Index of content:
Volume 104, Issue 11, 15 March 1996

Electronic spectral diffusion in glasses: The influence of coupling to the medium on experimental observables
View Description Hide DescriptionThe theory of electronic dephasing in low temperature glasses is extended to include the possibility that the strength of coupling of the chromophore to the solvent medium depends on the nature of the bath dynamical processes and the nature of the chromophore and, therefore, the chromophore‐bath coupling can vary as a function of the rate of the dynamics of the medium. In the context of the sudden jump two‐level system (TLS) model of low temperature glasses, this theory is used to reconcile the apparent contradiction implied by differences observed in spectral diffusion data for cresyl violet and metal‐porphyrins in deuterated ethanolglass at 1.5 K. Previously, the coupling strength of a chromophore to the TLS has been assumed to be independent of rate of the transition between TLS states. Within the context of this approximation, spectral diffusion data yield, P _{ i }(R), the intrinsic TLS fluctuation rate distribution. With the inclusion of the rate dependent coupling, C(R), it is shown that the spectral diffusion observables actually yield P _{ i }(R)C(R). Therefore, the observed lack of spectral diffusion for a particular chromophore over some range of times can imply C(R) is zero rather than the current interpretation that P _{ i }(R) is zero.
To illustrate the importance of C(R), a hueristic model is analyzed. A fluctuation rate distribution is introduced that consists of the sum of three log‐normal functions each associated with a specific class of dynamics occurring over three overlapping ranges of rates. The uncharged and nonpolar metal porphyrins is taken to couple to TLS strain dipoles, while the charged and polar cresyl violet also couples to TLS electric dipoles. By taking one of the types of TLS dynamics to only give rise to electric dipole fluctuations, it is possible to fit all of the experimental data in deuterated ethanol with a single intrinsic distribution of TLS fluctuation rates. This analysis of previously reported data is supported by the presentation of new stimulated photon echo data on both cresyl violet and zinc meso‐tetraphenyl porphine in deuterated ethanol.

Mid‐infrared spectra of He–HN^{+} _{2} and He_{2}–HN^{+} _{2}
View Description Hide DescriptionMid‐infrared vibrational spectra of He–HN^{+} _{2} and He_{2}–HN^{+} _{2} have been recorded by monitoring their photofragmentation in a tandem mass spectrometer. For He–HN^{+} _{2} three rotationally resolved bands are seen: the fundamental ν_{1} transition (N–H stretch) at 3158.419±0.009 cm^{−1}, the ν_{1}+ν_{ b } combination band (N–H stretch plus intermolecular bend) at 3254.671±0.050 cm^{−1}, and the ν_{1}+ν_{ s } combination band (N–H stretch plus intermolecular stretch) at 3321.466±0.050 cm^{−1}. The spectroscopic data facilitate the development of approximate one‐dimensional radial intermolecular potentials relevant to the collinear bonding of He to HN^{+} _{2} in its (000) and (100) vibrational states. These consist of a short range potential derived from an RKR inversion of the spectroscopic data, together with a long range polarization potential generated by considering the interaction between the He atom and a set of multipoles distributed on the HN^{+} _{2} nuclei. The following estimates for binding energies are obtained: D _{0} ^{″}=378 cm^{−1} [He+HN^{+} _{2}(000)], and D _{0} ^{′}=431 cm^{−1} [He+HN^{+} _{2}(100)]. While the ν_{1} band of He_{2}–HN^{+} _{2} is not rotationally resolved, the fact that it is barely shifted from the corresponding band of He–HN^{+} _{2} suggests that the trimer possesses a structure in which one of the He atoms occupies a linear proton‐bound position forming a He–HN^{+} _{2} core, to which a second less strongly bound He is attached.

A molecular dynamics analysis of resonance emission: Optical dephasing and inhomogeneous broadening of CH_{3}I in CH_{4} and Ar
View Description Hide DescriptionThe spontaneous resonance emission of CH_{3}I in high pressures (800–1600 psi) of CH_{4} and Ar excited in the region of the RydbergB‐state origin (∼201 nm) are reported. These emission spectra consist of narrow Raman‐like (RL) and broad fluorescence‐like (FL) spectral features. The observed ratio of the Raman/fluorescence intensity in these high pressure solutions is a function of the excitation wavelength as the incident radiation is tuned through the pressure broadened electronic origin band. Molecular dynamics simulations are implemented for the analysis of the observed emission spectral shapes and their excitation frequency dependence. The four‐time dipole correlation functions required for the calculation of this nonlinear polarization derived signal are successfully approximated by a product of two two‐time dipole correlation functions for these chromophore‐bath systems (factorization approximation). The complex emission band shapes and their excitation frequency dependence are captured by this approach. The dispersion in the RL/FL emission redistribution is due to the multiple time scales inherent to the decay of the resonant optical coherence of these pressure‐broadened absorptions. The wavelength dependent pure‐dephasing rate is determined by the nonlinear shape of the solute–solvent difference potential. The observational time scale dependence of the spectroscopic homogeneous and inhomogeneous line broadening labels is clearly demonstrated and contrasted here for absorption and Raman scattering.

Stark effect and dipole moments of the ammonia dimer in different vibration–rotation–tunneling states
View Description Hide DescriptionIn this paper we present Stark measurements on the G:K=−1 vibration–rotation–tunneling (VRT) transition, band origin 747.2 GHz, of the ammonia dimer. The observed splitting pattern and selection rules can be explained by considering the G _{36} and G _{144} symmetries of the inversion states involved, and almost complete mixing of these states by the applied electric field. The absolute values of the electric dipole moments of the ground and excited state are determined to be 0.763(15) and 0.365(10) D, respectively. From the theoretical analysis and the observed selection rules it is possible to establish that the dipole moments of the two interchange states must have opposite sign. The theoretical calculations are in good agreement with the experimental results: The calculated dipole moments are −0.74 D for the lower and +0.35 D for the higher state. Our results, in combination with the earlier dipole measurements on the G:K=0 ground state and the G:K=1 transition with band origin 486.8 GHz, confirm that the ammonia dimer is highly nonrigid. Its relatively small and strongly K‐dependent dipole moment, which changes sign upon far‐infrared excitation, originates from the difference in dynamical behavior of ortho and para NH_{3}.

Electronic transition moment and rotational transition probabilities in CH. II. B ^{2}Σ^{−}–X ^{2}Π system
View Description Hide DescriptionVibrational transition probabilities of the B ^{2}Σ^{−}–X ^{2}Π band system of the CH radical, for the bound levels v′=0 and 1, have been measured in an 8 Torr flame using laser‐induced fluorescence. These new branching ratio measurements together with Rydberg–Klein–Rees wave functions furnish an empirical electronic transition moment for this system. A quadratic form compares very well with previous ab initio calculations. The model predicts significant variations in lifetime with rotational quantum number. As a consequence, the Einstein emission and absorption coefficients are strongly dependent on rotational quantum number, and are needed for quantitative temperature and nonthermal population distribution determinations.

Higher‐order effects in the vibration–torsion–rotation spectra of a molecule with an internal rotor
View Description Hide DescriptionThe sequential contact transformation technique is applied to the vibrational–torsional–rotational Hamiltonian for molecules containing a threefold symmetric internal rotor, which makes it possible to analyze various higher‐order vibration–torsion–rotation interactions and their effects in the vibration–torsion–rotation spectra of the molecule. The formalism for the calculation of Hamiltonian coefficients is developed in terms of the fundamental molecular parameters. The resulting formulas are applicable to analyze the molecular structure. The theory presented is applied to the calculation of the quartic and sixtic centrifugal distortion constants of the molecule.

Vibrationally resolved photoelectron angular distributions and branching ratios for the carbon dioxide molecule in the wavelength region 685–795 Å
View Description Hide DescriptionMeasurements of vibrational branching ratios and photoelectron angular distributions have been made in the regions of the Tanaka‐Ogawa, Lindholm, and Henning series for the CO_{2} molecule. The behavior of these parameters was found to be sensitive to which particular resonance is excited, with considerable intensity going into vibrational modes other than the symmetric stretch. An initial analysis of some of the data taken is presented.

High resolution electronic spectroscopy of 1‐aminonaphthalene: S _{0} and S _{1} geometries and S _{1}←S _{0} transition moment orientations
View Description Hide DescriptionFluorescence excitation spectroscopy at both vibrational and rotational resolution has been used to probe the changes in energy, electronic distribution, and geometry that occur when 1‐aminonaphthalene (1AN) absorbs light at ∼332 nm. The 0^{0} _{0} band of the S _{1}←S _{0} transition of 1AN is red shifted by nearly 2000 cm^{−1} with respect to the corresponding band of naphthalene. Additionally, it is mainly b‐axis polarized, unlike the corresponding bands of naphthalene and other 1‐substituted naphthalenes. Thus, ^{1} L _{ a }/^{1} L _{ b } state reversal occurs on 1‐substitution of naphthalene with an NH_{2} group. The S _{0} state of 1AN is pyramidally distorted at the nitrogen atom. Additionally, the NH_{2} group is rotated by ∼20° about the C–NH_{2}bond. Excitation of 1AN to the zero‐point vibrational level of its S _{1} state reduces the C–NH_{2}bond length by ∼0.2 Å and flattens the NH_{2} group along both out‐of‐plane coordinates. Other vibronic bands in the S _{1}←S _{0} transition exhibit significantly different rotational constants, inertial defects, and transition moment orientations. An explanation for these findings is given that is based on the well‐known conjugative properties of the NH_{2} group in chemically related systems.

Effects of different population, orientation, and alignment relaxation rates in resonant four‐wave mixing
View Description Hide DescriptionWe present a combined theoretical and experimental study on the effects of different population, orientation, and alignment relaxation rates in resonant four‐wave mixing (RFWM). Signal generation in RFWM can be viewed as the formation of and scattering from laser‐induced population, orientation, and alignment gratings. We show that the relative contributions from the upper‐state and lower‐state population, orientation, and alignment gratings to the observed output signal can be changed by varying the polarizations of the three input fields. A theory is developed to account for these changes in collisional environments where the three multipole moments of the total angular momentum distribution, i.e., the population, the orientation, and the alignment, relax unequally. This theory is applied to the OH radical in an atmospheric‐pressure H_{2}/O_{2}/He flame for which we have measured the line profiles using high‐resolution degenerate and nearly degenerate four‐wave mixing. We find that orientation and alignment gratings relax more rapidly than population gratings for low rotational levels of OH in the presence of He but at essentially the same rate for high rotational levels. A discussion is presented of the importance of this effect in the interpretation of RFWM experiments.

Calculations of the low temperature pressure broadening of HCO^{+} rotational spectral lines by H_{2}
View Description Hide DescriptionCalculations on the pressure broadening by H_{2} of HCO^{+} rotational spectral lines have been performed in the temperature range 11–300 K. Recent low temperature measurements of the pressure broadening of the J=3–2 line at 11–30 K are reproduced to within 10–20% by the results of a capture theory. The results of Anderson theory in this temperature range, on the other hand, are low by a factor of up to ≊3 compared with experiment. A hybrid method is proposed, which converges to the capture theory at low temperatures and the Anderson theory at high temperatures.

Monte Carlo simulation of n‐member associating fluids: Application to antigen–antibody systems
View Description Hide DescriptionAn association biased Monte Carlo (ABMC) method of simulating associating systems with two bonding sites on each particle is described. The method includes a strategy for frequently forming two simultaneous bonds by a single particle during a Monte Carlo move. This strategy is employed to obtain adequate configuration statistics at each state point and is particularly important when ring formation is possible. A variety of thermodynamic and physicochemical parameters of the associating system were monitored including the compressibility factor, internal potential energy, isothermal compressibility, chain and ring number, and shape were monitored. Our analysis indicated that there is a strong dependence of these monitored quantities upon the angle between vectors representing the bonding sites on each particle. Also presented are results which suggest the existence of a two phase region, which we believe is a gas–liquid coexistence, which is dependent upon density, bonding energy, and the relative angle between the bonding sites.

Guided ion beam studies of the reactions of Fe^{+} _{ n } (n=2–15) with D_{2}: Cluster–deuteride bond energies as a chemical probe of cluster structures
View Description Hide DescriptionThe kinetic energy dependencies of the reactions of Fe^{+} _{ n } (n=2–15) with D_{2} are studied in a guided ion beammass spectrometer. The only products observed are Fe_{ n }D^{+} (n=2–15) and Fe_{ n }D^{+} _{2} (n=9–15). All reactions are observed to exhibit thresholds, except for formation of Fe_{9}D^{+} _{2}. Threshold analyses of the endothermic reactions lead to binding energies for the first deuterium atom to the cluster ions as a function of cluster size. The Fe^{+} _{ n }–D bond energies are compared to previously determined metal–metal bond energies, D _{0}(Fe^{+} _{ n }–Fe). The bond energies of Fe^{+} _{ n }–D vary nonmonotonically with n, and parallel those for Fe^{+} _{ n }–Fe except for notable differences at n=5, 8, 12, and 14. These trends are rationalized in terms of electronic and geometricstructures for the Fe^{+} _{ n }clusters. Arguments are presented to suggest that the thresholds measured for Fe_{ n }D^{+} _{2} production correspond to barriers for chemisorbtion.

Quantum chaos in collinear (He, H_{2} ^{+}) collisions
View Description Hide DescriptionThe quasibound spectrum of the transition state in collinear (He, H_{2} ^{+}) collisions is obtained from time‐dependent wave packet calculations. Examination of short‐ and long‐range correlations in the eigenvalue spectra through a study of the nearest neighbor spacing distribution, P(s), and the spectral rigidity, Δ_{3}(L), reveals signatures of quantum chaotic behavior. Analysis in the time domain is carried out by computing the survival probability 〈〈P(t)〉〉 averaged over initial states and Hamiltonian. All these indicators show intermediate behavior between regular and chaotic. A quantitative comparison of 〈〈P(t)〉〉 with the results of random matrix theory provides an estimate of the fraction of phase space exhibiting chaotic behavior, in reasonable agreement with the classical dynamics. We also analyse the dynamical evolution of coherent Gaussian wave packets located initially in different regions of phase space and compute the survival probability, power spectrum and the volume of phase space over which the wave packet spreads and illustrate the different behaviors.

Heterogeneously catalyzed hydrolysis of chlorine nitrate: Fourier‐transform ion cyclotron resonance investigations of stratospheric chemistry
View Description Hide DescriptionHigh resolution Fourier‐transform ion cyclotron resonance (FT‐ICR) mass spectroscopy is used to investigate reactions of large ionic water clusters H^{+}(H_{2}O)_{ n } and X^{−}(H_{2}O)_{ n } (n=1−100, X=O or OH). Reactions of the clusters with chlorine nitrate, important ‘‘reservoir compound’’ involved in the stratosphericozone chemistry, are investigated to evaluate the importance of heterogeneously catalyzed reactions for ozone depletion. It is found that reactions of both cationic and anionic clusters result in effective hydrolysis of chlorine nitrate and return of the more active hypochlorous acid, HOCl into the gas phase. The chemistry of clusters is discussed, and its validity and relevance as a model for ‘‘real life’’ processes in the so‐called polar stratospheric clouds (PSC’s) is assessed.

A quantum‐classical study of the reaction CO(v _{1},j _{1})+OH(v _{2},j _{2})→CO_{2}+H
View Description Hide DescriptionThe dynamics of the complex‐forming reaction OH+CO→CO_{2}+H is investigated using a recently reported quantum‐classical approach for diatom‐diatom reactive scattering. In the present study, the OH and CO vibrations are treated quantum mechanically using the time‐dependent wave packet approach and their relative translational and rotational motions are treated classically. Results of total reaction probabilities, total reaction cross sections and thermal rate constants obtained from our calculations are compared with those from quasiclassical trajectory and different reduced dimensional quantum mechanical calculations.

Reaction of tungsten clusters with molecular nitrogen
View Description Hide DescriptionReactions of tungstenclusters with molecular nitrogen have been investigated by using a fast‐flow reactor equipped with a laser vaporizationcluster source and time‐of‐flight mass spectrometerdetector. Absolute rate coefficients are reported for reaction of W_{ n }clusters in the range n=4–26, at temperatures 277, 300, and 370 K in He buffer gas at 1 and 2 Torr pressure. For smaller clusters with n<15, complexes with N_{2} are formed with binding energies near 16 kcal mol^{−1}, and act as precursors to dissociation of N_{2} on the clusters. A sharp jump in the binding energy occurs at a cluster size of 15 metal atoms, and may signal the onset of atomic as opposed to molecular binding of N_{2} on the cluster. It is suggested that the change in reactivity at n=15 is correlated with a structural transition of the clusters, from relatively close‐packed to more open structures. The reactivity of tungstenclusters with N_{2} is compared with that of molybdenum clusters and tungsten metal surfaces.

Photofragment imaging of methane
View Description Hide DescriptionThe photolysis of methane is studied using photofragment imaging techniques. Our study reveals that the photolysis of methane proceeds via many different pathways. The photofragment imaging technique is used to resolve and characterize these various pathways and provides therefore unique insight into the dynamical processes that govern this photodissociation. The formation of H‐atom photofragments following absorption of a Lyman‐α photon, and H_{2} photofragments following absorption of two ultraviolet photons (λ=210–230 nm) are studied. The measured H‐atom photofragment images reveal that a channel that produces fast H atoms concomitant with methyl fragments is dominant in the Lyman‐α photolysis of methane. This channel leads to an anisotropic recoil of the fragments. A secondary channel is observed leading to the formation of somewhat slower H atoms, but an unique identification of this second channel is not possible from the data. At least part of these slower H atoms are formed via a channel that produces H atoms concomitant with CH and H_{2} photofragments. The recoil of these slower H atoms appears to be isotropic. The measured, state‐resolved H_{2}(v,J), photofragment images reveal that two channels lead to H_{2} photofragments from the two‐photon photolysis of methane: a channel that leads to H_{2} products concomitant with methylene fragments; and a channel that leads to H_{2} products concomitant with CH and H fragments.
H_{2}(v,J) rotational and vibrational distributions are measured for each of these two channels separately. The H_{2} products formed via the H_{2}+CH_{2} channel are rotationally and vibrationally highly excited, whereas those formed via the H_{2}+CH+H channel are rotationally and vibrationally cooler. Rotational distributions of H_{2} formed via the H_{2}+CH+H channel are well reproduced by Boltzmann distributions. Results on D_{2} elimination following two‐photon photolysis of CD_{4} are in general similar and in qualitative agreement with the results on CH_{4}.

A simplified approach to optimally controlled quantum dynamics
View Description Hide DescriptionA new formalism for the optimal control of quantum mechanical physical observables is presented. This approach is based on an analogous classical control technique reported previously [J. Botina, H. Rabitz, and N. Rahman, J. Chem. Phys. 102, 226 (1995)]. Quantum Lagrange multiplier functions are used to preserve a chosen subset of the observable dynamics of interest. As a result, a corresponding small set of Lagrange multipliers needs to be calculated and they are only a function of time. This is a considerable simplification over traditional quantum optimal control theory [S. Shi and H. Rabitz, Comp. Phys. Comm. 63, 71 (1991)]. The success of the new approach is based on taking advantage of the multiplicity of solutions to virtually any problem of quantum control to meet a physical objective. A family of such simplified formulations is introduced and numerically tested. Results are presented for these algorithms and compared with previous reported work on a model problem for selective unimolecular reaction induced by an external optical electric field.

Multidimensional semiclassical tunneling between asymmetric wells via two channels
View Description Hide DescriptionWe have developed a semiclassical method, based on the models proposed by Miller and co‐workers, for calculating tunneling effects in asymmetric double‐well systems. The procedure can be easily implemented within standard classical trajectory simulations and thus allows for explicit treatment of the full‐dimensional dynamics. We have applied the method to HSiOH cis–transisomerization.

Ab initio study of the ammonia ion–ammonia reaction paths
View Description Hide DescriptionThe three reactions NH^{+} _{3}+NH_{3}→NH_{2}+NH^{+} _{4}(proton transfer), NH^{+} _{3}+NH_{3}→NH^{+} _{4}+NH_{2} (atom transfer) and NH^{+} _{3}+NH_{3}→NH_{3}+NH^{+} _{3}(charge transfer) are studied in an ab initio framework. All geometry optimizations are carried out at the MP2 level, and a SDCI(TQ) calculation is performed at the optimized geometry. For the charge transferreaction, the energy is calculated as a function of the N–N internuclear distance. The intermediate complex is found to have D _{3d } symmetry. The geometry of the NH_{3}+NH^{+} _{3} system is optimized for each value of the N–N distance. For the proton transferreaction, the energy is calculated as a function of two variables which are the two N–H internuclear distances of the central part N–H–N of the complex. For each N–H–N configuration, other coordinates of the system are completely optimized. This approach shows that the atom transfer reaction can be interpreted as a charge transfer process followed by a proton transfer. The influence of the vibrational excitation of the NH^{+} _{3} reagent on the reaction is discussed.