Volume 98, Issue 3, 01 February 1993
Index of content:

Spectroscopic characterization of the hydrogen bonded OC–HI in supersonic jets
View Description Hide DescriptionThe hydrogen bond OC–HI has been characterized using high resolution microwave and infrared spectroscopies in supersonic seeded molecular jets. Ground state molecular parameters of the ^{16}O^{12}C–HI and ^{16}O^{13}C–HI isotopic species determined by the pulsed‐nozzle Fourier transform microwave supersonic jet technique include: for ^{16}O^{12}C–HI, B _{0} (MHz)=900.9522(1), D _{ J } (kHz)=2.519(1), C _{ N } (kHz)=0.94(18), χ(MHz)=−1346.238(13), χ_{ J } (kHz)=−8.27(31). The corresponding values for ^{16}O^{13}C–HI are 882.5997(2), 2.404(2), 0.87(19), −1349.481(17), and −7.76(28). This analysis is consistent only with a linear equilibrium dimer structure in which the proton is bound to the carbon atom of carbon monoxide. Other derived dimer parameters include: r(C–I)=4.271(2) Å, α_{av}=24.8°, k _{σ}(N m^{−1})=1.713. Infrared diode laser investigations provide a band origin frequency ν_{0} of 2148.549 040(29) cm^{−1} for the ν_{2} C≡O stretching fundamental vibration. This corresponds to a blue shift of 5.277 28(37) cm^{−1} relative to free monomer CO. Excited state molecular constants B _{2}=898.2728(33) MHz. and D _{ J } ^{(2)}=2.614(24) kHz are also determined. Line profiles are consistent with an excited state lifetime ≥0.54 ns.

Optical spectroscopy of the dimer system Cs_{3}Yb_{2}Br_{9}
View Description Hide DescriptionSingle crystals of Cs_{3}Yb_{2}Br_{9} were grown using the Bridgman technique. Well‐resolved absorption, luminescence, excitation, and Raman measurements down to 5 K are reported and analyzed. The crystal‐field splitting of Yb^{3+} in Cs_{3}Yb_{2}Br_{9} was determined and can be rationalized in terms of a trigonally distorted YbBr_{6} ^{3−} octahedron. Under near‐infrared (NIR) cw‐laser excitation of the ^{2} F _{7/2}→^{2} F _{5/2} transitions, Cs_{3}Yb_{2}Br_{9} shows up‐conversion luminescence in the green spectral region. Conversely, NIR ^{2} F _{5/2}→^{2} F _{7/2}luminescence can be excited at twice the energy of the ^{2} F _{7/2}→^{2} F _{5/2} transitions in the green spectral region. These effects are shown to result from cooperative processes within the Yb_{2}Br_{9} ^{3−} dimers. The cooperative dimer‐absorption oscillator strength of 4.0×10^{−12} corresponds to the square of the NIR absorption oscillator strength of 1.9×10^{−6}, as expected for weak coupling within the Yb_{2}Br_{9} ^{3−} dimer. By a numerical deconvolution the cooperative dimer luminescence spectrum can be quantitatively correlated with the NIR luminescence spectrum in which reabsorption effects are important.

Infrared spectral evidence of N≡C–C≡C–N≡C: Photoisomerization of N≡C–C≡C–C≡N in an argon matrix
View Description Hide DescriptionUltraviolet (UV) laser photolysis of dicyanoacetylene, N≡C–C≡C–C≡N, isolated in an argon matrix at 16 K produces the iso‐nitrile, N≡C–C≡C–N≡C, in sufficient concentration for direct Fourier‐transform‐infrared measurements of all five stretching vibrational fundamentals. The assignments for ν_{1} (2287.1 cm^{−1}), ν_{2} (2203.6 cm^{−1}), ν_{3} (2044.8 cm^{−1}), ν_{4} (1202.3 cm^{−1}), and ν_{5} (610.1 cm^{−1}) are supported by a normal coordinate analysis using 20 vibrational frequencies from 7 isotopomers and by ab initio results of Botschwina et al. The di‐isonitrile C≡N–C≡C–N≡C is also produced in small amounts and its infrared‐active asymmetric stretches assigned as ν_{4} (2114.9 cm^{−1}) and ν_{5} (1287.5 cm^{−1}). As isotopic vibrational frequencies for N≡C–C≡C–C≡N were measured for the first time, the disputed assignments for the three totally symmetric stretching vibrations were re‐examined through a normal coordinate analysis. Laser‐induced fluorescencemeasured by a Fourier‐transform‐spectrometer during photolysis reveals the ^{1}Σ_{ g } ^{+}←^{3}Σ_{ u } ^{+} electronic transition in dicyanoacetylene at T _{00}=25 607 cm^{−1} and four vibronic transitions involving two totally symmetric stretching states of the ground electronic state.

Continuous wave multiquantum electron paramagnetic resonance spectroscopy. III. Theory of intermodulation sidebands
View Description Hide DescriptionThe interaction of a spin 1/2 system with two continuous wave transverse electromagnetic fields is studied using the dressed‐atom formalism and the Floquet theory. The equation of motion of the density matrix in the presence of two fields is solved and used to analyze the response of the spin system to double irradiation under steady state. In the average frequency rotating frame, the diagonal elements of the density matrix oscillate at even harmonics of the frequency difference, while the off‐diagonal elements oscillate at odd harmonics. The spectral response of the spin system can be predicted by applying the conservation rules: The frequency spectrum is a consequence of the conservation of total angular momentum, while the resonance condition is the result of the conservation of energy. The interpretive and predictive nature of the theoretical framework presented is illustrated by the treatment of the classical Anderson experiment and the simulation of the splitting of the multiquantum signal at high frequency difference. Approximate expressions for the population differences and coherences are derived and graphic representation is used to study the general nonlinear dependence on spectral parameters. At low values of the saturation parameter S, the n‐quantum absorption is proportional to dT _{2} S ^{(n−1)/2}, where d=1/2γH. Therefore, the signal amplitude is proportional not only to T _{2}, but also to powers of T _{1} T _{2}, which makes the multiquantum signals more sensitive to relaxation rates than conventional one‐photon displays. The frequency difference swept line shape of the multiquantum signal depends on both T _{1} and T _{2}. However, when T _{2}≪T _{1}, the new spectroscopic dimension, namely, the frequency difference, can be used to determine the spin–lattice relaxation time. Several spectroscopic features of multiquantum signals are discussed in the context of the general mathematical equivalence of double irradiation and amplitude modulation spectroscopy.

The one‐atom cage effect: Continuum processes in I_{2}–Ar below the B‐state dissociation limit
View Description Hide DescriptionOptical spectra recorded with Ar and I_{2} in a He expansion exhibit fluorescence from an excitation continuum through a broad region of the discrete B←X transitions of I_{2} and I_{2}–Ar. This fluorescence emanates from B‐state I_{2} and arises from excitations of a bimolecular I_{2}–Ar van der Waals complex. These results were obtained in order to test a proposed mechanism for the one‐atom cage effect in I_{2}–Ar, whereby continuum excitation to the repulsive Π_{ u } state precedes coupling onto the B state, dissociation of the complex, and fluorescence from B‐state I_{2}. The variation of the relative intensity of the observed fluorescence with excitation wavelength can be adequately reproduced with this model, but the Π_{ u }←X transition is much too weak to explain the observed absolute intensities. We consider the possible existence of a linear I_{2}–Ar isomer in the expansion along with the well‐documented T‐shaped isomer. Large geometry changes for the linear isomer upon B←X excitation would result in highly dispersed Franck–Condon factors and thus split this stronger transition over a continuum. Both absolute intensities and wavelength dependences observed for fluorescence from continuum excitation fit well to the linear isomer model. Linear isomers could also be responsible for the one‐atom cage effect observed at higher excitation energies.

Resonance enhanced multiphoton ionization photoelectron spectra of CO_{2}. III. Autoionization dominates direct ionization
View Description Hide DescriptionIn (3+1) resonance enhanced multiphoton ionizationphotoelectron spectra (REMPI‐PES) of CO_{2}, photoionization competes with dissociation. In addition to direct photoionization,autoionization is possible through accidental resonances embedded in the continuum at the four‐photon level. Photoabsorption from these long‐lived autoionizing states leads to resonance enhanced above threshold absorption (REATA). REATA produces photoelectron terminations on the C̃ state of CO_{2} ^{+}. Previous experiments did not indicate whether the dissociation occurred at the three‐photon level or four‐photon level. REMPI‐PES of CO_{2} via several Rydberg states have been collected at a number of laser intensities, and it was found that the photoelectron spectra terminating on each individual ionic state do not change over the range of experimentally available laser intensities. This indicates that the dissociation of CO_{2} occurs at the four‐photon level. The long vibrational progressions in the PES indicate that the dominant ionization process is autoionization rather than direct ionization. Relative intensities of the X̃ and C̃ state components of the PES do change with intensity, confirming the C̃ state assignment and its five‐photon mechanism.

Intramolecular vibrational energy redistribution and trapping in centro‐symmetric chains by Fermi resonance
View Description Hide DescriptionA model of intramolecular vibrational energy redistribution (IVR) in regular chain or ring molecules has been studied under conditions where both Fermi resonance coupling between different modes and symmetric linear coupling between similar oscillators on neighboring groups are important. The relation between coupling parameters and the time scale for decay of an initial state, the rate of energy transfer through the molecule, and the appearance of energy localization is examined.

Variation of the electronic wave function with internuclear separation: High‐resolution spectroscopy of the A ^{3}Π(1) state of I ^{35}Cl near the dissociation limit
View Description Hide DescriptionFluorescence excitation spectra of the A ^{3}Π(1)−X ^{1}Σ^{+} electronic transition of I ^{35}Cl were acquired using a single‐frequency dye laser and a well collimated molecular beam. Data were collected for eleven vibrational levels in the range v’=11–34. Of these, ten were well enough resolved for the hyperfine structure of both nuclei to be observed. For the highest vibrational level studied, v’=34, only the splitting due to the iodine nucleus could be resolved. The measured hyperfine constants for the A state were found to vary significantly with vibrational energy, and were found to be inconsistent with an analysis based on a single electronic configuration [linear combination atomic orbital/molecular orbital (LCAO/MO)]. Instead, it is found that the unpaired electron on the iodine atom is primarily oriented perpendicular to the bond axis for large internuclear separations and only achieves the orientation predicted by the LCAO/MO description for r<3.0 Å. The chlorine orientation is roughly in accord with the LCAO/MO prediction. These conclusions were reached by analyzing the hyperfine constants in a separated atom basis set. Additionally, a perturbation was found in the spectra for the v’=27 level. Evidence is presented that the cause of this perturbation is an interaction with the weakly bound a ^{3}Π_{1} electronic state of ICl.

Molecular beam pump/probe microwave‐optical double resonance using a laser ablation source
View Description Hide DescriptionThe first successful pump/probe microwave‐optical double resonance experiment using a laser ablation/reaction scheme for molecular beam production has been performed. Pure rotational transitions at frequencies up to 52 GHz have been recorded for the transient refractory compounds YF, YO, and SrOH at a resolution of <30 kHz [full‐width at half‐ maximum (FWHM)]. The observed three lowest pure rotational transition frequencies of YF (X ^{1}Σ^{+}) were analyzed to produce an improved set of rotational constants,B=8683.6156(11) MHz and D=0.007 521(74) MHz. The three lowest pure rotational transitions of SrOH (X ^{2}Σ^{+}) were analyzed to give the spectroscopic parameters (in MHz), B=7470.8180(4), D=0.006 25(3), γ=72.706(1), γ_{ D }=−0.0021(2); b _{ F } (H)=1.713(2) and c (H)=1.673(5). The proton magnetic hyperfineinteractions were interpreted in terms of a molecular orbital description for the X ^{2}Σ^{+} state.

Rovibrational spectra of open‐shell van der Waals complexes: H_{2}–OH (X ^{2}Π)
View Description Hide DescriptionA variational basis function approach is described for the calculation of the rovibrational bound states and infrared spectrum of a van der Waals complex of a closed‐shell diatom and a ^{2}Π diatom using no angular momentum decoupling approximations. Using an ab initiopotential surface, the method is then applied to calculate the bound states and spectra of the system H_{2}–OH (X ^{2}Π), and results are given for the complex containing both para and ortho H_{2}. Interesting similarities and differences are discovered on comparing the results with those for the related Ar–OH and H_{2}–HF complexes.

Photodissociation spectroscopy of Nb_{ n }Ar_{ m } complexes
View Description Hide DescriptionThe optical absorption spectra of niobium clusters containing 7 to 20 atoms have been measured from 336 to 634 nm by way of photodissociation action spectroscopy of the corresponding van der Waals complexes with argon atoms: Nb_{ n } Ar_{ m } → ^{ hν} Nb_{ n } + m Ar. The clusters in this size range do not display discrete absorption bands characteristic of molecular behavior, but rather absorption cross sections which increase monotonically with decreasing wavelength. This behavior is in qualitative accord with the absorption behavior predicted by the spherical Mie model for small niobium spheres, however, the measured cross sections are 2–5 times larger than predicted over this wavelength range, with the smallest clusters displaying the largest deviations. Interpreted within the classical electrodynamic framework, these observations suggest that the absorption spectra derive oscillator strength from an incipient surface plasmon, redshifted from its predicted resonance frequency in the vacuum ultraviolet.

Photodissociation spectroscopy of the Mg^{+}–CO_{2} complex and its isotopic analogs
View Description Hide DescriptionMg^{+}–CO_{2} ion–molecule cluster complexes are produced by laser vaporization in a pulsed nozzle cluster source. The vibronic spectroscopy in these complexes is studied with mass‐selected photodissociationspectroscopy in a reflectron time‐of‐flight mass spectrometer. Two excited electronic states are observed (2) ^{2}Σ^{+} and ^{2}Π. The ^{2}Π state has a vibrational progression in the metal–CO_{2} stretching mode (ω_{ e } ^{’}=381.8 cm^{−1}). The complexes are linear (Mg^{+}–OCO) and are bound by the charge–quadrupole interaction. The dissociation energy (D _{0} ^{‘}) is 14.7 kcal/mol. Corresponding spectra are measured for each of the 24, 25, and 26 isotopes of magnesium. These results are compared to theoretical predictions made by Bauschlicher and co‐workers.

Solvation effects on the molecular 3s Rydberg state: AZAB/CYCLO octanes clustered with argon
View Description Hide DescriptionTwo color, 1+1, mass resolved excitation spectroscopy (MRES) is used to obtain molecular Rydberg (3s←n) spectra of azabicyclo[2.2.2]octane (ABCO) and diazabicyclo[2.2.2]octane (DABCO) clustered with argon. Nozzle/laser timing delay studies are employed together with time‐of‐flight mass spectroscopy to identify cluster composition. Population depletion techniques are used to differentiate between clusters with the same mass, but different geometries. A Lennard‐Jones 6–12 potential is used to model the intermolecular interactions and predict minimum energy cluster geometries and cluster binding energies. The experimental results are combined with the cluster geometry calculations to assign spectral features to specific cluster geometries. Three different excited state interactions are required to model the experimentally observed line shapes,spectral shifts, and cluster dissociation. The relationship between these model potentials and the cluster binding sites suggests that the form of the cluster intermolecular potential in the Rydbergexcited state is dictated by the distance between the argon and chromophore atoms. A comparison of results for ABCO(Ar)_{1} and DABCO(Ar)_{1} leads to the conclusion that the nitrogen 3sRydberg orbital in clusters of DABCO is delocalized.

A three‐dimensional study of NeICl predissociation resonances by the complex scaled discrete variable representation method
View Description Hide DescriptionA study of the vibrational predissociation of the van der Waals complex NeICl on the B electronic surface, NeICl (B, v=2) to Ne+ICl (B, v=1, j), was conducted using the discrete variable representation (DVR) formulation of the complex coordinate method. The resonance positions, widths, and wave functions were inferred from the complex eigenvalues and eigenvectors of the complex scaled Hamiltonian matrix. Assignment of the various states was done by the natural expansion analysis. The natural expansion analysis provides a rigorous criterion as to the separability of the different modes in a given coordinate system, and allows for quantum number assignment to the vibrational, stretching, and bending modes of NeICl. The rotational distribution of the ICl fragment after dissociation was obtained by the asymptoticanalysis of the tail of the complex scaled square‐integrable resonancewave functions and was found to be in good agreement with theoretical results previously obtained for this system by Roncero et al. [J. Chem. Phys. 92, 3348 (1990)] and with experimental measurements by Skene, Drobits, and Lester [J. Chem. Phys. 85, 2329 (1986)].

Resonance‐enhanced one‐ and two‐photon ionization of water molecule: Preliminary analysis by multichannel quantum defect theory
View Description Hide DescriptionExperimental results are presented for one‐ and two‐photon ionization of the water molecule, obtained using a near transform‐limited xuv laser. The single‐photon ionization results show rotationally resolved autoionizing resonances corresponding to members of Rydberg series (nd←1b _{1}; n=6–11) converging on the H_{2}O^{+}(100) vibrational state. The two‐color (1+1) multiphoton ionization results show rotationally resolved structure corresponding to Rydberg series (nd←1b _{1}; n≥6) converging on the H_{2}O^{+}(000) vibrational state. Typical linewidths below and above the H_{2}O^{+}(000) ionization threshold are 1 and 2 cm^{−1}, respectively. The experimental results are simulated by multichannel quantum defect theory (MQDT). The main features in the spectrum are reproduced in a treatment of the rotational channel interactions with partial l mixing. It is argued that remaining discrepancies between experiment and theory arise from perturbative interactions between the (nd←1b _{1}) levels and members of the (nd←3a _{1}) Rydberg series. Also, it is argued that in the (1+1) multiphoton ionization spectra lines may be missing due to selective predissociation.

Hyperfine structure of the NaK a ^{3}Σ^{+} state: Interaction of an electron spin with the sodium and potassium nuclear spins
View Description Hide DescriptionThe hyperfine structures of the NaK a ^{3}Σ^{+} state are observed for the first time by using a Doppler‐free spectroscopy named perturbation facilitated polarization (PFP) spectroscopy. The hyperfine constants A _{Na} and A _{K} of the a ^{3}Σ^{+} (v=4, 5, 9, and 12) levels are determined. The observed hyperfine splittings are found to be well explained by the Fermi contact interactions of an electron spin with the sodium and potassiumnuclear spins. The intensities and signs of the PFP spectra are calculated by using the determined hyperfine constants, and the results also agree with the observed ones. From the width of the observed spectra, energies of the spin–spin and spin–rotation interactions are obtained to be <5×10^{−3} and 5×10^{−5} cm^{−1}, respectively.

New electronic spectra of the CHFCl radical observed with resonance enhanced multiphoton ionization
View Description Hide DescriptionThe structures and optical spectroscopy of the CHFCl radical and cation were studied by ab initio molecular orbital calculations and by experiment. Ab initio calculations at the MP2/ 6–311++G** theory level found that the optimum structure of the CHFCl^{+} (X̃ ^{1} A’) cation is planar with r(C–H)=1.092 Å, r(C–F)=1.254 Å, r(C–Cl)=1.599 Å, ∠H–C–F=116.85°, and ∠H–C–Cl=122.14°. CHFCl (X̃ ^{2} A’) radical is nonplanar with r(C–H)=1.083 Å, r(C–F)=1.335 Å, r(C–Cl)=1.705 Å, ∠H–C–F=113.49°, ∠H–C–Cl=116.68°, and ∠F–C–Cl=114.44°. The ab initio angle between the F–C–Cl plane and the C–H bond is Φ_{ e }=38° and the inversion barrier is B _{inv}=1190 cm^{−1}. Using isogyric reactions to obtain empirical corrections, we calculate IP_{ a }(CHFCl)=8.37±0.05 eV. Ab initio vibrational frequencies are reported. The electronic spectrum of the CHFCl radicals was observed between 340–420 nm using one color, mass resolved, 2+1 resonance enhanced multiphoton ionization (REMPI) spectroscopy. The spectrum arises from two‐photon resonances with the planar F̃(3p) [λ_{laser}=406.7 nm, ν_{00}=49 160(20) cm^{−1}] and J(3d) [λ_{laser}=361.9 nm, ν_{00}=55 250(20) cm^{−1}] Rydberg states. A third laser photon ionized the radicals. Both states produced the same vibrational constants: ν_{2} (C–H deformation)=1280(30) cm^{−1}, ν_{4} (C–Cl stretch)=910(30) cm^{−1}, ν_{5} (CFCl scissors)=440(30) cm^{−1}, ν_{6} (OPLA)=980(30) cm^{−1}. The REMPI spectrum exhibited v ^{‘} _{6} = 1–6 hot bands of the CHFCl (X̃ ^{2} A) radical. Modeling of these hot bands with a double‐well potential gives the inversion barrier, B _{inv}=1180 cm^{−1}, and Φ_{ e }=42°.

The phenol dimer: Zero‐kinetic‐energy photoelectron and two‐color resonance‐enhanced multiphoton ionization spectroscopy
View Description Hide DescriptionThe two‐color, two‐photon (1+1’) resonance‐enhanced multiphoton ionizationspectrum of the hydrogen‐bonded phenol dimer has been recorded in reasonable agreement with previously reported spectra. However, more features are obtained in this work and this has allowed a more detailed analysis of the spectrum. Five intermolecular vibrations (out of a possible six) are observed for the S _{1} ^{donor} state, while only two modes are obtained for the S _{1} ^{acceptor} state. Zero‐kinetic‐energy (ZEKE) photoelectron spectra were recorded via different intermediate vibronic states. The spectrum recorded via the vibrationless level of the S _{1} ^{donor} state is rich in structure and indicates a large change in the geometry on ionization. Progressions in the intermolecular stretch mode and at least one other mode are obtained. ZEKE spectra were also recorded via a number of S _{1} ^{donor} vibronic levels, and the S _{1} ^{acceptor} vibrationless level. The lowest value measured for the ionization energy of the donor is 63 649±4 cm^{−1} (7.8915±0.0005 eV); this is over 2000 cm^{−1} lower than the previously reported value. No structure is resolved in the ZEKE spectrum of the acceptor, and it is suggested that this could be due to rapid internal conversion between the S _{1} ^{acceptor} and S _{1} ^{donor} states.

Statistical modeling of capture, association, and exit‐channel dynamics in the CH_{3} ^{+}/CH_{3}CN system
View Description Hide DescriptionThe ion–molecule reaction CH_{3} ^{+}+CH_{3}CN is known to have an association channel leading to CH_{3}CNCH_{3} ^{+} in competition with the exothermic binary channels H_{2}CN^{+}+C_{2}H_{4} and C_{2}H_{5} ^{+}+HCN. This reaction has been modeled using a master equation treatment incorporating weak collisions. The parameters required for the Rice–Ramsberger–Kassel–Marcus (RRKM) treatment have been found from an ab initio investigation of the CH_{3} ^{+}/CH_{3}CN energy surface. A means of including capture rate coefficients in the RRKM approach is developed, in which only the hindered dipole rotation is coupled with the reaction coordinate at large separations. Existing experimental data from ion cyclotron resonance (ICR) spectroscopy and a selected ion flow tube are fitted by the model in the pressure range 10^{−7}–0.3 Torr. The low pressure experimental results are accounted for by weak collisions of the complex with the bath gas (when M=He, <ΔE _{down}≳ and <ΔR _{down}≳∼100 cm^{−1}) corresponding to a collision efficiency β=0.05 for M=He and 0.14 for M=CH_{3}CN. Unimolecular rate coefficients for the (CH_{3}CNCH_{3} ^{+})* complex are calculated for all product channels at a range of temperatures from 300 to 600 K. The rate coefficient for radiative stabilization was found to be 225 s^{−1} at the conditions of the ICR experiment. The average lifetime of the complex was calculated to vary between 29 μs at 600 K to 0.47 ms at 300 K and the termolecular association rates from 3.4×10^{−24}–9.8×10^{−23} cm^{6} s^{−1} (M=He) and from 6.7×10^{−23}–2.2×10^{−21} cm^{6} s^{−1} (M=CH_{3}CN) over the temperature range 600–300 K.

Vibrational predissociation of methylnitrite using phase‐locked ultrashort laser pulses
View Description Hide DescriptionWe solve numerically the time‐dependent Schrödinger equation to study the behavior of a molecule interacting with two phase‐locked ultrashort laser pulses. The two‐dimensional model used in the calculations mimics the properties of the CH_{3}ONO molecule. The two pulses are identical except for their relative phase and are tuned to excite an upper electronic state of the molecule. After excitation the molecule predissociates and we calculate the dependence of the NO yield and of the NO vibrational population on the delay time between the pulses. Because the molecular wave functions representing the excited states created by each pulse interfere, the total product population created by the joint action of the pulses differs from the total population obtained when the two pulses act independently on the molecule. This type of experiment provides a means of using quantum interference for a modest control of photodissociation.