Laser-induced desorption of polyatomic molecules with a CO2 laser

380

Applied Surface Science 29 (1987) 380-390 North-Holland, Amsterdam

LETTER LASER-INDUCED DESORPTION OF POLYATOMIC MOLECULES WITH A CO2 LASER * E.G. SEEBAUER * *, A.C.F. KONG and L.D. SCHMIDT Department of Chemical Engineering and Materials MN 55455, USA

Science, University of Minnesota,

Minneapolis,

Received 13 April 1987; accepted for publication 3 July 1987

Laser-induced thermal desorption (LITD) has been increasingly employed as a tool for investigating surface processes. In LITD, a pulsed laser beam that is focused onto a surface induces a rapid temperature rise that causes desorption. In spite of the success enjoyed by the CO, laser in studies of diatomic molecules, its use with polyatomic molecules is shown to be severely limited by laser-induced dissociation. In desorption experiments with CHsOH, HCOOH, CH,NH2 and NH, dissociation occurs only when the laser frequency coincides with an infrared absorption band of the molecule. Fragmentation may take place either on the surface or in the dense gas phase present above the surface during the laser pulse.

Laser-induced thermal desorption (LITD) has been shown recently to be an effective local probe of surface coverage on metal surfaces. This method employs a pulsed laser beam which is focused on an adsorbate-covered surface in ultrahigh vacuum. The beam produces very rapid (lo* to 10” K/s) and localized ( - 10 pm step) heating of the surface which induces thermal desorption. LITD has been used as a probe of surface adsorption properties and kinetics by Cowin et al. for deuterium on W [l], and more recently by Hall et al. in their extensive studies of methanol decomposition on Ni [2,3]. The adsorption and desorption kinetics, thermodynamics [4], and surface diffusion [5-91 of adsorbed species have also been investigated. There has been some question about how the extremely high laser power densities at the surface (up to 150 MW/cm’ in some cases) might affect the surface and the desorbing species. Although surface damage has been observed [6,10] and partially characterized [4,11] in some systems, laser-induced chemistry during the desorption is relatively unexplored. Aside from studies in which the power densities were so high that surface vaporization and plasma formation were observed, we know of only one chemisorption system in which * This research partially supported by NSF under Grant No. DMR82126729. ** Present address: Department of Chemical Engineering, University of Illinois, Urbana, IL 61801, USA.

0169-4332/87/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

E.G. Seebauer et al. / Laser-induceddesorptionofpo&atomicmolecules

381

such chemistry has been demonstrated [12]. Several physisorption systems have also been examined. However, most LITD investigations have involved diatomic molecules such as NO, CO and H, which are either very strongly bound or, in the case of homonuclear molecules, can form no stable products. Since LITD has demonstrated its capability for selecting surface chemical pathways that are not normally observed [2,3], we expect that LITD will be used increasingly for studies of the surface chemistry of polyatomic molecules that are much more likely to undergo laser-induced dissociation. We have examined with a CO, laser several such adsorption systems on Pt(lll), including CH,OH, HCOOH, CH,NH, and NH,. The CO, laser was chosen because the relatively long pulse width makes surface damage less of a problem than with other lasers [4] and because of the fairly flat beam profile. Our goal was to explore the surface chemistry but at the same time watch for evidence of unwanted laser-induced processes. We have found that for multilayers of all these adsorbates such processes are unavoidable, while in the submonolayer region only NH, may be studied with confidence. Resonance of a vibrational mode of the adsorbed or gaseous species with the laser frequency appears to be the critical factor. The apparatus has been described previously [4,7]. Briefly, it consisted of a stainless steel six-way cross (1.5 inch OD tubing) in which the sample was suspended. The system was pumped by a 170 e/s turbomolecular pump and a small titanium sublimation pump, giving a base pressure of 4 X lo-” Torr. The partial and total pressures of gases in the chamber were monitored with a quadrupole mass spectrometer and an ionization gauge. Adsorption was accomplished by background gas dosing. The sample was a 99.999% pure Pt single crystal disk 5 mm in diameter and 0.5 mm thick, oriented to within 0.25” of the (111) direction and polished by standard metallographic techniques. The sample could be heated resistively through its Ta support wires or by electron bombardment and could be cooled to 80 K. After cleaning, TPD spectra of Hz, CO, and NO were identical to those reported for Pt(ll1). Light from the 9.6 pm P(20) line of a Lumonics TEA CO1 laser (130 ns FWHM pulse, A = 9.552 pm, Y= 1046.85 cm-‘) was apertured and focused onto the crystal through a CaF, plano-convex lens (12 cm focal length) and a coated ZnSe plate that served as the vacuum seal into the system. No experiments were performed with the laser output at 10.6 pm because of strong absorption by the lens. The sample was oriented at a 45” angle with respect to the incoming beam so that outgoing light was reflected out at 90” through a CaF, window into a beam stop. The spatial profile of the beam in multimode operation was essentially flat over the apertured area. The beam spot on the sample was typically a 1.2 mm x 0.4 mm rectangle. Signals were recorded and stored using a digital multichannel analyzer. Since the experi-

382

E.G. Seebauer et al. / Laser-induced desorption ofpolyatomic

molecules

mental arrangement did not permit direct line of sight from the sample to the mass spectrometer, the transient background pressure rise in the system after a desorption pulse was used as a measure of surface coverage. Hence, only stable, neutral molecules could be detected with this setup. CH,OH (HPLC grade), NH, (99.99%), and CH,NH, (98.0%) were used without further purification. However, the HCOOH that was available contained about 5% H,O, which was removed by repeated fractional recrystallization until a melting point of 8.4” C was attained. Dosing of HCOOH was performed with a pressure in the gas manifold of about 100 mTorr and T = 300 K in order to ensure that the gas entering the vacuum system was the monomer and not the dimer, which apparently has different surface chemistry [16]. Under the above conditions the ratio of monomer to dimer can be calculated to be about 30 [17]. Signal versus exposure curves were obtained when possible by both TPD and LITD at coverages 0 ranging several times above and below one monolayer. The sample was first dosed as desired at 100 K, and a single LITD signal was obtained. Since the desorption area was very small compared with the total sample area, a TPD spectrum could be taken immediately thereafter without another dose. Signal versus exposure curves for CH,OH, HCOOH, and CH,NH, are displayed in figs. 1, 2, and 3, respectively. For LITD the signal represents the desorption peak height corrected for pumping speeds and cracking patterns, while for TPD the signal is the integrated desorption area similarly corrected. Sample TPD spectra near 1 L exposure are inset. Fig. 4 shows LITD signal versus exposure for NH,; reliable TPD data could not be obtained with background dosing because of desorption interference from the electrical leads supporting the sample. For CH,OH the TPD spectra were similar to those obtained by Sexton [18]. As can be seen from fig. 1 the TPD signals for the decomposition products CO and H, saturate near an exposure of 2 L, where multilayers begin to form. Larger exposures simply result in the desorption of the CH,OH ice. On the other hand, the LITD signals for CO and H, continue to rise even in the multilayer regime. Furthermore, the CH,OH is smaller than in TPD by a factor of nearly 250 compared to CO and H,. In fig. 2 a similar pattern for HCOOH is observed. The TPD signals for the decomposition products CO, COz, H,, and H,O rise to saturation values while the LITD signals continue upward in the multilayer region. It should be noted that large amounts of the dehydration products CO and H,O are ratio was less reliable observed in TPD (CO/CO, = 2/l). The H,O/H, because of daily variations in pumping speed for H,O. The CO TPD peak shape was identical to that of CO adsorbed alone. This result contradicts that of Avery [19], who claimed that all the CO desorbing from Pt(ll1) during

E.G. Seebauer et al. / Laser-induced

desorption of polyatomic molecules

383

8-

Exposure

IL)

Fig. 1. TPD and LITD signals as a function of gas exposure for CH,OH.

HCOOH TPD may be accounted for by desorption from the sample leads. We have no explanation for the discrepancy, but we note that the dehydration is a major decomposition pathway on other surfaces such as Ru(OO1) [20,21], Ni(ll1) [16], Ni(ll0) [22,23], and Ni(100) [24]. In LITD the distribution of desorbing species is markedly different. The relative HCOOH signal is down by a factor of 10’ from its value in TPD, while the CO signal rises by a factor of 50. The results for CH,NH, (fig. 3) are not as dramatic but follow a somewhat similar pattern. The main TPD products from adsorption at 100 K are H, (380 K), HCN (380, 500 K) and CH,NH, (140, 220 K). Very small amounts of C,N, (700, 1070 K) can also be detected, but if the sample is annealed to 350 K, the C,N, signal becomes comparable to that of HCN. A more detailed study of CH3NH, adsorption on Pt(ll1) will be published shortly [25]. The H, and HCN signals saturate near 1.5 L, while CH,NH, continues to adsorb

384

E.G. Seebauer et al. / her-induced

desorption of polyatomic molecules

6

HCOOI

I

I

I

I,

2

3

4

5

6

Exposure Fig. 2. TPD and LITD signals as a function

I

I

,

I

,

7

8

9

10

11

'

(L) of gas exposure

for HCOOH.

into multilayers. On the other hand, in LITD the H, and HCN signals continue up. No C,N, desorption was seen, although this is not unexpected because C,N, desorbs at a fairly high temperature even in TPD. As discussed previously [2,4], the temperature of maximum desorption rate T, rises as the heating rate rises. Hence at the heating rates of 10” K/s characteristic of LITD, T, for the 1070 K state increases to - 2700 K (above the melting point of Pt) if first order kinetics and a pre-exponential factor of 1013 s-l are assumed. If the desorption is second order as seems likely, T, rises even further, At the laser power levels used in these experiments (- 100 MW/cm2), such high temperatures are not attained. If the power levels are increased, severe surface damage results, as discussed in ref. [4]. When desorption is induced from a very smooth spot on the sample, the CH,NH, signal as shown in fig. 3 generally remains comparable to those of H, and HCN, in contrast to the cases of CH,OH and HCOOH. However, if a

E.G. Seebauer et al. / Laser-induced

desorption of polyatomic

molecules

385

HCN

a6-

I

1

I

1

2

3

1

I

I

1

4

5

6

7

Exposure

I

a

(L)

Fig. 3. TPD and LITD signals as a function of gas exposure for CH,NH,.

small scratch or rough spot is present in the laser irradiated area, a plasma (small spark) can be observed on the surface, and the H, and HCN signals rise enormously. The onset of plasma formation on such a spot is very sharp, occurring at just above 1 L exposure and a surface temperature at or below 141-143 K. LITD results for NH, are displayed in fig. 4. TPD of NH, from Pt(ll1) results in desorption of NH, alone; decomposition into H, and N, is not observed [26-281. Neither is this decomposition observed in LITD for exposures less than 1 L. However, for larger exposures a large amount of molecular N, is desorbed, although the H, signal does not increase significantly. Exposures as large as 70 L were employed to produce a significant amount of multilayer adsorption, since solid NH, desorbs between 95 and 110 K [27], while the adsorption temperature was 100 K. For comparison we also

386

E.G. Seebauer et al. / Laser-induced desorption ofpolyatomic

.

NH3 .

. N2

5-

//

I -I’

I

n

801 “r

=

H2

100 O.’

’

lo - NO

4-

Exposure Fig. 4. LITD signals as a function

show in fig. 4 LITD exposures.

molecules

of gas exposure

I

I

3

4

(Ll for NH,

(top) and NO (bottom).

results for NO [4]; only the parent mass is seen at all

Clearly a considerable difference exists for these polyatomic molecules between TPD and LITD with a COz laser. In all cases decomposition products are observed with LITD even in multilayer adsorption where surface catalysis should no longer be important. In many cases the decompositions appeared nonstoichiometric, but because the mass spectrometer did not have direct line of sight to the sample, we could not detect ionic or radical species. Several investigators have observed that CO, laser-induced dissociation correlates strongly with the linear infrared absorption spectrum [13-15,291. Hence, in table 1 are presented vibrational frequencies within 100 cm-’ of the laser frequency for all the adsorbates in the gas and solid phases, and where possible on a Pt(ll1) surface. CH,OH, HCOOH and CH,NH, all have gas phase absorption peaks within 15 cm -i of the laser frequency 1047 cm-‘. For

E.G. Seebauer et al. / Laser-induced

Table 1 Vibrational frequencies (cm-‘) frequency (1047 cm-‘) Molecule

vaas

HCOOH

1105 rs [36]

&sorption

of polyatomic molecules

of the fundamental bands within 100 cm-’

1033 Ys

vsolid

970 v(C0) [31] wH) ]321 r(OH) P91

387

of the CO, laser

“surfacs

Chemisorbed species is a for-mate that has no absorption bands near 1047 [19]

1070 n(CH) [19] CH,OH

CH,NH,

1008 1039 v(C0) [30] 1058

1130 CH, rock [38] 1044 CN stretch

1035 v(C0) [37]

970 v(C0) (155 K) [18] 1060 v(C0) (300 K)

1182 CH, rock [39] 1048 CN stretch

1220 1195 1185 1190 1030

Ni(100) Ni(lll)CHs Cr(100) Cr(ll1) Ni(100)

lo10 Ni(lll)cN

rock 1401

stretch

1000 Cr(100) 1020 cr(lllj

NH,

968 Y* [36] 932 (inversion components)

1060 S,(HNH) [41]

1140 G,(HNH) (0 i 0.4) [26] 1190 &(HNH) (0 > 0.4)

CH,OH and HCOOH, these near-resonances are due to a C-O stretch, while for CH,NH, it results from the C-N stretch. Close examination of the published spectra reveals that the absorption peaks are fairly broad so that absorption at 1047 cm-’ is strong. Although NH, has significant rotational fine structure from 800 to 1200 cm- ‘, there is no strong resonance at 1047 cm-’ [30]. The situation is similar for solid phase CH,OH, CH,NH, and NH,. For solid HCOOH the intense band at 970 cm-’ observed with electron energy loss spectroscopy (EELS) again appears quite broad but has been variously assigned to C-O and O-H vibrations [19,31,32]. The C-H band at 1070 cm-’ is much less intense, appearing almost as a shoulder on the 970 cm-’ band [19]. Of the chemisorbed adsorbates only the C-O of CH,OH and possibly the C-N of CH,NH, have vibrations near 1047 cm-‘. Avouris et al. have studied infrared laser multiphoton ionization (MPI) of several gas phase molecules [15], including CH,NO,, CH,OH, CH,CHCN, C2H4, C,H,O, ethyl-vinyl-ether, cyclopropane and NH,. All but the last two yielded ionic products, although, the identities of neither the ions nor the neutral fragments were determined. CH,OH was among the molecules most susceptible to MPI, but the ionic yield depended strongly on laser pulse energy. The chief mechanism for ion production was proposed to be chemi-

388

E.G. Seebauer et al. / Laser-induced desorption of polyatomic

molecules

ionization, in which a previously generated free radical (in the ground or an electronically excited state) reacts with another radical or stable molecule. The reaction is often exothermic enough to induce ionization of one of the products. Resonant absorption by NH, does indeed yield the necessary NH, radicals [29,33]. Related experiments were carried out by Mashni and Hess for thick layers of CH,OH adsorbed on an unspecified metal sample holder [14]. Again, ionization and fragmentation were possible only when the laser frequency corresponded’to a vibrational absorption band, in this case the C-O stretch. The ions were mass analyzed, and numerous decomposition and addition products were observed. Whether the species leaving the surface were neutral or ionic could not be determined, but collisions and photon absorption in the gas phase appeared to be critical. Similar results have been obtained by Hussla and Chuang for multilayers of NH, adsorbed on Cu(100) [13]. Frequency dependent photoionization into NH: and fragmentation into neutral NH,, NH, N and N, were observed together with molecular desorption. Apparently neither H nor H, were detected, which is consistent with our results. The active vibration of NH, was the 8, symmetric bending mode on the surface at 1056 cm-‘, but the fragmentation and ionization mechanism was not clear from the experiments. Taken together, the above results indicate that laser-induced chemistry may be induced when a resonance exists between the laser frequency and a vibrational mode of the molecule. Fragmentation and ionization may occur once the molecule has entered the gas phase, but pressures of several Torr may be required. However, such particle densities are easily attained in LITD even at submonolayer coverages because most desorption occurs during the laser pulse [34]. Hence, we cannot determine from our data for CH,OH and CH,NH, whether the fragmentation proceeds mainly in the gas phase or on the surface. Vibrational resonances exist in both places for submonolayer and multilayer coverages. For HCOOH there are also resonances for the molecular species. However, at 8 < 1 only a formate having no resonance exists on the surface [19]. We cannot determine whether the laser beam interacts strongly with this species because it may desorb under the influence of the laser as HCOOH which then dissociates in the gas phase. On the other hand, both our data and that of Hussla and Chuang [13] indicate that NH, dissociation must occur at the surface because no strong vibrational resonance exists in the gas phase. Furthermore, we observe no dissociation of NH, at submonolayer coverage, where again there is no resonance. The threshold for dissociation as 8 increases is quite sharp and corresponds almost exactly to the onset of multilayer formation. These observations lead us to believe that the dissociation does not result simply from the increase in surface density of adsorbed molecules but from a change in the nature of their vibrational character.

E.G. Seebauer et al. / Laser-induced desorption of polyatomic

molecules

389

Dissociation at the surface is actually somewhat surprising because V-V coupling to the surface and to other adsorbed molecules should dissipate energy deposited in the S, mode very quickly. This appears to be the case for several vinyl monomers, which did not appear to polymerize or decompose substantially when irradiated with a CO2 laser even though extensive decomposition occurred in the gas phase [35]. However, we note that because of the high reflectivity of Pt, the maximum power densities used in our experiments were loo-150 MW/cm2, which is at least an order of magnitude more than the power used by Mashni and Hess [14], Hussla and Chuang [13], and apparently by Gardini [35]. In conclusion, CO, lasers do not appear to be useful for LITD studies of polyatomic molecules because of dissociation caused by vibrational resonances and the high incident powers required. Similar experiments are now underway with a Nd-YAG laser (X = 1.06 pm, reflectivity = 81%) to avoid this problem. We gratefully acknowledge Professor R.W. Carr for the use of his CO, laser.

References [l] [2] [3] [4] [5] [6] [7] [8] [9] [lo] [ll] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]

J.P. Cowin, D.J. Auerbach, C. Becker and L. Wharton, Surface Sci. 78 (1978) 545. R.B. HaB and A.M. DeSantolo, Surface Sci. 137 (1984) 421. R.B. Hall, A.M. DeSantolo and S.J. Bares, Surface Sci. 161 (1985) L533. E.G. Seebauer, A.C.F. Kong and L.D. Schmidt, Surface Sci. 176 (1986) 134. R. Viswanathan, D.R. Burgess, P.C. Stair and E. Weitz, J. Vacuum Sci. Technol. 20 (1982) 605. S.M. George, A.M. DeSantolo and R.B. HaB, Surface Sci. 159 (1985) L425. E.G. Seebauer and L.D. Schmidt, Chem. Phys. Letters 123 (1986) 129. D.R. Mulhns, B. Roop and J.M. White, Chem. Phys. Letters, submitted. C.H. Mak, J.L. Brand, A.A. Deckert and S.M. George, J. Chem. Phys., in press. M.G. Sherman, J.R. Kingsley, D.A. DahIgren, J.C. Hemminger and R.T. McIver, Jr., Surface Sci. 149 (1985) L25. I. Hussla and R. Viswanathan, Surface Sci. 145 (1984) L488. M.G. Sherman, J.R. Kingsley, J.C. Hemminger and R.T. McIver, Jr., Anal. Chim. Acta 178 (1983) 79. I. Hussla and T.J. Chuang, Ber. Bunsenges. Phys. Chem. 89 (1985) 294. M. Mashni and P. Hess, Chem. Phys. Letters 77 (1981) 541. P. Avouris, I.Y. Chart and M.M.T. Loy, J. Chem. Phys. 72 (1980) 3522. J.B. Benziger and G.R. Schoofs, J. Phys. Chem. 88 (1984) 4439. AS. Coolidge, J. Am. Chem. Sot. 50 (1928) 2166. B.A. Sexton, Surface Sci. 102 (1981) 271. N.R. Avery, Appl. Surface Sci. 11/12 (1982) 774. L.A. Larson and J.T. Dickenson, Surface Sci. 84 (1979) 17. N.R. Avery, B.H. Toby, A.B. Anton and W.H. Weinberg, Surface Sci. 122 (1982) L574. J.L. Falconer and R.J. Madix, Surface Sci. 46 (1974) 473. J. McCarty, J. Falconer and R.J. Madix, J. Catalysis 30 (1973) 235. l

390 [24] [25] [26] 1271 [28] [29] [30] [31] [32] [33] [34] [35] [36]

E.G. Seebauer et al. / her-induced

desorption of polyatomic

molecules

J.B. Benxiger and R.J. Madix, Surface Sci. 79 (1979) 394. S.Y. Hwang and L.D. Schmidt, to be published. B.A. Sexton and G.E. Mitchell, Surface Sci. 99 (1980) 523. G.B. Fisher, Chem. Phys. Letters 79 (1981) 452. J.L. Gland and E.B. Kollin, J. Vacuum Sci. Technol. 18 (1981) 604. J.D. Campbell, G. Hancock, J.B. Halpem and K.H. Welge, Opt. Commun. 17 (1976) 38. Sadtler Standard Spectra (Sadtler, Philadelphia, 1962). B.A. Sexton, Surface Sci. 88 (1979) 319. J.L. Gland, B.A. Sexton and G.B. Fisher, Surface Sci. 95 (1980) 587. P. Avouris, M.M.T. Lay and I.Y. Chan, Chem. Phys. Letters 63 (1979) 624. D. Burgess, Jr., P.C. Stair and E. Weita, submitted. A. Gardini, Can. J. Chem. 55 (1977) 4045. G. Herxberg, Molecular Spectra and Molecular Structure, Vol. 3, Electronic Structure of Polyatomic Molecules (Van Nostrand-Reinhold, New York, 1966). [37] H.A. Szymanski, Interpreted Infrared Spectra, Vol. 2 (Plenum, New York, 1966). [38] H. Wolff and L. Ludwig, J. Chem. Phys. 56 (1972) 5278; A.P. Gray and R.C. Lord, J. Chem. Phys. 26 (1957) 690; K. Tamagaka, M. Tsuboi and A.Y. Hirakawa, J. Chem. Phys. 48 (1968) 5536. [39] J.R. Durig, S.F. Bush and F.G. Baglin, J. Chem. Phys. 49 (1968) 2106. ‘1401 A.G. Baca, M.A. Schulz and D.A. Shirley, J. Chem. Phys. 83 (1985) 6001. [41] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds (Wiley, New York, 1978).