Volume 98, Issue 7, 01 April 1993
Index of content:

Predissociation linewidths in O_{2} B ^{3}Σ_{ u } ^{−} (v=0,2)
View Description Hide DescriptionUsing laser‐induced fluorescence techniques applied to Schumann–Runge absorption transitions from vibrationally excited O_{2}(X ^{3}Σ_{ g } ^{−}), we have measured the rotational and fine‐structure level‐specific linewidths in v=0 and v=2 of the B ^{3}Σ_{ u } ^{−} state. These linewidths represent the first measurement of fine‐structure level‐specific predissociation rates in B ^{3}Σ_{ u } ^{−}(v≤10), and they are found to vary considerably among the various rotational and fine‐structure levels, encompassing a range of 0.09–0.34 cm^{−1} in v=0, N’≤36, and 0.4–1.4 cm^{−1} in v=2, N’≤24. Orbit‐rotational coupling in the B ^{3}Σ_{ u } ^{−}–^{3}Π_{ u } interaction, in addition to spin‐orbit coupling in the B ^{3}Σ_{ u } ^{−}–^{1}Π_{ u }, −^{3}Π_{ u }, −^{5}Π_{ u } interactions, is found to be crucial to explaining the relative predissociation rates among the fine‐structure levels, even in low rotational levels. Measurements were made in the (v’,v‘)=(0,9), (0,10), (0,21), (2,10), and (2,22) Schumann–Runge [B ^{3}Σ_{ u } ^{−}(v’)←X ^{3}Σ_{ g } ^{−}(v‘)] bands without presumption as to the molecular constants in either the X or B states. The use of high ground state vibrational levels as a starting point for the photoexcitationmeasurements produces a spectral separation among the previously blended triplet components of the absorption lines. All lines in these bands are found to be broadened by predissociation, with those terminating in v’=0 and in the N’=J’−1 (F’_{1}) levels exhibiting the smallest effect.

Double electron–electron resonance spin–echo modulation: Spectroscopic measurement of electron spin pair separations in orientationally disordered solids
View Description Hide DescriptionA DEER (double electron–electron resonance) spin–echo technique was applied to measure the electron–electron dipolar spectrum of a frozen toluene solution of the biradical, 2,6‐bis[(((2,2,5, 5‐tetramethyl‐1‐oxypyrrolin‐3‐yl)carbonyl)oxy)]‐anthracene. Modulation of the DEER spin–echo envelope was observed and identified as originating from the dipolar coupling between the two nitroxide spins of the biradical. Fourier transformation of the modulated components of the echo envelope yielded a dipolar spectrum from which a spin–pair separation of 19.73±0.14 Å was calculated. Constraints on the relative orientation of the two nitroxide spin moieties were obtained by analysis of the effect of the microwave pulse orientational selectivity on the DEER modulation amplitudes. Molecular models of the studied compound exhibit structures that correspond well with the structural information deduced by DEER spectroscopy.

Pulsed and continuous wave electron nuclear double resonance patterns of aquo protons coordinated in frozen solution to high spin MN^{2+}
View Description Hide DescriptionFor the water protons that coordinate to Mn^{2+}, the frozen solution ENDOR(electron nuclear double resonance)spectra are made complex by the anisotropic electron–proton hyperfine interaction and by multiple contributions of the electron spin 5/2 manifold. A spin 5/2 Mn^{2+} ion having magnetic quantum numbers M _{ s }=±1/2, ±3/2, ±5/2 and small zero‐field splittings has overlapping electron spin EPR transitions. Proton hyperfine couplings to each of these electron spin states have yielded overlapping ENDOR patterns whose interpretation is nontrivial, even in so simple a system as Mn^{2+} ion having hexaaquo coordination. We have experimentally obtained and theoretically explained these protonENDOR patterns and in so doing have laid the foundation for interpreting and sorting out frozen solution ENDOR patterns in more complex (enzyme) environments. Pulsed and cw ENDOR experiments showed features of metal‐coordinated water protons occurring not only within a few MHz of the free proton frequency (as will happen for an electron spin 1/2 system) but extending over a range of up to 35 MHz. The EPR line of the Mn^{2+} S=5/2 manifold was broadened by zero‐field splitting for hundreds of Gauss away from g=2.00, and the relative intensity of different ENDOR features reflected couplings to differing M _{ s } spin states at varying fields across this EPR line.
An expression was derived to show the dependence of protonENDOR frequencies on the electron spin quantum number, M _{ s }, upon the principal values of the intrinsic proton hyperfine tensor, upon the direction of the magnetic field, and upon the free proton frequency. This expression provided the starting point for powder simulations of the overall ENDOR pattern. These ENDORpowder pattern simulations were not elementary extensions of first‐order theory as often applied to single‐crystallike ENDORspectra obtained at frozen solution EPR extrema. These simulations addressed the interpretation and the conditions for obtaining consistent hyperfine information from nuclei hyperfine coupled to Mn^{2+} where all M _{ s } levels of the Mn^{2+} ion can furnish contributions to ENDOR and EPR. The aquo protons yielded an isotropic coupling A _{ s }=0.8 MHz and a dipolar coupling A _{ d }=3.4 MHz, where A _{∥}=2A _{ d }+A _{ s } (=7.6 MHz) and A _{⊥}=A _{ s }−A _{ d } (=−2.6 MHz). Such couplings are in agreement with those obtained by protonENDOR of [Mn(H_{2}O)_{6}]^{2+} in single crystals [R. DeBeer, W. DeBoer, C. A. Van’t Hof, and D. Van Ormondt, Acta Cryst. B29, 1473 (1973)].

Spectroscopic characterization of the lowest Π and Σ bending states of ArHCN
View Description Hide DescriptionThe lowest excited bending states, Σ_{1} and Π_{1}, of the ArHCN complex have been measured by millimeter‐wave electric resonance optothermal spectroscopy. The principal molecular constants determined for the Σ_{1} state are ν_{0}=164 890.790(12) MHz; B=1958.8571(37) MHz; D=−0.075 23(29) MHz; eq _{ aa } Q=0.825(27) MHz; and μ_{ a }=−0.521(30) D. For the Π_{1} state, the constants are ν_{0}=181 984.4126(47) MHz; B=2031.3624(17) MHz; D=0.153 35(16) MHz; eq _{ aa } Q=0.904(11) MHz; and μ_{ a }=0.273 02(63) D. The leading Σ_{1}–Π_{1} coupling constants are the Coriolis coefficient β_{0}=1016.998(13) MHz and the transition dipole moment μ_{ b }=2.2535(57) D. The rotational constants for the two bending states indicate that the average separation between the argon and the HCN center of mass contracts by roughly 0.5 Å compared to the linear ground state. This is consistent with the nearly T‐shaped average geometry for each state established by analysis of the dipole moments and quadrupole coupling constants. Agreement between this work and prior theory confirms attribution of the anomalous distortion and isotope effects in the ground state to extreme angular–radial coupling. The relative sign of the dipole moments for the Σ_{1} and Π_{1} states is resolved in this work, allowing an unambiguous interpretation of the angular information. Assuming Laguerre angular distributions, we obtain that the Σ_{1} state wave function has a maximum at an angle of 108° with a halfwidth of 49°, and that the Π_{1} state maximum is at 80° with a halfwidth of 37°. This estimate for the Σ_{1} state angular distribution indicates that although the state is not antilinear (ArNCH), as was expected, it does approach this configuration. The Π_{1} state is nearly a free rotor eigenstate, showing that the angular part of the potential surface near 90° is extremely flat. The combined data from the ground, Σ_{1}, and Π_{1} states reflect virtually the entire angular coordinate along the radial minimum of the potential, and should provide a reliable benchmark for ab initiopotential energy surfaces for ArHCN near the bottom of the well. We compare the data to predictions from available models.

Reinvestigation of the HCP electronic spectrum: Experimental determination of D _{0} for the X̃ state and observation of hyperfine quantum beats in the B̃ state
View Description Hide DescriptionIn the laser‐induced fluorescencespectrum of HCP, a sharp decrease in fluorescence intensity was observed at 41 680 cm^{−1}. This diminuation in emission intensity is due to a rotationally mediated coupling of the excited vibronic level with the dissociative continuum of the X̃ state; this has allowed us to place an upper limit on D _{0} for the ground electronic state. A lower limit for D _{0} can be placed at 41 650 cm^{−1} since strong emission was observed below this excitation energy. Using the assignments of Johns et al. [Can. J. Phys. 47, 893 (1969)] as a guide, fluorescence decays were recorded from the two B̃ state vibronic bands to determine lifetimes. In addition to determining an unusually long lifetime for both states, the decays were seen to be modulated at four frequencies resulting from the coherent excitation of four nuclear hyperfine levels. This increase in lifetime and magnetic hyperfine interaction arises from a mixing of three singlet and triplet states. The contributing states are tentatively identified.

Quantum dynamics of overtone relaxation in benzene. III. Spectra and dynamics for relaxation from CH(v=3)
View Description Hide DescriptionThis series is concerned with the quantum dynamics of overtone relaxation in benzene and in reduced mode benzene fragments. In part III of this series, emphasis is placed on the CH(v=3) overtone spectrum and the survival probability for both 16 mode (five CH stretch modes are forced to remain inactive out of 21 planar modes) and 21 mode planar benzene models. Through use of the wave operator contraction algorithm, primitive vibrational basis sets containing up to 9×10^{9} states are contracted to active spaces containing 2000–7000 states. The exact dynamics within the active space is developed with the recursive residue generation method (RRGM). Specific results reported in this study include the following: (a) a comparison is made between dipole spectra and residue spectra, the former explicitly involving the CH dipole function; (b) for the 16 mode model, the following quantities are displayed: overtone spectrum, survival probability of the initial state, complex‐valued autocorrelation function, number of phase space cells explored as a function of time, and the rate of exploring phase space cells; (c) sensitivity of the overtone spectrum to the size and composition of the active space built with the wave operator contraction algorithm; (d) for 21 mode benzene, we consider the sensitivity of the overtone spectrum to ‘‘softening’’ the anharmonic potential; (e) comparisons are made with recent jet cooled beam experimental overtone spectra.

Highly vibrationally excited HCN/HNC: Eigenvalues, wave functions, and stimulated emission pumping spectra
View Description Hide DescriptionWe report the large scale calculation of accurate vibrational eigenstates of X̃ state HCN/HNC up to ∼37 000 cm^{−1}. The discrete variable representation‐distributed Gaussian basis (DVR‐DGB) approach is used. In the final major computational step, large dense Hamiltonian matrices (≥11 000 rows or columns) must be diagonalized. The use of new software for out‐of‐core eigenproblems is described in some detail. Representative wave functions of high lying vibrational eigenstates near the isomerization barrier and at energies up to 39 313 cm^{−1} are presented. Theoreticalstimulated emission pumping (SEP) spectra for several initial (Ã) vibrational states are presented along with calculations of the quantum survival probability for these spectra. The spectra range from being dominated by localized eigenstates to being dominated by delocalized eigenstates, depending on the geometry of each initial state. In addition, time‐dependent expectation values of the coordinates of the evolving wave packet are plotted.

Theoretical spin–rovibronic ^{2} A _{1}(Π_{ u })–^{2} B _{1} spectrum of the H_{2}O^{+}, HDO^{+}, and D_{2}O^{+} cations
View Description Hide DescriptionBased on extensive ab initio multiconfiguration reference configuration interaction (MRCI) electronic structure calculations, three‐dimensional potential energy functions for the A ^{2} A _{1}(Π_{ u })–X ^{2} B _{1} linear/bent Renner–Teller systems of H_{2}O^{+}, HDO^{+}, and D_{2}O^{+} have been generated and used in beyond Born–Oppenheimer calculations of the spin–rovibronic energy levels by a variational approach. The effects of anharmonicity, rotation–vibration, electronic angular momenta, and spin coupling effects have been accounted for. For H_{2}O^{+}, HDO^{+}, and D_{2}O^{+} vibronic band origins for the bending levels to K _{ a }=5 and for selected stretching and combination levels are given. Almost all experimentally known data have been reproduced with an accuracy better than 10 cm^{−1} after minor modifications of the ab initio potential energy functions. Our calculated values will facilitate further experimental assignments. A consistent interpretation of the photoelectron spectra for the different isotopes of water is given. Previous assignments of the bending modes in the A ^{2} A _{1} state have been revised by two quantum numbers.

Nuclear hyperfine interactions and dynamic state of H_{2}O in Ar–H_{2}O
View Description Hide DescriptionRotational spectra for the Ar–H_{2}O dimer have been measured with the Balle/Flygare Mark II pulsed nozzle, Fourier transformmicrowave spectrometer. The present study extends that of Fraser et al. [J. Mol. Spectrosc. 144, 97 (1990)] to higher resolution, enabling analysis of the hyperfine structure and spin statistics for the Σ(0_{00}) and Σ(1_{01}) internal rotor states of water in Ar–H_{2}O and Ar–D_{2}O and of the Σ(0_{00}) state in Ar–HDO. The results indicate the two internal rotor states to be quite similar, especially for Ar–D_{2}O.

Rotational distributions of molecular ions following core excitation and decay
View Description Hide DescriptionThe rotational distribution in the N_{2} ^{+} (B) state produced by core excitation and decay is reported for three different core excitation energies. In contrast to the case of valence excitation, the rotational branching ratios do not exhibit a significant dependence on excitation energy. This similarity is attributed to the uniformly high kinetic energies of the emitted electron for the three cases and provides evidence for the production of singly charged molecular ions in the core ionization continuum through an interchannel coupling mechanism.

Auger electron spectroscopy of molecules: Theory for spin polarization following photoabsorption in rotating linear molecules
View Description Hide DescriptionThis paper develops theoretical expressions to study angular distribution and spin polarization of those Auger electrons which are emitted in the decay of a vacancy created by the absorption of a photon in a rotating linear molecule. Identical expressions except, of course, for different decay amplitudes, in both the Hund’s coupling schemes (a) and (b), are obtained for the differential Auger current emitted in the transition J→J _{ f } measured by an electron spectrometer sensitive to spin detection. The structure of these angular distributions is exactly the same as that of the spin‐resolved photoelectrons from unoriented atoms and molecules. The present paper thus puts the angle‐ and spin‐resolved Auger and photoelectron spectroscopies on the same footing wherein identical geometrical and kinematical analysis is applicable. The four parameters needed to completely characterize such distributions depend, in the present case, on rotational orientation and/or alignment of the photoexcited molecule, in addition to its Auger decay amplitudes. The use of parity‐adapted molecular states separates the Auger spectra into even and odd partial wave components of the ejected electron continuum in both of the coupling schemes. Our analysis shows that the integrated Auger current is spin resolved provided it is produced in the decay of oriented vacancies. We further find that Auger electrons which leave the molecular ion in J _{ f }=0 state may have nonzero degree of spin polarization if they follow absorption of only circularly polarized light. In this case, both the angular distribution and spin polarization of emitted electrons become totally independent of Auger dynamics. Thus, angle‐ and spin‐resolved Auger electron spectroscopy can be used to produce polarized electrons, to determine rotational orientation and alignment of linear molecules, to study their structure and dynamics, and to prepare ions of such molecules in selective ro‐vibronic states.

Photoacoustic Fourier transform infrared spectroscopy of gases: A nonequilibrium approach
View Description Hide DescriptionThe first order approximations of the nonequilibrium mass balance and momentum balance equations allow predictions of the nonequilibrium temperature–pressure relationships in a photoacoustic effect. This is achieved by setting appropriate boundary conditions for the case when frequency of the normal vibrational modes in infrared is significantly greater than the photoacoustic modulation frequency. Based on the internal energy balance equations, the temperature distributions in the gas phase are derived and tested experimentally, allowing quantitative analysis of the photoacoustic FT‐IR spectra.

Photoelectron spectroscopy of rare gas dimers revisited: Vibrationally resolved photoelectron spectrum of argon dimer
View Description Hide DescriptionPhotoelectron spectra of rare gas dimers Ar_{2}, Kr_{2}, and Xe_{2} have been measured using the He i radiation at 584 A with a resolution of 13 meV. All the six ionic states in the He i region have been resolved for the first time, including some of the purely repulsive states. Ionization energies are presented to an accuracy ≤0.003 eV. Dissociation energies (D _{0}) of the ionic states are calculated using the best estimate adiabatic ionization energies. The D ^{2}Σ_{1/2g } ^{+} states of all the dimers are weakly bound, contrary to an earlier theoretical prediction. Part of the vibrational structure of two of the electronic states of Ar_{2} has been resolved. For the A ^{2}Σ_{1/2u } ^{+} state, as many as 14 vibrational excitations are observed. These excitations are assigned to transitions to the higher vibrational levels of v=32–45. For the B ^{2}Π_{3/2g } state, three vibrational structures are resolved and are assigned to v=2–4 of the ionic state. Accurate values of the spectroscopic constants of these states are presented. The present values are compared with the values available in the literature. The D _{ e } values for the A ^{2}Σ_{1/2u } ^{+} and the B ^{2}Π_{3/2g } states are estimated to be 1.361 and 0.104 eV, respectively.

Fluorescence properties of submonolayers of rhodamine 6G in front of a mirror
View Description Hide DescriptionFluorescence properties of submonolayers of rhodamine 6G have been measured as a function of distance to an aluminummirror. For distances less than 5 nm (spacer layers of 1 nm thickness were used) a broadening of the emission spectrum is observed. The fluorescence lifetime has been measured for distances up to 6 nm. The broadening of the fluorescence spectra and the shortening of the lifetime observed when the sample is close to the mirror are attributed to efficient energy transfer from the monolayer to the mirror. The experiments clearly demonstrate that both volume and surface contributions must be considered in this energy‐transfer process.

Solvation of Hg(^{3} P _{1}) in the HgAr_{ n } clusters through resonant enhanced multiphoton ionization: Evidence for nonadditive effects in the excited state potentials
View Description Hide DescriptionMass resolved resonance enhanced excitation spectra of HgAr_{ n } have been measured in the vicinity of the Hg(6s6p) ^{3} P _{1}→(6s ^{2}) ^{1} S _{0} transition. The spectra are compared with calculations based on additive potentials for both the ground and the excited state. These potentials give reasonable agreement with the experiments. Deviation from this simple model occurs in the excited electronic state mainly for structures where the argon atoms are in the closest approach from the Hg.

Microwave spectra of the D_{2} ^{+} and HD^{+} ions near their dissociation limits
View Description Hide DescriptionWe have observed and measured seven microwave transitions in D_{2} ^{+}, which are vibration–rotation components of the 2pσ_{ u }–1sσ_{ g } electronic band system. We also report two microwave rotational components involving the 1sσ v=21 level of HD^{+}, which lies close to the first dissociation limit. Our experiments use ion beam techniques in which state selection is achieved by electric fielddissociation. The techniques and theory of electric fielddissociation are discussed, and fragment ion kinetic energy spectra described. Ab initio calculations of the transition frequencies for both D_{2} ^{+} and HD^{+} are in excellent agreement with experiment, and the hyperfine structure of the rotational transitions in HD^{+} confirms previous demonstrations of the increasing asymmetry in the electron distribution as the dissociation limit is approached. The electronic transition in D_{2} ^{+} which involves the highest bound level of the ground state shows an unexpected hyperfine splitting, which is interpreted in terms of g/u symmetry breaking by the deuteron Fermi contact interaction.

Approximations for the inertial defects of planar molecules
View Description Hide DescriptionThe observed zero‐point inertial defect Δ_{0}=I _{0} ^{ cc }−I _{0} ^{ aa }−I _{0} ^{ bb } is often used as a test of the planarity of a molecule, but this test is difficult to make quantitative when the information on the vibrations and vibration–rotation interactions is limited. The general formula for the inertial defect to order κ^{2} I _{ e } is separated into harmonic and Coriolis contributions Δ_{0} ^{harm} and Δ_{0} ^{Cor}, which can be expressed in various ways by use of sum rules. It is shown here that the zero‐point harmonic term is given approximately by Δ^{harm} _{0}≊Δ^{tau} _{0}= 3K(1/ω_{ A }+1/ω_{ B }+1/ω_{ AB }−1/ω_{ C }), where K is ℏ^{2}/2hc and the effective vibrational frequencies are given in terms of the centrifugal tensor τ_{αβγδ} by τ_{ aaaa }=−16A ^{3}/ω_{ A } ^{2}, τ_{ bbbb }=−16B ^{3}/ω_{ B } ^{2}, τ_{ cccc }=−16C ^{3}/ω_{ C } ^{2}, and τ_{ abab }=−16ABC/ω_{ AB } ^{2}, so that Δ_{0} ^{harm} can be estimated from centrifugal distortion constants. A fit of empirical data gives Δ_{0}≊Δ_{0} ^{tau}[1.0292−0.0911(N−3)], where N is the number of atoms and the correction factor makes approximate allowance for Δ_{0} ^{Cor}. Thus an observed Δ_{0} that is significantly negative relative to this approximation may indicate that the molecule is nonplanar or has unusually low out‐of‐plane frequencies.

Far infrared absorption by electrolyte solutions
View Description Hide DescriptionThe far infrared absorption spectra of aqueous solution of LiCl have been investigated in a frequency range 8–40 cm^{−1} at room temperature. In low frequency range (8–20 cm^{−1}), the absorption is mainly controlled by Debye relaxation of the electric dipole rotation, and the absorption decreases with the increase of concentration.

Nonlinear response functions for birefringence and dichroism measurements in condensed phases
View Description Hide DescriptionThe fourth‐rank tensorial nonlinear response function which controls the response of a chromophore in the condensed phase to three arbitrarily polarized external fields is calculated. Internal (vibrational and rotational) degrees of freedom are incorporated using wave packets in Liouville space (the doorway/window picture). The rotational contribution of the chromophore is expressed in terms of a conditional probability for the rotational diffusion model. We apply this formalism to transient dichroism and birefringence spectroscopies.

van der Waals rovibration levels and the high resolution spectrum of the argon–benzene dimer
View Description Hide DescriptionThe van der Waals vibrations of Ar–benzene are calculated from two different intermolecular potentials, which are analytic fits to the same ab initio potential. The rovibrational Hamiltonian was derived earlier; the wave functions of the large amplitude vibrations are expanded in products of harmonic oscillator functions. The rotational structure of each van der Waals state is obtained from perturbation theory, as well as from variational calculations of the complete rovibrational states for J=0, 1, and 2. The degenerate bending modes and combinations have a large vibrational angular momentum; for their rotational structure it is important to include all first, second, and higher order rovibrational (Coriolis) coupling. The calculated vibrational frequencies, the information about rovibrational coupling, and the PI(C _{6v }) selection rules for van der Waals transitions, in combination with the vibronic 6_{0} ^{1} transition on the benzene monomer, lead to a partially new assignment of the three van der Waals sidebands observed in high resolution UV spectra.