Laser-induced resonant desorption and ablation on ultra-thin carbon films

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Surface Science 189/190 (1987) 946-953 North-Holland, Amsterdam

LASER-INDUCED RESONANT DESORPTION AND ABLATION ON ULTRA-THIN CARBON FILMS J. HEIDBERG, C. LANGOWSKI, G. N E U B A U E R lnstitut ff~r Physikalische Chemie und Elektrochemie, Universitiit Hannover, Callinstrasse 3-3,4, D-3000 Hannover 1, Fed. Rep. of Germany

and M. FOLMAN Technion, Department of Chemistry, Haifa 32000, Israel Received 31 March 1987; accepted for publication 13 April 1987

Adsorption and laser-induced resonant photodesorption and ablation of fluorinated benzene derivatives (a,a, ct-trifluorotoluene C6H~CF3, p-difluorobenzene C6H4F2) on NaCI(100) crystals and carbon films at a temperature of 77 K under ultra-high vacuum have been studied. The desorption and ablation signals show a strong dependence upon the laser excitation frequency, the frequency of the maximum coinciding with the frequency of the internal C - F vibration of the adsorbate and the condensate, respectively. The laser-energy absorption spectra have been measured.

1. Introduction

Desorption and ablation can be induced by laser irradiation. A laser beam striking a substrate can cause desorption by (a) direct heating of the substrate, (b) resonant excitation of an internal adsorbate vibration or (c) resonant excitation of the external adsorbate-adsorbent vibration. Desorption induced by resonant laser-adsorbate coupling has already been observed on dielectrics, semiconductors and metals in the adsorption systems of CH3F-NaC1 [1-3], CO-NaC1 [4], SFr-NaC1 [5], C2Ha-NaC1 [6], pyridine-KC1 [7], SFr-Si [8], CC14-Ge [9], pyridine-Ag [10], NH3-Cu [11]. A detailed theory has been developed [12-21]. The aim of this work is to show resonant desorption, occurring in monolayer region, as well as ablation, occurring in condensed multilayer systems on carbon films for the first time. NaCI(100) crystals are chosen as supports for the carbon films due to the transparency of NaC1 in the CO2-1aser frequency 0039-6028/87/$03.50 9 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

J. Heidberg et al. / Laser-induced desorption and ablation

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region. The fluorinated aromatics are used as "model systems" because of the expected high efficiency of desorption of C - F compounds [1-3] and because of the alternative way of detection via multiphoton ionization of the benzene ring.

2. Experimental The samples were cleaved ultra-pure NaCI(100) single crystals with a diameter of 3.5 cm. Carbon films were prepared in situ by resistive thermal evaporation (a few seconds) of graphite rods (Ringsdorff) leading to a maximum pressure rise of - 10 -7 mbar and deposition on the NaC1 single crystal at room temperature. The adsorptives (a,a,a-trifluorotoluene, p-difluorobenzene, Fluka AG, pro analysi grade) were further purified by vacuum distillation. An ultra-high-vacuum cryostat has been used in the experiments. The base pressure has been below 2 • 10 - 9 mbar, residual gas being mainly carbon monoxide. The carbon films have a thickness of 30 nm as determined with an electron microscope. Scanning electron microscopy also shows the. continuity of the film and the smoothness of the surface within 5 nm as well as the electrical conductivity of the films. No crystal structure could be resolved within 5 nm. The transmittance at a wavelength of 10/~m was determined to be 80%, the absorption of CO2-1aser radiation of these films was below detection limit ( < 1%). Admission of the adsorptives onto the samples was carried out via a capillary installed in front of the sample. The amount adsorbed on the surface is given by the pressure drop in the chamber of admission and the pressure rise above the sample. Sticking coefficients higher than 98% at 77 K have been inferred. For the desorption and ablation experiments a tunable pulsed TEA-CO 2laser with a pulse width of 60 ns and laser fluences of 0.5-2 J cm -2 at a distance of 9 m has been used. Time-of-flight curves were recorded with a quadrupole mass spectrometer tuned to m/e = 51 for C6H 5 CF 3 or m/e = 50 for CeHaF 2, respectively. Ionization occurred via electron impact. The measured quantity has been the number of molecules per unit volume in dependence on time, the number of molecules being proportional to the QMS signal in mV, the time resolution being 15 tts. Ions produced in experiments on multiphoton ionization with a frequencydoubled dye laser could be detected with a channel-electron-multiplier, also sensitive to UV light.

J. Heidberg et aL / Laser-induced desorption and ablation

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3. Results 3.1. Resonant processes and time-of-flight curves

Fig. 1 shows at the top the linear IR absorption spectra of gaseous C6HsCF3 in comparison to the laser IR absorption spectra of the condensed multilayer system of C6HsCF 3 on NaCI(100). At the bottom of fig. 1 the ablation signal of the multilayer system (3 • 1017 molecules cm -2) versus laser frequency o b t a i n e d a t the first CO2-1aser pulse is shown. A remarkable coincidence of the ablation maximum with the laser absorption maximum at the CO2-(001-020)-R8 transition can be seen. These observations are also made in the corresponding measurements of the ablation of C6HsCF 3 on carbon film as demonstrated in fig. 2. Fig. 3 shows the QMS signal of C6H5CF3 obtained at a reduced coverage resulting after the first two CO2-1aser pulses. This signal now can be referred to as a desorption

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J. Heidberg et al. / Laser-induced desorption and ablation

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Fig. 4. Laser IR absorption spectrum and photon-induced resonant ablation of a multilayer system of C6H4F2 on carbon film. signal since a coverage less than 10 monolayers ( - 1016 molecules c m - 2 ) c a n be estimated (see below). Fig. 4 shows the corresponding measurements made with a multilayer of C6H4F 2 on carbon film and for comparison the linear IR absorption spectra of the adsorption system at 70 K (the admission of 8 x ]015 molecules leading to the coverage of 1015 molecules cm -2) [22] is shown in the top. The off-resonant part of the ablation (admission of 8 x 1017 molecules leading to the coverage of 3 x 1017 molecules cm -2) is below detection limit at lower fluences whereas it is raised to - 15% of the maximum value of the resonant part at higher fluences, the frequency again coinciding with the laser absorption maximum. Time-of-flight (TOF) curves of C6HsCF3 on carbon film are shown in fig. 5 for three subsequent laser pulses and the 8th pulse at the same fluence and frequency. A shift to longer times for the third and subsequent laser pulses in

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the maximum of the QMS signal is seen which is also found in the T O F curves of C6HsCF 3 on NaCI(100). Further laser pulses do not result in a further shift in time, the maximum being constant at approximately 1 ms for desorption from carbon film and 700 /~s for NaCI(100). Thus, a possibility is given to distinguish between ablation, as thought to occur in the condensed multilayer system (3 • 1017 molecules cm-2), and desorption occurring in the monolayer region ( < 1016 molecules cm-2). From the total amount of molecules obtained by summing all ablation signals Until total depletion of the surface is reached, a coverage of about 1016 molecules cm -2 has been estimated before the third laser pulse. Assuming a Maxwell-Boltzmann behaviour which is in fact only valid in very coarse approximation here, from Ttr = Ml2/4Rt 2 (where M = molecular mass, l = distance to QMS = 12 cm, R = gas constant, t m = time at maximum) one obtains translational temperatures in the range of the substrate temperature ( - 6 5 K) for the desorption and higher temperatures ( - 100-150 K) for the ablation process. However, significant deviations are observed at flight-time >> t m so that the temperatures given here can be considered as rough estimates only. Fig. 6 shows the dependence on fluence of the ablation yield of C6HsCF 3 in a condensed multilayer system (3 x 1017 molecules cm - z ) on the carbon film. A constant slope is reached at higher fluences. The yield has been obtained from Y = A0/00 with 0o = initial coverage, A0 = change in coverage due to ablation. It has to be pointed out that Y correctly is given by Y = - I n ( 0 / 0 0 ) for adsorbates leading to the above equation for A0 << 00 [2]. A critical point in this work is the evaluation of 00 since in the condensed multilayer systems

J. Heidberg et aL / Laser-induced desorption and ablation

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investigated here it is not certain whether the lowest layers can contribute to the ablation. The process involves a multiphoton excitation since the desorption and ablation threshold is equivalent to at least 4 laser photons. The non-linearity of the yield at low fluences is in concert with the predictions for multiphoton excitation and also the relatively small line width of the laser absorption and desorption spectra, though, unfortunately, not the complete spectral lineshape could be detected due to lack of laser emission at the respective frequencies.

3,2. Additional phenomena On CO2-1aser irradiation light emission (continuous spectrum down to wavelengths below 250 nm) and ion emission have been found on NaCI(100) and carbon film as well as the adsorption system even at fluences as low as 0.08 J era-2.

4. Discussion

The most important feature in this work is the clear frequency dependence of photoablation (fig. 2) and especially photodesorption (fig. 3) of molecules on carbon films. The photon-induced ablation occurs from condensed multilayers of C6HsCF3 as well as C6H4F2 (3 x 1017 molecules cm -2) on the carbon film whereas photon-induced desorption (fig. 3) occurs at coverages

J. Heidberg et al. / Laser-induced desorption and ablation

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below 10 layers. Also the laser-energy absorption spectra of condensed multilayer systems on carbon film were measured for the first time. The frequency at the absorption maximum coincides with the frequency of the ablation and desorption maxima for both adsorbates, clearly showing the strong resonance of the photon-induced ablation and desorption by multiquantum excitation of the internal C - F vibration.

Acknowledgement "Gef~3rdert mit Forschungsmitteln des Landes Niedersachsen"

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