G. Zumofen

Apriori calculation of the vibron band and of the vibron–phonon and two‐phonon combination bands in the Raman spectrum of solid α‐N2

The observed shift, splitting, and intensity ratio of the vibrational Raman band in solid α‐nitrogen as reported by Anderson et al. have been reproduced in a calculation which is based on the quadrupole moments and polarizabilities of N2 in the gas phase. Furthermore, the intensities and shapes of the weak phonon sideband in the N2 stretching region, as well as of the two‐phonon sideband in the lattice region, have been calculated by treating these as combination bands which derive their intensities from electrical anharmonicities, i.e., from quadratic dependences of the polarizability on internal and external nuclear displacements. The contributions from mechanical anharmonicities, i.e., from vibron–phonon coupling through third and higher order terms in the vibrational potential, are found to be negligible in these band intensities. The computations of the lattice dynamics are based on a Lennard‐Jones plus quadrupole–quadrupole potential.

Levy distribution of single molecule line shape cumulants in glasses

We investigate the distribution of single molecule line shape cumulants, kappa(1),kappa(2),ellipsis, in low temperature glasses based on the sudden jump, standard tunneling model. We find that the cumulants are described by Levy stable laws, thus the generalized central limit theorem is applicable for this problem.

Calculation of the vibron band and of the vibron–phonon combination bands in the ir and Raman spectra of solid α‐CO

The observed gas‐to‐solid shift, splitting, and intensity ratio of the vibrational band in solid α‐CO and the shifts of CO isotopes suspended in solid 12C16O and in N2 matrices have been studied by calculations based on known gas‐phase and matrix properties. Equations and numerical results for the crystal sums of the various intermolecular interactions are tabulated. It has been shown that the random distribution of the two possible orientations of the CO molecules modifies the dipole–dipole interaction in such a way that the ir spectrum is not affected whereas the Raman spectrum is spread over the entire vibron band. The splitting in the ir spectrum reported by Dubost is interpreted as LO–TO splitting which becomes visible because of nonnormal incidence. The vibron–phonon combination bands have been calculated using electrical anharmonicities, i.e., the bilinear dependence of the polarizability and of the dipole moment on internal and external coordinates. An ellipsoidal cavity has been assumed to comput...

HIGH PHOTO-STABILITY OF SINGLE MOLECULES IN AN ORGANIC CRYSTAL AT ROOM TEMPERATURE OBSERVED BY SCANNING CONFOCAL OPTICAL MICROSCOPY

tum eA ciency lower than 1.2 10 8 . On average, with a detection eA ciency of 6.5%, a stable ¯uorescence signal of 10 5 photons s 1 could be detected fromsingle molecules during 1min of continuous photo-excitation. Both irreversible and reversible abruptuorescence intensity jumps to the background have been observed. The experimental data indicate that diA using quenchers at low concentration in the crystal contribute touorescence bleaching of single molecules. All molecularuorescence signals follow a typical power saturation law with a mean saturation count rate of 4.3 10 5 photons s 1 . The large photo-stability allows for long illumination times and high emission rates of single molecules trapped in a crystal at room temperature. A potential application as single organic quantum light sources under ambient conditions is conceivable.

Ab initio CI calculation of spectroscopic constants of the ground and electronically excited states of CN

Ab initio configuration interaction calculations have been carried out for the ground state (1Σ) and for two excited states (1Π and 3Π) of CN-. The calculated energies of Te = 56 423 cm-1 and 47 470 cm-1 for the 1Π and 3Π states are in reasonable agreement with the solid state values. Spectroscopic constants, static and transition dipole moments and Franck-Condon factors have been derived and compared with available solid state experimental data.

Transition from simple to complex behavior of single molecule line shapes in disordered condensed phase

We use the Kubo–Anderson sudden jump approach to investigate line shapes of single molecules (SMs) interacting with randomly distributed two level systems (TLSs). Depending on their random environment, SMs exhibit a wide variety of behaviors. Under certain conditions, given in the text, line shapes exhibit simple behavior, e.g., cases where lines are Lorentzian with a width which varies from one molecule to the other. As control parameters are changed a transition to complex line shape phenomena is observed (i.e., the line shapes have random structures, each with a random number of peaks). We investigate these behaviors for two cases—(i) the case when all TLSs are identical though randomly distributed in space and (ii) the standard tunneling model of low temperature glass where the TLSs are nonidentical. We show that, in certain limits, both models can be analyzed using Levy-stable laws. For the glass model we compute the distribution of line shape variance and discuss a previous proposition, that distrib...

Tip-induced spectral dynamics of single molecules

Manipulation of spectral dynamics of single molecules (SM) by a metallized scanning probe tip is demonstrated. The Stark effect of the zero-phonon lines of single pentacene molecules in a p-terphenyl host at 1.8 K is investigated by applying a voltage to the tip in contact with the sample. The measured Stark shifts exhibit a plateau and the line widths depend on the electric field. These anomalies are explained by a model based on two-level systems with field-dependent double-well potentials. The experimental data show that the two-level systems are induced by the tip.

Photon statistics in single-molecule fluorescence at room temperature

The fluorescence of single terrylene molecules in a crystalline host is investigated at room temperature by scanning confocal optical microscopy. Photon arrival times are analyzed in terms of interphoton time distributions, second order correlation functions, and the variance of the photon number probability distribution. Antibunching at short times and bunching behavior for longer times is observed, associated with sub- and super-Poissonian statistics, respectively. A rate-equation analysis of the molecular level populations indicates an accelerated reverse intersystem crossing.

Symmetry breaking and spectra of diphenyloctatetraene in n-alkanes

One- and two-photon excitation spectra, as well as absorption and emission spectra of diphenyloctatetraene (DPOT) in n-alkanes are investigated at low temperatures. For DPOT in n-octane we report on the measurements of one-photon excitation and emission spectra and for DPOT in n-tetradecane (TD) on the measurements of one- and two-photon excitation spectra and emission spectra. The spectra are governed by the transitions between the electronic ground (S0) and the two lowest electronic excited singlet states (S1,S2). The interpretation is based on allowed transitions and transitions induced by the S1–S2 coupling due to Herzberg–Teller promoting modes or due to static lattice-induced distortions of DPOT. A single noncentrosymmetric site is observed for DPOT in n-octane. For DPOT in TD a centrosymmetric and a noncentrosymmetric site are reported. The analysis indicates that there is a dynamical equilibrium in the population of these two sites. The experimental data are quantitatively studied by comparison wi...

Non-Lorentzian spectral diffusion line shapes in glasses: Analysis based on the two-level-system model

The dynamics of tunneling two-level systems (TLSs) or other slowly relaxing objects in glasses cause a diffusional broadening of the line shapes of chromophore impurity molecules. The standard tunneling TLS model for glasses predicts that the diffusional broadening is Lorentzian. Recently non-Lorentzian absorption line profiles of chromophores were discovered in a series of glasses by means of optical hole burning spectroscopy. This paper attempts to analyze possible reasons for the non-Lorentzian line shapes. The standard model is modified to account for the dispersion of the chromophore-TLS coupling strengths, for TLS concentration inhomogeneities, and for chromophore-TLS interactions other than dipolar. It is found that in general the above modifications lead to line shapes, which are more peaked at the center and show more pronounced wings than a Lorentzian of the same width, in agreement with the experimental observations.