Index of content:
Volume 93, Issue 10, 15 November 1990

Nuclear spin relaxation in paramagnetic solutions. Effects of large zero‐field splitting in the electron spin Hamiltonian
View Description Hide DescriptionExpressions are derived describing nuclear spin relaxation in paramagnetic salt solutions under conditions where the electron spin Hamiltonian is dominated by a uniaxial quadratic zero‐field splitting (zfs) interaction. In this situation, the electron spin vector is quantized along molecular axes rather than along the external magnetic field. By expressing the time dependence of the electron spin operators, written in the molecular coordinate frame, in the Heisenberg representation and then transforming these expressions to the laboratory coordinate system, simple closed form expressions for the paramagnetic nuclear relaxation increment have been derived. Electron–nuclear dipole–dipole and scalar relaxation mechanisms are considered. The resulting expressions parallel those of Solomon–Bloembergen–Morgan theory, but are valid in the zfs limit rather than the Zeeman limit. Nuclear relaxation rates in the zfs and Zeeman limits exhibit characteristic qualitative differences, some of which have been noted in earlier studies. Of particular note is the fact that the scalar contribution to T ^{−1} _{1p } is much larger in the zfs than in the Zeeman limit. In most circumstances, T ^{−1} _{1p }=T ^{−1} _{2p } in the zfs limit, while in the Zeeman limit, scalar relaxation usually contributes significantly only to T ^{−1} _{2p }. A vector model of this phenomenon is suggested. The results are valid for arbitrary values of the electron spin quantum number but they assume that electron spin relaxation is in the Redfield limit, i.e., that the correlation times of the coupling between electron spin and the lattice be short on the time scale of electron spin relaxation. This condition is probably satisfied widely when the static zfs is large.

The emission spectrum of helium hydride. IV. Bands near 4100 and 4600 Å
View Description Hide DescriptionThe emission bands of helium hydride near 4100 and 4600 Å were analyzed for ^{4}HeH and ^{3}HeD. They are assigned to the emission of the three coupled states 4p ^{2}Π, 4d ^{2}Π, and 4d ^{2}Δ to the A ^{2}Σ^{+} (4100 Å) and B ^{2}Π (4600 Å) states. These bands are analogous to the emission of n=3 states near 5300 and 6000 Å analyzed previously. Only the A‘parity components of n=4 states were observed, the states of A’parity decay predominantly by predissociation. Several weak lines around 4100 Å were assigned to the transition 5d→B ^{2}Π.

The emission spectrum of helium hydride. V. Characterization of low‐lying Rydberg states (n=2–5)
View Description Hide DescriptionThe low‐lying Rydberg states of helium hydride (n=2–5) are characterized. Absolute binding energies and quantum defects are derived from spectroscopic data. Isotope shifts and scaling of molecular constants with principal quantum number n are discussed. Comparison with available a b i n i t i o calculations shows good agreement. For n d states the transition from Hund’s case (b) to (d) is treated. Their splitting is mainly caused by the quadrupole moment of HeH^{+}. Some so far unassigned groups of spectral lines were shown to involve n=2–4 Rydberg states with vibrational excitation.

Decoupling in the line mixing of acetylene infrared Q branches
View Description Hide DescriptionThe Q‐branch profiles of the ν_{1} +ν_{5} , ν_{3} +ν_{4} and ν_{2} +2ν_{4} +ν_{5} Π_{ u } –Σ_{ g } combination bands in the 2.5 μm C–H stretch‐bend region of acetylene have been recorded with a difference‐frequency laser spectrometer at pressures from 1 to 500 Torr (0.13 to 66.7 kPa). The broadening coefficients, obtained from the ν_{1} +ν_{5} band at pressures low enough to avoid significant spectral overlap, can be well fit with empirical rotationally inelastic energy‐gap scaling laws or satisfactorily modeled with semiclassical line broadening theory using known intermolecular potential parameters. At pressures when lines are overlapped, collisional interference or line mixing is manifest as a deviation of the Q‐branch profiles from an additive superposition of individual transition components. However the line coupling given by the state‐to‐state collisional scaling laws used to fit the broadening coefficients predicts far more collisional narrowing or Q‐branch collapse than is observed. We find that only about one‐third of the collisions that broaden the individual lines effectively couple the lines within the f sublevel of the l‐doubled excited Π vibrational state observed in the Q branch. This decoupling indicates that there is little or no propensity for preserving the vibrational angular momentum sublevel upon collision, and that elastic reorientational and dephasingcollisions may also be‐significant. Additionally, we find that the collisional parameters and decoupling are independent of the vibrational state despite dramatically different spectral overlaps exhibited by the three bands studied and a close Fermi resonance between the lower two vibrations. This implies that vibrational relaxation and dephasingcollision rates are negligible compared with rotationally inelastic and reorientational rates and usually can be ignored for infrared spectral broadening.

Raman spectroscopic study of dilute HOD in liquid H_{2}O in the temperature range − 31.5 to 160 °C
View Description Hide DescriptionWe present Raman data for the OD stretch mode of 10 mol % HOD in H_{2}O for the liquid phase from −31.5 to 160 °C. We find that an exact isosbestic does not hold, but rather the crossing of isotherms slowly but uniformly changes with temperature. We present an analysis based on Boltzmann statistics which gives evidence for a distribution of deuterium hydrogen bond strengths with minimum energy near the frequency (2440 cm^{−} ^{1}) also found in the solidice and amorphous solid phases. This analysis also gives evidence for a band of frequencies above 2630 cm^{−} ^{1} due to OD oscillators all at essentially the same high energy relative to the strongest hydrogen bonds, and we interpret this band as due to broken hydrogen bonds. This allows us to calculate hydrogen bond probabilities, and we find this probability increases with decreasing temperature and approaches a value equal to the four bonded percolation threshold near the singular temperature T _{ s } ≂−45 °C for the anomalies of supercooled water. Peak frequency and full width at half‐maximum of the OD stretch band are found to drop precipitously to the amorphous solid values as T→T _{ s } implying the ultimate state of supercooled water is similar to the amorphous solid.

Line‐mixing effects in Ar‐broadened doublets of a hot band of OCS
View Description Hide DescriptionThe spectra of several Ar‐broadened symmetric doublets in the hot band ν_{1}+ν_{2}−ν_{2} (at 850 cm^{−} ^{1}) of OC ^{3} ^{2}S have been recorded with a tunable diode‐laser spectrometer. The pressure‐broadening coefficients measured for sufficiently separated lines belonging to 7 doublets are in agreement with the results previously obtained in the ν_{1} band. From these data, we have determined, for different pressures of argon, line‐coupling coefficients for 15 doublets in the P and R branches of the hot band. These coefficients appear to be small and negative, at the limit of our estimated accuracy, which implies very weak line‐mixing effects in the doublets, in agreement with the results of a semiclassical calculation as well as with results of coupling cross sections estimated at large‐j values from the infinite order sudden (IOS) approximation.

Rotational spectra and structures of the OC– and H_{3}N–HCN–HF trimers: Coaxial mixing nozzle for reactive species
View Description Hide DescriptionRotational spectra are reported for several isotopic species of the OC–and H_{3}N–HCN–HF heterotrimers, detected with a pulsed nozzle, Fourier transform, Balle/Flygare microwave spectrometer.Rotational constants for the main isotopic species of the OC trimer are a B _{0} of 615.574 MHz and D _{ J } of 251 Hz, and for H_{3}N, a symmetric top, a B _{0} of 1067.161 MHz and D _{ J } and D _{ J K } of 0.40 and 63 kHz. Their structures are composites of those reported for the X–HCN and HCN–HF dimers. They are effectively axially symmetric but have some shrinkage from the distances in the dimers. The shrinkages found in r _{1}, the c.m. to c.m. distance for X–HCN are 0.070 and 0.098 Å for X=OC and H_{3}N, respectively, and in r _{2} for HCN–HF, 0.033 and 0.027 Å. The ^{1} ^{4}N and H–F hyperfine interactions in OC–HCN–HF are the same as those reported for the HCN–HF dimer. Detection of the X=OC and H_{3}N trimers out of the many species possible required care in their generation. Both were favored by the strongly bonded HCN–HF subunit. The OC–HCN–HF was further enhanced by using a high concentration of CO in the gas expansion. For H_{3}N–HCN–HF a coaxial mixing nozzle was developed to avoid the formation of NH_{4}F(s). The selectivity and simplicity of the nozzle should be helpful in extending the range of species observable with pulsed nozzles.

The spectroscopy and dynamics of π hydrogen‐bonded complexes: Benzene–HCl/DCl and toluene–HCl/DCl
View Description Hide DescriptionThe benzene–HCl/DCl and toluene–HCl/DCl complexes have been studied using both fluorescence and multiphoton ionization detection. These complexes are prototypical of π hydrogen‐bonded complexes involved in the chemically important process of electrophilic aromatic attack. Laser‐induced fluorescence(LIF)etalon scans of the 6^{1} _{0} rotational band contour are used to determine the S _{1} state geometry of the benzene–HCl as one in which HCl is on the sixfold axis with a center‐of‐mass separation of 3.64±0.03 Å. The lack of significant van der Waals’ intensity points to the complex having a hydrogen‐bonded geometry similar to that found in the ground state. Dispersed fluorescence scans are used to put crude bounds on the S _{0} and S _{1} binding energies of the benzene–HCl complex of 1.8≤D ^{‘} _{0} ≤ 3.8 kcal/mol and 1.5≤D ^{’} _{0}≤3.5 kcal/mol. The fluorescence lifetimes of bound levels of the complexes are factors of 7–12 times shorter than the corresponding levels of the free molecules. In contrast, the C_{6}H_{6}–CH_{3}Cl complex, which has a similar geometry and binding energy, has a fluorescence lifetime nearly as long as C_{6}H_{6}. We argue that the differences observed are consequences of the hydrogen bonding interactions present in benzene–HCl and toluene–HCl. Two‐color multiphoton ionization experiments in which the delay between the S _{0}–S _{1} laser and the ionization lasers is continuously scanned give evidence that the hydrogen bonding interactions lead to enhanced intersystem crossing to the triplet state. One‐color RE2PI scans show that fragmentation of the [benzene–HCl]^{+} and [toluene–HCl]^{+} ionic complexes proceeds nearly quantitatively (≥98%) from the hydrogen‐bonded S _{1} state. This fragmentation occurs by virtue of the repulsive geometry formed for the ionic complex in Franck–Condon excitation from the S _{1} state.

Photoelectron spectroscopy of metal cluster anions: Cu^{−} _{ n }, Ag^{−} _{ n }, and Au^{−} _{ n }
View Description Hide DescriptionNegative ion photoelectron spectra of Cu^{−} _{ n }, Ag^{−} _{ n }(n=1–10), and Au^{−} _{ n } (n=1–5) are presented for electron binding energies up to 3.35 eV at an instrumental resolution of 6–9 meV. The metal cluster anions are prepared in a flowing afterglowion source with a cold cathode dc discharge. In the spectra of Cu^{−} _{2}, Ag^{−} _{2}, and Au^{−} _{2}, the M_{2} X ^{1}Σ^{+} _{ g }←M^{−} _{2} X ^{2}Σ^{+} _{ u } transitions are vibrationally resolved. We analyze these spectra to yield the adiabatic electron affinities, vibrational frequencies, bond length changes, and dissociation energies. The a ^{3}Σ^{+} _{ u } triplet states of Cu_{2} and Ag_{2} are also observed. Using experimental and theoretical data, we assign the major features in the Cu^{−} _{3} and Ag^{−} _{3} spectra to the transition from the linear ground state of the anion (M^{−} _{3} ^{1}Σ^{+} _{ g }) to an excited linear state of the neutral (M_{3} ^{2}Σ^{+} _{ u }). The Au^{−} _{3} spectrum is attributed to a two‐photon process, photodissociation followed by photodetachment of the Au^{−} or Au^{−} _{2} fragment. For larger clusters, we measure the threshold and vertical detachment energies as a function of size. Trends in the electron affinities and excited state energy levels as a function of cluster size and composition are discussed in terms of simple models.

H+O_{3} Fourier‐transform infrared emission and laser absorption studies of OH (X ^{2}Π) radical: An experimental dipole moment function and state‐to‐state Einstein A coefficients
View Description Hide DescriptionThe relative intensities of 88 pairs of rovibrational transitions of OH (X ^{2}Π) distributed over 16 vibrational bands (v’≤9, Δv=−1,−2) have been measured using Fourier transform infrared (FTIR) emission/absorption spectroscopy. Each pair of transitions originates from a common vibrational, rotational, and spin–orbit state, so that the measured relative intensities are i n d e p e n d e n t of the OH number density and quantum state distribution. These data are combined with previous v=1←0 relative intensity absorptionmeasurements and v=0, 1, and 2 permanent dipole moments to determine the OH dipole moment function as a cubic polynomial expanded about r _{ e }, the equilibrium bond length. The relative intensities provide detailed information about the shape of the OH dipole moment function μ(r) and hence the a b s o l u t e Einstein A coefficients.
The intensity information is inverted through a procedure which takes full account of the strong rotation–vibration interaction and spin uncoupling effects in OH to obtain the dipole moment function (with 95% confidence limits): μ(r)=1.6502(2) D+0.538(29) D/Å (r−r _{ e })−0.796(51) D/Å^{2} (r−r _{ e })^{2}−0.739(50) D/Å^{3} (r−r _{ e }), ^{3} with a range of quantitative validity up to the classical turning points of the v=9 vibrational level (i.e., from 0.70 to 1.76 Å). The μ(r) determined in this study differs significantly from previous empirical analyses which neglect the strong effects of rotation–vibration interaction and spin uncoupling. The present work also permits distinguishing between the various a b i n i t i o efforts. Best agreement is with the dipole moment function of Langhoff, Werner, and Rosmus [J. Mol. Spectrosc. 1 1 8, 507 (1986)], but their theoretical predictions for higher overtone transitions are still outside of the 2σ experimental error bars. Absolute Einstein A coefficients from the present μ(r) are therefore presented for P, Q, R branch transitions for Δv=1, 2, 3, v’≤9, J’≤14.5, in order to provide the most reliable experimental numbers for modeling of near IR atmosphere OH emission phenomena.

Dative circular dichroism: An overlap‐dependent perturbation analysis for magnetic dipole allowed transitions
View Description Hide DescriptionThe conventional independent systems/perturbation (ISP) treatment for the circular dichroism of magnetic dipole allowed transitions of an achiral chromophore (A) in a chiral environment (C) is extended to include analytically the effect of weak overlap between the A and C systems. The derivation exploits the recently developed simplified group function (SGF) approach, which resolves the interaction between two weakly overlapping groups into separable and additive overlap‐independent long range contributions (excitonic effects on which the ISP analysis is based) and overlap‐dependent dative terms. A theoretical justification for the group (chromophore) descriptions implicit in the SGF analysis is presented through the concept of equivalent representations. The results provide clear criteria for when the ISP results must be augmented with the dative contributions, and also provide a clear rationale for why the ISP/dative approach may be expected to be of relatively broad validity.

Microwave spectrum, structure, barrier to internal rotation, dipole moment, and deuterium quadupole coupling constants of the ethylene–sulfur dioxide complex
View Description Hide DescriptionThe microwave spectra of the complex between ethylene and sulfur dioxide and nine of its isotopic species have been observed in a Fourier transformmicrowave spectrometer. The spectra exhibit a and c dipole selection rules; transitions of the normal species and several of the isotopically substituted species occur as tunneling doublets. The complex has a stacked structure with C _{ s } symmetry; the C_{2}H_{4} and SO_{2} moieties both straddle the mirror plane with the C_{2} axis of SO_{2} crossed at 90 ° to the carbon–carbon bond axis (i.e., only the S atom lies in the symmetry plane). The distance between the centers of mass (R _{cm}) of C_{2}H_{4} and SO_{2} is 3.504(1) Å and the deviation of their planes from perpendicular to R _{cm} is 21(2) ° and 12(2) °, respectively. The tunneling splittings arise from a rotation of the ethylene subunit in its molecular plane. The barrier to internal rotation is 30(2) cm^{−1}. The dipole moment of the complex is 1.650(3)D. The deuterium nuclear quadrupole coupling constants for C_{2}H_{3}D⋅SO_{2} are χ_{ a a }=−0.119(1) MHz, χ_{ b b }=0.010(1) MHz, and χ_{ c c }=0.109(1) MHz. The binding energy is estimated to be 490 cm^{−1} from the pseudo‐diatomic approximation. A distributed multipole electrostatic model is explored to rationalize the structure and binding energies.

Collision‐induced double resonance studies of HN^{+} _{2} and HCN
View Description Hide DescriptionWe have investigated collision‐induced rotational transitions of HN^{+} _{2} and HCN using infrared‐microwave four‐level double resonance spectroscopy. These two isoelectronic molecules were studied in collisions with He, Ar, and N_{2}. For all cases studied, we have observed that the collision‐induced rotational transitions exhibit collisional ‘‘selection rules.’’ The selection rules can be explained using the symmetry properties (i.e., parity) of the dominant terms in the interaction potential. This represents the first observation of selection rules for rotational energy transfer of a molecular ion. This study has allowed us to directly compare the difference between ion–neutral and neutral–neutral collisions which cause rotational transitions. We have experimentally observed that ion–neutral and neutral–neutral collisions differ because of the presence of the Langevin force in the ion–neutral interaction potential, which has two unique effects. The Langevin force produces a charge‐induced dipole in the collision partner which is parallel to the ion’s electric field. This charge‐induced dipole interacts with the electric charge of the molecular ion which creates an attractive force between the ion and neutral. This interaction therefore decreases the ion–neutral distance and produces strong collisions which randomizes the rotational states. The second effect occurs when the molecular ion has a permanent electric moment. The charge‐induced dipole in the collision partner will interact with an electric moment of the molecular ion creating a long‐range interaction. For HN^{+} _{2}, a molecular ion with a permanent dipole moment, this interaction produces ‘‘dipole‐type’’ collisional selection rules.

High‐resolution spectrum of the C–O stretch overtone band in methyl alcohol
View Description Hide DescriptionThe Fourier‐transform spectrum of the CH_{3}OH overtone C–O stretching band (v _{CO}=2←0) has been recorded in the range 1950–2060 cm^{−1}, at a resolution of 0.004 cm^{−1}. The spectrum is resolved into J multiplets, each displaying complicated substructure due to strong torsion–vibration–rotation effects. Within this structure, it has been possible so far to identify R and P branch transitions for K=0 and K=1 for both A and E torsional symmetry species for J≤24. The assignments are supported by combination differences derived from known ground‐state frequencies. The identified branches have been analyzed in terms of effective state‐dependent series‐expansion parameters, and the leading terms in the torsion–rotation Hamiltonian have been derived for the second excited C–O stretching state. The v _{CO}=2 torsional barrier height is obtained as 395.5±0.2 cm^{−1}, and the effective B value as 23 671.9±1.3 MHz. In addition, the 34.946 cm^{−1} far‐infrared laser line pumped by the 10R(48) CO_{2} laser line has been tentatively identified as the (nτK,J)^{ v }=(031^{+},16)^{2}→(010^{+},15)^{2} transition, and its parent IR pump absorption as the P(031^{+},17) v _{CO}=2←1 hot‐band line.

(2+1’) rotationally resolved resonance enhanced multiphoton ionization via the E ^{2}∑^{+}(4s,3d) and H ^{2}∑^{+}(3d,4s) Rydberg states of NO
View Description Hide DescriptionThe results of studies of ionic rotational branching ratios and photoelectron angular distributions resulting from (2+1’) resonance enhanced multiphoton ionization of NO via various high J (≊21.5) rotational branches of the E ^{2}∑^{+}(4s,3d) and H ^{2}∑^{+}(3d,4s) Rydberg states are presented. The rotational branching ratios show the expected ΔN=even rotational propensity rule with very small ΔN=odd signals. The branching ratios for the E ^{2}∑^{+} state are seen to be independent of photoelectron energy with the ΔN=+2 signals strongest and no appreciable higher rotational transfer peaks (‖ΔN‖≥3). The higher rotational transfer signal for ionization of the H ^{2}∑^{+} state are also negligible but the rotational branching ratios are strongly energy dependent due to a Cooper minimum in the l=3 partial wave of the kσ‐ and kπ‐continua at a photoelectron kinetic energy of 2.6 eV and 2.9 eV, respectively. This leads to a strong rotational selectivity that can be exploited to produce ions in a specific rotational level. These consequences of Cooper minima close to threshold are quite general and their influence on rotational distributions should be readily observable in other molecular systems. The photoelectron angular distributions via both states show a strong energy dependence with a rapid change in the angular distributions around the Cooper minimum associated with the H ^{2}∑^{+} state.

The atmospheric water continuum in the infrared: Extension of the statistical theory of Rosenkranz
View Description Hide DescriptionThe statistical theory proposed by Rosenkranz to calculate the continuous absorption by water molecules in the high‐frequency (infrared) wing of the pure rotational band is reviewed and extended. In the review there is a discussion, in particular, of the approximations that are made, including those that are necessary and which limit the applicability of the theory to other spectral regions, and those that are made for calculational convenience. Then, several extensions to the theory are discussed, including increasing the number of rotational states used to calculate the band‐average relaxation parameter, modifying the definition of this parameter to account for near‐wing effects, and eliminating the boxcar approximation. This last modification, effected by using asymmetric‐top functions instead of symmetric‐top functions to calculate matrix elements of the density operator and to diagonalize the dipole–dipole interaction, results in significant enhancement of the relaxation parameter. This improvement, in turn, allows one to eliminate an inconsistency in the original formulation of Rosenkranz while obtaining substantially the same final results. The implications of the present results for the calculation of the absorption in the high‐frequency wing of the ν_{2} fundamental vibration‐rotational band of H_{2}O are discussed briefly.

The sites of Gd^{3+} in the luminescent matrix La_{1−x }Gd_{ x }MgAl_{11}O_{19}: Single crystal structure determination and site‐selective excitation of Gd^{3+}
View Description Hide DescriptionSingle crystals of La_{1−x }Gd_{ x }MgAl_{11}O_{19} for x=0.02 to 1 have been grown from the melt by the Verneuil (flame fusion) method. The localization of Gd^{3+} ions in the matrix has been obtained using x‐ray diffraction and Gd^{3+}fluorescence techniques giving the average and the local structure of the material. The resolution of the crystal structure, of La_{0.4}Gd_{0.6}MgAl_{11}O_{19} homolog, indicates that this compound is of the distorted magnetoplumbite (MP) type (hexagonal P6_{3}/ m m c). Lanthanide ions lie in the mirror plane in two kinds of sites: the (2d) regular MP one (D _{3h } symmetry) occupied by La^{3+} ions, the distorted (12j) one (C _{ s } symmetry) partially filled up and containing only Gd^{3+}. Some oxygen ions of the Ln coordination polyhedron may be missing leading to a lowering of the true symmetry of the sites. Site selective excitation of the fluorescence of Gd^{3+} and emission spectra have been carried out on crystals with different x values. It indicates that Gd^{3+} ions are distributed mainly among two sites, A and B. Crystal field analysis of the splitting of the ^{6} P terms of Gd^{3+} determined on the excitation spectra show that site A is close to the ideal D _{3h } symmetry while site B is a strongly distorted site. This leads to the identification A=(2d), B=(12j). The occupancy is larger for site B than for site A in agreement with the refinement of the structure. Selective laser excitation into the A site induces emission of the B sites as a result of energy transfer which is demonstrated by the fluorescence decay. No evidence of energy migration is found for gadolinium content up to 100%. Fluorescence spectroscopy and crystal structure determination appear complementary to obtain a detailed description of the sites of Gd^{3+} in La_{1−x }Gd_{ x }MgAl_{11}O_{19}.

Ground‐ and excited‐state dipole moments of some nitroaromatics: Evidence for extensive charge transfer in twisted nitrobenzene systems
View Description Hide DescriptionElectro‐optical absorption measurements have been made on four model nitroaromatics to determine the effect of twisting of the donor–acceptor single bond on the charge‐transfercharacteristics in the Franck–Condon excited states. Observed ground‐ and excited‐state dipole moments of nitromesitylene, which has been treated experimentally as the nonplanar analogue of planar nitrobenzene, indicate that electronic excitation of twisted nitrobenzene results in a nearly full unit charge transfer from donor (benzene) to the acceptor (nitro) group (Δμ=18.3 D). On the other hand, in planar nitrobenzene and nitronaphthalene the charge transfer is more delocalized over the whole molecular skeleton, resulting in normal changes in dipole moment (Δμ=5–10 D). In the analogous anthracene system, i.e., 9‐nitroanthracene, the charge transfer upon electronic excitation is extremely low (Δμ=1.7 D), which is reflected by its very small change in the dipole moment. Therefore, it is evident that the charge‐transfer processes in the twisted molecules are quite different for different aromatic ring systems. Simple molecular‐orbital calculations satisfactorily explain the reason for such differences on the basis of their highest occupied molecular‐orbital (HOMO) and lowest unoccupied molecular‐orbital (LUMO) characteristics.Transition moment directions have also been obtained experimentally and compared with the theoretically predicted directions based on the symmetry properties of the HOMO and LUMO. Agreement is found in all cases studied.

Spectroscopic observation of the b ^{1}Σ^{+}→X̃ ^{3}Σ^{−} transition of AsH
View Description Hide DescriptionEmission lines of the b ^{1}∑^{+}→X̃ ^{3}∑^{−} transition of AsH radicals have been detected in the fluorescence of a dc‐glow‐discharge of arsine in hydrogen. From measurements of line positions of the (0,0), (1,1), and (2,2) Q branches and the (0,0) P and R branches, the molecular constants of the b ^{1}∑^{+} state were determined: T _{ e }=14 178.0 cm^{−1}, B _{0}=7.2467 cm^{−1}, D _{0}=3.1528⋅10^{−4} cm^{−1}, ω_{ e }=2213 cm^{−1}, ω_{ e } x _{ e }=47.5 cm^{−1}, r _{ e }=152.937 pm.

Overtone intensities and dipole moment surfaces for the isolated CH chromophore in CHD_{3} and CHF_{3}: Experiment and a b i n i t i o theory
View Description Hide DescriptionThe band strengths of fundamentals (N=1) and overtones (up to N=6) of the strongly coupled CH stretching and bending vibrations in CHD_{3} and CHF_{3} are calculated using high level a b i n i t i o (SCF‐CI) dipole moment functions and potential surfaces in one and two (three) dimensions. The calculations are performed in approximate normal coordinate and internal coordinate subspaces, the former giving generally superior results. The overall prediction of relative and absolute intensities ranging over many orders of magnitude is often accurate to within a factor of 2, but not to within experimental accuracy. Different dipole model functions and potential surfaces are investigated and an empirical adjustment of the dipole function to experiment is proposed for CHF_{3}. The comparison of experimental and a b i n i t i o overtone intensities for the Fermi resonance system is discussed in some detail, as well as the importance of the results for IR spectroscopy and IR multiphoton excitation.