Transient Structures of Solids and Liquids by ... - ACS Publications

Downloaded by OHIO STATE UNIV LIBRARIES on September 14, 2012 | http://pubs.acs.org Publication Date: June 3, 2002 | doi: 10.1021/bk-2002-0820.ch006

Chapter 6

Transient Structures of Solids and Liquids by Means ofTime-ResolvedX-rayDiffraction and X-ray Spectroscopy D.

A.

*

Oulianov, I. V. Tomov, and P. M. Rentzepis

Department of Chemistry, University of California, Irvine, CA 92697-2025

We describe atime-resolvedx-raydiffraction and EXAFS experimental system, suitable for the study of ultafast processes in liquids and solids, with picosecond time resolution. The system uses a laser pulse to excite the sample and a delayed ultrashort hardx-raypulse, generated in an x-ray diode driven by the same laser, to probe the structure of intermediate species produced by the excitation. A few time -resolved experiments are described including the picosecond time-resolvedx-raydiffraction of laser heated metal and semiconductor crystals, and time-resolved EXAFS of carbon tetrabromide in ethanol solution before and after photoinduced dissociation.

© 2002 American Chemical Society

In Liquid Dynamics; Fourkas, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

71

72


Introduction Currently, ultrafast time-resolved optical spectroscopy remains the most popular technique for the study of ultrafast processes in physical, chemical and biological systems. It is based on the 1968 picosecond pump/probe technique (1): the process is initiated by a pump pulse, and then the changes in the sample are probed by a delayed probe pulse. In ultrafast time-resolved optical spectroscopy, the probe pulse is optical (it is either an ultrashort monochromatic laser pulse or a non-coherent pulse with a wide spectrum, e.g. supercontinuum laser pulse). The use of this technique provides the spectra and kinetics of intermediate species produced during the photoinduced process and by performing the appropriate quantum mechanical molecular calculations the structure of the intermediate states may be calculated. These calculations, however, are rather complicated and usually limited to just a handful of simple molecules. Nevertheless, in many cases the knowledge of the transient structures of the molecules involved in the core of a chemical or biological process is very important, if not mandatory, for understanding thoroughly the process occurred. Ever since the discovery of the x-rays, x-ray diffraction has been the most extensively used method to study the structure of materials (2). The x-ray diffraction from a single molecular crystal provides the most accurate lattice parameters and the molecular structure information. This technique, however, requires very high order of sample periodicity, and therefore in most cases, cannot be used for accurate structure determination of amorphous solids and liquids. One popular method that can be used in these cases is extended x-ray absorption fine structure (EXAFS) spectroscopy (3). This method measures the x-ray absorption spectrum in the vicinity of the absorption edge of a specific atom. Analyzing the low amplitude oscillations in the x-ray absorption spectrum in the range of 40- 1000 eV energies higher than the absorption edge, one can calculate the structure of the first few coordination layers around the atom. There are several other methods not discussed in this paper, which have also been successfully applied for structure determination of both solids and liquids. They include the electron and neutron diffraction. In the past, only static structures were determined by x-ray diffraction and EXAFS spectroscopy. In recent years, the advances in ultrashort pulsed X-ray sources have opened the possibility to extend these methods to the ultrafast timeresolved domain. Several ultrafast time-resolved x-ray diffraction experiments with nano- and picosecond time resolution have been reported recently (4-10) (excellent reviews of the most important results could be found in Ref. 11), however, to the best of our knowledge, no ultrashort time-resolved EXAFS experiment has ever been performed previously. Several types of ultrafast x-ray sources have been used for time-resolved xray diffraction. They are laser generated plasma, laser driving photodiodes and



73 pulsed synchrotron sources. The synchrotron sources are the most powerful sources, however they are not easily accessible and have pump/probe synchronization problems. To generate the ultrashort x-ray pulses we have used a laser driving diode. Because the generated x-ray beam is divergent the x-ray photon flux at the position of the sample is quite low. Therefore, the accumulation of many pulses is required for a reasonably good quality x-ray diffraction pattern or an EXAFS spectrum. In the x-ray diffraction experiments presented here, the typical exposure time was of the order of 1 hour when a 300 Hz repetition rate x-ray source was used. For the EXAFS data the average experimental collection time was about 100 hours. We expect to improve the experimental time required for future experiments by using x-ray monolithic polycapillary focusing optics, which we have just received for testing from X-ray Optical Systems Inc. (XOS). A similar system used in conjunction with a micro x-ray source was reported by XOS to result in an intensity gain of 4400 times at 8 keV and 2400 times at 17.4 keV (12). In this report we present our ultrafast time-resolved x-ray diffraction and EXAFS experimental systems with picosecond time resolution. First we will present some of our time-resolved x-ray diffraction studies on laser induced heat and strain propagation in metal and semiconductor crystals with nanosecond and picosecond time resolution (5, 6). In the second part of this report we will present our data on structure determination of liquids, specifically on the EXAFS spectroscopy of carbon tetrabromide/ethanol solution before and after photoinduced dissociation (13).

Time-Resolved X-ray Diffraction In this section we present an application of ultrafast time resolved x-ray diffraction for the study of lattice behavior during pulsed laser illumination, by means of time-resolved Bragg-profile measurements. When energy from a laser pulse interacts with a material, it generates a non-uniform transient temperature distribution, carrier concentration and other effects, which alter the lattice structure of the crystal. The deformed crystal lattice will change the angle of diffraction for a monochromatic x-ray beam by ΔΘ = -(Ad!/d>tan Θ , where d is the spacing of the diffracting planes, ad is the change of the spacing due to an outside influence, and Θ is the Bragg angle. Thus, for a divergent incoming xray beam, the diffracted signal from the inhomogeneous crystal will consist of signals scattered over a range of angles which are related to the depth distribution of the strain. Using a CCD detector, we were able to detect the distribution of the diffracted x-ray beam along the Bragg angle coordinate with high resolution and consequently measure the lattice deformation accurately. Β

Β



74 This suggests that the transient crystal structure changes, induced by low energy short laser pulses, can be measured directly. The experimental system, which we used for time-resolved x-ray diffraction experiments, is shown in Figure 1. It consists of the laser system, which produces nanosecond or picosecond UV pulses, the laser driving x-ray diode and the detection system. The laser system has two functions: it excites the sample and drives the x-ray diode, which generates the x-ray pulses used to probe the changes in the sample. Detail description of the experimental system can be found elsewhere (14). For nanosecond x-ray diffraction experiments we employed the ArF excimer laser, which produces 193 nm laser pulses with duration of 12 ns (FWHM) at 300 Hz repetition rate. For picosecond diffraction the laser system consists of the ArF laser used as an amplifier seeded by 193 nm picosecond pulse generated by the dye laser system (see Ref. 14 for details). This system produces 193 nm laser pulses with duration of 1.8 ps (FWHM) and energy up to 0.5 mJ/pulse at 300 Hz repetition rate. The x-ray diode consists of two flat electrodes: copper anode and aluminum photocathode. The characteristic Cu Κα radiation (λ = 1.54 Â) is used for these measurements. The x-ray probe pulse duration was 12 ns and 8 ps for the nanosecond and picosecond systems respectively. In both experiments, the x-ray pulses generated by the x-ray diode, after passing through two parallel slits separated by 180 mm, were directed to the sample crystal at the Bragg angle for Cu Κα lines. The sample was mounted on a four-axis Eulerian cradle in order to be accurately oriented. The scattered x-ray radiation was recorded by a liquid nitrogen cooled 2Kx2K CCD camera (15 μηι pixel). The CCD camera is made specifically for direct x-ray imaging, and the geometric resolution of this experimental system is about 15 ujad. The advantage of the large-area CCD detector is that it allows for the simultaneous recording of the reflected x-ray radiation from different parts of the sample. Thus, a signal from the laser interaction spot and a reference signal from not excited part of the sample are recorded at the same time. First we have performed the nanosecond time-resolved x-ray diffraction experiment in order to study the deformation of the structure in GaAs (111) crystal after nanosecond-pulsed UV laser irradiation. The GaAs crystal absorbs strongly 193 nm radiation, and because only 10 nm of the surface is penetrated by the UV photons, the bulk of the crystal is heated by diffusion. The energy of the heating laser pulse on the crystal surface was about 3 mJ corresponding to an energy density of 30 mJ/cm . This is several times smaller than the melting threshold of GaAs, which was reported to be 225 mJ/cm for nanosecond pulses at 193 nm (15). The size of the UV spot, on the crystal, was much larger than the x-ray penetration depth in the crystal, therefore one-dimensional distribution of the temperature and stress in the probed bulk of the crystal was assumed. Only very high quality GaAs(lll) crystals were used in these experiment. Several crystals of varying sizes were cut from 0.5 mm thick, 50 mm diameter GaAs wafer. Figure 2 shows the experimental K a l and Ka2 rocking curve (the 2

2



Figure 1. Experimental system for pump/probe time resolved x-ray studies. A, anode; C, cathode; ( ) laser pulses; ( ) x-ray pulses; ( ) electron pulses; ( ) diffracted x-ray pulses.



οάΠΣΣ

W

——

Unhealed

—

Heated

Ο

Thewy

ttiu

110

120

130

140

150

Pixel Figure 2. Experimental rocking cui~ve for a cold and hot GaAs crystal. The delay time is 10 ns. The points are the calculated rocking curve for the cold crystal. Both Cu Kal (stronger) and Cu Ka2 lines are recorded. 1 pixel = 50 μταά.



77 intensity profile of the diffracted spot) for a cold and hot (measured at 10 ns after excitation) GaAs crystal. The shift and increase in intensity of the excited crystal rocking curve can be clearly seen. Figure 3 represents the measured and calculated integrated reflectivity for delays up to 50 ns after excitation. The increase in the integrated intensity of the rocking curve can be explained by the increase in the acceptance angle of the excited crystal due to small lattice deformations. These data show directly a histogram of the evolution of the transient structure of the crystal and its eventual return to the original lattice spacing. We have calculated the rocking curves for all delays used in this experiment (the dynamical equations for slightly deformed crystal were used, see Ref. 5 for details). By fitting the calculated and experimental rocking curves the lattice deformation profile in the crystal over a 100 ns time window was obtained (Figure 4). We have also performed similar measurements for a Au(lll) crystal using our picosecond time-resolved x-ray diffraction system. The gold crystal was 150 nm thick, grown on a 100 μηι thick mica crystal. Electron diffraction patterns showed a well ordered Au(lll) crystal over several mm parallel to the surface. Thus we assume that there is a mosaic structure along the surface, but along the thickness it is practically a single crystal. Heating solid materials with picosecond laser pulses, when neither melting nor vaporization were induced, has been studied theoretically in detail (16). We have calculated the heat distribution in the gold crystal using the optical, thermal and other properties of gold and mica, which are relevant for our experiment. From these data we find that for a 1.8 ps laser pulse, the diffusion length is L = 22 nm. This length represents the crystal depth heated during the pulse illumination. From the above calculations we find that 0.1 mJ of absorbed energy in a spot size of S = 0.1 cm will increase the temperature of the volume SL of gold by about 190°C. The heat from this volume spreads within picoseconds to the rest of the 150 nm thick crystal. According to the heat diffusion theory it takes about 90 ps for heat equilibrium to be established inside the SL crystal volume. The heat dissipation from this volume may take three directions: along the gold film, to the air or through the mica substrate. The above estimates show that for the first 100 ps after the heat inducing laser pulse, the UV irradiated area of the crystal is in a non-equilibrium transient stage. Inside the crystal there is a thermal strain associated with a heated surface layer. Since the heated spot is much larger than the crystal thickness, it is reasonable to assume a one-dimensional strain distribution. A temperature gradient generated in the crystal lattice will alter the x-ray diffracted pattern. In our experiments the x-ray pulse is probing the entire thickness of the crystal. The absorption loss, for the diffracted x-rays, traveled the longest distance through the gold crystal is about 30%. Therefore, the recorded diffracted pattern is an integration, over the probed crystal volume, for d

2

d



78

-.V)

-20

-10

0

10

20

30

40

90

Delay (ns)

Figure 3. Calculated integrated reflectivity of laser pulse heated GaAs(lll) crystal as a function of time The points are the experimental results.



5

-50

Depth (micrometer)

Figure 4. Lattice spacing evolution within the GaAs(lll) crystal heated by a 12 ns laser pulse.



80 the time of the x-ray pulse duration. We used the theory of x-ray scattering from a one dimensionally strained crystal to calculate the diffracted x-ray intensity for a given temperature distribution. Figure 5 shows the experimentally measured and calculated rocking curves of the Au(lll) crystal for several time delays before and after laser excitation. In Figure 6, the shift of the peak of the rocking curve as a function of delay time is shown. The transition through a thermally nonuniform crystal lattice, in the first 50 ps, is clearly seen. In this transition time, the width of the rocking curve is also slightly larger than the one at equilibrium. The spread of the experimental points is partially due to the shot to shot fluctuations in the UV pulse energy which results from the jitter in the triggering of the excimer amplifier. After about 100 ps and up to 500 ps, which was the longest delay used in this experiment, no change in the shift was observed. These measurements show conclusively that our experimental system is capable of 10 ps time resolution and can easily detect transient lattice structure deformations caused by temperature changes of about 20°C.

Time-Resolved EXAFS EXAFS spectroscopy is based on the analysis of low amplitude oscillations in the region of 40-1000 eV higher than the absorption edge of a specific atom in the x-ray absorption spectrum of the material. After absorption of an x-ray photon with the energy Ε exceeding the edge energy E , the atom most probably emits a photoelectron with a kinetic energy equal to Ε^Ε-Ε * The photoelectron wave propagates from the absorber atom and scatters from the orbital electrons of the surrounding atoms. The scattered electron waves travel back to the absorber atom and interfere with the outgoing electron wave resulting in modulations in the absorption cross section of the absorber atom. Analyzing the EXAFS spectrum one can calculate the structure information of a few shells surrounding the absorber atom. This information includes the average distances between the absorber and the scatterers, the average square deviations from these distances (which result from the thermal oscillations and disorder of the system), the coordination number for each of the shells and in some cases even the high order anharmonic force cumulants. The raw EXAFS data are represented by the dependence of an x-ray mass absorption coefficient times the length of the sample μχ (μχ= ln(7

Transient Structures of Solids and Liquids by ... - ACS Publications

Recommend Documents