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Volume 104, Issue 21, 01 June 1996

The optical and optical/Stark spectrum of iridium monocarbide and mononitride
View Description Hide DescriptionSupersonic molecular beam samples of iridium monocarbide IrC and iridium mononitride IrN were generated using a laser ablation/reaction source and characterized using high resolution (Δν<30 MHz FWHM) laser induced fluorescence spectroscopy. This is the first identification of gaseous IrN. Numerous strong band systems in the 18 800 to 14 360 cm^{−1} spectral range were assigned as the (v′,0) progression of the A ^{1}Π–X ^{1}Σ^{+} band system of IrN. The (1,0) and (0,0) bands were analyzed to produce a set of fine and hyperfine parameters. The electric field induced effects on the R(0) line of these bands were analyzed to produce permanent electric dipole moments:A ^{1}Π(v=0) 2.78(2) D, A ^{1}Π(v=1) 2.64(2) D, X ^{1}Σ^{+}(v=0)=1.66(1) D. The (0,0) band of the D ^{2} Φ_{7/2}−X ^{2}Δ_{5/2} system of IrC was recorded and analyzed to produce a set of fine and hyperfine parameters. The electric field induced effects on the R(2.5) branch feature were analyzed to produce permanent electric dipole moments:D ^{2}Φ_{7/2}(v=0) 2.61(6) D and X ^{2}Δ_{5/2}(v=0) 1.60(7) D. Plausible electronic configurations consistent with the experimental observations are given.

Isotopic substitution of a hydrogen bond: A near infrared study of the intramolecular states in (DF)_{2}
View Description Hide DescriptionHigh resolution near infrared spectra of the two high frequency intramolecular modes in (DF)_{2} have been characterized using a slit‐jet infrared spectrometer. In total, four pairs of vibration–rotation–tunneling (VRT) bands are observed, corresponding to K=0 and K=1 excitation of both the ν_{2} (‘‘bound’’) and ν_{1} (‘‘free’’) intramolecular DF stretching modes. Analysis of the rotationally resolved spectra provides vibrational origins, rotational constants,tunneling splittings and upper state predissociation lifetimes for all four states. The rotational constants indicate that the deuterated hydrogen bond contracts and bends upon intramolecular excitation, analogous to what has been observed for (HF)_{2}. The isotope and K dependence of tunneling splittings for (HF)_{2} and (DF)_{2} in both intramolecular modes is interpreted in terms of a semiclassical 1‐D tunneling model. High resolution line shape measurements reveal vibrational predissociation broadening in (DF)_{2}: 56(2) and 3(2) MHz for the ν_{2} (bound) and ν_{1} (free) intramolecular stretching modes, respectively. This 20‐fold mode specific enhancement parallels the ≥30‐fold enhancement observed between analogous intramolecular modes of (HF)_{2}, further elucidating the role of nonstatistical predissociation dynamics in such hydrogen bonded clusters.

Zeeman studies of the first excited states of palladium and metal free phthalocyanines in Shpol’skii matrix
View Description Hide DescriptionPhthalocyanines (Pcs) are planar molecules of relatively high symmetry and exhibit magneto‐optic effects in many ways. We report the Zeeman effect of the first excited states of palladium (Pd–) and free‐base (H_{2}–) Pcs in a 5 K Shpol’skii matrix by direct observation of their fluorescence position shifts. From the experimental data an apparent angular momentum integral, coupling the lowest two Jahn–Teller stabilized crystal field split first excited states, Λ′, was found to be 2.7±0.1 ℏ for PdPc. No conclusive Λ′ was deduced for H_{2}Pc but its value can be estimated to be around the theoretical value of 3 ℏ. The experimental data of H_{2}Pc revealed a complex structure in the fluorescence band examined, which complicated the analysis of Λ′ but helped unravel the biexponential decay problem of the structure. We infer one narrower band in the structure has the previously reported lifetime of τ 6.3 ns, and the other broader band being ∼1.8 cm^{−1} higher, has τ 3.7 ns. We believe this composite situation also exists for the other tautomeric structure since it also exhibited biexponential decay.

Theory of thermal effects in nuclear magnetic resonance spectra of metal hydrides undergoing quantum mechanical exchange
View Description Hide DescriptionThermal effects in nuclear magnetic resonance spectra of transition metal hydrides exhibiting resolved quantum mechanical exchange splittings are consistently explained. Interactions of the relevant spatial degrees of freedom of the hydride protons with a quantum mechanical thermal bath are described in terms of Wangsness–Bloch–Redfield (WBR) theory. Upon elimination of the vibrational modes which relax too quickly to be observed on the NMR time scale, the WBR equation for the remaining, slowly relaxing modes (exchange modes) is shown to be equivalent to the Alexander–Binsch equation for classically exchanging nuclei, where the standard spin–spin coupling term is replaced by (or augmented with) quantum exchange term. Numerical calculations were performed for a one‐dimensional model of the relevant spatial motions, where the vibrational relaxation effects were described in terms of two adjustable parameters only. The assumed motion includes correlated rotation of a pair of the hydride protons, where the interproton distance may vary with the rotation angle. These calculations confirm that the present approach affords a consistent theoretical reproduction of the effects observed experimentally, i.e., an increase of the effective splitting with increasing temperature, with a gradual emergence of stochastic exchange that ultimately leads to motionally narrowed NMR spectrum lacking any fine structure.

Dicarbocyanine dyes in methanol solution probed by depolarized Rayleigh and hyper‐Rayleigh light scattering
View Description Hide DescriptionThe hyper‐Rayleigh scattering (HRS) intensity of two symmetric carbocyanine dyes (1122 DEDC and 1144 DEDC, full names given in the text) in methanol is measured as a function of dye concentration. These dye molecules at equilibrium show a negligible permanent dipole moment. The low concentration data showing that the HRS intensity is proportional to the dye concentration are used to determine the first hyperpolarizability for each of these dyes. However, above a concentration ρ_{ b }=0.1×10^{−3} M, the HRS intensity shows an anomalous concentration dependence. Above ρ_{ b }, the HRS intensity shows a saturation behavior and it even decreases with increasing concentration at high dye concentration. The depolarization ratio of the HRS intensity is also measured as a function of dye concentration. At lowest concentration, the depolarization ratio is 0.18. As the dye concentration increases, the depolarization ratio also rapidly increases but the increase quickly saturates as the concentration exceeds ρ_{ b }. The concentration dependence of the HRS intensity and depolarization ratio are interpreted as due to formation of molecular aggregates. The depolarized Rayleigh scattering (DRS) intensity is also measured as a function of dye concentration. The result of DRS corroborates well with that found in HRS.

Theory of Raman scattering with pulses: Application to continuum Raman spectroscopy
View Description Hide DescriptionA theory of real‐time dependence of Raman scattering for a pulse‐mode laser is developed within second‐order perturbation theory and using the wavepacket terminology. The rate of spontaneous Raman emission with a pulse correctly reduces to the dynamical equivalent of the Kramers‐Heisenberg‐Dirac expression in the monochromatic limit. We apply the theory to continuum Raman scattering for short and long pulses and varying pulse carrier frequency. The rate of Raman emission as a function of time and pulse carrier frequency, from an initial ground vibrational state to various final vibrational states, is shown to be structureless for all pulses, and for pulses that are longer than the dissociation time the rate also rises and decays with the pulses. This is contrary to recent reports of recurring resonance fluorescence‐type structures at long times after the pulse has vanished. We explain why such structures are unphysical for continuum Raman scattering. Results are also presented for excitation from an initial first excited vibrational state.

New Rydberg–Rydberg transitions of the ArH and ArD molecules. I. Emission from np states of ArD
View Description Hide DescriptionThe ground electronic state of argon hydride has a repulsive potential apart from a long‐range van der Waals minimum, but the Rydbergexcited states have bound potentials similar to those of the ion ArH^{+}. These states can be described approximately in terms of united‐atom quantum numbers nl. We report here rotational analyses of the bands 5p→5s, 5p→6s, and 6p→5s of ArD, which help to further characterize the npRydberg series. In ArH the bands 5p→5s and 6p→5s have broad lines because of predissociation in the lower state, and 5p→6s is difficult to analyze without further information. The present data are fitted with a Hund’s case (d) effective Hamiltonian. In previous work the 4p state was found to have a very small σ‐π splitting, but this does not hold for the higher np states, and is probably due to an accidental cancellation between electrostatic and polarizability contributions. On the other hand, the spin–orbit coupling decreases monotonically with n. Features of the rotational levels are discussed in terms of the high‐J limiting quantum numbers l _{ J }=N−R and s _{ J }=J−N, where R≡N ^{+}, in particular the effect of spin–orbit coupling on the levels with (l _{ J },s _{ J })=(−1,1/2) and (0,−1/2), which produces a tendency to Hund’s case e behavior in 4p, and a sharp avoided crossing in 6p. The corresponding avoided crossing in 5p would occur beyond the present range of observed J values.

Isotropic second‐order dipolar shifts in the rotating frame
View Description Hide DescriptionAn experiment is described that utilizes the truncation of the Hamiltonian in the rotating frame by a radio‐frequency field designed to yield an isotropic shift for the dipolar coupling. This approach allows the measurement of a normally orientation‐dependent coupling constant by a single isotropic value. The dipolar isotropic shift is closely related to the field‐dependent chemical shift in solids due to the second‐order dipolar perturbation observed in magic‐angle spinning experiments. In the rotating frame, larger shifts of up to 1000 Hz can be observed for the case of a one‐bond C–H coupling compared to a shift of a few Hertz in the laboratory‐frame experiment. In addition to the isotropic shift, a line broadening due to the P _{4}(cos β) terms is observed when the experiment is carried out under magic‐angle sample spinning (MAS) conditions, leading to the requirement of higher‐order averaging such as double rotation (DOR) for obtaining narrow lines. As an application of this new experiment the separation of CH, CH_{2}, and CH_{3} groups in a 2D spectrum under MAS is demonstrated. Implemented under DOR it could be used as a technique to select carbon atoms according to the number of directly attached protons.

Evolution of excitonic energy levels in Ar_{ N } clusters: Confinement of bulk, surface, and deep valence shell excitons
View Description Hide DescriptionThe evolution of excitonic energy levels (Wannier and Frenkel type) is investigated for Ar_{ N } clusters in the range N=200–10^{6} using fluorescence excitation spectroscopy. In the case of Wannier excitons, a pronounced blue shift of the absorption bands relative to the position in the infinite solid is observed. As a consequence of the lower dimensionality, the shift of the transition energy of surfaceexcitons is considerably smaller than that of the bulk states of clusters. The evolution with size is discussed within several theoretical models for exciton confinement. In addition, model calculations are performed for bulk excitons which give good quantitative agreement with the experimental results. In the case of n=1 Frenkel or intermediate type excitons, there are blue and red shifts observed. The spectral shift of (3p→4s) and deep valence (3s→4p) excitations differs considerably. From the shift of the transition energies the exciton mass of the (3p→4s) exciton is derived.

Radiative and nonradiative decay of electronically excited NCO
View Description Hide DescriptionA study to observe higher vibrational levels in NCO(Ã ^{2}Σ^{+}) and the onset of predissociation in this molecule has been carried out. Laser fluorescence spectra have been recorded over the wave number range 27 300–32 900 cm^{−1}, from the Ã(0,0,2)–X̃(0,0,0) band up through the B̃(1,0,0)–X̃(0,0,0) band. Vibrational assignments have been made for a number of newly observed Ã–X̃ bands, and band origin wave numbers and upper level rotational constants have been derived from comparison of experimental spectra with simulations. Decay lifetimes for excitation of a large number of both assigned and unassigned excited vibronic levels have been determined. The onset of predissociation appears to occur at energies slightly below that of the B̃(0,0,0) level.

Infrared laser jet spectroscopy of transition metal hexacarbonyl‐rare gas dimers
View Description Hide DescriptionHigh resolution infrared absorption spectra of nine van der Waals complexes M(CO)_{6}⋅Rg (M=Cr, Mo, W; Rg=Ar, Kr, Xe) formed in a supersonic jet expansion have been recorded near the 5 μm carbonyl stretching fundamental bands of the hexacarbonyl monomers. In each case a single red‐shifted perpendicular band was observed. It is shown that the spectral results are only consistent with a C _{3v } symmetric top structure for each dimer; no effects due to internal motions are seen in the spectra. The M–Rg separations deduced from analysing the spectra are slightly larger than the separations calculated from the van der Waals radii. Red‐shifts of the band origin are partly explained by a simple vibrational dipole‐induced dipole model.

Orientation and energy dependence of NO scattering from Pt(111)
View Description Hide DescriptionA classical molecular dynamics study is applied to simulate the scattering of NO from Pt(111) in the energy range of 0.3–1 eV. The solid consists of a large number of crystal atoms that interact via an anharmonic nearest‐neighbor potential. The NO–Pt(111) interaction potential is constructed as a pairwise additive potential with a well depth of 1 eV for the N end of the molecule towards the surface and purely repulsive for the O end. The in‐plane scattering results obtained with this model potential are compared with recent experiments for NO–Pt(111). The angular intensity distributions, the final translational energy, as well as the rotational energy distributions with the corresponding alignment are in qualitative agreement with those experimental results. A detailed examination of the collisiondynamics shows that multiple collisions with the surface results predominantly in superspecular scattering. The rotational angular momentum of the scattered molecules exhibits a preference for cartwheeling alignment and the rotational energy distributions for specular and normal exit angles can be described with a Boltzmann distribution, whereas for grazing exit angles they are distinctly non‐Boltzmann. The latter structure results from a cutoff in the rotational excitation by the attraction of the well. The high rotational excitation clearly originates from molecules that initially are oriented with the O end towards the surface, whereas for the low rotational excitation this orientation preference disappears.

Two‐color multiphoton transitions in molecular beam electric resonance studies: Rotating wave versus Floquet, and on‐ versus off‐resonance, calculations
View Description Hide DescriptionA nonzero difference, d, between the diagonal dipole moment matrix elements, μ_{ jj }, of two molecular states involved in either one‐ or two‐color multiphoton transitions, can have substantial impact on the temporal evolution and spectral behavior of the states. The effects of d≠0 are investigated in this paper for two‐color transitions in a two‐level system previously studied in one‐color molecular beam electric resonance (MBER) experiments on symmetric top molecules. The calculations suggest a two‐color analog to the one‐field experiments, where the flexibility furnished by the field parameters of the two continuous wave electric fields, including relative phase, can be used to advantage. Both exact Floquet calculations and the rotating wave approximation (RWA) are used in this study. Analytic RWA expressions for the one‐ and two‐color molecule‐laser(s) couplings are particularly useful in helping to interpret and/or predict the effects of d≠0. The novel aspects of two‐color laser‐molecule interactions, relative to the one‐field case, are emphasized. In addition to investigations related to MBER studies, this work contributes to the more formal aspects of two‐color laser‐molecule interactions. It is shown that very useful analytical two‐level RWA solutions for the on‐resonance temporal behavior of the molecular states are available, even in the presence of competing resonances, whereas off‐resonance numerically useful analytical results are available only when one multiphoton resonance dominates a transition.

Theoretical study of bath‐induced coherence transfer effects on a time‐ and frequency‐resolved resonant light scattering spectrum. II. Energy mismatch effects
View Description Hide DescriptionBath‐induced coherence transfer effects on a time‐ and frequency‐resolved resonant light scatteringspectrum is theoretically investigated using the Markoff master equation. According to Eberly and Wódkiewicz, a general expression for an experimentally observable spectrum in terms of a molecular response function is derived within the density matrix formalism. To generalize our previous results of the bath‐induced coherence transfer which were derived based on a displaced harmonic oscillator model [Y. Ohtsuki and Y. Fujimura, J. Chem. Phys. 91, 3903 (1989)], an eigenstate basis is used to represent a relevant system for investigating characteristics of the transfer. By the present model, we clarify the dependence of the bath‐induced coherence transfer on the energy‐level structure of the intermediate states associated with the transfer, i.e., energy mismatch effects. It is shown that if the energy mismatch of these states is smaller than dephasing rates, the bath‐induced coherence transfer occurs resonantly. In the other cases, the energy mismatch brings about a modulation in the time evolution of the superposition state created by the bath‐induced coherence transfer, which strongly diminishes the efficiency of the transfer. The resonance condition is derived analytically and is confirmed by numerical calculations of quantum beats induced by the bath‐induced coherence transfer. The possibility of very rapid dephasing of a quantum beat signal which cannot be explained in terms of dephasing rates is also shown, when the transition moments have such values that give π‐phase‐shifted quantum beats in bath‐induced fluorescence.

Acid–base chemistry in the gas phase: The trans‐1‐naphthol⋅NH_{3} complex in its S _{0} and S _{1} electronic states
View Description Hide DescriptionWe deduce information about the dynamics of a proton transferreaction between an acid and a base. Our probe is the fully resolved S _{1}←S _{0}fluorescence excitation spectrum of the 1:1 complex of 1‐naphthol and ammonia in the gas phase. Analysis of this spectrum shows that the complex is planar in both electronic states, with the NH_{3}forming a nearly linear hydrogen bond to the hydroxy hydrogen atom of 1‐naphthol. The O–H...N heavy atom separation is R=2.86 Å and the barrier to rotation of the NH_{3} group about its C _{3} axis is V _{3}=39.9 cm^{−1} in the S _{0} state. Excitation of the complex to its S _{1} state increases the acidity of 1‐naphthol, decreases the heavy atom separation to R=2.72 Å, and increases the torsional barrier to V _{3}=46.5 cm^{−1}. Modeling these changes using the Lippincott–Schroeder potential for the hydrogen bond shows that the photoinitiated heavy atom motion produces a significant decrease in the barrier to proton transfer in the S _{1} state.

Single and multiple photon ionization of triethylamine
View Description Hide DescriptionSingle and multiple photonionizationphotoelectron spectroscopy of triethylamine (TEA) was studied using a newly developed high‐resolution electron spectrometer which utilizes position sensitive detection. The adiabatic ionization potential of TEA was accurately determined using both single (7.47±0.04 eV) and multiphoton (7.53±0.10 eV) ionizationphotoelectron spectroscopy. Although excitation to both the S _{1} and S _{2} states can occur, multiphoton ionization always occurs out of the S _{1} state. When the cation dissociates, the distribution of photoelectron energies similarly reflects this partitioning between S _{1} and S _{2}.

Quasiclassical trajectory calculations of photodissociation of Ar–H_{2}O(X̃–Ã) and H_{2}O(X̃–Ã)
View Description Hide DescriptionWe present results of full‐dimensional quasiclassical trajectory calculations of the photodissociation of H_{2}O(3ν_{OH},X̃–Ã) and Ar–H_{2}O(3ν_{OH},X̃–Ã) at 243 and 218 nm, and compare the resulting OH rotational distributions, and also relate them to recent experiments of Nesbitt and co‐workers [D. F. Plusquellic, O. Votava, and D. J. Nesbitt, J. Chem. Phys. 101, 6356 (1994)]. The dynamics calculations make use of a new six degree‐of‐freedom potential for Ar–H_{2}O(Ã), which is reported here. The potential is based on a previously reported ab initio H_{2}O Ã‐state potential, a semiempirical Ar–OH(^{2}Π) potential, and a semiempirical Ar–H potential, together with an appropriate switching function to ensure permutation symmetry with respect to the two H atoms. Initial conditions for the trajectories are obtained from a product of a Husimi phase‐space density for the Ar–H_{2}O(X̃) intermolecular modes and a Wigner/classical phase‐space density for the H_{2}O(X̃) intramolecular modes. The Husimi phase‐space density is derived from the ground‐state wave function for Ar–H_{2}O(X̃), using a previous spectroscopically empirical potential. To assess the accuracy of the trajectory approach, trajectory calculations are also reported for X̃–Ã photodissociation of H_{2}O in the ground vibrational state at 166 nm and compared with the corresponding full‐dimensional quantum wave packet calculations of von Dirke and Schinke. To further assess the accuracy of the Ã‐state potential surface for H_{2}O, calculations for H_{2}O(4ν_{OH},X̃–Ã) are also reported at 218 nm and compared with experiment. Rotation/vibration distributions of the OH fragment are also calculated for photodissociation of Ar–H_{2}O(4ν_{OH},X̃–Ã) at 218 nm.

Theoretical study of the unimolecular dissociation HO_{2}→H+O_{2}. II. Calculation of resonant states, dissociation rates, and O_{2} product state distributions
View Description Hide DescriptionThree‐dimensional quantum mechanical calculations have been carried out, using a modification of the log‐derivative version of Kohn’s variational principle, to study the dissociation of HO_{2} into H and O_{2}. In a previous paper, over 360 bound states were found for each parity, and these are shown to extend into the continuum, forming many resonant states. Analysis of the bound states close to the dissociation threshold have revealed that HO_{2} is a mainly irregular system and in this paper it is demonstrated how this irregularity persists in the continuum. At low energies above the threshold, these resonances are isolated and have widths that fluctuate strongly over more than two orders of magnitude. At higher energies, the resonances begin to overlap, while the fluctuations in the widths decrease. The fluctuations in the lifetimes and the intensities in an absorption‐type spectrum are compared to the predictions of random matrix theory, and are found to be in fair agreement. The Rampsberger–Rice–Kassel–Marcus (RRKM) rates, calculated using variational transition state theory, compare well to the average of the quantum mechanical rates. The vibrational/rotational state distributions of O_{2} show strong fluctuations in the same way as the dissociation rates. However, their averages do not agree well with the predictions of statistical models, neither phase space theory (PST) nor the statistical adiabatic channel model (SACM), as these are dependent on the dynamical features of the exit channel. The results of classical trajectory calculations agree well on average with those of the quantum calculations.

Novel technique for real‐time monitoring of electron attachment to laser‐excited molecules
View Description Hide DescriptionWe report a new experimental technique that is capable of monitoring electron attachment to laser‐excited molecules in real time; the time resolution is limited only by the time constant of the detection circuit and was ∼100 ps for the experiments reported here. This technique provides information on the lifetime of the excited states responsible for electron attachment, and also allows determination of electron attachment cross sections involved. Results on dissociative electron attachment to ArF‐excimer‐laser‐irradiated NO are reported: Electron attachment occurred to the A ^{2}Σ^{+}(ν=3) state populated via the absorption of a single photon, and to highly excited states populated via two‐photon absorption; the cross section for low‐energy electron attachment to the A ^{2}Σ^{+}(ν=3) state was ∼3 orders of magnitude larger compared to that for the A ^{2}Σ^{+}(ν=0). Decay of the electrons over the ∼200 ns lifetime of the A ^{2}Σ^{+}(ν=3) state was directly monitored. Negative‐ion formation that occurred via the A ^{2}Σ^{+}(ν=3) state was suppressed in the presence of CO_{2} due to collisional quenching of that state by CO_{2}, and the reduction in the A ^{2}Σ^{+}(ν=3) state lifetime with increasing CO_{2} pressure was also observed.

The sulfur reaction in small ionized carbonyl sulfide clusters
View Description Hide DescriptionIonic species obtained by electron impact on (OCS)_{ n } clusters produced by expansion of OCS mixed with argon at two different (OCS/argon) concentrations are studied as a function of stagnation pressure and electron energy. Ionization efficiency curves of homogeneous cluster ions (OCS)^{+} _{ n }, homogeneous sulfur cluster ions S^{+} _{ m } and inhomogeneous ions (OCS)_{ n }S^{+} _{ m } are analyzed. Several reactions paths, ending in S^{+} _{ m }, have been identified. The strong correlation between reaction path and cluster size is discussed. Finally, by minimization of the potential function created by dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions between OCS molecules, we propose the geometrical configurations of neutral (OCS)_{ n } (n=2–5) clusters.