Peter Schellenberg

Spin‐conversion relaxation in dimethyl‐s‐tetrazine doped n‐octane: Deuteration and symmetry breaking

Spectral hole burning is used to measure spin‐conversion relaxation of methyl groups attached to a dye probe. The dye is dimethyl‐s‐tetrazine, the host is n‐octane, which is, at low temperatures, a Shpol’skii‐matrix. The temperature dependence of the spin‐conversion relaxation follows the same pattern for the protonated and the perdeuterated methyl‐rotor. However, the relaxation rates of the perdeuterated rotor are higher by almost two orders of magnitude. Partial deuteration destroys the rotor symmetry and leads to a breakdown of the correlation between nuclear spin and rotational wave function.

A compact versatile femtosecond spectrometer

A compact apparatus for femtosecond pump–probe experiments is described. The apparatus is based on a cavity-dumped titanium:sapphire laser. Probe pulses are generated by focusing weak (∼1 nJ) pulses into a microstructure fiber that produces broadband continuum pulses with high efficiency. With the pump pulses compressed and probe pulses uncompressed, the rise time of the pump–probe signals is <100 fs. The 830 nm pump pulses are also frequency doubled to generate light for excitation at 415 nm. The versatility of the spectrometer is demonstrated by exciting molecules at either 830 or 415 nm, and probing at wavelengths ranging from 500 to 950 nm. Some results on the green fluorescent protein are presented.

Spectral hole burning in polymorphic systems: Single site pressure phenomena and glassy behavior

We measured the pressure‐induced shift and broadening of spectral holes for 14 different impurity sites in a polycrystalline solid solution of chlorin (17,18‐dihydro‐21H,23H‐porphine) in benzophenone. In addition, we investigated the glassy phase of the same system in a frequency range which covered the entire inhomogeneously broadened band. The temperature range of the experiments was between 1.5 and 4.2 K. Pressure levels up to 10 MPa were applied. In the glassy phase, we observed a linear frequency dependence of the shift per pressure. However, in the crystalline phase, the general trend of the data clearly indicated a nonlinear frequency dependence. Yet, the individual sites, especially those with lower frequencies, showed a kind of stochastic scattering around the general trend behavior. The results were interpreted within the frame of existing models. The experiments demonstrate that matrix isolation combined with hole burning spectroscopy of large molecular impurities in polymorphic host materials ...

Enhancing visibility of graphene on arbitrary substrates by microdroplet condensation

In order to take advantage of the enormous potential of graphene for future electronic microcircuits and other applications it is necessary to develop reliable, rapid, and widely applicable methods to visualize graphene-based structures. We report here on a microdroplet condensation technique, which allows for quick visual identification of graphene on a variety of substrates, including some which were previously considered unsuitable for the visualization of carbon layers. The technique should also be applicable to visualize artificially patterned graphene structures which are expected to be key technologically enabling components in electronic microcircuits and other applications.

Symmetry species conversion in rotational tunneling systems observed by hole burning: high resolution spectroscopy of dimethyl-s-tetrazine in n-octane

We present a summary of our results on methyl group spectroscopy and relaxation measurements of the dye molecule dimethyl-s-tetrazine and its CD3- and CDH2-substituted derivatives in a n-octane host. In the CH3- and CD3-substituted derivatives, two hole burning mechanisms occur: one is based on nuclear spin-transformation, the other is a structural transformation. The mechanism based on spin transformation leads to sharp antiholes, spaced by 37 and 20 GHz from the burning laser frequency for CH3 and CD3, respectively. The structural burning mechanism leads to side holes. Surprisingly, the splitting of the side holes is different from that of the antiholes. This phenomenon is interpreted in terms of two different dye species, which are distinguished through their local environment. Both species have very different yields for the structural and the nuclear spin phototransformation process. From the recovery of the central hole, the relaxation of the rotational tunneling states was measured as a function of temperature. The data support a Raman-type phonon scattering process. Deuteration does not slow down the relaxation but, instead, increases it by almost two orders of magnitude. According to our knowledge these are the first measurements of symmetry species conversion times of isotopic derivatives of methyl groups. Within the assumption of a Raman-type conversion mechanism we estimate a rather low hindering potential barrier for the rotors.

THERMAL RECOVERY AND SPECTRAL DIFFUSION OF PHOTOCHEMICAL HOLES IN POLYMORPHIC SYSTEMS

We present comparative investigations of doped benzophenone in its glass and crystal phases by using thermal cycling hole burning techniques. Chlorin and s‐tetrazine were used as probe molecules. We measured the distribution of reactive barriers and the spectral diffusion behavior in both phases. The distribution in the glass is broad and is well described by a Gaussian. In the crystal, there are rather well‐defined barriers, yet they are different for different sites. The respective kinetics is exponential and its temperature dependence is Arrhenius‐like in contrast to the glass phase. Thermally induced spectral diffusion broadening in the glass phase is strong and shows the pattern well known from other organic glasses. In the crystal, it is much weaker, yet it is not completely absent. The experiments seem to indicate a threshold temperature ∼40 K.

On the distribution of reactive barriers in disordered materials

We investigated the hole‐burning photoreaction of a dye complex in alcohol glass which undergoes both photochemical and photophysical transformations. By measuring separately the disappearance of the photoproduct at the respective wavelength ranges under thermal cycling conditions, we found that the photochemically transformed species recovered according to a Gaussian distribution of barrier heights, whereas the photophysically transformed species recovered in accordance with a 1/(V)1/2 distribution. It is suggested that this behavior is rather general and is intimately related to the nature of the phototransformation process.

A pressure tuning hole burning study of resorufin in protic solvents

Abstract Resorufin in protic solvents is present in two forms. They have a strongly temperature and solvent dependent equilibrium. The two forms can be interchanged photochemically which seems to be the major hole burning mechanism of this chromophore. We performed pressure tuning hole burning experiments in the long wavelength origins of both forms. In the range where the spectral bands of the two forms overlap, holes split under pressure and a bifurcation of the color effect (frequency dependence of the pressure induced shift) occurs. We could identify two well-defined absorption frequencies with vanishing pressure shift.

Enhancement of graphene visibility on transparent substrates by refractive index optimization.

Optical reflection microscopy is one of the main imaging tools to visualize graphene microstructures. Here is reported a novel method that employs refractive index optimization in an optical reflection microscope, which greatly improves the visibility of graphene flakes. To this end, an immersion liquid with a refractive index that is close to that of the glass support is used in-between the microscope lens and the support improving the contrast and resolution of the sample image. Results show that the contrast of single and few layer graphene crystals and structures can be enhanced by a factor of 4 compared to values commonly achieved with transparent substrates using optical reflection microscopy lacking refractive index optimization.

Temperature broadening of impurity transitions in the crystalline and glassy phase of benzophenone

Abstract The temperature dependence of the width of spectral holes in the crystalline and glassy phase of benzophenone:chlorin solid solutions is measured between 1.5 and 10 K. In the crystal, the holewidth levels on Γ(0) = 45 MHz for T T > 5 K, it exhibits a rapid increase. In the glassy phase, the holewidth exceeds that in the crystalline state by more than a factor of ten. For T