Volume 99, Issue 7, 01 October 1993
Index of content:

Low‐J rotational spectra, internal rotation, and structures of several benzene–water dimers
View Description Hide DescriptionLow J (0–4) rotational transitions have been observed for the benzene–water dimer of which high J (≥4) transitions were reported recently by Blake [Science 257, 942 (1992)]. Our experiments used a modified Balle/Flygare Fourier transformmicrowave spectrometer, with a pulsed supersonic nozzle as the sample source, and examined a variety of isotopic species in the ground and first excited internal rotor states (m=0 and 1). The dimers of the parent C_{6}H_{6} benzene with H_{2}O, HDO, D_{2}O, and H_{2} ^{18}O have symmetric top spectra characteristic of two coaxial rotors with a symmetric top frame and a very low effective V _{6} barrier. The dimers of H_{2}O and D_{2}O with the ^{13}C and D monosubstituted benzenes have asymmetric top spectra of which only the m=0 state was assigned. However, doublets in the m=1, J=0→1 transitions show that there is a V _{2} term of ∼0.5 MHz in their barriers. A substitution analysis was made of the rotational constants found for the m=0 state of the dimers with H_{2} ^{18}O, D_{2}O, and the ^{13}C and D monosubstituted benzenes. It shows that the oxygen is at the a axis of the dimer, well outside (0.48 Å) the hydrogens. However, the C _{2} axis of the H_{2}O is not coincident with the a axis but is at an angle β of 37° to it, rotated so that the two hydrogens are equivalent. The sixfold axis of the benzene corresponds to the a axis, there is little or no tilt (γ) of the benzene. The c.m. (C_{6}H_{6}) to c.m. (H_{2}O) distance R is 3.329 Å. The closely spaced hyperfine structure from the proton–proton magnetic dipole interaction and the deuterium quadrupole interaction was resolved for several dimers and transitions, principally J=0→1 and 1→2. The results demonstrate effective nuclear equivalence in dimers with H_{2}O and D_{2}O. Also, the symmetries found for their nuclear spin functions correlate with the lowest rotational levels of free water, the m=0 state with 0_{00} and m=1 with 1_{01} and 1_{11}. For the m=1, K=0 transitions of C_{6}H_{6}–H_{2}O the correlation is with 1_{11} and for the K=±1, with 1_{01}. These assignments are reversed for C_{6}H_{6}–D_{2}O.

Complex formation between water and formamide
View Description Hide DescriptionThe infrared spectrum of the water–formamide complex in argon matrices has been recorded from 10 to 4000 cm^{−1}. The interaction energy of the complex forming molecules has been calculated from a theoretical potential. One global and three different local minima have been found for this potential. Intermolecular vibration frequencies have been calculated for each minimum. The results are compared with the experimentally observed far infrared spectrum. In agreement with microwave measurements and ab initio calculations, the global minimum of the complex is found, both from calculations and experiment, to have a cyclic structure with water forming a hydrogen bond to the amide oxygen and receiving a hydrogen bond from an amide hydrogen. In addition to the cyclic complex, we observe one of the local minimum structures of the complex, where water accepts a hydrogen bond from the amide NH on the CH side of the amide.

Time domain information from resonant Raman excitation profiles: A direct inversion by maximum entropy
View Description Hide DescriptionA direct method for the inversion of resonant Raman excitation profiles to the time domain is proposed. The inversion procedure is implemented within the maximum entropy (ME) formalism. The constraints used in the ME procedure are the values of the Fourier transform of the Raman excitation profile at a given set of values of times t _{ r }. It is shown that the ME functional form of the Raman cross section can be expressed in terms of a Raman amplitude, depending on the Lagrange multipliers and on the constraints. The Fourier transform of the Raman amplitude yields the time dependent cross correlation function. Another route to direct inversion, using a Fourier series expansion of the dispersion relation, is also considered. The analytical requirements that must be satisfied by the input excitation profile for a successful inversion to be possible are discussed. The optimum values of the times t _{ r } and of the Lagrange multipliers which determine the Raman amplitude are computed using a new algorithm (the min–max algorithm). The proposed ME numerical procedure is implemented for computed resonant Raman excitation profiles of the B̃ electronic state of the iodobenzene molecule and of a model anharmonic system. In addition, the analytical implications of the ME functional form of the excitation profile are discussed with special reference to the separation of time scales in the dynamics.

Microscopic theory of solvation dynamics in dipolar liquids
View Description Hide DescriptionA self contained microscopic theory is described for ion solvation dynamics in dipolar fluids. The theory takes account of inertial and non‐Markovian effects which are of critical importance for the fast initial phase of the relaxation of the ion–solvent energy. The theory also includes diffusional effects important at long times. The results obtained are shown to be in good agreement with recent computer simulation studies of ions in Stockmayer solvents.

The Jahn–Teller effect in 9‐fluorotriptycene
View Description Hide DescriptionThe vibronic structure in the S _{1}(E)←S _{0}(A _{1}) resonant two‐photon ionization (R2PI) spectrum of supersonically cooled 9‐fluorotriptycene is assigned using three different Jahn–Teller (JT)model Hamiltonians for the excited ^{1} E state—E⊗e, (A⊕E)⊗e, and (A⊕E)⊗(a _{2}+e). The basic E⊗e interpretation is satisfactory. However, the fitted vibronic band frequencies and intensities are improved by including coupling to a second excited state^{1} A _{1} in an excitonmodel. Some further observed absorption bands are only assignable by invoking a molecular Barnett effect (momentum coupling to an a _{2} vibration). The measured fluorescenceemission spectra from different S _{1} vibronic levels are quantitatively reproduced within all three coupling schemes by the parameters fitted to the R2PI spectrum. Results are compared to previous calculations on unsubstituted triptycene. The JT stabilization energy is decreased by ∼10% upon fluoro bridgehead substitution, which is rationalized by the electron‐withdrawing effect of the F atom. For the same reason, the exciton splitting between the S _{1} and the S _{2} states, as calculated in the (A⊕E)⊗emodel, is reduced relative to triptycene. The ground state vibrational frequencies in the range 0–700 cm^{−1} are calculated using the semiempirical MOPAC 6.0/AM1 method and compared with the measured S _{0} frequencies, as well as those of triptycene. A 273 cm^{−1} degenerate C–F bending mode predicted by the AM1 calculation may explain several unassigned features in the higher‐energy (200–360 cm^{−1}) part of the R2PI spectrum, and may represent an example of e⊕e multimode coupling within a degenerate electronic state.

A spectroscopic investigation of the nonrigid–rigid transition of (benzene)_{13} during free jet expansion
View Description Hide DescriptionWe have traced the evolution of the benzene‐h _{6} transition in (C_{6}H_{6})(C_{6}D_{6})_{12} as a function of time (distance from the nozzle) in a supersonic expansion. Taking advantage of the special properties of the isotopically substituted (benzene)_{13} resonant two‐photon ionizationspectrum, we present the first experimental evidence for a nonrigid–rigid transition in a single‐size molecular cluster. Initially, under the conditions of our free‐jet expansion, (benzene)_{13} condenses in a nonrigid form, then solidifies into a well‐defined structural configuration.

An electron paramagnetic resonance study of Er^{3+} in Bi_{4}Ge_{3}O_{12} single crystals
View Description Hide DescriptionThe X‐band electron paramagnetic resonance study of Er‐doped Bi_{4}Ge_{3}O_{12}single crystals at 5 K is reported. The spectra have been attributed to Er^{3+} ions in the Bi^{3+} site, confirming the assumed trigonal site symmetry of Er^{3+} in previous crystal‐field analysis. A detailed study of the measured g‐factor values indicates that corrections to the existing crystal‐field parameters B _{ n } ^{ m } should be performed. It is suggested that corrections to the sixth‐rank parameters are particularly important, in agreement with spin‐correlated crystal‐field (SCCF) effects for Er^{3+}.

The effects of molecular geometry on the depolarized stimulated gain spectra of simple liquids
View Description Hide DescriptionDepolarized Rayleigh‐wing spectra of simple liquids, including CS_{2}, chlorinated methanes, benzene and benzene derivatives have been recorded using stimulated gain spectroscopy. To adequately interpret these spectra of simple liquids, a dephasing dynamical contribution in an intermediate time scale is needed in addition to a slower Debye reorientation relaxation and a faster broad inhomogeneous oscillator. The curve‐fit dynamical characteristic parameters of these simple liquids are tabulated for comparison. The microscopic molecular properties, shape anisotropy, and dominating moment of inertia are found to play an important role in determining the rates of interaction‐induced dynamics; smaller shape anisotropy and smaller moment of inertia give rise to faster collision rate in the intermediate frequency regime and faster oscillation in the higher frequency regime, respectively. In the low frequency region, the shear viscosity (a macroscopic property) of the liquid and the general molecular shapes dictate reorientation diffusion; for molecules within a family, the Debye relaxation rate is faster for liquids with smaller viscosity and shape anisotropy. Halogenation of benzene breaks the symmetry of the benzene ring giving two distinct oscillator frequencies. In the case of iodobenzene, three principle molecular axes are clearly distinct; it is necessary to use two Debye relaxation rates and two broad inhomogeneous oscillators to fit its stimulated gain spectrum.

HNSi: Hot bands in emission around 3 μm
View Description Hide DescriptionFour new hot bands of HNSi, 2ν_{1}−ν_{1}, ν_{1}+ν_{3}−ν_{3}, 2ν_{1}+ν_{3}−(ν_{1}+ν_{3}), and ν_{1}+ν_{2}−ν_{2}, are observed in emission from a radio frequency excited plasma with a high resolution Fourier transforminterferometer. The equilibrium rotational and vibrational parameters are determined for the first time. About 400 lines positions, including improved data on the ν_{1} band, are provided.

Continuous wave multiquantum electron paramagnetic resonance spectroscopy. IV. Multiquantum electron–nuclear double resonance
View Description Hide DescriptionWe report a theoretical and experimental investigation of the interaction of a coupled electron–nuclear spin system with three electromagnetic fields: two equally intense microwave fields resonant with the electron spin, and one radio‐frequency field resonant with the nuclear spin. This is an electron–nuclear double‐resonance experiment where the effect of the nuclear transition is detected via changes of the electron multiple photon transitions (MQ‐EPR) rather than steady‐state saturation and, therefore, is called multiquantum electron–nuclear double resonance (MQ‐ENDOR). The theoretical framework previously developed for the description of multiple photon phenomena in a two level system is extended to the case of a four level system. The equation of motion of the density matrix is solved in the presence of three fields, which results in five master equations relating various populations and coherences. A ten photon approximation is used to study the functional dependence on spectral parameters and determine the sensitivity of this technique to spin relaxation rates. The experimental investigation is carried out on a sample of tri‐t‐butyl phenoxyl radical dissolved in mineral oil. At low values of the electron saturation factor S _{ e }, the rf‐swept MQ‐ENDOR is a de‐enhancement of the 3Q‐EPR signal (i.e., the first intermodulation sidebands). As the microwave field strength increases, the MQ‐ENDOR signal changes phase by 180° due to dominance of generalized saturation. Higher order MQ‐EPR signals (i.e., higher order sidebands) have larger negative enhancement and tend to display smaller positive enhancement. Line splitting results if the microwave frequency difference or field strength is increased. The dependence of MQ‐ENDOR displays on various spectral parameters was found to be consistent with the general trends predicted by the theory. These displays provide a convenient way to estimate electron and nuclear relaxation rates.

Ion‐pair formation in the photodissociation of HCl and DCl
View Description Hide DescriptionThe vibrationally resolved excitation functions for Cl^{−} formation from jet‐cooled HCl and DCl, following photoabsorption in the region 75–86 nm (HCl/DCl+hν→H^{+}/D^{+}+Cl^{−}), are reported and compared with ab initio calculations. This completes our understanding of the various possible decay channels available to superexcited states of HCl and DCl that lie above the first ionization limit. Our experimental and theoretical results suggest that the Rydberg states responsible for ion‐pair formation in HCl and DCl via predissociation by the V ^{1}Σ^{+} ion‐pair state correspond to ^{1}Σ^{+} states that are only weakly autoionized. These same Rydberg states appear to act as, at least, one class of precursor states for neutral dissociation to yield excited H* and Cl* atoms.

Computer simulation study of the Rayleigh and Raman spectra of fluid N_{2}
View Description Hide DescriptionComputer simulations of the Rayleigh and Raman spectral time‐correlation functions (TCFs) are reported for N_{2} at 323 K and densities ranging from 0.48 to 2.5 times the critical value. The results are compared with experiment. Particular attention is paid to the density dependence of the spectral intensity due to the correlated permanent polarizability and to the interaction‐induced contributions to the polarizability. The partial cancellations that occur between various two‐, three‐, and four‐body terms in the cross and collision induced (CI) parts of the spectral TCFs are evaluated. It is shown that these terms are significant in both the Raman and the Rayleigh spectra, but cancellation greatly reduces their net contribution at all densities studied. The weak but uncancelled TCFs that are associated with orientational correlations of the molecular polarizabilityanisotropies are shown to be a significant part of the high density Rayleigh TCFs. It is argued that the long‐range nature of this TCF means that its simulated values are poorly known.

Theory of fluorescence excitation spectra using anharmonic‐coriolis coupling in S _{1} and internal conversion to S _{0}. I. General formalism
View Description Hide DescriptionA treatment of one‐ or two‐photon fluorescence excitation spectra is described using the vibration–rotation coupling of zeroth order states in the excited electronic state and nonadiabatic coupling to the ground state. Using perturbation theory, experimental harmonic frequencies, an anharmonic force field, and various theoreticalCoriolis coupling constants, a quasistationary molecular eigenstate in an excited electronic stateS _{1} is first calculated. The S _{1} eigenstate is then coupled via the nonadiabatic nuclear kinetic energy operator (internal conversion) to rovibronic states in the ground state manifold, the latter states approximated in a simple manner. A search algorithm is used to select the S _{1} dark states and the S _{0} states. Both the perturbation theory coefficient and the Franck–Condon factors are employed in the evaluation function used in the search. The results are applied in part II to the channel three problem in benzene.

Theory of fluorescence excitation spectra using anharmonic‐Coriolis coupling in S _{1} and internal conversion to S _{0}. II. Application to the channel three problem in benzene for the 14^{1}1^{2} band
View Description Hide DescriptionRotational lines in the fluorescence excitation spectra of the 14^{1}1^{2} band of the first excited singlet state (S _{1}) of benzene are calculated for various J and K. For this purpose, perturbation theory is used to obtain an ‘‘eigenstate’’ in S _{1}. Internal conversion to S _{0} via Franck–Condon (FC) factors is then calculated. A search procedure is used to obtain the important contributors to this S _{1} state and to this internal conversion process S _{1}→S _{0} using the perturbation theory coefficients and the FC factors in the evaluation function. At low J, the calculated lines with K=0 are sharp, other lines being broadened and diminished in intensity. The calculated K=0 lines have a linewidth proportional to J(J+1). For high J, the lines with K=J remain sharp, the other lines being broadened and diminished in intensity. These various results are in general agreement with the experimental findings. The onset of channel three in benzene occurs in the present mechanism via anharmonic‐Coriolis coupling in the S _{1} state plus internal conversion to S _{0}. The calculations suggest that, at low J, parallel Coriolis coupling causes mixing of the in‐plane mode‐excited ‘‘light state’’ with in‐plane modes that are anharmonically coupled to out‐of‐plane modes.
Dark states with certain excited out‐of‐plane mode contributions possess large FC factors for the internal conversion to S _{0}. At high J, on the other hand, the in‐plane modes are coupled directly to these out‐of‐plane modes by perpendicular Coriolis coupling. Paths involving two perpendicular Coriolis operators are important at high J in the present calculation—their matrix elements are larger at high J and so they become more competitive relative to purely anharmonic coupling operators. Such two‐Coriolis paths at high J are expected to yield multiple excitation in the out‐of‐plane modes and further enhance the internal conversion. The perpendicular Coriolis coupling is least at J=K and so these lines survive at high J. Two‐Coriolis operator paths are calculated to be relatively unimportant at low J. The present calculations, using the same electronic matrix element, account for both the low J _{ K=0} and high J _{ K=J } sets of lines being the dominant ones. Aspects regarding further study are discussed.

On the infrared‐active overtones nν_{3} of uranium hexafluoride
View Description Hide DescriptionA perspective is provided of the infrared‐active fundamental mode ν_{3} and the band structure of its overtones relevant for uranium hexafluoride. The treatment is based on nonlinear (Morse‐type) creation and annihilation operators. The spectrum generating algebra provides ready access to anharmonic spectral features which incorporate dissociationcharacteristics.

Hyperfine structures of the triplet states of ^{23}Na^{85}Rb
View Description Hide DescriptionThe sub‐Doppler high resolution excitation spectrum of NaRb was measured by selectively monitoring the fluorescence intensity from perturbed levels to the (1)^{3}Σ^{+} lower state. Many transitions were observed to the (2)^{3}Σ^{+} state perturbed by B ^{1}Π. Energy shifts of the perturbed levels were analyzed and the spin–orbit coupling constant between the (2)^{3}Σ^{+} and B ^{1}Π states was determined, together with molecular constants of each state. Fully resolved hyperfine splittings, induced by the coupling of electron spin with both ^{23}Na and ^{85}Rb nuclear spins, were observed in the transitions. By analysis of the observed splittings, the mechanism of perturbation was confirmed as due to Fermi contact interaction and hyperfine constants of the ^{23}Na^{85}Rb (2)^{3}Σ^{+} state were determined. The ratio of electron spin densities at ^{23}Na and ^{85}Rb in the (2)^{3}Σ^{+} state is estimated to be 0.84:0.16.

Slit‐jet near‐infrared diode laser spectroscopy of (DCl)_{2}: ν_{1}, ν_{2} DCl stretching fundamentals, tunneling dynamics, and the influence of large amplitude ‘‘geared’’ intermolecular rotation
View Description Hide DescriptionThe first high resolution spectra of (DCl)_{2} are reported using direct IR laser absorption spectroscopy in a slit supersonic expansion. The spectral data are analyzed to obtain vibrational frequencies, rotational constants, and tunneling (interconversion) level splittings for isotopically symmetric (D^{35}Cl)_{2} and (D^{37}Cl)_{2}, and mixed D^{35}Cl–D^{37}Cl dimers. Six dimer absorption bands are observed and analyzed for both (D^{35}Cl)_{2} and D^{35}Cl–D^{37}Cl. These include two perpendicular K _{ a }=1←0, v _{1}=1←0 (i.e., ‘‘free’’ DCl stretch) bands, one each originating from the lower (+) and the upper (−) tunneling sublevels in the ground vibrational state. Four parallel v _{2}=1←0 (i.e., ‘‘bound’’ DCl stretch) bands are also observed, one for each of the K _{ a }=0←0 and K _{ a }=1←1 subbands originating from both the lower (+) and upper (−) tunneling components.
In addition, two bands are observed only for the isotopically mixed dimer (i.e., complexes from D^{35}Cl and D^{37}Cl), which acquire oscillator strength by virtue of the breaking of inversion symmetry. This complete set of bands provides the necessary data to determine interconversion splittings for the mixed dimer in the ground [5.9595(6) cm^{−1}] and the two DCl vibrationally excited states [3.2286(6) cm^{−1} for v _{1}=1 and 2.9935(6) cm^{−1} for v _{2}=1], as well as to make accurate predictions for the symmetric (D^{35}Cl)_{2} dimer. These experimental splittings for the ground state are compared to results from (i) a 1D quantum calculation for adiabatic motion over a minimum energy tunneling path; and (ii) a 3D variational calculation in a basis set of free DCl rotors which treats all three internal bend and torsion angles (Θ_{1}, Θ_{2}, and φ_{1}–φ_{2}). These calculations, performed on an approximate dipole and quadrupole model of the electrostatic potential surface, reproduce the ground statetunneling splittings to within 15%. The corresponding eigenfunctions provide direct evidence for highly correlated, ‘‘geared’’ internal rotation of the two DCl subunits over a low barrier. The fivefold decrease in tunneling splitting for the symmetric (DCl)_{2} upon v _{1}=1 or v _{2}=1 excitation is qualitatively consistent with previous models of vibrationally diminished tunneling rates due to intramolecular V→V energy transfer at the C _{2h } transition state. However, this decrease is nearly identical to the 4.8‐fold decrease observed in (HCl)_{2}, which is quantitatively inconsistent with a simple dipole–dipole vibrational energy transfer mechanism. Measured linewidths in these dimer spectra are all at the resolution limit of the diode laser apparatus, which translates into vibrational predissociation lifetimes in excess of 3 ns.

A (3+1) resonance enhanced multiphoton ionization study of the C ^{1}Σ^{+} and E ^{1}Π states of CO: Polarization dependence used to probe electronic excitation routes and electronic character
View Description Hide DescriptionThe three‐photon excitations of the C ^{1}Σ^{+} and E ^{1}Π states have been investigated with resonance enhanced multiphoton ionizationspectroscopy as a function of the polarization of the excitation light. From the observed polarization dependence the contributions of the various excitation routes have been determined, and the electronic character of these states has been analyzed. The results indicate that the v’=0 and 1 vibrational levels of the E ^{1}Π state have slightly different electronic characters.

About origin of Q component on the vibration–rotation band of HF in simple solvents
View Description Hide DescriptionThe evolution of the fundamental ν_{HF}absorption in the spectra of HF/Ar, Xe, CO_{2}, CO mixtures at densities varying from dilute gas to liquid has been studied. The spectral region of the HF librational mode has been simultaneously examined. The results show that molecular complexes play the crucial role in the mechanism of formation of the Q component in the spectra of HX in simple solvents.

High resolution S _{1}←S _{0} fluorescence excitation spectra of hydroquinone. Distinguishing the cis and trans rotamers by their nuclear spin statistical weights
View Description Hide DescriptionHydroquinone (HYQ) has two rotational isomers, differing in the orientation of the two O–H bonds with respect to the benzene ring. In the S _{1}←S _{0}fluorescence excitation spectrum of HYQ, two electronic origins, one for each rotamer, are present with a separation of 34.7 cm^{−1}. Rotationally resolved spectra of both origins have been obtained and analyzed. This analysis reveals that the rovibronic lines in the two origins exhibit different alternating intensity patterns due to nuclear spin statistical weights, thereby providing a means for distinguishing the two rotamers. On this basis, we assign the origin at 33 500 cm^{−1} to the trans, C _{2h } rotamer and the origin at 33 535 cm^{−1} to the cis, C _{2v } rotamer of HYQ. Coordinate determinations of the hydroxy–hydrogen atoms of both rotamers, in both electronic states confirm this assignment.