Extensive ab initio calculations of the phenol⋅H₂O complex were performed at the Hartree–Fock level, using the 6‐31G(d,p) and 6‐311++G(d,p) basis sets. Fully energy‐minimized geometries were obtained for (a) the equilibrium structure, which has a translinear H bond and the H₂O plane orthogonal to the phenol plane, similar to (H₂O)₂; (b) the lowest‐energy transition state structure, which is nonplanar (C₁ symmetry) and has the H₂O moiety rotated by ±90°. The calculated MP2/6‐311G++(d,p) binding energy including basis set superposition error corrections is 6.08 kcal/mol; the barrier for internal rotation around the H bond is only 0.4 kcal/mol. Intra‐ and intermolecular harmonic vibrational frequencies were calculated for a number of different isotopomers of phenol⋅H₂O. Anharmonic intermolecular vibrational frequencies were computed for several intermolecular vibrations; anharmonic corrections are very large for the β₂ intermolecular wag. Furthermore, the H₂O torsion τ around the H‐bond axis, and the β₂ mode are strongly anharmonically coupled, and a two‐dimensional τ/β₂ potential energy surface was explored. The role of tunneling splitting due to the torsional mode is discussed and tunnel splittings are estimated for the calculated range of barriers. The theoretical studies were complemented by a detailed spectroscopic study of h‐phenol⋅H₂O and d‐phenol⋅D₂O employing two‐color resonance‐two‐photon ionization and dispersed fluorescence emission techniques, which extends earlier spectroscopic studies of this system. The β₁ and β₂ wags of both isotopomers in the S₀ and S₁ electronic states are newly assigned, as well as several other weaker transitions. Tunneling splittings due to the torsional mode may be important in the S₀ state in conjunction with the excitation of the intermolecular σ and β₂ modes.

A combined experimental and theoretical study of the 2‐naphthol⋅H₂O/D₂O system was performed. Two different rotamers of 2‐naphthol (2‐hydroxynaphthalene, 2HN) exist with the O–H bond in cis‐ and trans‐position relative to the naphthalene frame. Using Hartree–Fock (HF) calculations with the 6‐31G(d,p) basis set, fully energy‐minimized geometries were computed for both cis‐ and trans‐2HN⋅H₂O of (a) the equilibrium structures with trans‐linear H‐bond arrangement and C_s symmetry and (b) the lowest‐energy transition states for H atom exchange on the H₂O subunit, which have a nonplanar C₁ symmetry. Both equilibrium and transition state structures are similar to the corresponding phenol⋅H₂O geometries. The H‐bond stabilization energies with zero point energy corrections included are ≊5.7 kcal/mol for both rotamers, ≊2.3 kcal/mol stronger than for the water dimer, and correspond closely to the binding energy calculated for phenol⋅H₂O at the same level of theory. Extension of the aromatic π‐system therefore hardly affects the H‐bonding conditions. The barrier height to internal rotation around the H‐bond only amounts to 0.5 kcal/mol. Harmonic vibrational analysis was carried out at these stationary points on the HF/6‐31G(d,p) potential energy surface with focus on the six intermolecular modes. The potential energy distributions and M‐matrices reflect considerable mode scrambling for the deuterated isotopomers. For the a′ intermolecular modes anharmonic corrections to the harmonic frequencies were evaluated. The β₂ wag mode shows the largest anharmonic contributions. For the torsional mode τ (H₂O H‐atom exchange coordinate) the vibrational level structure in an appropriate periodic potential was calculated. On the experimental side resonant‐two‐photon ionization and dispersed fluorescence emission spectra of 2HN⋅H₂O and d‐2HN⋅D₂O were measured. A detailed assignment of the bands in the intermolecular frequency range is given, based on the calculations. The predicted and measured vibrational frequencies are compared and differences discussed.

The minimum energy structure of the cyclic water trimer, its stationary points, and rearrangement processes at energies <1 kcal/mol above the global minimum are examined by ab initio molecular orbital theory. Structures corresponding to stationary points are fully optimized at the Hartree–Fock and second‐order Møller–Plesset levels, using the 6‐311++G(d,p) basis; each stationary point is characterized by harmonic vibrational analyses. The lowest energy conformation has two free O–H bonds on one and the third O–H bond on the other side of an approximately equilateral hydrogen‐bonded O...O...O (O₃) triangle. The lowest energy rearrangement pathway corresponds to the flipping of one of the two free O–H bonds which are on the same side of the plane across this plane via a transition structure with this O–H bond almost within the O₃ plane. Six distinguishable, but isometric transition structures of this type connect six isometric minimum energy structures along a cyclic vibrational‐tunneling path; neighboring minima correspond to enantiomers. The potential energy along this path has C₆ symmetry and a very low barrier V₆=0.1±0.1 kcal/mol. This implies nearly free pseudorotational interconversion of the six equilibrium structures. The corresponding anharmonic level structure was modeled using an internal rotation Hamiltonian. Two further low‐energy saddle points on the surface are of second and third order; they correspond to crown‐type and planar geometries with C₃ and C_3h symmetries, respectively. Interconversion tunneling vibrations via these stationary points are also important for the water trimer dynamics. A unified and symmetry‐adapted description of the intermolecular potential energy surface is given in terms of the three flipping coordinates of the O–H bonds. Implications of these results for the interpretation of spectroscopic data are discussed.

The well-resolved absorption, excitation, and luminescence spectra of [Ir(ppy)2bpy]+ (ppyH = 2-phenylpyridine, bpy = 2,2'-bipyridine) in different media at cryogenic temperatures are presented. In solutions and glasses at ambient temperature the lowest energy excited state corresponds to an Ir - bpy charge-transfer excitation whereas in the crystalline host lattice [Rh(ppy)2bpy]PF6 the lowest excited state at 21 450 cm-1 is assigned to a 37r-r* excitation localized on the cyclometalating ppy- ligands. The next higher excited Ir - bpy charge-transfer state has shifted to 21 820 cm-', only 300 cm-I above the 3LC excited state. The close proximity of the 3LC and 3MLCT excited states and the large spin-orbit coupling constant of Ir3+ induce a strong mixing of charge-transfer character into the 3LC lowest excited states, resulting in increased oscillator strengths, reduced lifetimes, short axis polarized transitions, and a large zero-field splitting of 10-15 cm-1.

A model is presented to explain the formation and morphologies of surfactant-silicate mesostructures. Three processes are identified: multidentate binding of silicate oligomers to the cationic surfactant, preferential silicate polymerization in the interface region, and charge density matching between the surfactant and the silicate. The model explains present experimental data, including the transformation between lamellar and hexagonal mesophases, and provides a guide for predicting conditions that favor the formation of lamellar, hexagonal, or cubic mesostructures. Model Q²³⁰ proposed by Mariani and his co-workers satisfactorily fits the x-ray data collected on the cubic mesostructure material. This model suggests that the silicate polymer forms a unique infinite silicate sheet sitting on the gyroid minimal surface and separating the surfactant molecules into two disconnected volumes.

Making use of the phenomenon of light-induced spin-crossover in the [Zn1−xFex(ptz)6](BF4)2 spin-crossover system, very high diffraction efficiencies η can be achieved in non-degenerate four-wave-mixing. In the mixed crystal with x=0.1 and at 76 K, i.e. at a temperature where the system is predominantly in the low-spin state, a value for η of 43% was obtained. This is attributed to a phase grating due to the large difference in metal---ligand bondlength between the low-spin ground state and the light-induced high-spin state. The rate constant for the decay of the laser-induced grating as a function of temperature is found to be exactly twice the one of the high-spin→low-spin relaxation, as expected for a dilute system in the absence of cooperative effects.

The two title compounds were synthesized and investigated with the inelastic-neutron-scattering (INS) technique. They contain mixed YbMBr93- (M=Cr3+, Ho3+) dimers as discrete units, and the magnetic excitations of mixed Yb3+-Cr3+ and Yb3+-Ho3+ dimers could thus be observed. The Yb3+-Cr3+ dimer has three INS transitions, for which anisotropic exchange, as well as zero-field splitting of Cr3+, has to be included in the exchange Hamiltonian. For the Yb3+-Ho3+ dimer the effect of the exchange interaction manifests itself as a broadening and a splitting of the crystal-electric-field levels of the isolated Ho3+ ion. Taking into account the full (2J+1) ground-state multiplet of Ho3+, as well as anisotropic exchange, gives a satisfactory description of this dimer.

We present the results of a crystallographic and optical study realized on the system Sr_yBa_1-yFCl_xBr_1-x where x and y vary from zero to one. All mixed crystals studied were of tetragonal symmetry with the parent PbFCl structure. Complete structure determinations were performed for two single crystals with y=0.3 and x=1 respectively 0.7 (nominal composition). This system forms complete solid solutions, in contrast to previously published results.

Spectral hole-burning studies of nile red and cresyl violet in polyvinylbutyral and polyvinylformal films have been performed. From the shape of spectral holes under the influence of an electric field, the dipole moment difference between the ground and excited state of both dyes has been determined. The Stark effect was investigated at different positions in the inhomogeneously broadened absorption band of the guest molecules. The observed dipole moment difference decreases with increasing wavelength. This variation is caused by the matrix induced dipole moment. For nile red, which is a neutral and polar molecule, the distribution of induced dipole moments is strongly correlated with the orientation of its ground state dipole moment. In the case of cresyl violet perchlorate, which is a salt, this distribution is anisotropic for guests absorbing in the blue part of the inhomogeneous band but becomes more isotropic as the absorption wavelength increases. The wavelength dependence of the observed dipole moment is much stronger and is ascribed to the existence of the cresyl violet perchlorate salt in different states of solvation.

Holographic detection of spectral holes is demonstrated in a crystalline host material with signal-to-noise ratios of up to 104. Hole burning occurs in two Pr3+ sites in the Y2SiO5 lattice, in both cases due to population redistribution between the ground-state quadrupole levels. The signal contains contributions due to a resonant hole and several side holes and antiholes, a phenomenon not previously observed using the holographic technique. The diffracted spectrum is modeled in two ways. In the first case the transmission spectrum is used to determine the population gratings and thus the diffraction efficiency. In the second case the transition probabilities between ground- and excited-state Kramer's doublets are used to model the population gratings. The technique is applied to pseudo-Stark-effect measurements from which the crystallographic sites as determined by x-ray analysis are matched to the spectroscopic data presented here. The time decay of the diffracted signal is used to study nuclear spin-lattice relaxation. It is shown that at 1.6 K temperature-dependent phonon-induced processes make no contribution to this decay. The nonexponential time decay of the population upon radio-frequency irradiation resonant with a ground-state quadrupole splitting is attributed to Pr-Pr cross relaxation

A detailed study of the separation efficiency in the photoinduced electron transfer reaction between 9,10-dicyanoanthracene and biphenyl in acetonitrile is presented. Both transient absorption and photoconductivity indicate a separation efficiency of about 0.4. This value is in discrepancy with two of three previously reported efficiencies. The problems arising with too large donor concentrations and with the use of a secondary donor to determine the separation efficiency are discussed.

A study of the hole-burning mechanisms of bis[4-(diethylamino)-2-hydroxyphenyl]squaraine (DEAH) and bis[4-(disethylamino)-phenyl]squaraine (DEA) in hydrogen-bonding and non-hydrogen-bonding polymers is presented. Intramolecular H-bonding is only possible for DEAH. In all systems, the spectral holes are not persistent and decay with a distribution of rates ranging from 10-5s-1 to about 1 s-1, the time resolution of the experiment. In H-donating matrices, this distribution varies with the burning wavelength. From the hole-burning efficiencies and the kineticsof the hole refilling, four different types of nonphotochemical hole-burning mechanisms are postulated. The efficiency of these mechanisms depends mainly on the occurrence of processes slowing down the relaxation to the initial product state.

The rotational dynamics of nile red has been studied in polar protic, polar aprotic and non-polar solvents. In the non-polar and the aprotic solvents, with the exception of long alkanenitriles, the rotation dynamics is consistent with the prediction of the Stokes—Einstein—Debye hydrodynamics theory for slip and close to the stick boundary condition, respectively. However in protic solvents, the rotation dynamics can be explained in terms of the Stokes—Einstein—Debye hydrodynamics theory under stick boundary condition only if solvent attachment via hydrogen bonding is assumed. The anomalous behaviour observed in longer alkanenitriles has been assigned to the formation of a reverse micelle-like solvation layer around nile red.

The conformational analysis of TMABN by three different methods X-ray analysis, photoelectron spectroscopy, and UV molar absorption coefficient yields a twist angle of the dimethylamino group of 60-70° in the ground state, whereas DMABN is not far from planar in qualitative agreement with the predictions from force field calculations (QCFF/PI and MM3). Dipole moment determinations by the thermochromic method agree with those from other methods (solvatochromism, electrochromism and time resolved microwave absorption) in that the excited state dipole moment of TMABN is very large, as well as that of the TICT state of DMABN. Its value increases somewhat with solvent polarity. This is explained by a nuclear polarizability model. The force field calculations are used to predict twist angle values for various sterically hindered DMABN derivatives

Copper and silver, respectively, were introduced into single crystals of CsCdF₃. Our detailed electron paramagnetic resonance (EPR) study showed that both elements enter the Cd lattice site—copper as Cu²⁺, silver as Ag⁺, which then was converted into Ag²⁺ by x raying the corresponding samples. Cu²⁺ and Ag²⁺ were shown to present in their ground state a pseudostatic Jahn–Teller effect. Motional effects were observed in the respective EPR spectra and studied in some detail for Cu²⁺ as they are seen over a wide temperature range. Predictions of a stochastic Kubo model [J. Phys. Soc. Jpn. 9, 935 (1954)] were compared with the temperature dependent linewidths of the motionally averaged EPR spectrum. A power law (Tⁿ with n≂1.9) was determined for the temperature dependence of the reorientation frequency between 30 and 90 K.

Ab initio electronic structure calculations for phenol and the hydrogen-bonded complexes phenol · H₂O and d-phenol · D₂O were performed at the Hartree-Fock 4-31G and 6-31G^** levels. Both phenol and phenol · H₂O were fully structure optimized. Based on the minimumenergy structures so obtained, full normal coordinate analyses were carried out. The resulting harmonic frequencies were scaled and compared to available experimental data. The agreement is satisfactory and allows for an assignment of a majority of the bands observed in the experimental spectra. Comparison with previous calculations on (H₂O)₂ reveals a considerable increase in the strength of the hydrogen bond on going from (H₂O)₂ to phenol · H₂O.

We report on crystal growth and about physico-chemical studies on Sr_yBa_1−yFCl_xBr_1−x (y = 0, 0.5, and 1) compounds doped with Sm. Persistent spectral hole burning at 300 K is further reported on Sr_0.5Ba_0.5FCl_0.5Br_0.5:Sm single crystals.

The high-spin to low-spin (HS→LS) relaxation in the [Fe(ptz)6](BF4)2 spin-crossover system deviates strongly from first-order kinetics because of cooperative effects of elastic origin. The shift in horizontal and vertical displacement of the potential wells of the initial and final state relative to each other due to the build-up of an "internal" pressure is estimated from spectroscopic measurements. The HS→LS relaxation as such is described by the theory of nonadiabatic multiphonon relaxation in the strong-coupling limit, with a Huang—Rhys factor S ≈ 45 which is much larger than the reduced energy gap p. The sigmoidal relaxation curves in [Fe(ptz)6](BF4)2 result when a change in S of ≈ −1 and in p of 1 during the relaxation is taken into account.

The constructive interference between two Stark-effect-broadened holograms produced by spectral hole burning is discussed. The holograms are burned at the same frequency but at different external electric-field values. The phase difference is selected to be zero so that constructive interference between the waves diffracted by each grating occurs. Experimentally it is found that a dip in the hologram efficiency that is not predicted by previous theory occurs for all reconstruction external electric-field values in the region between the original burn values. This dip is interpreted as being due to the time nonlinearity of the hologram burn process. The dip corresponds to those molecules, oriented in a specific direction with respect to the electric field, for which no Stark shift occurs and that are therefore resonant with the laser during the production of both holograms. The width of the anomalous feature is close to that of the hologram when the hologram is reconstructed at the original burn external electric-field strength. Other molecular orientations may be selected by burning pairs of holographic gratings at other combinations of the frequency and the electric field.

The electron transfer reaction between 9,lO-anthraquinone (AQ) excited to the lower triplet state and 1,2,4trimethoxybenzene (TMB) in solvents of different polarity has been studied by nanosecond time-resolved resonance Raman spectroscopy. In no solvent was there evidence for the formation of a triplet exciplex. All the observed vibrations were characterized as AQ* or TMB" modes. The frequency of the band assigned to the C-C stretch of TMB" has been found to be very dependent upon the environment and this effect has been used to observe the separation of the geminate ion pair into free ions in polar solvents. The data strongly suggest that the ions are at van der Waals contact in the geminate ion pair and not separated by solvent molecules.

A series of [Cr(benzene)(CO)2L] complexes with L = PPh3, P(OMe)3, PPh2 ((−)-menthyl), P(OPh)2(O-(−)-menthyl), P(O-(−)-menthyl)3 were subjected to a nueleophile addition/acylation sequence to give trans-5,6-disubstituted cyelohexadienes. Low-to-moderate asymmetric induction was observed with the chira] ligands. Experimental and theoretical evidence for an alkylation at the metal center trans to the P ligand is presented, and a crystal structure determination of a [Cr(η5-cyclohexadienyl)(P(OMe)3)(CO)2SnPh3] complex is included.

The ion Ag²⁺ introduced into NaF shows a tetragonal electron paramagnetic resonance spectrum at 4.2 K which dynamically averages above ≂40 K. Uniaxial stress is used to show that the ground state is a strongly coupled E⊗ϵ Jahn–Teller state. The well‐resolved superhyperfine structure due to the F⁻ neighbors is analyzed with a linear combination of atomic orbitals picture. Optical absorption of as‐grown and treated crystals is further presented. The former ones show peaks at 202, 213, 219 nm due to Ag⁺. The latter ones present complex absorption spectra related to silver.

The 5T2(HS)1A1(LS) intersystem crossing rates have been determined for a number of Fe(II) coordination compounds between 10 and 270 K using time-dependent optical spectroscopy. Strong deviations from Arrhenius kinetics with nearly temperature independent tunneling at low temperatures and a thermally activated behavior at elevated temperatures with apparent activation energies smaller than the classical energy barrier were found. The tunneling rates range from ~10−6 s−1 for the doped spin crossover system [Zn1−xFex(ptz)6](BF4)2 to ~106 s−1 for the doped low-spin (LS) system [Zn1−xFex(bipy)3](PF6)2. The large range of 12 orders of magnitude in the low temperature tunneling rates as well as the activated region can be understood in terms of nonadiabatic multiphonon relaxation. Values for the Huang–Rhys parameter S of 40–50 and for the reduced energy gap p of 1–12 are estimated for the present series of compounds. The validity of an inverse energy gap law in the strong vibronic coupling limit with Sp is borne out by experiment.

The temperature dependent Raman spectra of the title compound confirm the presence of some rotational disorder of the NH₃ end groups below 112 K. The central carbon-carbon stretching mode around 865 cm^-1 is coupled to the order parameter of the incommensurate phase transition at 168 K. No other clear evidence of the incommensurate modulation appears in the Raman spectra between 112 and 168 K. In the Abma phase (above 168 K) a dynamic conformational gauche-trans equilibrium is observed. The corresponding enthalpy difference is estimated to be 18.4+or-6.5 kJ mol^-1.

Due to the fact that for d6systems there are a number of low-lying ligand field (LF) states the relaxation from excited states of Fe(II) coordination compounds is, in general, a very fast and radiationless process. In Fe(II) spin-crossover systems, however, the zero point energy difference between the two lowest states, namely the low-spin (LS) 1A1 and the high-spin (HS) 5T2 state, is of the order of kBT, and some systems can be converted quantitatively to the HS state well below the thermal transition temperature by irradiating either into MLCT or LF absorption bands of the LS species, with HS→LS relaxation rates as small as 10−6 s−1 at XXX10 K. It is also possible to achieve a light-induced transient population of a HS state in Fe(II) LS compounds, but in this case the HS→LS relaxation rates can be larger than 106 s−1 even at low temperatures. The HS→LS relaxation rates show strong deviations from Arrhenius kinetics with nearly temperature independent tunnelling below ˜70 K and a thermally activated behaviour above ˜100 K. The range of 12 orders of magnitude in the low temperature tunnelling rate can be understood in terms of nonadiabatic multiphonon relaxation, where in the strong coupling limit, with the Huang-Rhys parameter S much larger than the reduced energy gap p, an inverse energy gap law holds.

[Fe(ptz)6](BF4)2 (ptz=1-propyltetrazole) is an Fe(II) spin crossover system, which shows a light-induced low-spin (1A1)-->high-spin (5T2) conversion below ~50 K by irradiating into the spin allowed 1A1-->1T1 d–d absorption band. This phenomenon, known as light-induced excited spin state trapping (LIESST), is reversible, and a subsequent irradiation into the 5T2-->5E band results in a light-induced 5T2-->1A1 conversion (reverse LIESST). Single crystal absorption spectra of the title compound in the region of d–d transitions are reported. In addition to the well-established spin allowed 1A1-->1T1 and 1A1-->1T2 transitions of the low-spin species and the 5T2-->5E transition of the high-spin species two weak bands in the NIR are assigned to the spin forbidden 1A1-->3T1 and 1A1-->3T2 transitions. Direct irradiation into the 1A1-->3T1 absorption band at 20 K results in a quantitative 1A1-->5T2 conversion, proving that this low lying triplet state plays an important role in the mechanism of LIESST. A full kinetic scheme for LIESST and reverse-LIESST with the 3T1 state as intermediate state is developed, and the quantum efficiencies for the various intersystem crossing steps involved are given: they are of the order of unity for the first step from the initially excited 1T1 and 5E states to the intermediate 3T1 state, respectively. The branching ratio from the 3T1 state to the 1A1 and the 5T2 states is 1:4.

The observatiqn of photon-gated hologram formation in a boric acid glass doped with triphenylene is reported. The first photon excites triphenylene to its first singlet excited state and, through intersystem-crossing, populates the first triplet stateTI. The second photon excites TI to T,, where autoionization occurs, leading to the formation of a radical cation. The gatinglight populating TI via SI is spatially uniform, while the light exciting TI to T, is spatially modulated. The long lifetime of the first triplet state allows Recording with low light intensities (mW/cm2). The spatially modulated excitation light forms three gratings (educt, intermediate state, and product). The extent of the interaction between these gratings depends on the overlap between educt, intermediate, and product absorption and refraction spectra as well as on the reading wavelength. The holograms were read at 363.8 and 632.8 nm. When the gating light is blocked, the holographic efficiency stays constant when read at 632.8 nm but increases substantially when read at 363.8 nm.

In conditions of laser flash photolysis, the kinetics of decay of the absorption of the benzophenone radical anion show that free, solvated ions are formed after electron transfer between the title compounds in neat, dry acetonitrile. Furthermore, it is shown that the opposite conclusion claimed by Devadoss and Fessenden (J. Phys. Chem., 1990, 94,4540), Le., no ion pair dissociation, results from a misinterpretation of the transient decay rate.

It is shown that using an ESR spectrometer with magnetic field modulation and sweeping the temperature across T_c (at a constant and a very low magnetic field), is equivalent to temperature modulation. The signal intensity obtained when crossing T_c is proportional to 1/( delta H_c2/ delta T) at T=T_c. Using the WHH relation H_c2(T=0)=0.7 T_c( delta H_c2/ delta T)_T=Tc enabled the measurement of the relative angular variation of H_c2 in single crystals of YBaCuO with T_c approximately 85 K. The data fit the Ginzberg-Landau theory. This very sensitive technique can be used to characterize properties of high T_c superconducting materials. Results on thin films is also be presented.

It is shown that measuring microwave absorption in high-T_c superconductors at constant and very low magnetic fields, using magnetic-field modulation, is, under some conditions, equivalent to temperature modulation when sweeping the temperature across T_c. Using an ESR spectrometer, the derivative of microwave absorption is measured close to T_c. This allows a determination of the relative angular variation of dH_c2/dT at T=T_c in single crystals of Y-Ba-Cu-O. The data fit the Ginzburg-Landau theory on the relative angular variation of H_c2. The ratio (dH_c2/dT)_T=Tc parallel and perpendicular to the Cu planes was found to be 2.7 and 5.3 for two Y-Ba-Cu-O single crystals with T_c=89 and 86 K, respectively. These values obtained at 10¹⁰ Hz are close to the values obtained by conventional dc methods.

Recently, we have discovered a fascinating photophysical effect in spin crossover complexes of iron(II) : Light-Induced Excited Spin State Trapping (LIESST). At sufficiently low temperatures, the low spin state (1A1) can be converted quantitatively to the high spin state (5T2) by irradiating the sample into the 1A1 → 1T1 d-d absorption band (—540 nm). The resulting metastable HS state has a very long lifetime at low temperatures, in some cases it does not decay noticeably over a period of several days at 10 K. Only at temperature above some critical temperature does thermal relaxation back to the LS state set in. The sample can also be reconverted to the LS state by irradiating into the 5T2 → 5E absorption band (∞50 nm). The system thus behaves like an optical switch. The relative positioning - horizontally and vertically - of the potential wells of the two spin states is crucial for the lifetime of the metastable HS state.

Polarized Raman measurements on single crystals of CuO yield the symmetry assignments of the three predicted Raman active lattice modes : 297cm⁻¹ (A_g, 344cm⁻¹ (B_g) and 629cm⁻¹ (B_g). These results are compared to literature data, including IR spectra. Our measurements confirm at low temperature the appearance of an additional Raman band around 240 cm⁻¹. The temperature dependence of the linewidth of the A_g mode presents an anomalous behavior near the magnetic phase transition, suggesting the possible presence of magnon-phonon couplings in the antiferromagnetic phase.

We report on the growth of Nd_2-xCe_xCuO_4-δ single crystals (0<x<0.2) from Cu₂O flux. Free separated crystals with maximum size of 5x8x0.15 nm³ have been obtained. Magnetic AC susceptibility measurements show a sharp superconducting transition at temperatures up to 23 K. The temperature dependence of the lattice parameters has been measured by means of X-ray powder diffraction between 10 K (a=3.9413(3) Å, c=12.0290(18) Å) and 290 K (a=3.9482(3) Å, c=12.0590(18) Å). Room temperature Raman spectra reveal a new band at 320 cm^-1 which is not observed in Nd₂CuO₄. Raman spectra of crystals withT_c ranging from 7 to 22 K show a systematic intensity change of the broad band at 590 cm^-1.

The polarized Raman spectra of four different beryl crystals were studied at room temperature in the range from 30 to 4000 cm^-1. The spectra show significant differences between the samples studied, and corrections are proposed for the reference Raman spectra of beryl previously reported by Adams and Gardner (1974). Type II water is observed in two crystals; the corresponding symmetric Raman stretching band at 3595 cm^-1 is extremely strong for an impurity (about 20% of the strongest beryl lattice mode). Another, sharper, band of similar intensity at 3605 cm^-1 could possibly originate from a hydroxyl stretching mode. Additional weaker bands are observed around 1600 cm^-1 and 3600–3750 cm^-1. The first polarized Raman spectra of bazzite are presented and discussed.

A study of viscosity and temperature effects on the rate of back electron transfer (BET) within an exciplex (9,10-dicyanoanthracene/N,N-dimethylaniline) with a strong charge transfer character in six non-polar solvents is reported. The extent of charge transfer has been estimated from the solvatochromic and thermochromic shifts of the fluorescence. Conformational changes are a prerequisite to the BET. In non-viscous solvents, where they are much faster than the ET step itself, the observed rate can be explained within the theory of non-adiabatic ET reactions, while in more viscous solvents, a time-dependent electronic coupling constant V has been introduced. In decalin and butylbenzene, a transition from a "solvent independent" to a "solvent controlled" non-adiabatic regime is observed.

The construction of a new high-resolution multichannel low-cost pulse generator is reported. It is fully computer controlled through a simple RS232 serial interface. Its features are 1 ns resolution within any time, pulse delays up to 16 ms and pulse lengths up to 65 mu s, and pulse sequence repetition rate from 66 Hz up to 250 kHz. It has fully programmable sequencing, including step increments for any pulse delay or length. It governs a pulsed ESR spectrometer, which is also described, but it could be used in a very wide range of experimental set-ups. A few examples of spin-echo detected ESR and ESEEM of some paramagnetic centres are shown.

Massive amounts of coordinate data result from molecular dynamics calculations. The animation program MDKINO is a simple but powerful tool for previewing or reviewing the results. In recent simulations of elastase, we have examined hydrogen bonding patterns, conformational changes involving shifts in ring positions and rotations of amino acid side chains, electric fields in interatomic space, and electric forces acting on chosen nuclei. Animation is also useful for checking on the stability of calculations in progress. Simple programming techniques achieve acceptable levels of animation with readily available hardware (PS330 or PS390 display with a serial interface to a laboratory VAX). In about half an hour, it is possible to make and watch a color stereo “movie” of a selected subsystem of a simulation (up to 1 000 frames of about 100 atoms each).

Temperature-dependent Raman measurements of n-propylammonium chloride solutions in water, concentrated hydrochloric acid and methanol yield experimental gauche-trans enthalpy differences of 0.21 ± 0.13, 0.51 ± 0.26 and 0.37 ± 0.11 kcal mol⁻¹, respectively. Raman spectra in different aqueous solutions show that the gauche-trans equilibrium in solution is affected by the total chloride concentration. The gauche-trans enthalpy difference for CH₃CH₂CH₂NH₃⁺ is found by 4-31G ab initio calculations to be 0.26 kcal mol⁻¹.

Electron spin resonance of a high-T_c Bi superconductor sample is reported. The d.c. susceptibility, d.c. resistivity and a.c. susceptibility show two superconducting transitions at 105 K and 75 K. The ESR spectra show a main resonance line whose temperature dependence is studied in detail. The g value shows a maximum of 2.24 at 230 K and decreases to 2.12 at 100 K. The line width also shows a maximum of 520 G at the same temperature and drops to 200 G at 100 K. An unusual behaviour is observed in the decrease of the integrated intensity from a maximum at 230 K to below noise at level 100 K. It is possible that the origin of this signal is due to impurity phases. However, the unusual behaviour of its intensity (disappearance of the signal below T_c) may indicate that it arises from pair formation much above T_c.

We have characterized as-grown and thermally treated YBaCuO single crystals by ESR, Raman spectroscopy, magnetic susceptibility, X-ray and neutron diffraction measurements. The as-grown crystals are tetragonal and are superconducting with an onset temperature of 30 K. They show an ESR signal which behaves as a localized Cu²⁺ ion with tetragonal symmetry and presumably originates from copper chain atoms that are octahedrally coordinated by six oxygen neighbors. The temperature dependence of the ESR between 150 and 270 K shows paramagnetic behavior and also dynamical features. Below 90 K, the ESR signal disappears reversibly. A likely explanation is that the onset of local superconductivity frustrates the spins responsible for the ESR signal.

Vibrational spectroscopic methods are widely used to characterize semicrystalline polymers in terms of crystallinity. The temperature coefficient of crystallinity, an important and fundamental quantity, is seldom determined for lack of a sensitive method. In this paper, we describe an infrared approach to the measurement of the temperature coefficient of crystallinity. We start from the well-known observation that the integrated intensities of the bands in the spectrum of a semicrystalline polymer change with temperature. It is also known, though less appreciated, that only part of the change is due to changes in crystallinity, the remaining part being due to changes in the intrinsic intensity of the bands. We outline a method for separating these overlapping effects. The method has been applied to a variety of semicrystalline polyethylene samples. The temperature coefficients are found to be highly dependent both on the temperature and on the morphology of the sample. In addition we report crystallinity measurements on a solution crystallized low molecular weight (A& = 13 600) sample, discuss the origin of an apparent anomalous temperature dependence of band intensity cited in the literature, and offer quantitative evidence that the temperature dependence of specific volume is, at temperatures above 0 "C, largely determined by partial melting.

The energy balance of a photoinduced electron transfer reaction is given by the Rehm-Weller equation which combines the oxidation potential Eox(D) of the electron donor, the reduction potential Ered(A) of the electron acceptor, an electrostatic correction term C and the excited state energy of the light-absorbing species: It is shown that if light carries a thermodynamic entropy the excitation energy term must be given by ηE*, η being the efficiency of the conversion of the energy of light into chemical free energy. Measurements of fluorescence quenching through electron transfer at very low light intensities show that the Rehm-Weller equation remains valid in spite of its implied assumption that η = 1; it is concluded that contrary to much current thinking light is a form of high grade energy which can be converted in principle entirely into chemical free energy and electrical energy.

A study of the temperature dependence (from 233 to 353 K) of the rate of back electron-transfer reactions within geminate radical pairs by measurement of the free radical yield is reported. The radical pair is generated by photoinduced electron transfer with rhodamine 6G and oxazine 118 cations as electron acceptors and aromatic amines and methoxy-benzene derivatives as electron donors in acetonitrile, methanol and ethanol. In acetonitrile, the back electron transfer is non-adiabatic and apparent negative activation energies are observed for barrierless reactions. In alcohol solvents, an anomalously large temperature dependence is observed, which is attributed to a solvent-controlled adiabatic behaviour.

The effect of externally applied stresses of e_g and t_2g symmetry on the cubic E_g(X)e_g Jahn-Teller (JT) system Y²⁺ in SrCl₂ is studied. Coupling constants were obtained with the aid of EPR and by relying on a Ham-type cluster JT Hamiltonian, in conjunction with a random strain distribution. This is established from a critical review of the contributing strain sources (including possible percolation effects due to the isotopic composition of natural SrCl₂). The stress of t_2g symmetry shows strong effects. The authors conclude that the cubic Ham factor q is nearly one and that the experiments allow, as a function of this latter stress, one to pass gradually towards a trigonal JT effect. The optical absorption spectrum assigned to Y²⁺ is given in addition.

The ESR of small concentration of Gd 0.03<y<0.06 substituting for Y in single crystals of Gd_yY_1−yBa₂Cu₃O_6−x has been measured. In the insulating compound, with x ~ 0.1, and the superdconducting materials with 30 K < T_c < 80 K, the measurements were performed at X-band, 9.3 GHz, and K_α-band, 36 GHz, over a large temperature range above T_c. Angular dependence measurements exhibit a spectrum which is fully resolved in certain directions, but only partially resolved, because of exchange narrowing, in other directions. Comparisons between the spectra in the insulating and superconducting compounds shows similar angular dependent behavior. This seems to indicate that the origin of the exchange narrowing is the same in both compounds. Since this narrowing in the insulating compound arises from interaction with, or via, the Cu magnetic system, it is implied that there is a similar, perhaps fluctuating, system in the superconducting state. Preliminary measurements of the temperature dependence of the line widths may indicate the presence of spin pairing at about 110 K, above the actual T_c of 70 K. The crystal field parameters are D = 3B⁰₂ = 1307 MHz, B⁰₄ = 3.014 MHz and B⁴₄ = -11.43 MHz, for the semiconducting sample. The g-value is 1.989 ± 0.005. These values change only slightly in the superconducting crystals.

ESR results are reported on the Ag²⁺ ion introduced into alkaline earth fluoride crystals. In SrF₂ (as in CaF₂) a trigonal centre is present which tunnels between four equivalent C₃ directions as shown by ESR under uniaxial stress. A T_2g ⊗ t_2g Jahn-Teller model describes adequately the paramagnetic and stress effects when the strong coupling case is considered. Ag²⁺ in BaF₂ forms a static tetragonal cluster similar to Ag²⁺ and Cu²⁺ in SrCl₂.