Index of content:
Volume 91, Issue 9, 01 November 1989
Infrared spectroscopy of carbo‐ions. V. Classical vs nonclassical structure of protonated acetylene C2H+ 391(1989); http://dx.doi.org/10.1063/1.457612View Description Hide Description
The problem of classical vs nonclassical structure of protonated acetylene (vinyl cation) C2H+ 3 has been studied using high resolution infrared spectroscopy. The spectrum has been observed in the 3.2 μm region in air‐cooled and water‐cooled plasmas using C2H2:H2:He mixtures and in liquid nitrogen‐cooled plasmas using CH4:H2:He mixtures. The difference frequency spectrometer with the velocity modulation method has been used to conduct the Doppler‐limited, high sensitivity spectroscopy.
The observed vibration–rotation pattern with the band origin at 3142.2 cm− 1 has been identified as due to the antisymmetric CH stretching ν6 band of the C2H+ 3 ion with the nonclassical (bridged) structure. The observed spectral pattern was anomalous, but definitive assignments could be made for a part of the spectrum using the ground state combination differences which fit to the usual asymmetric rotor pattern. The discrimination between the classical and nonclassical structures is based on the observed spectral intensity pattern due to spin statistical weights. Agreement of vibrational band patterns and the rotational constants with ab initio values gives supporting evidence. The anomaly of the spectrum is at least partly ascribed to the small energy difference between the classical and nonclassical structures and possible rearrangement between them, the idea used by organic chemists over the years in wet chemistry. Systematic splittings with the intensity ratio of 2:1 have been noticed in some parts of the spectrum indicating that the protons tunnel between the apex and the two end equilibrium positions of the bridged structure. Using a simplified internal rotation model proposed by Hougen, the barrier height of the tunneling has been estimated. Chemical kinetics in plasmas related to C2H+ 3 is also discussed.
We conclude that (1) the nonclassical structure is lower in energy than the classical structure, and (2) the apex proton and the two end protons exchange their positions with a measurable time scale.
An a b i n i t i o semirigid bender calculation of the rotation and t r a n s‐tunneling spectra of (HF)2 and (DF)291(1989); http://dx.doi.org/10.1063/1.457613View Description Hide Description
Using a purely a b i n i t i o minimum energy path for the t r a n s‐tunneling motion in the HF dimer, the energy levels for the K‐type rotation and t r a n s‐tunneling motion for (HF)2 and (DF)2 are calculated with a one‐dimensional semirigid bender Hamiltonian and no adjustable parameters. The transition moments for rotation‐tunneling transitions are calculated, using our a b i n i t i o value for the dipole moment of an isolated HF molecule, and we also calculate B̄ values. The energy levels we obtain are in close agreement with experiment; for example, the K=0 tunneling splitting in (HF)2 is calculated as 0.65 cm− 1 compared to the experimental value of 0.658 69 cm− 1. As well as showing that our a b i n i t i o minimum energy path is very good, the calculation demonstrates that the semirigid bender formalism is able to account quantitatively for the unusual Kdependence of the rotational energies resulting from the quasilinear behavior, and that the t r a n s‐tunneling motion is separable from the other degrees of freedom. We use the results to predict the locations, and transition moments, of the ΔK=0 and ±1 subbands in the tunneling spectra of (HF)2 and (DF)2, many of which have not yet been observed.
91(1989); http://dx.doi.org/10.1063/1.457614View Description Hide Description
A new u p p e r b o u n d for the dissociation energy of acetylene, D 0 0(HCC–H) =529.89(±0.01) kJ/mol, has been determined by Stark anticrossing spectroscopy. The zero‐pressure extrapolated (unimolecular) decay rates of levels of S 1 (Ã 1 A u ) v ’ 3=2 and 3 (quanta of the t r a n s‐bending vibration) i n c r e a s e upon application of an electric field of 113 kV/cm. We attribute this increase in molecular decay rate to p r e d i s s o c i a t i o n rather than any other Stark‐induced nonradiative or radiative phenomenon. The lowest level (v ’ 3 =2, J’=2, K’=1) from which we have observed an increase in decay rate (i.e., predissociation) has an internal energy of 44 295.65 cm−1 relative to v=0, J=0 of S 0 (X̃ 1∑+ g ). This corresponds to a value about 24 kJ/mol lower then the consensus u p p e r b o u n d result from four different recent experimental determinations (including one of ours) as well as current a b i n i t i o results. The new value agrees, however, with earlier work and with recent modeling studies of acetylene pyrolysis kinetics.
91(1989); http://dx.doi.org/10.1063/1.457615View Description Hide Description
The inelastic neutron scattering (INS) spectra of some ammonium halides and ammonium nitrate are analyzed in terms of the internal vibrations of the NH+ 4 ion convoluted with multiphonon lattice modes up to the eighth term. Due to the low mass of this ion, most of the intensity of the internal‐mode region of the INS spectra is in the ‘‘phonon wings,’’ differences between the salts in this spectral region being mainly due to differences in their lattice‐mode spectra. Refinement of the lattice Debye–Waller factor is included in the profile refinement of interatomic force constants.
91(1989); http://dx.doi.org/10.1063/1.457616View Description Hide Description
The absorption spectra of high‐nRydberg states of methyl iodide and benzene perturbed by varying number densities of hydrogen or argon, range 0.9×102 0–10.5×102 0 cm− 3 for H2 and 0.6×102 0–7.5×102 0 cm− 3 for Ar, have been investigated. The high‐n molecular states of both absorbers were found to shift linearly with the number density of atomic Ar and molecular H2scatterers. The Fermi formula modified by the Alekseev–Sobel’man polarization term provides an excellent fit of the shift data. The electron scattering lengths obtained are: 0.93 a 0 for H2 and −1.63 a 0 for Ar using the CH3I absorber; and 0.99 a 0 for H2 and −1.57 a 0 for Ar using the C6H6 absorber. The electron scattering lengths for H2 and Ar agree with the results of an empirical model that correlates scattering lengths and the polarizabilities α(spherical) for inert atoms and α2(nonspherical) for H2 molecule.
91(1989); http://dx.doi.org/10.1063/1.457617View Description Hide Description
Theory based on a configuration coordinate model is presented to clarify solute–solvent relaxation effect on the line shape of a time‐resolved hole‐burning spectrum. The time‐resolved hole‐burning spectrum is affected by both a hole created in a ground‐state population and an excess excited‐state population. The solute–solvent relaxation causes thermalization in both populations and then the shift and broadening of the spectrum is expected observable. The line shapes of three typical cases are mainly considered: (1) The excited state is assumed to have an infinitely long lifetime; (2) the excited state has a finite lifetime, the population being assumed to relax toward an electronic state other than the ground state, and (3) the population created in the excited state relaxes to the ground state, where a hole‐filling effect is taken into account. Results obtained by numerical calculations show that a time evolution of the ground‐state hole is clearly observed when the low‐energy tail of the absorption band is excited under the condition that the population decay to the other electronic state is very fast. Further, it is predicted that the hole‐filling effect causes a considerable deformation of the spectral shape when the solvent relaxation rate is comparable to the excited‐state population decay rate.
Multiphoton ionization of O2 X 3Σ− g , a 1Δ g , and b 1Σ+ g via the two‐photon resonant n sσ g , n dσ g , and n dπ g Rydberg levels91(1989); http://dx.doi.org/10.1063/1.457589View Description Hide Description
Multiphoton ionization spectra have been obtained and analyzed for excitation in the 215–360 nm region from the X 3Σ− g , a 1Δ g , and b 1Σ+ g states of O2. The 0–0 band of the C 1Π g state is reported for the first time. Measurements of other vibrational bands terminating in the C 3Π g and d 1Π g states are in good agreement with determinations by other groups. Several vibrational levels (v’=0–5) of the 3dπ g Rydberg complex have been assigned on the basis of (1) an analysis of the spin–orbit couplings between the (Λ,S) basis‐set states, (2) spectral simulation, and (3) the behavior of the states when the excitation radiation is changed from linear to circular polarization.
Rotational populations in OD formed in the reaction O(1D)+D2 investigated by infrared rotational absorption spectroscopy91(1989); http://dx.doi.org/10.1063/1.457590View Description Hide Description
Diode laser transient absorption/gain spectroscopy is used to monitor time‐dependent populations of high rotational levels in OD (v=0) produced in the reaction of O(1D)+D2. Pure rotational transitions on species with large dipole moments offer good sensitivity, full state resolution and μs time resolution in the present apparatus. Measured nascent populations of OD in the four highest rotational levels thermodynamically accessible in this reaction are in reasonable agreement with the reported results of earlier laser‐induced‐fluorescence measurements, in which corrections for transition moments and predissociation introduce increasing uncertainties at high rotational levels. The relaxation kinetics of the highest rotational levels are not hopelessly complex, and evidence is presented for strong, but not complete propensity for conservation of Λ doublet symmetry during rotational relaxation.
Molecular alignment from circular dichroic photoelectron angular distributions in (n+1) resonance enhanced multiphoton ionization91(1989); http://dx.doi.org/10.1063/1.457591View Description Hide Description
The theory for determination of molecular alignment from circular dichroism in photoelectron angular distributions is generalized to treat the case in which the excitation polarization direction and the laboratory z axis do not coincide. A new method of data analysis is presented here. Alignment created by surface scattering or photofragmentation should be obtainable by these procedures. For studies of orientation with elliptically polarized excitation, differential cross sections at a given collection angle are found to be, to a good approximation, independent of excited‐state alignment. Orientation can thus be obtained from d i f f e r e n t i a l cross sections by the methods developed by Kummel, Sitz, and Zare [J. Chem. Phys. 8 8, 6707 (1988)].
91(1989); http://dx.doi.org/10.1063/1.457592View Description Hide Description
Using Raman scattering we have studied the molecular vibrations in freely suspended liquid crystalfilms as thin as two molecular layers. The effect of the free surfaces on several aspects of molecular order were examined by comparing the Raman spectra obtained on both thin and thick films. The liquid crystal materials 8CB, 8SI, and 65OBC were studied in the crystal, hexatic, and fluid smectic phases. Changes were observed in both intramolecular and intermolecular vibrations as a function of thickness.
Interaction induced Raman spectroscopy of the dipole forbidden π u mode in neat CO2 and solutions with N2 as supercritical fluids91(1989); http://dx.doi.org/10.1063/1.457593View Description Hide Description
The interaction induced Raman spectra (IIRS) of 1:0, 1:1, and 1:3 CO2 :N2 solutions have been studied at 50 °C and 75 °C at densities up to 2.5 times the triple point density (90–3010 bars). M 0, M 2 , and M 4 evaluations have been made upon the π u ν2 Raman dipole forbidden signal. In addition, the absolute Raman scattering cross sections have been evaluated for the 1:0 and 1:1 solutions. A broad low intensity high frequency shoulder was seen in the 1:0 data of ν2 and is identified with a CO2 oscillating on a large cluster of CO2 ; this perturbs the isolated ν2 frequency; the shoulder is not found in the 1:1 or 1:3 solutions. Significant many body cancellation has been observed in the 1:0 data at 50 and 75 °C at near solid densities of the fluid. The nature of the ν2 signal shows that there is a significant difference between the neat and N2 –CO2 solutions. In particular, depolarization measurements upon ν2 in the 1:0 and 1:3 solutions at 50 °C and 1255 bars of pressure showed very different behavior. This would seem to indicate quite a different local field environment, i.e., local structures. All Raman intensity correlation functions show times in the 30–60 femtosecond regime, we believe that such fast times can be related to the behavior of individual CO2 moieties. Last, this IIRS (‘‘ears’’) study is consistent with the earlier findings of Amos, Buckingham, and Williams that angle averaged pair entities are the principal intermolecular scattering species and must be present to give rise to the quadrupole–induced‐dipole (QID) allowed ν2 signal.
A theoretical intensity study of the interaction induced Raman spectra of the ν2(π u ) mode of CO2 between 100 and 3000 bars of pressure91(1989); http://dx.doi.org/10.1063/1.457594View Description Hide Description
The density dependent integrated intensities of the interaction induced (ii) dipole forbidden Raman spectra of the CO2 ν2(π u ) mode have been calculated by the use of statistical mechanics based on the quadrupole–induced‐dipole (QID) mechanism. The calculated intensities were compared with experimentally determined intensities. The intensities examined were between 5.03 to 17.00×102 1 molecules/cm3 and the temperatures examined were 50 and 75 °C. The results indicated that at densities up to approximately 2.5× the triple point density, the integrated intensities can be expressed as a function of the density squared. This density square dependence of the integrated intensity may be due to almost perfect cancellation between constructive intensity originated from (CO2)2 dimers and the destructive intensity of classical three body interactions which have also been calculated. In any case, extra intensity was present as a high frequency shoulder at all densities except the lowest and highest values and was not accounted for by our calculations.
91(1989); http://dx.doi.org/10.1063/1.457595View Description Hide Description
The vibrational overtone spectrum of H2 O2 vapor between 7400 and 7600 Å is recorded under both bulk gas and supersonic beam conditions. An absorption band corresponding to a Δv=4 O–H stretch is observed in this spectral region. Rotational analysis indicates that it is a hybrid band with mainly parallel character. In addition to the 59 rovibrational transitions that could be assigned to this band, another 83 lines are observed that did not fit an interpretable asymmetric rotor pattern. Spectral linewidths are found to be Doppler limited in all cases (7 MHz FWHM with the supersonic beam).
Changes in electronic polarizability densities due to shifts in nuclear positions, and a new interpretation for integrated intensities of vibrational Raman bands91(1989); http://dx.doi.org/10.1063/1.457596View Description Hide Description
The nonlocal polarizability density α(r;r’,ω) is a linear‐response tensor that determines the electronic polarization induced at point r in a molecule, by an external electric field of frequency ω, acting at r’. This work focuses on the change in α(r;r’,ω) when a nuclear position shifts infinitesimally. We prove directly that the electronic charge distribution responds to the change in Coulomb field due to the nucleus via the same hyperpolarizability density that describes its response to external fields. This generalizes a result found previously for the static (ω=0) polarizability density. The work also provides a new interpretation for the integrated intensities of vibrational Raman bands: it proves that the intensities depend on the hyperpolarizability densities and the dipole propagator.
91(1989); http://dx.doi.org/10.1063/1.457571View Description Hide Description
The only large amplitude motion possible for an n‐alkane molecule in urea‐inclusion compounds is libration–torsion about the long axis of the chain. We present a quantitative model that incorporates the effect of this motion on the widths of the alkane vibrational bands. This model explains the difference in the widths of the different vibrations of the alkanes and their temperature dependence. Two effects are combined: (1) a modulation of the angles between the components of the polarizability in the space and the molecule‐fixed frames for Raman spectra or between the components of the dipole moment for the infrared spectra, and (2) a modulation of the frequency of the alkane vibration via anharmonic coupling terms with the libration–torsion. The first effect gives rise to a distinctly non‐Lorentzian band shape, which is convoluted with the approximately Lorentzian band of the second effect to produce the final result. The libration–torsional motion is modeled as that of a Brownian harmonic oscillator. Most of the parameters that enter the calculation are obtained from data other than that involving the bandwidths themselves. The libration–torsion relaxation time of about 1 ps obtained from fitting the observed bandwidths agrees with the value obtained from recent quasielastic neutron scattering experiments. Other bandwidth mechanisms that have been proposed are evaluated and it is shown that site hopping is too slow to account for the observations.
91(1989); http://dx.doi.org/10.1063/1.457572View Description Hide Description
The first excited electronic state (S 1) vibrational dynamics of aniline(Ar)1 and aniline(CH4)1 van der Waals (vdW) clusters have been studied using molecular jet and time resolved emission spectroscopic techniques. The rates of intramolecular vibrational energy redistribution (IVR) and vibrational predissociation (VP) as functions of vibrational energy are reported for both clusters. For vibrational energy in excess of the cluster binding energy, both clusters are observed to dissociate. The dispersed emission spectra of these clusters demonstrate that aniline(Ar)1 dissociates to all energetically accessible bare molecule states and that aniline(CH4)1 dissociates selectively to only the bare molecule vibrationless state. The emission kinetics show that in the aniline(Ar)1 case, the initially excited states have nanosecond lifetimes, and intermediate cluster states have very short lifetimes. In contrast, the initially excited aniline(CH4)1 states and other intermediate vibrationally excited cluster states are very short lived (<100 ps), and the intermediate cluster 00 state is observed. These results can be understood semiquantitatively in terms of an overall serial IVR/VP mechanism which consists of the following: (1) the rates of chromophore to vdW mode IVR are given by Fermi’s golden rule, and the density of vdW vibrational states is the most important factor in determining the relative [aniline(Ar)1 vs aniline(CH4)1] rates of IVR; (2) IVR among the vdW modes is rapid; and (3) VP rates can be calculated by a restricted vdW mode phase space Rice–Ramsberger–Kassel–Marcus theory. Since the density of vdW states is three orders of magnitude greater for aniline(CH4)1 than aniline(Ar)1 at 700 cm−1, the model predicts that IVR is slow and rate limiting in aniline(Ar)1, whereas VP is slow and rate limiting in aniline(CH4)1. The agreement of these predictions with the experimental results is very good and is discussed in detail.
Determination of the geometry of deuterated tryptamine by rotationally resolved electronic spectroscopy91(1989); http://dx.doi.org/10.1063/1.457573View Description Hide Description
The low and high resolution fluorescence excitation spectra of d 3‐tryptamine have been observed in the environment of a cold, supersonic molecular beam. As in the case of h 3‐tryptamine, six bands due to the origins of different conformers have been found in the low resolution spectrum of deuterated tryptamine. A previous paper reported that conformers labeled A and B and conformers labeled D and E of tryptamine have identical rotational structures. However, for deuterated tryptamine those conformers have distinguishable rotational structures. Analysis of the rotational structure in the high resolution electronic spectra of five of the bands was used to determine the geometries of the different conformers. Conformers A, B, and F have a g a u c h e conformation with respect to the rotation about the C α–C β bond while conformers D and E have an eclipsed conformation. The geometries of conformers A and B and conformers D and E differ only in the orientation of the amino group. In these structures the angles of the internal rotation of the amino group are 180°, 60°, 180°, 60°, and −60° for conformers A,B, and D–F, respectively. Feature C consists of two overlapped conformers, and it is suggested that these conformers have the amino group t r a n s to the indole ring.
91(1989); http://dx.doi.org/10.1063/1.457574View Description Hide Description
The technique of Brillouinspectroscopy has been used to determine the adiabatic elastic constants of single crystals of the orientationally disordered phase (phase I) of CBr4 .The values of the elastic constants at 333 K are C11 =42.2, C12 =33.6, and C44 =16.1 (in units of kbar) and suggest that there is strong rotation–translation coupling in the k=0 acoustic wave vector regime. Because x‐ray Laue diffraction was not possible in solid CBr4 , the necessary orientation of the single crystals was determined by also using the Brillouin scattering spectra.
91(1989); http://dx.doi.org/10.1063/1.457575View Description Hide Description
A theoretical model, based on the Lorentzequations for ion motion and the mass action law, is developed for two‐dimensional Fourier‐transform mass spectrometry known as 2D FT–ICR or 2D FTMS. The theory illustrates that the modulation of 2D FT–ICR ion signals in the additional time dimension comes from the modulation of the primary ion speed by the 2D excitation pulses. The modulation of the primary ion speed is found not to be sinusoidal and the modulation of the ion signals in 2D FT–ICR spectra is found to be complicated even in the simplest chemical system. The complex modulation creates higher harmonic components in the spectra. Based on the model, a data processing algorithm is proposed. The results show that the Fourier transformation should be performed stepwise in order to obtain complete information, and that the phase portion of the frequency domain generated by the second Fourier transformation should not be discarded since it contains useful information.
Computer simulation of depolarized light scattering from diatoms with hard core and square well interactions at low temperatures91(1989); http://dx.doi.org/10.1063/1.457576View Description Hide Description
We show that the depolarized light scattered from two interacting atoms is qualitatively different for a purely repulsive hard core potential and partially attractive square well potential. This difference is due to the bound states possible with attractive potentials and is used to obtain the spectra from these dimers. The spectra were calculated from the autocorrelation of the scatteredelectric field found by an effective field model. Values were phase space averaged using Gaussian integration. Molecular dynamics routines evolved the phase space points in time for the autocorrelation. The hard core spectra showed an exponential decay characterized by the mean passage time. The square well spectra consisted of a hard core‐type term, plus a harmonic component due to dimer contributions. Subtraction of the hard core spectra from the square well spectra gave the dimer component. The dimer component of the spectra was proportional to the square of the number of dimers and was characterized by the thermal rotational energy for temperatures below the saturation temperature.