Index of content:
Volume 90, Issue 10, 15 May 1989

Reinvestigation of the Raman spectra of dihydrogen trapped in rare gas solids. I. H_{2}, HD, and D_{2} monomeric species
View Description Hide DescriptionThe vibration–rotation and pure rotational Raman spectra of H_{2}, HD, and D_{2} trapped in Ar, Kr, and Xe matrices have been recorded at 9 K. The frequencies which have been measured within an accuracy of 0.3 cm^{−} ^{1} are compared to the results of recent calculations. Except for the S _{0}(0) line of HD the agreement between observed and calculated matrix shifts is satisfactory. The anomalous blue shift observed for the S _{0}(0) line of HD is well interpreted within the rotation translation coupling (RTC) framework.

Predissociation spectroscopy of cadmium dimethyl
View Description Hide DescriptionThe absorption and fluorescence excitation spectra of jet cooled Cd(CH_{3})_{2} and Cd(CD_{3})_{2} were measured in the structured region between 230 and 200 nm. The symmetric Cd–methyl stretch and methyl umbrella modes in the second excited electronic state of the latter molecule were found to the 304 and 790 cm^{−} ^{1}, respectively. Widths of the 0–0 lines (FWHM) were 101 and 25 cm^{−} ^{1}, respectively from which lifetimes of 53 and 265 fs were deduced. A weak fluorescence due to CdCH_{3} is excited by the absorption of Cd(CH_{3})_{2} and an even weaker fluorescence is excited in Cd(CD_{3})_{2}. The quantum yield of this fluorescence is below 10^{−} ^{4} at the electronic origin of the structured band and increases to about 10^{−} ^{3} above 2000 cm^{−} ^{1} vibrational energy.

Hyperfine and Zeeman quantum beats of the Na(3 ^{2} P _{3} _{/} _{2}–3 ^{2} S _{1} _{/} _{2}) transition, and the decay of coherence
View Description Hide DescriptionQuantum beats of the Na(3 ^{2} P _{3} _{/} _{2}–3 ^{2} S _{1} _{/} _{2}) transition and their change with the magnetic field were observed in pure Na vapor and Na+He gas by using a picosecond laser system. The energies of the hyperfine and Zeeman sublevels of the 3 ^{2} P _{3} _{/} _{2} and 3 ^{2} S _{1} _{/} _{2} states were calculated by using the reported hyperfine constants and g factors, and the theoretical curve of the quantum beats was calculated. The results were in good agreement with the observed profile of the quantum beats in phase of modulation, but the difference of the amplitudes increased with time. By comparing the amplitude of the theoretical curve of the quantum beats with that of the experimental curve, we determined the relaxation time of coherence. The decay of amplitude of the modulation was found to become faster as the pressure of the foreign gas (He) was increased. The decay of amplitude, i.e., the decay of coherence arises from the dephasing, which occurs from the fluctuation of the level energies induced by the interatomic interaction.

Excited state intermolecular proton transfer in matrix isolated β‐naphthol/ammonia complexes
View Description Hide DescriptionThe spectroscopy and proton transfer dynamics of matrix isolated β‐naphthol⋅(NH_{3})_{ n }, n=1,2...complexes have been studied. The complexes are formed by annealing of β‐naphthol/NH_{3}/argon matrices. The annealing studies indicate that the n=3, and probably n=4, complexes undergo excited state intermolecular proton transfer (ESPT), and this assignment is confirmed by comparison of experimental and simulated spectra. Time resolved emission studies indicate that the ESPT time is about 20 ps. These results are discussed in terms of simple tunneling theories.

Interatomic potentials for van der Waals complexes of group 13 metal atoms: AlAr, AlKr, and AlXe
View Description Hide DescriptionInteratomic potential parameters for the ground X _{1,2} ^{2}Π_{1/2,3/2} and excited B ^{2}Σ^{+} _{1/2} states of jet‐cooled van der Waals complexes of Al atoms with rare gases have been determined from fluorescence excitation and emission spectra. Vibrational numbering in the B states is established from isotopic and rotational broadening of fluorescence excitation line shapes. Bond distances are estimated by comparison with analogous states of alkali–rare gas molecules, and by modeling observed relative emission intensities with calculated Franck–Condon factors. Morse potentials are found to adequately describe the data for the ground states, but for the excited states there are indications of departures from Morse functions at large internuclear distances. Dissociation energiesD _{ e } are estimated from Birge–Sponer extrapolations. Multiplet splittings in Al–rare gas complexes are compared to those in In–rare gas complexes and are discussed in terms of an empirical treatment of spin–orbit coupling, which assumes that the spin–orbit coupling constant for the molecule is the same as that for the atom. The ground X _{1} state of each Al–rare gas molecule is more stable than that of the analogous In complex, whereas the spin–orbit excited X _{2} state is less stable. This is explained by the greater mixing between the X _{1} state and the dissociativeA ^{2}Σ^{+} _{1/2} state for indium over aluminum, caused by the greater spin–orbit coupling constant for the heavier metal.

Polarization fluctuations as a source of electron spin resonance linewidths and relaxation of spin probes in KH_{2}PO_{4}‐type of polar solids
View Description Hide DescriptionWhile the CrO^{3−} _{4} and SeO^{3−} _{4} radicals have been used as spin probes in numerous electron spin resonance(ESR) studies of phase transitions in the KH_{2}PO_{4}‐type of solids, discrepancies exist regarding the nature of molecular reorientations sensed by these probes, in particular, on whether the detected motion is thermally activated, with a well‐defined activation energy, ΔE. Our new ESR data on these probes show that their linewidth, Γ, varies as Γ=Γ_{0}+A exp(ΔE/k T)+B exp(−ΔE/k T). Here Γ_{0} is the temperature‐independent, residual linewidth and A and B are parameters that depend on the ESR line splittings and the Zeeman field. ΔE has been estimated to be ≈0.22±0.02 eV (20±2 kJ/mol) for CrO^{3−} _{4} in KD_{2}AsO_{4}, KD_{2}PO_{4}, NH_{4}H_{2}AsO_{4}, ND_{4}D_{2}AsO_{4}, and ND_{4}D_{2}PO_{4}, and SeO^{3−} _{4} in KD_{2}PO_{4}. The results suggest that the omission of the term B exp(−ΔE/k T) in all of the earlier analyses of the ESRlinewidth data from these probes was the major cause of the reported non‐Arrhenius behavior, especially in the high temperature (T≫T _{ c }) regime. Measurements utilizing externally applied electric fields indicate that this new, Orbach‐type, ESR line broadening process originates from thermally activated spin–flips via vibronic transitions, and ΔE provides a measure of the barrier height for the intrabond hydrogen transfer along the O–H⋅ ⋅ ⋅O bonds.

Simulation of aggregating colloids in shear flow
View Description Hide DescriptionComputer simulation is carried out for the kinetics of aggregating colloids in shear flow. The colloidal particles are assumed to form a cluster when they touch. The clusters can deform, rupture, and aggregate with other clusters. The motion of the clusters is determined by solving the equation of motion of individual particles. Cluster size and macroscopic viscosity are calculated as a function of shear rate and particle concentration. It is found that (i) at low concentration the particles form compact clusters, while at high concentration they form loose networks, and that (ii) despite the change in the cluster type, the steady state viscosity obeys the same power law for all concentrations. These results suggest that the key structural parameter governing the macroscopic viscosity is not the size of the clusters, but the size of the flowing unit which is a rigid block in the cluster.

An explanation of the ^{1}D_{2} anomalous crystal field splitting in PrCl_{3}
View Description Hide DescriptionTaking into account configuration interaction between 4f ^{2} and 4f ^{1}5d ^{1} v i a the odd rank crystal field parameters eliminates most of the well‐known discrepancy of the ^{1}D_{2} multiplet of trivalent praseodymium in PrCl_{3}, found when ‘‘standard’’ crystal field treatment is applied to experimental data. A b i n i t i o values of crystal field parameters agree with the new parameters including the configuration interaction.

The overtone spectrum of acetylene in the vibron model
View Description Hide DescriptionThe overtone spectrum of acetylene including all bending and stretching vibrations up to 20 000 cm^{−} ^{1} is calculated within the framework of the vibron model. Properties of the local to normal transition are discussed. It is indicated how the method can be extended to even larger molecules.

Two‐dimensional zero‐field nutation nuclear quadrupole resonance spectroscopy
View Description Hide DescriptionWe introduce a new two‐dimensional nuclear quadrupole resonance experiment, in which the first time period (t _{1}) is the duration of the radiofrequency exciting pulse; and the second (t _{2}) is the normal free precession of a quadrupolar nucleus at zero field. After double Fourier transformation, the result is a 2D spectrum in which the first frequency dimension is the nutation spectrum for the quadrupolar nucleus at zero field. For single crystals, this spectrum contains narrow lines, whose frequency, for axially symmetric tensors, is proportional to sin θ, where θ is the angle between the unique axis of the quadrupolar tensor and that of the transmitter/receiver coil. For polycrystalline samples we obtain powder line shapes which are reminiscent of high‐field nuclear magnetic resonance(NMR)powder patterns, and which allow determination of the asymmetry parameter η, which has previously only been obtainable using Zeeman perturbed nuclear quadrupole resonance (NQR) methods. Both theoretical spectra and several experimental examples are presented.

Measurement of vibrational frequencies of the H_{3} molecule using two‐step photoionization
View Description Hide DescriptionA two‐step photoionization scheme is used to determine vibrational frequencies of the n=3 Rydberg states of triatomic hydrogen. Symmetric‐stretch frequencies in orthotrihydrogen of 3212.1 and 3168 cm^{−1} are measured for the N=1 levels of the states 3s ^{2} A ^{’} _{1} and 3d ^{2} E‘, respectively. Transitions to bending‐mode excited levels in the 3d states and overtone levels of the 3p ^{2} E’ state are observed also.

The vibron–phonon sidebands in the Fourier‐transform infrared spectra of the molecular crystal CO_{2}
View Description Hide DescriptionFTIR investigations on polycrystalline CO_{2} at various temperatures show Stokes and anti‐Stokes sidebands coupled to the vibrational modes v _{3}, v _{2}, and v _{+}/v _{−} (Fermi dyad). The latter is infrared forbidden and may be eliminated from spectra of good quality polycrystalline films. The main peaks in the structure of the sidebands may be assigned to phonons from points of highest symmetry in the Brillouin zone. The vibron–phonon coupling process is mode specific and driven by electrical anharmonicity in case of v _{+} and v _{−}. For the infrared allowed fundamentals, and especially at the Γ point, the coupling process is also driven by mechanical anharmonicity. The dispersion of these internal modes at k=0 influences the coupled phonons and thus the one phonondensity of states is correctly mirrored by the v _{+} and v _{−} sidebands only.

General transform technique including mode mixing and non‐Condon coupling in resonance Raman scattering
View Description Hide DescriptionUsing our exact finite temperature time‐correlator model expressions for the optical absorption and resonance Raman (RR) excitation profiles including full linear plus quadratic electron–phonon coupling a n d linear non‐Condon coupling, we have generalized the absorption→RR profile transform technique to normal modes that have both mode mixing (Dushinsky rotation) and linear non‐Condon coupling. The new approximate transform provides a useful and convenient method for extracting information on mode mixing from measured RR excitation profiles. Compared with full m o d e l calculations, which are often prohibitively difficult for complex multimode systems owing to the fact that values of a l l of the model parameters are needed, the new transform is advantageous. This is because it allows one to compute a mode’s finite temperature profile using just the measured finite temperature absorption and model parameters of o n l y the Raman mode plus those modes d i r e c t l y mixed with it. The information about the remaining modes is carried through the calculation via the use of the measured absorption. Numerical illustrations will be presented on the use of the new transform as a probe of mode mixing. Comparison with a previous transform relation which includes only mode mixing and is restricted to T=0 K will also be discussed.

Longitudinal and transverse waves in a viscoelastic fluid
View Description Hide DescriptionWe have studied both the vertical–vertical and vertical–horizontal dynamic light scatteringspectra of triphenylphosphite over a wide range of viscosities, temperatures, densities, and pressures. In particular, we have focused on the propagating modes, both the longitudinal (Brillouin) and the transverse (shear) modes. We find that the linewidths of the associated spectral side peaks can be described over the entire range, from low viscosityhydrodynamic to high viscosityviscoelastic behavior, as a function solely of viscosity. The same is true for the frequency shifts of the shear waves but not for those of the longitudinal waves. We give an interpretation of these results. We also find that at a given temperature the shifts are unexpectedly linear in pressure over the entire range; this, we believe, is not a consequence of the pressure dependence of the adiabatic sound speed, but rather of a frequency‐dependent viscosity with a broad distribution of relaxation frequencies. Our analysis suggests that the low‐frequency longitudinal and shear viscosities are rather similar.

Rotational analysis of the 7000 Å (A ^{3}Φ→X ^{3}Δ) electronic emission system of diatomic vanadium mononitride (VN)
View Description Hide DescriptionThe (0,0) band of the electronic emission system of the diatomic molecule VN at ∼7000 Å has been generated and rotationally analyzed. The system is ^{3} Φ_{ r } →^{3} Δ_{ r } and is the vanadium analog of the niobium nitride system centered ∼6029 Å. The constants for the upper and lower (almost certainly the ground) states have been determined, including estimates of the spin–orbit coupling constants, despite the absence of satellite bands. The subband origins are somewhat asymmetrically located, probably due to the interaction of the ^{3} Δ_{2} and the higher lying (by ∼3000 cm^{−1} ) ^{1} Δ_{2} state. There is no evidence of localized perturbations in any of the subbands. VN has the shortest bond length (r _{0} =1.566 Å ) observed for any diatomic molecule containing a transition metal (apart from some hydrides). The (1,1) sequence bands have also been observed but have not been rotationally analyzed at this time.

Experimental determination of the Einstein coefficients for the N_{2}(B–A) transition
View Description Hide DescriptionWe have used a branching‐ratio technique to measure the relative variation in the transition‐dipole moment with internuclear separation for the N_{2}(B–A) transition. Our spectral observations cover the range from 500 to 1800 nm, and use several different detectors and excitation sources. The data from different sets are consistent in the regions of spectral overlap. Using well established values for the radiative lifetimes of N_{2}(B,v’≥5) allows the relative dipole‐moment function to be placed on an absolute basis. From the dipole‐moment function and a set of RKR‐based Franck–Condon factors which we have computed, we derive Einstein coefficients covering the range v’=0–12 and v‘=0–20. Our results indicate that currently accepted lifetimes for N_{2}(B,v’=0–2) should be revised upwards by 20% to 40%.

Optogalvanic observation of the CO W ^{1}Π–B ^{1}∑^{+} transition
View Description Hide DescriptionThe rotational structure of the W ^{1}Π–B ^{1}∑^{+} (0–0) band was resolved and the values of spectroscopic parameters were derived as B ^{’} _{0} =1.5549±0.0068 cm^{−} ^{1}, r _{0}=0.1257±0.0003 nm, and ν_{0} _{0}=15 891.6±0.6 cm^{−} ^{1}. The observation of the W ^{1}Π–B ^{1}∑^{+} (0–0) band supports Ogawa and Ogawa’s assignment that the W ^{1}Π state is the first member of the n sσ Rydberg series converging to the A ^{2} Π_{ i } state of CO^{+} ion. The W ^{1}Π–B ^{1}∑^{+} (1–0) band, on the other hand, was not observed with expected intensity and the presence of dissociative perturbation has been suggested.

The laser photoelectron spectrum of gas phase p‐difluorobenzene
View Description Hide DescriptionGas phase p‐difluorobenzene (PDFB) has been laser ionized from several vibronic levels of its ^{1} B _{2u } first excited electronic state. The vibrational state distribution of the ions generated depends on the vibronic level from which ionization occurs. The fundamental frequencies of five symmetric vibrations; ν_{2}, ν_{3}, ν_{4}, ν_{5}, and ν_{6}, and three asymmetric vibrations; ν_{8}, ν_{2} _{9}, and ν_{3} _{0}, of the lowest electronic state of PDFB^{+} have been assigned. Effects due to electronic configuration interaction are observed in the photoelectron spectra. Small peaks are also observed in the spectra which may reflect a change in geometry of the ground state cation.

The permanent electric dipole moment of iron monoxide
View Description Hide DescriptionThe Stark effect of the R(2), Q(2) and P(3) lines of the Ω’=2−X ^{5} Δ_{2} subband of the ‘‘orange’’ system of iron monoxide at 5919 Å has been measured. Values for the permanent electric dipole moments μ of 4.7±0.2 and 2.6±0.2 D for the ground and excited electronic states, respectively, were derived from the analysis of the Stark shift. The results are discussed in terms of the various bonding models and compared with theoretical estimates.

The 157 nm photodissociation of OCS
View Description Hide DescriptionThe photodissociation of OCS at 157 nm has been investigated by using tunable vacuum ultraviolet radiation to probe the CO and S photoproducts. Sulfur is produced almost entirely in the ^{1}S state, while CO is produced in its ground electronic state and in vibrational levels from v=0–3 in the approximate ratio (v=0):(v=1):(v=2):(v=3)=(1.0):(1.0):(0.5) :(0.3). The rotational distribution for each vibrational level is found to be near Boltzmann, with temperatures that decrease from 1350 K for v=0 to 780 K for v=3. Measurements of the CO Doppler profiles demonstrate that the dissociation takes place from a transition of predominantly parallel character (β=1.8±0.2) and that the CO velocity and angular momentum vectors are perpendicular to one another.