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Volume 106, Issue 6, 08 February 1997

Magnetoinfrared spectra of matrixisolated NiH and NiH_{2} molecules and theoretical calculations of the lowest electronic states of NiH_{2}
View Description Hide DescriptionTwo vibronic transitions, ^{2}Δ_{3/2}(v=0)←^{2}Δ_{5/2}(v^{″}=0) and ^{2}Π_{3/2}(v=0)←^{2}Δ_{5/2}(v^{″}=0) at 923 and 2560 cm^{−1}, respectively, were observed for NiH in solid argon (and krypton) at 4 K. These Ω=3/2←Ω=5/2 transitions were shifted and broadened by magnetic fields of up to 4 T. Also, its ground state vibrational frequency has been observed in neon, argon, and krypton matrices. Bands in the 600–2000 cm^{−1} region were assigned as frequencies of the NiH_{2} (NiD_{2}, NiHD) molecule. Contrary to earlier ab initio calculations, this assignment implies that the molecule is strongly bent in its ground electronic state. This discrepancy was explored theoretically by considering the lowest triplet (linear) and singlet (bent) states of NiH_{2}. The relative energies of the two states have not been satisfactorily resolved, but the calculated structural and vibrational properties of the singlet state are in good agreement with the observations.

Optical detection of yttrium dicarbide, a “Tshaped” molecule
View Description Hide DescriptionThe yttrium dicarbide molecule, YC_{2}, has been identified in the reactions of laserablated yttrium with methane and other small hydrocarbons under supersonic jetcooled expansion conditions. A parallelpolarized electronic band system with origin at 12 889.5 cm^{−1} has been tentatively identified as an A_{1}–X̃ ^{2}A_{1} transition of a “Tshaped’’ isomer where the Y atom is bonded to the side of a C_{2} molecule. Weak vibronically induced perpendicular bands, representing Δυ_{3}=odd transitions, are also present. Extensive vibrational progressions in the excitation and the resolved fluorescence spectra have been analyzed to give the two lowest vibrational frequencies of the two electronic states: X̃ ^{2}A_{1} , ν_{2} (a_{1})=561.1(±2.8 cm^{−1}), ν_{3} (b_{2})=369.6 (±1.7 cm^{−1}); excited A_{1} , ν_{2}=494.5(±5.0 cm^{−1}), ν_{3}=331.6(±2.2 cm^{−1}). No bands involving the C–C stretching vibration (ν_{1},a_{1}) have been found. The frequency of the ν_{3} vibration in the ground state indicates that the barrier to internal rotation of the C_{2} group against the Y atom is quite high, but large anharmonic coupling between the ν_{2} and ν_{3} vibrations suggests that internal rotation is facilitated by excitation of the Y–C_{2} stretching vibration.

Collisioninduced double transition effects in the 3ν_{3} CO_{2} band wing region
View Description Hide DescriptionIR absorption beyond the head of the 00^{0}3–00^{0}0 (3ν_{3}) band of CO_{2} near 7000 cm^{−1} has been analyzed. This absorption is found to consist of two comparable intensity contributions, namely, the allowed band wing and a collisioninduced absorption (CIA) band. The band wing profile has been described by using a nonMarkovian theory and the rotational perturbation densities for CO_{2}–CO_{2} collisions, which was previously calculated from the intensity distribution in the highfrequency wing of the 00^{0}1–00^{0}0 CO_{2} band. The CIA component has a typical shape of CO_{2} CIA bands with the maximum at the double transition (00^{0}1+00^{0}2)–(00^{0}0+00^{0}0) frequency. The integrated binary coefficient of this CIA band was estimated to be B_{2}=(1.0±0.6)×10^{−5} cm^{−2} Amagat^{−2}. The CIA spectral moment theory has been used for the intensity calculation, which takes into account for the first time the collisioninduced vibrational force field in CO_{2} pairs. By comparing the calculated and measured intensity for the double transition, the polarizabilityanisotropy matrix element for the 2ν_{3} band has been estimated as β_{20}=0.26±0.08 a.u., value in reasonable agreement with an independent estimation from previous results of polarizability matrix elements of CO_{2}.

Molecular beam optical Zeeman spectroscopy of iridium mononitride
View Description Hide DescriptionThe Zeeman effect on the F1/2←3/2 hyperfine component of the R(0) line in the (0,0) band of the A^{1}Π–X^{1}Σ^{+} transition of iridium mononitride, IrN, was investigated using a supersonic molecular beam optical spectrometer. The magnetic g_{J} factor for IrN in the A ^{1}Π state was determined to be 0.96±0.05. The general applicability of Zeeman measurements is described and the g_{J} factor which is determined is related to plausible molecular orbital descriptions for IrN in the A ^{1}Π state.

First, third, and fifthorder resonant spectroscopy of an anharmonic displaced oscillators system in the condensed phase
View Description Hide DescriptionWe have obtained thorder response functions for a twolevel system described by displaced anharmonic potential surfaces coupled to a heat bath. The anharmonicity of the potentials has been taken into account as a perturbation of harmonic potentials. The heatbath was assumed to be an ensemble of harmonic oscillators. Coupling between the twolevel system and the bath was assumed to be bilinear. The calculations were done analytically using the Liouvillespace generating functional, which had been obtained by way of the pathintegral approach. The response functions have been defined in terms of lineshape functions with these lineshape functions being expressed in terms of the bath spectral density and the temperature. We have carried out model calculations of the first, third, and fifthorder optical processes for various parameters of anharmonicity to show that anharmonicity plays a minor role in linear absorption, impulsive pump–probe, and photon echoexperiments, but plays a major role, in some cases, in fifthorder twodimensional resonant spectroscopy which is proposed in this paper.

Onedimensional nutation nuclear quadrupole resonance spectroscopy for measurement of the electric field gradient tensorasymmetry parameter
View Description Hide DescriptionA novel onedimensional nuclear quadrupole resonance experiment to measure the asymmetry parameter of the electric field gradient tensor in polycrystalline specimens using nutation spectroscopy is reported. The complete data set, necessary for the reconstruction of the powder nutation lineshape, is recorded in a single scan provided by a train of short and intense onresonance radiofrequency pulses. A modified scheme is presented to deal with multiple wellresolved resonances to provide a frequency selective nutation method. The technique is successfully demonstrated for the two ^{35}Cl resonances in cyanuric chloride. A reduction of 1 or 2 orders of magnitude in the data acquisition time, relative to the usual twodimensional method, is achieved.

Spectroscopy of massselected rhodium dimers in argon matrices
View Description Hide DescriptionThe absorption(scattering depletion) spectrum and Raman spectra for Rh_{2} in an argon matrix prepared by the mass selected ion deposition technique have been obtained. The absorptionspectrum in the visible region shows a single broad transition centered near 495 nm Resonance Raman spectra obtained by exciting with eight visible lines (457.9–514.5 nm) of an argon ion laser give a single, sharp progression with up to four Stokes transitions. These data give cm^{−1} with ω_{e}x_{e} =1.83 (33) cm^{−1}, leading to an experimental dissociation energy of 1.4±0.3 eV. The Raman results are discussed in the context of previous theoretical predictions for the dimer.

Bounds on spin dynamics and the design of multiplepulse NMR experiments
View Description Hide DescriptionThe “universal bound on spin dynamics’’ proposed by So/rensen is examined in detail and shown to be of great assistance in the design of novel multiplepulse NMRexperiments. The efficiency of coherence transfer between all possible states of a spin system, including populations, singlequantum coherences, and multiplequantum coherences, is investigated. Examples are drawn from coherence transfer processes in quadrupolar coupled spin I=1 and 3/2 nuclei and weakly J coupled systems of two and three spin I=1/2 nuclei. It is found that many of the most commonly used NMR pulse sequences fail to achieve the maximum coherence transfer efficiency when applied to spin I=3/2 or to three spin I=1/2 nuclei. However, it is shown that, with knowledge of the universal bound, novel multiplepulse NMRexperiments that achieve optimal efficiency can be easily derived using computer optimization. The application of the universal bound to twostep coherence transfer experiments presents a number of conceptual difficulties. In particular, examples are presented where the product of the universal bounds on the two individual coherence transfer coefficients is larger than the universal bound on the overall transfer from the initial to the final state. These difficulties are resolved and explained in terms of the presence of a “residue’’ that is created together with the intermediate state. The universal bound is used to examine the conditions under which the effect of this residue can be suppressed and the constraints that this places on the design of optimal multistep coherence transfer NMRexperiments.

Timeresolved infrared diode laser spectroscopy of the ν_{1} band of the iron carbonyl radical (FeCO) produced by the ultraviolet photolysis of Fe(CO)_{5}
View Description Hide DescriptionThe infrared spectrum of the iron carbonyl radical FeCO generated by the 193 nm excimer laserphotolysis of iron pentacarbonyl Fe(CO)_{5} was observed by timeresolveddiode laser spectroscopy. The 85 lines, mostly observed as triplets split by the electron spinspin interaction, were assigned to the ν_{1} (CO stretch) band of FeCO. The electronic ground state of FeCO was confirmed experimentally to have ^{3}Σ^{−} symmetry. Molecular constants in the ground and ν_{1}vibrational states were derived from an analysis of the infrared spectrum combined with pure rotational lines in the lowest spin component Ω=0 observed by Fourier transform microwave (FTMW) spectroscopy. The rotational and centrifugal distortion constants in the ground state were determined as MHz and kHz, where the figures in parentheses are standard errors to be attached to the last digit. The spinspin and spinrotation coupling constants are λ_{0}=663.1(40) GHz and γ_{0}=−974(27) MHz, respectively. The origin of the ν_{1} vibrational band determined is 1946.470 60(12) cm^{−1}, which is consistent with the value derived from photodetachment spectroscopy of the FeCO^{−} anion. The collisional quenching rate of FeCO by Fe(CO)_{5} was measured to be 1.13(4)×10^{−10} cm^{3} molec ^{−1} s^{−1} by a kinetic study.

Molecular theory of electronic spectroscopy in nonpolar fluids: Ultrafast solvation dynamics and absorption and emission line shapes
View Description Hide DescriptionWe present a theory of time and frequencydomain spectroscopy of a dilute nonpolar solute in a nonpolar liquid or supercritical fluidsolvent. The solute and solvent molecules are assumed to interact with isotropic pair potentials. These potentials, together with the solute and solvent masses, are the only input in the theory. We arrive at expressions for the absorption and emission line shapes, which include the possibility of motional narrowing, and for the timeresolved fluorescence and transient holeburning observables, by assuming that the solute’s fluctuating transition frequency describes a Gaussian process. These expressions depend only on the average and variance of the transition frequency distributions in absorption and emission and on the normalized frequency fluctuation timecorrelation functions. Within our formalism the former are obtained from the solutesolvent and solventsolvent radial distribution functions, which are calculated using integral equations. The timecorrelation functions involve the timedependent solutesolvent Green’s function. Its solution depends upon the solute and solvent diffusion constants, which in turn are determined from the radial distribution functions. The theory compares favorably with computer simulation results of the same model. We then investigate the dependence of the various spectroscopic observables on the solvent density, the temperature, and the difference between the ground and excitedstate solute’s pair interaction with the solvent molecules. For example, since our theory for the timecorrelation functions captures both their short and longtime behavior, we can see how the crossover from inertial to diffusive dynamics depends on these variables. Our results are similar to a variety of experiments on solutes in both nonpolar and polar solvents.

Resonant iondip infrared spectroscopy of benzene–(methanol) _{m} clusters with m=1–6
View Description Hide DescriptionResonant iondip infrared spectroscopy has been employed to record clustersizespecific spectra of C_{6}H_{6}–(CH_{3}OH) _{m} with m=1–6 in the OH stretch fundamental region. The comparison of the spectra with the results of ab initio calculations on the pure methanol clusters enables the assignment of the hydrogenbonding architecture in the clusters. In all cases, the methanol molecules aggregate together in a single subcluster. With m=1, a single infrared transition is observed, redshifted from that of a free methanol momomer by 42 cm^{−1} due to π hydrogen bonding between benzene and methanol. The m=2 spectrum features two strong transitions at 3506 and 3605 cm^{−1}. The lower frequency peak is redshifted from the free monomer value by 175 cm^{−1} and is assigned to the proton donor in the methanol dimer subcluster. The proton acceptor, which would be a free OH stretch in the absence of benzene, is redshifted by 76 cm^{−1} due to a strengthened π hydrogen bond. In benzene–(CH_{3}OH)_{3}, three sharp OH stretch transitions are observed at 3389, 3435, and 3589 cm^{−1}. The comparison of these absorptions with ab initio calculations and with experiments on the pure methanol trimer leads to a structure for benzene–(CH_{3}OH)_{3} which incorporates a π hydrogenbonded methanol trimer chain, confirming the earlier assignment based on its ultraviolet spectrum. The 3589 cm^{−1} transition, due to the π hydrogen bond of the terminal methanol, is redshifted from the free monomer by 93 cm^{−1}, a value approaching that of the donor methanol in methanol dimer (−107 cm^{−1}). The lower frequency transitions in the m=3 spectrum arise from the donor–acceptor and donor OH stretches in the methanol trimer chain. The spectral characteristics change when m=4. The OH stretch transitions are all located in a region around 3320 cm^{−1} and are significantly broadened compared to the smaller clusters. By comparison with ab initio calculations, the methanol tetramer structure in benzene–(CH_{3}OH)_{4} is deduced to be a cyclic methanol tetramer. The spectra for m=5 and 6 are slightly redshifted but similar to m=4 and point toward cyclic structures as well.

Exact sixdimensional quantum calculations of the rovibrational levels of
View Description Hide DescriptionResults of comprehensive fulldimensional (6D) quantum calculations of the rovibrational levels of , for total angular momentum are presented. The calculations employed two 6D potential energy surfaces (PES)—the ab initio PES of Bunker and coworkers, and the semiempirical PES of Elrod and Saykally. This 6D study provides the first rigorous, approximationfree description of the bound stateproperties of , including the dissociation energy,tunneling splittings and their , dependence, frequencies of intermolecular vibrations and associated spacings, and quantum number assignments of the 6D eigenstates. Detailed comparison with 4D bound state calculations (for fixed HCl bond length) was made in order to assess the importance of including the intramolecular vibrations of the two HCl subunits for accurate calculation of various spectroscopicproperties of . Comparison of the 6D results with experimental data, while confirming that the ES1 PES is substantially more accurate than the ab initio PES, shows that there is room for further refinements, preferably using 6D bound state calculations.

Spectroscopic characterization of the and Rydberg states of the MgAr van der Waals molecule
View Description Hide DescriptionThe Mg(3s4dσ ^{3}D_{J})⋅Ar(^{3}Σ^{+}), Mg(3s4dπ ^{3}D_{J})⋅Ar(^{3}Π), ⋅Ar(^{3}Δ), and Mg(3s5pπ ^{3}P_{J})⋅Ar(^{3}Π) Rydberg states have been characterized via Resonance Enhanced TwoPhotonIonization (R2PI) spectroscopy of transitions from the longlived metastable states of the MgAr van der Waals molecule. The 4dπ, 4dδ, and 5pπ states are all strongly bound (D_{0}=1230±50, 1200±40, 1270±50 cm^{−1}, respectively). These bond energies are very similar to that of the groundstate MgAr^{+} “core’’ ion (D_{0}=1240±40 cm^{−1}), indicating very effective penetration of transversely aligned, diffuse Rydberg electron clouds on the Mg atom by the Ar atom, even for the lowlying n=4,5Rydberg states. The Mg(3s4dσ ^{3}D_{J}) ⋅Ar(^{3}Σ^{+}) state was substantially less bound, D_{0}=800±40 cm^{−1}, showing there is still some residual Mg(4dσ)/Ar(3pσ)^{2} electron–electron repulsion preventing penetration of the axially aligned Mg(4dσ)Rydberg electron cloud by the Ar atom. Successful computer simulations of the rotational structure of several of the vibrational transitions to the 4dΔ and 4dΣ states, assuming Hund’s case “b’’ upperstate character, resulted in values of 2.80±0.04 Å and 2.90±0.05 Å, respectively (compared to R_{0}=2.82±0.01 Å determined previously by others for the MgAr^{+} ion). Because the 4dπ and 5pπ states have similarly large bond strengths as well as similar asymptotic Mg(4d ^{3}D_{J}) and Mg(5d ^{3}P_{J}) atomic energies, they have “mixed’’ 4dπ/5pπ character.

Electronic structure of vanadium cluster anions as studied by photoelectron spectroscopy
View Description Hide DescriptionPhotoelectron spectra of vanadiumcluster anions, V _{n} ^{−} (3⩽n⩽100), were measured at a photon energy of 3.49 eV (355 nm) by using a magneticbottletype photoelectron spectrometer. The electronic densityofstate profiles and the photoelectron spectra of V_{4} ^{−} and V_{3} ^{−} were calculated by the spinpolarized DV (discrete variational)Xα method for several plausible geometrical structures. The most plausible structure was determined so that the calculated photoelectron spectrum based on this structure well reproduces the observed one. The calculation showed that V_{4} ^{−} has a square planar geometrical structure and a charge distribution of D_{4h} symmetry. On the other hand, V_{3} ^{−} was found to possess an equilateral triangle geometrical structure, but C_{2v} symmetry for the charge distribution. The electronic states in the vicinity of the Fermi energy were found to consist of 3d atomic orbitals. The population differences between the minority and the majority spins per atom turned out to be 4.6 and 1.7 for V_{4} ^{−} and V_{3} ^{−}, respectively. The result implies that the delectron spins are coupled ferromagnetically in these cluster anions. In addition, the sizedependence of the electron affinity of V _{n} was explained by a spherical conducting droplet model in the n≳9 range.

Mobility of atomic hydrogen in solid krypton and xenon
View Description Hide DescriptionAtomic hydrogen is produced in xenon and krypton matrix by in situ xray induced photolysis of the dopants water, butane, acetone, or methane and trapped interstitially forming the “caged hydrogen” with characteristic VUV absorption bands. Their thermal bleaching as investigated between 8 K and 45 K cannot be described by a firstorder process. A random walkmodel is introduced considering the hydrogen atoms to move in a thermally activated diffusive motion to traps where they become “invisible.’’ The temperature dependence of the mean trapping time is Arrheniustype and characteristic only for the matrix. Infrared experiments have confirmed a partial recovery of the dopant molecule only in krypton. Annealinginduced IR absorption bands are assigned to a new complex which involves two hydrogen atoms.

electric dipole moment measurement by Stark level crossing and e – f mixing spectroscopy
View Description Hide DescriptionThe paper presents the first permanent electric dipole momentmeasurements for state rovibronic levels. Two different methods were applied to obtain values. Stark effect induced level crossing registered as the changes of fluorescence linear polarization with external electric field yielded from one fit both the electric dipole moment value and the doubling splitting between substates of an individual rotational state. Another method consisted of obtaining the ratio from dependence of the forbidden line appearing in fluorescence as a result of – Stark mixing, along with direct measurement by RF – optical double resonance. The respective dipole moment values obtained are 5.9 – 6.4 D for the state as well as 4.5 – 4.8 D for the typical errors being ca. 12%–20%. The value for the latter state reflects diminution expected due to the admixture of the state caused by intramolecular interaction. Signal simulation and data fitting have been accomplished using direct Hamiltonian diagonalization accounting for Stark interaction within rotational states and in the initial, excited and final state.

Spectroscopy and ultrafast dynamics of the 2A _{1} state of Zhexatriene in gas phase
View Description Hide DescriptionAs in longer polyenes, the strong 1A_{1}→1B_{2} band in the UVspectrum of 1,3Z,5hexatriene (Zhexatriene) has a weak precursor, the 1A_{1}→2A_{1} transition. It was measured in this work by a conventional spectrometer. Whereas the wave number of the 0–0 transition is by 5400 cm^{−1} lower than that of the 1B _{2} origin, the vibrational contour indicates that the vertical transitions of thetwo bands nearly coincide. From the fast decay of the rotational anisotropy in the timeresolvedmeasurements we conclude that this band is perpendicularly polarized. We measured the lifetime of the 2A _{1} state after pumping it directly by 250 fs pulses and probing the excited molecules by ionizing it by delayed pulses. The lifetimes decreased from several ps to 730 fs, when the excess energy was increased from near 0 to 4000 cm^{−1} and more. From the temperature dependence we infer a barrier of about 170 cm^{−1} (2 kJ/mol).

Time–frequency theory of pumpprobe absorption spectroscopy
View Description Hide DescriptionA variation of density matrix formulation based on the nature of field–matter interference in a mixed time–frequency domain is developed to study molecular pumpprobe absorption spectra in condensed phases. Considered are both the integrated probe transmitant signals and the frequencydispersed transient absorption coefficients for molecular systems with either two or three electronic surfaces involved. The present formulation is exact and applicable to any field with arbitrary timescale and shape, and it is valid when the pulses are overlapped as well as separated. The dual temporalspectral effect of both the excitation and detection fields can be clearly elucidated via a transformation that preserves causality. The resulting field–matter interference picture of transient absorption in the correlated timefrequency domain is conceptually natural and physically transparent. The molecular dynamics, and the field–matter temporal/spectral coherence and interference phenomena can all be clearly demonstrated.

An adiabatic exponential perturbation theory for rotationally inelastic scattering
View Description Hide DescriptionWe develop a perturbation theory to treat rotationally inelastic scattering using an adiabatic basis set. The results for Ar+N are twice as accurate as those using a diabatic basis set. The theory can be trivially extended to include closed channels. It can also be simply recast into the exact integration of a set of semiclassical coupled equations. In this mode it agrees to better than 1% with the exact quantal results.

Photodissociation of binary metal metallocarbohedrenes
View Description Hide DescriptionThe 532 nm photodissociation of the three singly substituted, binary metal containing Metallocarbohedrenes, Ti_{7}MC (M=Y, Zr, or Nb), is investigated. The photofragments are identified through energy analysis of the fragment ions in a reflectron timeofflightmass spectrometer, a new technique in photodissociation studies that results in a more accurate identification of the photofragment mass than what would normally be achievable through mass analysis using timeofflight determinations. The findings reveal that the dominant mechanism proceeds through the loss of neutral Ti atoms for all clusters studied.