One of the most important achievements of the Laboratory of electronic spectra of molecules was the development of methods of “selective spectroscopy of molecules in solid matrices”, that allowed to increase the real spectral resolution in studies of disordered systems up to 10⁵ times. These discoveries were the results of experimental investigations in 1972-1973 of the nature of broad spectral bands in doped solid solutions. In this experiments it was found, that at low temperatures the broadening of such spectra is inhomogeneous and spectra consist of a big number of narrow zero-phonon lines. It was shown that the fine structure in such spectra can be detected by using selective laser excitation (R.I. Personov, E.I. Al'shits, L.A. Bykovskaya [1,2,3]). The developed technique, Fluorescence Line Narrowing (FLN), was recognized and widely used as a powerful technique for detection of fine-structure spectra. The above mentioned experiments have shown that laser excitation can cause the photochemical and photophysical transformations, which could provide changes of spectra of the phototransformed molecules. These effects were taken as a principle for another selective spectroscopy method discovered in the Laboratory in 1974. It is Persistent Hole-Burning (HB) method [4],which was discovered independently in Estonian Institute of Physics.

At present the investigations being conducted in the Section can be conditionally separated in the two associate directions. The first is related to the perfection of the FLN and HB techniques and theirs application for the investigations of spectral diffusion processes in disordered solids (Prof. B.M. Kharlamov, N.I. Ulitsky, V.N. Krasheninnikov, A.A. Gorshelev). The second direction is related to the development and applications of new experimental techniques: photon echo (PE) and single molecule spectroscopy (SMS), for the study of spectral relaxation processes in doped amorphous system (Dr. Yu.G. Vainer, Dr. A.V. Naumov, Dr. M.A. Kol'chenko). The activity in the second direction was initiated by Dr. Yuri Vainer who joined the Laboratory in 1989. The method of incoherent PE, which was developed by Yu.G. Vainer and N.V. Gruzdev (1991) was used for investigations of fast dynamic processes in doped molecular systems. Since 2000 the improved SMS technique is also being used for investigations of glass dynamics (Yu.G. Vainer, A.V. Naumov).

All above mentioned experimental techniques allow to obtain a lot of unique information about the properties of impurity molecules and the nature of a matrix. In such studies the problems of molecular spectroscopy are deeply intertwined with the problems of solid state spectroscopy. In particular, one of the important fields of applications of developed selective spectroscopy techniques is the obtaining of information about dynamical processes in disordered solids. The impurity molecules in this case can be treated as microscopic spectral probes embedded in a transparent amorphous matrix. By using these methods the principal differences of dynamics of glasses and crystals were discovered. The complex investigations via above listed selective spectroscopy techniques allowed to obtain the unique data about low-temperature glass dynamics.

In addition to fundamental studies in the Section the new selective and highly sensitive methods of luminescent spectral analysis and spectral instrumentation are being developed.

1 Personov R.I., Al'shitz E.I., Bykovskaya L.A. Appearance of fine structure in fluorescence spectra of laser-excited complex molecules. Pis'ma v ZhETF 15 (10), 609 (1972)

2 Personov R.I., Al'shits E.I., Bykovskaya L.A. The effect of the structure appearance in laser-excited fluorescence spectra of organic conpounds in solid solutions. Optics Commun., 6, 169 (1972).

3 Personov R.I., Al'shits E.I., Bykovskaya L.A., Kharlamov B.M. Fine structure of luminescence spectra of organic molecules at laser excitation and nature of broad spectral bands of solid solutions. ZhETF, 65, 1825 (1973).

4 Kharlamov B.M., Personov R.I. , Bykovskaya L.A.. Stable “gap” in absorption of solid solutions of organic molecules by laser irradiation. Optics Commun., 12, 191, (1974). 

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Persistent hole burning spectroscopy
(B.M. Kharlamov, N.I. Ulitsky, V.N. Krasheninnikov, E.P. Snegirev, A.A. Gorchelev ) 

Persistent spectral hole burning (HB) technique proved to be a powerful tool for investigation of spectral relaxation in impurity glasses and is being successfully used by many research groups. Measurements of spectral dynamics in organic polymers and glasses using this method are continued in the Section. Combination of the results obtained by HB and photon echo (PE) methods gives important information about relaxation processes in the unique time scale from 10^-11 s to at least 10⁶ s. During last several years new interesting results of the investigation of low temperature relaxation in organic polymers were obtained [2,3] using HB technique. Experimental studies of non-equilibrium spectral diffusion (SD) were carried out [4], unique investigations of non-equilibrium SD, induced by the electric field were performed [5,6], light induced SD effect was discovered and interpreted [7]. The results of these investigations are summarized in [8]. Check-up criteria were worked out for finding the limitations of the two-level system (TLS) model for describing the relaxation processes in glasses, and experimental deviations from the TLS model predictions were detected [9,10]. Conventional description of SD by the TLS model was generalized for the case of non-equilibrium relaxation processes. 

One of the latest discoveries in the field of HB spectroscopy is the revealing of non-lorentzian shape of the holes in amorphous matrices [11]. The holes have a very characteristic profile with sharper peak and slower drop-down wings as compared to a Lorentz profile. It was performed a detailed theoretical analysis of impurity-matrix interaction mechanisms and the role of dispersion of parameters of this interaction in forming of spectral holes shape [12]. The obtained results provide great possibilities for studying of impurity-matrix interaction mechanisms in glasses and dispersion of parameters of this interaction by the precision measurements of spectral line profiles. 

We continue the investigations of kinetics of photoreactions, which are responsible for the HB mechanism. In particular, a new technique for the measurement of hole burning kinetics removing the influence of inhomogeneous broadening was worked out [13]. This method allows reliable measurements of such factors as dispersion of photoreaction efficiency, revelation of manifold channels of photoreaction, etc. 

The further examination of glass dynamics is aimed to determine the limits of TLS model in different glasses, to investigate in details “non-TLS” relaxation, to find correlation between relaxation characteristics and physicochemical structure of glasses and to develop a new model, describing the glass properties at low temperatures as well as near vitrifying temperature. 

One of the popular techniques for studying of impurity amorphous systems is the field experiment on the holes. Due to their small width, hole shapes are very sensitive to the external effects, which allows to investigate local field distributions in matrices as well as to determine a number of parameters of impurity molecules. During last several years there were performed experiments for studying of influence of external electric fields on the holes in spectra of centro-symmetrical molecules of H₂ -phtalocyanine and its metal-complexes in polymer matrices. It was the first time that tunable diode lasers of visible region were used in such experiments. A weak contribution of quadratic Stark effect was registered on the background of quasi-linear one [14,15], also an effect of metal atom “outlet” form molecular plane was examined [16,17]. Recently, the investigations of orbital Zeeman effect on degenerate S-S transitions of the molecules of the above mentioned class were proceeded. These experiments were started as early as in 80ths. First results of these new measurements showed to be very promising for studying of Jan-Teller dynamic effect and “crystalline” field effect in disordered molecular systems. 

Literature.

1  Maler H., Kharlamov B.M., Haarer D. Two-level system dynamics in the longtime limit: a power-law time dependence. Phys.Rev.Lett., 76 (12), 2085 (1996). 

2  Muller J., Haarer D., Khodykin O.V., Kharlamov B.M. Investigation of spectral diffusion in PMMA on time scales from 10 -5 to 10 4 seconds via transient and photophysical hole burning. Chem.Phys., 237, 483 (1998). 

3  Muller J., Maier H., Hannig G., Khodykin O.V., Haarer D., Kharlamov B. M. Long/time scale spectral diffusion in polymer glass. J. Chem. Phys., 113 (2), 876 (2000).

4  Fritsch K., Friedrich J., Kharlamov B.M. Non-equilibrium phenomena in spectral diffusion physics of organic glasses. J.Chem.Phys., 105, 1798 (1996). 

5  Maier H., Wunderlich R., Haarer D., Kharlamov B.M., Kulikov S.G.. Optical Detection of Electric Two Level System Dipoles in a Polymeric Glass. Phys.Rev.Lett., 74, 5252 (1995). 

6  Wunderlich R., Maier H., Haarer D., Kharlamov B.M. Optical investigation of low-temperature electric-field-induced relaxations in amorphous solids. J. Phys. Chem. B, 102 (50), 10150 (1998). 

7  Wunderlich R., Maier H., Haarer D., Kharlamov B.M.. Light-induced spectral diffusion in heavily doped polymers. Phys. Rev. B, 75, 1 (1997). 

8  Ěŕiĺr Í., Ęharlamov Â.Ě., Íŕŕrĺr D. Investigation of tunneling dynamics by optical hole-burning spectroscopy, in: “Tunneling Systems in Amorphous and Crystalline solids”, ed. by P.Esquinazy, (Springer, Berlin ). 1988. 

9  Muller J., Haarer D., Khodykin O.V., Kharlamov B.M. Long-time scale spectral diffusion in PMMA: Beyond the TLS model? Phys. Lett. A, 255, 331 (1999). 

10  Kharlamov B.M. Investigation of relaxation processes in organic glasses at low temperatures via hole burning spectroscopy. Examination of the limits of the two level system model. Opt. Spectrosc., 91, 490 (2001). 

11  Muller J., Haarer D., Kharlamov B.M. Non-lorentzian hole profiles in organic glasses caused by a distribution of optical line widths. Phys. Lett. A, 281, 64 (2001). 

12  Kharlamov B.M., Zumofen G. Non-Lorentzian spectral diffusion line shapes in glasses: Analysis based on the two-level-system model. J. Chem. Phys., 116, 5107 (2002). 

13  Gorshelev A., Snegirev E.P., Ulitsky N.I., Kharlamov B.M. Broadband hole burning: a new technique for the measurement of hole burning kinetics, removing the influence of inhomogeneous broadening. Chem. Phys., 285, 289 (2002). 

14  Snegirev E.P., Ulitsky N.I., Khodykin O.V., Personov R.I. Nonlinear Shtark effect on the holes in the spectrum of Zn- phtalocyanine in polymer films. Opt. Spectrosc., 84 (1), 47 (1998). 

15  Ulitsky N.I., Snegirev E.P., Khodykin O.V., Personov R.I. Diode laser stark spectroscopy on the persistent holes. Spectrochimica Acta Part A, 55 (10), 2015 (1999). 

16  Altmann R.B., Haarer D., Ulitsky N.I., Personov R.I. Matrix-induced and permanent dipole moment of Zn-octaethylporphin in polyvinylbutyral. J. Lumin., 56, 135 (1993). 

17  Ulitsky N.I., Snegirev E.P., Personov R.I. Shtark effect on the holes in absorption spectra of phtalocyanine and its aluminium complex in polymer matrices. Opt. Spectrosc., 92 (6), 931 (2002). 

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Photon echo spectroscopy
(Yu.G. Vainer, M.A. Kolchenko, A.V. Naumov)

Spectroscopic studies of doped disordered solids by using photon echo (PE) technique started in the Laboratory of electronic spectra in 1990. These studies became possible due to the creation of the experimental set-up and the development of the incoherent PE (IPE) technique (Yu.G. Vainer, 1989-1990) for the low-temperature measurements of the optical de-phasing and investigation of fast spectral diffusion processes in doped glasses. The first measurements on the created experimental setup resulted in observation of nanosecond spectral diffusion [1-3]. This experimentally observed effect is very important for understanding of the nature of elementary excitations in glasses. Similar results were simultaneously observed by the group of American researches (L.R. Narasimhan et al , Chem. Phys. Lett. (1991) v.176, N3,4). At present the systematic experimental and theoretical investigations of mixed glass dynamics by using of two versions of PE technique: IPE with broadband laser source and two-pulse picosecond PE (2PE) are carried out. 

The essential advantage of the IPE method is the opportunity to realize very high temporal resolution, which makes possible to study ultrafast relaxation processes in mixed glasses. In particular, ultrahigh temporal resolution (25-30 fs) permits to surely separate the zero-phonon line and phonon wing of PE decay curves (see Fig. 1) and thus to perform the measurements of the optical de-phasing times up to 100 K and more. 

Fig. 1. Decay curves of two-pulse picosecond echo (a) and incoherent photon echo (b-d) for the system zink-octaethylporphyne in toluene. Dotted line in (a) corresponds to exponential fit, which was used for determination of optical dephasing time, T₂ . Insert in (d) demonstrates the separation of decay curve parts, which correspond zero-phonon line (ZPL) and phonon wing (PW).

In collaboration with the researches of Bayreuth University (D. Haarer, S. Zilker, Germany), whose experimental setup is equipped by He-3 optical cryostat, the unique measurements of optical de-phasing processes in mixed glasses at very broad temperature region (0.35 -100 K) were performed for the first time. This enabled to obtain unique information about relaxation processes in glasses in a broad temperature region [4-10] (see Fig. 2). These measurements allowed determining for the first time the temperatures, at which optical de-phasing due to interaction of impurity molecules with quasilocal low-frequency vibration modes of amorphous matrix becomes apparent. In the course of these measurements the dispersion of optical de-phasing times Ň₂ in doped amorphous system tetra-tertbutylterrylene in polyisobuthylene was discovered for the first time. The analysis of the Ň₂ - dependence in systems under study has shown the deviation of the low-temperature part of this dependence from the predictions of the PE theory of low-temperature glasses. 

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Spectroscopy of single chromophore molecules, embedded in disordered organic matrix
(Yu.G. Vainer, A.V. Naumov)

This research area was started in the Section of electronic spectra of molecules since end of 2000 and at present it is one of the main fields of activity in the Section (see, for example [1,2]). Availability of single molecule spectroscopy (SMS) for researches of dynamics of amorphous solids is defined by the fact, that contrary to the traditional techniques SMS gives the information on the parameters and dynamics of a microscopic environment of an impurity molecule, undisturbed by averaging over ensemble of dye molecules. So, for example, SMS-experiments in amorphous polymers presented the first direct proof of existence of two-level systems (TLSs) in the disordered condensed matter and demonstrated that the interaction of chromophores with tunneling TLSs causes the jumps of spectral line of these dye molecules. 

The important features of the spectra of single impurity molecules (SM) in low-temperature disordered solid matrices are very complex shapes of spectra and their variations with time [1,2]. The reason is random nature of local dynamics of disordered media. Figure 1 demonstrates the measured SM spectra of tetra-tert-butylterrylene embedded in pure amorphous polyisobutylene matrix. One can see that SM spectra can be very complicated and consist of several peaks. A number of the fundamental questions arise: How to derive the information about general properties of a glassy matrix from very individual and random SM spectra? How to characterize adequately such complicated spectra? What are the statistical properties of SM spectra in amorphous solids?

In our Section a new approach has been developed for the SMS-studies of low-temperature glass dynamics. This approach allowed to overcome the above mentioned problems [3,4] and opened the possibilities of the wide using of SMS-technique for investigations of low-temperature doped amorphous systems. 

Fig. 2.  The example of temporal evolution of spectra of single tetra-tert-butylteerylene molecules in polyisobutylene matrix registered repeatedly at T  = 2 K (a,b). The results of measurements are presented in the form of three-dimensional (3D-plot) and two-dimensional (2D-plot) plot, (a) and (b) correspondingly. The color scale of the points on (a) and (b) is the function of the fluorescence intensity in the corresponding poins of fast spectra. The spectrum of the molecule A does not jump noticeably during the time of observation The spectrum of the molecule B jumps between 4 spectral positions (interaction with two “strong interacting” two-level systems). The resulting integral spectrum (c) corresponds to the sum of all “fast” spectra inside of selected time interval 120 seconds shown on (b) by the solid horizontal lines. One can see that sum spectrum corresponds to two single molecules: molecule A (singlet) and molecule B (triplet). 

The general idea of the developed approach is based on the measurement of a big number of SM spectra followed by theirs statistical analysis. The averaging algorithm in this approach can be chosen depending on the addressed problem. It allows saving the desired information about the dynamics of the system under study during data processing. This approach principally differs from the traditional spectroscopic techniques (HB and PE), which are usually used for the study of the doped glasses. The ensemble averaging intrinsic to HB and PE methods leads to the irretrievable losses of large part of the information about the spectral dynamics of doped systems. For example, HB-experiments demonstrate quasi-lorentian integral spectra of the impurity molecules; hence the information about individual asymmetry of SM spectral line is lost, whereas in SMS-experiments such information is saved. 

The special method of SM-spectra detection was developed to solve the problem of the identification of experimental complicated spectra. The main ideas of this method are the relatively fast (in comparison with total time of signal accumulation) repeated detection of fluorescence excitation spectra in some selected spectral region and the subsequent analysis of a temporal behavior of the registered spectra. The main point of the developed methodic is demonstrated in Fig. 2 [ 3 , 4 ]. 

We have suggested using the well-known concept of moments of distributions for adequate characterization of complicated SM spectra [3,4]. Using of moments (or cumulants, which are the derived functions of moments) allows to take into account correctly all details of complicated spectral shape, including both broadening and splitting of spectral lines. Furthermore it allowed discovering the common statistical regularity, to which the individual SM spectra in low-temperature glasses obey. In collaboration with the colleagues from Massachusetts Institute of Technology (E. Barkai, R. Silbey, Cambridge, USA) we have shown theoretically and experimentally that SM spectra in low-temperature glasses obey the Levy stable law (generalized Gauss statistics) [5]. This fact is clearly demonstrated in Fig. 3, which displays the distributions of the first and second cumulants of measured SM spectra and the fittings of both cumulants based on Levy statistics.

Fig. 3.  Probability density of the first (a) and second (b) cumulants of spectra of single tetra-tert-butylterrylene molecules embedded in polyisobutylene matrix (temperature 2 K, acquisition time 120 s). The dots are experimental results; the curves are one-parameter fits to the functions, which are the partial cases of general Levy statistics: a Lorentian (a) and Smirnov's probability density (b). 

Using the developed approach a series of researches of low-temperature dynamics of organic molecular system tetra-tert-butylterrylene in polyisobutylene were performed in collaboration with the colleagues from the Department “Experimental Physics IV” of the University of Bayreuth (L. Kador, M. Bauer, Bayreuth , Germany ). The unique information about the optical de-phasing and spectral diffusion processes on the microscopic level was obtained. In particular the effective value of minimal distance of interaction between an impurity center ( dyes) and two-level systems was estimated [3,4,6]. The distribution of frequency shifts, generalized widths and asymmetries of the individual SM spectra in the system under study were determined [3,4]. At the first time the contributions of tunneling TLSs and quasi-localized low-frequency vibration modes of matrix to the broadening processes of spectral lines of impurity centers were clearly separated on the microscopic level [7].

Fig. 4.  (ŕ) The spectral trail of one of the SM, measured at T  = 4.5 K and (b,c) the integral spectra. The corresponding time intervals (60 s) are shown on (a) by the horizontal dashed lines. (d) The scheme of three-level system, which “strong interaction” with a chromophore allows to explain the observed temporal evolution of SM spectrum. 

Recently Yu.G. Vainer and A.V. Naumov in collaboration with L. Kador and M. Bauer form the University of Bayreuth (Germany) have slightly changed the research area. We have started the investigations of the temporal evolution of individual SM spectra with the goal to investigate the interaction mechanisms of elementary low-energy excitations with impurity molecules in disordered doped systems. Such analysis allowed, for example, discovering the examples of temporal [1] and temperature [8] behavior of SM spectra, which can not be explained in the frame of modern conception of low-temperature glasses nature. In particular, some observations can be treated as a result of interaction between a single chromophore and “three-level system” (Fig. 4). The temporal evolution of some SM spectra can be interpreted as a direct observation of mutual TLSs interaction. Thus, the SMS-researches, which are performed in the Section, highlight the fundamental questions, related to the dynamics of disordered condensed matter. 

References

1  Vainer Yu.G., Naumov A.V., Bauer M., Kador L.. Dynamics of amorphous polymers at low temperatures and temporal evolution of spectra of single impurity molecules: I. Experiment. Optics and Spectroscopy, 94, 864 (2003). 

2  Vainer Yu.G., Naumov A.V., Bauer M., Kador L. Dynamics of amorphous polymers at low temperatures and temporal evolution of spectra of single impurity molecules: II. Model calculations and analysis of results. Optics and Spectroscopy, 94 (6), 873 (2003). 

3  Naumov A.V., Vainer Yu.G., Bauer M., Zilker S.J., Kador L.  Distributions of moments of single-molecule spectral lines and the dynamics of amorphous solids. Phys. Rev. B, 63, art. no. 212302 (2001). 

4  Naumov A.V., Vainer Yu.G., Bauer M., Kador L. Moments of single molecule spectral lines in low temperature glasses: measurements and model calculations. J. Chem. Phys., 116, 8132 (2002). 

5  Barkai E., Naumov A.V., Vainer Yu.G., Bauer M., Kador L. Levy Statistics for Random Single-Molecule Line Shapes in a Glass. Phys. Rev. Lett., 91 (7), art. no. 075502 (2003). 

6  Naumov A.V., Vainer Yu.G. Minimal distance between chromophore and two-level systems in amorphous solids: effect on photon echo and single molecule spectroscopy data. J. Lumin., 98 (1-4), 63 (2002).

7  Vainer Yu.G., Naumov A.V., Bauer M., Kador L. Dynamics of a doped polymer at temperatures where the two-level system model of glasses fails: study by single-molecule spectroscopy. J. Chem. Phys., 119, (13), 6296 (2003). 

8  Bauer M., Kador L., Vainer Yu.G., Naumov A.V. Thermal activation of two-level systems in a polymer glass as studied with single-molecule spectroscopy. J. Chem. Phys., 119 (7), 3836 (2003). 

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Spectral instrumentation
(Dr. A.M. Pyndyk, V.N. Krasheninnikov, N.I. Ulitskii, V.P.Vinogradov) 

The development of the technique of luminescence analysis of low concentrations of organic and inorganic species has demanded the design and manufacturing of the compact spectrometers for 190 – 1100 nm spectral range. For the analytical applications these newly designed spectrometers had to possess low level of background light scattering, sensitive and fast scheme for detection of spectra, friendly interface with the PC and, finally, low price. 

The thorough developmental work was done to realize the high contrast (“spectral purity”) of the compact short-focal spectrometers. The vertical Ebert-Fasty scheme provides better contrast as compared to the horizontal one, but labor consuming design work had to be done to realize this vertical scheme. Different prisms and gratings could be used as the dispersive elements in the spectrometer. The inverse dispersion from 20 nm/mm to 1.7 nm/mm was realized. The double pass compact monochromator with high contrast was also designed. These monochromators could perform both with the schemes of the dispersion subtraction and summation. The contrast ration of 10^-7 was realized with this monochromators. 

The multi-channel photodetector based on microchannel electro-optical detector (EP-10, EP-16) and sensitive super-videcon (LI-702) was designed for the detection of low intense spectra (for example the spectra of Raman scattering). This detector enables to detect in real time weak optical spectra (at the level of the photon counting). This is essentially important for the detection of Raman spectra of the biological samples in UV spectral range. This detector can also perform in a pulse mode (repetition rate 10³ Hz, acquisition time 10^-6 s), which is also very important for the detection of Raman spectra and the luminescence of high temperature samples. 

The designed multi-channel detection scheme in combination with the spectrometer (Ebert-Fasty scheme, f = 150 mm) was used for the detection by luminescence technique of low concentrations of uranyl in water ( C~10^-7 g/L ), for registration of Raman spectra and investigation of aurora and airglow.

The second registration scheme was designed based on the fast mechanical scanning of the exit slit of the monochromator and the fixed optical fiber along the output spectral plane. A photomultiplier, photodiode cooled by liquid nitrogen could be used as the detectors. In the third design a CCD array (spectral range 190 – 1100 nm) was used. A portable spectrometer with CCD-detector was successfully used for investigations of the emission spectra of the turbulent reacting flows ignited by electric discharge in aerodynamic tubes (in collaboration with Central Aerodynamic Institute). 

During last 10 years about 20 compact spectrometers were designed and manufactured for 23 institutes and organizations in Russia . An instrument with a multi-channel detector was used for the investigations in the frame of a contract with LLL (USA). Different instruments were presented at the exhibitions in Germany , USA , Japan . Two instruments were supplied to a university in USA. 

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