Volume 91, Issue 6, 15 September 1989
Index of content:

Reduced potential curves of the excited states of alkali diatomic molecules
View Description Hide DescriptionThe reduced potential curve (RPC) method has so far been successfully employed for the discussion of the ground states of diatomic molecules, however, its efficacy in the complicated world of the excited states has been doubted by some spectroscopists. The present paper should prove that such doubts are in general unjustified. It is shown that, for an excited state of definite symmetry and order (e.g., 1 ^{1}Π_{ u }, 2 ^{1}Π_{ u }, etc.) in a group of affiliated molecules, e.g., the group of homo‐ or heteronuclear alkali diatomic molecules, the same rules hold in the reduced potential curve (RPC) scheme as have been shown before to hold for the ground state. Small deviations and also anomalies with respect to this rule exist for some excited states as must, of course, be expected. The RPC method just seems most suited to visualize such anomalies. Rydberg–Klein–Rees (RKR) and theoretical a b i n i t i o calculated potentials are studied in reduced form. The RPC scheme makes possible a systematic comparative study of excited states of diatomic molecules. The RPC method may be also used for detection of errors (inaccuracies) in the analysis of the spectrum or of deficiencies in the theoretical calculation, and for estimation of the potentials of excited states.

Discussion of a ‘‘coherent artifact’’ in four‐wave mixing experiments
View Description Hide DescriptionIn this paper, we discuss the nonlinear optical effects that arise when stochastic light waves, with different correlation times, interfere in an absorbing medium. It is shown that four‐wave mixing signals are generated in several directions that spectrally track the incoming light fields. This effect is particularly relevant to transient hole‐burning experiments, where one of these signals could easily be misinterpreted as a genuine hole‐burning feature.

Optical–optical double resonance spectroscopy of Br_{2} through the A ^{3}Π(1_{ u }) state: Analyses of the 1_{ u } and 0^{−} _{ u }(^{3} P _{1}) ion‐pair states
View Description Hide DescriptionThe technique of optical–optical double resonance is applied to study the ion‐pair states of Br_{2} correlating to Br^{−}(^{1} S)+Br^{+}(^{3} P _{1}). The Br_{2} molecules in the ground state are pumped to the A ^{3}Π(1_{ u }) state as an intermediate, and subsequently excited into the ion‐pair states by a coherent two‐photon transition. The double resonance transition is detected through the dispersed fluorescence terminating on the lower valence states. The 1_{ u }(^{3} P _{1}) ion‐pair state is observed for the first time and the energy level analyses are made on the v=0 to v=10 levels in detail. The 0^{−} _{ u } (^{3} P _{1}) state is also identified through the heterogeneous interaction with the 1_{ u }(^{3} P _{1}) state, which is interpreted in terms of the pure precession approximation. The molecular parameters thus obtained in the Dunham expansion are Y _{0} _{0}=53 257.605(10), Y _{1} _{0}=164.1571(69), Y _{2} _{0}=−0.7588(15), Y _{3} _{0}=5.650(98)×10^{−} ^{3}, Y _{0} _{1}=0.040 011 6(40), Y _{1} _{1}=−1.7488(42)×10^{−} ^{4}, and Y _{0} _{2}=−8.40(53)×10^{−} ^{9} for the 1_{ u }(^{3} P _{1}) state of ^{7} ^{9}Br_{2}isotope species (all in cm^{−} ^{1} with 3σ in parentheses). The main parameters of the 0^{−} _{ u }(^{3} P _{1}) state are Y _{0} _{0}=53 479.72(25), Y _{1} _{0}=148.620(54), and Y _{0} _{1}=0.038 021 5(79).

Infrared and microwave study of angular–radial coupling effects in Ar–HCN
View Description Hide DescriptionMicrowave and infrared spectra of Ar–HCN have been obtained using an electric‐resonance optothermal spectrometer. The microwave measurements extend to higher J the previous results of Leopold e t a l. and Klots e t a l., allowing the determination of higher‐order centrifugal distortion constants for this quasilinear, highly nonrigid complex. A Padé approximant fit to the microwave data indicates a significant rotation‐induced asymptotic increase in the zero‐point center‐of‐mass separation between the Ar and the HCN, above that expected from pure radial distortion. This results from the large coupling between the angular and radial degrees in the intermolecular potential forcing the centrifugal alignment of the HCN. Infrared spectra are reported for the C–H streching fundamental ν_{1} and the combination band ν_{1}+ν^{1} _{5}, where ν_{5} is the van der Waals bending vibration. The band‐origin difference between these two bands gives ν_{5}=7.8 cm^{−} ^{1}, in rough agreement with the 10 cm^{−} ^{1} harmonic value predicted from the microwave‐determined nuclear quadrupole coupling constant. The complexation‐induced red shift of the C–H stretching vibration is 2.69 cm^{−} ^{1} and the vibrational predissociationlinewidths Γ are <10 MHz (FWHM). The vibrationally excited complex predissociates before striking the bolometer detector, implying that the predissociation lifetime τ<1 ms.

The torsional Raman spectra of C_{2}H_{6} and C_{2}D_{6}
View Description Hide DescriptionThe torsional Raman spectra of C_{2}H_{6} and C_{2}D_{6} have been experimentally investigated with an improved conventional Raman spectrometer. Wave numbers and absolute cross sections of all observed bands have been satisfactorily interpreted within the experimental accuracy by means of a simple, pure‐torsional Hamiltonian and a model for the torsional dependence of the molecular polarizability. The effective threefold potential barrier parameters for C_{2}H_{6} have been taken from the recent literature, whilst for C_{2}D_{6} the values V _{3eff} (C_{2}D_{6})=990±2 cm^{−} ^{1} and V _{6eff} (C_{2}D_{6})=7.9±1 cm^{−} ^{1} were obtained.

Diode laser spectroscopy of the hydrogen bond vibration ν_{2} OC‐‐‐HF in a continuous wave supersonic jet
View Description Hide DescriptionThe high resolution spectrum of the ν_{2} (C≡O) stretching vibration in the hydrogen bonded dimer, OC‐‐‐HF, has been recorded in a continuous wave (cw) supersonic molecular jet using a diode laser spectrometer. Spectroscopic analysis gives the following rovibrational parameters (in cm^{−} ^{1}): ν_{0}=2167.69 9 04(11); B _{0}=0.102 200 647(13); D ^{0} _{ J } =3.244(18)×10^{−} ^{7}; B _{2}=0.101 552 5(15); D ^{2} _{ J } =3.449(36)×10^{−} ^{7}. Investigation of observed line profiles allows a lower limit of 0.68 ns to be made for the excited state vibrational predissociative lifetime.

Multiphoton ionization of SiH_{3} and SiD_{3} radicals: Electronic spectra, vibrational analyses of the ground and Rydberg states, and ionization potentials
View Description Hide DescriptionThe electronic spectra of silyl radicals, SiH_{3} and SiD_{3}, were observed between 310 and 430 nm (46 000–64 000 cm^{−} ^{1}) by resonance enhanced multiphoton ionization (REMPI) mass spectroscopy. The spectra were generated through a 2+1 REMPI mechanism. Two Rydberg series originating from planar, D _{3h } point group states were observed. One series, of quantum defect δ=1.45(2), is comprised of the Ẽ ^{2} A ^{‘} _{2}
(4p), J̃ ^{2} A‘_{2}(5p), and M̃ ^{2} A‘_{2}(6p) Rydberg states which have origins at ν_{0–0} =48 438, 56 929, and 60 341 cm^{−} ^{1} in SiH_{3} and at ν_{0–0} =48 391, 56 874, and 60 267 cm^{−} ^{1} in SiD_{3}. In SiD_{3} the
P̃ ^{2} A‘_{2}(7p) Rydberg origin was observed at ν_{0–0} =62 002 cm^{−} ^{1}. The H̃, K̃, and Ñ states observed in the SiD_{3} spectrum comprise the second Rydberg series, δ=2.09, and were tentatively assigned as n s ^{2} A ^{’} _{1}Rydberg states (n=5, 6, 7). The K̃ and Ñ origins were observed at ν_{0–0} =58 417 and 61 005 cm^{−} ^{1}. A fit of the Rydberg formula to the n p ^{2} A‘_{2}(n≥5) origins found the adiabatic ionization potential of the SiH_{3} and SiD_{3} radicals to be IP_{ a }=8.135(+5,−2) eV and IP_{ a }=8.128(1) eV, respectively. Detailed vibrational analyses of these Rydberg states are presented. Analysis showed that in the Ẽ ^{2} A ^{‘} _{2} (4p) state of the SiH_{3} radical ω_{2} (a ^{‘} _{2} symmetric bend)=796(7) and 2ω_{4} /2(e’ degenerate bend)=870(5) cm^{−} ^{1} and that in SiD_{3} radical ω^{’} _{1} (a _{1} SiH_{3} symmetric stretch)=1576(3), ω^{’} _{2} =589(3), and 2ω_{4} /2=635(6) cm^{−} ^{1}. The REMPI spectra exhibited ν^{‘} _{2} hot bands from vibrational levels as high as E _{ v } =2073 cm^{−} ^{1} in the X̃ ^{2} A _{1}state. Modeling calculations, which fit the numerous ν‘_{2} hot bands, predicted barriers to inversion of B _{inv}
=1935 cm^{−} ^{1} and B _{inv} =1925 cm^{−} ^{1} for SiH_{3} and SiD_{3} X̃ ^{2} A _{1} radicals, respectively.

Dynamic light scattering observation of droplet aggregation in a Winsor type W/O microemulsion system
View Description Hide DescriptionWe have performed scattered light intensity autocorrelation measurements on a Winsor type microemulsion system composed of brine, cyclohexane, SDS and a mixture of 1‐butanol and 1‐pentanol. At high cosurfactant concentration, where the microemulsion phase was considered to consist of individual, spherical water‐in‐oil droplets of relatively low droplet volume fraction, the autocorrelation functions were observed to be essentially single exponential, as expected. Above a certain droplet volume fraction, however, a d d i t i o n a l decay modes were observed to enter the correlation data. These modes were interpreted to be due to rotation and/or internal motion of dropleta g g r e g a t e s.

The generation and trapping of ^{2} ^{8}SiH^{+} _{2} and ^{2} ^{9}SiH^{+} _{2} in neon matrices at 4 K: Electron spin resonance and theoretical investigations
View Description Hide DescriptionThe ^{2} ^{8}SiH^{+} _{2} ^{2} ^{9}SiH^{+} _{2} cation radicals have been generated under neon matrix deposition conditions at 4 K by the photoionization (16.8 eV) of SiH_{4}(g). A resolution of the ^{2} ^{9}Si and hydrogen Atensors was possible which enables a complete characterization of the unpaired electron in this highly reactive ionic species for its X ^{2} A _{1}ground electronic state. The Atensor assignments were facilitated by the occurrence of preferential orientation of the isolated SiH^{+} _{2} molecule in the neon lattice. A b i n i t i otheoretical calculations of the nuclear hyperfinetensors were conducted which showed good agreement with the experimental results. The experimental magnetic parameters in neon at 4 K are: g _{ x }=2.0043(3), g _{ y }=1.9970(3), g _{ z } =2.0018(3); A _{ x }(H) =110.4(3), A _{ y } (H)=124.1(3), A _{ z } (H)=116.6(3); for ^{2} ^{9}Si, A _{ x } =−762.9(3), A _{ y } =−760.8(3) and A _{ z }=−1005.0(3) MHz where X is perpendicular to the molecular plane and Z lies along the C _{2} axis of the C _{2v } radical. No experimental evidence for SiH^{+} _{4} or the complex, SiH^{+} _{2} ⋅⋅⋅H_{2} was obtained. Theoretical calculations indicate that hyperfine structure from the ‘‘complexed’’ H_{2} should be resolvable if this species was present. However, rotational averaging of H_{2} above the SiH^{+} _{2} plane would reduce the hfi of H_{2} and might prevent its detection.

N(^{2} D) production from predissociation of n s and n d Rydberg levels in NO
View Description Hide DescriptionN(^{2} D) can be generated by two‐photon dissociation of NO above the threshold wavelength of 279 nm. A comparison of the three‐photon NO^{+}ionization signal with the N(^{2} D) excitation spectrum resulting from its detection by 2+1 resonance‐enhanced multiphoton ionization (REMPI) shows both similarities and differences. The NO Rydberg states that are generated in the initial two‐photon process either ionize or predissociate. The NO^{+} and N^{+} signals are indicative of predissociative lifetimes and channels, since an NO molecule that does not predissociate to N(^{2} D) and O(^{3} P) can separate to two lower atomic limits. In the 268–279 nm region there are three two‐photon absorption bands, 5sσ‐X 3‐0, 4dπ^{−}‐X 2‐0, and 5dπ^{−}‐X1‐0, which show very high N(^{2} D) predissociative yields from the upper states. A fourth band, 4dδ‐X2‐0, behaves in a distinctly different manner from its 4dπ^{−} complex partner. The presence of the valence G ^{2}Σ^{−} state, which lies within the experimental energy range, is not in evidence from either the three‐photon NO^{+} spectrum, or from predissociation in the N(^{2} D) channel. It appears that this state predissociates rapidly to the lower N(^{4} S)+O(^{1} D) or N(^{4} S)+O(^{3} P) limits, even above the N(^{2} D)+O(^{3} P) threshold.

Infrared laser spectroscopy of the 2^{1} _{0} and 2^{2} _{1} bands of H_{2}O^{+}(X̃ ^{2} B _{1})
View Description Hide DescriptionInfrared laserabsorption spectra of H_{2}O^{+} have been recorded in an ac discharge using velocity modulation detection. Eighty‐five lines of the 2^{1} _{0} fundamental and 20 lines of the 2^{2} _{1} hot band were used to derive rotation, spin‐rotation, and distortion parameters for the ground state and the first two vibrationally excited states of the bending mode. The results are compared with those from analysis of the Ã–X̃ band system and from rotational and vibration–rotation laser spectra, as well as with recent a b i n i t i o results obtained using MRCI wave functions.

Reactive islands as essential mediators of unimolecular conformational isomerization: A dynamical study of 3‐phospholene
View Description Hide DescriptionIn this paper we focus on the detailed nonlinear classical dynamics of conformationalisomerization. In particular we concentrate on systems which admit phase space structures we call ‘‘reactive islands.’’ Our calculations are on a two degree of freedom model of the molecule 3‐phospholene with an experimentally fit potential energy surface by Harthcock and Laane. The reactive islands (RIS) are embedded within and are part of chaotic regions of phase space. We find that the RIS are constructed from a linear stability analysis of the period 1 orbit at the transition state or approximated by a similar analysis on reactive periodic orbits. The two approaches converge as the order of the reactive periodic orbit increases. It is found that the fully constructed RIS have well defined regions of reactivity and thus mediate the process of conformational isomerization.The overlap areas of the RIS give important kinetic information such as probabilities for trapped to reactive motion, reactive to trapped motion, and primary back reaction. The implications of this work lie in precisely identifying the microscopic dynamical pathway for isomerization. The results of this work and its relation to the related work of Gray and Rice as well as its relation to contemporary work in modern dynamics is also discussed.

Order in chaos and the dynamics and kinetics of unimolecular conformational isomerization
View Description Hide DescriptionA high degree of structure and therefore order in chaos is found to exist in the detailed dynamical pathways to conformationalisomerization. It is shown that this structure can be used to determine the probabilities associated with the dynamical pathways to reaction, trapping, and back reaction. An earlier publication described the mediation of the dynamics of 3‐phospholene by phase space structures we called ‘‘reactive islands’’ (RIS)^{2} ^{1}. In this paper we extend the physical and mathematical properties of RIS and develop the corresponding kinetic theory. RIS theory is applied to a model of a hindered rotor and 3‐phospholene. It is shown that the RIS kinetic model accurately predicts trajectory simulations of conformer population decay. Comparisons with standard RRKM theory are included. A discussion on the extension of RIS theory to quantum reactive dynamics and its relevance to laboratory experiments is also included.

The millimeter‐ and submillimeter‐wave spectrum of dichlorocarbene CCl_{2}: Electronic structure estimated from the nuclear quadrupole coupling constants
View Description Hide DescriptionThe pure rotational spectrum of CCl_{2} in the ground state was observed for the first time in the millimeter‐ and submillimeter‐wave region. The CCl_{2} molecule was generated directly in a free space cell by a dc glow discharge in CCl_{4} . Several transitions showed hyperfine splittings arising from the nuclear quadrupole coupling of two Cl atoms. Analysis of the observed spectrum yielded the rotational, centrifugal distortion, and nuclear quadrupole coupling constants of the C^{35} Cl_{2} species. The r _{0} structure was derived from the rotational constants to be r _{0} (C–Cl)=1.7157 Å and θ_{0} (ClCCl)=109.2°. The ionic and π character of the C–Cl bond were estimated from the nuclear quadrupole coupling constants to be 0.26 and 0.32, respectively.

Molecular orbital and resonance Raman studies of the structures of N,N’‐disubstituted indigo dyes
View Description Hide DescriptionMolecular orbital calculations have shown that the structures of both t r a n s and c i s isomers of N,N’‐disubstituted indigo dyes are considerably twisted about the central C=C bond. The twisting angle of the c i s isomer is not always larger than that of the corresponding t r a n s isomer, indicating that the large hypsochromic shift on the t r a n s→c i sisomerization of N,N’‐disubstituted indigo dyes cannot be explained by the twisted structure of the c i s isomer. It is shown that the hypsochromic shift can be ascribed to a larger contribution of the zwitterionic form to the resonance hybrid structure in the lowest excited singlet state S _{1} than in the ground stateS _{0} .This makes the repulsion between the negative charges on the carbonyl oxygen atoms enhanced and the instability of the c i s configuration compared to the t r a n s configuration much larger in the S _{1} state than in the S _{0} state and leads to a larger vertical S _{1} ←S _{0} transition energy for the c i s isomer than for the t r a n s isomer.

Infrared spectra of diatomic polar molecules in rare‐gas liquids. I. Spectral theory
View Description Hide DescriptionA theoreticalmodel for the infrared (0–1) band of dilute solutions of diatomic polar molecules in nonpolar rare‐gas liquids is presented. The model is based on decomposition of the rotational motion of the diatomic molecule into two limiting cases, according to the Bratos model: quasifree rotation and rotational diffusion. Contribution to the infrared absorption coefficient due to quasifree rotation is analyzed within a non‐Markovian formalism using a stochastic directing intermolecular field (DIF) model to describe the diatomic molecule–solvent interaction. The P and R branches appear as a consequence of the quasifree contribution, which is also important in the Q‐branch region. The contribution due to rotational diffusion is calculated making use of the Debye model and is mainly significant in the Q‐branch region.

Infrared spectra of diatomic polar molecules in rare‐gas liquids. II. Application to the HCl in Ar, Kr, and Xe solutions
View Description Hide DescriptionIn paper I (the preceding paper), a spectraltheory describing the near infrared fundamental band spectra of diatomic polar molecules impurities in nonpolar solvents has been developed in terms of a reduced set of parameters. In this paper, this spectraltheory is applied to HCl in Ar, Kr, and Xe liquid solutions. The parameters obtained by fitting the experimental and calculated profiles are in good agreement with those calculated using several microscopic theories.

Pressure dependence of the polarized reflectance spectrum of the solid charge‐transfer complex, perylene–TCNQ: Estimation of microscopic parameters
View Description Hide DescriptionThe polarized reflectance spectrum of the perylene–TCNQ complex has been measured at high pressure up to 27 kbar using a diamond anvil cell incorporated into a microspectrophotometer. The degree of charge transfer (ρ), the transfer integral (t), and the site‐energy difference (Δ) are estimated at each pressure from analysis of the charge‐transfer band. The pressure dependences of t and Δ are found to be ∂ ln t/∂P=+2.3% kbar^{−} ^{1} and ∂Δ/∂P=−6.4 MeV kbar^{−} ^{1}, respectively. It is shown that the observed pressure dependence of the stabilization energy of the charge‐transferexcited state is mainly attributable to the change in the electrostatic energy accompanying the lattice contraction.

Triplet dipoles in the absorption spectra of dense rare gas fluids. II. Long range interactions
View Description Hide DescriptionA model calculation is presented to evaluate the long range dispersion dipole occurring when three nonoverlapping dissimilar atoms interact. Deviation from pairwise additivity is taken into account by a simplified theory based on electrostatic arguments. By comparing the long range triplet dispersion dipole with the one resulting from exchange overlap effects previously investigated [J. Chem. Phys. 9 0, 650 (1989)], it is shown that a cancellation between both contributions occurs in the vicinity of the van der Waals separation. The far infrared spectra of dense rare gas fluids are next generated by molecular dynamics simulation including long range dispersion and short range overlap dipoles. An estimate of the expected absorption in liquid krypton is given, α_{max} =3.6×10^{−9} cm^{−1} am^{−2}, which might give some hope to experimentalists. As far as rare gas mixtures are concerned, the model calculation reproduces surprisingly well the experimental absolute spectral density of a Kr–Ar liquid mixture (15% Kr, 135 K) over a large domain of frequency but fails to reproduce the low frequency dip. Nevertheless, irreducible three‐body dipoles contribute significantly to the spectrum. The difficulty of obtaining accurate results from an approximate model is emphasized and a guideline for further improvements is sketched.

Hole and antihole profiles in nonphotochemical hole‐burned spectra
View Description Hide DescriptionThe shapes of the real‐phonon and pseudophonon sideband holes (PSBH) which occur in hole‐burned spectra in amorphous solids are both wavelength and burn time dependent. The theory previously proposed to simulate hole shapes in the short burn time limit is extended to examine the PSBH shapes at arbitrary times and at different burn wavelengths. The simulated spectra are compared with experimental data for tetraphenyl porphin in polystyrene. It is also shown how the simulated spectra may be used to deconvolve the antihole spectrum from the hole spectrum.