Volume 94, Issue 8, 15 April 1991
Index of content:

Rotational strengths and dissymmetry factors for molecular rotation of NHDT, PHDT, and the chiral deuterated oxiranes
View Description Hide DescriptionThe known experimental molecular gtensors for NH_{3}, PH_{3}, and oxirane are translated and rotated to give gtensors in the new principal axis coordinate systems produced by isotopic substitution for hydrogen. The new gtensors are used to calculate rotational strengths and Kuhn’s dissymmetry factors for transitions among the eight lowest rotational energy levels of NHDT, PHDT, and the chiral deuterated oxiranes. Predictions are made of the rotational strengths and dissymmetry factors (including sign) for specified absolute spatial configurations of the model molecules. The dissymmetry factors range from 10^{−5} to 10^{−9} and some of the larger ones are associated with transitiondipole moments greater than 0.02 Debye. Dissymmetry factors of some of the transitions are no more than an order of magnitude smaller than current vibrational circular dichroism capability.

Matrix‐isolated OCS: The high resolution infrared spectra of a cryogenically solvated linear molecule
View Description Hide DescriptionSpectra of the ν_{3} vibrational mode of OCS (around 2000 cm^{−1} ) isolated in solid Ar, Xe, and N_{2} at cryogenic temperatures and high dilution (1:50 000 typically) have been measured with an interferometric spectrometer at high resolution (0.01 cm^{−1} ). The spectra have in common a sharp feature (for any one isotopic form) to the red of a much broader feature. They differ in the way these features move and change width with changes in matrix temperature. The environment of the solute, which is deduced in detail in a companion paper, can be assigned to these features, implying that precision line shape measurements can be used to probe the immediate environment of a highly dilute solute. Applications of this technique to the study of photodissociation in cryogenic solids and to the study of diffusion are discussed.

The effect of solvent configuration on matrix‐isolation solute line shapes
View Description Hide DescriptionA theory of the vibrational spectral line shape for matrix‐isolated species is presented which is based on the perturbing influence of the solvent packing on a given normal mode of the solute. The theory is shown to be remarkably accurate in predicting relative line shapes in detail and in predicting absolute shifts of lines from site to site. Application of the theory to experimental spectra of OCS in Ar and Xe matrices has allowed assignment of the major features of the spectra to solvent sites of two general classes: one that is cage or crystalline in nature and the other that is cluster or amorphous in nature.

Third‐order nonlinearity and two‐photon‐induced molecular dynamics: Femtosecond time‐resolved transient absorption, Kerr gate, and degenerate four‐wave mixing studies in poly (p‐phenylene vinylene)/sol‐gel silica film
View Description Hide DescriptionFemtosecond response and relaxation of the third‐order optical nonlinearity in a newly developed poly (p‐phenylene vinylene)/sol‐gel silica composite are investigated by time‐resolved forward wave degenerate four‐wave mixing, Kerr gate, and transient absorption techniques using 60 fs pulses at 620 nm. Using a theoretical description of two‐ and four‐wave mixing in optically nonlinear media, it is shown that the results obtained from simultaneous use of these techniques yield valuable information on the real and imaginary components of the third‐order susceptibility. In the composite material investigated here, the imaginary component is derived from the presence of a two‐photon resonance at the wavelength of 620 nm used for the present study. This two‐photon resonance is observed as transient absorption of the probe beam induced by the presence of a strong pump beam. It also provides fifth‐order nonlinear response both in transient absorption and in degenerate four‐wave mixing. The fifth‐order contributions are derived from the two‐photon generated excited species which can absorb at the measurement wavelength and therefore modify both the absorption coefficient and the refractive index of the medium.

Density matrix calculation of population transfer between vibrational levels of Na_{2} by stimulated Raman scattering with temporally shifted laser beams
View Description Hide DescriptionWe compare the results of full density matrix calculations with recently reported experimental results describing the population transfer in Na_{2} using Raman scattering with the fundamental laser beam temporally shifted relative to the Stokes laser beam. Our calculations confirm the conclusion that almost total transfer of the population into the terminal level of the Raman transition can be achieved when the laser frequencies are tuned to resonance and the fundamental laser beam is temporally delayed relative to the Stokes shifted laser beam but still partially overlapping it. We analyze our results when the frequencies of the lasers are tuned off‐resonance as a function of the Rabi frequencies and the detuning, and compare with experimental and theoretical results. The alignment of the terminal level as a function of time displacement of the Stokes and pump beams is predicted.

Angle‐resolved photoelectron spectroscopy of the core levels of N_{2}O
View Description Hide DescriptionWe have measured photoionization cross sections and photoelectron asymmetry parameters for each of the core levels of N_{2}O. We have also carried out frozen‐ and relaxed‐core Hartree–Fock studies of these cross sections so as to better understand the underlying shape resonant structure and the role of electronic relaxation in these processes. A broad shape resonance is observed in each of the core‐hole cross sections at 10‐20 eV kinetic energy and there is some evidence of a second shape resonance near the thresholds, an energy region which is not accessible experimentally. The cross sections also exhibit site‐specific behavior with maxima at widely separated photoelectron kinetic energies. These differences probably arise from the fact that photoelectron matrix elements for different core orbitals probe different regions of the shape resonant orbital which extends over the entire molecule. Although the higher energy shape resonances appear quite similar, Hartree–Fock studies show that the central nitrogen resonance is more sensitive to effects of electronic relaxation than the terminal nitrogen or oxygen resonances. Large differences are also seen between the photoelectron asymmetry parameters for the central and terminal atoms.

Rotational spectrum and structure of the complex Ar–CH_{3}CN
View Description Hide DescriptionThe microwave spectrum of the weakly bound complex Ar–CH_{3}CN has been observed using a pulsed‐nozzle Fourier‐transform microwave spectrometer. The spectrum is characteristic of an asymmetric rotor with nearly free internal rotation of the methyl group. Spectroscopic constants for the ground internal rotor state, in megaHertz, are 3:[R W3:A=9323.7769(22),:B+C=3439.5578(15),:B–C=326.6860(12)].

Spectral moment method versus least‐squares Franck–Condon analysis for vibrationally resolved absorption spectra
View Description Hide DescriptionAn accurate, back‐of‐the‐envelopespectral moment method is described to obtain molecular parameters of excited state molecules and ions from vibrationally resolved absorption spectra. Since it is not an iterative procedure, the spectral moment method is clearly much faster than the least‐squares Franck–Condon analysis for obtaining molecular parameters, and the procedure described here can even be implemented on a hand‐held calculator. Some of the drawbacks of Franck–Condon analysis are overcome by the spectral moment method. Comparison of molecular parameters for a diverse range of excited state molecules and ions obtained by the spectral moment method versus Franck–Condon analysis or rotational spectroscopy reveal the high accuracy of the procedure here. Simulations of vibrationally resolved absorption spectra show that the spectral moment method with the Morse potential approximation gives an envelope that matches very well the observed spectra.

A new Raman cross section measurement technique monitors the tyrosine environmental dependence of the electromagnetic field strength
View Description Hide DescriptionWe have developed a new Raman spectroscopic technique which can be used to determine relative Raman intensities as a function of the liquid medium refractive index. We utilize teflon as an external standard. The refractive index dependence of the solid angle of light collection disappears in our sampling geometry and the self‐absorption bias is minimized. We observe a refractive index dependence for the Raman intensity for N‐acetyl‐L‐tyrosinamide (NATYR) close to that calculated using previously derived relations which determine the effect of the refractive index on the electromagnetic field strength and upon the Raman scattered intensities. This dependence gives a ca. 25% increased resonance Raman intensity of NATYR in ethylene glycol compared to water for ca. 243 nm excitation. The strong dependence of the Raman intensity upon the environment suggests that UV resonance Raman intensity measurements can be used to monitor the average environmental refractive index of aromatic amino acids in proteins.

Multiphoton ionization of SiH_{3} and SiD_{3} radicals. II. Three‐photon resonance‐enhanced spectra observed between 450 and 610 nm
View Description Hide DescriptionThe electronic spectra of silyl radicals, SiH_{3} and SiD_{3}, were observed between 450 and 610 nm (49 200‐65 200 cm^{−1} ) by resonance enhanced multiphoton ionization (REMPI) spectroscopy. The spectra were produced through a 3+1 REMPI mechanism. Spectra of four new planar Rydberg states were observed and assigned. In SiH_{3} the observed states and spectroscopic constants are D̃ ^{2} A ^{’} _{1}(3d)■ T _{0}=49 787(30), ω_{2}=810(31) cm^{−1}; Ĩ’ (4d): T _{0}=56 253(30), ω_{2}=814(25) cm^{−1}; J̃’ (4d): T _{0}=57 726(30), ω_{2}=835(26) cm^{−1}; and L̃ (5d): T _{0}=59 615(30) cm^{−1}, ω_{2}=839(26) cm^{−1}. In SiD_{3} the observed states and spectroscopic constants are D̃ ^{2} A _{1}(3d): T _{0}=49 685(30), ω_{2}=600(28) cm^{−1}; Ĩ’ (4d):T _{0}=56 205(30), ω_{2}=600(17) cm^{−1}; and J̃’ (4d):T _{0}=57 840(30), ω_{2}=603(20) cm^{−1}. The differences between 2+1 and 3+1 REMPI spectra are explained by a Δl rule, where l is the electronic orbital angular momentum. The H̃, K̃, and Ñ states observed in the 2+1 REMPI spectrum of SiD_{3} spectrum and reported in J. Chem. Phys. 9 1, 3340 (1989) are reassigned as the H̃ ^{2} E’ (4p), K̃ ^{2} E’ (4f), and Ñ ^{2} E’ (5f).

Comprehensive theory of the Fourier transform ion cyclotron resonance signal for all ion trap geometries
View Description Hide DescriptionAn analytic solution for the charge induced on each of the detection electrodes of a Fourier transform ion cyclotron resonance (FT/ICR) ion trap has been derived from basic electrostatics for both tetragonal and cylindrical traps of arbitrary aspect ratio, by use of a Green’s function formalism. Dunbar has shown that the result of that calculation is in general equivalent to that obtained from prior ‘‘reciprocity’’‐based methods (see text). A primary advantage of the present treatment is its variety of functional forms arising from the various forms of the Green’s function, some of which may converge much more rapidly in numerical evaluation. (Moreover, because the Green’s function is the potential field of a unit point charge, the Green’s function must be employed in any treatment of ion–ion repulsions.)
The present results (a) exactly confirm prior analyses of the cubic and tetragonal traps; (b) provide the first complete analysis of the cylindrical trap; and (c) may be extended to any trap geometry for which the Green’s function is known. In the absence of an available Green’s function, the reciprocity‐based treatment, either analytic or numerical, is the method of choice to solve for the induced charge for any ion trap geometry (e.g, unbroken or segmented hyperbolic). For circular orbits centered on the longitudinal axis of the trap, the presence of spectral components at odd multiples of the fundamental ICR orbital frequency is explained and a closed form solution for the relative magnitudes of these components is presented for tetragonal and cylindrical traps. The ratio of the spectral peak height at the third harmonic to that at the first (i.e., the ratio of the third to first Fourier coefficients) is a strong monotonic function of orbital radius; thus, measurement of that ratio provides a simple and direct means for determining the cyclotron orbital radius and hence its orbital translational energy.
The presence and location of magnetron and trapping sidebands of the fundamental peak are also predicted. In addition, we show that a cylindrical trap whose ring electrode is divided into equal quadrants is only slightly more sensitive than a tetragonal trap of the same aspect ratio. Finally, we develop a general circuit model which relates the charge induced on one or more detection electrodes to the detected voltage (i.e., the unamplified signal). Since the effect of trapped‐ion motion on each detection electrode is modeled as a charge (or current) source relative to ground, the net signal from any given electrode arrangement and interconnection scheme can be accommodated simply by adding, subtracting, or grounding the signal from each detection electrode, e.g., single‐electrode detection, Comisarow’s differential detection between two electrodes, or any of various multiple‐electrode configurations. From the measured ICR signal and ICR orbital radius, the number of coherently orbiting ions may be determined.

Electronic transition moment for the B ^{2}Σ^{+}–X ^{2}Σ^{+} emission of CN. Analysis of dependence on the internuclear distance
View Description Hide DescriptionThe dependence of the electronic transition moment R _{ e }(R) for the B ^{2}Σ^{+}–X ^{2}Σ^{+} transition of CN on the internuclear distance R is determined by an analysis of the relative intensity of its emission spectrum. In the present analysis of the spectrum, the relative values of R _{ e }(R) are directly evaluated by use of the vibrational integrals, 〈v’‖R ^{ n }‖v‘〉 (n=1,2,...), instead of the r‐centroid approximation, which is found to be inadequate particularly in the tail bands (v’≥10). The validity of the r‐centroid approximation is discussed on the basis of the Rydberg–Klein–Rees (RKR) potentials of the B ^{2}Σ^{+} and X ^{2}Σ^{+} states. The R dependence of the R _{ e }(R) value is interpreted in terms of the electronic wave function of B ^{2}Σ^{+}; the configuration‐interaction (CI) coefficients for the electronic configurations of the B ^{2}Σ^{+} state are estimated. The CI coefficients obtained in the present study, as well as those for the A ^{2}Π_{ i } state derived previously from a similar analysis of the electronic part of the spin–orbit constant A(R) for A ^{2}Π_{ i }, are used to evaluate the electronic part of the spin–orbit matrix element between the B ^{2}Σ^{+} and A ^{2}Π_{i} states H ^{SO} _{el}. The H ^{SO} _{el} values are predicted to be −25 and −21 cm^{−1} at R=0.97 and 1.3 Å, respectively, being consistent with those determined experimentally in our previous study, −31 and −21 cm^{−1}, respectively.

Molecular reorientation dynamics of polar dye probes in tertiary‐butyl alcohol–water mixtures
View Description Hide DescriptionThe rotational motion of the four dye probe molecules—nile blue and thionine (monocations), resorufin (monoanion), and nile red (polar but neutral) have been investigated in aqueous mixtures of tertiary‐butyl alcohol using picosecond fluorescence depolarization spectroscopy. The simple hydrodynamic theory due to Stokes–Einstein–Debye is unable to describe the entire profile of the plot of rotational reorientation time vs viscosity, but is adequate in the water rich region (low viscosity). If the dielectricfriction theory proposed for pure solvents is considered, then it is possible to mimic the bivalued profile seen in the alcohol rich region (high viscosity). The inability to produce a satisfactory fit over the entire composition range possibly demands a suitable dielectricfriction theory for binary mixtures.

Intensities and asymmetry features of electronic Raman spectra of TmPO_{4}. Third‐order one‐particle parametrization scheme
View Description Hide DescriptionThe intensities and polarization characteristics of the transitions from the ground state to the crystal field levels of the ^{3}H_{6} multiplet of TmPO_{4} are analyzed and compared with experiment. Emphasis is directed toward the asymmetry features of the Raman spectra. The discussion is based on the results of a b i n i t i o calculations performed within the third‐order electron correlationtheory formulated in terms of perturbed functions. The various contributions to the scatteringtensor have been evaluated for the complete radial basis sets (including continuum states), and they represent the impact of the singly excited configurations 4f ^{ N−1} l’, for l’=d,f,g.

Infrared laser spectroscopy of CNC^{+}
View Description Hide DescriptionThe CNC^{+} ion has been detected in the gas phase by tunable diode–laser spectroscopy in a hollow cathodedischarge. The (001)–(000), (011)–(010), (021)–(020), and the (031)–(030) transitions were observed and analyzed. The derived spectroscopic constants for the ground vibrational state are: ν_{0}=1974.071 72(65) cm^{−1} and B _{0}=0.460 609(39) cm^{−1}.

The chemical nature of the nitrogens in polypyrrole and polyaniline: A comparative study by x‐ray photoelectron spectroscopy
View Description Hide DescriptionThe chemical nature of the nitrogens corresponding to various intrinsic redox states of polypyrrole (PPY) and polyaniline (PAN) has been critically compared using x‐ray photoelectron spectroscopy(XPS) as a primary tool. Proton modifications of nitrogens in PPY give rise to a number of intrinsic redox states analogous to those observed in PAN. The behavior of the corresponding oxidation states in both polymers towards oxidation/reduction, deprotonation/reprotonation, or charge–transfer interactions with electron acceptors are grossly similar. However, the nitrogens of the two oxidizedpolymer complexes do differ in their thermal degradation behavior which suggests that the oxidized pyrrolylium nitrogens are more susceptible to deprotonation than their oxidized counterparts.

Saturation studies of H_{2}O and HDO near 3400 cm^{−1} using intense picosecond laser pulses
View Description Hide DescriptionThe effect of significant decrease of water absorptivity for the intense picosecond laser radiation at λ=2.79 and 2.94 μm being near the center of the OH stretching mode absorption band was discovered. In case of pure water a thermal mechanism dominated: A very fast temperature rise led to weakening of H‐bonds and consequently to the absorption band shift towards higher frequencies. As a result, a considerable (up to 10 times) decrease in the optical density at the laser frequency was obtained. In the second case of HDO diluted in D_{2}O the temperature effects were eliminated and a pure spectroscopicsaturation of the v=0 to v=1 vibrational transition was displayed. The saturation intensity as high as I _{ s }=2.5.10^{11} W/cm^{2} in this case gives the value of energy relaxation time of the OH excited state to be in the range 0.3 ps <T _{1} <0.6 ps. The width of the homogeneously broadened component of the fundamental OH band in HDO spectrum is evaluated to be ≥ 40 cm^{−1}.

Far‐infrared spectra and two‐dimensional potential energy surface for the out‐of‐plane vibrations of 2‐cyclopenten‐1‐one and its deuterated isotopomers
View Description Hide DescriptionThe far‐infrared spectra of 2‐cyclopenten‐1‐one and its 5‐d _{1} and 5,5‐d _{2} derivatives have been recorded and analyzed. Each molecule has a ring‐puckering series of bands characteristic of mixed quartic–quadratic potential energy functions. The fundamental puckering frequencies were observed at 94.4, 89.0, and 84.9 cm^{−} ^{1}, respectively, for the d _{0}, d _{1}, and d _{2} species. In addition to the main puckering series for each molecule, side bands arising from the ring‐twisting excited states were also observed. Bands resulting from ring‐twisting transitions from various puckering states were observed for all three species in the 280–290 cm^{−} ^{1} region. The data demonstrate that the molecular skeleton is planar. A two‐dimensional potential energy function of the form V=a _{1} x ^{4} _{1} +b _{1} x ^{2} _{1}+a _{2} x ^{4} _{2}+b _{2} x ^{2} _{2} +c x ^{2} _{1} x ^{2} _{2}, where x _{1} and x _{2} are the ring‐puckering and ring‐twisting coordinates, was determined. For all three isotopomers, this gives rise to calculated frequencies which are in excellent agreement with the observed values. The interaction term c is larger than in related molecules, reflecting the effects of asymmetry and conjugation.

Jahn–Teller coupling in Rydberg series of benzene
View Description Hide DescriptionJahn–Teller coupling effects in doubly degenerate Rydberg series (n p _{ x,y } and n d _{ x z,y z }, respectively) converging towards the doubly degenerate X̃^{2} E _{1g } ion core of benzene are analyzed in a multichannel quantum‐defect description. The model includes Jahn–Teller coupling (via the lowest‐frequency E _{2g } mode ν_{6} ) in the ion core and in the Rydberg orbitals, as well as the interaction of degenerate electronic channels. Fast internal‐conversion processes in the Rydbergmanifold of benzene are included in a phenomenological manner via an absorptive imaginary part of the reactance matrix. The model parameters are determined from the analysis of existing multiphoton absorptionspectra of low Rydberg members. Absorption spectra of the complete n p _{ x,y } and n d _{ x z,y z } series and near‐threshold photoionization cross sections are calculated, including hot‐band spectra (one quantum of ν_{6} excited in the initial electronic state). The results are of relevance for the assignment of the enigmatic vibronic structure of the 3p ^{1} A _{2u } and 3p ^{1} E _{2u }Rydberg states and for the understanding of autoionization dynamics in benzene.

Quasistable extreme motion vibrational states of HFCO above its dissociation threshold
View Description Hide DescriptionThe anharmonic mixing of highly excited vibrational states of HFCO above its dissociation threshold is studied by stimulated emission pumping spectroscopy. At 0.05‐cm^{−1} resolution, individual molecular eigenstates were resolved and state mixings observed by the distribution of oscillator strength to nearby dark states of the background. Most of the zero‐order vibrational levels observed in the energy range between 13 000 and 23 000 cm^{−1} are assigned to long Franck–Condon‐allowed progressions of extremely high overtones of the C–H out‐of‐plane bending mode (ν_{6}) in combination with the C■O stretching mode (ν_{2}). The extent of state mixing of highly excited vibrational states is strongly mode dependent. For vibrational states with almost the same total vibrational energy, states with the most quanta in ν_{6} show the least coupling with other states. More strikingly, as the total energy is increased by adding ν_{6} quanta, the state mixing becomes weaker. For the vibrational states observed above 18 000 cm^{−1}, those that have all of the energy in ν_{6} or at most one quantum of excitation in ν_{2} are extremely stable against state mixing. These appear to be the regular states or quasiperiodic trajectories predicted in some theoretical studies of two‐dimensional systems. Extreme motion of the C–H out‐of‐plane bending mode seems to localize and prevent coupling with other modes.