Infrared-laser-induced desorption by resonant multiphoton excitation of adsorbate virbation in CH3FNaCl at different coverages

Surface Science 126 (1983) 183-191 North-Holland Publishing Company

183

INFRARED-LASER-INDUCED DESORPTION BY RESONANT MULTIPHOTON EXCITATION OF ADSORBATE VIRBATION IN CH,F-NaCI AT DIFFERENT COVERAGES J. HEIDBERG,

H. STEIN and E. RIEHL

Institut fiir Physikalische Chemie und Elektrochemie D-3OGO Hatmover, Fed. Rep. of Germany

der Universitiit Harmover, Callinstrasse

Received

8 November

24 August

1982; accepted

for publication

3 - Sa,

1982

Desorption induced by resonant COs laser pulses in CHsF-NaCl(film) on NaCl(100) single crystal surfaces at different coverages has been observed. The primary activation step is the multiphoton excitation of the most intense internal adsorbate vibration, the C-F stretching mode vs. The variation of desorption yield with laser frequency was measured at constant fluence and coverage and was correlated with the corresponding linear absorption spectra. From desorption yield versus laser fluence plots, rate coefficients and characteristic minimum energy fluence values & for desorption at different surface coverages have been determined, showing increasing $I~with decreasing coverage at constant excitation frequency. By appropriate choice of laser frequency and fluence, it is possible to separate different adsorption phases by desorption.

1. Introduction For many years high resolution transmission and reflection infrared spectroscopy has extensively been used due to its high specificity in the chemical analysis of atomic and molecular adsorbates. The vibrational spectra can yield information on surface coverage, adsorption sites, adsorption phases and isotopic species, as well as intermolecular dynamic vibrational coupling. Infrared-laser-induced multiphoton dissociation of polyatomic molecules in the gas phase has been studied to a large extent in recent years. Various theories have been proposed. One of the mechanisms [l] assumes step by step one-photon transitions between levels having many closely spaced sublevels, as well as collisional interaction. In this case the excitation appears to be incoherent (with no oscillations present in coherent absorption) with smoothly changing, coarse-grained populations of the levels. At high excitation and moderate yield a steady state is reached by competition between pumping, molecular dissociation and deactivation after sufficiently long irradiation time. Then the yield becomes fluence dependent only without being intensity dependent. 0039-6028/83/0000-0000/$03.00

0 1983 North-Holland

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desorption

Only very recently laser-stimulated desorption of molecules from solid surfaces, after resonant excitation of a normal internal adsorbate mode, has been established in the adsorption systems SF,-NaCl [2], CH,F-NaCl [3-51, pyridine-KC1 [6], pyridine-Ni [7] and pyridine-Ag film [8,9] at low temperatures under ultra-high vacuum (UHV) conditions. Theoretical models on laser-stimulated desorption have been discussed, e.g., by Lin and George [lo], Jedrzejek, Freed, Efrima and Metiu [ 111, Kreuzer and Lowy [ 121, and Lucas and Ewing [ 131. In ref. [ 121 it is proposed that the energy of the IR laser photons absorbed by the adsorbed molecule in an internal mode is efficiently transferred via phonons into bound and continuum states of the adsorption potential. The unimolecular rate coefficient for the energy transfer is given. Fast optical pumping and rate-determining energy transfer into the adsorption potential states are assumed. In ref. [ 131 the residence time of a vibrationally excited adsorbate is given and interpreted in terms of Franck-Condon overlaps between the wavefunctions of the adsorbate and the desorbate. In the adsorption system CH,F-NaCl(film) three different adsorption phases have been identified by IR cryospectroscopy [ 141, the internal adsorbate frequencies vs being shifted to smaller values with respect to the gas and solid phase frequencies. At low laser fluence, + < 0.1 J cm-*, molecular desorption occurs in the system CH,F-NaCl after excitation of the v3 normal mode of CH,F [3-51. In this work we report on measurements of the desorption yield as a function of laser frequency or at constant laser fluence and surface coverage. The results clearly indicate the resonant character of the multiphoton activation process and the existence of different CH,F phases on NaCl film at low temperatures. Desorption rate coefficients and characteristic minimum fluences have been determined at constant laser frequency for different surface coverages in order to obtain information on the influence of quasiresonant transfer of the vibrational energy between adsorption potential states of different molecules. The energy transfer should be dependent on intermolecular distance, and therefore on surface coverage.

2. Experimental The experimental setup and procedure for studying laser-induced surface processes will be described very briefly. The main components are an UHV cryostat with a flanged analyzer cell [ 151, a TEA CO, laser running in TEM,, mode and a quadrupole mass spectrometer located 12 cm from the sample at 60” to the surface normal. The entire UHV system is pumped to a base pressure in the low lo- lo mbar range after bakeout. NaCl films are evaporated onto NaCl(100) single crystal disks at 77 K. CH,F gas is deposited onto the

I. Heidberg et al. / IR -laser-induced

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185

NaCl film at 77 K, the gas stream being controlled by an UHV leak valve. The CO, laser beam is focused ( f = 7 1 cm) onto the sample, the angle of incidence being 60” to the surface normal, except for the desorption from solid CH,F on a NaCl(100) single crystal with an angle of incidence of 0”. The pulse energy is determined with a pyroelectric energy meter, and the laser frequency w,_ with a grating spectrum analyzer. After each laser pulse the rise g(7) of the CH,F’ ion current due to desorbed CH,F molecules was monitored with the mass spectrometer, operated in time of flight mode, and displayed on a storage oscilloscope. r is the time of flight of the desorbed molecules between the sample and the ionization chamber of the mass spectrometer. The maximum increase j of g(7) turned out to be approximately proportional to the integral jg( T)~T, which is proportional to the number of molecules desorbed per pulse.

3. Evaluation of measurements;

desorption yield and rate

If dki and B,, denote the coverage before and after the k th laser pulse, respectively, with fluence #k in a sequence starting at coverage Bli = 8, and ifj, is the corresponding maximum increase of the CH,F+ ion current, the desorption yield per k th laser pulse is defined by

with

k

where a is a constant and m is the number of pulses necessary for complete desorption. The desorption yield can be dependent upon the angle cw of desorption and the angle p of the incident laser radiation and its polarization. Of particular interest in this work is the dependence of the desorption yield Y* upon laser frequency wr, fluence (p and surface coverage Bi at constant angles cy and 8:

(3 (2c) The index k of the laser pulse is omitted here, because it is needed only in the determination of the coverages oi and of but has no other physical meaning. Actually, the fluence dependence of the yield was measured according to eqs. (2b) and (1) in a sequence of pulses. In the latter case the fluence dependence is mixed with the coverage dependence. However, it turned out that the latter

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may be neglected within our range of coverages prepared in a sequence of pulses. In section 4 the results of measurements of the desorption yield as a function of the laser frequency at constant fluence and surface coverage are reported, establishing the resonant character of the desorption process. The fluence dependence of the desorption yield at constant laser frequency and approximately constant coverage is described in section 5. The desorption yield was found to be significantly surface coverage dependent upon strong coverage variation at constant laser fluence and frequency. As shown in the following, information on the kinetics of the desorption process can be obtained from the plot Y* =f(NwL,Oi. 3.1. Relation between Y* and Y(t) In our experiments the total desorption yield measured as a function of the total fluence $J. The laser fluence G(t) is defined by +(t)

dt’.

= j-Z(P)

Y* per laser pulse has been

(3)

0

The total fluence + is obtained if the upper limit of the integration corresponds to the end t, of the pulse. Z(t) denotes the time-dependent laser intensity. The total yield Y* measured long after the laser pulse and the time resolved yield Y(t) at time t of laser irradiation are related in a simple way at steady state. A similar relation was found by Quack for IR laser-induced gas reactions [l]. It must be noted that the total desorption yield Y* can also include molecules desorbing after the laser pulse. At irradiation time t the surface concentration O(t) can be expressed as the sum e(t)=e*(t)+e**(t)

(4)

of the surface concentrations (e**) threshold of desorption.

e-/e*

=

of molecules with energy below (f?*) and above In steady state the ratio

b

is time-independent

e(t) = e*(t)

and the equation

(1 + 6)

(5)

is valid. The surface concentration e*(t,). Considering the desorption cient k, is defined by -d

ln[ e( t)/di]

/dt

= k,( $, 1, t)

long after the laser pulse 8(t s=-tp) equals as a unimolecular process, the rate coeffi-

(6)

J. Heidberg et al. / IR -laser-induced

desorption

187

or dY(l)/dt

= k,(+,

Z, t),

(7)

where Y(t) = -ln[

8(t)/e,].

In steady state according - d ln( 8/fZi)/dt results dY( t)/Z

= -d

and, dividing

to eq. (5) the relation ln( 8*/di)/dt

(8)

by the laser intensity

dt = dY*( r)/dcp(

I,

t) = k,(st)/Z,

(9)

where Y*(t) = -ln[

e*(t)/e,]

and

Y*(t,)

= Y*.

The rate coefficient should become a constant k,(st) at steady state and time-independent intensity. From our experiments it can be inferred that k”(st) is approximately .proportional to the laser intensity Z in an interval of intermediate intensities, in concert with theoretical considerations [ 11. Therefore, dY/d+

= dY*/d$

= c,

(IO)

where c is independent of $I and I. In this case the yield Y* per pulse is dependent only upon the fluence (p and is independent of the shape I( t ) of the laser pulse for a given adsorption system: dY*/d+

= k,(st)/Z.

(11)

k,(sf) can be evaluated from the limiting slope of the plot Y* versus +. The variation d+ of the fluence is mostly achieved by variation d(Zt,) of intensity Z(t) at constant pulse length t, and shape. From eq. (10) it can be seen that the limiting slopes of Y(+) and Y*(9) are the same. However, the values of the coverages e( fr) and c?*(fr) differ by a quantity proportional to the number of molecules which desorb after the laser pulse. Deviations from the simple fluence dependence of the desorption yield and a non-linear intensity dependence of the steady state rate coefficient are under certain conditions expected and observed. Nevertheless, the concept proposed seems to be suitable to plan and carry out desorption experiments and evaluate them.

4. Resonance characteristics of CH,F

desorption

The curves a and b in fig. 1 show the difference in dependency of the CH,F desorption yield on laser frequency (resonance characteristics) for two laser fluences $I at constant coverage and sample temperature q = 77 K. For

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J. Heidberg et al. / IR -laser-induced

I

1050

desorption

I

1

I

1000

950

LASER FREOUENCY km-’ 1

10 1050

I I 1 loo0 WL 950 FREQUENCY (cm-‘)

x)

for different adsorption phases of Fig. 1. CH,F desorption yield (CHsF + ion current) CH,F-NaCl(film) versus laser frequency at constant surface coverage; laser fluence + = 0.06 J T, = 77 K, total pressure cm-* (curve a) and I$ = 0.11 J cm-* (curve b). Sample temperature P = 1.6~ 10m9 mbar, 0, = 8. Curve c gives the desorption yield for solid CH,F on NaCl(100) single crystal, cp = 0.1 J cm-*, T, = 64 K and P = 1.9X low9 mbar. Fig. 2. Linear infrared absorption spectra of CH,F-NaCl(film) showing three adsorption a, /3 and y. The measured peak frequencies are w, =947cm-‘,ws=967cm-‘andw,=977cm-’ [ 141. The frequency of the exciting laser is a,_.

$I= 0.06J cm-*

phases

(curve a) the measured yield is small and restricted to a relatively narrow frequency range (- 15 cm-‘) at 976 cm-‘, whereas for + = 0.11 J cm-* (curve b) the yield increases with a factor of six together with the frequency range (- 55 cm-‘), the peakremaining at 976 cm-‘. In addition a shoulder is observed at 950 to 960 cm- ‘. The dotted curve c is due to desorption from thin layer solid CH,F on a NaCl(100) single crystal face at T,= 64 K, + = 0.1 J cm-*. The peak maximum of the desorption yield is shifted towards the fundamental IR absorption frequency of solid CH,F at 996 cm-‘. No laser pulses with the specified fluence values were available in the wavenumber region between 990 and 1030 cm-‘. At this point in the paper we should recall that three coexisting adsorption phases (Y,/?, y of CH,F-NaCl(film) have been detected together with the solid phase [ 141, the corresponding peak frequencies being w, = 947 cm- ‘, oa = 967 At high coverage /3 and y cannot clearly be cm-’ and wy = 977 cm-‘. distinguished whereas (Yis always observed (cf. fig. 2). The resonance characteristics obtained by laser-stimulated desorption indicate that all CH,F phases on the surface contribute to the desorption yield under the prerequisite of sufficiently high laser fluences and the choice of a

J. Heidberg et al. / IR

-laser-induceddesorprion

189

suitable excitation laser frequency as shown in fig. 1 (curve b). At low fluence (curve a) adsorption phase (Y,being most strongly bound, is not desorbed. Thus it is possible to desorb selectively and/or sequently adsorption phases of CH,F-NaCl(film) by proper choice of laser frequency, intensity and fluence.

5. Dependence of CH,F coverages

desorption yield on laser fluence for different surface

The fluence dependence of the desorption yield Y* is shown in fig. 3 for two relative surface coverages 6, = 170 (left-hand curve) and e,, = 8 (right-hand curve) at or = 975.9 cm-’ and T, = 77 K for the adsorption system CH,F-NaCl(film). From this figure one can see that the onset of desorption is shifted to higher fluence values with decreasing coverage. It was found that a given yield Yz = ln(8,i/8kf).p,UL is independent from the index k of the laser pulse within a given sequence. This means that the coverage dependence of Y* is weak and may be neglected in the small intervals of coverages within a sequence. For 0, = 130 the measured desorption yield is nearly identical with Y* for 0, = 170, indicating, in agreement with the k-independence of the yield, that within this coverage range no pronounced B-dependence of Y* exists. The rate coefficeint k”(st) can be estimated from the limiting slope (straight

I

I

I

I

I

e,=1iu 012-

CH,F - NaCl lfbd q .9759 cm-’

I

I

-

I 1

-

op

-

I

7 Oo 8/ 0

I

0’

/ 00

.; 008$

I

’ 0,=0

lI

6

Fig. 3. CH,F desorption yield Y* for CH,F-NaCl(film) as a function of laser fluence at constant laser frequency wL = 975.9 cm-’ for two different surface coverages f7,, T, = 77 K and total pressure P =1.6X 10m9 mbar. Under these conditions only adsorption phases, no solid, exist. Some points with higher desorption yield and fluence values on the righthand curve are outside this graph but important in determining the limiting slope [5].

190

J. Heidberg et al. / IR -laser - induced desorption

lines in fig. 3). The values factor 2, from Y* = Ek,(st)/4

(@ - #J

k,(st)l-’

have been obtained,

within

an error of

(12)

to be 1.7 X lo7 s-l MW-’ cm2 (right-hand curve) and 0.9 X lo7 SK’ MW-’ cm2 (left-hand curve). In the last equation &,,, the minimum characteristic fluence of the process, is the intercept of the straight line on the +-axis. At low fluence (p, giving poor yield Y*, the slope of the curve Y* versus # is smaller than at higher fluences #B> ~1,. This may be due to a time lag between the start of photon absorption and the onset of desorption, which is a consequence of the stepwise multiquantum excitation. There can also be a non-linear intensity dependence of k, expected for multiquantum processes at low laser intensity. To use efficiently the laser photons for desorption, the fluence applied should be at least of the order of the characteristic minimum fluence tp,. Taking into account the measured heat of adsorption E of CH,F-NaCl [ 141, E 2: 3 hv,, the quantum yield q has been estimated from q = 3AN/Ap,

(13)

where Ap = N*u~~/~~~ denotes the number of absorbed photons, a, = 3 x IO-l8 cm2 the absorption cross section of the v3 mode, N, the number of adsorbed molecules before the laser pulse and AN = &,@&)/I)(+

- 6,)

the number of desorbed molecules per laser pulse. As follows from eq. (13) the quantum yield is dependent on #,. This means (cf. fig. 3) that at higher coverage the quantum yield is higher. This could be due to intermolecular quasiresonant transfer of vibrational quanta between adsorption potential states of adjacent molecules leading to the accumulation of vibrational energy in the adsorption bond and/or to different activation energies of desorption at different coverages. Acknowledgement “Gefordert mit Hilfe von Forschungsmitteln des Landes Niedersachsen”. Supported by research funds of the state of Lower Saxony, Federal Republic of Germany. This work is dedicated to Professor Robert Haul on the occasion of his 70th birthday. References [I] M. Quack, Advan. Chem. Phys. 50 (1982) 395. [2] J. Heidberg, H. Stein, A. Nestmann, E. Hoefs and I. Hussla, in: Laser-Solid Interactions and Laser Processing, AIP Conf. Proc. SO,Eds. SD. Ferris, H.J. Leamy and J.M. Poate (Am. Inst. Phys., New York, 1979) pp. 49-54.

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-induceddesarption

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[3] J. Heidberg, H, Stein, E. Riehl and A. Nestmann, 2. Physik. Chem. (NF) 121 (1980) 145. [4] J. Heidberg, H. Stein and E. Riehl, in: Vibrations at Surfaces, Eds. R. Caudano, J.M. Gilles and A.A. Lucas (Plenum, New York, 1982) pp. 17-38. [5] J. Heidberg, H. Stein and E. Riehl, Phys. Rev. Letters 49 (1982) 666. [6] T.J. Chuang, J. Chem. Phys. 76 (1982) 3828. [7] T.J. Chuang and F.A. Home, J. Vacuum Sci. Technol. 20 (1982) 603. [S] H. Seki and T.J. Chuang, Solid State Commun. 44 (1982) 473. [9] T.J. Chuang and H. Seki, Phys. Rev. Letters 49 (1982) 382. [IO] J. Lin and T.F. George, Chem. Phys. Letters 66 (1979) 5. [l l] C. Jedrzejek, K.F. Freed, S. Efrima and H. Metiu, Surface Sci. 109 (1981) 191. [12] H.J. Kreuzer and D.N. Lowy, Chem. Phys. Letters 78 (1981) 50. [13] D. Lucas and G.E. Ewing, Chem. Phys. 58 (1981) 385. [ 141 J. Heidberg, I. Hussia and 2. Sziliigyi, in: Proc. 3rd Intern. Conf. on Vibrations at Surfaces, Asilomar, CA, 1982, to be published. [IS] J. Heidberg, H. Stein and E. Hoefs, Ber. Bunsenges. Physik. Chem. 85 (1981) 300.