, ions have the effect of lowering the appearance energies for the Kr^{+} _{2}[II(1/2)_{ u }] and Xe^{+} _{2}[II(1/2)_{ u }] PIPECO bands.

Vibrational studies of reactive intermediates of aromatic amines. IV. Radical cation time‐resolved resonance Raman investigation of N, N‐dimethylaniline and N, N‐diethylaniline derivatives
View Description Hide DescriptionThe radical cation time‐resolved resonance Raman spectra of various isotopic derivatives of N, N‐dimethylaniline (DMA), N, N‐diethylaniline (DEA), N, N‐dimethyl‐p‐toluidine (4MDMA) and 3, 5, N, N‐tetramethylaniline (3,5DMDMA) are reported in the 300–1800 cm^{−} ^{1} range. Excitation was in the weak radical cation absorption around 480 nm. Complete vibrational assignments are proposed. The band activity and the changes in frequency with respect to the neutral molecules are consistent with a quinoidal‐type conformation of the framework close to planarity. Stabilization of this conformation is observed when the phenyl ring contains methyl substituents. The analysis of the Raman enhancements suggests that the quinoidal character of the radical structure is significantly lowered in the resonant excited state. An obvious analogy is found between the spectra of DMA^{+} _{ ⋅} and of the biphenyl radical cation, which clearly indicates that (i) a nearly common chromophore structure characterizes these two radical cations and (ii) the distortion of this chromophore structure in the resonant excited state is comparable in both compounds, i.e., the biphenyl^{+} _{ ⋅}* ←biphenyl^{+} _{ ⋅} and DMA^{+} _{ ⋅}* ←DMA^{+} _{ ⋅} transitions are of similar nature. These results are consistent with structural previsions from simple molecular orbital considerations and a comprehensive interpretation of the Raman spectra is given in terms of HOMO population.

An ultraviolet photoelectron spectroscopic study of BF_{3}–donor complexes
View Description Hide DescriptionHe ispectra of strong n–v type adducts of BF_{3} with H_{2}O, CH_{3}OH, (C_{2}H_{5})_{2}O, and CH_{3}CN as well as of weak complexes of BF_{3} with NO and H_{2}S are reported along with assignments based on MO calculations. The energy of the fluorine orbitals of BF_{3} is shown to be shifted in proportion to the strength of the donor–acceptor interaction. BF_{3} seems to form a contact pair with CS_{2}.

Assignment of the Raman active lattice vibrations in various phases of 1,2,4,5‐tetrabromobenzene and 1,2,4,5‐tetrachlorobenzene crystals
View Description Hide DescriptionResults of polarized Raman spectroscopy are reported and used to obtain complete assignment of the optical lattice modes of 1,2,4,5‐tetrabromobenzene crystals for both the beta and gamma phases. Similar information is provided for the beta phase of 1,2,4,5‐tetrachlorobenzene crystal. The results are compared with lattice‐dynamics calculations. Discrepancies with previously reported measurements and calculations are discussed and rationalized.

Nonresonant hyper‐Raman and hyper‐Rayleigh scattering in benzene and pyridine
View Description Hide DescriptionNonresonant hyper‐Raman and hyper‐Rayleigh spectra excited at 1064 nm are reported for neat benzene and pyridine. The theory of Herzberg–Teller vibronic coupling in nonresonant and preresonant hyper‐Raman scattering is developed. Nonresonant hyper‐Raman scattering is shown to be vibronically induced by modes that efficiently couple strongly allowed one‐photon and two‐photon transitions. A weak and broad (55 cm^{−} ^{1}) hyper‐Rayleigh band was observed in benzene and attributed to collective scattering, while in pyridine, a much more intense and much narrower hyper‐Rayleigh band was observed. Only the a _{2u } vibration (ν_{1} _{1}) was observed in the hyper‐Raman spectrum of benzene, while several strong bands were observed in pyridine. Possible vibronic‐coupling pathways are discussed for these modes. In addition, the observed hyper‐Raman spectrum of pyridine is compared to a recent calculation.

Rotation–vibration spectra of icosahedral molecules. I. Icosahedral symmetry analysis and fine structure
View Description Hide DescriptionIcosahedral symmetry analysis is developed for analyzing eigensolutions of rovibrational tensor Hamiltonians for molecules such as B_{1} _{2}H_{1} _{2} ^{−} ^{2}, C_{2} _{0}H_{2} _{0}, and C_{6} _{0}. Simplified asymptotic formulas and procedures are developed for obtaining rotational spectral fine structure for high angular momentum.J=100 eigenlevels for sixth‐ and tenth‐rank icosahedral tensors are discussed using different approximations and visualization schemes.

Rotation–vibration spectra of icosahedral molecules. II. Icosahedral symmetry, vibrational eigenfrequencies, and normal modes of buckminsterfullerene
View Description Hide DescriptionThe icosahedral symmetry of molecules such as buckyball, B_{1} _{2}H_{1} _{2} ^{−} ^{2}, and C_{2} _{0}H_{2} _{0}, is analyzed using subgroup chain defined projection operators. The icosahedral analysis is used to determine the eigenvalues and eigenvectors of a classical spring mass model of buckyball. A spectrum of Raman and dipole active modes is given using the spring constants of benzene. Corresponding dipole active and Raman active normal modes are displayed stereographically. Several choices for springs constants are discussed and a comparison with spring mass systems of reduced symmetry is made.

Infrared fluorescence from NO_{2} excited at 400–500 nm
View Description Hide DescriptionNO_{2} has been electronically excited to the ^{2} B _{2}/^{2} B _{1} states, using pulsed dye laserradiation at 400–500 nm. Strong mixing of the electronically excited state with the ground electronic state (^{2} A _{1}) leads to highly vibrationally excited NO_{2}(^{2} A _{1}), from which infrared emission has been observed. The time dependence of the IR fluorescence at several wavelengths has been observed, and quenching rate constants for NO_{2} and other gases have been measured. In addition to IR fluorescence at wavelengths identifiable as vibrational transitions (3.0–4.0, 6.1–6.8, 7.4–8.5, and 10.0–14.0 μm), emission at wavelengths <3.0 μm has been observed and attributed to a transition with electronic character. The emission observed in these experiments has been compared with that of chemiluminescent NO_{2} produced in the O+NO and O_{3}+NO reactions.

Nondipole light scattering by partially oriented ensembles. I. Numerical calculations and symmetries
View Description Hide DescriptionWe consider the elasticscattering of light by an ensemble of scatterers with radius of gyration greater than about one‐tenth of a wave; e.g., visible lightscattered by an aqueous suspension of viruses or bacteria. We model the scattering molecule as an asymmetric rigid array of interacting point polarizabilities, and we include, as the source of anisotropy in the scattering ensemble, a permanent dipole moment on the molecule and a uniform electric field E° in the scattering cell. We calculate the entire Müller matrix M(θ, E ^{○} _{ x }, E ^{○} _{ y }, E ^{○} _{ z }), for scattering angles θ from 0° to 360° and for E ^{○} _{ x }, E ^{○} _{ y }, E ^{○} _{ z } nonzero one at a time, where z is the incidence axis, y is perpendicular to the scattering plane, and x is perpendicular to y and z. Our first major finding is that the nondipole elements of M are enormously more sensitive to partial orientation than are the dipole elements. The second major finding is a set of new symmetries governing the scattering matrix for the case of axially anisotropicscattering clouds. The Perrin reciprocity symmetries for isotropic clouds may be symbolized by M(θ)=PM(θ), where P stands for matrix transpose plus negation of third row and column (with double negation of element 3,3). Using this operator our new axial symmetries may be expressed as PM(+θ, 0, E ^{○} _{ y }, 0)=M(+θ, 0, −E ^{○} _{ y },0)=M(−θ, 0, E ^{○} _{ y }, 0). The second symmetry may be generalized to fields in any direction as M(+θ,−E ^{○} _{ x }, −E ^{○} _{ y }, E ^{○} _{ z })= M(−θ, E ^{○} _{ x }, E ^{○} _{ y }, E ^{○} _{ z }). We also show in Appendix A how the nonlocal polarizabilities used in the theory may be calculated by the inversion only of symmetric matrices, with a significant saving in calculation time when the number of subunits is large.

State‐to‐state dynamics of H+HX collisions. I. The H+HX→H_{2}+X (X=Cl,Br,I) abstraction reactions at 1.6 eV collision energy
View Description Hide DescriptionThe rotational and vibrational state distributions of the H_{2} product from the reactions of translationally excited H atoms with HCl, HBr, and HI at 1.6 eV are probed by coherent anti‐Stokes Raman scatteringspectroscopy after only one collision of the fast H atom. Despite the high collision energy, only the very exoergic (ΔH=−1.4 eV) hydrogen atom abstraction involving HI leads to appreciable H_{2} product vibrational excitation. For this reaction the H_{2} vibrational distribution is strongly inverted and peaks in v’=1, with 25% of the total available energy partitioned to vibration. For the mildy exoergic (ΔH=−0.72 eV) reaction with HBr and the nearly thermoneutral (ΔH=−0.05 eV) reaction with HCl, very little energy appears in H_{2} vibration, 9% and 2%, respectively, and the vibrational state distributions peak at v’=0. However, in all three reactions a significant fraction, 18% to 21%, of the total energy available appears as H_{2} rotation. All three reactions show a strong propensity to conserve the translational energy, that is the translational energy of the H_{2}+X products is very nearly the same as that of the H+HX reactants. For the reactions with HCl, HBr, and HI the average translational energy of the products is 1.3, 1.7, and 1.7 eV, respectively, and the width of the translational energy distribution is only about 0.5 eV full width at half maximum. The energy disposal in all three reactions is quite specific, despite the fact that this high collision energy is well above the barrier to reaction in all three systems and a large number of product quantum states are energetically accessible. Only a few of these energetically allowed final states are appreciably populated. Although detailed theoretical calculations will be required to account completely for the state specifity, quite simple models of the reaction dynamics can explain much of the dynamical bias that we observe.

State‐to‐state dynamics of H+HX collisions. II. The H+HX→HX^{°}+H (X=Cl,Br,I) reactive exchange and inelastic collisions at 1.6 eV collision energy
View Description Hide DescriptionWe report measurement of product state distributions for the rotationally and/or vibrationally excited HX formed in collisions of translationally hot H atoms with HX (X=Cl, Br, and I) at 1.6 eV collision energy. The product state distributions are probed after only one collision of the fast H atom, using coherent anti‐Stokes Raman scatteringspectroscopy. Whether proceeding by inelastic collisions or reactive exchange, the transfer of translational energy to vibrational and rotational energy is quite inefficient in H+HX collisions at 1.6 eV. For all three hydrogen halides only 2–3% of the initial translational energy appears as HX vibration. For H+HCl only 6% of the initial energy is converted to HCl rotational energy, while for H+HBr and H+HI, this percentage is twice as large, 11–12%, but still small. The indistinguishability of the two H atoms involved makes it impossible to distinguish reactive exchange from inelastic energy transfer in these H+HX collisions. However, the difference in rotational energy partitioning for H+HBr and H+HI as compared with H+HCl, suggests that reactive exchange is dominant in the former and inelastic energy transfer dominates in the latter. The total cross sections for the combined energy transfer/reactive exchange do not change much with the identity of X, being 13±3, 11±2, and 11±2 Å^{2}, for H+HCl, H+HBr, and H+HI, respectively.

Analysis of optical emissions produced by dissociative electron impact on CCl_{2}F_{2}
View Description Hide DescriptionWe analyzed the optical emissions in the wavelength region 2000–8000 Å produced by dissociative electron impact on CCl_{2}F_{2}. Absolute photoemission cross sections have been determined for a variety of neutral and ionic fluorine and chlorine lines as well as for the strong diatomic CCl and CCl^{+} bands at 2778 and 2368 Å, respectively. In many cases comparisons between experimentally determined appearance potentials and spectroscopic and thermochemical data enabled a unique identification of the underlying break‐up mechanism of the parent molecule upon electron impact. Atomic fluorine emissions which are the result of the total fragmentation of the parent molecule dominate the spectrum from 6000 to 8000 Å with absolute emission cross sections in the range of 0.1 to 3.5×10^{−} ^{1} ^{9} cm^{2} at 100 eV for individual 3p→3s fine structure lines. The prominent continuous emission between 2200 and 4000 Å was found to consist of two contributions, the D ^{2} B _{2}→X ^{2} B _{2} emission of the CCl_{2}F^{+} _{2} parent ion with an appearance potential of 14.2±1.0 eV and a second emission feature with an appearance potential around 42 eV which has not been uniquely identified.

Branching ratios and lifetimes for the dissociative decay of triplet H_{2}*
View Description Hide DescriptionMolecular hydrogen is selectively excited to low vibrational levels in the triplet 3sσ and 3dλ states. We have observed the decay to both the b ^{3}Σ^{+} _{ u } and the c ^{3}Π^{+} _{ u } state for all excited levels. The dissociative final states have been investigated simultaneously using translational spectroscopy with a time and position sensitive detector. In this way the branching ratios were determined which depend on the mixing between the n=3 states. For some excited states we also measured the radiative lifetime. The experimental results are compared to a model which, in its most elaborate form, contains some transition dipole moments that depend on the internuclear coordinate. Even though the absolute values of the dipole moments are varied to fit the experiment, overall quantitative agreement cannot be obtained.

Theory of rotational transition in atom–diatom chemical reaction
View Description Hide DescriptionRotational transition in atom–diatom chemical reaction is theoretically studied. A new approximate theory (which we call IOS‐DW approximation) is proposed on the basis of the physical idea that rotational transition in reaction is induced by the following two different mechanisms: rotationally inelastic half collision in both initial and final arrangement channels, and coordinate transformation in the reaction zone. This theory gives a fairy compact expression for the state‐to‐state transition probability. Introducing the additional physically reasonable assumption that reaction (particle rearrangement) takes place in a spatially localized region, we have reduced this expression into a simpler analytical form which can explicitly give overall rotational state distribution in reaction. Numerical application was made to the H+H_{2}reaction and demonstrated its effectiveness for the simplicity. A further simplified most naive approximation, i.e., independent events approximation was also proposed and demonstrated to work well in the test calculation of H+H_{2}. The overall rotational state distribution is expressed simply by a product sum of the transition probabilities for the three consecutive processes in reaction: inelastic transition in the initial half collision, transition due to particle rearrangement, and inelastic transition in the final half collision.