Volume 99, Issue 11, 01 December 1993
Index of content:

High resolution emission spectroscopy of AlCl at 20 μ
View Description Hide DescriptionThe high resolution infrared emission spectrum of aluminum monochloride has been recorded with a Fourier transformspectrometer. A total of 1747 rovibrational transitions, v=1→0 to v=8→7, for the most abundant isotopomer ^{27}Al^{35}Cl and 708, v=1→0 to v=4→3, for the least abundant isotopomer ^{27}Al^{37}Cl have been assigned. This new set of infrared data was combined with existing microwave and millimeter‐wave data to refine the Dunham Y _{ ij } constants for the X ^{1}Σ^{+} electronic ground state. In addition two sets of mass‐reduced Dunham U _{ ij } constants have been determined from separate fits. In the first fit all of the U _{ ij } constants that could be statistically determined were treated as adjustable parameters. In the second fit only the constants satisfying the condition j<2 were treated as adjustable parameters while the values for the remaining constants were fixed to constraints imposed by the Dunham model. Finally, in order to fully utilize the information provided by this extensive data set in an attempt to improve the prediction of energies for higher lying v, J levels of the X ^{1}Σ^{+} state, the combined data set, consisting of microwave, millimeter, and infrared (IR) data were fitted directly to the eigenvalues of the Schrödinger equation containing a parametrized internuclear potential energy function.

High resolution infrared emission spectra of AlH and AlD
View Description Hide DescriptionHigh resolution infrared emission spectra of aluminum monohydride and monodeuteride have been recorded. Gaseous AlH and AlD were generated by reacting molten aluminum metal with hydrogen and deuterium gas. Approximately 265 AlH lines with v=1→0 to v=5→4 and 470 AlD lines with v=1→0 to v=7→6 are reported. Dunham Y _{ ij } constants were obtained by fitting the data of each isotopomer separately to the Dunham energy level expression while mass‐reduced Dunham U _{ ij } constants were obtained from a combined fit of all isotopomer data. A second set of Dunham U _{ ij } constants was obtained from a fit where U _{ ij }’s with j<2 were treated as adjustable parameters and all remaining U _{ ij }’s fixed to values that satisfy the constraints imposed by the Dunham model. Finally, an effective Born–Oppenheimer potential was determined by fitting all the data directly to the eigenvalues of the radial Schrödinger equation containing a parametrized potential function.

High resolution infrared emission spectra of GaH and GaD
View Description Hide DescriptionThe high resolution infrared emission spectra of gallium hydride and gallium deuteride have been recorded with a Fourier transformspectrometer. There were 1045 lines observed including those from the v=1→0 to v=7→6 bands for the ^{69}GaD and ^{71}GaD species and v=1→0 to v=4→3 bands for the ^{69}GaH and ^{71}GaH species. Dunham Y _{ ij }’s for each isotopomer were obtained by fitting the data set of each isotopomer separately to the Dunham energy levels of the X ^{1}Σ^{+} electronic ground state. The mass‐reduced Dunham U _{ ij }’s were determined using two independent methods. In the first fit the U _{ ij }’s constants were determined by the traditional method where all the constants were treated as adjustable parameters and determined statistically. In the second fit the U _{ ij }’s which satisfied the condition j<2 were treated as adjustable parameters and the remaining constants were fixed by constraints imposed by the Dunham model. In order to predict the positions of transitions with v’s and J’s much higher than those observed the entire data set was fit directly to the eigenvalues of the Schrödinger equation containing a parameterized internuclear potential energy function.

Free‐jet infrared absorption spectroscopy of the C_{2}H_{2}–Ar complex in the doubly degenerate monomer C–H bending region
View Description Hide DescriptionVibration–rotation transitions of the C_{2}H_{2}–Ar complex have been observed in the doubly degenerate monomer C–H bending (ν_{5}) region via direct IR absorption of tunable diode laserradiation in a pulsed supersonic free jet. The spectrum is composed of in‐plane and out‐of‐plane C–H bending bands, which are coupled with each other through a strong Coriolis interaction. Anomalous Coriolis coupling and A rotational constants indicates the failure in a semirigid‐molecule description of this highly nonrigid system in the doubly degenerate bendingmanifold. A dynamical model calculation, where the large‐amplitude van der Waals bending motion is treated as an internal rotation of the C_{2}H_{2}monomer hindered by an anisotropic intermolecular potential, has revealed that the characteristic energy‐level structure originates from its specific angular momentum coupling scheme involving an additional angular momentum for the intramolecular degenerate bending. The difference potential providing the parity doubling in the C–H bendingmanifold is estimated to be only about −0.1 cm^{−1}.

The intermolecular vibrations of Ar–styrene and Ar–4‐fluorostyrene complexes
View Description Hide DescriptionOne‐color (1+1) resonance enhanced multiphoton ionization (REMPI) spectra are reported for styrene–Ar_{ n } clusters with n=2, 3, and for 4‐fluorostyrene–Ar_{ n } clusters with n=2–5. These spectra are compared with previously recorded spectra of the mono‐Ar complexes and discussed in relation to the structures of the clusters. For the mono‐Ar complexes we have performed practically exact quantum calculations of the van der Waals vibrational frequencies and properties, starting from two different empirical atom–atom potentials. The intermolecular potentials are strongly anharmonic and, due to the low symmetry of these dimers, we find considerable mode mixing. As a consequence of the kinematic coupling between the Ar motion and the internal rotation, the bending frequencies depend considerably on the different rotational constants of the molecules. The order of the fundamental frequencies is the same for both dimers. For Ar–4‐fluorostyrene the calculated vibrational frequencies agree well with the observed spectrum; the van der Waals side bands can thus be assigned in detail. For Ar–styrene the observed frequencies are less well reproduced, so we must conclude that the atom–atom potential used is substantially better for Ar–fluorostyrene than for Ar–styrene.

Multiple quantum nuclear magnetic resonance of solids: A cautionary note for data analysis and interpretation
View Description Hide DescriptionThe conventional method of data analysis and interpretation of time‐resolved multiple quantum (MQ) nuclear magnetic resonance(NMR)spectra of solids is closely examined. Intensity profiles of experimental ^{1}H MQ NMRspectra of polycrystalline adamantane and hexamethylbenzene serve to test the Gaussian statistical model approach. Consequences of this model are explored with a least‐squares fitting procedure, transformation of data to yield linear plots, and a scaling analysis. Non‐Gaussian behavior of the MQ NMR spectral intensity profiles, as a function of order of coherences, is demonstrated with all these methods of analysis. A heuristic argument, based on the multiplicative properties of dipolar coupling constants in the equation of motion of the density operator, leads to the prediction of exponentially decaying MQ NMR spectral intensity profiles. Scaling analysis and semilog plots of experimental time‐resolved MQ NMRspectra of adamantane and hexamethylbenzene support this deduction. Dynamical scale invariance in the growth process of multiple spin coherences is revealed with this new approach. The validity of spin counting in solids with MQ NMR is discussed in light of the present results.

Spectroscopy of XeF in Ar and Ne matrices
View Description Hide DescriptionSpectroscopic constants for the B ^{2}Σ^{+} and C ^{2}Π charge transfer states of XeF in Ar and Ne matrices are derived from vibrational progressions in excitation and emission spectra.Polarization effects on the T _{ e } values are discussed and Rittner potentials are fitted to the constants. Emissions at 389 and 411 nm in Ne and Ar, respectively, are attributed to a distorted B state denoted as B* with a strong red shift of T _{ e } and an about 30% larger ω_{ e }. B* is assigned to a XeF center with an additional F atom which can be a precursor to XeF_{2} according to a comparison with Xe_{2}F spectra and the concentration dependence of the intensities. The B* vibrational bands display two well resolved fine structure progressions with a common ω_{ e } of ∼60 cm^{−1} in Ar and Ne which is close to the maximum matrix phonon density and one with 30 cm^{−1} in Ne and 15 cm^{−1} in Ar. A reversible change in the Ne fine structure pattern with temperature can be correlated to a face‐centered‐cubic–hexagonal close‐packed (fcc–hcp) phase transition.

The role of the anisotropic interaction on collision induced absorption of systems containing linear molecules: The CO_{2}–Ar case
View Description Hide DescriptionNumerical calculations are presented of spectral moments of collision induced absorption (CIA) coefficient of CO_{2}–Ar mixtures at various temperatures. Three spherical components have been used for the induced dipole moment all of which arise from pure multipolar induction. The calculations have been performed with six different potentials which differ with respect to the amount of anisotropy. The comparison of experimental and numerical results shows that the potential proposed by Preston and Pack [J. Chem. Phys. 66, 2480 (1977)] is the most suitable to describe CIA spectra. In addition, it is shown that the role of the anisotropy of the potential with regard to its contribution to the spectral moments, particularly the amount of mixing of dipole components of different symmetry, cannot be neglected.

Zero‐kinetic‐energy photoelectron spectrum of carbon dioxide
View Description Hide DescriptionThe zero‐kinetic‐energy (ZEKE) photoelectron spectrum of carbon dioxide has been measured between 111 000 and 112 000 cm^{−1} at a resolution of 1.5 cm^{−1} using a coherent source of XUV radiation based on four‐wave mixing in krypton. The spectrum consists of six bands corresponding to transitions from the ground X ^{1}Σ^{+} _{ g }(v _{1},v _{2},v _{3}=000) state of the neutral to the two spin–orbit components of the (000) vibrational level and the four Renner–Teller states associated with the (010) vibrational level of the ground electronic state (X ^{2}Π_{ g }) of the ion. The analysis of the partially resolved rotational structure of the various bands leads to a detailed picture of the photoionization process. The propensity rules for angular momentum transfer during photoionization are strongly dependent on the symmetry (^{2}Π_{ g,3/2}, ^{2}Π_{ g,1/2}, ^{2}Δ_{ u,5/2}, ^{2}Δ_{ u,3/2}, ^{2}Σ^{+} _{ u }, and ^{2}Σ^{−} _{ u }) of the different ionic states probed and on the Hund’s coupling case they follow [case (a) for the Π and Δ states and case (b) for the Σ states]. A comparison of the experimental ZEKE line intensities with theoretical predictions and conventional photoelectron spectra reveals a series of anomalies which are discussed in terms of final state interactions. The ionization potential of CO_{2} is estimated to be 111 111.0±3 cm^{−1}, somewhat lower than the value of 111 121±2 cm^{−1} determined from extrapolation of the Rydberg series by Cossart‐Magos et al. [Mol. Phys. 61, 1077 (1987)].

Adiabatic ionization energy of CH_{3}SSCH_{3}
View Description Hide DescriptionThe ionization energy (IE) for CH_{3}SSCH_{3} has been measured by the pulsed molecular beamphotoionization mass spectrometric method. The experimental IE of 8.18±0.03 eV is in excellent agreement with the theoretical prediction of 8.15 eV calculated using the ab initio Gaussian‐2 procedure, indicating that the experimental ionization onset can be assigned as the adiabatic IE for CH_{3}SSCH_{3}. The observation of the adiabatic IE(CH_{3}SSCH_{3}) is attributed to the low potential energy barrier for rotation about the S–S bond, which allows CH_{3}SSCH_{3} to exist dynamically in a wide range of CSSC dihedral angles.

Photoionization mass spectrometric studies of Sb_{2} and Bi_{2}
View Description Hide DescriptionThe photoion yield curves of Sb^{+} _{2}(Sb_{2}) and Bi^{+} _{2}(Bi_{2}) are presented. In both cases, two autoionizing series (designated pσ and pπ) are observed, converging on the excited ^{2}Σ^{+} _{ g } state. From this information, the ionization energy of the ^{2}Σ^{+} _{ g } state in Sb_{2} is lowered to 9.247 eV. The difference in quantum defects, δ_{ pπ}−δ_{ pσ}, is shown to be related to the quadrupole moment of the molecular ion core of the A ^{2}Σ^{+} _{ g } state in Pn^{+} _{2}. The adiabatic ionization energies (AIP) are also decreased: AIP(Sb_{2})≤8.43 eV, AIP(Bi_{2})≲7.34 eV. Although the uppermost occupied orbital is nominally a bonding pπ orbital, an analysis leads to the surprising conclusion that D _{0}(Pn^{+} _{2})≳D _{0}(Pn_{2}), where Pn=P, As, Sb, and Bi.

Ab initio calculations of circular dichroism for (S)‐2‐deuteriopropanoic acid and (R)‐2‐deuterioglycine
View Description Hide DescriptionAb initio calculations using the random‐phase approximation are reported for the natural circular dichroism spectra (ellipticity vs wavelength) of (S)‐2‐deuteriopropanoic acid (CH_{3}CHDCOOH) and (R)‐2‐deuterioglycine (NH_{2}CHDCOOH). The calculated results agree reasonably well with previously reported experimental measurements when consideration is given to the environment in which the calculation (vapor) and measurement (liquid) are made. The results also show that the structure of the molecules under study deviate slightly from the planar states of their parent compounds propanoic acid (CH_{3}CH_{2}COOH) and glycine (NH_{2}CH_{2}COOH). The latter feature has not been tested experimentally.

Local order and transition dipole coupling in liquid methanol and acetone as the origin of the Raman noncoincidence effect
View Description Hide DescriptionModel calculations are performed on the Raman noncoincidence effect (frequency difference between the isotropic and anisotropic components) observed for the C–O stretching band of liquid methanol and the C=O stretching band of liquid acetone. Microscopic liquid structures are obtained by Monte Carlo simulations, and coupling between molecular vibrations is introduced by the transition dipole coupling mechanism. Ab initio molecular orbital calculations are also performed to check the validity of the assumed direction of the transition dipole for the C–O stretching mode of methanol. The different signs of the Raman noncoincidence between the C–O stretching band of liquid methanol and the C=O stretching band of liquid acetone can be explained by the transition dipole coupling mechanism. The calculated magnitudes of the frequency separations between the isotropic and anisotropic components are in good agreement with the experimental results. Pressure dependence of the Raman noncoincidence is also calculated and compared with the experimental results. In the case of the C–O stretching band of liquid methanol, local anisotropy in the pressure‐induced changes of the liquid structure is shown to be important for the pressure dependence of the Raman noncoincidence.

Nonlinear optical line shapes of disordered molecular aggregates: Motional narrowing and the effect of intersite correlations
View Description Hide DescriptionWe theoretically investigate nonlinear optical line shapes of linear molecular aggregates with Gaussian disorder in the molecular transition frequencies. A perturbative treatment in the disorder is used, within which the joint stochastic distribution function of the frequencies of all multiexciton states of an aggregate can be determined analytically. It is shown that motional narrowing, which is characteristic for the linear absorption spectra of aggregates, also occurs for nonlinear line shapes. An important aspect of our disorder model is that it allows for general correlations between the transition frequencies of molecules within one aggregate, thereby interpolating between continuous energy disorder and a segment or kink model. The general theory is applicable for nonlinearities of any order. Specific applications are discussed for linear absorption, nonlinear absorption, and two‐color pump–probe spectra. Our theory suggests that pump–probe experiments provide a novel and very promising approach to obtain microscopic information on aggregate systems; in particular, this technique can be used to determine both the magnitude of the molecular disorder and its degree of intersite correlation within aggregates.

The 3 ^{1}Σ^{+} _{ g } ‘‘shelf’’ state of Na_{2}
View Description Hide DescriptionThe 3 ^{1}Σ^{+} _{ g } state of Na_{2} has been extensively studied using an optical–optical double resonance technique and a shielded cylindrical space‐charge‐limited ionizationdetector. A wide range of vibrational quantum numbers 0≤v≤141 was observed which covered over 99.9% of the potential well depth. A ‘‘shelf’’ on the potential energy curve was found near v=23. A hybrid potential was constructed based on the Rydberg–Klein–Rees (RKR) potential curve and a local inverse perturbation approach (IPA) for the shelf region. For this potential, there is a shallow second local minimum ∼5 cm^{−1} in depth at the shelf region. The long range energy also compares well with long range theory.

Infrared diode laser spectroscopy of the LiO radical
View Description Hide DescriptionThe fundamental vibrational band of the ^{7}LiO radical in the ground electronic stateX ^{2}Π_{ i } was observed in a region from 720 to 850 cm^{−1} using a sourcefrequency modulation infrared diode laserspectrometer. Radicals were generated in a high‐temperature cell by the reaction of lithium metal vapor with nitrous oxide. The observed spectrum was analyzed together with the radio‐frequency and microwave spectra already reported. It was found that the vibration‐rotation Hamiltonian employed in a previous paper was insufficient to fit all of the observed spectra simultaneously. The Hamiltonian was thus extended to include higher‐order corrections for the centrifugal distortion and Λ‐type doubling terms, and was used to derive molecular parameters.

Raman spectroscopic investigation of the dynamics of urea–water complexes
View Description Hide DescriptionThe hydration of urea via hydrogen‐bond formation was found to have a significant effect on the observed Raman band shapes. An analysis indicates that, on the average, one molecule of water is carried along with a solute molecule during its reorientational motion (τ_{ R }≊2.5 ps). Vibrational dephasing of the ν_{4} vibration (τ_{ω}≊0.2 ps) is independent of concentration and appears to decrease slightly with increasing temperature. No evidence of urea dimer formation was detected.

Electronic spectroscopy of the niobium dimer molecule: Experimental and theoretical results
View Description Hide DescriptionRotationally resolved electronic spectra of the niobium dimer molecule are reported for the first time. The molecules were produced by laser vaporization of a niobium target rod and cooled in a helium supersonic expansion. The molecular beam containing niobium dimer molecules was interrogated in the range 400–900 nm using a pulsed dye laser to excite fluorescence. Numerous Ω=0←Ω=0 and Ω=1←Ω=1 vibronic transitions were discovered in the region 630–720 nm and investigated at 200 MHz resolution using the cw output of a single mode ring dye laser. The principal features were classified into five Ω=0←Ω=0 systems originating from a common lower state of 0^{+} _{ g } symmetry, and three Ω=1←Ω=1 systems originating from a common lower state of 1_{ g } symmetry. The two lower states were assigned as the Ω=0 and Ω=1 spin–orbit components of the X ^{3}Σ^{−} _{ g }ground state, which is derived from the electron configuration 1π^{4} _{ u }1σ^{2} _{ g }2σ^{2} _{ g }1δ^{2} _{ g }. The two spin–orbit components are split by several hundred cm^{−1} due to a strong, second‐order isoconfigurational spin–orbit interaction with the low‐lying ^{1}Σ^{+} _{ g } state. Evidence for significant 4d orbital participation in the Nb_{2} bond is furnished by the short bondlength [r _{ e }=2.077 81(18) Å] and large vibrational frequency [ω_{ e }=424.8917(12) cm^{−1}] determined for the X ^{3}Σ^{−} _{ g }(0^{+} _{ g }) state (2σ error bounds). The electronic structure of niobium dimer was investigated using density functional theory. For the electronic ground state, the predicted spectroscopic properties were in good agreement with experiment. Calculations on excited states reveal congested manifolds of triplet and singlet electronic states in the range 0–3 eV, reflecting the multitude of possible electronic promotions among the 4d‐ and 5s‐based molecular orbitals. The difficulties of correlating the experimentally observed electronic transitions with specific valence electronic promotions are addressed. Comparisons are drawn between Nb_{2} and the isoelectronic molecule V_{2}.

A general discrete variable method to calculate vibrational energy levels of three‐ and four‐atom molecules
View Description Hide DescriptionWe present a general variational method to calculate vibrational energy levels of polyatomic molecules without dynamical approximation. The method is based on a Lanczos algorithm, which does not require storage of the Hamiltonian matrix. The rate‐determining step of each Lanczos iteration is the evaluation of the product of the matrix and a trial vector. We use simple product basis functions and write the Hamiltonian as a sum of factorizable terms. With n one‐dimensional functions in each of f dimensions, the matrix‐vector product requires no more than cn ^{ f+1} multiplications for a single term involving c coordinates. Choosing a (potential optimized) discrete variable representation (DVR) in each dimension, the potential energy matrix is diagonal. The rate‐determining step is now the multiplication of a vector by the kinetic energy matrix and c is effectively (with rare exceptions) at most two. The n ^{ f+1} scaling holds for both diagonal and mixed second derivative operators. The method is directly applicable to any three‐atom and any nonlinear four‐atom molecule. We use a variety of coordinate systems (Jacobi, Radau, a hybrid of the two, and bond), for which the total number of factorizable terms in the exact kinetic energy operator is never large, to calculate very well‐converged band origins of H_{2}O up to 22 000 cm^{−1}, of H^{+} _{3} up to 18 000 cm^{−1}, and of CH_{2}O up to 5700 cm^{−1}; and low‐lying levels of H_{2}O_{2}. The results for CH_{2}O are new, and those for H^{+} _{3} clarify the causes of discrepancies in published work. The product basis results in very large matrices (up to 500 000×500 000 for four atoms), but the cost is within an order of magnitude of that of contracted‐basis approaches using explicit diagonalization. While contracted basis approaches are molecule and Hamiltonian specific, it was possible to apply the DVR‐Lanczos method to all the examples presented here with a single computer program. The principal advantage of our method is thus its generality, and in this context it is efficient, with the cost scaling slowly with basis size. It is also easily parallelized.

Photoelectron spectroscopy of nickel group dimers: Ni^{−} _{2}, Pd^{−} _{2}, and Pt^{−} _{2}
View Description Hide DescriptionNegative ion photoelectron spectra of Ni^{−} _{2}, Pd^{−} _{2}, and Pt^{−} _{2} are presented for electron binding energies up to 3.35 eV at an instrumental resolution of 8–10 meV. The metal cluster anions are prepared in a flowing afterglowion source. Each dimer exhibits multiple low‐lying electronic states and a vibrationally resolved ground state transition. Franck–Condon analyses yield the anion and neutral vibrational frequencies and the bond length changes between anion and neutral. The electron affinities are determined to be EA(Ni_{2})=0.926±0.010 eV, EA(Pd_{2})=1.685±0.008 eV, and EA(Pt_{2})=1.898±0.008 eV. The electronic configurations of the ground states are tentatively assigned. Comparison of the nickel group dimers to the coinage metal dimers sheds light on the d orbital contribution to the metal bonding in the nickel group dimers.