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Volume 93, Issue 3, 01 August 1990

Raman overtone intensities measured for H_{2}
View Description Hide DescriptionThe Raman spectra of the vibrational fundamental, first overtone and second overtone transitions of the H_{2} molecule were recorded using visible and ultraviolet argon–ion laser excitation. The ratios of transition polarizability matrix elements, α_{01,21}/α_{01,11} and α_{01,31}/α_{01,11}, were determined from the measured intensities of the Q(1) Raman lines v,J=0,1→v’,1 for v’=1,2,3. The experimentally determined value of the Raman first overtone matrix element is in good agreement with the value from the best a b i n i t i o calculation.

Raman Q‐branch line shapes as a test of the H_{2}–Ar intermolecular potential
View Description Hide DescriptionUsing an intermolecular potential determined from spectra of van der Waals complexes [R. J. Le Roy and J. M. Hutson, J. Chem. Phys. 86, 837 (1987)] and accurate close coupling scattering matrices, line‐shape cross sections were calculated for vibrational Raman Q‐branch spectra of H_{2} and D_{2} in Ar to compare with recent experimental values. While predicted room temperature linewidths are in moderate (≊25%) agreement with experiment, predicted line shifts are small by nearly a factor of 2. The calculations show a strong collision speed dependence in the shift cross sections, in qualitative if not quantitative accord with the model developed by R. L. Farrow et al. [Phys. Rev. Lett. 63, 746 (1989)] to explain their H_{2} data. However, the calculations predict a strong temperature dependence in the linewidth cross sections as well, whereas the data show little variation between room temperature and 1000 K. It is suggested that these discrepancies point to inadequacies in the vibrational dependence of the intermolecular potential at close intermolecular separations (approximately 2.8 to 3.1 Å).

Hydrogen bond dynamics in isotopically substituted benzoic acid dimers
View Description Hide DescriptionThe hydrogen pair transfer in the hydrogen‐bonded dimers of benzoic acid and its carboxyl‐deuterated species is investigated in the solid.Measurements are reported for the temperature‐dependent NMRrelaxation timeT _{1} in single crystals containing dimers with one or two carboxylic deuterons. Combined with previous data, the temperature dependence of the measurements is analyzed in terms of a master‐equation description for a one‐ or two‐dimensional quantum‐mechanical model of the transfer motion. The description by a one‐dimensional model is found to be inadequate as it yields unrealistic isotope effects in the model parameters. On the other hand, reasonable results are obtained for a two‐dimensional model which includes, apart from the transfer motion of the hydrogens, a heavy atom mode with properties suggested by x‐ray structural data. This model explains the thermal activation of the transfer process mainly as a result of promotion of tunneling by heavy atom rocking. Activation energies remain considerably smaller than the barrier height and indicate predominance of tunneling even at room temperature.

Multiphoton spectroscopy of Rydberg states of tetrachloroethylene
View Description Hide DescriptionSpectroscopic studies of two‐photon resonant vibronic bands of the (π,n p) and (π,n f) Rydberg series of jet‐cooled C_{2}Cl_{4} are reported. Polarization ratio measurements for each transition and comparisons of experimental and calculated chlorine isotope shifts permitted the unambiguous assignment of vibronic band structure.A b i n i t i o self‐consistent field calculations of the harmonic frequencies for the optimized ground state geometry of C_{2}Cl_{4} were performed for each of the seven isotopic chlorine compositions to assist interpretation of the isotope shift measurements. The 4p‐Rydberg bands are quite weak and strongly perturbed, suggestive of localized Rydberg–valence state configuration mixing. The n f‐Rydberg series provides a measurement of the first ionization potential for C_{2}Cl_{4} of 75 216±5 cm^{−1} in good agreement with previous measurements. Mass analysis of ion fragmentation patterns reveals a multiphoton ladder switching excitation mechanism.

Photoionization studies of transition metal clusters: Ionization potentials for Fe_{ n } and Co_{ n }
View Description Hide DescriptionThe photoionization efficiency (PIE) spectra for cold iron clusters (Fe_{6}–Fe_{90}) and cobalt clusters (Co_{3}, Co_{7}–Co_{92}) have been measured using a tunable ultraviolet laser combined with time‐of‐flight mass spectrometry. Vertical ionization potentials (IPs) assigned from the observed photoionization thresholds decrease rapidly but nonmonotonically for both Fe_{ n } and Co_{ n } up to n≅20, beyond which IPs evolve more slowly and smoothly. The measured IPs for both iron and cobalt clusters are lower than those predicted by the conducting spherical droplet model. The evolution of IP with increasing cluster radius R is observed to occur more gradually than the R ^{−1}dependence predicted by this model. This suggests that the Fermi energy for clusters in this size range decreases with increasing cluster size. The shapes of the PIE spectra near threshold are observed to continuously evolve with increasing cluster size, with the smallest clusters displaying photoionization efficiencies which rapidly increase with energy above threshold, and larger clusters displaying spectra with more gradual slopes. It is proposed that this trend reflects the evolution in the density and distribution of low‐lying electronic states which occurs with increasing cluster size.

State selective doubly enhanced sum‐frequency mixing in CO
View Description Hide DescriptionDoubly enhanced sum‐frequency mixing (DESMI) of two collinear laser beams is studied using a pulsed jet of carbon monoxide as the nonlinear medium. The pump laser frequency (ω_{1}) is in two‐photon resonance with a selected rotational component of the A ^{1}Π–X ^{1}Σ^{+} (5,0) transition. Tuning the probe laser frequency (ω_{2}) to a sequential rotational component of the B ^{1}Σ^{+}–A ^{1}Π (1,5) transition leads to strong enhancement of VUV generation at frequency 2ω_{1}+ω_{2}. The double enhancement is shown to be subject to firm overall selection rules. It is demonstrated that DESMI can be accurate and efficient for the study of molecular states in the VUV and XUV provided that low laser powers are used. The high selectivity of the four‐wave mixing processes investigated allows a better understanding of the competition between four‐wave mixing, reabsorption of the sum frequency, and multiphoton absorption. In particular, it is shown that resonance‐enhanced multiphoton absorption is an important saturation mechanism at high laser powers.

Photon echo decays in optically dense media
View Description Hide DescriptionIn this paper we investigate the influence of optical density or superradiance on the photon echo decay using the coupled Maxwell–Blochequations. Our theory applies to optically thick but dilute media, where the transition dipole–dipole interaction between nearby chromophores is negligible, but the far‐field interaction is not. Corrections to the (normalized) echo decay in the optically thin limit, I ^{ N } _{ e }(t _{π})=exp [−4t _{π}/T _{2}] (t _{π} and T _{2} are the interpulse separation times and the coherencerelaxation times, respectively), are found by expanding the polarization,excited state population and pulse electric fields in powers of ζ=αz, where α is the inverse Beer’s absorption length, and z is the coordinate along the pulse propagation axis. Numerical solutions for optically thick samples are also presented, and when ζ≫1 the normalized echo intensity is found to obey the simple analytic form: I ^{ N } _{ e }(t _{π})=2 exp [−t _{π}/T _{1}]−1, when t _{π}/T _{1}<ln 2 and I ^{ N } _{ e }(t _{π})=0 for longer times. (T _{1} is the population relaxation time.) In this limit the echo decay i s i n d e p e n d e n t o f t h e c o h e r e n c e r e l a x a t i o n t i m e T _{2}, illustrating the profound effect intermolecular coupling may have on any nonlinear optical measurement. We show that at low temperatures, when T _{2}=2T _{1}, the echo decay rate increases as the optical density increases. However, for higher temperatures, when T _{2} is sufficiently shorter than 2T _{1}, the opposite behavior results, that is, the echo decay rate d e c r e a s e s as the optical density i n c r e a s e s.

An experimental potential energy surface for internal rotation in glyoxal
View Description Hide DescriptionThe torsional potential energy surface (TPES) for internal rotation of the CHO group in glyoxal CHOCHO has been derived experimentally by fitting observed energies for the torsional vibration ν_{7} in both t r a n s‐ and c i s‐glyoxal to those calculated with the hindered rotation formalism of Lewis, Malloy, Chao, and Laane. The experimental energies were obtained from S _{1} → S _{0} single vibronic level fluorescence (SVLF) spectra of jet‐cooled glyoxal. SVLF from the t r a n s levels 7^{3} and 7^{4} plus the c i s levels 0^{0} and 5^{1} yield the torsional vibrational energies of all 7_{ n } t r a n s levels with n≤14 (except for 7_{1} _{3}) and that of the c i s level 7_{2}. The energies of odd t r a n s 7_{ n } levels with n≥7 as well as spectroscopic values of any c i s 7_{ n } level were previously unknown. The best derived TPES fits the observed t r a n s and c i s levels to within 0.4 cm^{−1} except for two cases where the mismatch is 1.0 cm^{−1}. The TPES is defined by the potential energy 2V=∑^{6} _{ n=1} V _{ n } (1−cos nφ) where φ is the torsional angle. For the best TPES, coefficients are (in cm^{−1}) V _{1}=1719.4, V _{2}=1063.5, V _{3}=−53.2, V _{4}=−81.9, V _{5}=21.3 and V _{6}=2.9. For this TPES, the energy separation between the t r a n s and c i s potential wells is 1688 cm^{−1}, the barrier to t r a n s → c i s internal rotation is 2077 cm^{−1} and the barrier to c i s → t r a n s rotation is 389 cm^{−1}. As one moves from the t r a n s well at φ=0°, the torsional barrier occurs at 110°.

Molecular‐beam optical Stark spectroscopy of ScO
View Description Hide DescriptionThe molecular‐beam optical Stark spectrum of the A ^{2}∏(v=1)–X ^{2}∑^{+}(v=0) band system of ScO has been recorded and analyzed. The experimentally determined permanent electric dipole moments of the A ^{2}∏_{3} _{/} _{2}, A ^{2}∏_{1} _{/} _{2}, and X ^{2}∑^{+} states are 4.06(3), 4.43(2), and 4.55(8) D, respectively. The determined hyperfine parameters for the A ^{2}∏ state are a=135(1) MHz, d=177(2) MHz, and e Q q _{0}=−83(12) MHz. The spectroscopic parameters are interpreted in terms of plausible models for the electronic nature of the A ^{2}∏ and X ^{2}∑^{+} states and are compared with theoretical values.

Diode laser and Fourier transform infrared spectroscopy of jet‐cooled nickel carbonyl
View Description Hide DescriptionInfrared absorption spectra of Ni(CO)_{4} in the region of its 5^{1} _{0} band have been measured in supersonic jets using diode lasers and FTIR. The 5^{1} _{0} band consisted of an intense Q branch and well‐resolved P and R branches which clearly showed the effects of nuclear spin statistics. The analysis of 114 fundamental band transitions yielded the following effective parameters, which include small Coriolis effects (3σ uncertainties in parenthesis) ν̃_{0}=2061.314 4(45) cm^{−1}, B _{0}=0.035 721 3(30) cm^{−1}, B _{5}=0.035 670 6(30) cm^{−1}. The band center is blue‐shifted by 3.5 cm^{−1} from the room temperature value due to the contribution of hot bands and higher‐rotational distributions in the room temperature spectra. Weaker features near the 5^{1} _{0} Q branch in the diode laser spectra are assigned to the Q branches of two hot bands. Their spectra, optimized using higher concentrations of Ni(CO)_{4} in He or Ar seed gas, were analyzed to give rotation, vibration–rotation and anharmonicity parameters. The effective hot band centers are: ν̃_{0}(5^{1} _{0}4^{1} _{1}) =2060.996 4(50) cm^{−1}, ν̃_{0}(5^{1} _{0}8^{1} _{1}) =2061.114 8(48) cm^{−1}.

Fourier transform infrared study of tricarbon hydride radicals trapped in Ar at 10 K
View Description Hide DescriptionThe vibrational spectra of tricarbon hydride radicals formed by trapping the products of the vacuum ultraviolet photolysis of methylacetylene and various deuterated isotopomers in argon at 10 K have been investigated by Fourier transform spectroscopy in the range 200–3400 cm^{−1}. Supplementary studies have examined the photolysis products of allene as well as C_{3}H_{6} and C_{3}D_{6}, cyclopropane. The results reveal new information on the vibrations of C_{3}H; five C_{3}H_{2} isomers, cyclopropenylidene, vinylidenecarbene, triplet propargylene, t r a n s‐ and c i s‐propenediylidene; two C_{3}H_{3} isomers, propargyl and cyclopropenyl; and C_{3}H_{5}, allyl.

Fourier transform spectroscopy of C_{4}H (butadiynyl) in Ar at 10 K: C–H and C≡C stretching modes
View Description Hide DescriptionA Fourier transform infrared (FTIR) isotopic study of the C_{4}H (butadiynyl) radical produced by trapping the products from the vacuum ultraviolet photolysis of diacetylene (C_{4}H_{2}) and 1,3‐butadiene (C_{4}H_{6}) in solid argon at 10 K has been carried out in the midinfrared from 400–3900 cm^{−1}. A variety of carbon‐13 and deuterium substituted C_{4}H_{2} and C_{4}H_{6} parent molecules were used to produce various isotopomers of C_{4}H. On the basis of their isotopic behavior two absorptions at 3307.4 and 2083.9 cm^{−1} have been assigned to the ν_{1}, C–H stretching and ν_{2}, C 3/4 C stretching modes. The previous identification of the other C 3/4 C stretching mode, ν_{3}=2060.6 cm^{−1}, which was based on the effects of deuteration, has been confirmed and the corresponding frequencies for various ^{13}C‐substituted isotopomers have been measured. The following frequencies have been obtained for ν_{1}, ν_{2}, and ν_{3} for C_{4}H isotopomers: 3307.2, 2078.9, and 2023.8 cm^{−1} for H–C 3/4 C–C 3/4 ^{13}C; 3307.2, 2069.9, and 2032.5 cm^{−1} for H–C 3/4 C–^{13}C 3/4 C; 3307.2, 2066.8, and 2054.0 cm^{−1} for H–C 3/4 ^{13}C–C 3/4 C; 3293.3, 2048.9, and 2057.4 cm^{−1} for H–^{13}C 3/4 C–C 3/4 C; 3292.5, 2050.6, and 2024.6 cm^{−1} for H–^{13}C 3/4 C–C 3/4 ^{13}C; 3292.5, 2050.6, and 2030.0 cm^{−1} for H–^{13}C 3/4 C–^{13}C 3/4 C; 3306.3, 2063.3, and 2019.5 cm^{−1} for H–C 3/4 ^{13}C–C 3/4 ^{13}C; 3306.3, 2050.6, and 2018.0 cm^{−1} for H–C 3/4 ^{13}C–^{13}C 3/4 C; 3290.1, 2007.9, and 1981.6 cm^{−1} for H–^{13}C 3/4 ^{13}C–^{13}C 3/4 ^{13}C; and 2579.3, 2056.5, and 2049.7 cm^{−1} for D–C 3/4 C–C 3/4 C.

Magnetic circular dichroism and ground state splittings
View Description Hide DescriptionRecently reported measurements of the ground state splitting of atomic rhenium in a krypton matrix by the observation of MCDsaturation phenomena are considered. It is shown that the suggestion that this splitting comes from the admixing, via the spin–orbit interaction, of some ^{4} P _{5} _{/} _{2} character into the ^{6} S _{5} _{/} _{2} ground state cannot lead to a ground state splitting and higher order mechanisms must be invoked. The special properties of half‐filled shells are considered and the splitting mechanism outlined. Detailed calculations show the significance of configuration interaction.

Rotational analysis of the (2000) stretching vibrational band system of ^{1} ^{1} ^{6}SnH_{4}
View Description Hide DescriptionA high resolution Fourier transforminfrared spectrum, resolution (FWHM) 10.5×10^{−} ^{3} cm^{−} ^{1}, of the (2000, A _{1}/F _{2}) stretching vibrational band system of ^{1} ^{1} ^{6}SnH_{4} has been measured and analyzed in detail up to J=20. The rotational levels of the two vibrational statesA _{1} and F _{2} are treated simultaneously by including vibration–rotation coupling terms in the Hamiltonian. 1290 infrared transitions from the ground vibrational state to the excited (2000) states have been included as data in the least squares optimization of 21 Hamiltonian parameters. The standard deviation of the fit obtained is 1.1×10^{−} ^{3} cm^{−} ^{1}. The upper state vibrational energy level separation has been determined to be 0.0908 (5) cm^{−} ^{1} with the A _{1} state being higher in energy than the F _{2} state. The effective rovibrational parameters are found to satisfy closely the local mode constraints.

Complex kinetics of systems with multiple stationary states at equilibrium
View Description Hide DescriptionWe consider chemical reactions which at equilibrium have multiple stationary states due to nonidealities of chemical species. When such reactions are included in a simple reaction mechanism open to mass flow, without autocatalysis or feedback steps, there may occur complex dynamics such as relaxation oscillations, as reported earlier for regular solutions. Here we consider both regular solution and ionic species (Debye–Hückel nonideality), show that chemical oscillations may occur arbitrarily close to chemical equilibrium, and trace the topological structure of the complex dynamics of relaxation oscillations, sustained oscillations, stable focus, and stable nodes to the multiplicity of equilibrium states, for stated constraints. Relaxation oscillations occur around an unstable stationary state which, on approach to equilibrium, connects to an unstable equilibrium state. Thus, there is no bifurcation to oscillations on removing the systems from equilibrium. Neither is there a region where linear irreversible thermodynamics is valid close to equilibrium. Earlier work on ionic systems is found to be in error.

Radiative relaxation rate determination by competitive photodissociation of n‐butylbenzene ions
View Description Hide DescriptionThe radiative relaxation rate of n‐butylbenzene ions was measured using competitive photodissociation as a thermometric tool. Cooling curves showing the loss of internal energy of the ions with time were obtained at three pressures. An infrared radiative cooling rate of 0.5 s^{−1} was obtained from the data. This value is comparable to other cooling studies done in polyatomic ions having internal temperatures of the order of 200 K above room temperature. The radiative cooling of polyatomic molecules is compared with that of diatomics. Observations are in accord with theoretical expectations in showing that for polyatomics the infrared radiative cooling rate constants are slow and strongly dependent on internal temperature, in contrast to diatomics where they are fast and temperature independent.

Extensions to the instantaneous normal mode analysis of cluster dynamics: Diffusion constants and the role of rotations in clusters
View Description Hide DescriptionFor the instantaneous normal modeanalysis method to be generally useful in studying the dynamics of clusters of arbitrary size, it ought to yield values of atomic self‐diffusion constants which agree with those derived directly from molecular dynamics calculations. The present study proposes that such agreement indeed can be obtained if a sufficiently sophisticated formalism for computing the diffusion constant is adopted, such as the one suggested by Madan, Keyes, and Seeley [J. Chem. Phys. 9 2, 7565 (1990)]. In order to implement this particular formalism, however, we have found it necessary to pay particular attention to the removal from the computed spectra of spurious rotational contributions. The utility of the formalism is demonstrated via a study of small argon clusters, for which numerous results generated using other approaches are available. We find the same temperature dependence of the Ar_{13} self‐diffusion constant that Beck and Marchioro [J. Chem. Phys. 9 3, 1347 (1990)] do from their direct calculation of the velocity autocorrelation function: The diffusion constant rises quickly from zero to a liquid‐like value as the cluster goes through (the finite‐size equivalent of) the melting transition.

The He–Co_{2} interaction revisited: Approximate rotational energy transfer efficiency indices and computed transport coefficients
View Description Hide DescriptionOf the several anisotropic interactions suggested over the years for the title system, two of the most recent ones have been tested against experimental total differential cross sections, energy lossangular distributions, and transport coefficients. In the present work, they are examined again in order to compare them in a calculation over a broader range of transport properties, using the approximate infinite order sudden approximation (IOSA) scheme for the dynamics and getting first‐ and second‐order contributions to the Chapman–Cowling expansion in the kinetic theory of dilute gaseous mixtures. The various observables are found to be very close to the experiments for both potential functions, thereby making it still difficult to establish clearcut differences between them from the study of the above properties alone. The novelty of the present study, however, is to show that various recently proposed efficiency indices for the rotational energy transfer (RET) collisions are able to exhibit marked differences between the two surfaces and to unequivocally select one of them as the more anisotropic of the two. Because of the low value of such indices, one is also able to explain why the IOSA approximation should work well for computing the transport coefficients of this system.

Configuration dependent crossed beam reactive scattering in an intense laser field
View Description Hide DescriptionWe model the reaction between crossed beams of atoms A and diatoms BC in an intense nonresonant laser field. Cross sections are in particular determined for laser switching between a pair of one‐dimensional potential curves which are free to rotate in space. The cross sections are dependent on the configuration or orientation of the laser field with respect to the initial relative velocity vector of the reactants; for computational simplicity, many of our calculations are performed for coincident field and relative velocity vectors. Contributions to the cross sections arising from interference between the phases of transition matrix elements, are highlighted by comparing with ones determined by random phase approximation. We also make comparison with the corresponding configuration averaged (and phase independent) cross section. Photon energy dependencies for the various cross sections are obtained at several field intensities and collision energies. We find that the shifting with photon and collision energy, of the range of partial waves from which a cross section draws, can dramatically affect the relationship between phase dependent and independent cross sections; the shifting modifies in particular the interference between transition elements. For a single field intensity and photon and collision energy, we also generate cross sections as a function of configuration angle. The structure of the configuration dependence is elucidated by decomposition into terms which are characterized by numbers of contributing photons.

Dynamics on potential energy surfaces with a conical intersection: Adiabatic, intermediate, and diabatic behavior
View Description Hide DescriptionThe nuclear dynamics on potential energy surfaces which are strongly vibronically coupled through a conical intersection is investigated by exact (numerical) integration of the time‐dependent Schrödinger equation. Results for realistic model systems including three nuclear degrees of freedom are presented: C_{2}H^{+} _{4}, pyrazine and NO_{2}. It was found previously for C_{2}H^{+} _{4} that the wave packet moves after an initial decay mainly on the lower adiabatic surface. This observation could be confirmed also for pyrazine and NO_{2}. By varying the coupling strength λ also other types of behavior could be identified. A transition from adiabatic via intermediate to a diabatic nature of the system dynamics is found upon decreasing λ. Population of diabatic and adiabatic states in the long time limit are calculated by classical phase‐space statistics. Analytic relations between the population of the diabatic states and the dynamical observables, coordinates and momenta, are derived. They allow for a deeper understanding of their long time limits.