J. Kikas

Low-temperature spectroscopy of organic molecules in solids by photochemical hole burning

Characteristic features of photochemical hole-burning (PHB) in the impurity spectra of low-temperature solids and PHB applications in molecular spectroscopy are considered. The evolution of a no-phonon hole and a phonon sidehole in excitation and fluorescence spectra is analysed on the basis of model calculations. Some more complex models for PHB are considered, which take into account reverse reactions, the optical thickness of the sample, the inhomogeneous dispersion of both homogeneous linewidths and transition energies of the photoproduct and quasi-static impurity-impurity interactions. The effects of PHB on fluorescence line narrowing are discussed. By PHB homogeneous linewidths of purely electronic and vibronic no-phonon lines in the spectra of some porphine and phthalocyanine derivatives in various matrices are measured and their temperature dependence is studied. The latter is found to be essentially different in polycrystalline and glassy matrices. Line-broadening mechanisms are discussed. By PHB the existence of an inhomogeneous distribution of vibrational frequencies in molecular impurities is established. The applications of PHB in the studies of photochemical reactions in solid solutions of phthalocyanine derivatives and tetracene are regarded and the mechanisms of these reactions are discussed. The results obtained by PHB for chlorophyll and its derivatives are presented.

Inhomogeneous broadening of local vibrations in spectra of organic molecules in solid matrices

Abstract The existence of the inhomogeneous distribution of local vibrational frequencies in addition to the inhomogeneous distribution of pure electronic frequencies in the spectra of impurity molecules is demonstrated by the hole-burning technique. The half-width of the inhomogeneous vibrational distribution is determined.

Effects of inhomogeneity and site selective impurity—phonon coupling In solid solutions

Abstract Inhomogeneously broadened fluorescene spectra of low-temperature impurity molecules are considered. It is demonstrated that the dependence of the spectra on monochromatic excitation frequency is caused first of all by different ratios of the molecules excited in no-phonon lines to those excited in phonon wings at different frequencies. There is no need to suppose any drastic alteration of the electron—phonon coupling. A possibility of testing the existence of such an alteration is supposed.

Spectral hole burning at high hydrostatic pressure

Abstract The influence of hydrostatic pressure up to about 5 kbar at 4.2 K on the spectral holes burnt and measured at fixed values of pressure has been studied for chlorin molecules embedded in a polystyrene matrix. The narrowing of spectral holes by nearly a factor of two takes place at 5.1 kbar as compared with the results obtained at normal pressure. An incomplete restoration of the widths of the holes burnt at normal pressure after pressure cycling is observed. Possible mechanisms of these phenomena are discussed.

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 ...

Hydrostatic pressure effects on spectral hole burning in a Shpol’skii system

Hydrostatic-pressure effects on persistent spectral holes in a polycrystalline solid solution of chlorin (Chl) in n-octane (C8:Chl, a Shpol’skii system) at 4.2 K were studied at high-pressure [up to 8.4 kbars (1 kbar = 750.06 kTorr)] isobaric (burning and recording at the same pressure) conditions as well as in low-pressure (21–39 bars) nonisobaric (burning and recording at different pressures) experiments. A significant decrease of the hole width was seen at high pressures, with the FWHM of holes in the 15 730-cm−1 line being 130 and 45 MHz at 1 bar and 5.5 kbars, respectively. The latter value approaches the normal-pressure lifetime limit of 40 MHz. A pressure-induced site-dependent instability of spectral holes was found at pressures above 5.5 kbars. High-pressure shifts of Shpol’skii lines and low-pressure hole shifts gave the same pressure coefficient, −4.8 cm−1/kbar, for the S1 ← S0 (0–0) transition energy. This coefficient yielded the value of 13.2 Mbars−1 for the local compressibility. Reversible non-Gaussian non-area-conserving hole broadening was observed in nonisobaric low-pressure experiments.

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.

Determination of homogeneous vibronic spectra and the inhomogeneous distribution function of impurity molecules in solids by fluorescence saturation

Abstract Methods are developed for determining homogeneous vibronic spectra and the inhomogeneous distribution function of impurity molecules in low-temperature solid matrices by optical saturation of no-phonon transitions. The possibilities are demonstrated with solid solutions of perylene in benzophenone and n-hexane at 4.2 K.

Anomalous impurity spectra in low-temperature glasses

Spectral holes have been measured for chlorin dopant educt and photoproduct states in five different glasses, o‐terphenyl, benzophenone, polystyrene, ethanol, and triethylamine at 5 K. In contrast to the conventional picture of zero‐phonon holes and phonon sideholes observable in the educt state, spectral holes in the photoproduct state reveal rather unconventional features. No zero‐phonon hole is observable yet the spectra exhibit a peculiarity resonant with the burning frequency (‘‘Λ‐spectra’’). The low‐temperature vibronic spectra of an impurity in a 1D elastic lattice are calculated and demonstrated to match qualitatively well the Λ‐spectra in the case of weak electron‐phonon coupling. The applicability of a 1D model is argued to be due to the formation of a local structural defect (planar crack) caused by dopant phototransformation.

High pressure effects on low temperature relaxation in solids

Abstract Persistent spectral hole burning (SHB) was used to probe the influence of high hydrostatic pressure (up to 8.4 kbar) on optical relaxation in chlorin (Chl)-doped glassy (polystyrene, PS) and (poly)crystalline (n-octane, C 8 Shpolskii system) samples at 4.2 K. Pressure induced width reduction of isobarically burnt and measured holes was found, FWHM of holes being for PS:Chl 4.3 and 2.4 GHz at 1 atm and 5.1 kbar, respectively, and for C 8 :Chl (15730 cm −1 line) 130 and 45 MHz at 1 atm and 5.5 kbar, respectively. The latter value approaches the lifetime limit of 40 MHz. For the glassy matrix the effect of hole narrowing gives evidence of the dominant role of low frequency quasilocal vibrations in determining the hole width. Pressure induced instability of photoproduct was found in C 8 :Chl at pressures above 5.5 kbar.