Index of content:
Volume 105, Issue 20, 22 November 1996

Sumâoverâstates density functional perturbation theory: Prediction of reliable ^{13}C, ^{15}N, and ^{17}O nuclear magnetic resonance chemical shifts
View Description Hide DescriptionSumâoverâstates density functionalperturbation theory (SOSâDFPT) has been used to calculate ^{13}C, ^{15}N, and ^{17}O NMRchemical shifts of 20 molecules, for which accurate experimental gasâphase values are available. Compared to HartreeâFock (HF), SOSâDFPT leads to improved chemical shift values and approaches the degree of accuracy obtained with second order Mo/llerâPlesset perturbation theory (MP2). This is particularly true in the case of ^{15}N chemical shifts where SOSâDFPT performs even better than MP2. Additional improvements of SOSâDFPT chemical shifts can be obtained by empirically correcting diamagnetic and paramagnetic contributions to compensate for deficiencies which are typical of DFT.

On the theoretical investigation of vibronic spectra of ethylene by ab initio calculations of the FranckâCondon factors
View Description Hide DescriptionThe vibronic spectra of ethylene have been studied using ab initio molecular orbital methods. Geometries of the singlet ÏâÏ*, Ïâ3s, and Ïâ3pexcited electronic states have been optimized at the CIS and CASSCF levels of theory with the 6â311(2+)G* basis set. Vertical and adiabatic excitation energies, calculated by the multireference configuration interaction (MRCI) and equationâofâmotion coupled cluster (EOMâCCSD) methods are in quantitative agreement with experiment. Vibrational frequencies and normal coordinates for the ground and excited states are used for the calculations of vibrational overlap integrals and FranckâCondon factors, taking into account distortion, displacement, and normal mode mixing (up to four modes). Major features of the observed absorptionspectrum of ethylene have been interpreted on the basis of the computed FranckâCondon factors. The role of each electronic state in the spectra has been clarified; the Ïâ3s transition corresponds to the distinct intensive peaks in the 57â000â61â000 cm^{â1} energy region, the less intensive distinct bands in the interval of 62â000â65â000 cm^{â1} are due to the Ïâ3p _{Ï} states and the ÏâÏ* peaks constitute the continuum underlying the spectrum. The theoretical vibronic spectrum is in qualitative agreement with the experimental one, except of some details. Possible reasons for the discrepancies between theory and experiment are also discussed.

Exciplex vacuum ultraviolet emission spectra of KrAr: Temperature dependence and potentials
View Description Hide DescriptionThe temperature dependence of the emissions from the 0^{+}(^{3} P _{1})and 1(^{3} P _{2}) Kr*Ar exciplex states in the range 85â350 K was studied using time resolved techniques, vacuum ultraviolet synchrotron radiation, and argon samples doped with minimal amounts of krypton. As the temperature is increased, the emission shifts to the blue, its width increases by almost a factor of 2, and the line shape becomes asymmetrical. The experimental line shapes have been simulated by means of FranckâCondon density calculations using the available ground state potential of Aziz and Slaman [Mol. Phys. 58, 679 (1986)] and by modeling the exciplex potentials as Morse curves. The potential parameters for the 0^{+} and 1 states are r _{ e }=5.05Â±0.01 and 5.07Â±0.01 a _{0}, respectively; D _{ e }=1150Â±200 cm^{â1} and ÎČ=1.4Â±0.1 a _{0} ^{â1} for both states. The latter two values yield Ï_{ e }=140 cm^{â1} and Ï_{ ex } _{ e }=4.3 cm^{â1}. The energy positions of the exciplexesâs wells and their depths are compared with published results.

Coriolisâdependent Stark effect of the 2Îœ_{3} band of methane observed by saturated absorption spectroscopy
View Description Hide DescriptionWe studied the vibrationâinduced dipole moment of methane by observing Starkâmodulation spectra of the 2Îœ_{3} band centered at 1.66 ÎŒm. The spectral purity of an external cavitysemiconductor laser and radiation density in a FabryâPerot cavity absorption cell are sufficiently high to record subâDoppler saturated absorption lines with a spectral resolution of 0.8 MHz. Firstâorder Stark shifts of the Eâtype symmetry components in the P(2), Q(2), Q(4), Q(5), R(2), R(4), and R(5) transitions were measured, and the vibrationâinduced dipole moments for the v _{3}=2 state were determined. Their values clearly demonstrate the Coriolisâinduced dipole term, yielding [5.79(5)+0.440(18)Ă(2Lâ3)] mD for the J _{ R } level, where L is defined to be â(R+1), 0, and R for the R=Jâ1, J, and J+1 levels, R and J are rotational and total angular momentum quantum numbers, and numbers in parentheses are 90% confidence intervals in unit of the last digit.

High resolution photoelectron spectroscopy of sulfur 2p electrons in H_{2}S, SO_{2}, CS_{2}, and OCS
View Description Hide DescriptionHighâresolution photoelectron spectra for the 2pelectrons in H_{2}S, SO_{2}, CS_{2}, and OCS show the effects of vibrational excitation in the coreâexcited species as well as the splitting of the 2p _{3/2} hole state by the molecular field. Theoretical calculations at the HartreeâFock level account reasonably well for the vibrational structure. The molecularâfield splitting is calculated with a configuration interactionâbased method using large basis sets. This produces values for the 2p _{3/2} splitting of 108, 96, 129, and 144 meV for the title molecules, to be compared with experimental values of 110, 105, 140, and 150 meV. Thus all observed features in the spectra are quantitatively accounted for by theoretical modeling.

Transformations of styrene at high pressure
View Description Hide DescriptionPhase transformations of styrene (C_{8}H_{8}) at room temperature and up to 32 GPa have been studied by visual observations, Fourier transform infrared spectroscopy, and Raman scattering. At room temperature, styrene freezes at 0.3 GPa and undergoes a solidâtoâsolid transformation around 3.5 GPa. Around 15 GPa the extent of the reaction monostyreneâpolystyrene becomes significant. At 32 GPa the reaction is nearly completed and, contrary to the general trend, the phenyl group is still stable at this pressure. This uncommon behavior is very likely correlated with the amorphous character of the polystyrene formed under pressure.

Recovery of broad and weak electron spin echo envelope modulation (ESEEM) lines in disordered systems with mixingâfrequency ESEEM method
View Description Hide DescriptionThe mixingâfrequency electron spin echo envelope modulation (MIFâESEEM) spectroscopy method has been used to recover broad and weak lines in disordered systems by means of choice of an appropriate increment ratio Îł. A Îłâirradiated powder alanine sample was used to demonstrate the advantage of this new scheme, in which two new ESEEM lines were observed.

Ab initioâstudy of NO_{2}. V. Nonadiabatic vibronic states and levels of the XÌâ ^{2} A _{1}/Ăâ ^{2} B _{2} conical intersection
View Description Hide DescriptionWe have computed 1500 nonadiabatic levels of the XÌâ ^{2} A _{1}/Ăâ ^{2} B _{2} conical intersection of NO_{2}, up to 18â700 cm^{â1}. By using a bond lengthsâbond angle Hamiltonian, the molecular states have been expanded in a diabatic electronic basis and in primitive, optimized, and BornâOppenheimer vibrational basis functions. We have optimized the diabatic potentials with respect to 191 observed bands up to 10â000 cm^{â1}, with a root mean square deviation (RMSD) of 17.8 cm^{â1}, and 691 nonadiabatic bands up to 15â000 cm^{â1} and 1060 up to 17â000 cm^{â1} have been converged within 1.9 and 4.4 cm^{â1}, respectively, by using 6117 basis functions per symmetry, and several states have been assigned. Up to 9500 cm^{â1} we have essentially found ^{2} A _{1}vibrational states, some of them mixed by the DelonâJost resonances. The nonadiabatic coupling then begins near the ^{2} B _{2} (0,0,0) origin, which we assign to an electronically mixed band at 9747 cm^{â1}, and gradually increases via the interaction between bending states of ^{2} A _{1} and ^{2} B _{2}. The vibronic mixing is more important above 12â000 cm^{â1}, where both electronic species contribute to several nonadiabatic states, but the ^{2} B _{2} bending progressions can be followed up to about 16â000 cm^{â1}, since they give rise to clumps of strongly mixed vibronic bands. Above 16â000 cm^{â1} finally, the nonadiabaticinteractions are very strong, masking all the vibrational progressions of both electronic states, and giving a fully chaotic spectrum which follows a Wignerâtype distribution. Our results thus explain the beginning and the development of the ^{2} A _{1}/^{2} B _{2}nonadiabaticinteraction, from the regular farâinfrared region up to the chaotic yellow portion of the spectrum. They are in good agreement with the available experimental data, allowing the assignment of several observed bands up to 16â000 cm^{â1}, and increase remarkably the number of known NO_{2} vibronic levels.

Bond energy oscillation in the cluster ion NO^{+}(NO)_{ n }
View Description Hide DescriptionThe gasâphase equilibria of the clusteringreaction of NO^{+} with NO and F^{â} with NO were measured with a pulsed electronâbeam highâpressure mass spectrometer. Vanât Hoff plots of equilibrium constants lead to the determination of the thermochemical stabilities for NO^{+}(NO)_{ n } with n=1â10 and F^{â}(NO)_{ n } with n=1â3. The equilibrium constantsK _{ nâ1,n } for the former reaction with n=4, 6, 8, and 10 were found to be larger than K _{ nâ2,nâ1} with (nâ1)=3, 5, 7, and 9, respectively. That is, the cluster ions NO^{+}(NO)_{ n } with even n are thermochemically more stable than the smaller ones, NO^{+}(NO)_{ nâ1}, under the present experimental conditions. The measured enthalpy (âÎH ^{0} _{ nâ1,n }) and entropy changes (âÎS ^{0} _{ nâ1,n }) show oddâeven oscillation. This is due to the electronâspin pairing effect, i.e., dimer pair formation in the cluster ions. The sudden decrease in the bond energies for the cluster NO^{+}(NO)_{ n } between n=2 and 3 suggests that the core in the cluster NO^{+}(NO)_{ n } is NO^{+}(NO)_{2}. The bond energy oscillation is also likely for the negative cluster ion F^{â}(NO)_{ n }.

Oneâatom cage effect in collinear I_{2}(B)âAr complexes: A timeâdependent wave packet study
View Description Hide DescriptionTwoâdimensional timeâdependent wave packet calculations are carried out on a collinear model of the I_{2}(B)âAr complex to investigate the possible kinematic origin of the oneâatom cage effect in small van der Waals molecules. Three different excitation wavelengths are considered (496.5, 488, and 476.5 nm), and the dynamics are assumed to be restricted to the I_{2} B state electronic surface, with no nonadiabatic transitions following the pump excitation. Good agreement with experiment is obtained. To investigate the sensitivity of observable final state distributions on the weak intermolecular potential between I_{2} and Ar, three slightly different B state IâAr interactions are employed for the case of 488 nm excitation. It is found that relatively small changes in the form and magnitude of the weak van der Waals interactions can have a large effect on the final state distributions. These results suggest that the experimental data on I_{2}âAr photodissociationârecombination can be explained by a purely kinematic oneâatom cage effect on the B state electronic surface for a collinear population of I_{2}âAr clusters, without the need to introduce nonadiabatic electronic effects.

Numerical simulation of the isomerization of HCN by two perpendicular intense IR laser pulses
View Description Hide DescriptionIsomerization of HCN is studied numerically for a laser excitation configuration of two perpendicular intense IR pulses. This scheme confines the molecule to a plane and promotes proton transfer along the curved reaction path. It is shown that internal rotation of the CN group enhances isomerization when compared to a fixed CâĄN orientation model. Isomerization rates with rotation exceed those without rotation of the CN by about a factor of 3. Internal rotation also enhances dissociation and destroys phase control of the isomerization. It is found that at intensities IâŒ10^{13} W/cm^{2}, maximum isomerization occurs with negligible dissociation for a 2 ps pulse excitation. Maximum isomerization is also found for one field frequency resonant with the CH bend frequency Ï_{bend} and the other perpendicular frequency at 2Ï_{bend}.

Variational transition state theory for electron transfer reactions in solution
View Description Hide DescriptionVariational transition state theory is used to compute the rate of nonadiabaticelectron transfer for a model of two sets of shifted harmonic oscillators. The calculations provide new insight on the suitability of the energy gap as a reaction coordinate. The relationship to the standard generalized Langevin equation model of electron transfer is established, and provides a framework for the application of variational transition state theory in a realistic simulation of electron transfer in a microscopic (nonlinear) bath.

Avoiding long propagation times in wave packet calculations on scattering with resonances: A hybrid approach involving the Lanczos method
View Description Hide DescriptionWe investigate the usefulness of a hybrid method for scattering with resonances. Wave packet propagation is used to obtain the timeâdependent wave function Îš(t) up to some time T at which direct scattering is over. Next, Îš(t) is extrapolated beyond T employing resonance eigenvalues and eigenfunctions obtained in a Lanczos procedure, using Îš(T) as starting vector to achieve faster convergence. The method is tested on one twoâdimensional (2D) and one fourâdimensional (4D) reactive scattering problem, affected by resonances of widths 0.1â5 meV. Compared to long time wave packet propagation, the hybrid method allows large reductions in the number of Hamiltonian operations N _{ H } required for obtaining converged reaction probabilities: A reduction factor of 24 was achieved for the 2D problem, and a factor of 6 for the 4D problem.

Twoâpulse laser control of bondâselective fragmentation
View Description Hide DescriptionWe elaborate on a twoâpulse (pumpâpump) laser control scheme for selective bondâbreaking in molecules [Amstrup and Henriksen, J. Chem. Phys. 97, 8285 (1992)]. We show, in particular, that with this scheme one can overcome the obstacle of intramolecular vibrational relaxation. As an example, we consider an ozone molecule with isotopic substitution, that is, ^{16}O^{16}O^{18}O. It is shown that asymmetric bond stretching can be created in simple (intense) laser fields. We predict that an alternating high selectivity between the channels ^{16}O+^{16}O^{18}O and ^{16}O^{16}O+^{18}O can be obtained when such a nonâstationary vibrating ozone molecule is photodissociated with short laser pulses (âŒ10â15 fs) with a time delay corresponding to half a vibrational period (âŒ17 fs).

Rotational population distribution of KH (v=0, 1, 2, and 3) in the reaction of K(5â^{2} P _{ J }, 6â^{2} P _{ J }, and 7â^{2} P _{ J }) with H_{2}: Reaction mechanism and product energy disposal
View Description Hide DescriptionUsing a pumpâprobe method, we have systematically studied the rotational distribution of KH (v=0â3) produced in the reaction of K (5P, 6P, and 7P) with H_{2}. The resulting rotational states fit roughly a statistical distribution at the system temperature, while the vibrational populations are characterized by a Boltzmann vibrational temperature of 1800, 3000, and 3100 K for the 5p, 6P, and 7P states, respectively. These results provide evidence that the reaction follows a collinear collisional geometry. This work has successfully probed KH from the K(5P) reaction, and confirms that a nonadiabatical transition via formation of an ionâpair K^{+}H^{â} _{2} intermediate should account for the reaction pathway. The available energy dissipation was measured to be (68Â±4)%, (26Â±2)%, and (6Â±3)% into the translation, vibration, and rotation of the KH product, respectively. The energy conversion into vibrational degree of freedom generally increases with the principal quantum number, indicating that the electronâjump distance elongates along the order of 5P<6P<7P. The result is different from the Cs(8P,9P)âH_{2} case, in which the electronâjump distances were considered roughly the same. Furthermore, a relatively large distance is expected to account for highly vibrational excitation found in the KH product. According to the classical trajectory computation reported by Polanyi and coâworkers, the strong instability of the H^{â} _{2}bond, inducing a large repulsion energy, appears to favor energy partitioning into the translation.

The intermolecular potential energy surface for CO_{2}âAr: Fitting to highâresolution spectroscopy of Van der Waals complexes and second virial coefficients
View Description Hide DescriptionTwo potential energy surfaces for CO_{2}âAr are obtained by leastâsquares fitting to the highâresolution spectra of Van der Waals complexes and the second virial coefficients of Ar+CO_{2} gas mixtures. The potentials incorporate a repulsive wall based on monomerab initio calculations and the assumption that the repulsion potential is proportional to the overlap of the monomercharge densities. The dispersion energy is represented in a twoâsite model, with dispersion centers located along the CâO bonds of CO_{2}. The resulting potentials give a good representation of all the experimental data with only three or four adjustable parameters. They are quite different from previous empirical CO_{2}âAr potentials, which all have either a poor representation of the attractive well or a poor representation of the repulsive wall.

A study of conical intersection effects on scattering processes: The validity of adiabatic singleâsurface approximations within a quasiâJahnâTeller model
View Description Hide DescriptionConical intersections between BornâOppenheimer potential energy surfaces create singularities which are known to have a direct effect on the symmetry of the nuclear wave functions. In this article is presented a quasiâJahnâTeller model to study the symmetry effects of these singularities on nonreactive and reactive scattering processes. Applying this model, we were able to determine in what way and to what extent the conical intersection affects the relevant Sâmatrix elements. Having the results of this study available, conclusions concerning more realistic systems were derived.

193.3 nm photodissociation of acetylene: Nascent state distribution of CCH radical studied by laserâinduced fluorescence
View Description Hide DescriptionThe nascent rovibronic distribution of CCH radicals in the 193.3 nm photolysis of acetylene has been measured by laserâinduced fluorescence in a supersonic jet. CCH fragments in the XÌâ ^{2}ÎŁ^{+} state are vibrationally hot, but rotationally cold. Predominant CCH fragments were observed at levels of the XÌ state with large mixing of Ăâstate character, particularly levels near the potential minimum of Ăâ ^{2}Î . This indicates that a nonadiabatic transition near the exit channels plays an important role in the 193.3 nm photodissociation of acetylene. Some, but not all, of the K=1 levels have distinctively bimodal rotational distributions. The relative vibrational energy distributions obtained from this work were used to simulate the translational energy distribution of the hydrogen atom by Balko, Zhang, and Lee [J. Chem. Phys. 94, 7958 (1991)] to extract the population distribution of CCH. It is thus determined that the majority of CCH radicals are formed in the ground electronic state (XÌ). Less than half of the CCH population was detected at K=1 levels, and the rest was distributed among K=0, 2, and 3 stacks. The bondenergy of HCCâH is estimated as 131.5Â±0.5 kcal/mol from the vibronic energy of the most populated CCH fragments determined in this work and the translational energy of the recoiled hydrogen atom reported previously by Balko, Zhang, and Lee and Segall, Wen, Lavi, Singer, and Wittig [J. Phys. Chem. 95, 8078 (1991)].

Coherent control of bimolecular collisions: Collinear reactive scattering
View Description Hide DescriptionA recently proposed approach to the coherent control of bimolecular reactions is applied to collinear models of H+H_{2}, D+H_{2}, and F+H_{2} scattering. Reactive scattering probabilities above the reaction threshold are shown to be controllable in these systems over a wide range, often nearing total yield control.

Dissociation dynamics of Na^{+} _{ n } in collision with rareâgas atoms
View Description Hide DescriptionDissociationdynamics of a sodium cluster ion, Na^{+} _{ n } (n=2â9 and 11), in collision with a rare gas atom (He or Ne) was investigated by measuring the absolute cross sections for the production of fragmented ions by using a tandem massâspectrometer equipped with several octapole ion guides. The mass spectra of the fragmented ions show that release of Na and/or Na_{2} from Na^{+} _{ n } occurs dominantly. The absolute total cross section for the dissociation of Na^{+} _{ n } and the absolute partial cross sections for the Na and/or the Na_{2} release were determined at different collision energies and cluster sizes. The absolute total dissociation cross sections were calculated by a scheme that collisionally excited Na^{+} _{ n } dissociates with leaving Na and Na_{2} unimolecularly. On the other hand, the partial cross sections for the Na and the Na_{2} release were successfully explained by the orbital correlation diagram for the dissociation system; the dissociation channel involving an adiabatic transition was found to be influenced significantly by the collision energy and the cluster size.