Pulsed 266 nm Laser Desorption of Aluminum, Hydrogen, and Water

The pulsed laser desorption of monoatomic, Al, H2, and H2O was studied at 266 nm with the time-of-flight (TOF) method at laser fluences between 63.3 a...
3 downloads 5 Views 558KB Size
Download PDF
14074

J. Phys. Chem. 1996, 100, 14074-14080

Pulsed 266 nm Laser Desorption of Aluminum, Hydrogen, and Water from H-Y Zeolites Studied with Time-of-Flight Mass Spectrometric Analysis Katsumi Tanaka,* Kazutaka Ishigoh, and Ryohei Nakata Department of Electronic Engineering, The UniVersity of Electro-Communications, Chofu, Tokyo 182, Japan ReceiVed: January 17, 1996; In Final Form: June 4, 1996X

The pulsed laser desorption of monoatomic, Al, H2, and H2O was studied at 266 nm with the time-of-flight (TOF) method at laser fluences between 63.3 and 90.2 mJ cm-2. TOF data were analyzed with shifted Maxwell-Boltzmann distributions taking collisions of desorbed species into account. Kinetic energy at TOF peak (KEp) of ionic Al species increased, while that of neutral Al species decreased as the laser fluence increased. However, their center of mass velocities were constant. These results clearly indicate that collisions occur between desorbed species. KEp values of ionic and neutral Al species were similar (∼8 eV) when they were extrapolated to zero fluence. Such a high KE value cannot be explained by a temperature rise due to a pulsed laser shot but was explained by two-photon excitation process followed by surface relaxation, which will be dissipated either by an exciton formation or by the minimum energy necessary to form ionized Al species, ionization energy Ei minus work function φ. A new method to estimate work function of materials is proposed. From XPS results and thermodynamic considerations, it is concluded that laser-desorbed Al species are from zeolite framework, and not from agglomerated Al species. It was confirmed that H2 and H2O are pulsed laser desorbed from OH species present in H-Y zeolites. The formation of H2O was dependent on the concentration of OH species. A model was proposed to interpret laser desorption of monoatomic Al and H2 from H-Y zeolite.

Introduction

SCHEME 1

Zeolites are aluminosilicates with three-dimensional cage structure of different sizes and are also applicable from catalysis to nanostructure confinement.1-3 Although thermal processes have been used to prepare these materials, recent technology requires, photochemical processes because less damage to material is needed. As an example, excimer lasers have been extensively applied to microelectronic fabrications, patterning, etching, and projection lithography.4,5 In this sense, laser photochemistry in zeolite cages is of great interest, whereas an understanding of the laser/zeolite interaction is still lacking in the literature. Under X-ray irradiation, a color change of the Na-Y zeolite has been reported due to Na cluster formation in Na metal vapor dosed zeolites.6-8 Recently we found that color change on Na-Y zeolite after irradiating 266 nm laser light at fluences below ablation threshold.9 Near ablation threshold, Na+ and Al+ with kinetic energies (KEs) higher than 266 nm quanta (4.66 eV) were observed, which is interpreted by a photochemical process based on electric excitation mechanism. Dealumination of tetrahedral Al from zeolite framework during the hydration-dehydration has been widely studied.10-16 In this sense, laser desorption of Al species from sapphire (Al2O3) and oxidized Al(111) studied with time-of-flight (TOF) methods17-19 may induce an insight upon dealumination of Al species from zeolite. Because Al3+ species situate in the place of Si4+ in zeolite framework, positive cations are necessary for charge compensation. H-type zeolites contain H+ as OH groups, which are studied on the basis of their structure and reactivity. In this paper, we report 266 nm pulsed laser desorption of Al+, H2, and H2O from H-Y zeolites, studied using the mass spectrometric TOF technique (Scheme 1). These observations will shed light on the photochemical desorption mechanism * Author to whom all correspondence should be addressed. E-mail: [email protected]. X Abstract published in AdVance ACS Abstracts, August 1, 1996.

S0022-3654(96)00199-2 CCC: $12.00

based on local structure of zeolite as well as interaction of laser light and the zeolite with three-dimensional structure. Experimental Section Zeolite H-Y is one of the synthetic faujasite with three dimensional channel system of 0.74 nm opening windows connecting spherical 1.18 nm cavities (supercages).20 We used two kinds of H-Y zeolites. Their chemical compositions are Na9H20(AlO2)29(SiO2)163‚xH2O (hereafter denoted as H-Y zeolite (68%)) and Na0.4H30.6(AlO2)31(SiO2)161‚xH2O (H-Y zeolite (99%)). The zeolite powder was pressed at a pressure of 200 kg cm-2 and cut to about a size of 10 × 15 mm2 with a thickness about 0.3 mm. The pellet was held on Ta foil holder and was transferred to a vacuum chamber. The zeolite pellet was gradually heated up to 473 K and finally heated at 673 K under vacuum. To eliminate the background pressure of H2, a cryopump (ANELVA CAP80MK II) was used. The background pressure was below 1 × 10-6 Pa after system bakeout. Experimental apparatus was the same as reported before.9 The fourth harmonic generation (FHG, 266 nm) of a Q-switched Nd/YAG pulsed laser (Continuum Surelite-10, 1064 nm, 5-7 ns fwhm pulse duration) was generated and selected by the wavelength separator. The pulsed laser with a near-Gaussian © 1996 American Chemical Society

Pulsed Laser Desorption of Al, H2 and Water from H-Y Zeolites

Figure 1. TOF spectra of pulsed laser desorbed Al species from H-Y zeolite (68%) at laser fluence of 85 mJ cm-2: (a) with QMS filament on, (b) with QMS filament off. Dotted lines are curve-fitting results using S-MBDs, which indicate slow and fast components with QMS filament on; see text for details. Note that flight time corresponding to Ei is ti (51 µs).

pulse shape was reflected by two UV mirrors to irradiate the sample surface through a sapphire viewport with an angle 45° normal to the sample. Laser fluence (mJ cm-2) was calculated with the absolute laser energy measured by a Joule meter (Laser Instrumentation, Model 5273) and the area of laser beam (0.14 cm-2) measured by a burn paper. In this experiment, no laserfocusing device was employed. A quadrupole mass spectrometer (QMS) (Spectra Metrics, Multi Quad, mass-to-charge ratio m/e < 300) was placed normal to the laser-irradiating point to measure KE distributions. The QMS analyzer acceptance angle of the desorbed species was below (1.6°. Ions and neutrals were discriminated with the filament on and off, respectively. The electron impact voltage and current used by the ionizer filament were 70 V and 1 mA, respectively. The QMS signal was multiplied by the secondary electron multiplier and directly transferred to a transient memory (Iwatsu, TS-8123 storage scope) through the amplifier in the Multi Quad system that can detect a high-frequency signal of 140 MHz with a rise time below 7 ns from the laser high-voltage power supply.21 Each single shot was irradiated on the zeolite surface after digitizing the previous TOF data, which took a few seconds. A number of shots were accumulated to obtain an average TOF spectrum while tuning a m/e and were triggered by the scattered light detected with an avalanche photodiode detector. Each statistical TOF datum was stored in the transient memory and was transferred to a personal computer. The distance from sample surface to the ionizer and the detector were 175 and 335 mm, respectively. The effects of 266 nm laser shots on the Al to Si molar ratio in H-Y zeolite was studied with X-ray photoelectron spectroscopy (XPS) (Vacuum Generator ESCA LAB-5). A H-Y zeolite pellet was placed on Ta holder and was evacuated at 673 K for 12 h in XPS chamber. The background pressure was below 1 × 10-7 Pa. Laser shots were introduced through a sapphire viewport with an angle about 45° without any focusing devices. Al 2p and Si 2p spectra were recorded with the Al KR radiation (hν ) 1486.6 eV) of 50 eV pass energy. The molar ratio of Al to Si was elucidated by their spectral area and their sensitivities. Results Aluminum-tuned TOF spectra (m/e ) 27) of H-Y zeolite (68%) were recorded at laser fluence of 85 mJ cm-2 at room temperature. Figure 1 shows the TOF with QMS filament on (a) and with off (b). It is noted that ionic species can be selectively detected in the filament off mode, while both neutral

J. Phys. Chem., Vol. 100, No. 33, 1996 14075

Figure 2. KE distributions of pulsed laser desorbed Al species. Note that these are obtained using TOF spectra of Figure 1; see Figure 1 for experimental conditions.

and ionic Al species can be detected in the filament on mode. TOF data were analyzed to be fitted in shifted MaxwellBoltzmann distribution (S-MBD) taking collision between particles into account as follows:22

f(t) ) At-4 exp{(m/2kTM)(z/t - Vcm)2}

(1)

Here f(t) is the mass spectrum intensity of the tuned species as a function of t, time after a laser pulse. In eq 1, A, m, k, z, and Vcm are scaling factor, mass of tuned species, Boltzmann constant, distance between sample and detector, center of mass velocity, and flow rate, respectively. TM is the MaxwellBoltzmann temperature (K); however, it does not necessarily mean the surface temperature or rather spread of a distribution. A, TM, and Vcm are fitting parameters. Spectrum a in Figure 1 can be deconvoluted into two species, for which times at the TOF peak maximum tp were 38 and 59 µs, and Vcm for both was 4.28 km s-1, while for the ionic species observed in b of Figure 1, tp and Vcm were 37 µs and 4.91 km s-1, respectively. It is significant to note that the TOF shape and the intensity of the fast component are almost the same as that of ionic species in spectrum b. Consequently, it can be deduced that the fast and slow components in Figure 1 are ionic and neutral Al species, respectively. TOF spectra of Figure 1 are replotted as a function of KE ()mz2/2t2) in Figure 2. The neutral and ionic Al species desorbed from H-Y zeolite (68%) in spectrum a have KEs at the TOF peak maximum (KEp) of 4.5 and 10.9 eV, while Al ionic species has 11.5 eV for KEp. The ionization potential of Al is 6.0 eV. As TOF signals obey S-MBDs, it is clear that collisions take place between desorbed species in flux. Therefore, one of the key points in deciding the fate of desorbed Al species whether neutral or ionic is the ionization potential (Ei). The slow Al component with KEp below Ei will be neutral and the fast Al component with KEp larger than Ei will be ionic. It is significant to note that KEp of ionic Al species exceeds the 266 nm photon energy (4.66 eV). This fact suggests that Al species will be desorbed photochemically via a multiphoton absorption process. To evaluate the origin of Al species desorbed from H-Y zeolite, their KEp values were measured as a function of laser fluences. As shown in Figure 3, the KEp value of neutral species detected as the slow TOF component with QMS filament on was 5.2 eV at 63.3 mJ cm-2 and the value decreased as laser fluence increased, while KEp values of ionic Al species detected as the fast component with QMS filament on and off increased as the laser fluence increased. Below 63.3 mJ cm-2 it was crucial to detect Al TOF signals, probably due to the sensitivity of our signal detection system. KE of desorbed species has a

14076 J. Phys. Chem., Vol. 100, No. 33, 1996

Figure 3. KEp (KE at TOF peak) values of pulsed laser desorbed Al species from H-Y zeolite (68%) (O, 4, 2) and from H-Y zeolite (99%) (double circle, 0, 9) as a function of laser fluence. O, double circle: neutral Al species. 4, 0: ionic Al species with QMS filament on. 2, 9: ionic Al species with QMS filament off.

Tanaka et al.

Figure 5. Vcm (center of mass velocity, km s-1) in S-MBD of pulsed laser desorbed Al species. Symbols are the same as in Figure 3.

TABLE 1: XPS Analyses of Al to Si Molar Ratios Induced by 266 nm Laser Shots on H-Y Zeolite (99%). Laser fluence Used: 76.7 mJ cm-2

a

Figure 4. Time at center of S-MBD (µs) of pulsed laser desorbed Al species as a function of laser fluence. Symbols are the same as in Figure 3.

distribution which will be reflected both by KEp value and the time at center of gravity in a S-MBD. Of course they should influence each other. In Figure 4, the time at the center of S-MBD is shown as a function of laser fluence. It is easily found that the center of S-MBD shifts to slower time for neutral Al species, while it shifts to faster time from ionic Al species as laser fluence increases. Therefore, it can be concluded that KE of ionic Al species increases while that of neutral Al species decreases as laser fluence increases. It may be important to report that KEp of ionic Al species reached about 8 eV at focused 532 nm pulsed laser shots with laser fluence of about 1 J cm-2. The decrease of KEp of desorbed neutral Al species will be not only due to collision among them but also due to collision with concomitantly desorbed hydrogen from OH species present as the charge compensator of Al3+ which situates the position of Si4+. It is interesting to note that KEp of desorbed neutral Al species and that of ionic Al species reach the same KE, about 8 eV, if they are extrapolated to zero fluence. This suggestion strongly invokes the speculation that both neutral and ionic desorbed Al species may have the same origin in the pulsed laser desorption process and neutral Al species may exist with KEp higher than Ei. As shown in Figure 3, KEp value of desorbed Al species from H-Y zeolite (99%) was also analyzed at laser fluence of 90.2 mJ cm-2. KEp values of the slow and fast Al TOF components measured with QMS filament on were 5.6 and 8.4 eV, respectively, while KEp of ionic Al species with QMS filament off was 8.3 eV. These results infer that KEp values of desorbed Al species are influenced by the concentration of OH species in zeolite. To study whether or not neutral and ionic Al species originate from the same Al in zeolite skeleton, their center of mass velocity Vcm values were compared. The results are shown in Figure 5. It is found that Vcm values are similar to neutral and ionic Al species and are constant for the laser fluences used. It

no. of laser shots

Al/Si intensity ratio by XPS

calcd Al/Si molar ratio

0a 1 × 102 1 × 103 3 × 103

0.55 0.52 0.46 0.34

0.19 0.18 0.16 0.12

Fresh sample.

is also noted that Vcm values of Al species do not depend on the concentration of OH groups in zeolite. From these results it is concluded that pulsed laser shot to H-Y zeolite induces the desorption of Al species from the same Al sites in zeolite. The effect of 266 nm laser shots on the chemical composition of zeolite was studied. Table 1 shows the XPS analytical results of Al/Si molar ratios induced by 266 nm laser shots on H-Y zeolite (99%). The laser fluence used was 76.7 mJ cm-2. The Al/Si molar ratio of fresh H-Y zeolite (0.19) was gradually decreased as the laser shot number increased. It is amazing that the Al/Si ratio decreased as much as 37% after 3 × 103 laser shots. The Al/Si ratio decrease is affected by selective Al desorption from H-Y zeolite skeleton since no Si desorption was detected by our QMS. We have also observed continuous Al desorption from Na-Y zeolite; however, Al to Si ratio was constant at the surface measured by SIMS.9 This result implies that Al migrates into zeolite and a constant composition of Al+ to Si+ is measured on the surface by sputtering with Ar+ during SIMS measurements. The same Al migration would occur concurrently with Al desorption in H-Y zeolite with 266 nm laser shot; however, the Al/Si ratio in XPS can be reflected by their escape depths. The effect of laser fluence on the desorption yield of Al was studied. The logarithmic plots between desorbed Al mass intensity and the laser fluence are shown in Figure 6. It is found that the relation is nonlinear and the dependence can be discussed by a factor n defined as

I ) BFn

(2)

Here, I, B, and F are mass intensity of desorbed species, a scaling factor, and laser fluence, respectively. To the ionic Al desorbed species measured as almost the same intensity with both QMS filament on and off, n became 8. Compared to the Al ionic species, the n value was 2.7 for the neutral Al species. These results are reflected by the fact that neutral Al species are dominant at low laser fluences and the intensity of ionic Al species increases with much the intense exponential factor as the laser fluence increases. Aside from the discussion whether pulsed laser desorbed Al species are ionic or neutral, it will happen that desorbed ionic Al species are accelerated by collision in the flux while decelerated ionic Al species lose their

Pulsed Laser Desorption of Al, H2 and Water from H-Y Zeolites

J. Phys. Chem., Vol. 100, No. 33, 1996 14077

Figure 6. Logarithmic plots of desorbed Al mass intensity to those of laser fluences. Symbols are the same as in Figure 3.

Figure 8. Logarithmic plots of pulsed laser desorbed H2 (O) and H2O (b) QMS intensities from H-Y zeolite (99%) to those of laser fluences.

Figure 7. KEp and Vcm of pulsed laser desorbed Al species as a function of laser fluence: (O, b) H-Y zeolite (68%), (4, 2) H-Y zeolite (99%).

Figure 9. Effects of sample temperature on KEp (4) and Vcm (2) of pulsed laser desorbed H2 from H-Y zeolite (99%) at laser fluence of 90.2 mJ cm-2.

KEs. This holds to the fact that KE values of Al ions are distributed widely. It is quite reasonable that KEp of ionic Al species will increase while KEp of neutral species will decrease since collision in the flux should increase as laser fluence increases. Hydrogen molecules were desorbed from H-Y zeolite (68%), while hydrogen and water molecules were desorbed from H-Y zeolite (99%), both concurrent with Al species after a 266 nm pulsed laser shot. Analysis was carried out with S-MBD fits. Best fits of KEp and Vcm of desorbed hydrogen are shown in Figure 7 as a function of laser fluence. It is found that KEp is too small even though assuming that mass of H2 is smaller than mass of Al by a factor of 2/27, which means that the peak of H2 TOF signal is observed at least 1 order of magnitude slower than that of Al species. It is noted that KEp increases linearly as a function of laser fluence; however, Vcm is extrapolated to zero at laser fluence of 40 mJ cm-2. The KEp increase due to laser fluence increase can be interpreted by the collision in the flux. If H2 molecules are desorbed to be involved in the same flux as of Al species, the Vcm should be equal or similar to that of Al species. However, it is noted that Al desorption was always detected at the laser fluence where H2 desorption was observed. This fact may indicate that pulsed laser desorption of H2 will be the secondary reaction of the photochemical desorption of Al species from H-Y zelite. The KEp and Vcm values of H2 desorbed from H-Y zeolite (99%) are smaller compared to those from H-Y zeolite (68%). The difference will be influenced by H2O molecules desorbed from H-Y zeolite (99%). The effect of laser fluence on the desorption yield of H2 is shown in Figure 8. From the logarithmic relation between desorption yield and laser fluence, the n value in eq 2 becomes 3.4. This value can be compared to the value (2.7) observed

to the pulsed laser desorption of Al. Taking the experimental error in our system into account, these values will be similar. If it is the case, H2 desorption will be involved as the secondary reaction in the primary photochemical Al desorption process. The dependency of laser fluence on H2O desorption yield observed from H-Y zeolite (99%) is also shown in Figure 8. It should be noted here that the H2O desorption originates from OH groups in H-Y zeolite but not from surface adsorbed H2O, which is clearly verified in Figure 10. As the concentration of OH groups in H-Y zeolite increases, H2O begins to desorb by a pulsed laser shot. In this sense, the fact that the n value of pulsed laser desorption of H2O is similar to that of Al means that pulsed laser H2O formation may also be involved in Al desorption process. The effects of sample temperature of H-Y zeolite on KEp and Vcm of pulsed laser desorbed H2 and H2O are shown in Figures 9 and 10, respectively. As shown in Figure 9, KEp and Vcm of desorbed H2 were studied at laser fluence of 90.2 mJ cm-2. It is found that KEp and Vcm were almost constant, ∼0.027 eV and 7.0 km s-1, respectively. The peak time of TOF signals should change in a thermal desorption process.23 At the same time, center of mass velocity Vcm does not depend on sample temperature but is constant. These results infer that the H2 formation reaction will be photochemical or will be involved in photochemical Al desorption process. However the surface temperature difference (400 K) may not be large enough to elucidate the process. The effect of laser fluence on KEp and Vcm of pulsed laser desorbed H2O from H-Y zeolite (99%) is shown in Figure 10. Especially noted is that H2O molecules are desorbed by pulsed laser shots at 673 K, where no H2O can be adsorbed on the surface. The fact implies that the pulsed laser desorption of H2O shown here is brought about by the reaction of OH groups in H-Y zeolite. Both KEp and Vcm

14078 J. Phys. Chem., Vol. 100, No. 33, 1996

Tanaka et al.

Figure 10. Effects of sample temperature on KEp and Vcm of pulsed laser desorbed H2O from H-Y zeolite (99%). Laser fluence used are 85.0 mJ cm-2 (4, 2) and 90.2 mJ cm-2 (O, b).

increased as laser fluence increased from 85.0 to 90.2 mJ cm-2, and they did not change when the sample temperature was changed from 300 to 673 K. Discussion The mean KE value (KEM) of laser-desorbed ionic Al species exceeds 266 nm photon energy (4.66 eV) since KEM exceeds KEp (>8 eV). This fact suggests a multiphoton process or an electronic excitation mechanism. Before the reason for photochemical high-energy species formation, we should pay attention to the thermal effect. A temperature rise (∆T) due to pulsed “infrared” laser is represented as follows:23

∆T ) (1 - R)It/CvF(2Dt)1/2

(3)

Here R, I, t, CV, F, and D are reflectivity, laser density (W cm-2), laser pulse of duration, heat capacity, density, and heat diffusivity ()κ/CVF; κ is thermal conductivity), respectively. We can estimate D to be 1.7 × 10-5 m2 s-1 by using Cp (1.0 J mol-1 deg-1), F (2.5 g cm-3), and κ (2.5 kJ m-1 h-1 deg-1) data of SiO2; the ∆T reaches 5.8 × 104 K at a laser fluence of 0.1 J cm-2. Here we estimate CV ) Cp because of solid. We use especially R ) 0 in this estimation even though the zeolite color is white. Although this estimated temperature rise may be surprisingly high, it corresponds to 5.1 eV and is still below KEp. It is noted that the ∆T estimation can only be applied to infrared laser experiments.23 If thermal process is dominant and the energy inserted changes to kinetic energy of desorbed species, ∆T should increase linearly with laser fluence and as a result KEp will be enhanced linearly with laser fluence and start from zero at zero laser fluence. However, in our experiments KEp of desorbed Al species is nearly constant with small increase by collision of desorbed species as shown in Figure 3. More strongly supporting result for photochemical process is that KEps of desorbed Al species are similar at wavelength of 266 and 532 nm and it exceeds both photon energy (4.66 and 2.33 eV). Such photochemical desorption mechanism in the laser ablation process has been accepted recently in Na-Y zeolite,9 CdWO4, and Si.24 Whether the origin of desorbed Al species is ionic or neutral is the next question. As shown in Figure 3, KE of desorbed ionic Al species increases as the laser fluence increases, while that of neutral Al species decreases. The change of KE by the laser fluence can be interpreted by the collision between desorbed species. S-MBD of translation energies assume thermally neutralized ejected particles to be dispersed by their collisions.22 Providing that photochemically desorbed species which should not have a thermal information by nature are equilibrated in the flight path of collisions, they will take S-MBDs. The possibility is suggested in the ground state neutral

species formed during the laser ablation of Na2SiO325 and our previous paper.9 Neutral species are desorbed from Na2SiO3 with KEs only below 1 eV. However, in our experiments laserdesorbed neutral Al species have a KE at TOF peak more than 5 eV even at lower fluences. Neutral species with such high KEp cannot be explained without supposing that Al species with high KEp collide with one another and some are more accelerated and the others are decelerated to be neutral species, or that ionic precursor species of the neutral atoms transfer their fast velocities, which they originally have, to the neutral atoms.26 At lower laser fluences, the amount of desorbed ionic Al species should be less. If this is the case, the number of collisions should also be less, and the area where ionic Al species is confined will be restricted and the density will be low. Then, the possibility of neutralization by the recombination between ionic Al species and electrons or that of reaction between ionic Al species and residual molecules in the gas phase will be enhanced during their flight to the detector. As a result, neutral Al species will be dominant at lower laser fluences. Therefore, we conclude that the origin of desorbed Al species should be ionic. The production of highly energetic ions in laser-produced plasmas has been explained by the energy transfer between electrons and ions, and the acceleration in transient electronic field is unlikely.27 This explanation means the inverse bremsstrahlung mechanism which produces highly energetic electrons. In fact, high KE ionic species observed in laser ablation processes have been explained by this mechanism28 and laser/plume interaction.25 However, these cannot assume KE values of desorbed species. Classical MGR29,30 and KF31 mechanisms cannot predict KE values either.32 Therefore, we need another mechanism which can estimate KE values. For the sputtered atoms, the minimum amount of energy required to ionize positively an atom at infinitely and deposit electron on the solid surface is defined as Ei - φ.33 Here Ei and φ represent ionization potential and work function, respectively. In the case of Al metal, Ei and φ are 6.0 and 4.2 eV, respectively, implying that the minimum energy corresponds to 1.8 eV. In laser ablation of Al(111) at 355 nm, the KE of desorbed Al+ is 5.1 eV.19 As a 355 nm photon energy is 3.53 eV, it can be assumed that two photon energies (7.1 eV) are required to ionize Al (1.8 eV) and to transfer the remaining energy to KE (5.3 eV) of Al+ thus formed. The coincidence of Al+ KEs will support the validity of this assumption. If this assumption is applicable to our system, the energy difference between two photon energies of 266 nm (9.32 eV)) and Ei + KEp of Al (14 eV) corresponds to the work function of zeolite (4.68 eV). So far, the work function of zeolite has never been measured. This assumption may give us one method to evaluate the work function of materials. However, high Al+ KEp desorbed from zeolite cannot be explained by the work function either. The KE value is related to bulk longitudinal plasmon energy to the desorption of Ag+ from Ag metal.32 Laser-induced desorption stimulated by surface plasmon excitations has been reported on metal particles especially in several tenths of a nanometer size.34 However, we consider that ionic Al species are desorbed not from agglomerated Al or Al2O3 species but from framework since Al species are desorbed from zeolite by 266 nm laser light as much as 37% as shown in Table 1. In addition, the bandgap control does not play a significant role on the population of excited state.35 In steady state excimer laser (248 nm ) 5.0 eV) ablation of sapphire, KE of Al+ evaluated from the laserinduced fluorescence at 396.2 nm36 reaches 4 eV, but regarding AlO species simultaneously formed, their rotational and vibrational energies are quite low (∼600 K), suggesting an electronic

Pulsed Laser Desorption of Al, H2 and Water from H-Y Zeolites

Figure 11. An electronic excitation model considering surface relaxation. Eth is the threshold energy for transition. EK is the KE of ionic Al species without surface relaxation. Surface ground state S is activated to the excited surface state S*. When surface relaxation occurs, S* is quenched to surface relaxation state S- through an activation barrier of ED for Al+ desorption. Then KEp value is added as much as ER, which reaches EK + ER. See text regarding the surface relaxation.

mechanism.37 Selective laser desorption of Al+ species from sapphire (bandgap ∼ 9 eV) is explained by desorption induced by electronic transition (DIET) considering surface relaxation based on MGR model.18 Although a multiphoton excitation is needed so that MGR model cannot be applied since the model is fundamentally based on a single photon ionization process,29,30 their explanation seems to be reasonable to interpret high KE of desorbed ionic Al species qualitatively by the surface relaxation model. The validity of the DIET mechanism will be claimed by the fact that KEp of pulsed laser desorbed Al+ species form H-Y zeolites are similar at 266 and 532 nm; that is, the KEp value is reflected by the potential energy difference between the ground state and the excited state. This will hold true to our case since ionic Al species were desorbed with KEp about 8 eV both at 266 and 532 nm. On sapphire KE of Al+ is 7 eV at laser wavelength both at 1064 and 355 nm.18 Zeolite is composed of both AlO4 and SiO4 tetrahedrals. Therefore, there is some discrepancy when comparing zeolite and SiO2 and Al2O3 since Si-O-Al bond exists and H species are present to compensate one deficient positive cation per one Al species. Our electronic excitation model proposed is shown in Figure 11. Without the surface relaxation, KEp of desorbed Al+ should be EK. Contrary to this, in the presence of surface relaxation KEp should be biased as much as ER, an additional energy due to surface relaxation, which explains the high KE values. If the two-photon process is dominant, ER corresponds to 1.32 eV. It will be dissipated by an exciton formation18 or by the ionization process based on Ei - φ as follows.

KEp ) 4.66 × 2 - {Ei - φ}

(4)

It has been suggested that dealumination will start on less acidic Brønsted associated Al species from faujasite like zeolite.10-13 An advancement of 27Al magic angle spinning NMR can distinguish tetrahedral framework Al and octahedral nonframework Al species,38 which clearly proves dealumination during steaming treatment14 and the process to remove NH3 from NH4form zeolite to obtain H-form zeolite.39 Therefore, it may be reasonable to imagine that there are nonframework Al species in our H-Y zeolites and the pulsed laser desorption selectively occurs on them. We reject this consideration for the following three reasons: (1) As mentioned before, Al species are desorbed by 266 nm laser light as much as 37%. (2) From thermodynamic data it can be assumed that Al species are less probable to the pulsed laser desorption than Si species. The heat of formation ∆Hf° of Al2O3 (∼-1.6 × 103 kJ mol-1)40,41 is quite large and exothermic compared to that of SiO2 (∼-2.8 × 102

J. Phys. Chem., Vol. 100, No. 33, 1996 14079 kJ mol-1)42-45 and the standard free energy formation, ∆Gf° of Al2O3 (∼-1.6 × 103 kJ mol-1) is also quite large compared to that of SiO2 (∼-8.0 × 102 kJ mol-1).46 Therefore, the energy to obtain Al species from Al2O3 should be larger, if it were formed. (3) Framework Al species will be less stable than framework Si species probably because of the less stable tetrahedral oxide unit. It will be reasonable to suppose that such less stable framework Al species are pulsed laser desorbed from the surface and can be supplied from the bulk zeolite through migration.9 Al species during migration may be associated with tetrahedral nonframework species detected by 27Al MAS NMR.14 The origins of Brønsted acid sites in H-type zeolite are two types of bridged OH species between framework Si and framework Al species in different cages, that is, in supercage (infrared νOH ) ∼3650 cm-1) and in β-cage (sodalite cage; νOH ) ∼3550 cm-1).47-49 The structure is as follows. H O Si

Al

The formation of OH species can be expected when the angle of Si-O-Al is smallest.13,50,51 If the angle is increased, the ionicity of OH (acidity) decreases and νOH should decrease. Therefore, OH species in β-cage are less stable due to lower polarity of the bond and are responsible for the homolytic dissociation to desorb H2.50 Pulsed laser desorption of H2 should occur facilely from OH species in β-cage. This expectation is reflected by the fact that dealumination takes place on less acidic sites.10-13 The Vcm value of desorbed H2 is larger than that of desorbed Al species. This fact will imply that the stream rate of H2 flux is increased by collision with Al species as much as their atomic ratio because H2 molecule is lighter than Al atom. KEp of desorbed H2 is too small compared to that of Al species. This fact will infer that pulsed laser H2 desorption process is not primarily a photochemical electronic excitation process but is probably involved in the ionic Al desorption process. After the desorption of Al species, the zeolite framework should experience intense stress and distortion. These structural deformations will induce H2 desorption and will be finally settled to a stable structure. Two cases can be imagined for the pulsed laser desorption of H2O from H-Y zeolite (99%). One case is that H2O is formed from OH groups both in the super cages and the β-cages. In this case, as number of H sites increases, the distance between two adjacent OH groups becomes shorter, then the possibility forming H2O from them increases. The other case is that H2O are formed distinctively from the super cage. This case is reflected by that Na species in Na-Y zeolite are replaced by H species in the order, first from β-cage and then from supercage. Finally proposed is the model to interpret pulsed laser desorption of Al+ and H2 as shown in Scheme 1. In this model, the total positive charge decreases from 20 to 12. The number of oxygen atoms is 16 to be constant. Pulsed laser desorption of Al+ and H2 causes the change of bonded oxygen from bridged to terminal one. Since the terminal oxygens formed in the second oxygen layer should cause distortion both of lattice structure and the electronic charge, they will further bring above successive desorption of Al species. References and Notes (1) Introduction to zeolite science and practice; van Bekken, H., Flanigen, E. M., Jansen, J. C., Eds.; Studies in Surface Science and Catalysis Vol. 58; Elsevier: New York, 1991. (2) Corcoran, E. Sci. Am. 1990, Nov., 74.

14080 J. Phys. Chem., Vol. 100, No. 33, 1996 (3) Supramolecular Architecture; Bein, T., Ed.; ACS Symposium Series 499; American Chemical Society: Washington, DC, 1992. (4) Herman, I. P. Chem. ReV. 1989, 89, 1323. (5) Rithschild, M.; Ehrlich, D. J. J. Vac. Sci. Technol. 1988, B6, 1. (6) Kasai, P. H. J. Phys. Chem. 1965, 43, 3322. (7) Harrison, M. R.; Edwards, P. P.; Klinowski, J.; Thomas, J. M. J. Solid State Chem. 1984, 54, 330. (8) Sun, T.; Seff, K.; Heo, N.; Petranovskii, V. P. Science 1993, 259, 495 and references therein. (9) Ishigoh, K.; Tanaka, K.; Zhuang, Q.; Nakata, R. J. Phys. Chem. 1995, 99, 12231. (10) Beaumont, R.; Barthomeuf, D. J. Catal. 1972, 26, 218. (11) Beaumont, R.; Barthomeuf, D. J. Catal. 1972, 27, 45. (12) Barthomeuf, D.; Beaumont, R. J. Catal. 1973, 30, 45. (13) Goursot, A.; Fajala, F.; Daul, C.; Weber, J. J. Phys. Chem. 1988, 92, 4456. (14) Samoson, A.; Lippmaa, E.; Engelhardt, G.; Lohhse, U.; Jerschkewitz, H.-G. Chem. Phys. Lett. 1987, 134, 589. (15) Bodart, P.; Nagy, J. B.; Debres, G.; Jacobs, P. A. J. Phys. Chem. 1986, 90, 5183. (16) Pelmenshilov, A. G.; Paukshris, E. A.; Edisherashvili, M. O.; Zhidomirov. J. Phys. Chem. 1992, 96, 7051. (17) Dreyfus, R. W.; Kelly, R.; Walkup, R. E. Appl. Phys. Lett. 1978, 49, 1478. (18) Schildbach, M. A.; Hamza, A. V. Phys. ReV. B 1992, 45, 6197. (19) Kim, H. S.; Helvajian, H. J. Phys. Chem. 1991, 95, 6623. (20) Meier, W. M.; Olson, D. H. Atlas of Zeolite Structure Types; 3rd ed. Butterworth-Heinemann: New York, 1992. (21) Zhuang, Q.; Tanaka, K.; Nakata, R. Jpn. J. Appl. Phys., 1995, 34, L1089. (22) Kelly, R. J. Chem. Phys. 1990, 92, 5047. (23) Cheung, J. T.; Sankur, H. CRC Crit. ReV. Solid State Mater. Sci. 1988, 15, 63. (24) (a) Tanaka, K.; Miyajima, T.; Shirai, N.; Zhuang, Q.; Nakata, R. J. Appl. Phys. 1995, 77, 6581. (b) Zhuang, Q.; Ishigoh, K.; Tanaka, K.; Kawano, K.; Nakata, R. Jpn. J. Appl. Phys. 1995, 34, L248. (25) Eschbach, P. A.; Dickinson, J. T.; Langford, S. C.; Pederson, L. R. J. Vac. Sci. Technol. 1989, A7, 2943. (26) Wiedeman, L.; Helvajian, H. 1991, 70, 4513.

Tanaka et al. (27) Allen, F. J. J. Appl. Phys. 1972, 43, 2169. (28) Otis, E.; Goodwin, P. H. J. Appl. Phys. 1993, 73, 1957. (29) Menzel, D.; Gomer, R. J. Chem. Phys. 1964, 41, 3329. (30) Redhead, P. A. Can. J. Phys. 1964, 42, 886. (31) (a) Knotek, M. L.; Feibelman, P. J. Phys. ReV. Lett. 1978, 40, 964. (b) Feibelman, P. J.; Knotek, M. L. Phys. ReV. B 1978, 18, 6531. (32) Helvajian, H.; Welle, R. J. Chem. Phys. 1989, 91, 2616. (33) Yu, M. L. In Sputtering by Particle Bombardment III; Topics in Applied Physics, Vol. 64; Behrisch, R., Wittmaack, K., Eds.; SpringerVerlag: Berlin, 1991; p 97. (34) (a) Hoheisel, W.; Jungmann, K.; Vollmer, M.; Weidenauer, R.; Trager, F. Phys. ReV. Lett. 1988, 60, 1649. (b) Hoheisel, W.; Schulte, U.; Vollmer, M.; Weidenauer, R.; Trager, T. Appl. Surf. Sci. 1989, 36, 664. (35) Thomas, G. E.; de Kluizenaar, E. E. Nucl. Instrum. Methods 1976, 132, 449. (36) Tsong, I. S. T.; Yusuf, N. A. Appl. Phys. Lett. 1978, 33, 999. (37) Dreyfus, R. W.; Kelly, R.; Walkup, R. E. Appl. Phys. Lett. 1978, 49, 1478. (38) Klinowski, J. Annu. ReV. Mater. Sci. 1988, 18, 189. (39) Yan, Y.; Vansant, E. F. J. Phys. Chem. 1995, 99, 14089. (40) Moeller, T. Inorganic Chemistry, an adVanced textbook, 2nd ed.; John Wiley: New York, 1958. (41) ComprehensiVe Inorg. Chem. 1995, 41, 1034. (42) Darling, C. L.; Schlegel, H. B. J. Phys. Chem. 1993, 97, 8207. (43) Hindenbrand, D. L.; Lau, K. H. J. Phys. Chem. 1994, 101, 6076. (44) Zachariah, M. R.; Tsang, W. J. Phys. Chem. 1995, 99, 5308. (45) Allendorf, M. D.; Melius, C. F.; Ho, P.; Zachariah. J. Phys. Chem. 1995, 99, 15285. (46) Kim, K. S.; Baitinger, W. E.; Amy, J. M.; Winograd, N. J. Electron. Spectrosc. Relat. Phenom. 1974, 5, 351. (47) White, J. L.; Jelli, A. N.; Andre, J. M.; Fripiat, J. J. Trans. Faraday, Soc. 1967, 63, 461. (48) Olson, D. H.; Dempsey, E. J. Catal. 1969, 13, 221. (49) Mortier, W. J.; Jacobs, P. A. Zeolites 1982, 2, 226. (50) O’Malley, P. J.; Dwyer, J. J. Phys. Chem. 1988, 92, 3005. (51) Czjzek, N.; Jobic, H.; Fitch, A. N.; Vogt, T. J. Phys. Chem. 1992, 96, 1535.

JP960199V