Femtosecond Laser-Induced Recombinative O+O=O2 Reaction on

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Femtosecond Laser-Induced Recombinative O+O=O2 Reaction on Single Crystal Pd(100) Surface Requires Thermal Assistance Sourav Banerjee, Anupam Bera, and Atanu Bhattacharya J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08653 • Publication Date (Web): 23 Oct 2018 Downloaded from http://pubs.acs.org on October 26, 2018

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The Journal of Physical Chemistry

Femtosecond Laser-Induced Recombinative O+O=O2 Reaction on Single Crystal Pd(100) Surface Requires Thermal Assistance Sourav Banerjee, Anupam Bera, and Atanu Bhattacharya* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012 *corresponding authors: [email protected], [email protected]

Abstract: The process of recombinative desorption of molecular oxygen (Oadsorbed + Oadsorbed = O2,gas) from the Pd(100) single crystal surface, under the femtosecond laser irradiation, has been investigated with the help of pre- and post-radiation temperature programmed desorption (TPD) measurements. This femtosecond optical pulse-induced surface chemistry is found to depend strongly on the initial surface temperature. The threshold temperature is observed to be 400 K above which this reaction remains active for the absorbed fluence of 2.86 mJ/cm2. Furthermore, the desorption-yield is observed to be linear with respect to the absorbed fluence. We explain our observations with the help of combined two-temperature model simulation and density functional theory computations. A two-step mechanism for the femtosecond optical pulseinduced recombinative desorption of molecular oxygen is evident: the first step involves the hot electron-mediated activation of the oxygen atoms and the second step involves the thermal activation (phonon-mediated) of the oxygen atoms leading to the recombination of oxygen atoms to form molecular oxygen which immediately desorbs from the Pd(100) surface. This is the first report on the femtosecond optical pulse-induced recombinative surface chemistry of adsorbed oxygen atoms on the Pd single crystal surface.

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Introduction: The interaction of atomic oxygen with transition-metal is of pivotal importance for a variety of industrially and environmentally relevant processes, including the CO oxidation in the automobile exhaust catalytic converters.1,2 In particular, the interaction of atomic oxygen with palladium (Pd) transition metal surfaces has attracted immense attention because Pd is one of the active oxidation catalysts used in the automobile catalytic converters1,2 and the oxygen reduction reaction (ORR).3 This is why, thus far, many fundamental investigations based on the surface science techniques have been carried out to explore the interaction of atomic oxygen with welldefined single crystal Pd surfaces [Pd(100)4-6, Pd(110)7-11 and Pd(111)12,13 and palladium nanoparticles14]. All these studies irrevocably have monitored the thermally activated recombinative desorption of molecular oxygen (Oadsorbed + Oadsorbed = O2,gas) to explore the interaction of atomic oxygen with the palladium surface. On the contrary, for the first time in the present work, we have explored the femtosecond optical pulse-induced recombinative desorption of molecular oxygen from palladium surface (more specifically from single crystal Pd(100) surface). Desorption induced by the femtosecond optical pulse excitation exhibits several distinctive features. Femtosecond optical pulse-induced desorption is very often characterized by highly nonlinear fluence-dependent desorption yield.15 Nonlinear fluence dependence of the femtosecond pulse-induced desorption yield is attributed to the influence of the very high electronic temperature attained in the metal substrate under the femtosecond laser excitation. This nonlinear fluence-dependent feature helped Misewich, Heinz, Ho, Wolf and others,15-19 for the first time about two decades ago, to explore surface chemical reactions on the ultrafast timescale (femtoseconds to picoseconds). Motivated by their seminal works, very recently, we have taken an initiative at the Chemical Dynamics Lab in the Indian Institute of Science to explore the femtosecond laser-induced surface chemical dynamics. As the recombination of two atoms forming a molecule, which then leaves the surface, represents one of the simplest surface chemical reactions, the recombinative desorption of molecular oxygen (Oadsorbed + Oadsorbed = O2,gas) from Pd single crystal surfaces attracted very early attention of femtosecond surface chemistry community. In particular, an early attempt was made by Heinz and coworkers, more than two decades ago, to monitor this reaction from the single crystal Pd(111) surface with the help of femtosecond laser excitation.20,21 However, no recombinative desorption of O2 was observed from the experiments performed with adsorbed atomic oxygen on the Pd(111) surface. It was concluded by Heinz and coworkers that the femtosecond laser excitation with fluences in the range of 2 to 5 mJ/cm2 does not initiate recombinative desorption of oxygen from the Pd(111) surface. In the present work, for the first time, we have used atomic oxygen-covered single crystal Pd(100) surface to initiate the recombinative O2 desorption reaction with the help of femtosecond laser excitation. This femtosecond laser-induced surface chemistry is found to strongly depend on the initial surface temperature. Furthermore, the O2 desorption-yield is found to be linear with respect to the absorbed fluence. We explain our experimental findings with the help of two temperature model-based simulation and density functional theory computations.


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Experimental Section: Earlier experiments directly confirm that an accurate measure of surface coverage of atomic oxygen can be obtained from the area under the temperature programmed desorption (TPD) peak corresponding to the recombinative desorption of molecular oxygen.4-13 Femtosecond laser excitation leads to the depletion of surface coverage of the atomic oxygen. Thus, femtosecond laser-induced surface chemical reaction yield can be easily estimated from the difference in TPD peak-areas obtained before and after the surface irradiation with the femtosecond laser. With this idea in mind, in the present work, we have employed the TPD measurements to monitor the femtosecond laser-induced recombinative desorption of the molecular oxygen from the single crystal Pd(100) surface. For the temperature programmed desorption (TPD) measurements, ultrahigh pure O2 (99.9%, Bhuruka Gases Limited) was delivered to the clean Pd(100) surface with an effusive beam formed in a stainless steel tube downstream from a 5 m aperture (VCR gasket, Lenox Laser) from a turbo-pumped gas manifold. Dosing gas pressure was monitored with the help of a capacitance manometer (Baratron, MKS Instruments). Immediately prior to the gas adsorption, the Pd(100) sample was flash-annealed to 1000 K. The desorption rates were measured with the help of a quadrupole mass spectrometer (Hiden EPIC) while the sample temperature was ramped under computer control (LabVIEW PID controls TDK Lambda DC Power Supply) at a constant 4.4 K s–1 rate. Prior to the TPD measurements, single crystal Pd(100) (MaTeck, GmbH) surface was cleaned in ultra-high vacuum (UHV of base pressure 2.5x10-10 Torr) by cycles of Ar+ bombardment followed by high temperature annealing in the O2 ambient. We find that the high temperature annealing in the O2 ambient was crucial to the removal of carbon impurity in the asreceived Pd(100) surface. The cleanliness of the Pd(100) surface was confirmed by auger electron spectroscopy (AES) and temperature programmed desorption (TPD) of molecular oxygen. Representative AES result obtained from a clean single crystal Pd(100) surface is shown in Figure 1(a). The intensity of AES spectrum is normalized to that of the PdMNN peak at 330 eV. Here, we note that the main PdMNN feature at 330 eV is accompanied by an additional feature at 279 eV which is difficult to resolve from the CKLL (272 eV) feature. Because of this reason, in spite of having very high sensitivity of the AES technique, it is difficult to analyze carbon impurity on a palladium surface with the help of only AES. In this regard, Yates and coworkers earlier pointed out that the CO and CO2 TPD measurements following oxygen adsorption can unambiguously confirm the purity of the palladium surface.22 We have employed the same methodology to confirm that the Pd(100) surface is carbon-free prior to the TPD measurements. Figure 1(b) shows the CO- and CO2-TPD results following oxygen adsorption at 400 K. Observation of no signal (beyond background) in CO and CO2 mass channels confirms that the Pd(100) surface is carbon-free. The Pd(100) substrate was mounted to a Ta plate with Ta clips. The plate was suspended from an electrical feedthrough (sample probe, McAllister Technical Services) by two tantalum (Ta, ESPI Metals) leads, affording resistive heating from room temperature to 1000 K. The temperature was monitored by a K-type thermocouple (Omega Engineering) spot welded to the back of the Ta plate.



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Femtosecond laser pulses (800 nm center wavelength, 40 fs temporal duration measured with GRENOUILLE, 1 kHz) were generated by a regenerative amplifier system (Solstice ACE 35F1K, Newport). A 75 cm quartz thin lens (Newport) focused the femtosecond laser beam onto the Pd(100) crystal through a UV grade quartz window. The laser beam was incident on the Pd(100) surface at a 70° angle off normal for 3 minutes. Oxygen atoms on the palladium surface are highly reactive towards typical residual impurities (such as CO and H2) in a UHV chamber. The palladium sample with oxygen atom adlayer surface can lose oxygen via reaction to form CO2 or water just by resting in the UHV chamber at even room temperature for a period of time. Furthermore, during femtosecond laserinduced photolysis, the temperature of the Pd(100) crystal increases from its initial surface temperature (this is caused by radiative heating). Therefore, it is imperative that a very controlled experiment must be performed to confirm the primary observed phenomenon originates from the femtosecond laser-induced chemistry and not from an artifact which includes loss of oxygen by the residual impurity in UHV and by the radiative heating. The contribution of loss of oxygen atoms due to the residual impurity in a UHV environment and the radiative heating during photolysis must be estimated carefully otherwise observed desorption yield can be mistakenly attributed to photodesorption. In our experiments, we have carefully monitored the substrate temperature rise during the photolysis (due to the radiative heating) and pre- and post-radiation TPD profiles have been carefully compared to estimate the pure femtosecond laser-induced desorption yield. Furthermore, we have also carefully maintained the time between dosing the sample and performing the experiment to obtain a set of reproducible pre- and post-radiation TPD profiles for a particular initial surface temperature. For example, 8 minutes was maintained for the initial surface temperature 560 K and 7 minutes was maintained for the initial surface temperature 400 K.

Computational Details: All density functional theory (DFT) calculations were performed within the generalized gradient approximation23 (with PBE functional) to explore the (O+O) adlayer on the Pd(100) surface using the CASTEP program.24,25 Ionic cores were described by ultrasoft pseudopotentials26 and the Kohn-Sham one electron states were expanded in a plane wave basis set up to 300 eV. We used the k-point sampling by using 2 x 2 x 1 Monkhorst-Pack mesh.27 We also tested higher energy cut-off and higher order Monkhorst-Pack mesh to verify the accuracy of the present method.28 The supercell approach was employed to model periodic geometries. The surface was modeled by two-layer slabs of Pd metal atoms. Particularly, a 4x2 supercell with two layers containing 16 metal atoms was used for the Pd(100) system. The slabs were separated by 20 Å of vacuum. In all the calculations, only the top surface layer of metal atoms was allowed to relax. As shown in one of our recent works,28 the above setup provides sufficient accuracy for the study of catalytic reactions on transition metal (particularly palladium) surfaces.


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Results and Discussion: Before we discuss the femtosecond optical pulse-induced O2 desorption, we first turn our attention to the thermally activated recombinative desorption of O2. Earlier experiments show that the molecular oxygen (O2) dissociates completely on the single crystal Pd(100) surface at and above room temperature (300 K), yielding the O-atomic adlayer structure.29,30 It has experimentally been observed that the room temperature adsorption of oxygen (O2) at the Pd(100) surface renders a well-ordered p(2X2)-O phase, when the coverage of atomic O is 0.25 monolayer (ML) and a c(2x2)-O phase is formed when the O-coverage is 0.50 ML.6,31 The O atoms in these resultant adlayer structures are bound to four-fold hollow sites. It has also shown experimentally that further increase in atomic oxygen coverage above 0.5 ML is not possible at the room temperature.6,31 A surface temperature of 470 K is required to form the higher coverage p(5x5)-O structure, corresponding to 0.68 ML coverage and a surface temperature of 550 K is necessary to form even higher coverage (√5X√5)R27O-O structure, corresponding to saturation coverage 0.80 ML. The p(5x5)-O and the (√5X√5)R27O-O structures result from the reconstruction of the Pd(100) surface. In our present work, first, we have monitored the thermally activated O+O=O2 reaction (which ultimately leads to recombinative desorption of molecular oxygen) on the single crystal Pd(100) surface by means of the temperature programmed desorption (TPD) measurements. Figure 2 shows representative O2 TPD profiles recorded for different atomic-O coverages on the Pd(100) surface. The atomic O-adlayers are prepared following adsorption of the molecular O2 at 400 K initial surface temperature. The coverage of the atomic oxygen in terms of monolayer (ML) is defined with respect to the 300 K saturation coverage. At the high coverage of atomic oxygen, two main molecular O2 desorption features are evident: the first with a steep rise (called β peak) beginning just above 600 K and a high temperature broad feature showing a peak near 850 K (called α peak). On the other hand, lower atomic-O coverage renders only single high temperature broad feature with a peak near 850 K (α peak) and the β peak is not observed at lower atomic-O coverage. Although interpretation of the TPD profiles of O2 obtained from the Pd(100) surface is not very relevant to the present study, we present a brief interpretation of the observed O2 TPD profiles to make the present article a self-contained one. Interpretation of the above TPD profiles can be easily found based on previous combined low energy electron diffraction (LEED) and temperature programmed desorption (TPD) studies.6,31 It has been shown with the help of LEED that a c(2x2)-O phase is formed following saturation coverage of 0.50 ML at the room temperature. This c(2X2) phase undergoes an irreversible phase transformation at about 470 K under vacuum to a mixed p(5X5)-O and p(2x2)-O phase without loss of (either molecular or atomic) oxygen to the gas phase. A further irreversible phase transformation at 550 K under vacuum results in the formation of a mixed (√5X√5)R27O-O and p(2x2)-O phase. Heating the Pd(100) crystal in vacuum at temperatures above 600 K results in partial desorption of molecular oxygen: the (√5X√5)R27O phase is decomposed to the p(2x2) with partial desorption of molecular oxygen. This partial desorption is manifested by the steep peak (β peak) observed at 650 K. Loss of the molecular oxygen from the p(2x2)-O phase subsequently occurs at temperatures above 650 K. This desorption is manifested by the broad feature observed in the 5 ACS Paragon Plus Environment


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range 650-1000 K. Therefore, a highly reactive (√5X√5)R27O-O phase is responsible for the steep β peak and relatively more stable p(2x2)-O phase is responsible for the broad envelope (α feature), shown in Figure 2. Femtosecond laser irradiation of the atomic-O covered Pd(100) surface causes loss of atomic oxygen only above 400 K initial surface temperature. Below 400 K, femtosecond laserinduced chemistry is completely suppressed. This temperature-dependent behavior is evident from a series of TPD profiles taken before and after the irradiation in order to probe the atomic oxygen left on the surface for different initial surface temperatures. To illustrate this point, Figures 3(a) and (b), respectively, depict pre- and post-radiation recombinative O2 TPD profiles recorded at 400 K and 530 K initial surface temperatures, respectively. A noticeable depletion of the recombinative O2 TPD profile due to femtosecond laser irradiation is evident for the initial surface temperature 530 K; however, almost no change in pre- and post-radiation TPD profiles is observed for the initial surface temperature 400 K. Here, we note that Figure 3(b) depicts two pre-radiation O2-TPD profiles. In the pre- and post-radiation TPD measurements, we have carefully monitored the rise of substrate temperature during the laser-beam exposure (This temperature rise is caused by radiative heating of the sample). If Ti and T f are the substrate temperatures recorded just before and immediately after the irradiation and if t represents the photolysis time, then pre-1 profile in Figure 3(b) features the O2-TPD profile recorded without radiation from the Ti initial surface temperature. Pre-2 profile, on the other hand, represents the O2-TPD profile recorded without radiation from Ti initial surface temperature only after ramping the temperature of the oxygen atom-covered surface from Ti to T f for t time (without laser exposure) followed by cooling to Ti . Ideally (without artifactual loss of adsorbates) in a photodesorption experiment, the A  photodesorption yield can be expressed as  ln  post  ; where, the this factor is closely related to  A   pre  desorption cross-section, Qdesp , as follows:    are the coverages of atomic oxygen (expressed as O2 molecules desorbed Qdesp  

Here, Apost and Apre

1  Apost ln  nt  Apre

following recombinative desorption process) on the surface after and before irradiation, respectively and nt is the total photon number per cm2 incident in time t . However, in our experiments, to take the contribution of the artifactual loss of oxygen atoms into account, Apre represents integrated area of the molecular oxygen desorption peak associated with pre-1 profile but Apost does not directly represent integrated area of the molecular oxygen desorption peak associated with the post profile. Due to presence of artifactual loss of oxygen atoms in our experiments, the post profile is a consequence of the joint effect of pure photodesorption and artifactual loss of oxygen atoms. The Apost area contributed by the pure photodesorption,


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however, can be accurately estimated by adding the integrated area difference between pre-1 and pre-2 profiles to the area of the post profile. Following the methodology mentioned above, we have estimated the temperatureA  dependent photodesorption yield, which is expressed as  ln  post  , for different initial surface  A   pre  temperatures. Results are depicted in Figure 4. These measurements were made with absorbed fluence 2.86 mJ/cm2. Visual examination of the threshold behavior in Figure 4 indicates that photodesorption has a threshold of initial surface temperature approximately 400 K. Only above 400 K, efficiency of photoinduced conversion of atomic oxygen to molecular O2 almost linearly increases as a function of the initial surface temperature. Furthermore, we have monitored the fluence-dependent behavior of the O2 desorption yield. Femtosecond laser-induced desorption feature, in general, exhibits highly nonlinear fluence dependence (as mentioned in the Introduction section). On the contrary, the photodesorption yield of molecular oxygen from the Pd(100) surface is found to be linear with respect to the absorbed fluence. The fluence dependence, shown in Figure 5, indicates a linear process. In the end, we note that the reproducibility of the experimental results presented in Figures 4 and 5 is featured with the error bars for those data points which are well-separated in the respective X-axis domain. By repeating the O2-TPD experiments with the same O2 exposure but without laser exposure on different days, we have found that the variation of the O2-TPD peak area falls within 5% of the mean value.

General Discussion: From the temperature programmed desorption measurements, it is unambiguously evident that only atomic O is present on the Pd(100) surface under the experimental conditions used in the present work (O2 was dosed at the surface temperature 400 K, which completely dissociates molecular oxygen to atomic oxygen on the palladium surface). No molecular oxygen can survive at 400 K on the palladium surface. Now, in order for the O2 molecules to form (and desorb) through the femtosecond laser-induced recombinative O2 desorption process, two steps must be followed: (a) vibrational excitation of the O atoms and then (b) recombination of vibrationally excited oxygen atoms to form a molecule before they can desorb as O2. The former process (step (a)) can be electron-mediated (may have femtosecond response); however, the later process (step (b)) cannot be an electron-mediated one. The later process must be a slow process which involves nuclear motion (diffusion) and an activation barrier (associated with diffusion of two oxygen atoms to recombine) must be overcome. To obtain insight into the microscopic mechanism of the possible electron-induced activation of atomic O adsorbates (the first step as mentioned above), we have carried out density functional theory calculations for the (O+O) adlayer on slab model Pd(100) surface. Previous 7 ACS Paragon Plus Environment


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experimental works evidence that the c(2x2)-O structure, in which O atoms occupy four-fold hollow sites, is the most stable oxygen atom adlayer structure on the Pd(100) surface. Therefore, we have first optimized this O-adlayer on model Pd(100) surface. Respective the most stable optimized (O+O) adlayer structure is depicted in Figure 6. Here, we note that among different (O+O) configurations, the most stable (associated with the lowest energy) (O+O)-structure on the Pd(100) surface is the one with O atoms residing at the four-fold hollow sites. Our motivation behind optimizing the c(2x2)-O structure is to judge whether femtosecond laser excitation can create hot electrons (with sufficient energy) which can be transferred to the affinity (antibonding) levels of the adsorbed atomic oxygen. Energy of hot electrons can be easily computed with the help of two-temperature model discussed below. Femtosecond laser-induced surface chemistry on extended single crystal metal surfaces is often analyzed using two coupled heat baths of the metal: the electron and phonon heat baths. Each is characterized by its own temperature (as two temperatures are involved, this is often referred to as two temperature model).15 Under the two-temperature model (2TM), it is assumed that absorption of photons ( h ) in the metal substrate creates a non-equilibrium electron–hole pair distribution which thermalizes by rapid electron–electron scattering. The electronic system can subsequently be described by an electron temperature Tel. The hot electron gas relaxes by electron–phonon coupling to the lattice phonons (Tph) and by heat diffusion into the bulk. The time evolution of the electron and lattice temperature (Tel and Tph) is finally written as,32 Cel

 Tel t

   Tel  g (Tel  Tph )  S ………. (1) z z  C ph Tph  g (Tel  Tph ) ………. (2) t



Here, Cel and C ph are the electron and lattice heat capacities, respectively.  and g refer to the thermal conductivity and the electron-phonon coupling constant, respectively. Furthermore, lateral diffusion is neglected here since the beam diameter is much larger than the electron diffusion length. This reduces the dimensionality to the distance z along the surface normal. The laser source term is given by,

S ( z, t ) 

 1  t  t1   t  t2    F2 sech 2  exp   z   F1 sech 2     ………. (3)  Z   2 BZ  B   B 

Here, t1 and t2 are the arrival times for two pulses, F1 and F2 are absorbed fluences, Z is optical penetration depth and finally, B is temporal width of the pulses. Because of its low heat capacity, the electronic system is heated by the femtosecond pulses to several thousands of kelvin before equilibrating with the lattice vibrational modes by electron–phonon coupling. This is shown in Figure 7(a) for initial surface temperature 300 K. Other thermodynamical and optical properties of palladium which were used to carry out the 2TM-based simulation are given in Table 1. We clearly find that because of its low heat capacity, the electronic system is heated to ~2215 K before equilibrating with lattice vibrational modes by electron-phonon coupling within ~1 ps, as shown in Figure 7(a). Furthermore, we note that 8 ACS Paragon Plus Environment

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higher initial surface temperature linearly increases the peak temperatures of the electron and phonon baths, as shown in Figure 7(b). After predicting the electron and phonon bath temperatures from the 2TM for the experimental conditions used in the present work, we have analyzed the local density of states (DOS) produced upon adsorption of the O atoms on the Pd(100) surface. Figure 6 depicts that an unoccupied antibonding state (with respect to the Pd-O bond) right above the Fermi level  F (within 0.5 eV from the Fermi level) appears following adsorption of the O atoms on the Pd(100) surface. Hot metal electrons (created by the femtosecond pulse excitation) can easily populate this antibonding state, as shown in Figure 6, by taking illustrative example of the Fermi-Dirac distributions ( f    ) at two different electronic bath temperatures (namely, 300 K and 2215 K: here we note that 2215 K electronic bath temperature can be achieved for the initial surface temperature 300 K). Thus far, resonant hot electron transfer to the low-lying metal-O antibonding states is believed to be involved in many ultrafast surface reactions.15,33 Based on the present DFT results, we also believe that the same mechanism must be involved in the femtosecond laser-induced activation of the oxygen atoms on the Pd(100) surface. Furthermore, the timescale for the Pd-O bond activation must also be on the order of a few hundred femtoseconds because it is purely hot electron-mediated (the same is evident in many instances pertaining to the ultrafast surface reactions).15,33 The hot electron-mediated activation of the O atoms immediately results in vibrational activation of the O atoms. This vibrational activation, in principle, may lead to two consequences: (1) the direct desorption of oxygen atoms and (b) the recombination of oxygen atoms followed by desorption of molecular oxygen from the surface. We argue that the second pathway is active and energetically preferred in our present work. Details of this argument are presented below. Generally, under femtosecond-pulse excitation, the direct photodesorption yield ( Y ) is found to depend on the incident fluence ( F ) in a highly nonlinear fashion (note examples of femtosecond laser induced O2 desorption already reported in literature).15,33 Empirically, this nonlinear fluence dependence is described by a power law, Y  F n . Typically, n > 2 for desorption induced by a femtosecond pulse is observed, consistent with a mechanism where high transient adsorbate temperatures, which may greatly exceed the thermal-desorption temperature, are needed to drive the desorption process. In the spirit of understanding this common observation, we argue that if direct desorption of oxygen atom pathway were active (via pure electron-mediated activation of the Pd-O bond) in the femtosecond laser-induced chemistry of (O+O) adlayer on the Pd(100) surface, a nonlinear photodesorption yield would have been observed by us. Quite on the contrary, we have observed a linear photodesorption yield (see Figure 5). This counter-intuitive observation in our femtosecond laser experiment directly suggests that the direct desorption of oxygen atoms due to the femtosecond laser excitation is not active for the system presented in this article. There is no doubt that oxygen atoms are vibrationally activated on the Pd(100) surface via hot electrons; however, the consequence of this vibrational excitation is not the direct desorption of oxygen atoms. Instead, we argue that the recombinative desorption of molecular oxygen is the consequence of this femtosecond laserinduced vibrational excitation of the oxygen atoms. We elaborate this point further below.



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Figure 4 shows that the femtosecond laser-induced chemistry is active only above initial surface temperature 400 K. This threshold behavior cannot be explained with the help of only electron bath temperature revealed by the 2TM-based simulation. Figure 6 shows that even for 300 K initial surface temperature, electronic bath temperature can transiently reach peak temperature of 2215 K (for the absorbed fluence used in our experiment). This high electron bath temperature generates hot electrons which can very easily excite surface-O bonds even for the initial surface temperature 300 K and which can lead to direct photodesorption of oxygen atoms (if this process is at all active). However, our experimental results presented in Figure 4 show that no desorption occurs for the initial surface temperature 300 K. Only above 400 K initial surface temperature, difference between the recoded pre- and post-radiation O2-TPD profiles starts to show up. This is because only at (or above) 400 K initial surface temperature, phonon bath temperature reaches almost 600 K (or above, see Figure 7(b)) which is a threshold temperature for the recombinative desorption of molecular oxygen, as revealed by the temperature programmed desorption measurements presented in Figure 3. Therefore, femtosecond laser-induced recombinative desorption of molecular oxygen process is a thermally activated (phonon-driven) process. This reaction becomes active only above certain threshold initial surface temperature. In our experiments, therefore, femtosecond laser acts as a high-tech local heater which eventually deposits heat energy to the Pd(100) surface to carry out recombinative desorption of molecular oxygen from (O+O)-adlayer.

Conclusions: In conclusion, we find that the photodesorption yield of recombinative O2 from the Pd(100) surface increases almost linearly with increasing substrate initial temperature only above 400 K. This result can be attributed to the influence of the initial vibrational state distribution in the recombinative desorption coordinate. We explain this observation with the help of twotemperature model-based simulation and density functional theory computation: the O2 desorption yield increases with increasing population of the excited vibrational levels on the ground potential energy surface by increasing substrate phonon temperature. Furthermore, observation of the linear fluence dependence of the photodesorption yield also corroborates the fact that the femtosecond laser-induced recombinative desorption of O2 from the single crystal Pd(100) surface is a thermally activated process (phonon-mediated). However, vibrational excitation of the oxygen atoms, which is the first step to the recombinative desorption process, occurs through the hot-electron mediated process.

Acknowledgments: Authors gratefully acknowledge that the financial support for the present work came jointly from the DST Nano Mission (Contract # SR/NM/NS-1117/2013(G)) and the 12th Plan Grant of the Indian Institute of Science (Contract # 12-0201-0243-01-403).


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Table 1: Values of parameters used in the two-temperature model simulation.

Parameter Electronic specific heat offset Electronic specific heat slope Thermal conductivity Electron-phonon coupling constant Initial temperature Absorbed Fluence Wavelength of the Incident light Pulse FWHM

Symbol γ0 γ1 κ g T F λ B

Value 7.8849 x 104 249.14 72 8.95 x 1017 300 2.86 800 40

Dimensions J K-1 m-3 J K-2 m-3 W K-1 m-1 W K-1 m-3 K mJ cm-2 nm fs



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Figure 1: (a) Auger electron spectrum of a clean single crystal Pd(100) surface. This spectrum has been normalized to the intensity of the 330 eV PdMNN peak. (b) O2, CO and CO2 TPD profiles obtained from a clean Pd(100) surface following exposure of the surface to molecular oxygen at 400 K. O2-TPD profile is clearly evident, while no change is observed in both the CO and CO2 mass channels. This confirms cleanliness of the Pd(100) surface used in our present work.


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Figure 2: Temperature-programmed desorption (TPD) profiles of molecular O2 from (O+O)/Pd(100) surface as a function of O atom coverage. The ramp rate was 4.4 K/s.



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Figure 3: TPD profiles of molecular oxygen recorded before and after irradiation of (O+O)/Pd(100) surfaces for (a) 400 K and (b) 530 K initial surface temperatures. Pre, Post, Pre-1, Pre-2 profiles are defined in the main text.


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A  Figure 4: O2 Desorption yield,  ln  post  , as a function of the initial surface temperature. The  A   pre  solid line represents average value to guide our eye. Apost and Apre are defined in the main text.

Absorbed fluence of 2.86 mJ/cm2 was used for each measurement. Error bar represents the standard deviation of the mean.



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Figure 5: O2 photodesorption yield as a function of absorbed fluence. The solid line is a linear fit to the data. All data points were recorded for the initial surface temperature 560 K. Error bar represents the standard deviation of the mean.


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Figure 6: DFT results: Most stable c(2x2)-O adlayer structure on the Pd(100) surface. Antibonding states of O (with respect to the Pd-O bond) appears just above the Fermi level (within 0.5 eV from the Fermi level); Fermi-Dirac distribution functions (right coordinate axis), obtained for electronic bath temperatures 300 and 2215 K, demonstrate that with increasing electron bath temperature, antibonding O states can be easily populated. Therefore, O atoms can be easily activated through direct coupling of the adsorbates with the hot electron bath even for initial surface temperature 300 K (we note that 2215 K peak electronic temperature is achieved for the initial surface temperature 300 K).



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Figure 7: (a) Simulation results obtained with the thermodynamical and optical parameters given in Table 1 (for initial surface temperature 300 K): variation of the electron and phonon temperatures at the Pd(100) surface is shown. (b) Simulated peak electronic (solid black square) and phonon (solid red circle) temperatures are plotted as a function of the initial surface temperature. We note that only at or above 400 K initial surface temperature, phonon bath temperature can rise above the threshold temperature 600 K for the recombinative desorption of molecular oxygen.


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Femtosecond Laser-Induced Recombinative O+O=O2 Reaction on

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