High-resolution electron energy loss spectroscopy and thermal

4318

J. Phys. Chem. 1987, 91, 43 18-4323

High-Resolution Electron Energy Loss Spectroscopy and Thermal Programmed Desorption Studies of the Chemisorption and Thermal Decomposition of Ethylene and Acetylene on Ni( 100) Single-Crystal Surfaces F. Zaera* Department of Chemistry, University of California, Riverside, Riverside, California 92521

and R. B. Hall Corporate Research Science Laboratories, Exxon Research and Engineering Company, Annandale, New Jersey 08801 (Received: January 12, 1987; In Final Form: March 13, 1987)

The chemisorption and thermal decomposition of acetylene and ethylene on Ni( 100) surfaces were studied by using thermal programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). Acetylene chemisorbs molecularly at 90 K forming a rehybridized (sp3) acetylenic moiety on the surface. This fragment is stable u p to 270 K, where dehydrogenation takes place to form CCH residues. Heating further, above 400 K, leaves only carbon on the nickel surface. Ethylene also adsorbs molecularly but with little rehybridization, H bonding to the metal. Stepwise dehydrogenation takes place as the crystal temperature is increased. Vinyl is formed as a product of a unimolecular reaction at 170 K for low coverages of ethylene (e I0.4), but this process is inhibited at higher coverages and only takes place around 200 K. A strong isotope effect was also observed, and C,D, decomposition only occurred at temperatures about 40 K higher. Vinyl decomposes further to form an acetylenic moiety at about 230 K. Further heating is followed by stepwise dehydrogenation similar to that observed when acetylene is adsorbed. No H-D scrambling is observed during the thermal treatment for partially or fully deuteriated molecules.

Introduction Detailed understanding of chemisorption of olefins on metal surfaces is essential for elucidating the mechanism of hydrogenation reactions. Extensive work has been done on the adsorption and decomposition of unsaturated hydrocarbons, especially of ethylene and acetylene, on single-crystal surfaces and under ultra-high-vacuum. The characterization of stable intermediates has been recently improved by the use of vibrational spectroscopies. For instance, ethylidyne formation at room temperature is now known to occur after ethylene chemisorption on Pt( 11l ) , Pt( IOO), Pd( 11 l ) , Rh(l1 I ) , or Ru( loo).' On Ni( 11 1) ethylene dehydrogenates to form acetylene above 230 K,* while on Ni( 110) C C H intermediates have been detected above 200 K.3 We have recently reported the formation of vinyl moieties at 170 K on Ni(100).435 Acetylene has been reported to rehybridize to a sp3 configuration on both Ni( 11 1) and Ni(1 In the present paper we extend our previous work to the study of chemisorption and decomposition of ethylene and acetylene on Ni( 100) as a function of temperature at several initial coverages. We find that the simple unimolecular reaction of chemisorbed ethylene to form a vinyl fragment at low coverage and 170 K gets complicated at higher coverages, where competition with molecular desorption takes place up to 230 K. Further heating of this surface results in the formation of an acetylenic fragment around 230 K, followed by CCH, and finally carbon on the surface above 400 K. Acetylene rehybridizes upon adsorption on Ni(100), as it does on the other nickel surfaces, and dehydrogenates stepwise as in the case of ethylene above 270 K. Experimental Section TPD and HREELS experiments were performed in a stainless steel ultra-high-vacuum chamber as described in detail e l s e ~ h e r e . ~ Briefly, the HREELS spectrometer consists of two fixed 127' cylindrical electrostatic deflectors in a double "c" configuration. (1) Kesmodel, L. L.; Dubois, L. H.; Somorjai, G. A. J . G e m . Phys. 1979, 70,2180. (2) Lehwald, S.; Ibach, H. Surf. Sci. 1979, 89, 425. (3) Stroscio, J. A.; Bare, S.R.; Ho, W. Surf. Sci. 1984, 148, 499. (4) Hall, R. B.; Bares, S.J.; DeSantolo, A. M.; Zaera, F. J . Vac. Sci. Technol. 1986, A 4 , 1493. ( 5 ) Zaera, F.; Hall, R. B. Surf: Sei. 1987, 180, L123.

0022-3654/87/2091-4318$01.50/0

Vibrational spectra were taken at 90 K after heating to the selected temperature for 100 s. Coadsorbed C O is seen in most vibrational spectra due to the large data acquisition times. This CO represents only less than 5% of the total coverage, and is believed not to alter the essential features of the ethylene surface chemistry. It has recently been reported that C O coadsorption helps ordering on the surface without changes in the molecular structure of the coadsorbed moiety.31 The chamber was also equipped with a shielded mass spectrometer for TPD, arranged so it detected desorption only from the front of the crystal face. In addition, exposures were done using dosers with capillary arrays facing the front surface. The nickel crystal was cut and polished in the (100) orientation by using standard procedures, and mounted in a manipulator so it could be cooled to liquid nitrogen temperature and resistively heated above 1500 K. It was cleaned by cycles of oxygen treatment, argon ion sputtering, and annealing until no impurities were detected by using auger electron or X-ray photoelectron spectroscopies. Finally, all gases were of the highest purity commercially available (less than 0.1% impurities, 98% isotopic purity for the deuteriated ethylenes). They were used as supplied, except for the use of an acetone trap for the acetylene.

Results Ethylene thermal desorption spectra from the Ni( 100) surface were obtained as a function of exposure. Only hydrogen and molecular ethylene were detected. TPD traces at 2 and 26 amu recorded after normal ethylene adsorption are shown in Figure 1. No ethylene desorption is observed below 1.3-langmuir exposure. A peak at 250 K appears in the 26 amu TPD trace above 1.3 langmuirs that grows and shifts to 200 K with increasing coverages. H2 desorption peaks at 380 K at low coverages, and shifts to lower temperature with increasing ethylene dose. A high-temperature shoulder starts growing above 1.O langmuir. TPD spectra from chemisorbed 1,l- and 1,2-C,DzHz were also obtained. The results from both compounds were similar; they are shown in Figure 2. It can be seen that the high-temperature shoulder at 400 K after saturation is mostly due to deuterium desorption. Vibrational spectra of ethylene at different coverages were obtained after heating to selected temperatures for the four isotopic isomers used in these studies, namely CzH4,1,1-C2H,D,, 1,2C2H2D2,and C2D4. An example of this annealing sequence is 0 1987 American Chemical Society

The Journal of Physical Chemistry, Vol. 91, No. 16, 1987 4319

Decomposition of Ethylene and Acetylene on Ni( 100) H P TPD, C2H4/Ni(100)

TABLE 11: C,X,/Ni(100) Saturation, 175 K

A

C2H4 370 620 775 930

.-lEn L

3

1150 1425

Exposure

2.00 L 1.33 L 1.00 L 0.67 L 0.33 L 200

0

600

400

1550 2920 3025 3090

800

TemperaturelDeg. K

l,l-C2D?H2 370 480 715 875 980 1200 1410

-

1600 2140 2240 2770 3000

lr2-C2D2H2 325 445 645 860 960 1120 1185 1370 1660 2260 30.50 -2100 2890

CzD4 430 670 930 1275 1650 2160 2270

TABLE III: C,X,/Ni(100) Saturation. 225 K

CZH4TPD, CzH4/Ni(100)

380 470 630

1,JA ,

,

,

,

,

,

,

$,? 1.33 L 1.00 L

0.67 L

0

200

800

600

400 TemperaturelDeg. K

Figure 1. Hydrogen (2 amu) and molecular ethylene (26 amu) thermal programmed desorption as a function of exposure after ethylene dose on Ni(100). Heating rate = 10 K/s. TDS from l,l-C2D2H2/Ni(100)

s 1-

\ 200

350

, ,:p.?,L

500

200

810 970

2180 3040

-

430 635 780 900 1070 1285 2210

C2D4 220 430 810 940 1065 2220

4 amu

\

350

1335 -1410 2940 3100

220 430 625

TABLE IV: C2X4/Ni(100) Saturation, 275 K C2H4 191-C2D2H2 1,2-C2D2Hz 390 355 220 480 405 420 540 600 590 805 900 960 970 1030 1060 1060 1240 1245 1350 1340 1380 2920 2185 2210 2920 2330 2800 TABLE V: C2Xd/Ni(100) Saturation, 325 K C2H4 1,1-C2D2H2 132-C2D2H2 230 260 220 390 380 365 490 470 450 730 750 730 820 980 980 1025 1070 1090 1360 1235 1250 2910 2160 2210 2900

,L*

1 . 0 L j ,

975

370 450 625 780 850 970 1060 1245 1380 2180 2340 2915 -3090

500

l . 0.7L

0

L

200

350

L

500

TemperatureIK

Figure 2. H 2 (2 amu), H D (3 amu), and D2 (4 amu) thermal programmed desorption as a function of exposure after dosing Ni(100) with I.I-CZD2H2. Heating rate = 10 K/s. TABLE I: CIXl/Ni(lOO) Saturation, 90 K mode l,l-C2D2H2 1,2-CzD2H2 C2H4 380 380 380 VMC 600 480 490 -830 640 710 (CD2) -880 825 955 (CH,) 1100 955 1090 1395 1165 (CD2) 1300 1390 (CH2) 1560 1575 2995 2225 2230 (CD2) 2995 2990 (CHJ

C2D4 -420

-

600 640

920 1230 1550 2260

shown in Figure 3 for a 0.5-langmuir exposure of the light ethylene. More detailed spectra recorded for all compounds at this coverage up to 225 K have been published previously.5 The data for saturation coverages is shown for all isomers in Figures 4-7, and

C2D4 430 780 860 1050 2230

the vibrational frequencies are listed in Tables I-V. Finally, HREEL spectra after dosing with 0.5 langmuir of acetylene were taken. They did not change with temperature from 80 to 270 K (Figure 8). Similar results were obtained with deuteriated acetylene.

Discussion A detailed study of the chemisorption and decomposition of ethylene on Ni( 100) has been performed using TPD and HREELS as a function of annealing temperature and ethylene coverage. The vibrational data indicate that ethylene adsorbs molecularly at liquid nitrogen temperature since the frequencies, as measured by HREELS, coincide with those reported for gas-phase ethylene. No appreciable rehybridization of the carbon atom orbitals was observed at any coverage. The details for this system have been already published elsewhere.5

4320 The Journal of Physical Chemistry, Vol. 91, No. 16, 1987

Zaera and Hall

, 0.5L CIH,/Ni

-

I

‘xloool

(100)

HREELS

I

C Z H ~ N (100) I Sat HREELS

375

XlOOO I

- N

0

1000

2000

Frequency (cm

3000

Figure 3. HREEL spectra of ethylene chemisorbed on Ni(100) after annealing at different temperatures for 100 s. The dose was 0.5 langmuir and all the spectra were taken at 90 K.

Only ethylene and hydrogen were detected from thermal desorption after ethylene chemisorption. At exposures below l .3 langmuirs only decomposition is observed, but at higher exposures some molecular desorption is seen as well. H2 desorption starts above 280 K even after ethylene saturation, but laser-induced desorption experiments have indicated that ethylene decomposition occurs at much lower temperatures, around 170 K for coverages below half-~aturation.~ HREEL spectra also show the appearance of a new species after annealing at 175 K for 100 s (Figure 3) that is believed to be a vinyl m ~ i e t y .This ~ decomposition displays a marked isotope kinetic effect. At 170 K, the decomposition of deuteriated ethylene is about 60 times slower than that of C2H4. Appreciable conversion of C2D4 is observed only after heating above 210 K. At coverages above half a monolayer the reaction slows down. While conversion to vinyl is nearly complete after annealing at 158 K for 200 s when starting with an initial coverage of Bo = 0.25, no decomposition is detected by LID after 400 s of heating at the same temperature for Bo = 0.8.4 A slowing of the reaction at high coverage is also observed by the vibrational spectra. The spectrum obtained after annealing a saturated surface at 175 K is composed of overlapping peaks from molecular ethylene and vinyl fragments. This is clearly seen in the C-H stretching region, where there are three peaks, two at 2920 and 3090 cm-’ corresponding to vinyl, and one at 3025 cm-’ due to ethylene (Figure 3). Similar results were obtained from the deuteriated compounds. Vinyl is eventually formed on the saturated surface above 200 K, but it starts to dehydrogenate further above 230 K. HREEL (6) Evans, J.; McNulty, G. S.J . Chem. Soc., Dalton Trans. 1984, 1 9 . (7) Demuth, J. E.; Eastman, D. E. Phys. Rev. Lett. 1974, 32, 1123. (8) Demuth, J. E. SurJ Sci. 1978, 76, L603. (9) Demuth, J. E. Surf. Sci. 1980, 93, 127. (10) Demuth, J. E.: Ibach, H.; Lehwald, S . Phys. Rea. Lett. 1978, 40,

1044.

0

j)

(11) Demuth, J. E.; Ibach, H. Surf. Sci. 1978, 78, L238.

1000

2000

3000

Frequency (cm - 1 ) Figure 4. HREEL spectra from Ni(100) saturated with ethylene after annealing to the indicated temperatures for 100 s. Spectra taken at 90

K.

spectra obtained after annealing to 225 K are similar regardless of the initial ethylene coverage. The peaks at 2940 and 3 100 cm-’ correspond to the different C-H stretching modes for vinyl, but their relative intensity suggests the growth of another peak at around 2930 cm-I due to an acetylenic moiety. Other detectable modes include peaks at 1410 cm-’ for a CH, scissors mode, 1335 cm-I for a C H rocking, and 975 cm-’ for a combination of CH2 twisting and rocking (1 380, 1245, and 970 cm-I, respectively, when starting from 1, 1-C2D2H2;similar, although less clear, assignments can also be made for 1,2-C2D2H2and C2D4). Total decomposition occurs below 275 K; there is no trace of the vinyl modes in the spectra recorded after annealing at that temperature. The vibrational data for the species formed after heating chemisorbed ethylene to 275 K are similar for all coverages studied. The spectra obtained from all four isotopic isomers after 0.5-langmuir ethylene exposure and 275 K annealing are shown in Figure 9. Also in the same figure are shown spectra of adsorbed acetylene, both normal and fully deuteriated. The spectra of the acetylenes did not change after annealing to 275 K; they are identical with those obtained immediately after chemisorption at 90 K. From the spectra in Figure 9 we can see peaks at 230 and 430 cm-I for carbon-nickel vibrational modes (220 and 425 for C2D2),970 cm-’ for a C-H bending mode (around 820 cm-I for C-D), 1340 cm-’ corresponding to the C-C stretch (1285), and 2950 cm-’ (2210) for C-H (C-D) stretches. The value for vcc is close to that observed for saturated hydrocarbons, suggesting that the C-C is a single bond. We assign the acetylene spectra to a rehybridized acetylenic fragment where the carbon atoms have close to sp3 character. The spectra recorded after annealing the chemisorbed ethylene to 275 K are similar to those obtained from acetylene. Thus, the spectrum after annealing when starting with C2H4 is similar to that for C2H2,and when starting with either C2D4 or 1,2-C2D2H2 we obtain spectra analogous to that from C2D2. The spectrum

The Journal of Physical Chemistry, Vol. 91, No. 16, 1987 4321

Decomposition of Ethylene and Acetylene on Ni( 100) l,l-CpDzHz/Ni(100) Sat HREELS

I

1

I

I

C2D4/Ni(100) Sat

I

~~

1000 2000 Frequency (cm 1 )

0

3000

I

Figure 5. HREEL spectra from Ni(100) saturated with I,I-C2D2H2. Same as Figure 4. 1,2.C2DzHz/Ni(100) Sat HREELS

co

TABLE V I Vibrational Mode Assignment for C2X2on Ni(100), 275 K. 0.5 lanemuir uMC

I

i

X333(

-730 980

vCC

1340 2925

YCH

j/

I

x 1000

0

1000

2000

3000

Frequency (cm-1)

Figure 6. Same as Figure 4 for 1,2-C2D2H2chemisorption.

corresponding to l,1-C2D2H2contains some of the peaks observed for C2H2and some for C2D2. These results indicate that, while

C2H4 435

8CH

,----

i

3000

Figure 7. Same as Figure 4 for C2D4chemisorption.

mode

I

1000 2000 Frequency (cm 1)

0

C2H2 230 430 970 1345 2950

l,l-C2H2D2 1,2-C2H2D2 C2D4 400 420 390 540 800 1060 1250 2200, 2915

585 795 930 1260 2205

555 700 980 2235

C2D2 220 425 590 820 930 1285 2210

the first dehydrogenation step involves a C-H bond breaking (preferentially to C-D bonds), the second hydrogen is lost from the (3 carbon. In the case of 1,2-C2D2H2this second bond rupture also occurs on a C-H bond, and therefore only a C-D stretching frequency is observed at 2205 cm-' and the spectrum is similar to that for C2D4. For 1,1-C2D2H2,on the other hand, a C-D bond is broken and a DCCH fragment is formed; both C-H (2915 cm-') and C-D (2200 cm-I) stretches can be seen in the vibrational spectrum. Surprisingly, no H-D scrambling occurs during these reactions:' the high-temperature shoulder in the TPD for the saturated surfaces contains almost exclusively deuterium. A summary of the acetylenic fragment vibrational data is given in Table VI. Warming the surface to 325 K induces additional changes in the vibrational spectra. There are still peaks present at 2910 and 1360 cm-' corresponding to C-H and C-C stretching modes, respectively, but two C-H bending modes at 730 and 820 cm-' replace the mode at 960 cm-' obtained from the acetylenic fragment. Similar trends are seen for the other isotopes. We assign these new features to CCH fragments. This assignment is consistent with the IR spectra obtained for a triosmium cluster with tilted ethynyl ligand, OS~(CO)~(~-H)(M~-~~-CCH),~ where vibrational frequencies were reported at 3157 cm-' ( Y ~ H ) ,1534 cm-' (vCc), 861 and 854 cm-' (&.,), and 762 and 759 cm-' ( 6 ~ ~ ) . Finally, heating above 400 K only leaves carbon on the surface. Ethylene and acetylene chemisorption and decomposition have been extensively studied in the past both on nickel and on other

4322 The Journal of Physical Chemistry, Vol. 91, No. 16, 1987

Zaera and Hall

TABLE VII: Vibrational Mode Assignment for Acetylene mode

gas phase"

"MC

634 759

768 894

2006 3287 3409 3510

1403 3086 31 16

6CH

wc *CH

C,H,CO(CO)~' 551, 605

Reference 13. Reference 19

C2H2C12' 711 570 (XI 1179 (a) 1303 (PI 1591 3086 3160

Reference 14. Reference 15.

Rh(lllId

Pt(lll)e

323

340 570 770 985

706 887 -1350 2984 3085

1310 3010

Reference 16.

e

C,H, chemisorbed on Pd(lll)/ Ni(1ll)g

5 00

Ni(100)'

370 470 675 745 890 1305 3015

230 430 -730 970

435

1345 2950

1340 2925

480 560 690 860 1080 1220 2920

673 872 1034 1402 2992

Reference 17. /Reference 18. XReference 12.

980

Reference 3. ' Reference 19.

x 333

I

0.5L C2Xz/Ni(100) HREELS

HREELS

XlOOO

CAHd NiilOO)

Ni(llO)h

I

Adsorbate

oI '

l

o

o

~

A~

(

~

c

z

D

z

\

x333

'

n I\

%\ 'xi000

.I

1 1

I ,

i

0

I

1

1000 2000 Frequency (cm ')

3000

Figure 8. HREEL spectra from Ni(100) dose with 0.5 langmuir of C2H2 after annealing to the indicated temperatures. Same as Figure 4.

metal surfaces. Lewald and Ibach concluded that ethylene dehydrogenates to acetylene on Ni( 111) above 230 K based on their HREELS results.2 Stroscio et al. reported the formation of CCH above 200 K and subsequent dehydrogenation to C H for ethylene lications to the formation of a vinyl moiety, but most of the work has been done at room or higher temperatures. However, it has been reported that chemisorption of vinyl fluoride yields the same LEED pattern sequence as ethylene when chemisorbed on Ni(10O),l2 a result easy to understand if the C-F bond breaks upon chemisorption. Several stable intermediates have been postulated to form on different metal surfaces. The formation of ethylidyne around room

A-

0

1000 2000 Frequency (cm 1 )

1

3000

Figure 9. HREEL spectra from Ni(100) after dosing with 0.5 langmuir of the indicated gases and annealing to room temperature Same as Figure 4.

temperature has been well documented on Pt(l1 l ) , Pt(100), Pd( 11 l ) , Rh(l1 l ) , and Ru( 100).132[t23 Ethylidyne further decomposes to form C H and C C H fragments at higher temperat u r e ~ . ~A ~mixture . ~ ~ of ethylidyne and C C H moieties form on Rh(100) between 200 and 300 K.26 Chemisorption on more reactive surfaces is not as well documented. Ethylene chemisorbed at room temperature transforms into an acetylenic fragment on Fe(1 but it decomposes under similar conditions on Fe(l1 1).28 Acetylene formation from chemisorbed ethylene has also been

(12) Casalone, G.;Cattania, M. G.;Simonetta, M.;Tescari, M. Surf. Sci. 1917, 62, 321.

(13) Sverdlov, L. M.; Kovner, M. A,; Krainov, E. P. Vibrational Spectra of Polyatomic Molecules; W h y : New York, 1974; p 316. (14) Iwashita, Y. Inorg. Chem. 1970, 9, 1178. (15) Herzberg, G.Molecular Spectra and Molecular Structure. I I . Infrared and Raman Spectra of Polyatomic Molecules; Van Nostrand: New York, 1945; p 330. (16) Dubois, L. H.; Castner, D. G.;Somorjai, G.A, J . Chem. Phys. 1980, 72, 5234. (17) Ibach, H.; Hopster, H.; Sexton, B. Appl. Surf. Sci. 1977, 1, 1. (18) Gates, J. A.; Kesmodel, L. L. Surf. Sci. 1983, 124, 68. (19) This work.

(20) Ibach, H. In Proceedings of the International Conference on Vibrations in Adsorbed Layers, Jiilich, 1978; pp 64-65. (21) Kesmodel, L. L.; Gates, J. A. Surf. Sci. 1981, 111, L747. (22) Dubois, L. H.; Castner, D. G.; Somorjai, G . A. J . Chem. Phys. 1980, 72, 5234. (23) Barteau, M. A,; Braughton, J. Q.; Menzel, D. Appl. Surf. Sci. 1984, 19, 92. (24) Steininger, H.; Ibach, H.; Lehwald, S . Surf. Sci. 1982, 117, 341. (25) Gates, J. A,; Kesmodel, L. L. Surf. Sci. 1983, 124, 68. (26) Bent, B. E. Ph.D. Thesis, University of California, Berkeley, 1986. (27) Erley, W.; Baro, A. M.; Ibach, H. Surf Sci. 1982, 120, 273. (28) Seip, U.; Tsai, M.-C.; Kuppers, J.; Ertl, G.Surf. Sci. 1984, 147, 65.

J . Phys. Ckem. 1987, 91, 4323-4329 reported on W(100) surfaces, but in this case the conversion occurs at temperatures as low as 135 K.29 Finally, the formation of vinyl has been suggested from the decomposition of ethylene on Pd( but the experimental data were not good enough to make this assignment conclusive. The reactivity observed here for Ni(100) seems to fall in between that seen for other platinum group metals and what has been obtained for more active transition metals to the left of the periodic table. A summary of results (29) Hamilton, J. C.; Swanson, N.; Waclawski, B. J.; Celotta, R. J. J . Chem. Phys. 1981, 74, 4156. (30) Stuve, E. M.; Madix, R. J. J . Phys. Chem. 1985,89, 105. (31) Mate, C. M.; Somorjai, G. A. Surf. Sci. 1985, 160, 542.

4323

regarding the vibrational spectra of acetylenic fragments on different surfaces is given in Table VII. In conclusion, we have studied the thermal decomposition of ethylene and acetylene over Ni(100) at low and saturation coverages. Acetylene rehybridizes to form an acetylenic moiety that does not react until around room temperature. The same fragment is obtained from ethylene following the formation of vinyl at lower temperatures. Vinyl forms in an unimolecular process at 170 K and coverages below 0.4 monolayers, but this reaction is inhibited to some extent at higher coverages. The acetylenic fragment forms in any case at room temperature and decomposes to CCH at about 325 K and to carbon above 400 K. No H-D scrambling was observed for the deuteriated or partially deuteriated molecules.

Mechanisms of Pulsed UV Laser Ionization of Molecules Adsorbed to Thin Metal Films J. R. Millard, M. Yang, and J. P. Reilly*+ Department of Chemistry, Indiana University, Bloomington, Indiana 47405 (Received: February 13, 1987)

The mechanism of UV-laser-induced ionization and desorption of molecules adsorbed to metal surfaces has been investigated. From a comparison of the gas-phase and surface ionization wavelength thresholds of aniline (C6H5NH,) and phenol (C6H50H) it is concluded that surface ionization by pulsed UV laser radiation depends primarily upon the absorption cross section of the adsorbed molecule. Thresholds are consistent with a two-photon process and do not differ for ionization from gold and aluminum substrates. In order to determine whether ionization of the adsorbate precedes or follows its desorption, the wavelength dependence of surface ionization of nitric oxide (NO) molecules on a gold film was recorded. A gas-phase-like excitation spectrum was obtained that exhibited a Boltzmann distribution of rotational states characterized by a rotational temperature of 475 i 60 K. This is definitive evidence that NO molecules are first desorbed and then ionized above the surface. The mechanism for aniline surface ionization under equivalent conditions is less clear. No resonances are evident in the aniline excitation spectrum. The disparity between the NO and aniline results may be due to thermal effects as well as differences in the strength of molecular adsorption to the metal.

1. Introduction Laser-stimulated ionization and desorption of molecules adsorbed to metal surfaces is a subject of considerable current interest. General mechanisms for stimulated desorption have been the topic of recent conferences,’ and laser-stimulated processes2 and laser-induced ion formation at surfaces in mass spectrometry3 have been reviewed. Commercial instruments such as the LAMMA 1000, which utilize UV-laser-induced surface ionization with mass spectrometric detection are finding wide application. Even so, the physical processes involved in the creation and desorption of molecular ions from adsorbed species by laser irradiation have not been completely determined. This is due at least in part to the wide range of experimental conditions that have been used in these studies4 The laser wavelength, the optical and thermal properties of the substrate and of the adsorbed species, and the laser fluence (power per unit area) can all profoundly affect the dominant channels for desorption and ionization. Research in our l a b o r a t ~ r y ~has - ~ involved the use of nanosecond pulse UV laser irradiation to ionize and desorb molecules adsorbed to metal films in a time-of-flight mass spectrometer (TOFMS). Herein, we report results that pertain to the ionization mechanism; laser desorption will be discussed in more detail e l ~ e w h e r e . ~Three questions are addressed in this study. First, what is the relative importance of substrate vs. adsorbate photoexcitation in leading to ion formation? Second, is strong resonant enhancement of surface ionization observed, as in gas-phase multiphoton ionization? Finally, are adsorbed molecules ionized at the surface and then desorbed, or are neutral molecules first desorbed and subsequently ionized in the laser field just above the surface? To approach these questions a method is needed to distinguish between ionization of desorbed neutral molecules and ionization Camille and Henry Dreyfus Teacher-Scholar.

0022-36 5 4/ 87 / 209 1-432 3$0 1.50/ 0

of ambient gas-phase molecules. For this reason, the internal reflection geometry5J0 has been used for all surface ionization studies described within. In this configuration, a thin metal film is first deposited upon the surface of a quartz prism. Laser pulses refract through the prism and irradiate the metal/quartz interface. Molecules at or near the metal/gas interface may be excited by the evanescent or transmitted portion of the beam or by transfer of energy from substrate excitations such as surface plasmons.] Because the laser does not pass directly through the sample gas (as in the usual “external reflection” geometry), concurrent ionization of gas-phase molecules well above the surface is eliminated. Coupling to plasmon and evanescent fields decreases rapidly away from the surface, so ions formed by these processes will arise from molecules in a region less than 1 km above the surface. During the laser pulse the density of desorbed neutrals in this region is several orders of magnitude greater than the density of ambient (1) (a) Tolk, N. H., Traum, M . , Tully, J. C., Madey, T. E., Eds. Desorption Induced by Electronic Transitions (DIET r); Springer-Verlag: Berlin, 1983. (b) Brenig, W., Menzel, D., Eds. Desorption Induced by Electronic Transitions (DIET I I ) ; Springer-Verlag: Berlin, 1986. (2) George, T. F.; Lin, J.; Beri, A. C.; Murphy, W. C. Prog. Surf, Sci. 1984, 16, 139. (3) Conzemius, R. J.; Capellen, J. M. In!. J . Mass Spectrom. Ion Phys. 1980, 34, 197.

(4) Hillenkamp, F.In Ion Formation from Organic Solids; Benninghoven, A., Ed.; Springer-Verlag: Berlin, 1983; p 190. ( 5 ) Yang, M.; Millard, J. R.; Reilly, J. P. Opt. Commun. 1985, 55, 41. (6) Chai, J.-W.; Rhodes, G.; Meek, J. T.;Reilly, J. P. Proc. SPIE Int. SOC. Opt. Eng. 1983, 426, 129. (7) Opsal, R. B.; Reilly, J. P. Chem. Phys. Lett. 1983, 99, 461. (8) Chai, J.-W.; Reilly, J. P. Opt. Commun. 1984, 49, 51.

(9) Millard, J. R.; Yang, M.; Reilly, J. P., manuscript in preparation. (10) Hansen, W. N. Adv. Electrochem. Electrochem. Eng. 1973, 9, 1. (11) Kurosawa, K.; Pierce, R. M.; Ushioda, S.; Hemminger, J. C. Phys. Rev. B 1986, 33, 789.

0 198 7 American Chemical Society