- Email: [email protected]
Structural relaxations in amorphous water
JOURNALOF
LUMINESCENCE EUEMER
Journal of Luminescence
72-74
(1997) 427429
Structural relaxations in amorphous water T. Giering, D. Haarer * Institute of’ Physics and BIMF,
University of Bayreuth. P. 0. Box 101?51, D-95440 BaJreuth,
German)
Abstract In order to study the structural relaxations of vapor condensed amorphous water (ASW) by means of spectral hole-burning water matrices were doped with free-base phthalocyanine as a molecular probe. Structural relaxations are found which change the energy landscape of the amorphous solid upon annealing to temperatures far below the glass transition. Keywords: Amorphous
The
capability
water; Spectral diffusion;
to form
network
Spectral hole-burning
structures
with
the main cause of the numerous anomalies of liquid water but also is responsible for the extraordinary behavior of amorphous water. The various structural modifications of this low-temperature phase of disordered water may be distinguished by their preparation techniques, and mainly, have a weakly pronounced glass transition above 130K in common. Strong relaxations are already found far below this temperature, especially for the two modifications which are prepared by condensing water onto a cryogenic substrate either from the gas phase leading to amorphous solid water (ASW) or directly from the liquid producing hyperquenched glassy water (HGW). For both the systems, it was previously shown that the technique of spectral hole-burning offers a suitable method to characterize the structural relaxations which are observed upon annealing the matrices above their preparation temperature [l, 21. To this end we doped ASW with the dye molecule free-base phthalocyanine (HzPc) as an optical probe using an experimental setup which was described behydrogen
bonds
is not only
* Corresponding author. Tel.: 49-921-55-3240; 3250; e-mail: [email protected].
fax: 49-92
I -55-
0022-23 13/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved PII SOO22-2313(96)00232-3
fore [l] and performed hole-burning experiments at the center of the purely electronic (0-0)-absorption band of HzPc. The homogeneous line width of HzPc which was always measured at a temperature of 4.2 K narrows considerably upon annealing of the matrix. It was reported that the reduction in line width becomes detectable at a temperature of about 70K [l], which is far below the glass transition temperature and on annealing towards the glass transition temperature reaches a final value which is only about 16% of the line width measured in the fresh matrix before annealing. To characterize the observed reduction of the homogeneous line width we measured the time-resolved spectral diffusion (SD) which broadens the spectral holes as a function of time. Upon annealing we found a pronounced decrease of the SD-effect which is due to the reduction of the TLS-number density. By combining the two experiments we were then able to show that the reduction of the homogeneous line width is indeed due to the reduction of the TLS-density [3], as was proposed by Small et al. in doped films of HGW on the basis of the temperature dependence of the homogeneous line width [2]. Further information about the TLS ensemble may be gained from temperature cycle experiments of
428
T. Giering. D. Haarerf Journal of Luminescence
I”.
1
0
20
Fig. 1. Thermally induced spectral diffusion measured 130 K (lower c&e)
‘.
.
72 -74 11997) 427-429
1.
40 cycling temperature [K]
.
1..
60
.
4
80
in a fresh matrix of ASW (upper curve) and in the same matrix after annealing
hole-burning spectra. To this extent, spectral holes are burnt at a temperature of 4.2K and are cycled to higher temperatures. To prevent the complications with the temperature-dependent dephasing, the spectral holes are read out at the bum temperature again and, thus, the experiment is exclusively sensitive to the TLS-dynamics [4]. It monitors the irreversible broadening of spectral holes as shown in Fig. 1, which gives directly the thermally induced spectral diffusion. Referring to the static potential hypersurface sketched along a highly dimensional configuration coordinate, the hole-burning process marks a specific configuration and the cycle experiment measures the probability that the system finds back to the same minimum when the temperature is raised between two successive scans. For an ideal crystal the system would always be trapped in the same (absolute) minimum and no irreversible broadening would be observed upon thermal cycling, whereas the rough potential landscape of amorphous solids enables the system to reach another (local) minimum resulting in an irreversible broadening. For the fresh matrix (i.e. directly after the preparation) a strong irreversible broadening may be
to
observed which is comparable to the behavior of thermodynamically fragile alcohol glasses. The fit to a model which considers tunneling and thermal activation as parallel processes [4] shows that tunneling dominates in the fresh matrix up to temperatures as high as 20K. However, in the annealed matrix, the irreversible broadening is not only reduced to a fraction of its initial value, but has also changed the broadening mechanism, and thermal activation already dominates the tunneling above 9 K. From the decreased thermal broadening it may be concluded that the potential hypersurface is irreversibly changed upon annealing from a rough surface in the fresh matrix to a smoothened surface, where only the deep and broad maxima survive. The reduction of the density of potential minima and maxima directly corresponds to the decrease of the TLS-density which may be measured via spectral hole burning. From these experiments it may be concluded that annealing close to the glass transition temperature not only reduces the density of those TLS which show dynamics at helium temperatures and govern the homogeneous line width in the fresh matrix. It also decreases the TLS which cause the strong
T. Giering, D. Haarer/ Journal of Luminescence
thermal broadening at higher temperatures and, thus, the thermally activated barrier crossing dominates nearly the whole temperature range. According to the classification by Angel1 [5] the observed reduction of the density of potential minima and maxima corresponds to the transition from a fi-agile to a strong glass as derived from the static properties of the glass potential. Amorphous water thus is another system that fits the correlation published by Sokolov et al. between the TLS-density as a lowtemperature property on the one hand and the fragility measured at the glass transition on the other [6]. The particularity of amorphous water is that both, the fragile and strong glass-like behavior may be observed in a single system. These findings may be understood on the basis of an H-bonded network which is strongly disturbed in the fresh matrices and forms tetragonal bonds upon annealing to higher temperatures.
72-74 (1997)
427429
429
This work was supported by the Deutsche Forschungsgemeinschaft (Sonder-forschungsbereich 213).
References [I] T. Giering and D. Haarer, J. Lumin. 66, 67 (1996) 299. [2] W.-H. Kim, T. Reinot, J.M. Hayes and G.J. Small, J. Phys. Chem. 99 (1995) 7300. [3] T. Giering and D. Haarer, Chem. Phys. Lett. 261 (1996) 677. [4] W. Kijhler, J. Zollfrank and J. Friedrich, Phys. Rev. B 39 (1989) 5414. [5] R. Biihmer and C.A. Angell, in: Disorder Effects on Relaxational Processes, eds. R. Richert and A. Blumen (Springer, Berlin, 1994) Ch. 2. [6] A.P. Sokolov, E. Riissler, A. Kisliuk and D. Quitmann, Phys. Rev. Lett. 7 I (1993) 2062.
LUMINESCENCE EUEMER
Journal of Luminescence
72-74
(1997) 427429
Structural relaxations in amorphous water T. Giering, D. Haarer * Institute of’ Physics and BIMF,
University of Bayreuth. P. 0. Box 101?51, D-95440 BaJreuth,
German)
Abstract In order to study the structural relaxations of vapor condensed amorphous water (ASW) by means of spectral hole-burning water matrices were doped with free-base phthalocyanine as a molecular probe. Structural relaxations are found which change the energy landscape of the amorphous solid upon annealing to temperatures far below the glass transition. Keywords: Amorphous
The
capability
water; Spectral diffusion;
to form
network
Spectral hole-burning
structures
with
the main cause of the numerous anomalies of liquid water but also is responsible for the extraordinary behavior of amorphous water. The various structural modifications of this low-temperature phase of disordered water may be distinguished by their preparation techniques, and mainly, have a weakly pronounced glass transition above 130K in common. Strong relaxations are already found far below this temperature, especially for the two modifications which are prepared by condensing water onto a cryogenic substrate either from the gas phase leading to amorphous solid water (ASW) or directly from the liquid producing hyperquenched glassy water (HGW). For both the systems, it was previously shown that the technique of spectral hole-burning offers a suitable method to characterize the structural relaxations which are observed upon annealing the matrices above their preparation temperature [l, 21. To this end we doped ASW with the dye molecule free-base phthalocyanine (HzPc) as an optical probe using an experimental setup which was described behydrogen
bonds
is not only
* Corresponding author. Tel.: 49-921-55-3240; 3250; e-mail: [email protected].
fax: 49-92
I -55-
0022-23 13/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved PII SOO22-2313(96)00232-3
fore [l] and performed hole-burning experiments at the center of the purely electronic (0-0)-absorption band of HzPc. The homogeneous line width of HzPc which was always measured at a temperature of 4.2 K narrows considerably upon annealing of the matrix. It was reported that the reduction in line width becomes detectable at a temperature of about 70K [l], which is far below the glass transition temperature and on annealing towards the glass transition temperature reaches a final value which is only about 16% of the line width measured in the fresh matrix before annealing. To characterize the observed reduction of the homogeneous line width we measured the time-resolved spectral diffusion (SD) which broadens the spectral holes as a function of time. Upon annealing we found a pronounced decrease of the SD-effect which is due to the reduction of the TLS-number density. By combining the two experiments we were then able to show that the reduction of the homogeneous line width is indeed due to the reduction of the TLS-density [3], as was proposed by Small et al. in doped films of HGW on the basis of the temperature dependence of the homogeneous line width [2]. Further information about the TLS ensemble may be gained from temperature cycle experiments of
428
T. Giering. D. Haarerf Journal of Luminescence
I”.
1
0
20
Fig. 1. Thermally induced spectral diffusion measured 130 K (lower c&e)
‘.
.
72 -74 11997) 427-429
1.
40 cycling temperature [K]
.
1..
60
.
4
80
in a fresh matrix of ASW (upper curve) and in the same matrix after annealing
hole-burning spectra. To this extent, spectral holes are burnt at a temperature of 4.2K and are cycled to higher temperatures. To prevent the complications with the temperature-dependent dephasing, the spectral holes are read out at the bum temperature again and, thus, the experiment is exclusively sensitive to the TLS-dynamics [4]. It monitors the irreversible broadening of spectral holes as shown in Fig. 1, which gives directly the thermally induced spectral diffusion. Referring to the static potential hypersurface sketched along a highly dimensional configuration coordinate, the hole-burning process marks a specific configuration and the cycle experiment measures the probability that the system finds back to the same minimum when the temperature is raised between two successive scans. For an ideal crystal the system would always be trapped in the same (absolute) minimum and no irreversible broadening would be observed upon thermal cycling, whereas the rough potential landscape of amorphous solids enables the system to reach another (local) minimum resulting in an irreversible broadening. For the fresh matrix (i.e. directly after the preparation) a strong irreversible broadening may be
to
observed which is comparable to the behavior of thermodynamically fragile alcohol glasses. The fit to a model which considers tunneling and thermal activation as parallel processes [4] shows that tunneling dominates in the fresh matrix up to temperatures as high as 20K. However, in the annealed matrix, the irreversible broadening is not only reduced to a fraction of its initial value, but has also changed the broadening mechanism, and thermal activation already dominates the tunneling above 9 K. From the decreased thermal broadening it may be concluded that the potential hypersurface is irreversibly changed upon annealing from a rough surface in the fresh matrix to a smoothened surface, where only the deep and broad maxima survive. The reduction of the density of potential minima and maxima directly corresponds to the decrease of the TLS-density which may be measured via spectral hole burning. From these experiments it may be concluded that annealing close to the glass transition temperature not only reduces the density of those TLS which show dynamics at helium temperatures and govern the homogeneous line width in the fresh matrix. It also decreases the TLS which cause the strong
T. Giering, D. Haarer/ Journal of Luminescence
thermal broadening at higher temperatures and, thus, the thermally activated barrier crossing dominates nearly the whole temperature range. According to the classification by Angel1 [5] the observed reduction of the density of potential minima and maxima corresponds to the transition from a fi-agile to a strong glass as derived from the static properties of the glass potential. Amorphous water thus is another system that fits the correlation published by Sokolov et al. between the TLS-density as a lowtemperature property on the one hand and the fragility measured at the glass transition on the other [6]. The particularity of amorphous water is that both, the fragile and strong glass-like behavior may be observed in a single system. These findings may be understood on the basis of an H-bonded network which is strongly disturbed in the fresh matrices and forms tetragonal bonds upon annealing to higher temperatures.
72-74 (1997)
427429
429
This work was supported by the Deutsche Forschungsgemeinschaft (Sonder-forschungsbereich 213).
References [I] T. Giering and D. Haarer, J. Lumin. 66, 67 (1996) 299. [2] W.-H. Kim, T. Reinot, J.M. Hayes and G.J. Small, J. Phys. Chem. 99 (1995) 7300. [3] T. Giering and D. Haarer, Chem. Phys. Lett. 261 (1996) 677. [4] W. Kijhler, J. Zollfrank and J. Friedrich, Phys. Rev. B 39 (1989) 5414. [5] R. Biihmer and C.A. Angell, in: Disorder Effects on Relaxational Processes, eds. R. Richert and A. Blumen (Springer, Berlin, 1994) Ch. 2. [6] A.P. Sokolov, E. Riissler, A. Kisliuk and D. Quitmann, Phys. Rev. Lett. 7 I (1993) 2062.