Anisotropy of the triplet spin—lattice relaxation in molecular crystals at low temperatures and high magnetic fields

The anisotropy of the triplet relaxation rates in molecular crystals at low temperatures (T = 1.6 K) and very high magnetic fields (B = 5.2–10 T) is explained by a model, which assumes time dependent matrix elements of the electronic dipole tensor and of the electronic g-tensor. The time dependencies may be due, for instance, to librations of the molecule or to changes in the electronic configuration; they arise from a direct process. This fact is used in order to reduce the number of parameters. The relaxation rates are given as functions of the direction cosines of the magnetic field and of eight parameters which are determined in a least-squares fit for the system quinoxaline in perdeuterated naphthalene. Some uncertainties concerning the numerical values of the parameters would be reduced by measurements with the magnetic field directed along the principal axes of the electronic dipole tensor.     

Librational model for the direct spin—lattice relaxation process in isolated triplet molecules in a naphthalene-mixed crystal

For the spin-lattice relaxation (SLR) in molecular crystals a model is presented, which attributes the SLR mechanism to phonon-stimulated molecular librations. Its applicability to the direct process is shown by comparison with the experimental data on quinoxaline guest molecules in a perdeuterated napthalene host crystal.     

The anisotropy of the spin—lattice relaxation in the lowest triplet state of naphthalene in durene in the presence of a magnetic field

We have measured the spin—lattice relaxation rates in the phosphorescent triplet state T_o of naphthalene in durene as a function of the direction of the magnetic field. The orientational dependence of the relaxation is explained by thermal activation to a close lying local phonon state where the spin axes are rotated with respect to those in T_o.     

Spin-lattice relaxation of linear triplet excitons in tetrachlorobenzene at high magnetic fields

Spin-lattice relaxation of linear triplet excitons in 1,2,4,5-tetrachlorobenzene has been studied by an optical method at high magnetic fields B and at low crystal temperature. We find the interesting result that the field dependence of the relaxation rate constants for the direct processes cannot be described by a simple B³ law. The experimental relaxation rate for the Δm = 2 transition increases as B², and for the Δm = 1 transition as B^2,6.     

Anisotropic spin—lattice relaxation in the triplet state of naphtralene-h8 in n-pentane

The temperature dependence of the individual spin—lattice relaxation rate constants (W_ij) between the lowest triplet sublevels of naphthalene-h₈ in a polycrystalline Shpol'skii matrix of n-pentane have been measured between 1.2 and 2.4 K in zero field. The total sublevel decay constants are evaluated and found to be independent of temperature. The W_ij are found to be highly anisotropic, but the anisotropy pattern differs from that observed previously in a durene matrix.     

ODMR in zero field of the photoexcited triplet state of naphthalene and quinoxaline dissolved in argon matrices

ODMR of zero-field transitions in the triplet states of naphthalene and quinoxaline in an argon host are reported. Excitation with a Hg are leads to linewidths considerably smaller than those obtained for the same molecules dissolved in glassy matrices. In naphthalene selective laser excitation reduces the linewidth to a value almost equal to those observed in single crystals.     

Magnetic field induced cross relaxation between two different spin transitions of triplet coumarin

At certain magnetic fields, when the energy separations between the τ_x — τ_y spin levels and between the τ_y — τ_z levels of triplet coumarin become equal, intensity changes are observed in the phosphorescence of coumarin doped in a single crystal of durence cooled to 1.6 K that can be attributed to cross relaxation (CR) between the τ_x — τ_y spin levels of one triplet coumarin molecule and the τ_y — τ_z spin levels of another triplet coumarin. Rate equations that describe the time behavior of the populations of the spin levels which are involved in the CR process are described and numerically solved. CR behavior is found to be strongly dependent on the total coumarin triplet concentration.     

Proton spin relaxation in a smectic liquid crystal

A proton spin relaxation study in the liquid crystal ethyl-[(methoxybenzylidene)-amino] cinnamate is presented. A “phase change” is observed at ≈ 103°C within the smectic A phase. Some liquid-like mobility exists below this temperature.     

Deuteron spin lattice relaxation in amorphous ices

Temperature-dependent deuteron spin lattice relaxation times T(1) have been obtained from water in its three amorphous states at ambient pressure: low density amorphous (LDA), high density amorphous (HDA), and very high density amorphous (VHDA). It is found that in all of these states the magnetization recovery is essentially monoexponential and that T(1) of LDA is significantly longer than that of the higher density forms. Thus, T(1) can be used as a monitor parameter to study the kinetics of the transitions from HDA to LDA and from VHDA to LDA. During the transformation of VHDA to LDA an intermediate state is formed, which, according to its T(1) at low temperature, is clearly determined to be HDA-like. However, and most significantly, the transition from VHDA to this HDA-like state and further on to LDA occurs at temperatures significantly above the kinetic stability limit of native HDA produced at 77 K. These findings contribute to the current discussion on the nature of HDA and VHDA by strengthening the view that the annealing of VHDA at ambient pressure produces a relaxed HDA-like state.     

Delayed fluorescence in a pyrene-doped naphthalene crystal: homofusion and heterofusion of triplet excitons

Prompt and delayed emissions from a naphthalene crystal doped with pyrene have been investigated in the temperature range 77–300 K. It is shown that delayed fluorescence (DF) in this crystal originates from heterofusion and homofusion of two different types of trap triplets at low temperature (T< 130 K) and from homofusion of pyrene triplets at high temperature (T >200 K). Pyrene concentration effects on DF suggest that free triplet excitons are trapped more efficiently at monomeric pyrene sites than at nascent excimeric pyrene sites.     

The anisotropy of the spin-lattice relaxation rate in the excited triplet state of quinoxaline

The anisotropy of the spin-lattice relaxation has been investigated at T = 2.6°K for the triplet state of quinoxaline in durene and in perdeuterated naphthalene (N-d₈) with the pulse saturation method. In the N-d₈ host crystal the anisotropy is identical for both molecular orientations A and B.     

Anisotropic oscillator strength of 1-hydronaphthyl radical in naphthalene single crystal

The absolute value of the optical absorption strength of the 539 nm intra-radical transition of the 1-hydronaphthyl radical in a naphthalene crystal was measured with light polarized along the a, b and c′ axes. The long and short axial components of the oscillator strength were determined by using the local field tensors obtained with several methods. It was found that the local field tensors obtained recently by Munn, assuming each molecule to be two polarizable points, yield good agreement between the molecular axial components of the oscillator strengths obtained experimentally and theoretically. The local field tensors obtained previously with the single point-dipole approximation were found to result in inconsistency.     

Spin—lattice relaxation of linear triplet excitons in 1,4 dibromonaphthalene

We report measurements on spin—lattice relaxation (SLR) of linear triplet excitons in 1,4 dibromonaphthalene (1,4 DBN) single crystals at liquid helium temperatures and at room temperature. SLR was found to be temperature dependent but isotropic with respect to the external magnetic field.     

Nuclear spin relaxation in biaxial liquid crystal phases

The orientation-dependent spin-lattice relaxation rates for biaxial liquid crystal phases are given explicitly in terms of spectral densities J_mLm'L (ω) described by Berggren et al. (1993, J. chem. Phys., 99, 6180). It is recognized that the 'biaxial' spectral densities are not observed in biaxial phases unless the director is oriented away from the external magnetic field.     

Nuclear spin relaxation in biaxial liquid crystal phases

Abstract

The orientation-dependent spin-lattice relaxation rates for biaxial liquid crystal phases are given explicitly in terms of spectral densities J _mLm′L (ω) described by Berggren et al. (1993, J. chem. Phys., 99, 6180). It is recognized that the ‘biaxial’ spectral densities are not observed in biaxial phases unless the director is oriented away from the external magnetic field.     

Crystal field effects on the ground and lowest excited triplet states of benzene isotopes in a borazine host crystal

The phosphorescence spectra of the C₆H₆C₆H₅D₁p-C₆H₄D₂symp-C₆H₃D₃, C₆D₆ and ¹³CC₅D in a borazine host crystal are analyzed at high resolution. The spectral lines are sharp (～2 cm^?1 wide) indicating that the impurity molecules occupy a unique site in the borazine lattice which is probably substitutional. The phonon sidebands are weak,giving clean, well-resolved spectra much like those of isotopic mixed crystals. In contrast, however, the crystal field effects on the ground state vibrational levels are much smaller than those found for isotopic mixed crystals. The gas-to-crystal shifts are very small, the vibrational degeneracies are not removed and orientational splittings are only observable for a few select vibrational levels. For most vibrational levels and for the derivation of selection rules one can asume the effective crystal site symmetry to be D_3d. The data provide the first conclusive evidence that the splitting observed in the benzene phosphorescence spectrum results from a distortion of the molecule when excited to the zeroth vibrational level of the T₁ state. Furthermore, the data suggest that the distortion is intrinsic in nature (i.e.,is not caused by the crystal field).     

On spin—lattice relaxation of tunnelling methyl groups

Spin—lattice relaxation of non-interacting tunnelling methyl groups is determined by seven coupled parameters. For interacting methyl groups in a solid the number of parameters is reduced, by symmetry-restricted spin diffusion, to four, describing Zeeman, dipolar, tunnelling and rotational polarization systems.     

Rotating frame spin—lattice relaxation in solid hydrated methaemoglobin

Proton T_1? relaxation times in two samples of methaemoglobin with different water contents showed that a process occurs with a correlation time of the order 10^?s s at 240 K. This is probably an exchange of molecules between bound and free water.     

The effect of vibronic relaxation on the spin coherence in photo-excited triplet states

We present a theoretical model for describing spin coherence in photo-excited triplet states in zero-field under the influence of vibronic relaxation. From a comparison with experimental results in parabenzoquinone we obtain information about the properties of the excited vibronic level.