Volume 92, Issue 10, 15 May 1990
Index of content:

Electronic spectra of jet‐cooled tropolone. Effect of the vibrational excitation on the proton tunneling dynamics
View Description Hide DescriptionThe laser fluorescence excitation spectra and single vibronic level fluorescence spectra have been measured for jet‐cooled tropolone (–OH) and tropolone (–OD). The ν^{’} _{11}(a _{1}), ν_{12}(a _{1}), and ν^{’} _{19}(a _{2}), modes have been newly identified in the excitation spectrum of tropolone (–OH), in addition to the previously observed ν_{13}(a _{1}), ν^{’} _{14}(a _{1}), ν_{25}(b _{1}), and ν^{’} _{26}(b _{1}) modes. The tunneling doublet splittings of these modes in the Ã ^{1} B _{2} state have been determined to investigate the effect of the vibrational excitation on the protontunneling dynamics. The tunneling splitting is highly mode specific. The ν_{13}(a _{1}) and ν^{’} _{14}(a _{1}) skeletal deformation modes are strongly coupled to the proton transfer coordinate and enhance tunneling, whereas the ν_{19}(a _{2}) skeletal twisting mode as well as ν^{’} _{25}(b _{1}) and ν_{26}(b _{1}) out‐of‐plane bending modes diminish tunneling. The ν^{’} _{11}(a _{1}) C–C stretching and ν_{12}(a _{1}) C–C–C bending modes are considered to be weakly coupled to the proton transfer coordinate, because the tunneling splittings of these modes are similar to that of the zero‐point level. The hydrogen/deuterium isotope dependence of the tunneling splitting of the vibronic level has been studied. In contrast with the ν^{’} _{13} mode, the isotope effect on the tunneling splitting is much smaller in the ν_{14} mode, although the magnitude of the tunneling splitting of ν^{’} _{14} is very similar to ν_{13}. This implies that both the average O–O distance and the potential energy barrier are significantly different between these modes.

Ion dip spectroscopy of van der Waals clusters
View Description Hide DescriptionWe report the implementation of ion dip spectroscopy in a supersonic molecular beam time‐of‐flight mass spectrometer as a powerful mass‐selective method for observing ground‐state vibrational levels in van der Waals clusters. Ion dip spectra of phenylacetylene and phenylacetylene‐NH_{3} are demonstrated in the range of 900–1100 cm^{−} ^{1}, showing prominent dips at 978.0, 1002.8, and 1028.0 cm^{−} ^{1}. These dips have been tentatively assigned as 13^{0} _{1} 35^{1} _{1}, 11^{0} _{1} 35^{1} _{0}, and 35^{1} _{2}, respectively, in phenylacetylene. Shifts in the 35^{1} _{2} and 11^{0} _{1}35^{1} _{0} vibrational bands of the complex are observed while the 13^{0} _{1}35^{1} _{1} band of the complex is either shifted or attenuated.

Zero field nuclear magnetic resonance in high field
View Description Hide DescriptionWe present a complete description of techniques for obtaining nuclear magnetic resonance(NMR) spectra of solids that have the appearance of zero field spectra but are obtained entirely in high field. With these techniques, we gain the resolution and simplicity of zero field NMR while preserving the full sensitivity and isotopic selectivity of high field NMR. Combinations of rapid sample rotation and the synchronous application of sequences of resonant radio frequency pulses are used to average nuclear spin couplings in solids from their usual orientation‐dependent forms in high field to scalar, zero field forms. We describe the theoretical basis for the techniques, the derivation of specific combinations of sample rotations and pulse sequences that produce the desired scalar average couplings, and the experimental implementation of the techniques. We use exact dynamical simulations to assess the effects of experimental nonidealities on the spectra and present new sequences designed to be less sensitive to the dominant nonidealities. Experimental proton NMR spectra of selectively deuterated, polycrystalline benzene samples are presented. The spectra show sharp lines with splittings that depend only on internuclear distances as in zero field spectra, rather than the inhomogeneously broadened ‘‘powder pattern’’ lines of traditional high field NMR spectra. Applications of the techniques to structural studies of polycrystalline and noncrystalline solids are discussed.

High‐resolution electronic spectroscopy of jet‐cooled hexafluorobenzene and 1,3,5‐trifluorobenzene cations, C_{6}F^{+} _{6} and C_{6}F_{3}H^{+} _{3}
View Description Hide DescriptionThe nearly completely rotationally resolved electronic spectra of two aromatic organic ions have been obtained. Rotationally cold ions, C_{6}F^{+} _{6} and C_{6}F_{3}H^{+} _{3}, are produced by laser ionization in a supersonic free jet expansion and probed via laser induced fluorescence with a very high resolution pulse‐amplified cw ring dye laser. The spectra are analyzed to obtain band origins, rotational constants,Coriolis and Jahn–Teller parameters.

Bound–free 1 ^{3}Π→1 ^{3}Σ^{+} emission from the NaK molecule: Determination of the 1 ^{3}Σ^{+} repulsive wall above the dissociation limit
View Description Hide DescriptionWe report the observation of bound–free emission on the 1 ^{3}Π→1 ^{3}Σ^{+} band of the NaK molecule. The spectra, which consist of oscillating continua in the near‐infrared, have been analyzed to determine parameters describing the repulsive wall of the 1 ^{3}Σ^{+} state above the dissociation limit. Spectra calculated using a potential of the form A e ^{−B R } +C for the 1 ^{3}Σ^{+} state were compared to experimental spectra to yield the following values: A=5.94×10^{5} cm^{−1}, B=1.605 Å^{−1}, C=−220.520 cm^{−1}. This potential, which is referenced to the bottom of the RKR 1 ^{3}Σ^{+} well (D _{ e } =209.1 cm^{−1}), is valid over the range R=3.4–4.5 Å (R=6.4–8.5 a.u.). The relative transition dipole moment of the 1 ^{3}Π→1 ^{3}Σ^{+} band has also been determined over a limited range in R (7.5<R<8.9 a.u.) through the study of relative intensities of various maxima within each oscillating spectrum. In the simulated spectra, the dipole moment was represented by a functional form D(R)=m(R−R _{0})+D _{0} where D _{0} was used to normalize the results to a recent theoretical calculation (D _{0}=1.07 a.u. R _{0}=8.034 a.u.). The best fit for the parameter m was determined to be 0.121±0.029 in atomic units.

Photoelectron spectroscopic studies of polyatomic molecules: Detection‐integrated cross sections for ionization in fixed T _{ d } systems
View Description Hide DescriptionWe have, in this paper, derived expressions for the partial photocurrent produced by ionization in the electric dipole approximation in an orbital of an oriented polyatomic molecule. The cross‐section formulas, which are integrated over all directions of ejection of the photoelectron but differential with respect to the direction of the fixed molecular axis, are obtained in their simplest possible forms by taking full account of the transformation properties of the point symmetry group of the target and are thus applicable to photoionization in any oriented molecule belonging to one of the 32 point groups. The theory, as an example, has been applied to photoionization in a _{1} orbital of those oriented nonlinear systems whose point symmetry group is T _{ d }. This application shows that the singly differential, detection‐integrated partial cross section for ionization by unpolarized, linearly or circularly polarized light in a _{1} orbital of a fixed T _{ d } molecule is (i) independent of the direction of its axis and (ii) equal to that averaged over all orientations of the target in space. Both conclusions are in agreement with recent experimental measurements on photoionization in 6a ^{2} _{1} orbital of fixed CCl_{4}. These results, in turn, are shown to mean that the variations with respect to the orientation of a molecule in space, found in theoretical calculations of photoelectron angular distribution for ionization in a _{1} valence orbitals of some of the T _{ d } targets, are due completely to the terms which stem from freezing both the molecular axis and the photoelectron detector in space. These terms completely vanish on integrating over all directions of propagation of the photoelectron, resulting in a current which is isotropic with respect to the orientation of the molecular axis in space. In such cases, it is therefore necessary to study the angular distribution of electrons ejected by photoionization in oriented molecules in order to obtain cross sections which will change with the direction of the target axis.

A pulsed optical–optical double resonance study of the 1 ^{1}Π_{ g } state of ^{7}Li_{2}
View Description Hide DescriptionThe results of an optical–optical double resonance study of the 1 ^{1}Π_{ g } state of ^{7}Li_{2} are presented. This completes the observation and characterization of all singlet states of Li_{2} correlating with the Li(2s)+Li(2p) dissociation limit. Data spanning the first 31 vibrational levels were used to obtain Dunham molecular constants and a Rydberg–Klein–Rees (RKR) potential corresponding to 96% of the estimated potential well. The 1 ^{1}Π_{ g } state, which lies at T _{ e }=21 998.25 cm^{−} ^{1}, can be characterized by a limited set of Dunham coefficients with ω_{ e }=93.354 cm^{−} ^{1}, ω_{ e } x _{ e } =−1.874 cm^{−} ^{1}, B _{ e }=0.291 89 cm^{−} ^{1}, and a dissociation energyD _{ e }=1422.5±0.3 cm^{−} ^{1}. The results are compared with the predictions of recent theoretical treatments.

CO_{2}‐laser induced photodissociation studies of size‐selected small benzene clusters
View Description Hide DescriptionThe infrared photodissociation of size‐selected, small benzene clusters has been investigated in the region of the ν_{18} CH in‐plane bend using a pulsed CO_{2} laser. By scattering the cluster beam with a secondary Ne beam and observing off‐axis the effect of the laser irradiation with a rotatable mass spectrometer, cluster‐specific spectroscopy is performed. The dependence of IR absorption and subsequent dissociation of (C_{6}H_{6})_{ n } clusters has been investigated as a function of laser frequency and laser fluence for n=2, 3, and 4. The absorption profiles are structureless and show only little variation with cluster size. If, instead of He, Ne is used as carrier gas, the absorption profiles are distinctively narrower. This effect is attributed to a lower internal temperature achieved with Ne. In contrast to the benzene dimer and tetramer, the fluence dependence for the trimer dissociation is stronger than linear suggesting that more than one photon is needed to dissociate this cluster. In a computational approach, the structures of the benzene dimer, trimer, and tetramer have been calculated employing an energy minimization program. For the trimer a cyclic ring structure is determined. The computational results are in perfect agreement with the experimental findings.

Two‐photon spectroscopy of the 3d Rydberg states of O_{2}: ^{1}Φ_{ g } and ^{3}Φ_{ g } states
View Description Hide DescriptionThe 3dδ ^{1}Φ_{ g } and ^{3}Φ_{ g }Rydberg states of O_{2} have been observed by (2+1) resonant enhanced multiphoton ionization from the metastable a ^{1}Δ_{ g } state and the resulting spectra rotationally analyzed. In contrast to all other observed Rydberg states of O_{2}, the ^{3}Φ_{2}, ^{3}Φ_{3}, and ^{3}Φ_{4} states show no sign of perturbation or predissociation by valence states. The ^{1}Φ_{3} state is apparently weakly predissociated. The energy level structure of those 3dRydberg states which have been reliably assigned is compared with theoretical calculations. The reasons for the failure to detect Π_{ g } states in these experiments are discussed.

Optimal phase modulation in stored wave form inverse Fourier transform excitation for Fourier transform mass spectrometry. I. Basic algorithm
View Description Hide DescriptionA new signal processing method has been proposed for generating optimal stored wave form inverse Fourier transform (SWIFT) excitation signals used in Fourier transform mass spectrometry(FTMS or FT‐ICR). The excitation wave forms with desired flat excitation power can be obtained by using the data processing steps which include: (1) smoothing of the specified magnitude spectrum, (2) generation of the optimal phase function, and (3) inverse Fourier transformation. In contrast to previously used procedures, no time domain wave form apodization is necessary. The optimal phase functions can be expressed as an integration of the specified power spectral profiles. This allows one not only to calculate optimal phase functions in discrete data format, but also to obtain an analytical expression (in simple magnitude spectral cases) that is for theoretical studies. A comparison is made of the frequency sweeping or ‘‘chirp’’ excitation and stored wave form inverse Fourier transform (SWIFT) excitation. This shows that chirp excitation and SWIFT excitation with a square magnitude spectrum and a quadratic phase are counterparts of the Fourier transformation. Therefore, the results of theoretical work on chirp excitation can be used for the analysis of the time domain excitation wave forms in the SWIFT technique.

A simple method analyzing ^{2}H nuclear magnetic resonance line shapes to determine the activation energy distribution of mobile guest molecules in disordered systems
View Description Hide DescriptionWe investigated the ^{2}H nuclear magnetic resonance(NMR)line shape of deuterated benzene and hexamethylbenzene as guest molecules in organic glasses in the temperature range of 10–150 K. A broad distribution G(ln τ) of correlation times determines the slowing down of the molecular reorientation around the sixfold symmetry axis of the guests. The line shape is described by a superposition of temperature‐dependent fractions F(T) of only two subspectra corresponding to fast and slowly rotating molecules; no spectracharacteristic for intermediate mobility as found in crystal matrices are observed. Assuming a thermally activated motional process, the temperature dependence of G(ln τ) comes from a temperature‐independent distribution of activation energiesg(E). In this case, the derivative of the fraction d F(T)/d T yields directly the distribution g(E). Using this method an asymmetric distribution g(E) with its maximum at the low energy side is found for the glasses. While the general shape of g(E) is similar for different matrices, the mean activation energy differs significantly. We used the same approach to discuss similar ‘‘two‐phase’’ spectra for the isotropic reorientation of toluene in polystyrene below the glass transition of the mixed system. Here, an alternative explanation is offered considering a distribution of glass transition points T _{ g } for the dynamics of toluene in the mixed system.

Laser vaporization generation of Al^{12}C, Al^{13}C, Al^{12}C_{2}, and Al^{13}C_{2} for rare gas matrix electron spin resonance studies: Experimental–theoretical comparisons
View Description Hide DescriptionThe metalcarbide radicals AlC and AlC_{2} have been generated by the laser vaporization of aluminumcarbide and trapped in neon and argon matrices at 4 K for electron spin resonance(ESR) characterization. These results provide the first experimental evidence showing that AlC has a ^{4}Σ ground electronic state and that AlC_{2} is X ^{2} A _{1}. A b i n i t i o theoretical calculations were conducted for the geometries and various nuclear hyperfine parameters in both radicals which yielded A values in reasonable agreement with the observed. In AlC, the three unpaired electrons reside primarily on carbon with the following neon matrix magnetic parameters (MHz): g _{∥}=2.000(1); g _{⊥}=2.0010(5); ‖A _{⊥}(Al)‖=33.2(5); ‖A _{∥}(Al)‖=3(3); A _{⊥}(^{13}C)=52.1(5); A _{∥}(^{13}C)=52(2); and D(zero field splitting)=374(1). For AlC_{2}, the spin density resides predominantly in an aluminum 3p _{ z }/3s hybrid directed away from C_{2}. The neon magnetic parameters (MHz) are: g _{∥}=2.0005(5); g _{⊥}=1.9965(3); A _{⊥}(Al)=941.5(5); A _{∥}(Al)=1067(1); ‖A _{∥}(^{13}C)‖=59(1); and ‖A _{⊥}(^{13}C)‖=52(1).

Absorption spectra of alkali halide molecules in the vacuum ultraviolet
View Description Hide DescriptionThe ultraviolet (UV) and vacuum ultraviolet (VUV)absorption spectra of three alkali halide molecules CsF, CsCl, and RbF have been determined experimentally. The longest wavelength absorption features are attributed to bound–free or bound–weakly bound transitions where the upper state correlates to ground state alkali and halogen atoms. The absorption at shorter wavelengths is due to transitions from the bound ground state to excited states that correlate to excited alkali and neutral halogen atoms. The covalent repulsive curves of CsF were found to have a close resemblance to the curve corresponding to the first ionization potential of CsF. The peak absorption cross sections are similar for the CsF and RbF molecules. CsCl molecules have a three to four times larger absorption cross section for the analogous upper states. The absorption oscillator strengths have been determined from the measured absorption cross sections, and are compared to those of other alkali halide molecules that have UV absorptions.

Calculations on the van der Waals spectrum of Ar–tetrazine
View Description Hide DescriptionThe van der Waals bound states of Ar–tetrazine are calculated by a method which treats the tetrazine fragment as a rigid rotor, but which is otherwise exact within the Born–Oppenheimer approximation. The results are used to obtain frequencies and intensities for transitions between van der Waals states in the S _{1}←S _{0} electronic spectrum of the complex. Selection rules for such transitions are derived using permutation–inversion symmetry arguments. The normal rigid molecule selection rules are relaxed due to coupling between the low frequency van der Waals vibrations and the overall rotation of the complex, leading to the possibility of observing single excitation of the nontotally symmetric van der Waals bending modes. Transition intensities for van der Waals stretching excitation are predicted much smaller than experiment and various possible explanations for this disagreement are discussed.

Photodissociation of KrF by intense ultrashort pulses
View Description Hide DescriptionA theoretical model is presented for studying the photodissociation of KrF in the presence of an intense ultrashort optical pulse. The spectrum of the translational kinetic energies of the dissociated products is calculated and shown to be a clear probe for the coherent interaction.

Electron‐ and nuclear‐spin relaxation in an integer spin system, t r i s‐(acetylacetonato)Mn(iii) in solution
View Description Hide DescriptionExpressions are derived for the intermolecular contribution to the nuclear‐spin relaxation rate in solutions containing dissolved paramagnetic ions with spin S≥1. The calculation assumes that the electron‐spin Hamiltonian is dominated by a large axial zero‐field splitting, and it accounts for effects of Zeeman interactions to first order. The expressions are used to analyze proton‐spin relaxation of the acetone solvent in solutions of t r i s‐(acetylacetonato)Mn(iii)/ acetone. The main objective was to measure electron‐spin relaxation times of Mn(iii), which in this complex is a high‐spin, d ^{4} ion with integer spin S=2. Spin‐lattice relaxationmeasurements were conducted over a range of magnetic field strengths (0.28–1.1 T) where the zero‐field splitting is large compared to the Zeeman energy. Electron‐spin relaxation times of Mn(iii) were found to be 8±2 ps, with little dependence on temperature over the range 215–303 K and on magnetic field strength up to 1.1 T. Use of the assumption that Zeeman splittings dominate zero‐field splittings (Solomon–Bloembergen–Morgan theory) resulted in computed electron‐spin relaxation times that are too short by a factor of 3–4.

Studies of alkaline earth and transition metal M^{+} ^{+} gas phase ion chemistry
View Description Hide DescriptionA breakthrough into the hitherto inaccessible alkaline earth and transition metal M^{+} ^{+} gas phase ion chemistry is reported. Ions M^{+} ^{+}(L)_{ n }, where M^{+} ^{+}(Mg^{+} ^{+}, Ca^{+} ^{+}, Sr^{+} ^{+}, Ba^{+} ^{+}, Mn^{+} ^{+}, Fe^{+} ^{+}, Co^{+} ^{+}, Ni^{+} ^{+}, and Zn^{+} ^{+}) and L=H_{2}O could be produced. The hydrate equilibria M^{+} ^{+}(H_{2}O)_{ n−1}+H_{2}O=M^{+} ^{+}(H_{2}O)_{ n } (n−1, n), were determined for Mg^{+} ^{+}, Ca^{+} ^{+}, Sr^{+} ^{+}, Mn^{+} ^{+}, and Co^{+} ^{+}. These lead to successive ion–H_{2}O binding energies for high n, i.e., n=8–13 which are in the 15 kcal/mol range. The above hydrates and many other ion–ligand complexes could be produced by transferring the ions from liquid solution into the gas phase by means of electrospray. The ions were detected with a triple quadrupolemass spectrometer. The much stronger inner shell ion–ligand interactions can be studied by collision‐induced dissociation in the triple quadrupole. Single ligand loss gives way to charge reduction at low n. Thus the M^{+} ^{+}(H_{2}O)_{ n } give MOH^{+}(H_{2}O)_{ k }+H_{3}O^{+} at a low n. The n for which reduction occurs decreases as the second ionization energy of M decreases. Ligands such as DMSO and DMF lead to charge reduction at a lower n than that observed for H_{2}O. For these ligands, the charge reduction occurs via simple charge transfer, i.e., Cu^{+} ^{+}(DMSO)_{3}=Cu^{+}(DMSO)_{2}+DMSO^{+}.

Proton and deuteron magnetic resonance study of the HD–He potential energy surface
View Description Hide DescriptionThe relaxation of hydrogen and deuterium nuclei in HD–He gas mixtures is studied both experimentally and theoretically in the temperature range 90–300 K. A rationalization is given for the temperature dependence of the proton and the deuteron relaxations in terms of the relative strengths of the proton and deuteron intramolecular couplings and the role played by those HD molecules in the ground rotational state. Using a recent a b i n i t i o potential, quantitative agreement is found between the temperature dependence of the spin–lattice relaxation time of the proton in HD, as calculated theoretically and determined experimentally. A similar comparison between the calculated and experimental temperature dependence of the spin–lattice relaxation time of the deuteron in HD gave only semiquantitative agreement. It is suggested that the difference in quantitative agreement may be attributed to the selectivity of the respective predominant relaxation mechanisms to slightly different aspects of the anisotropic components of the interaction potential.

Ammonia: Dynamical modeling of the absorption spectrum
View Description Hide DescriptionA quantum mechanical calculation involving a time‐dependent formalism is used to study the Ã–X̃ transition in ammonia. The experimental spectrum exhibits a single progression in the bend despite evidence that there is a displacement in both the bond lengths and bond angles in going from the ground to first excited state. Two models for the excited statesurface are presented which reproduce this single progression in the bend. The starting point for our study is an a b i n i t i osurface. The excited statesurface is first adjusted to match the equilibrium geometries and rotational constants derived from experiment. The surface is then further refined to produce agreement with the experimental absorption and emission frequency and intensity patterns.

Photodissociation of CO^{−} _{3}: Product kinetic energy measurements as a probe of excited state potential surfaces and dissociation dynamics
View Description Hide DescriptionThe photodissociation process CO^{−} _{3} +hν→O^{−}+CO_{2} has been investigated at photon energies of 2.41, 2.50, 2.54, 2.60, and 2.71 eV. Experiments were conducted by crossing a mass‐selected, 8 keV ion beam with a linearly polarized laser beam, and measuring the kinetic energy distributions of the charged photodissociation products. By varying the angle between the ion beam and laser polarization, angular distributions were obtained at photon energies of 2.41 and 2.54 eV. The photon energy dependence of the average photofragment kinetic energies shows conclusively that photodissociation at these photon energies does not proceed by a direct dissociation process on a repulsive potential surface, or by a statistical vibrational predissociation process on a bound surface. The photofragment angular distributions are isotropic, providing further evidence that precludes direct photodissociation on a repulsive potential surface.A b i n i t i o calculations were performed using the gaussian86 programs. These calculations indicate that ground state CO^{−} _{3} has a planar D _{3} _{ h } geometry, and ^{2} A ^{’} _{2} electronic symmetry. This ground state correlates adiabatically to the CO^{−} _{2} +O dissociation asymptote, not the lower energy O^{−}+CO_{2} asymptote. Taken together, these new experimental and theoretical results suggest that the photodissociation of CO^{−} _{3} at these energies occurs via the interaction of bound and repulsive excited statepotential surfaces. A new model of the potential surfaces of CO^{−} _{3} is proposed.