Volume 100, Issue 2, 15 January 1994
Index of content:
100(1994); http://dx.doi.org/10.1063/1.466558View Description Hide Description
Ten overtone and combination bands of iso‐dicyanoacetylene (NC3NC), produced by in situ pulsed ultraviolet laser photoisomerization of dicyanoacetylene in an argon matrix, have been assigned in our Fourier‐transform absorption spectra. Experimental band frequencies, relative band intensities, and derived anharmonicity constants are compared in detail to CEPA‐1 theoretical calculations. The agreement here, even for overtone band intensities, is excellent.
Optical–optical double resonance spectroscopy of the 5 1Σ+ g ‘‘shelf’’ state of Na2 using an ultrasensitive ionization detector100(1994); http://dx.doi.org/10.1063/1.466559View Description Hide Description
Using optical–optical double resonance and an ultrasensitive ionizationdetector, we have been able to extensively study a wide range of internuclear distances for the 5 1Σ+ g state of Na2. Using a combination of spectroscopic data and the ab initio calculations, we have constructed a hybrid potential which has an outer well at shelf region. For this hybrid potential, there are 54 vibrational levels in the inner well (which all have been observed in this work) and 12 vibrational levels in the outer well (of which only the last level has been observed in this work). Above the potential barrier, 78 additional vibrational levels have been observed in this work. These spectroscopic data represent about 99.8% of the potential well depth.
Slit jet infrared spectroscopy of hydrogen bonded N2HF isotopomers: Rotational Rydberg–Klein–Rees analysis and H/D dependent vibrational predissociation rates100(1994); http://dx.doi.org/10.1063/1.466560View Description Hide Description
High resolution IR laser direct absorption spectra in a slit jet are presented and analyzed for nitrogen (15N14N–HF, 14N15N–HF, 15N15N–HF), and deuterium (14N14N–DF) substituted N2HF isotopomers. Both 14N15N–HF and 15N14N–HF isomers are observed, indicating a sufficiently deep minimum in the hydrogen bondingpotential energy surface to quench internal rotation of the N2. The vibrationally averaged stretching potentials for each substituted species are recovered from rotational Rydberg–Klein–Rees (RKR) analysis. Features of the one‐dimensional (1D) potential surface such as hydrogen bond length (R H‐bond), harmonic force constant (k σ), and well depth (D e ) are then tested for isotopic invariance by direct comparison of the different isotopomers. Agreement among the various N substituted species for HF based complexes for either v HF=0 or 1 is excellent, and provides effective 1D potentials for the stretching coordinate between 3.39 and 3.75 Å. There is a 43 cm−1 (∼10%) strengthening of the hydrogen bond upon HF vibrational excitation, as quantitatively reflected in the experimental redshifts and the shape of the RKR potentials for v HF=0 and 1.
The hydrogen bond is further strengthened by D/H isotopic substitution; this is a result of reduced vibrational averaging over DF vs HF bending motion, yielding a more linear, and hence stronger, hydrogen bond geometry. In contrast to the nearly apparatus‐limited linewidths (Δνprediss∼7 MHz) observed for each of the N2HF isotopomers, the N2DF complexes yield significantly broadened lines with 73±9 MHz homogeneous linewidths due to vibrational predissociation. This tenfold increase in predissociation rates upon deuteration is in contrast to previous measurements in other HF/DF containing complexes, and indicates the importance of a near resonant vibrational channel to form N2(v=1)+DF(v=0). The energetic accessibility of this V→V channel would suggest an upper limit on the N2DF binding energy of D 0≤547 cm−1, which is also consistent with upper limits on D 0 from the rotational RKR analysis.
Picosecond time‐resolved multiplex coherent anti‐Stokes Raman scattering spectroscopy by using a streak camera: Isomerization dynamics of all‐trans and 9‐cis retinal in the lowest excited triplet state100(1994); http://dx.doi.org/10.1063/1.466561View Description Hide Description
A picosecond time‐resolved multiplex coherent anti‐Stokes Raman scattering(CARS)measuring system using a streak camera was constructed. Picosecond uv laser pulses (50 ps) were used for the photoexcitation and nanosecond laser pulses (7 ns) were used as ω1 and ω2 for CARS probing. The multiplex CARS signals were spectrally analyzed by a spectrograph, and time‐resolved and detected by a streak camera. This system enabled us two‐dimensional (the time and frequency domain) detection of CARS signals with picosecond time resolution. The cis–trans photoisomerization dynamics of all‐trans and 9‐cis retinal was investigated. In the case of all‐trans retinal, the CARS signals due to the ‘‘all‐trans‐like’’ T 1 state appeared in accordance with the decay of the S 1 fluorescence. In contrast, the appearance of the all‐trans‐like T 1 signals was markedly delayed in the case of the 9‐cis isomer. This indicates that there exists a T 1 state having ‘‘9‐cis‐like’’ conformation in the picosecond time region. The lifetime of this 9‐cis‐like T 1 state was obtained as 880±150 ps from the analysis of the rise of the all‐trans‐like T 1 state. The mechanism of retinal photoisomerization is discussed on the basis of the obtained picosecond time‐resolved data. It is concluded that the conformational change from the 9‐cis to the all‐trans form in the triplet manifold proceeds from the thermalized (not vibrationally excited) 9‐cis‐like T 1 state.
Structural measurements of hydrogen‐bonded van der Waals dimers and trimers by rotational coherence spectroscopy: 2,5‐diphenyloxadiazole with argon, methane, water, and alcohols100(1994); http://dx.doi.org/10.1063/1.466562View Description Hide Description
Picosecond time‐resolved polarized fluorescence experiments involving time‐correlated single‐photon counting have studied rotational coherence phenomena of hydrogen‐bonded and other molecular aggregates in their lowest excited singlet states. The experiments are supported by detailed simulations. Using the molecule 2,5‐diphenyl‐1,3,4‐oxadiazole (PPD) as a host species, experiments have compared van der Waals aggregates with Ar1,2, (CH4)1,2, (H2O)1,2, (CH3OH)1,2, (C2H5OH)1, and (1‐C3H7OH)1. Bare PPD, and the argon and methane aggregates all exhibit prominent J‐type recurrences. The resulting sums of rotational constants (B+C) are consistent with center‐of‐mass‐bound, three‐dimensional structures, having out‐of‐plane distances for the attached species of 3.3–3.5 Å. The 1:2 aggregates involving argon and methane exhibit additive spectral shifts and nearly additive rotational recurrence times. This shows that the sites for addition of consecutive species are equivalent.
Calculations of rotational constants confirm these findings. All except the Ar 1:2 cluster exist close to the prolate symmetric top limit. On the other hand, the excitation spectra of complexes involving hydrogen‐bonding species all show small complexation shifts at the 1:1 level and disproportionately larger shifts at the 1:2 level. Similar nonadditive behavior is seen for the rotational recurrence transients. Hydrogen‐bonded species differ from the nonpolar cases, since they show both prominent C‐type and J‐type transients. This shows that these species all differ significantly from prolate symmetric tops. Detailed simulations reveal that all of the hydrogen‐bonding species produce aggregates that involve a single hydrogen bond to one of the PPD nitrogen atoms. This imposes a planar type of structure on the 1:1 water and methanol complexes. On the other hand, the aggregates methanol 1:2, ethanol 1:1, and propanol 1:1 all involve a distinct out‐of‐plane twist, consistent with the increasing influence of dispersive interactions.
Hydrogen bond distances (N...H–O) are found to be in the range 2.7–2.9 Å, and the hydrogen‐bond angles (N–N...H–O), relative to the PPD long axis, range from 115° to 130°. In addition, the water and methanol 1:2 aggregates both contain hydrogen‐bonded dimer units that resemble the free dimers of each species as identified by infrared and microwave techniques. For example, we find the (O...H–O) distance in the methanol dimer complex to be ≊2.7 Å.
100(1994); http://dx.doi.org/10.1063/1.466563View Description Hide Description
In solid state nuclear magnetic resonance spectroscopy, several methods have recently been introduced for the purpose of restoring dipolar couplings into magic angle spinning experiments. These powerful techniques are useful for the measurement of internuclear distances and, more generally, for filtering and correlation experiments in noncrystalline materials. In the case of heteronuclear spins, the rotational echo double resonance experiment provides an elegant and practical approach to the spectrally nonselective reintroduction of dipolar interactions. In this article, we describe an approach to restore heteronuclear couplings which is spectrally selective—in particular we recouple the observed spins only with those nonobserved spins lying at exact resonance (and at multiples of the spinning frequency) in the triple resonance experiment. We refer to the technique as frequency‐selective dipolar recoupling.
100(1994); http://dx.doi.org/10.1063/1.466564View Description Hide Description
Magnetic circular dichroism(MCD), circular dichroism (CD), and axial absorption spectra of Na3Eu(ODA)3⋅2NaClO4⋅6H2O are recorded at 4.2 and 150 K between 4500 and 25 500 cm−1. MCD parameters, rotatory strengths, and intensities are calculated using a parametric model for 4f→ 4f transitions. It is shown that the sign and magnitudes of the MCD parameters are very sensitive to the sign and absolute values of the intensity parameters.
100(1994); http://dx.doi.org/10.1063/1.466565View Description Hide Description
The pure rotational spectrum of the MgF radical in its ground electronic state (X 2Σ+) has been recorded using millimeter/submillimeter direct absorption techniques. Transitions arising from the v=0, 1, 2, and 3 vibrational modes of the main magnesium isotopic species, 24MgF, have been observed. In addition, spectra of the isotopomers 26MgF and 25MgF in the natural abundances of magnesium have been detected. Rotational and fine structure constants have been determined for these species, as well as hyperfine parameters for the fluorine nucleus (I=1/2). For 25MgF, the hyperfine structure was also resolved arising from the magnesium nucleus, which has I=5/2, yielding the 25Mghyperfine and quadrupole constants. Comparison of these hyperfine parameters with those of the heavier alkaline–earth monofluorides and the free 25Mg+ atom suggests that there is an increase in covalent bonding in MgF vs its heavier fluoride counterparts. This behavior is also apparent in the hybridization of the wave function of the unpaired electron in MgF, which appears to consist of almost equal s and p character.
Rotational spectrum of a dark state in 2‐fluoroethanol using microwave/radio‐frequency‐infrared multiple resonance100(1994); http://dx.doi.org/10.1063/1.466566View Description Hide Description
Microwave/radio‐frequency‐infrared multiple resonance has been used with an electric‐resonance optothermal spectrometer to characterize a weak 21.6 MHz perturbation in the infrared spectrum of the ν14 C–O stretching vibration of 2‐fluoroethanol. The infrared spectrum of 2‐fluoroethanol was recorded at a resolution of ∼2 MHz using a tunable microwave‐sideband CO2 laser. The spectrum is fit by an asymmetric‐rotor Hamiltonian to a precision of 0.6 MHz, except for the transitions to the 413 upper state which are split into doublets by an interaction between the 413 level and a rotational level of a nearby background, or dark, vibrational state. Microwave/radio‐frequency‐infrared double and triple resonance reveals that the 413 level of the C–O stretching vibration is interacting with the 431 level of the dark state. The rotational constants determined for the dark state allow us to assign the perturbing state to the ν18+4ν21 combination vibration of the lowest energy conformer, where ν18 is the CCO bending vibration and ν21 is the C–C torsional vibration. From the weak ΔK a =2 matrix element between ν14 and ν18+4ν21 it is possible to derive a J=0 anharmonic interaction between these states of ∼3.5 GHz.
Intermolecular perturbation of a Jahn–Teller system: The triptycene⋅Ne n (n=1–3) van der Waals clusters100(1994); http://dx.doi.org/10.1063/1.466567View Description Hide Description
The effect of rare gas complexation on the electronically excited S 1(E’) state of triptycene (T), which is Jahn–Teller distorted, was investigated by two‐color resonant two‐photon ionization (2C‐R2PI) spectroscopy of the supersonically cooled van der Waals complexes triptycene⋅Ne n , n=1–3. These complexes afford unique possibilities to study the effects of weak intermolecular interactions on the intramolecular Jahn–Teller coupling. Since the atoms are adsorbed at high‐symmetry positions, the system symmetry is lowered from D 3h (n=0) to C 2v for n=1 and 2, but reverts to D 3h for n=3. A Jahn–Teller (A 1⊕E)⊗e coupling model including a uniaxial external strain component was applied successfully to calculate the S 1 state levels and S 1←S 0 electronic spectra of all three complexes. The spectrum of T⋅Ne3 was fully interpreted without inclusion of strain, implying a highly symmetric D 3h structure in which each of the three V‐shaped compartments of triptycene is occupied by a single Ne atom. In contrast, the vibronic spectra of T⋅Ne and T⋅Ne2 were fitted with a considerable uniaxial strain of −19.73 and 19.07 cm−1, respectively, confirming both the predicted C 2v geometry of the complexes as well as the equal magnitude, but opposite sign of the intramolecular distortion induced by one or two Ne atoms. These distortions correspond to a small change of the angle of the V‐shaped compartments by ∼0.8°. The spectra of T⋅Ne and T⋅Ne2 are much more complex compared to triptycene and T⋅Ne3 due to the splitting of the E vibronic levels in C 2v symmetry, and the appearance of additional transitions to levels that are weak or symmetry forbidden in bare triptycene. The calculated Jahn–Teller potential energy surfaces and vibronic wave functions are discussed and classified in terms of their symmetry and localization/delocalization properties.
100(1994); http://dx.doi.org/10.1063/1.466568View Description Hide Description
We apply two‐color laser‐induced gratingspectroscopy (LIGS) to obtain excitation spectra of the S 1(Ã 1 B 1)–S 0(X̃ 1 A 1) system of jet‐cooled azulene. The ground‐state depletion grating created by excitation with crossed beams is probed via either the S 2(B̃ 1 A 1)–S 0(X̃ 1 A 1) or S 4(D̃ 1 A 1)–S 0(X̃ 1 A 1) transitions. First‐order LIGS spectra of the S 1–S 0 origin band at several grating‐laser intensities reveal saturation‐induced line broadening and significant saturation dips at line center. We develop a simple model, based on the anharmonic grating created in the saturated regime, and use it to fit the first‐order data. From these fits we determine a saturation parameter and the linewidth of the S 1–S 0 origin absorption band, which is homogeneously broadened due to rapid internal conversion (τ∼1 ps) in the S 1 state. We also observe LIGS spectra of the azulene origin taken at the second Bragg order of the induced grating; the observed second‐order lineshapes are also fit with the anharmonic‐grating model.
Far infrared vibration‐rotation‐tunneling spectroscopy and internal dynamics of methane–water: A prototypical hydrophobic system100(1994); http://dx.doi.org/10.1063/1.466569View Description Hide Description
Thirteen vibration‐rotation‐tunneling (VRT) bands of the CH4–H2O complex have been measured in the range from 18 to 35.5 cm−1 using tunable far infrared laserspectroscopy. The ground state has an average center of mass separation of 3.70 Å and a stretching force constant of 1.52 N/m, indicating that this complex is more strongly bound than Ar–H2O. The eigenvaluespectrum has been calculated with a variational procedure using a spherical expansion of a site–site ab initio intermolecular potential energy surface [J. Chem. Phys. 93, 7808 (1991)]. The computed eigenvalues exhibit a similar pattern to the observed spectra but are not in quantitative agreement. These observations suggest that both monomers undergo nearly free internal rotation within the complex.
Interpretation of new 13CD3F submillimeter‐wave laser lines using high resolution infrared spectroscopy100(1994); http://dx.doi.org/10.1063/1.466570View Description Hide Description
A high resolution infrared spectrum of 13CD3F has been recorded in the wave number region 820–1350 cm−1 which covers four fundamentals ν2(A 1), ν3(A 1), ν5(E), and ν6(E). All strong and medium strong vibration‐rotation lines have been assigned in terms of a model which treats simultaneously these strongly Coriolis coupled four fundamentals. Fifty‐nine upper state parameters have been determined with the least squares method including 4390 infrared transitions as data. The standard deviation of the fit obtained is 0.56×10−3 cm−1. Ground state rotational parameters have been determined using ground state combination differences. Due to strong vibration–rotation interactions a large number of normally forbidden transitions have been observed which have allowed an accurate determination of the ground state axial rotational constantA 0 and the centrifugal distortion constant D K0. The results of the analyzedinfrared spectrum have made it possible to interpret many new laser emission lines from 13CD3F gas optically pumped with a continuously tunable high pressure pulsed CO2 laser. Making use of the coincidence of the 10 μm P and R branches of CO2 with the ν2, ν3, ν5, and ν6 bands, altogether 151 laser lines of 13CD3F between 10 and 61 cm−1 have been observed. These emission lines have been assigned as rotational transitions within the excited vibrational states and the ground vibrational state.
Spectral line shape parameters for HF in a bath of Ar are accurately predicted by a potential inferred from spectra of the van der Waals dimer100(1994); http://dx.doi.org/10.1063/1.466571View Description Hide Description
An interaction potential for Ar–HF, which was recently determined from extensive spectroscopic data for the van der Waals dimer and which includes dependence on the HF vibrational state, has been used with accurate close‐coupling molecular scattering calculations to predict line shape parameters for the pure rotational and the fundamental and first overtone vibrational bands of HF in a bath of Ar. Agreement with experiment is good; in fact, considering inconsistencies among the experimental values, the theoretical values may be the more reliable. This confirms the accuracy of the Ar–HF interaction potential, including the dependence on the HF vibrational level which is sensitively probed by the line shift cross sections.
100(1994); http://dx.doi.org/10.1063/1.466572View Description Hide Description
The X 2 A 2 ’ band of the photoelectron spectrum of NO− 3 is theoretically calculated in the framework of a multimode vibronic coupling model. Linear coupling between and within the X 2 A 2 ’ and the B 2 E’ electronic states of NO3 is found to be the important mechanism governing the dynamics. The necessary coupling constants are obtained from the ionization potentials of NO− 3 calculated at different geometries using ab initioGreen’s functions. Comparison of the theoretical with experimental results [A. Weaver, D. W. Arnold, S. E. Bradforth, and D. M. Neumark, J. Chem. Phys. 94(3), 1740 (1991)] for the PE spectrum shows good agreement if a temperature of the NO− 3 anion of 455 K in the experiment is assumed. A general theoretical treatment of thermal effects in vibronically coupled electronic states is presented. The ground electronic statepotential surface of NO3 is discussed in the framework of the vibronic coupling model. A shallow minimum at a C 2v geometry with a barrier height of 0.006 eV relative to the minimum energy D 3h configuration is found. It is too weak to deform the effective geometry of the molecule from D 3h to C 2v . Mode‐mode coupling is found to play a relevant role in the ground electronic state of NO3, in general.
High resolution infrared spectra of the ν3 vibron in natural sulfur and in the isotopically pure 32S crystal100(1994); http://dx.doi.org/10.1063/1.466573View Description Hide Description
We have studied by high resolution infrared spectroscopy the ν3 vibron band in natural and isotopically pure 32S sulfur crystals. From the comparison of the spectra we have assigned the bands due to the different molecules containing one or two atoms of 34S as well as one atom of 33S. We have measured the bandwidth and the frequency of the vibron as a function of temperature from 10 to 140 K both in the natural and in the isotopically pure crystal and we have interpreted the results in terms of decay and scattering processes. In particular we have explained the nonlinear evolution of the bandwidth and of the frequency shifts with temperature in terms of scattering processes controlled by quartic terms of the crystal Hamiltonian.
100(1994); http://dx.doi.org/10.1063/1.466574View Description Hide Description
Theoretical calculations of the low lying excited states of the Zn atom and ZnH molecule are reported. An averaged relativistic effective core potential is used for the 1s 2 2s 2 2p 4 3d 10 electrons of Zn and the polarization and intrashell correlationeffects are treated with a semiempirical core polarization potential. Spin–orbit splittings are incorporated into the correlated electronic states.
100(1994); http://dx.doi.org/10.1063/1.467253View Description Hide Description
A path‐integral (PI) approach to real‐time quantum dynamics is developed which is suitable to treat the short‐time dynamics of vibronic‐coupling systems involving many degrees of freedom. The theory is formulated for the case of two electronic states which are coupled by a single active vibrational mode and whose energy separation is modulated by many so‐called tuning modes. Time‐dependent correlation functions are expressed as sums over all possible paths in the space of two electronic states in discretized time. For each electronic path, the multi‐mode vibrational propagator factorizes into a product of single‐mode propagators. Introducing the concept of classes of approximately equivalent paths, the summation over paths is replaced by a summation over classes and the computation of propagator averages within each class. It is shown that the propagator averages can efficiently be calculated by a recursive scheme. The performance of the PI method has been tested for a two‐state four‐mode model representing S 1–S 2 vibronic coupling in pyrazine. The PI results (time‐dependent correlation functions and absorption spectra) are compared with numerically exact reference data which are available for this model. To demonstrate the potential of the path‐integral approach for multi‐mode problems, calculations are reported for a twenty‐four‐mode vibronic‐coupling model.
100(1994); http://dx.doi.org/10.1063/1.466575View Description Hide Description
Results of the first theoretical study of the electronic structure of all the molecular states of BaLi dissociating into the six lowest limits Ba+Li are reported. The method used is similar to that used previously with success to describe the molecule BaH [J. Chem. Phys. 96, 7646 (1992)]. For the bound states2,4Λ(±) (spin–orbit effects neglected) and Ω(±) (spin–orbit effects included), all previously unknown values of a set of spectroscopic constants are displayed. They have been very helpful in the analysis of the (2)2Π→X 2Σ+ system of BaLi recently observed in our laboratory and presented in the following paper. An agreement of ≊1.5% for the energy T e of the (2)2Π state, as well as for rotational constantsB v=0 of both states (2)2Π and X 2Σ+ and of ≊9% for the spin–orbit parameter A v=0 of the (2)2Π state, is obtained between theoretical predictions and experimental observations.
Electronic structure of BaLi. II. First observation of the Ba6,7Li spectrum: Analysis of the (2)2Π→X 2Σ+ system100(1994); http://dx.doi.org/10.1063/1.466576View Description Hide Description
The thermal emission at high temperature (1100 °C) of BaLi has been analyzedspectroscopically at high resolution with a Fourier transform spectrometer. Molecular emission in the infrared region is observed and is ascribed to a transition from the (2)2Π state towards the ground state X 2Σ+ of BaLi. These states (among several others) have been predicted from ab initio calculations [see part I, A. R. Allouche and M. Aubert‐Frécon, J. Chem. Phys. 100, 938 (1994)]. Very good agreement is observed between theoretical predictions and experiment for the energy of the (2)2Π state and spectroscopic constants of both states. Both isotopic species Ba7Li and Ba6Li have been investigated and molecular parameters are derived from the analysis of the (2)2Π→X 2Σ+(0,0) band (the one analyzable band). No irregularities appear in the rotational structure.