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J. Phys. Chem. 1995, 99, 7300-7310
Hyperquenched Glassy Films of Water: A Study by Hole Burning W.-H. Kim, T. Reinot, J. M. Hayes, and G. J. Small* Department of Chemistry and Ames Laboratory, U S . Department of Energy, Iowa State University, Ames, Iowa 50011-311 I Received: September 20, 1994; In Final Form: December 21, 1994@
The results of infrared absorption (-OH) experiments and nonphotochemical hole-burning experiments of aluminum-phthalocyanine-tetrasulfonate (ATP) in hyperquenched glassy films of water (HGW) are reported. Films were produced by deposition of liquid water clusters (-2 pm), generated by a thermal spray nozzle source, onto either a sapphire or polycrystalline copper cryoplate. Deposition temperatures (TD)in the -5150 K range were employed. TD= 5 K films were annealed at various temperatures (TA),up to 140 K. For each value of TA,the infrared and hole-burning properties (zero-phonon hole width and hole growth kinetics) of the film (annealed) are identical to those of unannealed HGW formed at TD = TA. Thus, HGW formed at a deposition temperature of TD' is kinetically accessible, by annealing of HGW formed at temperatures TD < To'. Dramatic irreversible manifestations of configurational relaxation in HGW are observed to onset at TA (TD)-90 K. This configurational relaxation progresses smoothly with temperature up to 150 K (highest TDand TA used). Zero-phonon hole widths were usually determined for a burning and reading temperature of 5 K. Hole growth kinetics were always monitored at a burning temperature of 5 K. It was found, for example, that HGW annealed or deposited at 140 K yields a zero-phonon hole width of 180 MHz, a factor of 3 times narrower than the hole of HGW formed at TD= 5 K. Decrease of the hole width with annealing power law for the dependence of the onsets at TA -90 K. Both unannealed and annealed films yielded a hole width on the burning temperature (510 K), proving that pure dephasinghpectral diffusion is governed by the electron-TLSint (intrinsic two-level systems) interaction. An interpretation of the aforementioned configuration relaxation, onsetting at -90 K, in terms of the TLSintmodel is given. ATP in HGW turns out to be the most efficient system for nonphotochemical hole burning yet discovered, with an average quantum yield as high as 0.18. (The SI lifetime of ATP is 4.8 ns.) Remarkably, the hole burning is essentially inoperative in cubic ice formed by warming of HGW. However, this cessation is consistent with the current mechanism for nonphotochemical hole burning.
I. Introduction The structural, thermodynamic, and spectroscopic (vibrational) properties of amorphous or glassy solid water have long been subjects of quite considerable interest.' Notwithstanding the importance of understanding the glassy state itself, the interest in amorphous solid water has stemmed, to a considerable extent, from the desire to understand the unusual physical properties of liquid water, e.g., the density maximum at 4 "C at normal pressure, the increase in fluidity and diffusivity which occur with increasing pressure (up to 150 m a ) , and the decrease in compressibility as the temperature is increased (0-50 "C). In this regard, it should be noted that the question of whether or not glassy water is connected to normal liquid water by a reversible thermodynamic path continues to be Prior to 1985, the only approach to production of amorphous solid water involved slow deposition of water vapor onto a cryoplate.' Henceforth, this substance will be referred to as vapor-deposited water (VDW). According to the traditional definition, VDW is not a glass because it is not produced by vitrification of the liquid. In 1985, Mayer demonstrated that hyperquenched glassy water (HGW) can be produced by deposition of micron-size liquid droplets onto a ~ryoplate.~ In his hyperquenching technique, liquid droplets are expanded through a pinhole with the droplets being generated either by an ultrasonic nebulizer or by an airbrush. In either case the droplets are then entrained in a carrier gas. The hyperquenching rate has been estimated to be 106-107 K SKI.Very recently, @Abstractpublished in Advance ACS Abstracts, May 1, 1995.
0022-365419512099-7300$09.00/0
we demonstrated that5 vitrification of liquid droplets can be achieved by replacing the nebulizer, carrier gas, and pinhole with a thermospray source.6 This approach avoids potential complicationsintroduced by condensation of the carrier gas and also by vapor of the nebulized liquid with which the carrier gas is saturated. The HGW films reported on here were produced using the thermospray approach. As recently emphasized by Angell,7HGW and VDW appear to be rather similar in their properties, at least when annealed in-vacuo at temperatures sufficiently close to the glass transition temperature, T,. Both types of amorphous solid appear to exhibit a very weak and reversible glass transition at Tg -136 K with crystallization to cubic ice (IC) occumng near 150 K.* The dielectric behavior of the two substances also indicates that the structures are very ~ i m i l a r .Conversion ~ to hexagonal ice (Ih) occurs at a higher temperature. The density of the amorphous solid water being consideredlo is 0.94 g ~ m - only ~, slightly higher than that of Ih. As reviewed by Sceats and Rice,' the major source of disorder in VDW is a dispersion (-8") in the 0-0-0 angle from 109.5" for Ih in which each oxygen is tetrahedrally connected to four nearest-neighbor oxygen atoms via hydrogen bonds. The above dispersion is correlated with departure from linearity of the 0-H* * -0bond of Ih (up to -5"). In addition, it appears that the nearest 0-0 distance in VDW carries a somewhat broader distribution than in Ih. The wellknown proton disorder of Ihll persists in low density VDW. According to the definition of Angell,12 water is a strong liquid because of the weakness (small AC,) of its glass transition and because both its short and long time relaxation dynamics 0 1995 American Chemical Society
Hyperquenched Glassy Films of Water follow an Arrhenius temperature dependence, and the temporal dispersion of these dynamics is, at best, weak. Methanol, for example, would be a fragile liquid since, for example, its short range order rapidly degrades with temperature above Tg.As remarked by Sceats and Rice,’ “It is the subtlety and complexity of the (strong) intermolecular forces in the condensed phases of water which yield the startling variety of structures in the several forms of crystalline and amorphous solid water.” In this regard, it is germane to note that a new high-pressure phase of amorphous solid water has recently been discovered.13J4One final point is that water is also atypical in that HGW is thermodynamically stable against crystallization and exists (by hyperquenching) only because of the sluggishness of the configurational relaxation dynamics involved in the transition from the liquid to ~ r y s t a l . ~ Aside from the fact that we were interested in developing a hyperquenching methodology for practical and reproducible production of glassy films from liquids at standard temperature and pressure, there were two main reasons for undertaking the studies which are reported on here. The first stems from our interest in the anomalous magnitudes and temperature depend‘encies of essentially all properties of glasses at very low temperatures. These properties include specific heat, thermal conductivity, and ultrasound ab~orption.’~ In the latter half of the 1970’s, it was also observed that the pure dephasinglspectral diffusion of the pure electronic transitions (zero-phonon lines) of molecules and rare earth ions imbedded in glassy hosts is also anomalous in its magnitude and temperature dependence.16J7 It is now firmly established that the pure dephasing frequency follows a weak 2-1.3 power law at low temperatures (510 K).’* Both spectral hole-burning and photon echo techniques have been used to study pure dephasing and spectral d i f f u s i ~ n . ’ ~By . ~ spectral ~ diffusion is meant the homogeneous broadening of the zero-phonon line of the probe molecule’s electronic transition due to configurational relaxations of the glass that occur at times longer (roughly speaking)20than the excited state lifetime of the probe. Spectral diffusion is most readily probed by the three-pulse stimulated photon echo technique.21 It is the immense structural disorder of amorphous solids that is responsible for the broad distribution of tunneling rates for the bistable configurations associated with pure dephasing and spectral diffusion. These configurations or intrinsic two-level systems (TLS,) of the host glass are coupled to the electronic transition and provide a density of low-energy (--kT) excitations which is normally absent in crystals. Transitions between the two tunnel states of the TLSintare phononassisted. This simple standard tunnel in which a static distribution of asymmetric intermolecular double-well potentials (TLSmt)is assumed, has been quite successful in accounting for the aforementioned properties of glasses. Nevertheless, the structural identity of TLSinthas generally proven to be elusive especially for molecular systems. Because water is a strong liquid, meaning that the structure of low-density HGW is a slightly randomized (vide supra) version of the structure of Ih, we reasoned that the study of TLSht behavior in HGW might ultimately lead to structural characterization of the TLSh,. Required in the end would be, of course, accurate molecular dynamics simulations coupled with statistical analysis and quantum mechanical calculation^.^^ Our first objectives were to determine whether nonphotochemical hole burning (NPHB) of probe molecule electronic transitions in HGW is operative and, if so, whether or not the temperature dependence of the zero-phonon line width is governed by the electronTLSint interaction. Because water is a strong liquid, the possibility existed that this interaction would be too weak to detect by hole burning, i.e., dominated by dephasing mechanisms
J. Phys. Chem., Vol. 99, No. 19, 1995 7301 Innova 90 Ar+
A-meter
f
Figure 1. Experimental scheme: S, sample holder on cold figer with He shield; D, beam targeting and attenuating diaphragm; BS, beam splitters, M, mirrors; P, prisms; L, lens; V, valve; F, long-pass filter; NDF, neutral density filters; PMT,photomultiplier tubes. The temperature of the sample, photon counter signal from the sample, and
reference signal were collected by computer which simultaneously controls the shutter and scanning of the laser. The wavelength of the dye laser was continuously monitored by a stand-alone wave meter. operative in crystalline hosts. It was also our intent to test the mechanism of Shu and Sma1125s26for NPHB since our hyperquenching apparatus allows for production of crystalline ice from HGW. This mechanism will be discussed later, but briefly, it entails “diffusion” of excess free volume from the outer shell of the host glass to the region of the probe molecule and its inner shell (similar, in spirit, to Onsager’s inverse snowball effect for liquid dynamics around an instantaneously created point charge or dip~le).~’At that point the relaxation dynamics of the TLS associated mainly with the probe and its inner shell of solvent molecules (TLS,,; ext = extrinsic) becomes important. The above NPHB mechanism predicts that the hole-burning efficiency should be markedly lower in ice than in HGW. The second main reason for our studies was to use NPHB (the magnitude and temperature dependence of the zero-phonon hole width and dispersive kinetics of hole growth) to study, in greater detail, configurationalrelaxation processes in HGW. One specific objective was to determine the extent to which HGW films distinguished by different histories of preparation would be similar at a given temperature and pressure (1 atm). Another was to ascertain whether significant ordering of HGW occurs at temperatures well below T,. In the broadest sense we are interested in the questions of kinetic accessibility and thermodynamic continuity between different states of HGW.
11. Experimental Section A block diagram of the apparatus used for measuring the fluorescence excitation spectra of thermospray-deposited samples is shown in Figure 1. In the present work, the samples were M aluminum-phthalocyanine-tetrasulfonate, (AFT, structure shown as inset in Figure 5 but without C1- ligated to Al) in water. APT was obtained from Porphyrin Products and used without further purification. HPLC analysis of the APT showed that the sample contained only trace amounts of mono-, di-, and trisulfonated species. Splitting of the tetrasulfonate peak into a doublet is probably. due to sulfonation of two different positions in the benzo rings. The water used was triply distilled
Kim et al.
7302 J. Phys. Chem., Vol. 99, No. 19, 1995 locally. For some infrared measurements, deuterated water (99.9% D) from Aldrich was used. HGW films are produced by thermospray deposition of the samples onto either a sapphire or copper substrate attached to the cold finger of a Janis ST-100 continuous flow helium cryostat. The substrate temperature is measured by a silicon diode (Lake Shore Cryogenics, DT-470) mounted on the reverse of the copper substrate or on the copper frame holding the sapphire substrate. The copper substrate was mechanically polished with 0.05 pm alumina powder; it was found, however, that better optical quality films were obtained if the polished surface was aged in air for several days before use. The lowest temperature attainable without pumping on the liquid He was 4.7 K. The manufacturer supplied vacuum jacket for the cryostat was not used. Rather, the cryostat body was mounted in a locally constructed steel vacuum chamber of dimensions 26 x 22 x 18 cm3. This is pumped by a 170 L s-l turbo pump (Leybold TP170). The base pressure of the chamber was -2 x loL7Torr. The chamber is equipped with several windows for fluorescence excitation or transmission measurements. For fluorescence excitation, as shown in Figure 1, BK-7 optical windows at 90" to each other and both at 45" to the plane of the sample were used. For infrared measurements, in-line sapphire windows were used. The thermospray apparatus is a Vestec VT-1499-13R vaporizer. This consists of a resistively heated, 150 p m i.d. capillary tube mounted in a 0.5 in. insulating tube which is inserted into the vacuum chamber through an "0"-ring-sealed compression fitting. Thermocouples are attached to the capillary inlet and outlet for monitoring these temperatures. The capillary is heated by applying a voltage along its length from a locally built 0-5 V power supply. Liquid samples are delivered to the thermospray with a LDC 396-89 pump. This is a reciprocating plunger pump operated at 89 strokes per minute. The pump is connected to the thermospray through a four-way valve with 0.02 in. i.d. PEEK tubing. No pulse suppression is used in the connections, so the thermospray output is quasi-pulsed at the pump rate. One port of the four-way valve allows the thermospray capillary to be connected to a mechanical vacuum pump so that residual liquid is quickly evacuated from the capillary after the liquid pump is turned off. If this is not done, this liquid will slowly diffuse into the sample chamber, and a portion of it will condense on the deposited film. By adjusting the voltage applied to the thermospray capillary, it is possible to vary the size of clusters emanating from the source over a small range. In the present work, the appropriate thermospray voltage was determined by visual observation of the thermospray output as the voltage was increased. The setting used was slightly higher than that at which the flow was observed to change from a stream to an aerosol. For water the measured inlet and outlet temperatures were 80 and 180 "C, respectively. From infrared measurements and from microscopic observation, the average cluster size was determined to be -2 pm in diameter. As stated above, the lowest attainable temperature was 4.7 K. From day-to-day this varied up to 4.9 K. Most hole burning reported here was done at the lowest attainable temperature. This is reported here as 5 K, but the actual temperature, when important, will be given. Annealing of the deposited HGW films to ASW films is accomplished by closing the helium inlet valve and then monitoring the substrate temperature. When the substrate temperature begins to rise, a 25 W heater wound around the cryostat cold finger is turned on. A typical annealing curve is shown in Figure 2. When the helium valve is first closed, there is a delay of -20 s as the residual helium in the cryostat vaporizes. The temperature then begins to rise as shown. At point A, the heater was turned on. During the
i 20
r
Time (s)
0 200 400 600 800 1000 Figure 2. Sample temperature vs time at maximum heating rate. At zero time, the He valve was closed after -20 s the temperature of the cold finger started to rise. At point A the heater was switched on at maximum possible current. Maximum heating rates were -100 Wmin at 0-40 K, -15 Wmin at 40-60 K, and -7 " i n at 60-300 K.
experiments, the temperature is continuously monitored and recorded by computer. Approximate heating rates for various temperature ranges are given in the caption to Figure 2. Samples annealed to a temperature TA were held at TA for -1 min. To obtain ice samples at 170 K, it was necessary to use a somewhat slower heating rate to prevent cracking of the sample following the amorphous to ice transition. The slower warming was done by setting the heater current to -65% of its maximum. The warming rate in the range 130-170 K was 2 K s-l. As film thicknesses for hole-burning measurements were -20 pm, the optical densities were too low for hole burning to be measured in absorption. All measurements were made using fluorescence excitation spectroscopy. The laser source is a Coherent 699-29 ring dye laser pumped with 6 W, all lines, from an argon ion laser. DCM Special dye (Exciton) was used, giving a tuning range from 615.0 to 706.0 nm. For long-range scans of the excitation spectrum, the intracavity etalon was removed from the ring laser and the wavelength scanned by rotating the birefringent filter stack. In this configuration, the laser line width was 0.1 cm-'. For hole-burning and highresolution scans, the intracavity etalons were reinstalled, giving a laser line width of 5 2 0 MHz. The laser wavelength was continuously monitored with a Burleigh wavemeter. For hole burning, laser power densities of 10 nW cm-* to 2 pW cmP2 were generally used. For scanning the fluorescence spectra before or after burning, the laser was attenuated to -10 nW cm-2. The laser illuminated a sample area of -1.0 cm2. Fluorescence from the sample was long pass filtered and detected with an RCA C31034 GaAs photomultiplier tube. The signal from the photomultiplier tube was amplified (Stanford Research Systems SR-445 preamplifier) and digitized with a Stanford Research Systems SR-400 photon counter. To normalize the signal for laser power variations, a portion of the beam was chopped and detected by a photomultiplier and an Ithaco 397E0 lock-in amplifier. Data from both sample and reference channels were stored in a personal computer along with sample temperature data. Film quality was routinely checked visually and by measuring the fluorescence intensity at 660 nm. Poor-quality films gave weak fluorescence at this wavelength. This was usually indicative of restricted flow in the thermospray capillary. For infrared measurements, the vacuum chamber was mounted in a Bruker IFS 120 Fourier transform spectrometer. With a tungsten source, CaF beam splitter, and cooled InSb detector, spectra over the range from 2000 to 9000 cm-' were acquired. These spectra were used to verify the amorphous nature of the HGW and annealed HGW films. Representative spectra of a
J. Phys. Chem., Vol. 99, No. 19, 1995 7303
Hyperquenched Glassy Films of Water Optical density
0.4
0.3
0.2
0.1
0 2800
3000
3200 3400 Wavenumbers (cm-l)
3600 I
Figure 3. IR absorption spectra of HGW detected at TA= Tdetwtion: (A) 5-20 K, (B)40 K, (C) 80 K, (D)100 K, (E)120-150 K,( F) 155 K, (G) 160-170 K, and (H) Tdetection = 5 K after annealing to 170 K. HGW film deposited at 5 K and annealed to various temperatures are shown in Figure 3. These spectra are for a film obtained from a single pulse of the thermospray. In order to obtain a representative film, the thermospray flow was turned on but the output mechanically blocked until stable flow was established. The flow was then unblocked for spray of a single pump pulse onto the cold window. The structured spectrum of trace G in Figure 3 is characteristic of ice E. Spectra A-F for the amorphous films are similar to those reported by o t h e r ~ . ~ ~ - ~ O Vapor-deposited water, VDW, deposited by allowing the thermospray to flow but baffling the output so that only water monomers that diffuse to the window are deposited, has a similar spectrum to HGW but with an absorption maximum at 3282 cm-' compared to 3265 cm-' for HGW. (Sample-to-sample variations in the maximum absorption frequency of -10 cm-' were observed.) The spectral differences between HGW and VDW observed by us are not as great as those reported by Mayer31 for deuterated VDW and HGW. We will discuss this further in section 1V.A.
111. Results Figure 4 shows the fluorescence excitation spectrum of APT in HGW for various deposition and annealing temperatures. Spectrum A is for HGW deposited at 5 K and detected at 5 K without any annealing (virgin sample). Spectrum B, which is for TA = 80 K, detected at 5 K, is practically identical. The same result was obtained for any TA < Tcryst.In contrast, in ice films, as shown in spectrum C , there is a 50 cm-' red shift of the spectrum. The same observations are also applicable to different deposition temperatures: for TD < Tcryst,the spectra are independent of TD,but for TD =- Tcryst,the spectra are redshifted. This is shown in spectra D-F for TD = 5 , 130, and 170 K. The absorption width, rat,s, is independent of Tfor both HGW and ice. As seen in the figure, rabs 575 cm-'. From the flat-topped shape of the spectrum, it is fairly clear that this width is due to the presence of unresolved vibronic absorption on the high-energy side. Taking 670 nm as the center of the origin band, the origin width is -300 cm-'. This width is primarily due to inhomogeneous broadening. Although the absorption red shifts on going to ice, there is no reduction in rid. This is in contrast to benzophenone in which a marked reduction in rinh was observed on going from the glass to the
1
7000 6900 6800 6700 6600 6500 6400 Figure 4. Fluorescence excitation spectra of M APT in HGW depending on differentdeposition and annealing temperatures (detected at 5 K): (A) TD = 5 K; (B)TO = 5 K, TA = 80 K (C) TD = 5 K, TA = 170 K (D)TD = 5 K, TA = 130 K; (E)TD = 130 K; (F)TO = 170 K. Spectra are normalized to the same peak intensity.
7000
6900
6800
6700 6600 Wavelength (A)
6500
Figure 5. Fluorescence excitation spectrum of APT (upper left comer) in HGW at T = 5 K. In the inset four stages of hole formation are shown (A, B, C, D) by using different burning times and power densities (see text). The wavelength scale in insets is the same as in fluorescence spectrum; the intensity scale is 2 times compressed. crystal for two different probe molecules (chlorin and stetrazine).3* Figure 5 shows the APT in HGW (TD= 5 K) fluorescence excitation spectrum following hole burning at 671.8 nm. The spectrum was obtained after burning for 15 min with -30 n W cm-2. From comparison of this spectrum with those of Figure 4, the only apparent difference is the sharp hole at the bum wavelength. This is shown more clearly in the hole spectra A-D which are obtained by subtracting the excitation spectrum following burning for various times from the excitation spectrum before burning. Inset B is the hole spectrum which corresponds to the excitation spectrum shown. Inset A, obtained after a 1 min burn with 30 nW cm-2, has a fractional hole depth of -0.3 and a zero phonon hole, ZPH, width of -600 MHz, with no trace of phonon sideband holes (PSBH). In hole spectrum B,
7304 J. Phys. Chem., Vol. 99, No. 19, 1995
Kim et al. 5 GHz
Fractional ZPH Depth 1.0 -
0.8 -
1
0.2
0.. , 6900
Wavelength (A) I
I
,
6800
I
I
I
I
,
6700
x
I
,
,
,
6600
Figure 6. Multitude of saturated ZPH's bumed at T = 5 K with -20 8, distance between holes in the fluorescence excitation spectrum between 6600 and 6900 A. Holes are actually deeper than shown due to laser line width being more than the hole width. The burning power density was -10 pW/cm2.Solid line is the saturated ZPH's fractional hole depth dependence on wavelength. burned for a 15 times longer period, the ZPH has become saturated (reached maximum depth) and broadened somewhat from spectrum A. There is a barely detectable pseudo-PSBH to lower energy of the ZPH. In spectrum C, burned for an additional 30 min (still at 30 nW cm-2), the pseudo-PSBH is now well developed although there is as yet no trace of the real PSBH to higher energy of the ZPH. The maximum of the pseudo-PSBH is 38 f 2 cm-I from the ZPH and exhibits a pronounced asymmetry which may be due to multiphonon transitions but seems more likely to indicate the involvement of a second (more weakly coupled) phonon with w ~ 1 0 cm-'. 0 Finally, spectrum D is obtained by burning with an increased power density (1 mW cm-*) for 1 min. This -lo4 increase in bum fluence produces no additional deepening of the ZPH but does broaden the ZPH, deepen the pseudo-PSBH, and produce a high-energy antihole. Close examination of the antihole indicates that there is interference between the antihole and the real PSBH, such as that described by Lee et for burning of the antenna chlorophyll of photosystem I. Figure 6 shows an excitation spectrum obtained after burning multiple holes, each spaced 2 nm from the previous bum wavelength throughout the APT absorption. Each hole was burned with -10 pW cmP2 for -10 min. Also shown, superimposed on the spectrum is a plot of the fractional hole depth as a function of bum wavelength. (The hole depths in the plot were not measured from the excitation spectrum shown but were measured from hole growth curves obtained during burning with the laser in single-frequency mode. The excitation spectrum, obtained with a laser width much broader than the hole width, does not accurately depict the hole depths.) For wavelengths longer than 668 nm, the saturated fractional hole depth is constant at -0.6. For shorter wavelengths, the hole depth falls markedly. This wavelength-dependent hole depth behavior reinforces the interpretation stated above that the absorption band includes a contribution from unresolved vibronic bands which onsets near 670 nm. The value of 670 nm used above for the center of the origin absorption is consistent with the observed dependence of ZPH depth on burn wavelength. We turn now to the dependence of hole properties on TAand TD. In Figure 7 is shown high-resolution scans of ZPH burned with 19 f 2 nW cm-* for 563 s. The sample was a film originally deposited at 5 K, bumed, and subsequently annealed
Figure 7. Hole shapes bumed and detected at 5 K, with the same power density (19 f 2 nW/cm2,t = 563 s). TD = 5 K; hole widths, rholey and annealing temperatures are (A) no annealing, rhole = 540 MHZ; (B) TA= 100 K,rhole = 480 MHz; (C) TA= 110 K,rhole= 360 MHz; (D)TA = 120 K, = 280 MHz; and (E) TA = 140 K, = 180 MHz. TABLE 1: Comparison of Hole Widths, Burned at the Same Wavelength (674.0 nm) and at the Same Temperature, T = 5 K, Using the Same (Low) Burning Power Density 17 f 2 nW/cm* and Burning Time 563 s, but Having Different Annealing Temperatures, TA
TD(K)
TA (K)
80
90 100 110 120 140
rhoie (MHz)
540 610 620 480 360 280
80
180 560
130 140
260
320
Different samples are separated with a horizontal space. Hole widths are accurate to 10%. Temperatures are f l K. at various TAand then cooled and burned at 5 K. The annealing, burning cycle was repeated a number of times, each time with a higher TA. During the annealing process, the previously burned holes are thermally filled. For TA > 50 K, the hole filling is complete. For TA < 90 K, the ZPH width is constant at 540 f 40 MHz. For TA > 90 K, the ZPH width narrows as shown in the figure, reaching a value of 180 MHz at TA = 140 K. The glass transition for amorphous water is reported to be at 136 Above this temperature, the sample begins to crystallize with a temperature-dependent rate.34 At 140 K, this rate is low enough that the degree of crystallization is negligible during the 1 min annealing time typically used. At temperatures higher than 140 K, however, the degree of crystallization becomes significant, and the sample composition becomes difficult to determine until T = 160 K, at which complete crystallization can be expected. Therefore, we do not report any hole widths for the 140-160 K range in which the sample may be partly ice and partly amorphous. However, for completely, crystallized films, e.g., with TA = 170 K, no hole burning is detected, even for power densities lOOOx higher than used to obtain the datu shown in Figure 7 . Table 1 lists the hole widths observed at various TA. Within experimental error,
J. Phys. Chem., Vol. 99, No. 19, I995 7305
Hyperquenched Glassy Films of Water Fractional holedepth
Fractional holedepth
A
B
0.1
1.o
10.
100.
Time (s)
Figure 8. Hole growth curves (with theoretical fits) and corresponding holes bumed at 5 K, with the same energy density 18 f 0.5 nW/cm* into the same sample deposited at 5 K. (A) HGW without annealing, rhole = 600 MHz; (B)HGW, annealed to TA = 140 K, rhoie = 180
0.4
1 0.1
I
I
1.o
10.
I
100. Time (s)
MHz.
Figure 9. Comparison of two holes and hole growth curves burned into different samples. Hole burning was done at T = 5 K with the
if the film is deposited at some TD other than 5 K, the ZPH width is the same as is observed for TA = TD. Another hole-burning parameter which has been shown to be quite sensitive to subtle changes in glass structure5 is the rate of hole formation. Because hole growth is generally d i s p e r ~ i v e , the ~ ~ ,rate ~ ~ cannot be described by a single rate parameter. Success has been achieved in modeling dispersive hole growth by assuming that the TLS parameters which govern the rate of tunneling in the rate determining step of hole formation are subject to a Gaussian distribution. This form of distribution for TLS asymmetry and tunneling parameter19 has been successful in explaining numerous anomalous glass proper tie^.^^ With regard to hole growth, this model leads to the following form for time-dependent hole depth, Oft):
same wavelength, 674.0 nm, and buming power density, 17 nW/cm2. One sample was deposited at T = 130 K; the other was deposited at T = 5 K but annealed to T = 130 K. The hole widths are 300 and 320 MHz, respectively.
where & = Pa!& with P, the bum photon flux; a, the peak absorption cross section; and z, the excited state lifetime; BO describes the average harmonic frequency of the extrinsic TLS, TLSext,the glass bistable configurations formed by interaction of the probe molecule with the glass. The integration variable x = (1 - Lo)/ai,and E(x) = exp[-2(& - a~x)].1 is the tunnel parameter for TLS,,,. Equation 1 is derived by assuming that A has a Gaussian distribution centered at 10 with a standard deviation ai, for the TLSext. Figure 8 shows experimental hole growth data for burning of APT in HGW with TD = 5 K and for HGW annealed to TA = 140 K. Also shown are fits to the data using eq 1 and the ZPH which are measured after 563 s of burning. Clearly, excellent fits to the data are obtained. The parameters used are listed in Table 2 for various TA. As observed for the ZPH width, there is also a significant change in the hole growth parameters for TA > 90 K. The change in 10is slight, but the change in ai, is -20%. The direction of the change corresponds to a narrowing of the 1 distribution. As was the case for hole widths, there is an equivalence of hole growth parameters determined for samples annealed or deposited at the same temperature. This is shown in Figure 9 which shows hole growth data with two different power densities for a sample deposited at 5 K and then annealed at 130 K before burning and for a sample deposited at 130 K and then cooled to 5 K for burning. As can be seen from the Figure, the two growth curves are nearly indistinguishable. In Figure 10, the temperature dependence of the hole widths as a function of temperature is shown for HGW, TD = 5 K,
/
1 Holewidth
2
Temperature (K) I
I
I
1
I
I
5
6
7
8
9
10
I
I
I
11 12 13
Figure 10. Hole width,
rhole, versus bum temperature, TB in full logarithmic plot. For (A) and (B), the sample is the same, TD= 5 K (A) no annealing, (B) annealed to TA = 130 K. TB = Tdetsction.In the insets the hole shapes for both samples are shown. The holeshapes are drawn to the same scale; hole depths at 5 K were -25%. Straight lines correspond to a rhole -F.3 law.
and for annealed HGW, TA = 130 K. The hole profiles at a bum temperature, TB = 5 K, and after warming to 11 or 12 K are also shown. It is apparent that the thermal broadening of the holes is well approximated by a T1.3power law for T 5 10 K. IV. Discussion
A. General. We consider first the nature of the deposited films. Three questions are relevant here: (1) Are the HGW films obtained by thermospray deposition equivalent to those of Mayer obtained by ultrasonic nebulization? (2) Is the HGW formed by thermospray deposition on copper or sapphire with TD % 5 K equivalent to low- or high-density amorphous ice? (3) Does the presence of the relatively large probe molecule alter the properties of the films? In answering these questions, we are guided primarily by infrared spectra (Figure 3).
7306 J. Phys. Chem., Vol. 99, No. 19, 1995
Kim et al.
Addressing first the equivalence of our procedure and that described by Mayer, the major difference in the two techniques is the absence of a carrier gas in thermospray deposition. The techniques probably also differ in the proportion of VDW which is codeposited along with HGW. Direct comparison of the two techniques is not possible due to the lack of infrared spectra obtained for equivalent deposition and observation temperatures. There is one spectrum by Mayer31 of 11% HOD with TD % 50-55 K measured at 16 K. This is compared in that paper with VDW, also deposited at 50-55 K and observed at 16 K. For HGW, there is a shoulder at 3115 cm-' in the OH stretch region which is absent in VDW. For both VDW and HGW there is a peak at -3290 cm-'. In our thermospray-deposited HGW, the shoulder observed by Mayer is absent, but the peak absorption is at -3265 cm-' rather than at 3290 cm-'. For VDW, we observe spectra similar to Mayer, with a peak at 3285 cm-'. We interpret these results as follows: the VDW peak is at 3285-3290 cm-' and the HGW peak at -3115 cm-' (at TD % 50 K). Our failure to observe a shoulder in this vicinity is due to a higher percentage of VDW in our samples so that the bands are not resolved but rather appear as a peak with maximum in between the VDW and HGW maxima. We note that the presence of VDW in our samples does not alter any of our conclusions regarding hole-burning results as all of the APT probe molecules must occur in the HGW fraction. Regarding the density of our HGW samples, we have been guided by the observation of Rice and c o - w ~ r k e r that s ~ ~high~~ density amorphous water is difficult to prepare. According to them, high-density amorphous water is only formed at low TD (< 15 K) and on a single-crystal copper substrate. They observed the low-density amorph on other substrates (sapphire, alkali halides, polycrystalline metals) regardless of TD. On the basis of these results, we have assumed that we form only the low-density material, especially since it is liquid clusters which are hyperquenched. This assumption is brought into question somewhat by reports of the production of high-density amorphous water on amorphous carbon In these reports very thin ( 100 K, the average for ai, = 0.80, which gives (R) = 8.4 x IO6 s-l. Thus, although the hole burned in the film annealed at higher temperatures reaches saturation more quickly, the average rate for lower temperature annealing is -3 times faster. Using these average rates to calculate the average quantum yield for hole burning, (4) = (R)/((R) UT,gives (4) = 0. I I for low-temperature annealing and (4) = 0.04 for higher-temperature annealing. According to the calculations of Kenney et u Z . , ~ ~one can classify the dispersion for the hole growth kinetics as weak and quite strong for oi, = 0.80 and 1.1, respectively. Comparing these average rates and quantum yields with the values reported for other efficient hole-burning systems shows that APT in HGW and annealed HGW have the highest average rates and quantum yields for hole burning. Kenney er U Z . ~ ~ previously claimed this record for the laser dye oxazine 720 in poly(viny1 alcohol) or in glycerol. Recalculating that data with the same value of QOused here gives (R) = 6 x los s-l and (4) = 1.6 x for glycerol and (R)= 1.9 x lo6 s-l and (4) =5 x for poly(viny1 alcohol). The high-temperature annealed HGW film is about an order of magnitude more efficient, while the low-temperature annealing gives a film -30 times more efficient.
+
Hyperquenched Glassy Films of Water From Figure 9, it can be seen that both the rate of hole growth and the ZPH width are the same for samples deposited at high temperatures as for samples annealed to the same temperatures. This is consistent with the hole width data of Table 1, which shows that films deposited at elevated temperatures are equivalent to those annealed to that temperature. Similar results were reported by HZirdle et aL48for ultrasonic attenuation measurements on VDW,"8 and we have observed the same behavior in comparing infrared spectra for the same TD and TA. D. Zero-Phonon Hole Width and Its Temperature Dependence. We begin with the ZPH widths listed in the upper half of Table 1. All holes were burned and read at 4.9 K. The width of 540 MHz given for TA = 4.9 K is for the virgin HGW film deposited at 4.9 K; i.e., the film was not annealed at a higher temperature. The other widths were obtained following annealing at the TA values listed and subsequent burning and reading at 4.9 K. Previous experiments had shown that the width is invariant for TA < -90 K, which explains why in this run the lowest annealing temperature was 80 K. A plot of the data in Table 1 versus TAreveals that the ZPH width undergoes a marked and smooth reduction (-x3) between 90 and 140 K. This reduction is discussed in the following subsection. Here we first want to emphasize that the width of 180 MHz is the sharpest yet observed for a molecular chromophore imbedded in a purely amorphous host at 4.2 K. Given the broadening of the ZPH (Figure lo), which appears to be universal for T 5 10 K,18 one can calculate that the lifetime limited value of the width would be attained at -2.3 K. This assumes the value of our measured APT fluorescence lifetime at room temperature, 4.8 ns. As discussed in refs 20,37, and 56, it is the amorphous host rather than the chromophore that is usually most important in determining the homogeneous width of the ZPH due to pure dephasinghpectral diffusion. We have performed an initial set of experiments with ATP in hyperquenched films of methanol (a fragile liquid). At 5 K the width of the ZPH is more than a factor of 10 times broader than 180 MHz, a value more in line with what is typically observed at 5 K for amorphous molecular hosts. Thus, the extremely narrow ZPH observed for ATP in HGW can be attributed to HGW. The narrow ZPH for ATP in HGW begs an interpretation. A definitive interpretation awaits the results of additional experiments. However, a plausible explanation at this point is that the number density of effective %Sint which couple to the optical transition in HGW is unusually low. This number density is significantly decreased for TA > 90 K; cf. following subsection. The low density of TLSi,, in HGW can be reasonably attributed to the fact that water is a strong liquid. There has been much interest in utilizing hole burning in doped molecular amorphous solids for high-density frequency domain optical storage (FDOS).57 The design of practical systems operative at 77 K is a formidable challenge, never mind at room temperature. We will report in the near future on the results of high-temperature hole-burning experiments with ATP in HGW. At this time, however, we can state that -50 ZPH with a fractional depth of -0.1 and width of -10 cm-I can be burned at 80 K within the inhomogeneously broadened origin absorption profile. The 100% holes were burned with a fluence of 200 mJ cm-2. To the best of our knowledge, this represents a marked improvement over what has been a c h i e ~ e d . ~ ~ . ~ ~ E. Configurational Relaxation in HGW and the Glass Transition. The key findings related to configurational relaxation are (1) HGW deposited at T, has the same properties (hole burning, infrared absorption) as HGW deposited at a lower temperature and annealed for short times at TA = TD;(2) for TA(TD)above 90 K there is a significant narrowing of the ZPH and the width of the distribution of the tunneling parameter I
J. Phys. Chem., Vol. 99, No. 19, 1995 7309 (Tables 1 and 2) as measured at a burning temperature of 5 K; and (3) the fluorescence excitation spectrum of APT is invariant to TA(TD)< Tcrystbut undergoes a 50 cm-' red shift in L.With regard to the structural changes which onset at -90 K, it should be noted that they are irreversible. This follows since hole burning is performed at 5 K following annealing at TA Z 90 K. If the changes were reversible, the hole widths and ui values for the annealed samples would be the same as those measured for the virgin sample deposited at 5 K. This is not the case (Tables 1 and 2). We emphasize also that we do not observe any significant structural relaxation for TA -90 K. Finding (3) should not be viewed in contradiction to ( 2 )but, rather, as testament to fluorescence excitation being a far less sensitive structural probe than hole burning. Recall that the 50 cm-' red shift upon the transition to I, is accompanied by a cessation of hole burning. The T1 power law for the ZPH width (Figure 10) establishes that it is the probe-TLSlnt interactions which dictate the homogeneous width. (For reviews of relevant theories see refs 60 and 61.) The reduction (-x3) in this width as TA is increased from 90 to 140 K (Table 1) could be due to a decrease in and/or a reduction in the deformation potentials associated with the TLSlnt. (The strain-induced dipoles of the TLS, are coupled to the transition dipole of the probe molecule with a RV3distance dependence.) We are not able to determine the relative importance of these two mechanisms, but we favor a significant reduction in emsmtbeing the primary cause since this is consistent with the data on UJ.in Table 2 and the finding that the average quantum yield for NPHB is reduced for TA > 90 K; cf. section 1V.D. Perhaps the strongest support for this view comes from the acoustic wave attenuation and velocity measurements of Htirdle et ~ 2 1on . ~-0.5 ~ pm VDW films. Their films were formed by slow deposition at 4.4 K. Following annealing at higher temperatures (as high as 144 K), the film temperature was reduced to 0.5 K to initiate temperaturedependent measurements. Dramatic effects of annealing near 90 K were also observed by Htirdle et ~ 1 Using . ~ firmly ~ established theory, they showed that it is primarily ens,n1that is affected by annealing, not the deformation potential. Alare consistent, a though our results and those of Hiirdle et quantitative comparison will not be possible until the minimum annealing temperature (T-) required for VDW to become structurally equivalent to HGW deposited at Tmn is accurately determined. Tmncould be higher than 90 K for thin films. As indicated in the Introduction, differential scanning calorimetry (DSC) measurements have indicated that annealed HGW and sintered VDW exhibit a weak (difficult to observe, with small ACp) glass transition at Tp 136 K. In unannealed HGW, the endothermic step in the DSC at Tg is obscured due to a broad exotherm which is attributed to rapid configurational relaxation to a structural state of lower enthalpy. This is an irreversible process which onsets at 125 K in both VDW and HGW.8,62It is no doubt the effects of this process which cause the narrowing of the ZPH for HGW annealed at TA > 90 K. That the hole narrowing onsets at a lower temperature than is observed in DSC might be due to a lower barrier for relaxation in the TLS,,, probed by hole burning, or it could be that hole burning is more sensitive than DSC. The latter argument is favored by the ultrasonic attenuation measurements of HZirdle et al.48on annealed VDW, who saw a considerable decrease in ultrasonic attenuation following annealing to 86 K. We note that although both hole burning and ultrasonic measurements may be more sensitive than DSC in detecting irreversible thermal effects, they are not able to speak to reversible effects such as the glass transition since the measurements must be made at low temperatures following the annealing step.
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7310 J. Phys. Chem., Vol. 99, No. 19, 1995 Acknowledgment. This article was made possible by support from the Division of Materials Research of the National Science Foundation under Grant DMR-9307034. The authors thank G. P. Johari, A. Mank, and K. Woo for helpful discussions. We also thank C. Smith and S. Savikhin for measuring the APT fluorescence lifetimes and P. Dumont for performing HPLC analysis of the APT. References and Notes (1) Sceats, M. G.; Rice, S. A. In Water, a Comprehensive Treatise; Franks, F., Ed.; Plenum Press: New York, 1982; Vol. 7, Chapter 2. (2) Johari, G. P.; Fleissner, G.; Hallbrucker, A.; Mayer, E. J. Phys. Chem. 1994,98,4719. (3) Speedy, R. J. J. Phys. Chem. 1992,96,2322. (4) Mayer, E. J. Appl. Phys. 1985,58, 663. (5) Kim, W.-H.; McPhillen, M.; Hayes, J. M.; Small, G. J. Chem. Phys. Len. 1993,207,443. (6) Blakely, C. R.; Vestal, M. L. Anal. Chem. 1983,55, 750. (7) Angell, C. A. Nature 1988,331,206. (8) Hallbrucker, A.; Mayer, E.; Johari, G. P. J. Phys. Chem. 1989,93, 4986. (9) Johari, G. P.; Hallbrucker, A,; Mayer, E. J. Chem. Phys. 1992,97, 5851. (10) As discussed in ref 1, there is also a high-density form of VDW. See also section W.A. (11) Pauling, L. In The Nature ofthe Chemical Bond, Come11 University Press: Ithaca, 1945; p 301. (12) Angell, C. A. J. Phys. Chem. 1993,97,6339. (13) Klug, D. D.; Mishima, 0.;Whalley, E. J. Chem. Phys. 1987,86, 5323. (14) Mishima, 0.;Calvert, L. D.; Whalley, E. Nature 1984,310,393. (15) Amolphous Solids-Low Temperature Properties; Phillips, W. A,, Ed.; Springer-Verlag: New York, 1981. (16) Avouris, P.; Campion, A.; El-Sayed, M. A. J. Chem. Phys. 1977, 67, 3397. (17) Morgan, J. R.; El-Sayed, M. A. Chem. Phys. Lett. 1981,84, 215. (18) Volker, S. In Relaxation Processes in Molecular Excited States; Funfschilling, J., Ed.; Kluwer: Dordrecht, 1989; p 113. (19) Jankowiak, R.; Hayes, J. M.; Small, G. J. Chem. Rev. 1993,93, 1471. (20) Narasimhan, L.R.; Littau, K. A,; Pack, D. W.; Bai, Y. S.; Elschner, A,; Fayer, M. D. Chem. Rev. 1990,90,439. (21) Meijers, H. C.; Wiersma, D. A. Phys. Rev. Lett. 1992,68,381. (22) Anderson, P. W.; Halperin, B. I.; V m a , C. M. Philos. Mag. 1972, 25, 1. (23) Phillips, W. A. J. Low Temp. Phys. 1972,7,351. (24) Heuer, A,; Silbey, R. J. Phys. Rev. B 1994,49, 1441. (25) Shu, L.; Small, G. J. Chem. Phys. 1990,141, 447. (26) Shu, L.; Small, G. J. J. Opt. SOC. Am. B 1992,9,724. (27) Maroncelli, M.; MacInnis, J.; Fleming, G. R. Science 1989,243, 1674 and references therein. (28) Hagen, W.; Tielens, A. G. G. M.; Greenberg, J. M. Chem. Phys. 1981,56,367. (29) Bertie, J. E.; Whalley, E. J. Chem. Phys. 1964,40, 1637. (30) Bergren, M. S.; Schuh, D.; Sceats, M. G.; Rice, S . A. J. Chem. Phys. 1978,69,3477. (31) Mayer, E. J. Phys. Chem. 1985,89,3474. (32) Schellenberg, P.; Friedrich, J.; Kikas, J. J. Chem. Phys. 1994,101, 9264.
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