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Volume 92, Issue 11, 01 June 1990

Theory and simulations of homonuclear spin pair systems in rotating solids
View Description Hide DescriptionThe theory of nuclear magnetic resonance(NMR) on a solid sample containing pairs of coupled homonuclear spins‐1/2, rotating in a large magnetic field, is presented. The time dependence introduced by the sample rotation, in conjunction with the spin–spin coupling, makes it appear that each of the central two levels in the four‐level system split into a pair of ‘‘virtual states.’’ Each of the eight possible single‐quantum coherences between the virtual states and the two outer levels in general contribute to the spectrum, although four of these contributions are forbidden unless a rotational resonance occurs (matching of an integer multiple of the spinning speed with the difference in isotropic shifts). Analytical line shapes for the case of vanishing shift anisotropy are given and techniques for numerical simulation in the general case demonstrated. The theory of Zeeman magnetization exchange in the presence of zero‐quantum dephasing is presented.

Vibrational analysis of the 460 nm band system of nickel dichloride produced in a free jet expansion
View Description Hide DescriptionUsing a free‐jet expansion which incorporates a heated nozzle, we have recorded the laser excitation spectrum of the 460 nm band system of NiCl_{2} at a rotational temperature of ∼40 K. ^{35}Cl/^{37}Cl isotope shifts were resolved which permit the assignment of progressions involving the symmetric stretching vibrational mode and identify a triplet splitting with spacings of 96 and 149 cm^{−1} which is believed to be due to spin–orbit coupling. Sequence bands involving the bending vibrational mode are also tentatively assigned. Only a small change in the symmetric stretching vibrational wave number is found between the electronic states involved in this transition (ν̃^{’} _{1} =356 cm^{−1}, ν̃^{″} _{1} =360 cm^{−1}). This result and the triplet splitting observed are discussed with respect to the possible electronic states involved and the assignment of this band system as either a Laporte forbidden g↔g transition involving the d orbitals on the Ni atom or an allowed u↔gcharge transfer transition.

Time frame viewpoint of resonance Raman enhancement of a non‐totally symmetric vibration
View Description Hide DescriptionThe time frame approach is presented to describe the resonanceRaman enhancement of a non‐totally symmetric vibration due to vibronic coupling between two nearby electronic states. It is shown that in contrast to the resonanceRaman enhancement of a totally symmetric vibration, the resonant contribution to the Raman scatteringtensor for a non‐totally symmetric vibration arises from the electronic state that is vibronically coupled to the electronic state first reached by the incident light. The second‐order differencing method of Kosloff is used to numerically solve the coupled time‐dependent Schrödinger equation in an application to pyrazine. In the case where the vibronically coupled states have the same oscillator frequency, it is shown that a unitary transformation of the wave packets leads to an immediate picture of the adiabatic states, as well as provide a qualitative explanation of various factors on the Raman intensity.

Ring‐puckering potential function for butadiene sulphone
View Description Hide DescriptionThe ring puckering in butadiene sulphone has been investigated using microwave spectroscopy and a b i n i t i o computations. Microwave spectra of the ground and first eight excited states of the ring‐puckering vibration have been observed. A two state analysis of the vibration–rotation interaction for the v=0 and v=1 states gives an energy separation between these states of 4.97 (3) cm^{−} ^{1}. This separation and the vibrational dependence of the rotational constants have been used to derive the reduced potential function V(X)=4.7(X ^{4}−6.5X ^{2}) for the ring‐puckering vibration. This potential function gives a barrier to ring inversion of 50 (11) cm^{−} ^{1} and an equilibrium dihedral angle of ≊20°. A b i n i t i o computations using STO 3‐21G* orbitals and full geometry optimization give a barrier height of 88 cm^{−} ^{1} and an equilibrium dihedral angle of 19.2°. The a b i n i t i o computations predict structural relaxation including a rocking of the sulphone group during the ring‐puckering motion. This is supported by calculations of the vibration–rotation interaction parameter (δμ_{ a c }/δQ). The μ_{ a } component of the electric dipole moment has been determined as 4.6 (1) D from Stark effect measurements. The a b i n i t i o computations and Stark effect measurements indicate that the transition moment 〈0‖μ_{ c }‖1〉 is 1–2 D.

Excited‐state dipole moments of SO_{2}
View Description Hide DescriptionHigh resolution ultraviolet spectroscopy of SO_{2} in a cold supersonic beam is reported. The behavior of single rovibronic lines of the E band (305 nm) and the 322 nm band was investigated in an electric field of up to 12 kV/cm. The dipole moment of the ^{1} A _{1} state was determined to be 1.85 (0.03) D. Problems of the dipole determination in SO_{2} are discussed.

The rotational spectrum and nature of the heterodimer in trimethylammonium bromide vapor
View Description Hide DescriptionThe ground‐state rotational spectra of three symmetric‐top isotopomers (CH_{3})_{3} ^{14}N⋅⋅⋅H ^{79}Br, (CH_{3})_{3} ^{14}N⋅⋅⋅H ^{81}Br, and (CH_{3})_{3} ^{14}N⋅⋅⋅D ^{79}Br of the heterodimer of trimethylamine and hydrogen bromide have been detected by pulsed‐nozzle, Fourier‐transform microwave spectroscopy. The spectroscopic constants B _{0}, D _{ J }, D _{ J K }, χ(^{14}N), and χ(Br) have been determined for each of the isotopomers and for (CH_{3})_{3} ^{14}N⋅⋅⋅H ^{81}Br have the values 1161.6294(2) MHz, 0.148(5) kHz, 7.77(2) kHz, −2.883(7) MHz, and 99.645(7) MHz, respectively. A comparison of the ^{14}N– and ^{81} Br–nuclear‐quadrupole coupling constants χ(^{14}N) and χ(Br) with those expected on the basis of a hydrogen‐bonded model (CH_{3})_{3}N⋅⋅⋅HBr and an ion‐pair model (CH_{3})_{3}NH^{+}⋅⋅⋅Br^{−} leads to the conclusion that in the heterodimer trimethylamine‐hydrogen bromide there is a significant extent of proton transfer from HBr to (CH_{3})_{3}N. The value of the intermolecular stretching force constant k _{σ}=82(3) N m^{−1} determined from D _{ J } is also compared with those expected for the limiting models of the dimer and is found to lie close to that associated with the ion‐pair limit.

The rotational and tunneling spectrum of the H_{2}S⋅CO_{2} van der Waals complex
View Description Hide DescriptionThe rotational spectra of H_{2}S⋅CO_{2} and two deuterated forms have been observed using a pulsed‐beam Fourier‐transform microwave spectrometer. For each of the three complexes we assign a‐type and c‐type transitions which are split into a ‘‘weak’’ and a ‘‘strong’’ intensity component. The analysis based on that previously used for the (H_{2}O)_{2} complex and modified for application to H_{2}S⋅CO_{2}, allowed us to assign internal rotation, inversion tunneling states of the H_{2}S and CO_{2} units in the complex. The following rotational constants were determined for the ground tunneling state of each species: for H_{2}S⋅CO_{2}, A=11 048.0(26) MHz, B=2147.786(4) MHz, and C=1806.468(4) MHz; for HDS⋅CO_{2}, A=10 769(35) MHz, B=2107.26(24) MHz, and C=1775.83(24) MHz; and for D_{2}S⋅CO_{2}, A=10 356.2(28) MHz, B=2065.376(8) MHz, and C=1746.122(8) MHz. The electric dipole moments were determined for the H_{2}S⋅CO_{2} and D_{2}S⋅CO_{2} species, resulting in the values μ_{ a }=0.410(14) D and μ_{ c }=0.822(10) D for the H_{2}S⋅CO_{2} species. The structure of the complex has the CO_{2} and the S atom of H_{2}S in a T‐shaped configuration. The H_{2}S plane is nearly orthogonal to the CO_{2}–S plane with an angle of about 92° and the H_{2}S⋅CO_{2} center‐of‐mass separation R _{c.m.} is 3.498(3) Å.

New electronic states of NH and ND observed from 258 to 288 nm by resonance enhanced multiphoton ionization spectroscopy
View Description Hide DescriptionThree new electronic states of NH and ND (imidogen radical) have been observed by REMPI (resonance enhanced multiphoton ionization)spectroscopy in the region of 258 through 288 nm and assigned. The NH (ND) was produced by the photolysis of HN_{3} (DN_{3}) (hydrazoic acid) in the same wavelength region. The observed two‐photon transitions are from the a ^{1}Δ state to 3pRydberg states. Transitions were also observed from the a ^{1}Δ state to the d ^{1}Σ^{+} state. The new state assignments are: f ^{1}Π (3pσ) at 86 378 cm^{−1}, g ^{1}Δ (3pπ) at 88 140 cm^{−1}, and h ^{1}Σ (3pπ) at 89 531 cm^{−1}. Rotational constants (B and D) and, where possible, vibrational spacings for the thirteen observed bands are also determined.

Four‐wave mixing spectroscopy of NO E ^{2}Σ^{+} state
View Description Hide DescriptionThe E ^{2}Σ^{+} state of NO (nitric oxide) has been probed by four‐wave mixing spectroscopy via the A ^{2}Σ^{+} state. In the present scheme, the pump laser frequency ω_{1} was fixed on the A ^{2}Σ^{+} (v’=0 and 1)←X ^{2}Π_{3/2}(v‘=0) two‐photon transition, while the probe laser frequency ω_{2} was remained variable to monitor the E ^{2}Σ^{+}←A ^{2}Σ^{+} electronic system. The intensity of the resultant coherent VUV radiation (ω_{VUV}=2ω_{1}+ω_{2}) was strongly wavelength dependent. The analysis of the spectrum revealed the following two respects: (1) The intensity of VUV radiation was enhanced by the resonance of ω_{2} to rotational levels of the E ^{2}Σ^{+} state as well as of 2ω_{1} to the A ^{2}Σ^{+} state. (2) The spectral structure corresponding to the E ^{2}Σ^{+}←A ^{2}Σ^{+} system was governed by different rotational selection rules from ordinary single‐photon transitions. These aspects were discussed in terms of the third order nonlinear process in isotropic media and of the two‐photon line intensities for the A ^{2}Σ^{+}←X ^{2}Π system.

Two dimensional nuclear magnetic resonance relaxation spectroscopy of molecular solids
View Description Hide DescriptionMolecular motions in solids cover a broad dynamic range, extending from the fast rotational to the ultraslow motional regime. Two dimensional (2D) NMR relaxation spectroscopy is designed to follow these motions and to differentiate the various motional modes. The method employs the pronounced anisotropy of the nuclear spin relaxation times, observed for polycrystalline or multidomain samples. Generally, 2D NMR relaxation spectra are obtained by recording the time signals S(t _{2}) after the last pulse as a function of successive incremented time intervals t _{1}, corresponding to the relaxation period of the particular sequence. A Fourier transformation in both time domains transformsS(t _{1},t _{2}) into a 2D representation S(ω_{1},ω_{2}) of the relevant relaxation experiment.
The normalized contour plot then displays the change of the corresponding relaxation rate 1/T _{ i } along the frequency spectrum. It turns out that this variation is very dependent upon the character of the molecular motion. Model calculations for deuterons, involved in planar motions, demonstrate the potential of 2D NMR relaxation techniques. Generally, the type of motion can reliably be deduced from the shape of the contour plots. A model independent analysis provides the geometrical parameters of the dynamic process, including the jump angle Δψ_{ K } and the orientation ϑ_{ K } of the rotation axis in the magnetic frame. In addition, from the separation of the contour lines the motional correlation times can be determined. The techniques are employed in the dynamical characterization of L‐alanine, specifically deuteriated at the methyl group. From an analysis of 2D quadrupole echo spectra geometrical parameters of Δψ_{ K }=(120±1)° and ϑ_{ K }=(70.5±1)° have been determined. Apparently, methyl group reorientation in L‐alanine occurs via three‐site jumps about a rotation axis, tilted by an angle of ϑ_{ K }=70.5° relative to the C–^{2}H bond direction. Computer simulations of 2D quadrupole echo and inversion recovery experiments provide the correlation times for this motion. The values range from τ_{ J }=5×10^{−10} s at T=353 K to τ_{ J }=3×10^{−5} s at T=140 K. An Arrhenius plot for these correlation times is linear over the entire dynamic range. From the slope of the straight line an activation of E _{ a }=20 kJ/mol has been determined.

A temperature effect in the luminescence emission from electron‐irradiated MgO
View Description Hide DescriptionThe temperature dependence of luminescence emissions from electron‐irradiated CaO and MgO single crystals has been studied by time resolved luminescencespectroscopy after pulsed nanosecond irradiation with 0.20 to 0.60 MeV electrons. Emissions from CaO at 375 nm at both 293 and 83 K, show similar threshold characteristics for atomic displacement. These have been attributed to the displacement of oxygen ions and subsequent electrontrapping, resulting in the formation of F^{+} centers. The threshold energy of 0.32±0.01 MeV corresponds to an oxygen displacement energy of 58±2 eV. A 380 nm emission from MgO, also attributed to oxygen displacement and F^{+} center formation, shows no temperature effect, with a displacement threshold virtually identical to that for CaO. A 235 nm emission from MgO shows a significant temperature effect. The threshold energy at 293 K is 0.31±0.01 MeV, whilst at 83 K two thresholds are observed, 0.31±0.01 and 0.41±0.01 MeV.

Heavy atoms and tunneling in the X̃ state of tropolone
View Description Hide DescriptionLarge (6.9 to 16.3 cm^{−1} ) tunneling splittings are uniquely observed for the ν_{27} (OD stretch), ν_{31} (carbonyl stretch), and ν_{34} (C=C–C stretch) fundamentals of tropolone‐OH and tropolone‐OD in the X̃ ^{1} A _{1} (ground) electronic state. These same three modes are predicted by the molecular geometry to interact strongly with tunneling because the dominant vibrational and tunneling displacements involve the same atoms. The heavy atom tunneling displacements (≊0.07 Å) are small enough to plausibly consider heavy atom tunneling phenomena—especially in appropriate excited vibrational states—and the tunneling splittings appear consistent with behavior expected at zero order for adiabatic reactionsurfacetheory with a 2D reactionsurface defined by C=O/C–O and C=C–C heavy atom coordinates. This model attributes tunneling in the X̃ state of tropolone to heavy atom motion followed adiabatically by H atom motion rather than the reverse. Energy balance equations are used to obtain estimates for the vibrational state‐specific tunneling barrier heights of tropolone‐OH (13.7 and 9.3 kcal/mol for the zero‐point and ν_{27} states) and tropolone‐OD (14.3 and 11.0 kcal/mol for the zero‐point and ν_{27} states).

Ã ^{1} B _{2}–X̃ ^{1} A _{1} 26^{ v } _{0} transitions of ^{1} ^{8}O‐enriched tropolone
View Description Hide DescriptionLaser excitation spectra with v=0, 2, 4, and 6 in the Ã ^{1} B _{2}–X̃ ^{1} A _{1} 26^{ v } _{0} progression of jet‐cooled ^{1} ^{8}O/^{1} ^{6}O isotopomers of tropolone are reported. The isotope shift for ν_{2} _{6}, an out‐of‐plane deformation mode at 39 cm^{−1} in the Ã state, is 2% for tropolone‐^{1} ^{8}O^{1} ^{8}O. This large ^{1} ^{8}O isotope effect indicates that Q _{2} _{6} for tropolone resembles the analogous normal mode of tropone, which is a ring deformation towards the boat conformation of 2, 4, 6‐cycloheptatriene accompanied by a large O atom displacement. Tunneling by tropolone in the Ã state is quenched by exciting the 26^{ v } overtone states and a mechanism for this quenching is proposed in terms of the indicated normal coordinate. Tunneling splittings are <0.3 cm^{−1} for the zero point levels of the X̃ state of the symmetrical isotopomers. In contrast, vibrational isotope effects dominate the tunneling interactions to split the corresponding levels of tropolone‐^{1} ^{6}O^{1} ^{8}O by 1.7 cm^{−1}. In the Ã state of this isotopomer the tunneling interactions are dominant. Because they are determined by the overlap between localized and delocalized wave functions, the Franck–Condon factors of tropolone‐^{1} ^{6}O^{1} ^{8}O are smaller than those of the symmetrical isotopomers.

Laser vaporization generation of PdCH_{3}, ^{1} ^{0} ^{5}PdCH_{3}, and Pd^{1} ^{3}CH_{3} for electron spin resonance neon matrix study at 4 K
View Description Hide DescriptionThe Pd^{1} ^{2}CH_{3}, Pd^{1} ^{3}CH_{3}, and ^{1} ^{0} ^{5}Pd^{1} ^{2}CH_{3} radicals have been generated by reactive laser vaporization and isolated in neon matrices at 4 K for electron spin resonance(ESR) investigation. Apparently no previous monomethyl metal radical has been characterized by ESR despite the importance of such species as reactive intermediates. These results allow an experimental description of the electronic structure in the valence region to be obtained. A direct electronic structure comparison between PdH and PdCH_{3} is also presented. A significant amount of s/d hybridization on Pd is observed which agrees with earlier calculations on the bonding in Pd(CH_{3})_{2}. The magnetic parameters (MHz) for PdCH_{3} in neon matrices are: g _{⊥}=2.273(1); for ^{1} ^{0} ^{5}Pd, A _{⊥}=−946(2), A _{∥}=−987(20); for H, ‖A‖=13(1); and for ^{1} ^{3}C, ‖A‖=10.2(4).

van der Waals vibrational dependence in the vibrational predissociation dynamics of OH–Ar
View Description Hide DescriptionThe OH–Ar vibrational predissociation lifetimes and OH product rotational state distributions are shown to change with van der Waals (vdW) state selection within the manifold of OH–Ar vibrational states correlating with OH A ^{2}Σ^{+}(v’=1)+Ar(^{1} S _{0}). Excitations to pure vdW stretching levels result in similar product state distributions, but predissociation lifetimes that vary from 30±8 ps at v ^{’} _{vdW}=0 to greater than 150 ps at v _{vdW}=6. Excitations to assigned vdW bend‐stretch combination bands result in product state distributions which differ from those observed after excitation of the pure vdW stretch and those differences are attributed to the form of the bending wave function.Rotational constants and band positions for OH–Ar features in the OH A ^{2}Σ^{+}–X ^{2}Π_{3/2} 0–0, 1–0, 1–1, 2–1, 1–2, and 2–2 regions are also presented. The spectroscopic analysis reveals details about the radial portion of the intermolecular potential between Ar (^{1} S _{0}) and hydroxyl radicals in the ground X ^{2}Π_{3/2} and excited A ^{2}Σ^{+} states. Vibrational excitation of the OH moiety induces measurable perturbations in the interaction potentials along the OH–Ar vdW stretching coordinate for both electronic states. These changes are reflected in the vibrational predissociation rates.

Rotational energy transfer in CH_{3}F: The ΔJ=n, ΔK=0 processes
View Description Hide DescriptionWe report the measurement of the rates o ΔJ=n, ΔK=0(‖n‖≤10) processes for CH_{3}F–CH_{3}F collisions at 300 K. The data are derived from a time‐resolved millimeter/submillimeter‐infrared double resonance investigation of both the ^{1} ^{2}CH_{3}F and the ^{1} ^{3}CH_{3}F isotopic species. The rates were obtained via a nonlinear least‐squares analysis of the data using a numerical simulation of rotational energy transfer in methyl fluoride. These rates are shown to be quantifiable in terms of the scaling law of infinite order sudden collision theory and the statistical power gap law. As a result, the numerous ΔJ=n, ΔK=0(‖n‖>1) rates can be understood in terms of only two parameters, independent of isotopic species. Using these results and the results of our earlier studies of K‐changing processes, we discuss how rotational energy transfer in the CH_{3}F system in general can be described in terms of a small number of collisional processes and parameters.

The dissociative recombination rate coefficients of H^{+} _{3}, HN^{+} _{2}, and HCO^{+}
View Description Hide DescriptionThe dissociative recombination rate coefficients for H^{+} _{3}, HN^{+} _{2}, and HCO^{+} are determined at 110, 210, and 273 K by monitoring the decay of the infrared absorption signals as a function of time. The rate coefficients are 1.8, 7.0, and 3.1 in units of 10^{−} ^{7} cm^{3} s^{−} ^{1} for H^{+} _{3}, HN^{+} _{2}, and HCO^{+}, respectively, at 273 K. These values agree very well with those obtained using the stationary afterglow or the merged beam techniques, but the values for H^{+} _{3} disagree with that obtained by Smith and co‐workers (≤2×10^{−} ^{8} cm^{3} s^{−} ^{1}) using the flowing afterglow/Langmuir probe method. The rate coefficients for H^{+} _{3} and HCO^{+} disagree with theory which has predicted very slow dissociative recombinations in the lower vibrational states. The temperature dependences obtained here, although the temperature range is rather limited, are consistent with those obtained previously using the stationary afterglow (for H^{+} _{3} and HCO^{+}) and the merged beam (for HN^{+} _{2}) techniques. The measurements are extended to several vibration–rotation levels and no significant rotation dependence of the rate coefficients is observed. It has also been found that the ions investigated here can be equally abundant at ice temperature as at liquid nitrogen temperature.

Spin–orbit state selectivity in KrF*and XeF* formation from ion‐recombination reactions of Kr^{+}(^{2} P _{3/2,1/2}) and Xe^{+}(^{2} P _{3/2,1/2}) with SF^{−} _{6} in the flowing afterglow
View Description Hide DescriptionAppropriate filter gases have been used to select one of the spin–orbit levels of Kr^{+} and Xe^{+}, ^{2} P _{3/2} or ^{2} P _{1/2} , in a flowing afterglow reactor. The separated reactions of Kr^{+} and Xe^{+} in the ^{2} P _{3/2} and ^{2} P _{1/2} levels with SF^{−} _{6} were examined at 300 K by observation of KrF* and XeF* formation. The Kr^{+} and Xe^{+} ions in the ^{2} P _{3/2} level give only the B and C states, while those in the ^{2} P _{1/2} level give the B and D states. The B:C distributions from Kr^{+}(^{2} P _{3/2}) and Xe^{+}(^{2} P _{3/2}) were estimated to be 0.62:0.38 and 0.68:0.32, respectively, while the B:D distributions from Kr^{+}(^{2} P _{1/2}) and Xe^{+}(^{2} P _{1/2}) were measured as 0.05:0.95 and 0.06:0.94. The high spin–orbit state selectively can be generally explained by the conservation of the Rg^{+}(^{2} P _{3/2})+F^{−} and Rg^{+}(^{2} P _{1/2})+F^{−} characters.

Structure and dynamics of small I_{2} . . . He_{ n } van der Waals clusters (n=1–9)
View Description Hide DescriptionEnergetics and dynamics of van der Waals (vdW) I_{2}⋅⋅⋅He_{ n } clusters are studied in an approximate way by using a model that considers 2n+1 degrees of freedom, that is, the I_{2} stretch and the 2n stretching and bending modes of the He atoms restricted to move along a plane perpendicular to the I_{2} axis. For n=2,3 a configuration‐interaction treatment is carried out to obtain energy levels. For n=4–8, ground‐level energies are estimated from those corresponding to n=2,3 and the geometric relationships among the n vdW bonds. A quasiclassical trajectory approach is used to study the dynamics of these clusters, and lifetimes and half‐widths for vibrational predissociation have been calculated for n=1–9. A large increase in the half‐width of n=9 with respect to the cases n=1–8 is observed, which would imply the existence of a first coordination shell for He about I_{2} containing eight atoms. Also, it is found that the mechanisms of dissociation for these clusters become statistical as the number of vdW bonds increases.

Rotationally inelastic scattering of glyoxal by H_{2} at E=80 meV
View Description Hide DescriptionUsing the Monte Carlo classical trajectory (CT) method and the azimuthal close‐coupled, infinite‐order sudden (ACC‐IOS) method, we have calculated cross sections for rotational excitation of S_{1} trans‐glyoxal by H_{2} at E=80 meV. The cross sections σ(k=0, j→k’) calculated with the CT method are nearly independent of j. The classical values of σ(k=0, j=5→k’) are in good agreement with the quantum values of σ(k=0→k’) for 2≤k’≤12, although the quantum calculations show a slight preference for odd Δk transitions which is not found in the CT calculations. Both the CT results and the ACC‐IOS results are in good agreement with results obtained in a recent crossed beam experiment. Rotational excitation to high k’ (k’=11,12) occurs by collisions of H_{2} with one of the H atoms of glyoxal, and the initial value of the orbitalangular momentum approximately equals the final value of k in such collisions. Since backward scattering is dominant in collisions leading to high k’, angular momentum constraints alone cannot explain the maximum observed in Δk experimentally (Δk=14).