Volume 98, Issue 12, 15 June 1993
Index of content:

Relaxation dynamics in the B(1/2) and C(3/2) charge transfer states of XeF in solid Ar
View Description Hide DescriptionDispersed laser induced fluorescence, and time domain measurements using the optical Kerr effect are applied to study the relaxation dynamics of Xe^{+}F^{−} (B ^{2}Σ_{1/2} and C ^{2}Π_{3/2}) charge transfer states in solid Ar. Very fast vibrational relaxation is observed in the C emitting site: excitation near v=20 leads to population of v=0 of the C state in 13(±2) ps. In the B emitting site, the lower vibrational states relax sequentially. Relaxation times of 800(±30) ps for 1→0 and 250(±30) ps for 2→1, are measured directly; and 150(±30) ps for 3→2 and <30 ps for 4→3 are estimated from spectral intensities. A new, much faster relaxation channel, which leads to B(v=1, and v=0) is open to states above v=3 in the B emitting site. This fast channel has a relaxation time of 7(±1) ps and must involve multiple internal conversions among the nested electronic states in the ionic manifold. Under intense pumping, the excited population relaxes by stimulated emission. Stimulated radiative relaxation rates larger than 1.5×10^{11} s^{−1} are observed for B(v=0).

Electric field effects in the near‐threshold photoionization spectrum of nitric oxide
View Description Hide DescriptionZero‐kinetic‐energy–photoelectron spectroscopy (ZEKE‐PES) and time‐of‐flight mass spectrometry are used to investigate the near‐threshold, two‐color photoionization of NO via the A ^{2}Σ^{+}, v=0 level. Pulsed‐field ionization of Rydberg states within 20 cm^{−1} of the ionization threshold is shown to proceed via a diabatic mechanism. Particular emphasis is given to the effect of predissociation on the signal produced by delayed field ionization. A small (∼3 V/cm) dcelectric field is shown to produce a dramatic decrease in the field ionization signal in both the NO^{+}‐ion and ZEKE‐electron channels. This decrease is thought to be due to an increase in the predissociation rate caused by the dcelectric field. The implications of these experiments for mass‐analyzed threshold ionization are discussed.

Fourier transform infrared observation of the ν_{2} stretching mode of the HCCCO radical in solid Ar
View Description Hide DescriptionThe identification of a fundamental vibration of the HCCCO radical has been made in a Fourier transform infrared (FTIR) study involving extensive isotopic measurements. An absorption at 2308.6 cm^{−1} has been assigned to the ν_{2}, antisymmetric CCCO stretching mode of HCCCO which was produced by trapping in Ar the products of the vacuum ultraviolet (VUV)photolysis of a mixture of acetylene and carbon monoxide. The assignment is confirmed by measurements for a variety of D‐, ^{18}O‐, and ^{13}C‐substituted isotopomers.

Incoherent cross‐correlation spectroscopy
View Description Hide DescriptionA new light scattering method is developed to measurescattering amplitude fluctuations resulting from either internal motions of flexible macromolecules or rotations of rigid particles. With a single‐beam two‐color cross‐correlation scheme, the technique becomes insensitive to rapid phase fluctuations of the scattered light produced by translational motions of the scattering particles. We frame the scattering theory so as to encompass rotations and number fluctuations of small particles in a steady laminar flow. Experiment verifies the theory and demonstrates its applications. The technique can be used to measure the magnitudes of the local velocity and the flow vorticity. It can also be used to measure internal motions and shape fluctuations of flexible macromolecules. The advantages of the technique are its high spatial resolution, fast temporal response, and ease of use.

State‐specific vibrational predissociation and interconversion tunneling quenching at 3ν_{1} and 3ν_{2} of (HF)_{2}
View Description Hide DescriptionWe report the second overtone (Δv=3←0) spectra of the free‐HF (ν_{1}) and bound‐HF (ν_{2}) stretches of (HF)_{2} using laser induced fluorescence. Subbands of K=0←0 and K=1←0 are detected near 900 nm with linewidths spanning almost two orders of magnitude. The line broadening (Δν_{ pd }) due to vibrational predissociation is not only mode specific but also is state specific. A fit of the spectral lines to a Voigt profile reveals Δν_{ pd }=10 GHz for the parallel band of 3ν_{2}, and 0.10 and 1.9 GHz for the parallel and the perpendicular bands of 3ν_{1}, respectively. The linewidths of these subbands are J and tunneling state independent. The K‐dependent vibrational predissociation is attributed to near‐resonant centrifugal interaction of the K=1 state with the K=1 combination mode of the bound HF stretch (3ν_{2}) and the antisymmetric bend (ν_{5}). The exceedingly state‐specific behavior is at variance with elementary density of states arguments. Spectroscopic constants of these two K subbands and two tunneling states (A ^{+} and B ^{+}) of 3ν_{1} are determined from their rotationally resolved manifolds. For the parallel band, we obtain band origins ν_{0}=11 273.501 cm^{−1} (A ^{+}), 112 73.499 cm^{−1} (B ^{+}), rotational constantsB̄=0.221 177 cm^{−1} (A ^{+}), 0.221 179 cm^{−1} (B ^{+}), and centrifugal distortion constants D=2.02×10^{−6} cm^{−1} (A ^{+}), 2.05×10^{−6} cm^{−1} (B ^{+}).
For the perpendicular band, ν_{0}=11 299.850 cm^{−1} (A ^{+}), 11 299.847 cm^{−1} (B ^{+}), and B̄=0.222 02 cm^{−1} (A ^{+}), 0.222 04 cm^{−1} (B ^{+}). The interconversion tunneling splitting is found to be 0.0024 cm^{−1}, showing that the tunneling motion of the dimer could be quenched entirely. For the 3ν_{2} where only the R branch is resolved, the breadth of the lines prevents accurate determination of its spectroscopic constants. The band is estimated to center at 11 043.09 cm^{−1} with a rotational B̄ constant of 0.2240 cm^{−1}. All the constants indicate that a stronger hydrogen bond is formed at higher valence vibrational states. The shifts of the free‐ and the bound‐HF stretching frequencies from that of the monomer are −99.306 and −329.72 cm^{−1}, respectively. Finally, we present an analysis for the rotational dependence of the tunneling in states of v _{1}, which suggests that the transition state, under the assumption of C _{2h } geometry, has the HF units oriented at 33° with respect to the F–F axis.

Large spectral bandwidth stimulated Rayleigh‐wing scattering in CS_{2}
View Description Hide DescriptionLarge spectral bandwidth stimulated Rayleigh‐wing scattering (SRWS) induced by picosecond laser pulse has been investigated in bulk liquid CS_{2}. The spectral broadening range of SRWS can reach up to 200 cm^{−1} on Stokes side. Many features of this scattering, such as the shift of the spectral maximum from excitation line and the temperature dependence, are presented. In order to explain the spectral broadening phenomenon, a possible mechanism based on collision‐induced light scattering is also discussed.

Analysis of CF_{3}I quasicontinuum states. III
View Description Hide DescriptionCF_{3}I quasicontinuum (QC) states are analyzed in the framework used previously for SF_{6} in papers I and II of this series. The existence of a hierarchy of intramolecular couplings V _{ k }, decreasing vs the number k of vibrational quanta exchanged, with an effective density of coupled states ρ_{ k }, is pointed out. The oscillator strength redistribution is described taking into account isolated resonances and perturbative redistribution associated to the lowest orders k, dissipative redistribution associated to higher orders, multiple transitions, and mode inhomogeneous spreading. Published homogeneous spectra of ^{12}CF_{3}I and ^{13}CF_{3}I at E=19 000 cm^{−1}, near the dissociation threshold, are reinvestigated in this framework and a new assignment of the resonances is proposed. Using fluorescence data at 〈E〉=3200 cm^{−1}, an interpolation of the position and weight of the main resonances ν_{1}, ν_{4}, ν_{2}+ν_{3}, 2ν_{5}, in the frequency range 900–1200 cm^{−1}, becomes possible. This interpolation allows to extract from IR multiphoton absorption data, at the laser frequency ν_{ L }=1072 cm^{−1}, an average Lorentzian half‐width γ̄ (〈n〉) vs the average number of photons absorbed 〈n〉. γ̄ increases from 1 to 5 cm^{−1} for 〈n〉 varying from 5 to 10, and varies slowly around 4–5 cm^{−1} for 10≤〈n〉≤18. Finally, the onset of the QC, near E∼6000 cm^{−1}, is explained using a modelization of the hierarchy V _{ k }, ρ_{ k }, without arbitrary parameters, and an intramolecular relaxation time ∼20 ps is predicted. This analysis leads to a very good agreement between the different sets of data concerning the CF_{3}I molecule.

Exchange interactions in trinuclear basic chromium(III) clusters: Direct observation of the magnetic spectrum by inelastic neutron scattering
View Description Hide DescriptionIncoherent inelastic neutron scatteringspectra are reported for salts of the complex [Cr_{3}O(OOCCH_{3})_{6}(OH_{2})_{3}]^{+}. The data are consistent with predominantly antiferromagnetic coupling between pairs of chromium ions. The complete spectrum of transitions between successive states with total spin S=1/2, 3/2, 5/2, 7/2, and 9/2 has been observed for the first time. Splittings of the ground stateS=1/2 have been observed directly and attributed to lowering of symmetry of the triangular cluster. For the chloride salt the data confirm that two sets of complex cations with different degrees of symmetry lowering are present in the crystal, at least at the lowest temperatures used (T=1.4 to 50 K). In principle, the relationship of J values for the symmetry‐lowered case could be described as ‘‘isosceles,’’ with J _{ ab }≳J _{ bc }=J _{ ac } or J _{ ab }<J _{ bc }=J _{ ac }; or as ‘‘scalene,’’ with all three J values different. We find that, for at least one of the two sets of metal ion clusters, the scalene case applies, with J values of −11.5±0.2, −10.5±0.2, and −9.7±0.2 cm^{−1}. Using appropriate linear combinations of J values, analogous to symmetry coordinates for a triatomic molecule, these values can be regarded as being midway between the two limiting isosceles cases. With these parameters, the Heisenberg spin‐only coupling model accounts well for the energies and intensities of the magnetic transitions.

Laser induced fluorescence and radiative lifetimes of the low‐lying electronic states of gaseous AgF
View Description Hide DescriptionAt the fringes of the visible region, two low‐lying 1 (Ω=1) electronic states A’Ω1 and aΩ1 of gaseous AgF located ∼4300 cm^{−1} below the previously known lowest excited A0^{+} state have been excited for the first time in a silver vapor–fluorine reaction system. The A’Ω1–X ^{1}Σ^{+} and a Ω1–X ^{1}Σ^{+} band systems (also observed in chemiluminescence) have been excited and studied using pulsed laser induced fluorescence spectroscopy. The band system associated with the A’Ω1–X ^{1}Σ^{+} transition has been rotationally analyzed. The UVfluorescence of the A0^{+} and B0^{+}–X ^{1}Σ^{+} transitions has also been excited. The radiative lifetimes of these four low‐lying electronic states have been measured as 7.1 μs (A’Ω1), 9.1 μs (aΩ1), 240 ns (A0^{+}), and 21 ns (B0^{+}), respectively, revealing that the two Ω=1 states are of triplet character, while the two 0^{+} states are of singlet character. The observed low‐lying states of AgF appear to dissociate adiabatically to neutral atoms in contrast to the apparent dissociation of the low‐lying electronic states in CuF to ion pairs. The observation of the low‐lying 1 states of AgF also indicates the existence of similar stable 1 states for the remaining silver halides, all of which should absorb visible photons. Major molecular constants of the newly observed A’Ω1 state of ^{107}AgF are T _{ e }=24 950.71(10) cm^{−1}, ΔG _{1/2}=506.74(8) cm^{−1}, B _{ e }=0.281 32(15) cm^{−1}, D _{ e }=0.116(60)×10^{−6} cm^{−1}, and r _{ e }=1.927 Å.

Refinement of the OH A ^{2}Σ^{+}(v=0)+Ar intermolecular potential energy surface
View Description Hide DescriptionSeveral intermolecular vibrational levels of the excited electronic state of OH–Ar correlating with OH A ^{2}Σ^{+}(v=0)+Ar have been characterized by laser‐induced fluorescence and hole‐burning experiments. The OH–Ar levels identified include the lowest intermolecular level, an intermolecular bending level with a lower degree of stretching excitation than previously observed, and intermolecular levels with two quanta of bending excitation. The intensities of electronic transitions to these levels from the lowest intermolecular level of the ground electronic state of OH–Ar (X ^{2}Π) are significantly weaker than those of transitions previously reported. These data are used to refine a semiempirical potential for OH A ^{2}Σ^{+}(v=0)+Ar proposed by Bowman et al. [J. Phys. Chem. 94, 2226 (1990)]. The potential parameters have been adjusted to increase the potential anisotropy and the steepness of the radial potential in the O–H–Ar well region. The bound states supported by the adjusted potential have been calculated by taking into account the electron spin angular momentum of the OH radical. The calculated vibrational energies and rotor constants reproduce the rovibrational structure observed experimentally. A theoretical simulation of the OH–Ar electronic excitation spectrum based on the adjusted intermolecular potential yields an intensity pattern which is consistent with experimental results.

Does chlorine peroxide exhibit a strong ultraviolet absorption near 250 nm?
View Description Hide DescriptionA theoretical investigation of the low‐lying singlet electronic states of chlorine peroxide (ClOOCl) is presented. Calculations of excitation energies and oscillator strengths at the equation of motion coupled‐cluster singles and doubles level predict that six excited states of ClOOCl are accessible at photon wavelengths above 200 nm. These states occur in three sets of pairs which can be approximately represented as symmetrized and antisymmetrized excitations of electrons from lone pair and Cl–O bonding orbitals to antibonding Cl–O orbitals. While small oscillator strengths are predicted for transitions to the lowest two ‘‘pairs’’ of states, sizable values are predicted for the X ^{1} A→E ^{1} A and X ^{1} A→F ^{1} B transitions (0.02 and 0.08, respectively). Furthermore, our estimation of the vertical excitation energy for these processes is ≊5.20 eV, in satisfactory agreement with the experimentally observed band maximum of ClOOCl at 5.04 eV. The results of this work lend strong support to the contention that photodissociation of ClOOCl plays an important role in the catalytic destruction of stratosphericozone.

Inter‐ring exchange interactions in the excited electronic state of bridged diaryl compounds: Exciton splitting in 9,10‐dihydroanthracene and dibenzosuberane
View Description Hide DescriptionThe fluorescence excitation spectra of jet‐cooled 9,10‐dihydroanthracene and dibenzosuberane, in the region of their lowest‐energy absorption band, exhibit two close‐lying electronic origins that can be identified as the two exciton components of the lowest‐energy benzene transition. The measured band positions and intensities of the two exciton components are in good agreement with the results of CNDO/S+CIS calculations based on the optimized QCFF/PI+CIS ground‐state geometry.

Spectroscopic cluster size effects
View Description Hide DescriptionIn this paper we examine the spectral moments of the absorptionline shapes of aromatic‐ molecule⋅(rare gas)_{ n } heteroclusters to provide analytic expressions for the spectral shifts (δν), which are determined by the first moment, and for the homogeneous linewidths (Γ), which are determined by the central second moment (Δ) of the line shape. δν originates from the cumulative contributions of dispersive pair interactions, while Δ and Γ manifest the short‐range dynamic nuclear fluctuations. Our analysis elucidates some of the general features of these spectroscopic observables, e.g., their dependence on the cluster structure and their size dependence, as well as some of their specific characteristics, e.g., their temperature dependence. We predict a weak temperature dependence of δν (=a+bT with b≪a/T) and a strong temperature dependence of Δ(∝T ^{1/2}), which is borne out by molecular dynamics (MD) simulations. We have derived cluster size equations (CSE) relating the spectroscopic observables of the finite cluster with those of the infinite bulk system. The excluded volume corrections for δν(n)∝n ^{−1} and for Δ(n)∝n ^{−3} are different, in accordance with the nonuniversability principle for CSEs. The predictions of the CSEs concur with experimental and MD simulation data for δν(n) and with MD simulation data for Δ(n). Finally, we have addressed dimensionality scaling of spectroscopic cluster properties, applicable for the situation of a common dimensionality (D≠3) for the cluster and for the infinite bulk. This analysis may be useful for the spectroscopic interrogation of doped clusters of low dimensionality, e.g., wire clusters (D=1) and planar clusters (D=2), as well as fractal clusters, e.g., clusters of porous materials.

Effects of methyl substitution on magnetic field effects on fluorescence of isolated molecules. 4‐ and 5‐methylpyrimidine
View Description Hide DescriptionMagnetic field dependence of fluorescence of jet‐cooled 4‐methylpyrimidine (4‐MP) and 5‐methylpyrimidine (5‐MP) has been examined following excitation into the individual rotational lines belonging to lower vibronic bands of the S _{0}→S _{1} transition. This work extends our earlier study of magnetic fieldeffects on photophysical processes of pyrimidine vapor. Rovibrational state dependence of the magnetic fieldeffects on fluorescence similar to pyrimidine is observed both in 4‐MP and in 5‐MP. Methyl substitution remarkably enhances the efficiency of magnetic quenching, and the efficiency at the ‘‘1e’’ methyl internal rotor level is higher than that at the ‘‘0a _{1}’’ level in both compounds. Further, the efficiency of magnetic quenching of 4‐MP is much higher than 5‐MP, suggesting that a high barrier height to methyl internal rotation in 4‐MP leads to a drastic increase of the effective number of the triplet state coupled to S _{1} as a result of a strong vibration/internal rotation coupling in the triplet states. In both 5‐MP and 4‐MP, fluorescence lifetime of the slow component becomes shorter with increasing the field strength on any excitation, which is interpreted in terms of the field‐induced enhancement of mixing between T _{1}(nπ*) and T _{2}(ππ*) states.

Lifetime measurements on ArH and ArD
View Description Hide DescriptionWe present results of lifetime measurements of rotational substates of the E ^{2}Π state in the Rydberg molecules ^{40}ArH and ^{40}ArD. We find reasonable agreement between our result and theoretical predictions in the case of ArH. However, for ArD we obtain a distinctively longer lifetime than for the lighter isotopic molecule, ArH, which is not unusual for hydrides but is not accounted for by theory. In addition, different lifetimes measured for the Q and R,P‐branch, respectively, emitting levels of the E ^{2}Π state in ArD, give interesting insight into the interaction between excited Rydberg states of this molecule. The dependence of the lifetime of levels emitting the Q‐branch on the rotational quantum number N for N≳19 is also investigated. Furthermore, the lifetime of the state(s) responsible for the continuous emission in the ultraviolet (UV) region was measured at discrete wavelengths for each of the molecules ^{40}ArH and ^{40}ArD. Our results, together with theoretical predictions, clearly favor the assignment of the UV continuum to two electronic states, namely B ^{2}Π and E ^{2}Π.

Third‐ and fourth‐order analysis of orbit‐ and spin‐forbidden transitions in two‐photon spectroscopy of lanthanide compounds
View Description Hide DescriptionThe role of spin–orbit and crystal field interactions in the intensity mechanism for orbit‐ and spin‐forbidden transitions in two‐photon absorption spectroscopy of lanthanide compounds is investigated. The existing third‐order analysis of the crystal field contribution to the transition moment has been restated in a more compact way using a 9j symbol. New general expressions for the fourth‐order combination of spin–orbit and crystal field interactions are developed. The highest recoupling is found to require a 12j symbol. The associated selection rules allow a distinction between the various coupling mechanisms.

Coherent anti‐Stokes Raman spectroscopy of shock‐compressed liquid nitrogen/carbon monoxide mixtures
View Description Hide DescriptionVibrational spectra of liquid nitrogen/carbon monoxide mixtures, shock compressed to several high‐pressure/high‐temperature states, were obtained using single‐pulse multiplex coherent anti‐Stokes Raman scattering(CARS). The experimental spectra were compared to synthetic spectra calculated with a semiclassical model for CARS intensities and using best fit vibrational frequencies, peak Raman susceptibilities, and Raman linewidths. Up to a maximum shock pressure of 9.3 GPa, both the N_{2} and CO vibrational frequencies were found to increase monotonically with pressure but depended strongly on the nitrogen/carbon monoxide mixture ratio. An empirical fit of the Raman frequency shifts incorporating previously published neat nitrogen and carbon monoxide data, using a functional form dependent on pressure, temperature, and mixture ratio, accurately describes both the N_{2} and CO shifts. The transition intensity and linewidth data suggest that thermal equilibrium of the vibrational levels is attained in less than 10 ns at these shock pressures and the vibrational temperatures obtained were used to improve the potential function used to calculate equation‐of‐state pressures and temperatures. The measuredlinewidths were different for N_{2} and CO and suggest that the vibrational dephasing time decreased to a few ps at the highest pressure shock state.

Conformational analysis of jet cooled tryptophan analogs by molecular mechanics: Comparison with experiment
View Description Hide DescriptionThe possibility of predicting the geometries of jet cooled conformers of tryptophan analogs via molecular mechanics, employing MM2 (1987), was studied by carrying out computations on six different analog molecules. For three of the analogs, tryptamine in particular, the computationally predicted gas phase conformer geometries could be compared with experimentally determined geometries obtained from jet cooled samples. Some conformer geometries agreed qualitatively with experiment but the match did not extend to all cases. Most notably, the MM2 calculations failed to predict the eclipsed conformers of tryptamine that were deduced experimentally via analysis of rotational constants. They similarly failed to predict a planar geometry conformation found for 3‐indole acetic acid. These two disagreements prompted a reconsideration of the possible geometries that yield the best match to the experimentally determined rotational constants for various jet cooled conformers. For tryptamine conformers D/E it was found that likely geometries might in fact be more than 30° from eclipsed structures. Some differences were also found for the C_{α}–C_{β} and C_{β}–C_{γ} dihedral angle values assigned to tryptamine conformers A/B and conformer F. Nonetheless, the overall agreement of the MM2 calculations with the refined tryptamine conformer geometries remains no more than qualitative. Reanalysis of the possible geometries of conformer A of 3‐indole acetic acid that are consistent with the experimental rotational constants yielded agreement with the previous planar geometry assignment. It was also possible to use the molecular mechanics data to predict relative conformer populations and from this relative peak sizes of 0–0 transitions in the jet cooled excitation spectra. Considering all six cases, the match with experiment is again at best partial. The agreement is not good enough for reliable association of definite computed geometries with particular peaks in an experimental excitation spectrum.

The microwave rotational spectrum of the Ar–CO dimer
View Description Hide DescriptionPure rotational spectra of four different isotopomers of the dimer Ar–CO have been investigated between 8 and 18 GHz using a pulsed beam cavity Fourier transformspectrometer. The spectra confirm that the complex is a prolate near‐symmetric rotor with an essentially T‐shaped structure, and that it undergoes large amplitude zero‐point motion. It is shown that on the average the argon is closer to the oxygen than to the carbon. The transitions measured obey a‐type selection rules with ΔJ=+1, ΔK _{ a }=0, and ΔK _{ c }=+1. For ^{40}Ar–^{12}C^{16}O, transitions have been observed for K _{ a }=0 and 1 with lower state J values of 1, 2, and 3. For ^{40}Ar–^{13}C^{16}O and ^{40}Ar–^{13}C^{18}O, a similar series was measured, but only for K _{ a }=0. For ^{40}Ar–^{13}C^{17}O, the ^{17}O quadrupolehyperfine pattern was resolved in the rotational transition J _{ K aK } _{ c } = 2_{02}–1_{01}. Determinations have been made for rotational and centrifugal distortion constants, as well as for the ^{17}O quadrupole coupling constant χ_{ aa }. Effective values have been obtained for the length of the line from the center of mass of the CO subunit to the argon nucleus, and for the angle between this line and the CO bond.

Predissociation effects in the A ^{2}Δ and B ^{2}Σ^{−} states of CD
View Description Hide DescriptionLifetimes have been measured for a large number of rotational levels of the A ^{2}Δ and B ^{2}Σ^{−} states in CD using the high frequency deflection technique. Four B‐state levels are found to be influenced by predissociation by rotation which demands a lower value of the CD dissociation energy. The B state predissociations have also been studied using ab initio calculations. The A state levels above the dissociation limit are found to be weakly predissociated through interaction with the ground state continuum. The results are compared with earlier lifetime investigations of CH.