Field effect in anthracene crystal

A transient and steady-state electric field effect has been observed at a freshly cleaved ab-surface of anthracene single crystal. Evaporation of naphthacene onto the crystal surface suppresses the field effect. Removal of the surface contaminant restores the field-induced change of the surface current.     

The 4000 Å transition of crystal anthracene

New low temperature determinations of ?₂(ω) for crystal anthracene locate the 0-0 transition origin at 25096 ± 6 cm^?1 for the b-polarization. There is no Stokes shift of the fluorescence origin. The exciton bands which are optically accessible by probing the (ab) crystal face with normal incidence light polarized either parallel or perpendicular to the b-axis have a temperature dependent splitting. The Davydov splitting at 77°K is 267 ± 12 cm^?1. The temperature dependence of the k = 0 polariton splitting and the exciton bandwidths (b-polarization and a-polarization) are also given and together with the Davydov splitting provide a test of the adequacy of theories of exciton interactions. The polarization ratio measured as the ratio of the oscillator strengths of the two transitions (f_b/f_a) is quite temperature dependent for both the region of the (0-0) transition and over the total vibronic transition. At 77°K, the value of f_b/f_a is 8.0 for the origin region close to that expected of an oriented gas but is 4.5 over the total transition. Some spectral structure observed only in the b-polarized reflectance spectrum at temperatures less than 77°K is related to the existence of defects localized near the surface.     

The absorption spectrum of the anthracene molecule in the vacuum ultraviolet

The optical absorption of anthracene vapour for photon energies from 5 to 8.5 eV was found to differ in finer structure from the spectra reported earlier for parts of this range. Above the strong ¹B_2u long axis polarized πlπ^* transition at 5.24 eV three short axis polarized ¹B_1u ππ^* transitions are assigned on the basis of the oriented gas model in comparison to spectra from anthracene single crystals. A tentative new assignment for most of the additionally observed sharp Rydberg bands leading to the first ionization potential at 7.47 eV is given.     

Far-infrared spectrum of anthraquinone single crystal

Far-infrared, polarized spectra of 9,10-anthraquinone as a single crystal have been measured at room and liquid-nitrogen temperatures. The observed bands have been assigned to translational lattice vibrations on the basis of polarization results of a normal-coordinate calculation. A peculiar effect of temperature on the spectra has been found and suggested to be due to Fermi resonance involving the lattice vibrations.     

First-principles study of the crystal structure and equation of state of naphthaline and anthracene

A first — principles method of density functional theory with a gradient approximation of the exchange-correlation potential in the form of PBE implemented in the PWscf program of the Quantum ESPRESSO software using the Grimme’06 scheme is used to calculate the crystal structure of naphthaline and anthracene at a hydrostatic pressure ranging from 0 GPa to 2 GPa and from 0 GPa to 20 GPa respectively; their equations of states are analyzed. It is shown that under pressure the volume decreases due to voids, and the molecules themselves are practically not deformed. The Grüneisen parameter is calculated in the Slater-Dugdale-MacDonald-Zubarev-Vashchenko model. This parameter decreases from the equilibrium values of 2.356 (anthracene) and 3.226 (naphthaline) with an increase in the pressure. With the use of the Mie-Grüneisen equation under the additional Hugoniot-Renkin condition the impact pressure is calculated, which increases compared to the cold one at a relative compression V/V ₀, below 0.7.     

Fluorescence from the second excited state of an anthracene crystal observed by two-step excitation

Weak fluorescence from the second excited state of an anthracene crystal has been observed by two-step excitation via the lowest excited state. Relaxation processes of a highly excited state to the lowest state are discussed.     

Phonon spectrum of a fluorene molecular crystal

The dynamics of the fluorene molecular crystal lattice is calculated by the atom-atom potential method. The fundamental lattice vibration frequencies are determined, their assignment is made, and shapes of the vibrations are studied over a pressure range of from 1 bar to 30 kbar. For acoustic phonon vibrations, the distribution of Debye frequencies over directions of the wave vector is estimated. The results obtained are used to discuss the photochemical reaction of detachment of a hydrogen atom in a fluorene crystal, a process occurring by the tunneling mechanism.     

Intermolecular intersystem crossing in single-molecule spectroscopy: Terrylene in anthracene crystal

We present a spectroscopic study of terrylene in anthracene crystals at the ensemble and single-molecule levels. In this matrix, single-molecule fluorescence is reduced by three orders of magnitude. Correlation measurements allow us to identify a new relaxation channel, matrix-enhanced intersystem crossing. This process starts with a singlet-to-triplet energy transfer from guest to host, after which the triplet exciton is transferred back to the guest. The intermolecular intersystem crossing is expected whenever the lowest triplet state of the host is located between the lowest singlet S(1) and lowest triplet T(1) excited states of the guest. It must be considered when searching for new host-guest systems for single-molecule spectroscopy.     

Low-temperature reflection spectrum of K-TCNQ single crystal

Polarized reflection spectra of a K-TCNQ single crystal have been measured at 27,295 and 413 K. The absorption spectra were derived from the observed reflection spectra by Kramers-Kronig transformation. At 27 K, the near-infrared absorption band which is associated with the charged transfer between TCNQ^? ions, splits into a strong sharp peak at 8 × 10³ cm^?1 and a weak broad band at 11 × 10³ cm^?1. These bands were interpreted in terms of the strongly correlated extended Hubbard model including the nearest-neighbor Coulomb interaction.     

The Raman spectrum of hexamethylbenzene single crystal

A set of polarized Raman spectra obtained from suitable hexamethylbenzene single crystals at room temperature is presented. All the observed frequencies are attributed to intramolecular vibrations or lattice modes on the basis of clear polarization data.     

Spectral shifts in the fluorescence spectrum of anthracene under the influence of laser driven shock compression

We report spectral shifts in the fluorescence emission of a solution of anthracene in benzene under the influence of laser driven shock compression. Red-shifts of up to 800 cm⁻¹ are observed in the peak of the 0–1 vibronic band of anthracene corresponding to maximum pressures of =10 kbar. These results are in agreement with literature values of spectral shifts obtained under conditions of high static pressure. Laser driven shock provides an attractive alternative to conventional shock generation schemes when used in conjunction with optical spectroscopy as a probe of molecular level changes.     

Reinterpretation of the absorption spectrum of UCl4 single crystal

The absorption spectra of UCl₄ single crystals have been measured from 4000 to 30 000 cm⁻¹ at temperatures ranging from 4.2 K to room temperature. The transitions were assigned by comparing the strong absorption features of UCl₄ to those of ThCl₄:U⁴⁺. The energy levels were fitted to a semiempirical Hamiltonian in D_2d symmetry.A r.m.s. deviation of 60 cm⁻¹ in the fit to 26 assigned levels was achieved using the following set of parameters (cm⁻¹): F² = 42 561 ± 235; F⁴ = 39 440 ± 634; F⁶ = 24 174 ± 185; ξ = 1805 ± 8; α = 31 ± 1; β = −576 ± 168; (γ = 1200); B²₀ = −903 ± 151; B⁴₀ = 766 ± 220; B⁴₄ = −3091 ± 185; B⁶₀ = −1619 ± 482; B⁶₄ = −308 ± 280.     

Single dibenzoterrylene molecules in an anthracene crystal: spectroscopy and photophysics.

We study single dibenzoterrylene molecules in an anthracene single crystal at 1.4 K in two insertion sites at 785.1 and 794.3 nm. The single-molecule zero-phonon lines are narrow (about 30 MHz), intense (the detected fluorescence rates at saturation reach 100,000 counts s(-1)), and very photostable. The intersystem-crossing yield is extremely low (10(-7) or lower). All of these features are hallmarks of an excellent system for high-resolution spectroscopy and nanoscale probing at cryogenic temperatures.     

Single dibenzoterrylene molecules in an anthracene crystal: main insertion sites.

We present a spectroscopic study of the properties of the two principal insertion sites (at 785.1 and 794.3 nm) of single dibenzoterrylene molecules in anthracene single crystals at cryogenic temperatures. We measured the temperature dependence of the line width, the orientation of the transition dipole moments, and the Stark effect. We performed molecular dynamics simulations, which show that one dibenzoterrylene molecule preferably replaces three anthracene molecules. From simulated annealing, we derive the molecular conformations in the most stable insertion sites and the orientations of the transition dipole moments. The good agreement between the spectroscopic results and the simulations allows us to propose unambiguous structures for the two principal spectroscopic sites.     

Infrared spectrum of hexafluorobenzene crystal at 77 K

Infrared spectra of polycrystalline C₆F₆ are analyzed in the 200 cm⁻¹ to 4000 cm⁻¹ region. The vibrational frequencies agree well with liquid data although many of the bands are split. Mutual exclusion between infrared active and Raman active fundamentals is found.     

Optical properties of anthracene single crystals in the excitonic region of the spectrum between 4 and 10.5 eV

The optical constants in the photon energy range between 4 and 10.5 eV for E ?b on (001) and E ?L and E ⊥ _L on the (010) plane are discussed. In particular the influence of macroscopic fields on the optical properties of anisotropic crystals is considered. For _E ? a on the (001) plane reflection spectra have been measured at various angles of incidence. These data and results obtained recently by Hymowitz and Clark for several artificially prepared crystal faces are discussed on the basis of the frequency dependend dielectric functions. New information on the directional dispersion of exciton bands is thus obtained.     

Moderate-resolution two-photon spectrum of the naphthalene crystal and naphthalene in durene

The two-photon absorption spectra at ～ 5 cm^?1 resolution are presented for the naphthalene crystal and naphthalene in durene at 2°K in polarized light. A rich vibronic spectrum of the ¹B_2u(3200 Å) naphthalene state is observed and some of the strongest vibronic origins are from in-plane vibrations.     

Singlet—triplet absorption spectrum of 9,10-dichloroanthracene crystal

The T₁ ← S_O absorption spectrum of 9,10-dichloroanthracene single crystal was measured by observing delayed fluorescence. The transition dipole is perpendicular to the molecular plane. The absorption coefficient at 694 nm is estimated to be 1 × 10^?3 cm^?1.