Volume 103, Issue 13, 01 October 1995
Index of content:
103(1995); http://dx.doi.org/10.1063/1.470555View Description Hide Description
Results are presented of a detailed experimental study of the infrared photofragmentation patterns of size‐selected SF6⋅Ar+ n cluster ions for n in the range 3 to 70. Line‐tuneable CO2 and N2O lasers have been used to excited the ν3 vibrational mode of the SF6 molecule which is followed by the loss of one and two argon atoms as the principal fragmentation routes. Which of the two processes is dominant depends quite strongly on the size of the cluster ion concerned, with very pronounced fluctuations in the relative intensities of photofragments being observed for cluster ions in the range SF6⋅Ar+ 3 to SF6⋅Ar+ 25. Only for SF6⋅Ar+ 3 is the fragmentation pattern markedly different from that found for the other ions; an observation that supports an earlier conclusion regarding the relative ionisation energies of the two constituents [Stace et al. J. Phys. Chem. 97, 11363 (1993)]. A summation of fragment ion intensities as a function of laser wavelength is used to determine infrared absorption profiles and these have been recorded for individual clusters containing up to 70 argon atoms. Clusters containing fewer than 40 argon atoms appear to form single structures, with both the absorption profile shapes and selected hole‐burning experiments suggesting that the number of isomers is small.
The presence of isomers only appears to become significant when the clusters contain more than 40 argon atoms. The observation of site splittings for the triply degenerate ν3 vibrational mode of SF6, together with the comparatively narrow linewidths seen for clusters containing between 15 and 40 rare gas atoms, indicates the presence of ordered structures. Such a conclusion implies that the clusters are solidlike rather than liquidlike. Overall, the results demonstrate that there is a clear correlation between those criteria previously used to identify the presence of stable cluster ion structures, i.e., mass spectra and unimolecular fragmentation patterns, and the corresponding infrared fragmentation patterns and absorption profiles. Of the ions studied, SF6⋅Ar+ 21 stands out as being particularly stable and worthy of future theoretical attention.
Random matrix treatment of intramolecular vibrational redistribution. II. Coriolis interactions in 1‐butyne and ethanol103(1995); http://dx.doi.org/10.1063/1.470556View Description Hide Description
A random matrix methodology has been applied to simulate the molecular eigenstate resolved infrared spectra of the 1‐butyne ν16 band and the ethanol ν14 band. In these methyl C–H stretch bands, each rotational transition is fragmented into a clump of molecular eigenstates. The frequencies and intensities of these discrete features carry information about the rate and mechanism of the intramolecular vibrational redistribution (IVR) which would follow the coherent excitation of the zero‐order state. The simulations include anharmonic and Coriolisx‐, y‐, and z‐type interactions. These interactions mix the bright state with the bath and also mix the bath states with each other. Since the vibrational identities of the bath states are assumed to be sufficiently mixed, the vibrational parts of the coupling matrix elements are treated stochastically following the development in Paper I of this series [J. Chem. Phys. 98, 6665 (1993)]. The rotational parts of the matrix elements are treated dynamically based on the known rotational quantum number dependence of the Coriolis effect. A stochastic treatment cannot expect to reproduce the detailed line positions and intensities of the experimental spectra, therefore three measures of IVR are used as the basis for comparison of the simulation with experiment.
The measures are the dilution factor φ d , the interaction width Δε, and the effective level density ρeff c . In the presence of multiple coupling mechanisms (near the best fit to the ethanol ν14 band), the correlations between φ d and Δε and the bright‐bath Coriolis coupling mechanisms follow the expected trends. It was also found that ρeff c is sensitive to the x, yCoriolis coupling among the bath states. The results were not sensitive to the z‐type Coriolis coupling among the bath states in the region of the ethanol simulation, but ρeff c was sensitive to it in the simulation of the 1‐butyne ν16 band. Best‐fit coupling parameters were obtained for both simulated bands. The rms bright‐bath z‐type Coriolis coupling was found to be 0.028±0.005 cm−1 which is about three times the value obtained from a naive approach which neglects the interaction of the multiple coupling mechanisms. A direct count vibrational level density, ρvib, provided good agreement with the experiments when a full treatment of the torsional modes was included and a 20% enhancement of the density from neglected diagonal anharmonicities was added. A method of quantifying the conservation of the rotational quantum number, K, is provided by the inequalities, ρvib≤ρeff c ≤(2J+1)ρvib. For 1‐butyne, ρeff c is closer to ρvib than for ethanol indicating that K is more nearly conserved. While this work treats only anharmonic and Coriolis coupling, the random matrix formalism provides the ability to treat a wide variety of coupling schemes.
Diffusion‐controlled electron transfer reactions: Subpicosecond fluorescence measurements of coumarin 1 quenched by aniline and N,N‐dimethylaniline103(1995); http://dx.doi.org/10.1063/1.470557View Description Hide Description
The fluorescence quenching of a 7‐aminocoumarin dye [coumarin 1 (C1)] by amine electron donors (aniline or N,N‐dimethylaniline) in methanol was examined by picosecond time‐resolved and steady‐state fluorescence measurements. The quencher concentration dependence of the data was analyzed using the classic Smoluchowski model and the Collins and Kimball model of diffusion‐controlled reactions. In addition, the Wilemski and Fixman model, which includes a distance‐dependent sink term, was used to analyze the data. We have conclusively shown that the Smoluchowski model does not describe either the C1‐aniline or the C1‐dimethylaniline fluorescence quenching data. It was found that the Collins and Kimball model accurately described the C1‐aniline data, but was inappropriate for the C1‐dimethylaniline results. The addition of a simple position‐dependent sink term to the Collins and Kimball model enabled both the C1‐aniline and the C1‐dimethylaniline time‐resolved data to be accurately described. Analysis with a model incorporating a nonadiabaticelectron transfer sink function revealed that both reactions have a strong distance dependence and that only the C1‐aniline reaction can be classified as solely nonadiabaticelectron transfer. Based on these analyses, we conclude that the C1‐dimethylaniline reaction encompasses both the adiabatic and nonadiabatic limits of electron transfer. We also analyzed the temperature dependence of the reaction rate using Marcus nonadiabaticelectron transfertheory to estimate the activation energy, the solvent reorganization energy, and the electronic coupling matrix element of the intrinsic electron transferreaction. The average bimolecular reaction rate found was 8.77×109 M−1 s−1 for the C1‐aniline reaction and 1.52×1010 M−1 s−1 for the C1‐dimethylaniline reaction.
103(1995); http://dx.doi.org/10.1063/1.470558View Description Hide Description
Resonance‐enhanced multiphoton‐dissociation spectroscopy has been used to measure the first highly resolved UVspectrum of the acetylene radical cation. The bent structure of the ionic Ã state could be proved. In the Ã–X̃‐spectrum progressions of all three bending modes of bent acetylene appear. Their frequencies, anharmonicities and estimated potential thresholds are given. Furthermore, for the origin and the 5a 2 0 band (out‐of‐plane cis‐bending mode) rotationally resolved spectra have been obtained and rotational constantsA, B, and D K have been determined as well as a directly measured value of the X̃‐state spin–orbit splitting. From rotational linewidths or envelopes information about single vibrational lifetimes has been deduced; a mode‐specific behavior of these lifetimes has been found. Several nonradiative processes involving isomerization to vinylidene seem to be important in the energy region of our spectrum.
103(1995); http://dx.doi.org/10.1063/1.470559View Description Hide Description
We use Stark spectroscopy to examine the nature of the excited states of CdSenanocrystallites. The Stark spectra we obtain are in the small coupling limit in which the changes induced by the electric field to the absorptionspectrum are small compared to the transition linewidths. Within this limit, we theoretically examine the dependence of the line shape of Stark difference spectra on the linewidth of the transitions involved. For systems such as CdSenanocrystallites, which have overlapping transitions coupled by the electric field, we find that the usual association of derivatives of absorption features with dipole moments and polarizabilities is problematic. We show that the Stark absorptionspectrum of the CdSenanocrystallites can be explained by polarizable and delocalized nonpolar excited states.
Dielectric response and a phenomenon of a narrow band absorption for a classical rotor in a double well potential103(1995); http://dx.doi.org/10.1063/1.470560View Description Hide Description
The theory of dielectric relaxation in a planar ensemble of polar molecules is presented for a model where dipoles may rotate in a conservative double well potential (DWP) U=U 0 sin2 ϑ. The evolution of wideband dielectricspectra as a function of the potential well depth U 0 is studied; both isotropic and anisotropic media being taken as examples. The spectra comprise the Debye relaxation and the quasiresonant Poley absorption regions. The exact theory is compared with a simple quasielastic bond/extended diffusion approximation. The latter is valid for a quantitative description for any value of the field parameter p=[U 0/(k BT )]1/2. For p≫1 the spectrum comprises one narrow absorption band, while the Debye relaxation region is markedly shifted to low frequencies. It is shown that in the long lifetime τ limit there exists a minimum absorption band Δν0(p). The quantity Δν0 becomes small for values of parameter p≫1. The dielectric relaxation in ice 1 is discussed in relation to this phenomenon.
103(1995); http://dx.doi.org/10.1063/1.470561View Description Hide Description
We use transient differential absorption experiments to investigate the ‘‘single dot’’ absorption line shape of CdSe quantum dots. We observe both a narrow (full width half maximum ∼5 meV) and a broad (∼50 meV) bleach component within the inhomogeneously broadened first absorption line of our samples. We deduce the single dot absorption line shape which is most consistent with the experimental results. This line shape, which contains structure in the lowest quantum dot absorption feature, explains the large ‘‘Stokes’’ shift seen in the full band edge luminescence of CdSe quantum dots. We discuss the structure within the context of several competing models. The data appear inconsistent with models which use surface states to explain the anomalous emission behavior of II–VI quantum dots. Instead they imply that exciton fine structure is observed in our samples.
103(1995); http://dx.doi.org/10.1063/1.470562View Description Hide Description
In a laser desorption jet‐cooling molecular beamspectrometer the concentration of translationally and internally cooled laser desorbed organic molecules that can be achieved is experimentally determined. Sensitive direct absorption detection of laser desorbed jet‐cooled diphenylamine (DPA) via cavity ring down (CRD) spectroscopy on the S 1←S 0 transition around 308 nm is used to measure the line‐integrated absolute absorption of the pulse of laser desorbed DPA molecules. The absolute cross section for the various vibrational bands of the electronic transition that is used, is determined in a separate two‐color ionization experiment. It is concluded that the optimum beam intensity that is obtained with laser desorption is comparable to the beam intensity that is obtained in the same spectrometer by conventional seeding of the desired species at a partial pressure of 10−4.
103(1995); http://dx.doi.org/10.1063/1.470563View Description Hide Description
The first experimental observation of the fundamentally important spin‐pair radicals, H...H, H...D, and D...D is reported. ESR(electron spin resonance) studies of rare gas matrices near 4 K containing extremely high concentrations of H and D atoms revealed the presence of these spin exchange‐coupled molecules whose ESRspectra are analyzed in terms of a 3Σ electronic ground state. The observed Dtensor (zero‐field splitting) for a specific isotopic spin pair was surprisingly constant in all four rare gases but showed large changes among the three isotopic species. As expected, the hyperfine splitting (A iso) for the spin‐pair was observed to be one‐half that of the free H or D atom.
103(1995); http://dx.doi.org/10.1063/1.470564View Description Hide Description
Dispersed fluorescence (DF) spectroscopy is used to explore the rovibrational structure of highly excited S 0 formaldehyde (H2CO). A narrowband laser excites formaldehyde molecules to a single S 1 rovibronic quantum state, and the resulting fluorescence is dispersed with a monochromator. DF spectra of ten vibrational levels with excitation in ν2, the carbon–oxygen stretch, and ν4, the out‐of‐plane bend, have been recorded, and the effective A, B, and Crotational constants are extracted. Five of the effective Arotational constants and seven of the effective B and Crotational constants are new to the literature. The dependence of these effective rotational constants on vibrational state are both calculated and discussed with regard to both the present and previous experiments. Particular attention is given to the manner in which that the effective Arotational constant depends on increasing excitation in ν4 due to the strong A‐axis ν4/ν6Coriolis interaction. For states where v 2 is less than two, quantitatively accurate values for the nonlinear dependence of the Arotational constant on quanta in ν2 and ν4 is achieved by a simultaneous consideration of the strong A‐axis ν4/ν6Coriolis interaction and the 11↔42, 11↔62, and 51↔2161 Fermi interactions.
103(1995); http://dx.doi.org/10.1063/1.470565View Description Hide Description
Excited state enol‐keto isomerization in salicylic acid (SA) monomer and dimer has been studied in a supersonic free jet expansion. Two carboxylic group rotamers of SA with significantly different photophysical properties are found in the expansion. Rotamer I, the major form of SA in the expansion, has an intramolecular hydrogen bond and can undergo excited state tautomerization reaction. Its S 1 origin is at 335.34 nm. Single vibronic level emission spectra are dominated by progressions in OH stretching (3230 cm−1), and in‐plane bending of the carboxylic group (240 cm−1). The spectra appear to consist of two components, normal (UV) and tautomer (BLUE) emissions, even at the origin. The intensity of the BLUE relative to the UV emission depends on the vibronic state, rather than the excess vibrational energy between the origin and 1100 cm−1. The fluorescence decay time profiles for both the emission components of rotamer I are identical within ∼1 ns experimental time resolution. A nonradiative decay process with an activation energy of ∼1100 cm−1 is deduced from an abrupt decrease in fluorescence lifetimes above this energy. The rotamer II cannot undergo excited state tautomerization. Its electronic origin is at 311.52 nm and emits only UVfluorescence. Upon increasing the concentration of the SA sample, a new spectrum is observed. Due to a nonlinear concentration dependence of the intensity and the propensity of SA to form dimers in solution, it is assigned to the SA dimer. This spectrum shows possible evidence of double proton transfer in the S 1 state.
Matrix isolation study of the interaction of excited neon atoms with SiF4: Infrared spectra of SiF+ 3 and SiF− 3103(1995); http://dx.doi.org/10.1063/1.470726View Description Hide Description
When a Ne:SiF4 mixture is codeposited at approximately 5 K with a beam of excited neon atoms, the infrared spectrum of the resulting deposit includes absorptions of SiF3, together with new absorptions which have been assigned to SiF+ 3 and SiF− 3. The results of ab initio calculations of the structure and ground‐state vibrational fundamentals of these two ion species are presented, and support the proposed assignments.
103(1995); http://dx.doi.org/10.1063/1.470566View Description Hide Description
Time resolvedelectron paramagnetic resonance(EPR) measurements on a photoexcitedchromiumdoped forsterite (Cr:Fo) single crystal are reported. The spectral changes with time, magnetic field, crystal orientation, microwave power and, in particular, photoexciting wavelength, provide a selective picture of the various chromiumdopants and the absorption–relaxation cycle associated with each optical excitation. Both Cr+4 ions lodged at tetrahedral (Td) and octahedral (Oh) sites as well as Cr+3 ions are detected. In particular, the laser-EPR technique enabled us to monitor the spin dynamics associated with the lasing center (Cr+4/Td) in the time regime of 200 ns–100 ms following a selective photoexcitation of the crystal between 532 and 1064 nm. The transient EPR signals associated with the lasing Cr+4/Td ions, exhibit a noticeable dependence on even small changes (∼0.5 nm) in the exciting wavelengths that correspond to the visible 3 A 2→3 T 1 and the near infrared 3 A 2→3 T 2 transitions. The transient magnetization associated with each absorption–relaxation cycle is quantitatively analyzed in terms of site selectivity due to the narrow band (i.e., low intensity) microwave detection following a narrow band optical excitation. Given this observed selectivity, it is suggested that laser-EPR may be employed to study intersite interactions and site structure versus optical function relationships in forsterite as well as other solids doped with transition metal ions.
An instanton approach to intramolecular hydrogen exchange: Tunneling splittings in malonaldehyde and the hydrogenoxalate anion103(1995); http://dx.doi.org/10.1063/1.470567View Description Hide Description
Calculations of hydrogen tunneling splittings are reported based on a combination of the instanton approach with quantum‐chemically computed potentials and force fields. The splittings are due to intramolecular hydrogen transfer in symmetric double‐minimum potentials in molecules such as malonaldehyde and the hydrogenoxalate anion. Potential‐energy curves along the tunneling coordinates and harmonic force fields at the stationary points are calculated at the HF/6‐31G** and HF/6‐31+G** level of theory, and combined to yield a complete multidimensional surface. All modes that are displaced between the equilibrium configuration and the transition state are included in the calculation. In the formalism, these modes are linearly coupled to the tunneling mode, the couplings being proportional to the displacements in dimensionless units. These couplings modify the instanton trajectory and subject it to fluctuations. It is argued that within the accuracy of the available potential‐energy surfaces, direct calculations of the instanton trajectory can be avoided and that the dynamics can be expressed with adequate accuracy in terms of the classical action integral calculated for the one‐dimensional potential along the reaction coordinate with corrections for the coupled modes. In addition, the fluctuations of the coupled modes which control the preexponential factor in the instanton rate equation are included in the adiabatic approximation. These approximations greatly simplify the tunnelingdynamics and permit its combination with real rather than model molecular potentials. It is shown that this approach accounts satisfactorily for the zero‐point level splittings in malonaldehyde and its monodeuterated isotopomer. Moreover, it yields a detailed picture of the effect of various skeletal modes, both symmetric and antisymmetric, on the observed splittings. The calculations are extended to produce predicted zero‐point level splittings for the hydrogenoxalate anion for which no experimental splittings are available as yet.
The structure of Nb3O and Nb3O+ determined by pulsed field ionization–zero electron kinetic energy photoelectron spectroscopy and density functional theory103(1995); http://dx.doi.org/10.1063/1.470568View Description Hide Description
The geometrical structures of the ground states of triniobium monoxide, Nb3O, and its cation, Nb3O+, have been determined by an experimental and theoretical study. Vibrationally resolved photoelectron spectra of an Nb3O cluster beam were obtained at 100 and 300 K using the pulsed field ionization‐zero electron kinetic energy technique. The spectra were simulated by calculating multidimensional Franck–Condon factors using the geometries and harmonic vibrational frequencies obtained from density functional theory for the minimum energy structures of the ion and neutral molecule. The rather remarkable agreement between the experiment and the simulated spectra establishes that Nb3O and Nb3O+ have planar C 2v structures with the oxygen atom bridging two niobium atoms. These are the most complex transition metal cluster structures to date to be characterized by gas phase spectroscopic techniques.
Computerized simulation and fitting of singlet–triplet spectra of orthorhombic asymmetric tops: Theory and extensions to molecules with large multiplet splittings103(1995); http://dx.doi.org/10.1063/1.470569View Description Hide Description
Motivated by our recent finding that the singlet–triplet bands of selenoformaldehyde involve an upper state with large zero field splittings, we have extended the theory and written a program for predicting and fitting such rotationally resolved spectra. Triplet state matrix elements for a case (A) basis have been developed, including corrections for centrifugal and spin–centrifugal distortion. The full Hamiltonian matrix has been symmetry adapted, simplifying the problem to four individual matrices of approximately equal size for molecules of orthorhombic symmetry. Diagonalization of these matrices yields triplet state energies that are in agreement with previous treatments using a basis in which the spin splittings are small relative to the rotational intervals. Methods have been developed for sorting the eigenvalues and assigning quantum labels regardless of the magnitude of the spin splittings. The calculation of the relative intensities of the rotational lines within a band has been programmed using transition moment matrix elements from the literature. The selection rules for various upper state symmetries have been developed in a form useful for the analysis of spectra. Band contour predictions of spectra for various coupling cases have been presented.
Dynamic studies of degenerate four‐wave‐mixing in an azobenzene‐doped polymer film with an optical pump103(1995); http://dx.doi.org/10.1063/1.470570View Description Hide Description
Degenerate four‐wave‐mixing was performed in azobenzene‐doped polymethyl methacrylate film under a low power continuous‐wave laser at 632.8 nm with a pulsed pump laser of 532 nm. Different temporal profiles of the degenerate four‐wave‐mixing signal were observed under different pump energies, which originated from the relaxation processes between the energy levels of azobenzene during the photoisomerization, especially the transferring processes between the triplet states of the two isomers, trans and cis.
Alignment of gas phase molecules by dynamic Stark effect with coherent narrow‐band ultraviolet laser pulses103(1995); http://dx.doi.org/10.1063/1.470571View Description Hide Description
Alignment of vibronically excited benzene (C 6 D 6) along a lab‐fixed axis in a selected rotational J, K, and m state is obtained by a UV–UV optical double‐resonance experiment without additional, static electric or magnetic fields. The dynamic Stark effect caused by a narrow band, coherent laser pulse leads to an energetic separation of the individual m levels and renders the selective excitation of ‖m′‖=J′ levels by a weaker probing laser pulse.
103(1995); http://dx.doi.org/10.1063/1.470572View Description Hide Description
Molecular beam depletion and fragment spectroscopy has been employed to study the absorption behavior of small hydrogen fluoride clusters [(HF) n , n=4–8] in the spectral region between 3100 and 3700 cm−1. As tunable infrared radiation source we used an IR‐seeded optical parametric oscillator(OPO) with a LiNbO3 crystal as nonlinear medium. Size‐specific information has been obtained by scattering the cluster beam from a secondary rare gas beam. In this way all spectral features are assigned to specific cluster sizes. While the qualitative agreement with earlier experiments is good, the new assignments differ by one or more monomer units. In general, it is found that the absorption bands must be assigned to larger clusters. For some cluster sizes a doublet structure is observed. This observation is discussed in terms of combination bands involving intra‐ and intermolecular modes; but the possibility of isomeric structures is considered, too.
103(1995); http://dx.doi.org/10.1063/1.470573View Description Hide Description
The dynamics of charge carrier trapping and recombination are measured as a function of ZnO cluster diameter by ultrafast pump–probe absorption spectroscopy. A finite spherical potential well model which shows good agreement with previous experimental work is employed to predict ZnO cluster diameters from absorption onsets. The rate of electron trapping is measured for clusters of 3.2 and 6.2 nm, and is found to increase with increasing cluster size. This increase in trapping rate for increasing cluster size is not consistent with either a diffusional or quantum mechanical picture of electron trapping. A mechanism for electron trapping involving trap‐to‐trap hopping is discussed whereby the number density of optically accessible deep traps must increase with increasing cluster size. Differences in the dynamics and in the ratio of interior to exterior atoms on the cluster are correlated and discussed. The time‐resolved absorption data of the subsequent electron–hole recombination shows the appearance of an early time signal which increases as the cluster size grows. The early time species decays away within the first 50 ps to a diameter‐independent plateau value via second‐order recombination, and is assigned to electrons trapped in the interior of the cluster. The electron–hole recombination is found to occur faster and to a greater extent in the largest nanoclusters.