Ultrafast electron crystallography of monolayer adsorbates on clean surfaces: Structural dynamics

Chemical Physics Letters 542 (2012) 1–7

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Ultrafast electron crystallography of monolayer adsorbates on clean surfaces: Structural dynamics Wenxi Liang, Sascha Schäfer, Ahmed H. Zewail ⇑ Physical Biology Center for Ultrafast Science and Technology, Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, CA 91125, USA

a r t i c l e

i n f o

Article history: Available online 1 June 2012

a b s t r a c t By combining the surface sensitivity and ultrafast temporal resolution of ultrafast electron crystallography (UEC) with in situ surface preparation, adsorbate deposition and surface characterization, we extend the capability of UEC to monitor structural dynamics of monolayer adsorbates. Here, we report investigation of the Ni(1 0 0)–c(2  2)–S, Ni(1 0 0)–pð2  2Þ–O, and Ni(1 0 0)–c(2  2)–CO systems. Their structural dynamics are quantified by comparing the temporal change of the experimental electron diffraction patterns to the theoretically-obtained multi-scattering simulations. This Letter paves the way to future exploration of monolayer reactivity under controlled conditions of ultrahigh vacuum and with spatiotemporal resolutions at the atomic scale. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Structural dynamics at interfaces are inherently different from the bulk. Interface-specific atomic and electronic rearrangements, localized electronic and vibrational excitation, and processes which involve the transport of energy, carriers and matter across the interface open up new pathways which make the compound system more than the sum of its constituent parts. Among interfacial systems, metal surfaces covered with monolayer or sub-monolayer adsorbate structures are of special interest since they provide a platform for the understanding of nature of heterogeneous reactions [1,2]. Whereas the study of the static (time-independent) properties of metal-adsorbate structures has a long tradition, experimental approaches that assess their dynamical characteristics on the ultrafast time-scale were only developed recently. For example, surface femtochemistry has shown the controlling processes for reactivity of adsorbates [3,4], four-wavelength mixing using infrared laser pulses has provided insights into the lifetime of vibrational excitations of adsorbates [5–7], time-resolved photoelectron spectroscopy has elucidated the electronic dynamics of adsorbate and substrate sub-systems [8–11], and femtosecond laser studies have provided valuable insights into the nature of the interactions involved [12,13]. Given the recent advancement of ultrafast diffraction techniques and their successful application to a variety of problems it seems highly desirably to use such an approach also for the

⇑ Corresponding author. E-mail address: [email protected] (A.H. Zewail). 0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2012.05.058

study of adsorbate systems. Ultrafast diffraction utilizing short electron pulses is well suited for such studies. The high scattering cross section of electrons by matter enables structural characterization of adsorbates with a thickness of just one atomic monolayer as it is well established for the static case [2]. At these low densities, sensitivity for X-ray diffraction is low and utility is limited [14]. In this laboratory, short electron probes have already been employed for the investigation of structural dynamics on the ultrafast time scale in various systems and using different experimental setups [15,16]. It was possible to study, e.g. the coherent restructuring of surface layers [17], the structural change of interfacial water or biological model bilayers on substrates in a highly nonequilibrium regime [18,19], the phonon dispersion in thin-film graphite with a sub-picosecond time resolution [20], thin-film morphology on the ultrashort time scale with nanometer resolution [21,22], and the dynamics of charge distributions at interfaces of semiconducting materials [23]. In this contribution, we extend the capability of UEC to explore the structural dynamics of monolayer adsorbate systems. In this first report, we investigate surface structures of sulfur, oxygen and carbon monoxide on a clean Nickel(1 0 0) surface. In these systems, the adsorbates form a crystalline mono-layer structure on top of the metal surface with a unit cell that is different from the substrate. In diffraction, the adsorbate mono-layer can be therefore identified by the appearance of new Bragg diffraction spots. We probed the adsorbate–substrate system in the reflection geometry, and characterized the subsequent change of the diffraction patterns. We also compared the experimental results with multi-scattering simulations and a modified two-temperature model (TTM).

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2. Methodology 2.1. The UHV-UEC apparatus

(a)

(b)

[11] (see (e))

(11)

(11)

(01)

(½½)

(½½) [01] (see (f))

(00) (10)

(½ ½)

sy

y

57

(1 1)

2.49 Å Ni(100)

(c)

2.52 Å

-1

( 1 1)

-1

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sx

x

(10)

(½½) 3.

The modified apparatus of UEC, whose basic components have been described in detail elsewhere [17,24,16], comprises three chambers, the sample preparation equipment, a sample transport system and the setup for time-resolved electron diffraction experiment. All chambers are maintained at ultrahigh vacuum (UHV) with a base pressure in the low 1010 torr range. For the studies reported here, surfaces were prepared and characterized in situ in the sample preparation chamber which is equipped with an ion sputter gun, a gas doser and a sample heater. The characterization is performed using low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). After preparation, the sample is transferred through a load lock chamber to the scattering chamber for ultrafast reflection high energy electron diffraction (URHEED) measurements. The URHEED design is capable of measuring surface dynamics with a temporal resolution on the sub-picosecond time scale. It consists of a femtosecond electron gun delivering ultrashort electron pulses [17,20] and a kinematic energy of 30 keV. The sample can be positioned by a 5-axes goniometer in order to record diffraction patterns at different zone axes and incidence angles. The diffraction patterns were recorded with a CCD camera together with a gated image intensifier assembly allowing for single electron detection. Structural dynamics are initiated by femtosecond laser pulses (800 nm wavelength, 100 fs pulsewidh, 1 kHz repetition rate) and characterized at different delay times Dt by the ultrashort electron pulses (Figure 1). In order to correct for the velocity mismatch between the laser and electron pulses a tilting scheme is utilized [25]. For every delay time we recorded 30 diffraction patterns, each one averaged over 3  104 electron pulses, with 3  103 electrons in each pulse. To assess possible contribution from transient electric fields (TEF), which can be generated due to the laser-induced photo-emission [16,26,27], we used a tangential electron pulse as a probe (see Figure 1, light green line and oval spot). The deflection of this electron pulse at different delay times permits the quantification of the TEF magnitude as discussed in the Appendix.

miscut, polished), was baked together with the chambers. The surface of the substrate was cleaned by repeated ion sputtering (300 eV, Ar+) and through annealing (900 K) cycles, as described in the literature [28]. The final cleanliness was verified by LEED and AES. Sharp diffraction spots with low background intensity were observed in the LEED patterns and the impurity levels were below the AES detection limits [29]. To form adsorbate overlayers, two methods were used. In the case of Ni(1 0 0)–cð2  2Þ–S, we utilized the small amount of sulfur impurities which were present in the nickel single crystal and which segregate to the surface after prolonged sample heating. Ni(1 0 0)–cð2  2Þ–O and Ni(1 0 0)–pð2  2Þ–CO were prepared by exposing the freshly cleaned Ni(1 0 0) surface to several Langmuir of oxygen or carbon monoxide, respectively, followed by an annealing procedure (10 min, 400–450 K) in the case of the oxygen overlayer [30–32]. The crystalline order of the adsorbate samples was verified by the sharpness of the additional diffraction spots in the LEED pattern which are characteristic of the surface superstructure (see Figure 2). AES was not performed on the monolayer adsorbate samples, since high electron currents lead to adsorbate desorption and disordering.

( 0 1)

Ni(100)-c(2×2)-S

(d)

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(01) (½½) (½½)

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(10)

2.2. Preparation of monolayer adsorbate structures

(10)

(10) (½ ½)

Prior to preparing the monolayer adsorbate specimen, the Ni(1 0 0) substrate (Accumet Materials Co., 4N5 purity, <0.5°

(½½) ( 0 1)

sy sx

nˆ fs laser

e ( Δ t) _

d if fr a

c ti o n

s tr e a

ks

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72 eV LEED

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sx

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[01]

sz

(00)

(10)

-1

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sx (10)

ta n g e b e a m n ti a l spot shado w edg e ‘d ir e c b e a mt’ spot

θ Figure 1. Geometry for ultrafast reflection high energy electron diffraction in UEC. The intensity front of the pump laser pulses is tilted to compensate for the velocity mismatch between the speed of light c and the velocity of the electron probe package (0.3c). From the difference of the direct electron beam direction and the diffraction streaks (green oval shapes) the scattering vector s can be obtained. For (nearly) elastic scattering, as considered here, a scattered electron has the same 0 energy as that of the incidence electron, i.e. jk j ¼ jkj, but a different direction. By recording the behavior after laser excitation of an electron beam which is tangential to the sample surface, we can quantify the impact of transient electric fields (see Appendix).

3.55 Å

-1

3.55 Å

-1

2.53 Å

2.53 Å

Figure 2. Diffraction of clean and adsorbate-covered metal surface. (a) The real space structure of Ni(1 0 0)–cð2  2Þ–S is shown (nickel atoms: blue, sulfur atoms: green). The p sulfur can be described by a cð2  2Þ unit cell (broken, green ffiffiffi pover-layer ffiffiffi line) or a ð 2  2Þ R45 unit cell. (b) Construction of the reciprocal lattice of Ni(1 0 0)–cð2  2Þ–S. Blue lattice points are common to the substrate and adsorbate lattice, whereas red lattice points are only due to the adsorbate layer. The broken lines indicate sections of the Ewald sphere for the RHEED pattern displayed in (e) and (f), respectively. (c and d) LEED pattern of a Ni(1 0 0) and Ni(1 0 0)–cð2  2Þ–S surface. The additional diffraction spots in (d) compared to (c) are due to the sulfur over-layer according to the construction shown in (b). (e) and (f) RHEED pattern of a Ni(1 0 0)–cð2  2Þ–S surface in the [1 1] and [0 1] zone axes, respectively.

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2.3. Dynamical scattering simulations In order to compare the experimental RHEED pattern and their change after laser excitation with structural models, we employ multiple scattering simulations, 1 since multiple consecutive electron scattering events are known to influence the intensities in RHEED diffraction patterns [33]. A full account of the importance of multiple scattering in URHEED experiments was given in a recent publication [29]. For the simulations, we used a previously implemented multi-slice algorithm, as described by Ichimiya [34],2 and extended it here to also allow for the simulation of adsorbate over-layers.

3. Results and discussion 3.1. LEED and RHEED diffraction patterns of monolayer adsorbate species In Figure 2c and d, we display the LEED pattern of a cleaned Ni(1 0 0) surface in comparison to the diffraction pattern as obtained after the growth of an ordered, sulfur over-layer. The most obvious change in the patterns is the appearance of a set of new diffraction spots which are commensurate with the substrate pattern and which are labeled with reference to the substrate lattice. By considering the symmetry of the additional spots (e.g. the appearance of ð12 12Þ, but not ð0 12Þ spots) the adsorbate structure shown in Figure 2a can be deduced. The adsorbate structure can be described with a centered unit mesh (cð2  2Þ, broken green line in Figure 2a) which is twice as large as the primitive unit mesh of the substrate. Alternatively, the adsorbate p structure ffiffiffi pffiffiffi can be constructed using a rotated primitive mesh (( 2  2) R45°) as also indicated in Figure 2a (solid, green line). The relative displacement of the surface and the adsorbate mesh, and thereby the coordination site of the adsorbate molecules, is not directly visible in the symmetry of the diffraction pattern. However, by analyzing the changing diffraction intensities at different electron energies, it was shown in an earlier study [35] that the sulfur atoms are located at a fourfold coordination site as shown in Figure 2b with a distance of 1.3 Å to the substrate layer (equivalent to a Ni–S bond distance of 2.2 Å). In Figure 2e and f, we show the corresponding RHEED patterns of Ni(1 0 0)–cð2  2Þ–S, recorded in the [1 1] and [0 1] zone axes. The diffraction pattern can be rationalized by considering the reciprocal surface net (Figure 2b) and the Ewald sphere construction. For the high electron energy (30 keV) employed here, the Ewald sphere is well approximated by a tangential plane with a normal vector along the electron incidence direction. The intersections of this plane with the surface net for the two zone axis, respectively, are shown as broken lines in Figure 2b. Since only reciprocal lattice points on the Ewald sphere contribute to the diffraction pattern (for elastic scattering), half-order diffraction rods are observed in the [1 1] RHEED pattern (Figure 2e) while only integer-order rods can be observed in the [0 1] direction (Figure 2f). The presence of diffraction rods, instead of diffraction spots, at grazing incidence is attributed to a certain degree of surface disorder (see below). At higher incidence angles (4°) sharp diffraction spots are observed in the RHEED pattern (see Figure 3). Ultrafast diffraction techniques offer the possibility to obtain quantitative information on structural dynamics on the fs and ps time-scale. The amount of information which can be obtained from temporal changes in the diffraction pattern are based on the 1

Also called dynamical scattering simulations. For details on the implementations see Ref. [29], software can be obtained from the authors. 2

theoretical understanding of the diffraction pattern itself. Recently for URHEED it was shown [29] that the incorporation of dynamical scattering effects in the data analysis provide a more quantitive diffraction maps for the determination of the changes involved. Here, in order to obtain these maps, we performed rockingcurve measurements in which RHEED patterns are recorded at different incidence angles h. The results are shown in Figure 3, left panels, for the [1 1] zone axis and the (0 0), ð12 12Þ, and ð1 1Þ diffraction rods. In the displayed rocking curves, each column of pixels corresponds to the diffracted intensity along a diffraction rod for a given incidence angle h. The position along the diffraction rod

10 Experiment

(00) 10

Simulation (008)

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Figure 3. Experimental rocking curves and dynamical scattering simulations. The experimentally recorded rocking curves of the (0 0), ð12 12Þ and ð1 1Þ rods for a Ni(1 0 0)–c(2  2)–S are in the left panels. Here, the scattered electron intensity is color-coded for a given incidence angle, h, and a scattering angle hs . The white lines indicate the expected position of the diffraction spots according to the Ewald construction (for the case when inelastic scattering and surface inhomogeneities can be neglected). In the right panels the simulated rocking curves for clean Ni(1 0 0) (blue curves) and Ni(1 0 0)–cð2  2Þ–S (green curves) are displayed. The simulated rocking curves correspond to the intensity along the white curves in the experimental data.

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(in the sz direction, see Figure 2e and f) is quantified by the vertical scattering angle hs ; hs is connected to the vertical scattering vector by sz =jkj, where k is the electron wave vector of the incident electron beam. We note that for a flat surface with non-interrupted transverse periodicity, the Ewald construction results in a direct relation between the incidence angle and the scattering angle which is marked in Figure 3 by white lines; for the specular spot ((0 0) rod), the scattering angle is hs ¼ 2h, whereas for the side rods qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi hs is given by hs ¼ h þ h2  ðsjj =kÞ2 3. Here, sjj is the scattering vec-

0.08

1

0

(00) 0.06 0.04 0.02

tor in the transverse direction (parallel to the surface, sjj ¼ 3:57 Å 1

3 For simplicity, we do not include the impact of the crystal potential, which is important for side-order rods at low exit angles, i.e. hs  h.

(½½) intensity [arb. units]

for the ð1 1Þ rod and 1.78 Å for ð12 12Þ). Any scattered intensity off the white curves which is visible in Figure 3 does not fulfill these relations and has to be attributed to inelastic scattering effects and defects on the surface. The experimental rocking curves depicted in the left panels of Figure 3 are compared to the results of dynamical scattering simulations shown in the right panels. For the simulations, we consider both a clean Ni(1 0 0) surface structure (blue curves) as well as a Ni(1 0 0)–cð2  2Þ–S surface (green curves) with the adsorbate geometry as described in Ref. [35]. Focussing first on the results for the (0 0) rod (upper panel), it can be observed that the general features of the experimental rocking curve are reproduced in the simulated intensity curves using either the clean surface or the adsorbate covered surface. Especially the intensity near the (0 0 6) Bragg condition is strongly reduced in both of the simulated curves, whereas a maximum of diffracted intensity would have been expected, if a kinematical diffraction theory would have been employed. Comparing the two simulated rocking curves, it can be observed that the presence of a single adsorbate-layer has a profound influence on the diffracted intensities at low incidence angles (<3°). Nevertheless, the regime of low incidence angles is of only limited value to extract information about the adsorbate structure and dynamics since there is a significant difference between the experimental rocking curves and the simulation results. This discrepancy can be attributed to the roughness of the sample surface which gives a shadowing effect at grazing incidence [33]. As expected, at high incidence angles the simulated rocking curves of a clean and adsorbate-cover surface are very similar due to the higher penetration depth of the probing electron beam in this regime. Half order rods are only observed for the adsorbate-covered surface. Here, the simulated rocking curve agrees well with the measured variation of the diffracted intensity (see Figure 3, middle panel), especially the location of the intensity maximum in the rocking curve which is predicted correctly. Furthermore, a shoulder in the experimental rocking curve (around h ¼ 3:5 ) can be identified with a second maximum as seen in the simulation. However, some peaks in the experimental rocking curve are less well defined than theoretically predicted, and we attribute this to a certain degree of surface disorder, either due to roughness of the substrate (see above) or due to incomplete ordering of the adsorbate over-layer. This idea is further corroborated by considering the rocking curve of the ð1 1Þ diffraction rod (Figure 3, lower panel) in comparison to the theoretical prediction for both surface structures. Interestingly, the maxima in the experimental rocking curve can be also found in both simulated rocking curves, although with considerably different, relative intensities and larger differences at low incidence angles (see above). However, one major maximum in the simulated rocking curve of Ni(1 0 0)–cð2  2Þ–S near a scattering angle of 5° is not reproduced in the experiment. Furthermore,

0.06

substr. cold, adsorb. cold substr. hot, adsorb. cold substr. cold, adsorb. hot substr. hot, adsorb. hot

0.04 0.02 0

(11) 0.02

0.01

0 0

1

2 3 4 incidence angle [deg.]

5

6

Figure 4. Non-equilibrium, temperature-dependent changes of rocking curves. Rocking curves for the (0 0), ð12 12Þ, and ð1 1) rods are calculated for T substr ¼ 300 and 600 K and T adsorb ¼ 300 and 600 K, both, in equilibrium (T substr ¼ T adsorb , blue and cyan curves) and out-of-equilibrium (T substr – T adsorb , green and red curves); see text.

the scattered intensity in the ð1 1Þ is larger than that of the halforder rod, in contrast with the theoretical simulation. Both facts can be explained by assuming an adsorbate structure with a coherence length that is shorter than that of the substrate. 3.2. Structural dynamics: theoretical After comparing the static rocking curves of Ni(10 0)–cð2  2Þ–S with a dynamical scattering simulation, we now turn to the question of how sensitive are different features of the rocking curve to changes in the temperature of the substrate, T substr , and the temperature of the adsorbate, T adsorb . In thermal equilibrium, both temperatures are equal but after laser excitation a non-equilibrium situation with T substr – T adsorb can occur.4 In Figure 4 we show the impact of substrate and adsorbate heating on the simulated rocking curves of Ni(1 0 0)–cð2  2Þ–S. In general, an increased temperature leads to a larger atomic 4 We note that describing the atomic mean-square displacement of the substrate and adsorbate system by employing temperatures T substr and T adsorb assumes a local equilibrium, which may not always apply. For molecular adsorbate systems it is known [7] that the various vibrational normal modes show a significantly different coupling time to the substrate, so that in general after excitation of the substrate the vibrational system of the adsorbate is not necessarily in a local equilibrium. A more detail account of the impact of mode-specific excitation on the RHEED rocking curves will be considered elsewhere.

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intensity [arb. units]

2 data fit substrate adsorbate background

1.5

relative intensity [arb. units]

×10

Ni(100)-c(2×2)-CO

(00)

Σ

1

(½½)

(11)

(½½)

( 1 1)

0.5

Ni(100)-p(2×2)-O 1

0.9

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TTM 13.3 mJ/cm 2 26.7 mJ/cm 2 40 mJ/cm 2

-20 0

100

200

0

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40

delay time Δ t [ps]

fit region [px] Figure 5. Quantification of surface-induced changes in the diffraction pattern. In the diffraction pattern (inset, here shown for Ni(1 0 0)–cð2  2Þ–CO) a region of interest (ROI) is defined and the transverse (parallel to surface) diffracted intensity profile (blue crosses) is obtained by summation of the ROI in the vertical (normal to surface) direction. Each peak in the transverse intensity profile corresponds to a diffraction rod in the RHEED pattern. The intensity of the individual peaks is quantified by fitting to Lorentzian functions (solid, blue and green curves for substrate and adsorbate rods, respectively) together with a Gaussian-shaped background (broken, black curve). The sum of all fitted functions (solid, red curve) describes the experimental data well.

mean-square displacement, which in turn gives a decrease in diffracted intensities due to the Debye–Waller effect. However, as it is evident in Figure 4, certain features in the rocking curves are predominately sensitive to only T substr or T adsorb . For example, at high incidence angles (>3°) the intensity of the (0 0) rod, almost only depends on the temperature of the substrate due to the larger electron penetration depth in this regime. On the other hand, the broad diffraction feature below <2° depends on both T substr and T adsorb in a complicated manner. Interestingly, the slope at h ¼ 1:75 is sensitive to the difference between adsorbate and substrate temperature, i.e. T substr  T adsorb . Focussing on the half order rod (Figure 4, middle panel) it can be observed that the maximum at low incidence angles, 1 < h < 2 , is predominantly sensitive to the temperature of the adsorbate ðT adsorb Þ while the temperature of the substrate leads only to minor intensity modulations. This can be well understood if a kinematical scattering picture is invoked since in this limit the half order

relative intensity [arb. units]

-10

300

Ni(100)-c(2×2)-S 40 mJ/cm 2

Figure 7. Ultrafast change of the diffracted intensity and the calculated TTM. The ultrafast intensity change of the (0 0) streaks of the Ni(1 0 0)–pð2  2Þ–O surface was measured employing excitation fluences of f ¼ 13:3 mJ=cm2 (green crosses), 26.7 mJ/cm2 (blue crosses) and 40 mJ/cm2 (cyan crosses). The displayed intensity change is normalized to the excitation fluence, i.e. DI=ðf :IÞ. All excitation fluences give rise to the same temporal behavior and the magnitude of intensity drop is linear with the excitation fluence as expected from in a thermal model [29]. Furthermore, the ultrafast intensity change can be well reproduced using a TTM (red line), as described in Ref. [29].

diffraction rod is only the result of scattering off the adsorbate over-layer. However, the impact of multiple scattering events can be observed in the two additional maxima in the ð12 12Þ rocking curve at h ¼ 2:75 and h ¼ 4:1 which are predominantly sensitive to the substrate temperature, contrary to the results of a simple kinematical scattering picture. For the ð1 1Þ diffraction rod (Figure 4, lower panel) the scattered intensity is mainly determined by the substrate temperature, except the maximum at h ¼ 3 which is due to a surface resonance involving the adsorbate overlayer (see also Figure 3, lower panel). In summary, the dynamical scattering simulations show that the temperature change of the adsorbate and substrate can be separately monitored by taking into account the diffraction intensity decrease of various parts of the rocking curve, the diffraction maps.

3.3. Structural dynamics: experimental In the preceding section we used dynamical scattering simulations to show that temperature changes in the adsorbate mono-

Ni(100)-c(2×2)-CO 26.7 mJ/cm 2

Ni(100)-p(2×2)-O 26.7 mJ/cm 2

(11) (½½) (00) (½½) ( 1 1)

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delay time Δ t [ps] Figure 6. Ultrafast dynamics of monolayer adsorbate on metal surfaces. The relative intensity change after laser excitation of Ni (0 0 1)–cð2  2Þ–S (left), Ni (0 0 1)–pð2  2Þ–O (middle) and Ni (0 0 1)–cð2  2Þ–CO (right) are displayed. The intensity change of all diffraction rods is normalized to the maximal intensity change of the (0 0) rod to account for different effective Debye–Waller factors. Transients of substrate and adsorbate diffraction rods show the same temporal behavior which can be explained by a TTM (see text for details). This indicates that the adsorbate and substrate equilibrate on a time-scale that is faster than 5 ps.

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(a)

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Ni-S, 70 μ m Ni-S, -30 μ m steel, 180 μ m steel, 10 μ m

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4. Conclusion

0

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for their different (effective) Debye–Waller factors. As can be observed, all diffraction rods show an ultrafast intensity decrease after laser excitation within 10 ps followed by a slower recovery. However, the scaled intensity traces of different diffraction rods, as well as different adsorbates, show the same temporal behavior. This indicates that the temperature equilibration between adsorbate and substrate system occurs on a time scale which is shorter than the time resolution of the experiment. In order to ascertain that the observed diffracted intensity change is due to a temperature jump, we compare in Figure 7 the temporal behavior of the intensity decrease of the (0 0) rod for the Ni(1 0 0)–p (2  2)–O sample to the predicted lattice temperature as deduced from a two-temperature model, described previously [29]. It can be observed that, taking into account a temporal resolution of 4 ps (standard deviation), the intensity change matches the predicted rise of the lattice temperature reasonable. Furthermore, since according to Figure 4, the half-order diffraction rods show the same transient, and since these are sensitive to the adsorbate temperature (see dynamical scattering simulation), we can conclude that diffraction changes measure the temperature of the mono-layer adsorbate structures on the ultrafast time-scale.

0

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delay timeΔt [ps] Figure 8. Transient electric field effects for Ni(1 0 0)–cð2  2Þ–S monolayer adsorbate and for steel. The probing electron beam, in tangential incidence, passes the sample with different distances above the surface; spot size of 200 lm. The distance between the tangential beam and the surface was measured from the center of the beam spot to the surface. (a) Images of beam spot at negative delayed times and difference images of beam spot at positive delayed times, referenced to an image at negative delayed time, for different setups. The legends at most left indicate the setups corresponding to time behaviors in (b). (b) Temporal behavior of the tangential beam for different setups, legends from top to bottom, Ni(1 0 0)– cð2  2Þ–S monolayer adsorbate with 70 lm, Ni(1 0 0)–cð2  2Þ–S monolayer adsorbate with 30 lm, steel (material of the sample holder) with 180 lm and steel with 10 lm, respectively. Three black dash lines are shown as guides to the eye for the delayed time in (a). The deflection is measured by the position of the fitted center of the beam spot.

Here, we demonstrated, using ultrafast electron crystallography (UEC), the first study of structural dynamics of monolayers adsorbed on clean surfaces. By combining UHV surface preparation techniques with UEC, ultrafast temperature changes of the metal substrate and the adsorbate layer were resolved in the diffraction maps. The comparison with dynamical scattering simulations quantify the changes in the rocking curves and elucidated the nature of the atomic motions involved. UEC now provides sub-picosecond time resolution of adsorbate structural changes, and for these reasons we envision that this approach should provide the means of studying reactivity on surfaces and with monolayer sensitivity. Acknowledgment This work was supported by the National Science Foundation and the Air Force Office of Scientific Research in the Center for Physical Biology at Caltech supported by the Gordon and Betty Moore Foundation.

Appendix. Effects of transient electric field on mono adsorbate layer and the underlying nickel substrate can be independently probed by considering the diffracted intensity change in the half order and zero order diffraction rod, respectively. Here, we demonstrate that ultrafast changes in the diffraction intensities are indeed observed experimentally after laser excitation. In order to quantify changes in the diffraction pattern, we summed the scattered intensity in a given region of interest along the perpendicular direction (see inset of Figure 5). The resulting intensity as a function of the transverse component of the scattering vector is shown in Figure 5. Here, each maximum in the scattered intensity corresponds to a diffraction rod in the diffraction pattern. By fitting a combination of Gaussian and Lorentzian functions to the intensity profile, we extract the intensity change of each diffraction rod for a given delay time after laser excitation. The position and width of the diffraction rods show no discernible dynamics. In Figure 6, we show the intensity change of half-order and integer-order diffraction rods after excitation for the three different surface structures which are studied here. All traces are normalized to the respective maximum intensity change to compensate

In UEC experiments, laser excitation might induce transient electric fields (TEF) [26,27]. It is important to carefully check for their presence due to their possible influence on probing electrons and scattered electrons. The magnitude of TEF depends sensitively on material under study and the wavelength and fluence of the excitation laser. To quantify the effects of TEF in our case, we employed a tangential probing geometry and detected the temporal change of the electron beam position on the detector as shown in Figure 8 for the Ni(1 0 0)–cð2  2Þ–S and the stainless steel sample holder. To account for the spatial dependence of the TEF we recorded the temporal change at different heights, obtaining the known shift to a faster transient if the TEF are probed closer to the sample. However, even if the probing tangential electron beam is positioned in a similar distance like it is approximately used in a grazing incidence setup [27], transients with time constants on the order of 100 ps are observed, significantly slower than the ultrafast structural dynamics obtained from the diffraction pattern changes. Furthermore, we focus in this Letter on changes in the diffraction

W. Liang et al. / Chemical Physics Letters 542 (2012) 1–7

intensity, and not spot movements. Therefore, TEF effects are not to be of significance.

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