Volume 74, Issue 7, 01 April 1981
Index of content:

General theory of the Raman scattering close to a plane surface. Evanescent Raman spectra
View Description Hide DescriptionWe present a general theory of Raman scattering near the surface of a plane dielectric medium. All possible cases are considered but only the ones in which evanescent waves are present are discussed (evanescent excitation or/and evanescent scattering). In each case the intensity and angular distribution patterns of the scattered light are discussed. The problem of measurement of depolarization ratios of Raman bands is also considered.

Calculation of Raman intensities for the ring‐puckering vibrations of trimethylene oxide and cyclobutane. The importance of electrical anharmonicity
View Description Hide DescriptionRaman intensities are calculated for the ring‐puckering vibration of trimethylene oxide (TMO) and cyclobutane using an anisotropic atom–point dipole interaction model to calculate the elements of the molecular polarizabilitytensor. Three different models for the ring‐puckering motion are examined: (i) a model in which the methylene groups are held rigid to the molecular frame as the ring puckers, (ii) a dynamical model in which the methylene groups rock (TMO and cyclobutane) and wag (TMO) as the ring puckers, and (iii) a second rigid model in which all of the polarizability of the molecule is localized on the atoms of the ring skeleton. All three models for the ring‐puckering motion predict unusually large second‐order terms in the expansion of the polarizabilitytensor elements in the ring puckering coordinate [‖∂^{2}a_{mn}/∂Z ^{2})_{0}‖≳0]. These terms result in intense Dv = 2 overtone transitions. The calculated relative intensities of the members of the Dv = 2 overtone progression are in good agreement with those observed for both molecules. The calculations also predict that for TMO the intensities of the Dv = 2 overtones are comparable to the Dv = 1 fundamentals. This result when evaluated in terms of the expected differences in the band shapes of the two types of transitions readily accounts for the predominance of the overtone transitions in the Raman spectrum of TMO.

A determination of the spin–rotation parameters for NO_{2} in the X̃ ^{2} A _{1} state by microwave–optical double resonance
View Description Hide DescriptionMagnetic dipole transitions between the spin components of rotational levels of NO_{2} in its ground state have been observed by the technique of microwave–optical double resonance.Fluorescence induced by an argon ion laser was used to monitor the process. Observations were made in the range 4 to 12 GHz and provide information on levels with a wide spread of N and K _{a} values. The results have been used in conjunction with the known electric dipole transition frequencies and some recent data from radio frequency–infrared double resonance to determine the rotational and spin–rotational parameters for the molecule in its ground state. It has proved possible to determine all the three quadratic and six quartic parameters in the spin–rotational Hamiltonian. The improvement in knowledge of the spin–rotation intervals allow a refinement of the Zeeman parameters for NO_{2} by refitting the data from the EPR and LMR spectra. The results are in good agreement with the relationships g _{ S } ^{aa} = g _{ S }−e_{aa}/2B _{aa}, etc.

Continuous wave intracavity dye laser spectroscopy. II. A parametric study
View Description Hide DescriptionThe sensitivity enhancement of cw intracavity dye laser spectra of I_{2} and of water vapor has been measured for four dye laser tuning systems, two cavity lengths, and a wide range of pumping powers. The longitudinal mode structure of the dye laser has also been examined. The sensitivity enhancement increases with the bandpass of the dye laser tuning element and is independent of the cavity length. It is only weakly dependent on pumping power except near threshold, where it climbs steeply. The experimental results are compared with predictions of several theories which seek to explain the basis for the enhancement. None of the predictions are in quantitative agreement with our results; and, except for the prediction that enhancement increases near threshold, the theories also give generally poor qualitative agreement with experiment.

Radiation‐induced color centers in p a r a‐terphenyl
View Description Hide DescriptionP a r a‐terphenyl single crystalsirradiated with g rays at liquid nitrogen temperature have been investigated by electron paramagnetic resonance and optical absorption experiments. In the optical absorptionspectra at 77 K, obtained with the light polarized to the a, b, and c′ axes of the crystals, four absorption bands were detected both in the longer wavelength region and in the shorter one. These absorptions were compared with the transition energies and moments of 2‐,3‐,4‐, and 9‐hydroterphenyl radicals calculated using SCF–CI molecular orbital theory. The 707 and 637 nm bands which are polarized along the c′ axis and stable at room temperature were assigned to 2‐ and 9‐hydroterphenyl radicals, respectively. The very weak transitions of 716 and 604 nm were ascribed to the transitions of 4‐ and 3‐hydroterphenyl radicals, respectively. The four bands in the shorter wavelength region were assigned to the four kinds of hydroterphenyl radicals considering the results of the thermal and optical annealing experiments. Upon annealing at room temperature, growths of four c′‐polarized clear absorption bands at 497, 536, 567, and 583 nm were observed. These bands were attributed to the absorptions of four kinds of hydrogen added terphenyl dimers judging from the different annealing behaviors and theoretical results. The EPR spectrum at room temperature exhibits triplet–triplet characteristics with splittings of 41 and 8.5 G at a rotation angle of 15 °. This spectrum was attributed to the signals arised from 2‐ and 9‐hydroterphyl radicals.

A ^{59}Co NMR study of the magnetic shielding of the cobalt nucleus in cobalt (III) complexes
View Description Hide DescriptionThe temperature dependence (in the range 0–90 °C) of the ^{59}Co magnetogyric ratios and of the ^{1} A _{1g }→^{1} T _{1g } electronic transition wavelengths for six octahedral cobalt (III) complexes were measured. For each complex a linear correlation between ^{59}Co magnetogyric ratios and the electronic transition wavelengths was obtained, in accordance with the prediction of ligand fieldtheory. All the correlation lines are of the same origin, thus stating that the diamagnetic shielding term is weakly influenced by ligands. This enables determination of the value of the ^{59}Co nuclear magnetic moment: m = (4.617±0.005)m_{ N }. The slope of the correlation line varies from one complex to another directly with the nephelauxetic ratio b_{35} of the complex considered. On this basis a more general correlation, between ^{59}Co magnetogyric ratios and electronic transition wavelengths reduced by nephelauxetic ratio, is suggested. As a consequence, it may be expected that m a g n e t o c h e m i c a l s e r i e s o f l i g a n d s in transition metal complexes is independent of the central ion.

Electron spin‐lattice relaxation times of transient free radicals
View Description Hide DescriptionThe saturation recovery method has been used to measure the electron spin relaxation times of short‐lived free radicals in solution. The radicals were produced by continuous i n s i t u radiolysis with a 2.8 MeV electron beam (except for two cases where stable radicals were formed by air oxidation). The ESRspectrometer used broad‐band amplification of the detected ESR signal (time constant of 100 ns) and signal averaging by means of a transient recorder and minicomputer. The magnetic field was stepped off the ESR line for alternate saturating pulses; the phase of the microwave saturating pulse could be changed by 180 ° to allow separation of the saturation recovery and free induction decay. The sensitivity was such that T _{1} values greater than 1 ms could be determined for samples of 10^{14} spins (i.e., 3×10^{−6} M radical in 50 ml) if the ESRlinewidth was 100 mG and transient curves from 40 000 saturating pulses were averaged. Somewhat smaller values of T _{1} could be determined with higher radical concentrations. To achieve this sensitivity, it was necessary to use observing microwave powers which significantly perturbed the recovery signal so that an analysis in terms of an exponential decay of the ESR signal to the steady state value was not possible. Curve fitting of solutions to modified Bloch equations was used. Values of T _{2} were determined from the damping of Torrey oscillations. Values of T _{1} were determined for p‐benzo‐semiquinone anion (2.0), the 2,5‐di‐t e r t‐butyl analog (11.5), ascorbate anion radical (2.3), chelidonic acid trianion radical (5.0), 1,2‐dicarboxylvinyl radical (9), and H atoms (13.5), all values in ms and accurate to about ±10%. The radicals were produced in aqueous solution except for the semiquinones which were in ethanol. The values of T _{2} were similar to the corresponding T _{1} except for ascorbate radical and 2,5‐di‐t e r t‐butyl‐p‐benzosemiquinone anion, where unresolved splittings made the apparent values smaller.

Threshold resonances in the electron photodetachment spectrum of acetaldehyde enolate anion. Evidence for a low‐lying, dipole‐supported state
View Description Hide DescriptionThe electron photodetachment spectra for the enolate anions of acetaldehyde and acetaldehyde‐d _{3} have been measured using an ion cyclotron resonancespectrometer in conjunction with a tunable dye laser. Sharp resonances at threshold are observed for both anions, and are attributed to a low‐lying excited electronic state of the anion, which we relate to postulated ’’dipole‐supported’’ states of electron–polar molecule systems. The electron affinity of the neutral enolate radicals CH_{2}CHO and CD_{2}CDO are found to be 1.824±0.005 and 1.819±0.005 eV, respectively.

The Q‐branch rotational spectrum in the ground state of CD_{4}, studied by laser–rf double resonance spectroscopy
View Description Hide DescriptionNear coincidences between CO_{2} and N_{2}O laser lines and transitions in the n_{2} and n_{4} infrared bands of CD_{4} have been investigated using laser–rf double resonance spectroscopy. Six rf transitions have been assigned to ground state DJ = 0 rotational transitions of CD_{4}, with J values of 7, 7, 10, 19, 19, and 21. A least squares fit of the observations using quartic and sextic tensor distortion constants only yields the parameters D _{ t } = 32649.7±1.7 Hz, H _{4t } = −1.9747±0.0044 Hz, and H _{6t } = 1.1027±0.0016 Hz.

Matrix interactions with rare‐gas oxide excimers
View Description Hide DescriptionMatrix interactions of rare‐gas oxide molecules formed in solid matrices were studied by photoluminescence.Excitation energies between 8–12 eV were provided by both synchrotron radiation at the Stanford Synchrotron Radiation Laboratroy (SSRL) and a hydrogen discharge source at UCSB. Photoluminescence emission and excitation spectra of Ar:CO_{2}(1%), Kr:CO_{2}(1%), Kr:N_{2}O(1%), and Kr:O_{2}(1%) mixtures were obtained. Radiative lifetimes of ArO and KrO emissions from CO_{2}doped Ar and Kr matrices were also measured. The direct photodissociative threshold of CO_{2} to CO(X ^{1}S^{+})+O(^{1} S) in an Ar matrix was found to be 10.6 eV, in agreement with that of the gas phase. Further unambiguous evidence for radiationless, dissociative energy transfer, between the matrix free exciton and CO_{2}, probably by a harpooning mechanism, was seen in Kr:CO_{2} below the direct photodissociation threshold. Temperature, matrix annealing, and character of the photofragment were found to have significant effects on the rare‐gas oxide molecular binding and production rate.

Raman spectroscopy of the superionic conductor pyridinium silver iodide (C_{5}H_{5}NH) Ag_{5}I_{6}, in the ordered phase
View Description Hide DescriptionThe Raman spectra of pyridinium silver iodide (C_{5}H_{5}NH)Ag_{5}I_{6} have been obtained in the ordered D _{6} phase. The spectra exhibit seven modes characteristic of pyridinium rings as well as two broad external modes characteristic of low‐frequency silver ion motion (97 and 32 cm^{−1}). The external modes broaden and disappear as the second‐order transition to a superionic phase at 323 K is approached from below.

Resonance effects on two photon absorption processes of molecules
View Description Hide DescriptionEffects of resonance on two photon absorption of molecules have been investigated. Using the expression for the photon processes which was derived by using the density matrix method in the Markoff approximation, the intensity of the two photon absorption can be separated into simultaneous, sequential, and their mixing terms. The last term vanishes in the case of zero pure dephasing constant between the initial and final electronic states. In order to study the resonance effects on the two photon absorption for a multilevel molecular system, an expression for the transition probability of the two photon absorption processes is derived in the Born–Oppenheimer basis set. Analytical expressions for both simultaneous and sequential processes are obtained for the displaced harmonic oscillator model. A temperature dependent simultaneous two photon absorption probability for the multimode case is formulated. Some model calculations of the vibrational structures in the low temperature limit for the molecular system are performed to illustrate the resonance effect.

The anharmonic force field of CClF_{3} and the determination of the Coriolis coefficients and anharmonic coefficients from the infrared spectrum
View Description Hide DescriptionThe spectroscopic parameters of CClF_{3} have been calculated from an assumed anharmonic Urey–Bradley force field. These calculated values have been used as an aid to the interpretation of the observed infrared spectrum. The Coriolis splitting of the K subbands of the parallel transitions to the combination states formed from two degenerate fundamentals, e.g., n_{4}+n_{5}, has been observed and used in the determination of the Coriolis coefficients.

Excited‐state dynamics of the isolated ultracold ovalene molecule
View Description Hide DescriptionIn this paper we report the results of an experimental study of the energetics and the dynamics of the first two electronically excited singlet states S _{1} and S _{2} of the ovalene molecule (C_{32}H_{14}) seeded in supersonic beams of Ar and of Kr. By an optimal choice of the stagnation pressure (100–300 Torr) of the Ar and Kr carrier gases expanded through a 200 m nozzle, efficient rotational–vibrational cooling of the large molecule was accomplished, while the spectrum was not obscured by the formation of van der Waals complexes. We have interrogated the energy‐resolved fluorescence action spectrum, the time‐resolved fluorescence, as well as the energy‐resolved fluorescence of the bare, ultracold, isolated, large molecule. The first spin‐allowed ^{1} S _{0}(^{1} A _{1g })→^{1} S _{1}(^{1} B _{3u } ^{−}) transition with an origin at 21 449 cm^{−1}, exhibits a sparse, well‐resolved, vibrational level structure at low excess vibrational energies, E_{ V }, while the higher energy range of S _{1} reveals a congested level structure corresponding to overlapping resonances within a single electronic manifold. The time‐resolved decay lifetimes of photoselected single vibronic levels of S _{1} are in the range 1.7–2.2 msec, exhibiting a very weak energy dependence on E _{ V } and being close to the pure radiative lifetimes of S _{1}. The second S _{0}(^{1} A _{1g })→S _{2}(^{1} B _{2u }) transition of ovalene reveals an irregular, closely‐spaced structure, resulting from strong interstate coupling between the S _{2} and the S _{1} electronic states, whose electronic origins are separated by ∼1800 cm^{−1}. The strength function for the scrambling of the S _{2} origin with the background S _{1}manifold was extracted from the spectroscopic data, providing information on the energetics and on the energy dependence of interstate coupling. The time‐resolved decay lifetimes of the scrambled S _{2} molecular eigenstates are longer by about two orders of magnitude than the pure radiative lifetimes of S _{2}, as estimated from the integrated oscillator strength, manifesting the dynamic consequences of the intermediate level structure. The energy‐resolved fluorescence resulting from excitation into the S _{2} state exhibits emission in the S _{1}→S _{0} energy region, which is in accord with the large dilution factor for the S _{2}–S _{1} scrambling. The structure of the energy‐resolved emission spectrum may provide some information regarding intrastate vibrational energy redistribution in this large molecule.

Calculation of ESR spectra and related Fokker–Planck forms by the use of the Lanczos algorithm
View Description Hide DescriptionThe applicability of the Lanczos algorithm in the general ESR (and NMR)line shape problem is investigated in detail. This algorithm is generalized to permit tridiagonalization of complex symmetric matrices characteristic of this problem. It is found to yield very accurate numerical solutions with a t l e a s t order of magnitude reductions in computation time compared to previous methods. It is shown that this great efficiency is a function of the sparsity of the matrix structure in these problems as well as the efficiency of selecting an approximation to the optimal basis set for representing the line shape problem as distinct from actually solving for the eigenvalues. Furthermore, it is found to aid in the analysis of truncation to minimize the basis set (MTS), which becomes nontrivial in complex problems, although the efficiency of the method is not very strongly dependent upon the MTS. It is also found that typical Fokker–Planck equations arising from stochastic modeling of molecular dynamics have the property of being representable by complex–symmetric matrices that are very sparse, so calculation of associated correlation functions can be very effectively implemented by the Lanczos algorithm. It is pointed out that large problems leading to matrices of very large dimension can be efficiently handled by the Lanczos algorithm.

Fluorescence detected linear dichroism. A new method for studies of molecular orientation in uniaxial systems
View Description Hide DescriptionA theoretical and experimental description of a new light spectroscopic method for determination of second rank order parameters is presented. The linear dichroism is obtained from the total fluorescent intensity, measured by using an integrating sphere. The method has been tested on a dichroic sample consisting of a stretched polymer film with an incorporated dye (2,2′‐diethyl thiocarbocyanide iodide). The main advantages of using emitted light are the high sensitivity and selectivity. With the method described the molecular orientation of a fluorophore in a mixture of other absorbing chromophores may be studied. It is also suggested that the integrating sphere should be used in determinations of excitation spectra and for measurements of fluorescence detected circular dichroism.

Depolarized light scattering in dilute solutions of alkanes: A comparison of the bond additive and interacting atom approximations to the molecular polarizability
View Description Hide DescriptionThe molecular polarizability of a few small alkane (4–10 bond) chains has been represented by (1) an interacting atom model (IAM), wherein the atoms are treated as isotropic point polarizabilitiesinteracting by the dipole tensor; and (2) the bond additive approximation (BAA) in which each bond is assigned an axially symmetric polarizabilitytensor, and the total molecular polarizability is the sum of the individual bond values. For selected values of the g a u c h e–t r a n s energy difference (0.3 kcal/mole), the calculated mean anisotropy per backbone atom 〈g^{2}〉/N increases linearly with N for the IAM and is essentially independent of N in the BAA. Orientational correlation functions have been determined for several second rank tensors characterizing the flexible chains using a modified version of Fixman’s Brownian dynamics programs. The orientational correlation functions displayed an effective nonanalytic decay for short times merging into an exponential for long times. Single particle correlation times for the IAM increased more rapidly with N than did those of the BAA. Relaxation of the end‐to‐end vector (actually its second rank analog) was found to be the slowest process, followed by the IAM and the BAA polarizabilities, and finally the fastest was a local rotational mode.

Effect of the oxygen protonation on the electronic structure of urea in the solid state: A ^{14}N NQR study
View Description Hide DescriptionThe ^{14}N NQR frequencies of urea complexes with H_{2}O_{2} (1:1), NH_{4}Cl (1:1), oxalic (2:1), phosphoric (1:1), and nitric acid (1:1) at 77 °K are reported. The analysis of the NQR data indicates that the population of the N nonbonding orbital decreases and that the population of the s N–H and N–C bonds increases as the degree of protonation of the O atom of urea increases. These changes are consistent with a larger weight of structures like C = N^{+}H_{2} as the protonation increases. The NQR results are in agreement with those obtained from a CNDO/2 calculation for the uronioum ion [Yu. A. Panteleev and A. A. Lipovskii, Zhu. Struk. Khim. 17, 2 (1976)].

Ultraviolet absorption spectrum of nitrous oxide as a function of temperature and isotopic substitution
View Description Hide DescriptionThe ultraviolet absorption spectra of nitrous oxide and its ^{15}N isotopes over the wavelength range 197 to 172 nm and between 150 and 500 K show a weak continuous absorption and a pattern of diffuse banding that became pronounced at higher temperatures. The temperature dependence of the absorptionspectrum results from the activation of the n^{″} _{2} bending mode. Deconvolution of the data shows that absorption by molecules in the (010) vibrational mode results in a spectrum of vibrational bands superimposed on a continuum. A weaker and nearly continuous spectrum results from the ultraviolet absorption by molecules in the (000) vibrational mode. Analysis of the structuring indicates n^{′} _{2} = (490±10) cm^{−1}. No rotational structure can be observed. Measurement of the n^{′} _{2}isotope shift is used to identify the quantum number of the upper state vibrational levels. Normal coordinate anaylsis of the excited state is used to determine a self‐consistent set of molecular parameters: bond angle (115°), the values of n^{′} _{1} and n^{′} _{3} (1372 and 1761 cm^{−1}, respectively), and the force constants of the upper state. It is suggested that the transitions observed are ^{1}S^{−}(^{1} A ^{′′})← ^{1}S^{+} and ^{1}D← ^{1}S^{+}.

Study of sulfur‐containing molecules in the EUV region. I. Photoabsorption cross section of SO_{2}
View Description Hide DescriptionUsing synchrotron radiation as a continuum background, the absorption cross sections of SO_{2} have been measured using a double ionization chamber. The cross sections range from 10 to a maximum value of 56 Mb in the 175–760 Å region. A possible window resonance series was observed and tentatively assigned to a Ryberg series converging to the (6a _{1})^{−1} ion state. From the fluorescence excitation function the vibrational progressions of the SO^{+} _{2} ? ^{2} B _{2}, ? ^{2} A _{1}, and ? ^{2} B _{1} ion states have been measured and found in good agreement with those observed from photoelectron spectroscopy. The same correspondence between peaks in the fragment fluorescence excitation function and the photoelectron spectrum suggests that these ion states of SO_{2} are dissociative in nature.