Volume 94, Issue 7, 01 April 1991
Index of content:

Lowest energy excited singlet states of isomers of alkyl substituted hexatrienes
View Description Hide DescriptionVibrationally resolved S _{0}→S _{1} excitation spectra for the alkyl substituted linear polyenes heptatriene, octatriene, and decatriene seeded into a supersonic He expansion have been measured by resonance enhanced multiphoton ionizationspectroscopy. As is the case for the parent compound hexatriene, the lowest energy excited singlet state in all of these molecules is the 2 ^{1} A _{ g } state. The measurement of S _{0}→S _{1} excitation spectra of three of the four double bond isomers of heptatriene gives a detailed picture of the dependence of the electronic structure of the 2 ^{1} A _{ g } state on molecular conformation. The three isomers for which spectra are presented have the c i s configuration at either the central or the alkyl substituted double bond, or both. For the case of the mono c i s species with the c i s configuration at the alkyl substituted double bond the spectra show the presence of two single bond conformers. Because of the increased number of distinguishable isomers and conformers for octatriene and decatriene we were unable to unambiguously separate the observed spectra of these molecules into contributions from single specific molecular conformations. However, the increased excitation intensity in the low frequency region relative to that in the C–C and C=C stretching region for octatriene and decatriene as compared to heptatriene suggests that vibrational relaxation is enhanced in the more complex molecules. In the case of unsubstituted hexatriene, previously reported spectra show that the 2 ^{1} A _{ g } state has lower symmetry than does the ground state (most likely due to nonplanarity at the terminal carbon atoms). There is no evidence for an analogous distortion in the excitation spectrameasured for the alkyl substituted hexatrienes.

An examination of the corrected centrifugal sudden approximation for the calculation of line broadening and shifting coefficients for HF in He
View Description Hide DescriptionBy utilizing the lineshape theory of Shafer and Gordon [J. Chem. Phys. 5 8, 5422 (1973)], pure rotational pressure broadening and shifting coefficients for the infrared spectral lines of HF in He have been calculated using the close‐coupled (CC) method, the centrifugal sudden (CS) approximation, and the corrected centrifugal sudden (CCS) approximation of McLenithan and Secrest [J. Chem. Phys. 8 0, 2480 (1987)]. Comparison of the results obtained from these three methods were made at the total angular momentum dependent, kinetic energy dependent, and temperature dependent levels. In the latter comparisons, a semiclassical method was also included. The CS results agree very well with the CC calculations for the pressure broadening coefficients, but deviate slightly for the pressure shifting coefficients. The CCS results were essentially identical to those obtained from the CS approximation calculations. The semiclassical method was accurate to within about 15% of the CC values for the pressure broadening coefficients. The pressure shifting coefficients have been found to be very sensitive to the number of closed channels included in the calculations. As well, while collisionally inelastic contributions to the effective cross sections were not found to be negligible, their effect on the final calculated pressure broadening and shifting coefficients was found to be insignificant for this particular system at room temperature.

Potential energy surfaces from highly excited spectra using the bootstrap fitting method: Two‐dimensional surfaces for water and ozone
View Description Hide DescriptionThe bootstrap fitting procedure is implemented to obtain molecular potential energy surfaces for the stretching degrees of freedom of ozone using experimental spectra of highly excited vibrational states probed in recent experiments. The method has three key features: first, the use of a flexible anharmonic potential with parameters optimized in a nonlinear least‐squares fit; second, the implementation of the bootstrap algorithm for fitting vibrational levels even in regions where the classical dynamics are globally chaotic; and third, the employment of a basis‐set reduction technique called algebraic resonance quantization for the computation of eigenvalues of the potential. The approach was tested by using the stretching eigenvalues from a realistic empirical potential for water as input. Excellent agreement between the bootstrap potential and the empirical surface was found using levels up to 25 000 cm^{−1} with as few as 4 parameters. The method was then applied to experimental stretching levels of ozone. The potential energy surface obtained by our technique for the stretching modes of ozone is believed to be the best surface currently available up to 6500 cm^{−1}.

Longwave properties of the orientation averaged Mueller scattering matrix for particles of arbitrary shape. II. Molecular parameters and Perrin symmetry
View Description Hide DescriptionIn Part I of this paper, we started from a dipole array model of elasticlight scattering, and found the longwave asymptotic formulas for all 16 elements of the orientation averaged Mueller scattering matrix. However, the Perrin symmetry of the scattering matrix was not obvious from the formulas obtained in Part I. In this paper, Part II, we carry the analysis further, finding the molecular parameter identities which result in the Perrin symmetries. The formulas we present provide a very practical method for model calculations in the longwave limit. After evaluation of ten sums over pairs of the dipolar subunits of the model, the orientation averaged Mueller scattering matrix, as a function of scattering angle, is given by simple trigonometric polynomials. For models containing several hundred subunits the computation is easily carried through by a desktop computer. We verify the asymptotic formulas by numerical comparison with our analytic orientation averaging program PMAT2. We use a helical model with spherical subunits, in which the first Born approximation gives excellent results for the dipole elements (symmetry DSE). In this same model the first, second, and third Born approximations are utterly worthless for calculating the helicity‐retardation elements M _{13} and M _{23} and their transposes, but fourth Born gives nearly the exact result. These observables therefore use four bounces to feel out the helicity of the array, and may be more sensitive to structural variations than the traditional circular difference observable M _{14}, which responds after only two bounces.

Electron spin resonance and optical studies on the radiolysis of carbon tetrachloride. II. Structure and reaction of CCl^{⋅−} _{4} radical anion in tetramethylsilane low‐temperature solids
View Description Hide DescriptionAn electron spin resonance(ESR) and optical study of carbon tetrachloride radical anion has been made to provide for a better understanding of the radiolysis of CCl_{4}, following CCl^{⋅+} _{4} cation previously studied. It was found that the anion was metastably trapped in tetramethylsilane (TMS) matrices γ irradiated at 4 or 77 K. The gtensor and the hyperfine coupling tensors of all atoms of the radical were determined from ESRspectral simulation by using ^{12} CCl_{4} and the ^{13}C enriched compound:g _{∥}=2.004–5, g _{1}=2.015,(A _{∥},A _{⊥}) =(24.3,18.3) mT for ^{13}C , (0.9, 0.2) mT for one ^{35}Cl atom, and (A _{1},A _{2}=A _{3})=(1.98,0.45) mT for the other three equivalent ^{35}Cl atoms. From these parameters and a consideration on the ganisotropy combined with the optical data, the anion was found to have a predissociating molecular structure (CCl_{3}⋅⋅⋅Cl)^{ ̇−} with C _{3v } symmetry, where the unpaired electron occupies A ^{*} _{1γ} antibonding orbital. The carbon atom has a large spin density and near s p _{3} hybridization: ρ_{ p }=0.62, ρ_{ s }=0.18, ρ_{ p }/ρ_{ s }=3.4, and three Cl atoms and the other Cl atom have the spin densities ρ_{ p }=0.10 and ρ_{ p }=0.05, respectively. The species had two optical absorptions at λ_{max}=265 and 370 nm which were assigned to the E _{γ}–A ^{*} _{1γ} and A _{1γ}–A ^{*} _{1γ} electronic transitions, respectively. The anion converted to CCl^{ ⋅} _{3} radical by warming to ∼150 K in the TMS matrix. The present results have given unequivocal ESR and optical spectroscopic evidence and support for the assignment of the 370 nm band reported in the radiolyses of organic solutions containing CCl_{4}.

The B←X electronic spectrum of N^{+} _{2}–Ne
View Description Hide DescriptionThe electronic spectrum of N^{+} _{2}–Ne has been measured in the region corresponding to the B ^{2}∑^{+} _{ u }←X ^{2}∑^{+} _{ g } origin and 1–0 transitions of N^{+} _{2}. Spectra were obtained by irradiating a mass selected population of N^{+} _{2}–Ne and monitoring the production of N^{+} _{2} as a function of wavelength. Low temperature N^{+} _{2}–Ne spectra exhibit several well resolved bands. From the shift of the N^{+} _{2}–Ne origin with respect to that of free N^{+} _{2} it is apparent that the complex dissociation energyD _{0} is 146.5 cm^{−1} greater in the B state than the X state. Pronounced changes in the complex’s spectrum occur as the effective temperature is increased. The hottest spectra resemble a broadened and truncated N^{+} _{2} spectrum. The breaking off at the high energy end of the spectrum at elevated temperatures allows us to establish a rough ground‐state dissociation energy of 300 cm^{−1}. Other conclusions resulting from this work are that the equilibrium geometry of the N^{+} _{2}–Ne molecule is probably linear in X and B electronic states, that the ΔG _{1/2} for the low frequency stretch in the B state is 104 cm^{−1}, and that the N–N stretching motion is affected only very weakly by the presence of the Ne atom.

Study of the 4 ^{1}Σ^{+} _{ g } ‘‘shelf’’ state of Na_{2} by optical–optical double resonance spectroscopy
View Description Hide DescriptionDue to an avoided crossing, the 4 ^{1}Σ^{+} _{ g } state of Na_{2} has an unusual potential energy curve with a ‘‘shelf’’ on its outer wall. By using optical–optical double resonance techniques, vibrational levels from v=0 to 130 have been observed and the shelf was found around v=48–60. From the experimental data we have successfully constructed a Rydberg–Klein–Rees (RKR) potential curve, which can reproduce measured data with a standard deviation of 0.064 cm^{−1}. A set of Dunham coefficients was obtained for the lower part v=0–45 of that state. Several interesting effects related to the shelf, such as the unusual behavior of B _{ v } and D _{ v }, of the vibrational wave functions and the semiclassical vibrational periods, and of the rotational effective potential, are discussed.

Relaxation of spin quantum number S=3/2 under multiple‐pulse quadrupolar echoes
View Description Hide DescriptionThe time evolution of the spin S=3/2 density operator due to relaxation in the presence of a sequence of (π/2) pulses has been calculated using perturbation theory in an interaction representation in which no external time‐dependent rf fields occur. It is shown that the relaxation under the effect of the pulse train is similar to T _{1ρ}relaxation. If the magnetization is sampled between the pulses, outside extreme narrowing conditions the observed signal is characterized by a biexponential decay. The amplitude ratio of the fast and slowly relaxing component equals 0.8:0.2. The rate of the fast relaxing component is sensitive to the pulse cycle time in the presence of slowly fluctuating electric‐field gradients. A full expression of the relevant spectral density function valid for (π/2) pulses with arbitrary pulse width has been derived. In this derivation, the processes involved in determining the loss of correlation of the quadrupolar interaction are assumed to be independent and characterized by exponential correlation functions. Due to relaxation under the effect of the (π/2) pulse train triple‐quantum coherences may be excited. The latter coherences can be monitored by applying an additional coherence transfer pulse after the pulse train. The relaxation of sodium in an ion exchange resin in the presence of the (π/2) pulse sequence has experimentally been studied and agrees with the theoretical analysis.

Measurement of the intermolecular vibration–rotation tunneling spectrum of the ammonia dimer by tunable far infrared laser spectroscopy
View Description Hide DescriptionOver 150 lines in six tunneling subbands of an intermolecular vibration located near 25 cm^{−1} have been measured with partial hyperfine resolution and assigned to (NH_{3})_{2}. The transitions sample all three types of tunneling states (A, G, E) and are consistent with the following assumptions: (1) G_{36} is the appropriate molecular symmetry group; (2) the equilibrium structure contains a plane of symmetry; (3) interchange tunneling of inequivalent monomers occurs via a t r a n s path; (4) the 2C_{3}+I limit of hydrogen exchange tunneling is appropriate; (5) tunneling and rotational motions are separable. A qualitative vibration–rotation tunneling energy level diagram is presented. Strong perturbations are observed among the states of E symmetry. This work supports the conclusions of Nelson e t a l. [J. Chem. Phys. 8 7, 6365 (1987)].

Fluorescence and absorption spectroscopy of the near‐infrared vibronic transitions in matrix‐isolated NpF_{6}
View Description Hide DescriptionThe ^{4}Γ_{8u } and ^{2}Γ_{7u }excited states of NpF_{6} have been characterized in a solid argon matrix using absorption,fluorescence, and fluorescence excitation spectroscopies.Analysis of the vibronic bands of these states gives excited state vibrational frequencies which are compared to those of the ground state. The ^{4}Γ_{8u } state fluoresces with a 4.6 ms lifetime and a quantum yield near unity.

Rotational analysis of the nonthermal NO(A ^{2}Σ^{+}, v’=0) distribution observed in the N_{2}(A ^{3}Σ^{+} _{ u }, v’=0)+NO(X ^{2}Π, v‘=0) energy transfer reaction
View Description Hide DescriptionA rapidly pumped discharge‐flow reactor was constructed to characterize the N_{2}(A ^{3}Σ^{+} _{ u },v’)+NO(X ^{2}Π, v‘=0) energy transfer(ET)reaction.Emission spectra of the NO γ bands from the product NO A ^{2}Σ state formed in the title reaction were collected with a 2.2 m vacuum‐ultraviolet spectrograph monochromator utilizing both photographic and photoelectric techniques. The rotational structure of the γ bands resulting from the title reaction is resolved with a spectral resolution of Δλ∼0.1 Å. The product NO(A,v’) emission spectra were measured as a function of total pressure (∼0.8≤p _{TOTAL}≤∼3.0 Torr) and initial N_{2}(A,v’) population distribution at 298 K. The product NO(A,v’,N’) rotational distributions yield temperatures much higher than room temperature (>1600 K) and are discussed in detail. Vibrational level specific bimolecular rate constantsk _{ v’} ’s for the N_{2}(A,4≤v’≤6)+NO reaction were measured using N_{2}(B ^{3}Π_{ g }–A ^{3}Σ^{+} _{ u }) laser‐excited fluorescence detection with a fixed reaction time. The k _{ v’} ’s are (9.5±1.0), (11.3±1.2), and (11.2±1.1)×10^{−11} cm^{3} molecule^{−1} s^{−1} for v’=4, 5, and 6, respectively. A comparison with previous measurements is presented.

Hydrogen tunneling in condensed media
View Description Hide DescriptionA microscopic description for reactions in condensed media involving hydrogen tunneling, valid over a large temperature range, is presented. The tunneling system, represented by a pseudospin (S=1/2), reaches equilibrium when coupled to its environment, modeled by a collection of harmonic oscillators that behave like a heat bath. The environment includes both modes of the lattice (or solvent) and those molecular vibrations which play an active role in the tunneling process. Analytical expressions for the reaction rate are given in various regimes.

Two and three dimensional laser induced fluorescence: Application to photodissociation of LiI
View Description Hide DescriptionA new method is proposed for the measurement of translational energy distributions in which a species with three energy levels is excited by two successive transitions with perpendicular laser beams. Simulations of the method are carried out for the photodissociation of H_{2} O and a pilot experiment was performed on Li atoms dissociated from LiI vapor.

Unusual photofragmentation dynamics in the multiphoton ionization of Cr(CO)_{6} /methanol van der Waals heteroclusters
View Description Hide DescriptionMixed van der Waals clusters containing Cr(CO)_{6} and methanol are generated in the free‐jet expansion of a pulsed beam of seeded helium and subjected to 248 nm multiphoton ionization (MPI) at moderate laser fluence, and the product ions are analyzed by time‐of‐flight mass spectrometry. We find that the multiphoton dissociation and ionizationdynamics of solvated Cr(CO)_{6} are s t r i k i n g l y different from those of the naked molecule. Two principal sequences of heterocluster ions are identified in the mass spectrum. A major sequence with the empirical formula S_{ n }Cr(CO)^{+} _{ x } (x=0,1,2), where S is a methanol molecule, first appears in the mass spectrum at n+x=6. A minor sequence with the empirical formula S_{ n }Cr(CO)^{+} _{ x } (x=5,6), first appears in the mass spectrum at n+x=7.
We discuss two possible dynamical schemes for MPI of Cr(CO)_{6} /methanol heteroclusters: One scheme, in which initial photoionization of neutral Cr(CO)_{6} ‐containing clusters gives rise to solvated Cr(CO)^{+} _{6} primary photoions, which subsequently photodissociate to yield the observed coordinatively unsaturated daughter fragments; and an alternative scheme, in which initial photodissociation in the neutral manifold gives rise to one or more coordinatively unsaturated primary photoproducts, each of which subsequently undergoes photoionization. We consider, in a qualitative fashion, the mass spectral fragmentation patterns predicted by these two alternative schemes under conditions of e x t r e m e l y h i g h laser fluence (where essentially all one‐photon processes are saturated), and compare these p r e d i c t i o n s with the o b s e r v e d MPI mass spectra following high‐fluence irradiation at both 248 and 350 nm. Assuming that any secondary photodissociation in the ionic manifold occurs statistically, our high‐fluence results are inconsistent with a dynamical scheme in which MPI p r e c e d e sphotodissociation.
We suggest that the dynamics are correctly described by initial photodissociation in the n e u t r a lmanifold, followed by MPI of the coordinatively unsaturated primary photoproduct clusters. Intracluster stabilization of nascent photoion internal energy, either through evaporative cooling or collisional V–Venergy transfer, is suggested to strongly influence subsequent metal–ligand bond dissociation.

Interactions energies associated with short intermolecular contacts of C–H bonds. II. A b i n i t i o computational study of the C–H⋅⋅⋅H–C interactions in methane dimer
View Description Hide DescriptionOn the basis of single‐and multireference a b i n i t i o calculations on (Be)_{2} and (H_{2})_{2}, we suggest that van der Waals systems with large highest occupied molecular orbital–lowest unoccupied molecular orbital gap can be reasonably well described by single reference MPn (n=2–4) calculations if the basis set is large enough. The binding energies of C–H⋅⋅⋅H–C contacts are then examined by performing single reference MPn calculations on (CH_{4})_{2} dimer. Our study shows that (CH_{4})_{2} is bound in all possible CH_{4}⋅⋅⋅CH_{4} arrangements, and that those arrangements with more than one C–H⋅⋅⋅H–C contact lead to a greater amount of stabilization than does the arrangement with one C–H⋅⋅⋅H–C contact. The potential energy curves obtained for staggered arrangements of (CH_{4})_{2} by the MP2 calculations with the 6‐311G(2d,2p) basis set are in close agreement with the experimentally deduced, isotropic potential curve.

Collision‐induced state‐to‐state flow of vibrational energy in S _{1} toluene
View Description Hide DescriptionState‐to‐state flow of vibrational energy in the S _{1} state of toluene vapor under ‘‘single collision’’ conditions has been measured. The 6b ^{1} level (ν^{’} _{6b }=530 cm^{−1} ) was pumped by pulsed laser excitation, and resolved fluorescence spectra revealed which vibrational levels in the S _{1} state were produced by collisions with three added gases. Energy flow was found to be selective and occurred into four channels. Endergonic flow of energy to identified, accessible vibrational levels is not observed. Collision cross sections increased with increasing molecular complexity of the collision partners, but similar energy flow patterns were observed with all collision partners. Energy flow between 6a ^{1} and 6b ^{1} levels, which have a spacing of about 90 cm^{−1}, was found to be very inefficient.

Laser induced fluorescence and vacuum ultraviolet spectroscopic studies of H‐atom production in the dissociative recombination of some protonated ions
View Description Hide DescriptionThe flowing afterglow technique, coupled with laser induced fluorescence(LIF) and vacuum ultraviolet (vuv)absorption spectroscopy, has been used to determine the fractional H‐atom contributions, f _{ H }, to the product distributions for the dissociative recombination of a series of protonated ions (N_{2}H^{+}, HCO^{+}, HCO^{+} _{2}, N_{2}OH^{+}, OCSH^{+}, H_{2}CN^{+}, H_{3}O^{+}, H_{3}S^{+}, NH^{+} _{4}, and CH^{+} _{5} ) with electrons. The measurements were made at 300 K in two separate ways in two laboratories by (i) directly determining the H‐atom number density using vuvabsorption spectroscopy at the L _{α} (121.6 nm) wavelength and (i i) converting the H atoms to OH radicals using the reaction H+NO_{2}→OH+NO followed by LIF to determine the OH number density. The agreement between the two techniques is excellent and values of f _{ H } varying from ∼0.2 (for OCSH^{+} ) to 1.2 (for CH^{+} _{5} ) have been obtained showing that in some of the cases recombination can lead to the ejection of two separate H atoms. Comparison of the oxygen/sulphur analogs, HCO^{+} _{2}/OCSH^{+} and H_{3}O^{+}/H_{3}S^{+} showed that the f _{ H } values were very different. Possible reasons for these differences are discussed. Comparison is also made with the available theory.

A b i n i t i o potential energy surfaces and trajectory studies of A‐band photodissociation dynamics: CH_{3}I*→CH_{3}+I and CH_{3}+I*
View Description Hide DescriptionA b i n i t i o contracted spin–orbit configuration interaction (SOCI) calculations have been carried out to obtain potential energy surfaces of ^{3} Q _{0} and ^{1} Q _{1}excited states of methyl iodide as functions of all the geometrical parameters except for the three C–H stretches. The results are fitted to six‐dimensional diabatic potential functions and their couplings. Classical trajectory calculations have been performed using these potential functions. The rotation of the CH_{3} product in the I channel has been calculated to be perpendicular to the top axis and to have a peak at N=5 and extend up to N=8, whereas it is cold in the I* channel, in good agreement with recent experiments. The CH_{3} rotation is excited by the time trajectories arrive at the conical intersection region; this excitation is retained in the I‐channel product because the ^{1} Q _{1}surface has a small bending force constant outside the conical intersection, whereas it is damped in the I* channel because ^{3} Q _{0} still has a large bending force constant. The calculated distribution in the ν_{2} umbrella vibrational mode of the CH_{3} product is hot and has a peak at v=2 for the I channel, and is cool for the I* channel, in good agreement with recent experiments. This channel selectivity is due to the difference in the preferred structure of the CH_{3} group outside the conical intersection region; while the ^{3} Q _{0}surface prefers a bent CH_{3} until the CH_{3}–I distance becomes very large, ^{1} Q _{1} wants a planar CH_{3}. The location of conical intersection and the ground‐excited energy difference there are in good agreement with those deduced from experiment if a dynamical effect is taken into account.

Polarization dependent translational energy release observed in the photodissociation of C_{2}F_{5}I at 304.7 nm
View Description Hide DescriptionUsing a state‐selective photofragment translational spectroscopy, we determined the angular distribution and the polarization dependence of the velocity distribution for the iodine atom in the ground ^{2} P _{3/2} and spin–orbit excited ^{2} P _{1/2} states produced in the photodissociation of C_{2}F_{5}I at ∼304 nm. Consistent with theoretical and experimental results on other alkyl iodides, the excited state iodine is found to be produced predominantly from the parallel ^{3} Q _{0}←N absorption (β_{lab} =1.63±0.06) with a high fraction of available energy released in translation (∼67%). The substantially lower anisotropy parameter (β_{lab} =1.08±0.03) and the polarization dependent velocity distribution observed for the ground state iodine atoms, however, suggest that they are formed from two different excited states, by direct dissociation from the ^{3} Q _{1} state (∼22%) and indirect dissociation via curve crossing from the ^{3} Q _{0} to ^{1} Q state (∼78%). The dissociation along the ^{3} Q _{1} state is found to release about 3.1 kcal/mol more energy in translation than the dissociation via curve crossing from the ^{3} Q _{0} state. This was blamed on the difference in the vibrational energy redistribution along the potential energy surfaces involved.

The photodissociation of O_{3}: A classical dynamical approach for the interpretation of the recurrences in the autocorrelation function
View Description Hide DescriptionFamilies of periodic orbits and the classical survival probability function are calculated for ozone in the excited ^{1} B _{2} state. It is shown that some of the observed oscillations in the experimentally extracted and quantum mechanically calculated time autocorrelation functions can be explained by certain types of periodic orbits. Disagreements between the calculated and experimental correlation function are attributed to inaccuracies of the potential function used.