Spin-lattice relaxation of radicals in the solid state. Part 3

Some causes of spin-lattice relaxation in the solid state are considered. It is found that anisotropic hyperfine interaction can make an appreciable contribution to the spin-lattice relaxation time, which becomes dependent on the orientation of the radical relative to the external field and on the number of HS components. There is no appreciable contribution from frequency modulation of the vibrations (of the radical as a whole or purely intramolecular) caused by the lattice vibrations.     

Vibronic coupling in the ground and excited states of oligoacene cations

The vibrational coupling in the ground and excited states of positively charged naphthalene, anthracene, tetracene, and pentacene molecules is studied on the basis of a joint experimental and theoretical study of ionization spectra using high-resolution gas-phase photoelectron spectroscopy and first-principles correlated quantum-mechanical calculations. Our theoretical and experimental results reveal that, while the main contribution to relaxation energy in the ground state of oligoacene systems comes from high-energy vibrations, the excited-state relaxation energies show a significant redistribution toward lower-frequency vibrations. A direct correlation is found between the nature of the vibronic interaction and the pattern of the electronic state structure.     

Vibrational relaxation of C-D stretching vibrations in CDCl3, CDBr3, and CDI3

We present time-resolved transient grating measurements of the vibrational relaxation rates of the C-D stretching vibrations of deuterated haloforms in benzene and acetone. We compare our results with previous measurements of excited C-H stretches in the same solvents to obtain insight into the solvent effect on the vibrational relaxation. In deuterated molecules, there are more low-order-coupled states and the states are closer in energy to the C-D stretch than in the unlabeled isotopologs. Therefore, the relaxation is faster for the deuterated molecules. The relaxation also shows a significant solvent dependence. Bromoform and iodoform form charge-transfer complexes with both benzene and acetone which enhance the relaxation rate. For chloroform, hydrogen bonding to acetone is expected to be a more favorable interaction. Surprisingly, however, the vibrational relaxation of CDCl(3) is slower in acetone than in benzene.     

Rotational mechanism for vibrational relaxation in rigid media. Interaction potentials

Using the assumption of pairwise additive forces we derive the interaction potential acting upon a substantial impurity in a crystalline lattice. The angle dependent parts of the force on the impurity internal vibrations are represented in the form of a Fourier series. Numerical calculations of the Fourier coefficients of the force are presented for a model system employing empirical argon—argon interactions. The higher Foutier coefficients are shown to decrease rapidly in conformity with the simple model potential used to describe the vibrational relaxation in a preceding paper.     

Microwave spectra of nitromethane and D3-nitromethane

The microwave spectra of isotopic nitromethanes are presently being reinvestigated in order to increase our understanding of the interaction between the almost free internal rotation and the vibrations.The presence of relaxation terms depending on cos3p proves that the symmetry of H_eff is D_3h and furthermore makes a determination of the structure of the methyl group possible.     

Vibrational spectroscopy of solid state molecular dimers

The IR and Raman spectra of α- and β-perylene crystal vibrations are investigated, β-perylene is monomeric and α-perylene has face-to-face dimers. Frequencies and forms of the lattice vibrations as well as vibron relaxation and localization are discussed. Polarization ratios in α crystals are perturbed by coupling to dimer vibrations.     

Small Al clusters on the Cu(111) surface: Atomic relaxation and vibrational properties

The relaxation and vibrational properties of both Al clusters and the (111) surface of a copper sub-strate were studied using the interatomic interaction potentials obtained in a tight-binding approximation. The presence of small aluminum clusters led to modification of the vibrational states of the substrate, a shift of the Rayleigh mode, and excitation of new Z-polarized modes. Hybridized modes localized on the cluster adatoms and the neighboring atoms of the substrate were found in the phonon spectrum. The localized dipole-active modes of the cluster and their strong hybridization with vibrations of the substrate points to desorption stability of the tri- and heptaatomic clusters.     

Applicability of the method of kinetic equations for describing nuclear quadrupole resonance in molecular crystals

The applicability of the method of kinetic equations for describing nuclear quadrupole resonance (NQR) in molecular crystals is demonstrated for the case in which the relaxation processes are produced by torsional vibrations of the molecules. The Bloch system of equations for NQR spins I=3/2 situated in an axisymmetric electrical field and expressions for spin-lattice relaxation times were obtained.     

Vibrational energy relaxation of the bend fundamental of dilute water in liquid chloroform and d-chloroform

The population lifetimes of the bend fundamental of dilute water in liquid chloroform (8.5 ps) and d-chloroform (28.5 ps) display an interesting solvent isotope effect. As the lowest excited vibrational state of the molecule, the water bend fundamental relaxes directly to the ground state with about 1600 cm-1 of energy released to the other degrees of freedom. The strong solvent isotope effect along with the large energy gap indicates the participation of solvent vibrational modes in this vibrational energy relaxation process. We calculate the vibrational energy relaxation rates of the water bend in chloroform and d-chloroform using the Landau-Teller formula with a new potential model developed and parametrized self-consistently to describe the chloroform-water interaction. The computed values are in reasonable agreement with the experimental results, and the trend for the isotope effect is correct. It is found that energy transfer to the solvent vibrations does indeed play an important role. Nevertheless, no single dominant solvent accepting mode can be identified; the relaxation appears to involve both the bend and the C-Cl stretches, and frequency changes of all of these modes upon deuteration contribute to the observed solvent isotope effect.     

The absorption and fluorescence properties of pyrene crystal: a theoretical approach. III. Relaxation from highly excited vibrational levels

It is shown that the model based on vibrational relaxation to the bath, while invalid for highly excited vibrational levels at low temperatures, is valid at high temperature. Thus relaxation time of ～10^?13 s previously estimated is acceptable. Using this time, known radiative lifetimes, and a reasonable value for relaxation times of the vibrations of the isolated molecules, a modified model for excimer absorption and emission is proposed.     

Angular correlation functions of spherical-top carbonyl complexes in gaseous N2 influenced by coriolis coupling

The Fourier transform of measured line shapes of the triply degenerate vibrations of carbonyl complexes in gaseous N₂ shows strong deviations from the angular correlation functions calculated by Steele for the rigid spherical-top molecules. These deviations are explained theoretically with the aid of Coriolis coupling and vibrational relaxation. For molecules in the gas phase it is possible to separate the Coriolis coupling from the vibrational relaxation.     

Relaxation of hot atoms following H2 dissociation on a Pd111 surface

We study the relaxation of hot H atoms produced by dissociation of H2 molecules on the Pd111 surface. Ab initio density-functional theory calculations and the "corrugation reducing procedure" are used to determine the interaction potential for a H atom in front of a rigid surface as well as its modification under surface-atom vibrations. A slab of 80 Pd atoms is used to model the surface together with "generalized Langevin oscillators" to account for energy dissipation to the bulk. We show that the energy relaxation is fast, about 75% of the available energy being lost by the hot atoms after 0.5 ps. As a consequence, the hot atoms do not travel more than a few angstroms along the surface before being trapped into the potential well located over the hollow site.     

Intraband relaxation in CdSe nanocrystals and the strong influence of the surface ligands

The intraband relaxation between the 1Pe and 1Se state of CdSe colloidal quantum dots is studied by pump-probe time-resolved spectroscopy. Infrared pump-probe measurements with approximately 6-ps pulses show identical relaxation whether the electron has been placed in the 1Se state by above band-gap photoexcitation or by electrochemical charging. This indicates that the intraband relaxation of the electrons is not affected by the photogenerated holes which have been trapped. However, the surface ligands are found to strongly affect the rate of relaxation in colloid solutions. Faster relaxation (<8 ps) is obtained with phosphonic acid and oleic acid ligands. Alkylamines lead to longer relaxation times of approximately 10 ps and the slowest relaxation is observed for dodecanethiol ligands with relaxation times approximately 30 ps. It is concluded that, in the absence of holes or when the holes are trapped, the intraband relaxation is dominated by the surface and faster relaxation correlates with larger interfacial polarity. Energy transfer to the ligand vibrations may be sufficiently effective to account for the intraband relaxation rate.     

Binding Sites,Vibrations and Spin-Lattice Relaxation Times in Europium(II)-Based Metallofullerene Spin Qubits

To design molecular spin qubits with enhanced quantum coherence, a control of the coupling between the local vibrations and the spin states is crucial, which could be realized in principle by engineering molecular structures via coordination chemistry. To this end, understanding the underlying structural factors that govern the spin relaxation is a central topic. Here, we report the investigation of the spin dynamics in a series of chemically designed europium(II)-based endohedral metallofullerenes (EMFs). By introducing a unique structural difference, i. e. metal-cage binding site, while keeping other molecular parameters constant between different complexes, these manifest the key role of the three low-energy metal-displacing vibrations in mediating the spin-lattice relaxation times (T₁). The temperature dependence of T₁ can thus be normalized by the frequencies of these low energy vibrations to show an unprecedentedly universal behavior for EMFs in frozen CS₂ solution. Our theoretical analysis indicates that this structural difference determines not only the vibrational rigidity but also spin-vibration coupling in these EMF-based qubit candidates.     

Dominance of methyl groups in picosecond vibrational relaxation in hydrocarbons

Picosecond relaxation times of CH stretching vibrations in a series of liquid hydrocarbons have been measured using the Raman scattering technique. The results indicate that the vibrational energy loss takes place primarily through the methyl groups in these molecules.     

Experimental (FTIR and FT-Raman) and ab initio and DFT study of vibrational frequencies of 5-amino-2-nitrobenzoic acid

The FTIR and FT-Raman spectra of 5-amino-2-nitrobenzoic acid (ANB) have been recorded in the region 400-4000cm(-1). The observed frequencies were assigned to different modes of vibrations on the basis of fundamental, combination and overtones. The geometry has been optimized with complete relaxation on the potential energy surface at HF, MP2 and B3LYP level of theories using 6-311++G(d,p) basis set and compared with the crystal data. The possible hydrogen bond interaction has been estimated taking a model compound. Further harmonic vibrational frequency calculations have been carried out at HF and B3LYP levels and the scaled values were in good agreement with majority of the experimental observations. The theoretically constructed spectra coincide satisfactorily with those of experimental spectra.     

面向有机光电材料的电子激发态结构与过程的计算方法

共轭聚合物与有机分子材料中的电子激发结构与过程决定了材料的光电功能：根据Kasha规则,低能级激发态的排序决定能否发光;最低激发态至基态的辐射跃迁与无辐射跃迁之间的竞争决定了发光效率,后者主要由非绝热耦合（声子作用）决定;电荷激发态载体的传输由电子分布与振动耦合或杂质和无序的散射弛豫过程决定．本文针对有机功能材料的发光性能,介绍两种理论方法的研究进展,即可用于计算共轭聚合物激发态结构的量子化学密度矩阵重整化群方法和计算发光效率的多模耦合无辐射跃迁速率方法．这些方法被应用于有机功能材料的性能预测和分子设计中．     

E ⊗ ϵ Jahn–Teller anharmonic coupling for an octahedral system

The coupling between doubly degenerate electronic states and doubly degenerate vibrations is analyzed for an octahedral system on the basis of the introduction of an anharmonic Morse potential for the vibronic part. The vibrations are described by anharmonic coherent states and their linear coupling with the electronic states is considered. The matrix elements of the vibronic interaction are built and the energy levels corresponding to the interaction Hamiltonian derived. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002     

Theoretical study of the chemisorption of CO on Al2O3(0001)

Density functional molecular cluster calculations have been used to study the adsorption of CO on the alpha-Al2O3-(0001) surface. Substrate and adsorbate geometry modifications, adsorption enthalpies, and adsorbate vibrations are computed. Despite the rather small size of the employed cluster, relaxation phenomena evaluated for the clean surface agree well with experimental measurements and periodic slab calculations and mainly consist of an inward relaxation of the Lewis acid site (Lsa). Different adsorbate arrangements, perpendicular and parallel to the surface, have been considered. Among them, the most state CO chemisorption geometry (delta Hads approximately -13 kcal/mol) is that corresponding to the adsorbate perpendicular to the surface, atop Lsa and C-down oriented. The C-O stretching frequency (nu C-O) computed for such an arrangement is 2158 cm-1, i.e., blue shifted by 44 cm-1 with respect to the free adsorbate. The lack of experimental evidence pertaining to CO interacting with a well-defined alpha-Al2O3(0001) surface prevents the possibility of a direct check of the computed quantities. Nevertheless, low-temperature IR data for CO on alumina powders (Zecchina, A.; Escalona Platero, E.; Otero Areán, C. J. Catal. 1987, 107, 244) indicate for the chemisorbed species a delta nu = 12 cm-1. The adsorbate-substrate interaction relieves some of the Lsa relaxation, even if the Lsa electronic structure is only slightly affected upon chemisorption.