Time resolved photofragmentation of Au+15 clusters

21 June 1996

CHEMICAL PHYSICS LETTERS ELSEVIER

Chemical Physics Letters 256 (1996) 77-82

Time resolved photofragmentation of C. Walther

a,

clusters

G. Dietrich b, M. Lindinger a, K. Liitzenkirchen h, L. Schweikhard a, j. Ziegler a

a Institutffir Physik, Johannes Gutenberg-Universitfit, D-55099 Mainz, Germany b lnstitutJfir Kernchemie, Johannes Gutenberg-Universit~t, D-55099 Mainz, Germany

Received 30 October 1995; in final form 27 March 1996

Abstract

Au~-5 ions were produced by laser vaporization and transferred into a Penning trap. They were illuminated by a 10 ns dye-laser pulse at photon energies covering the range from 2.7 to 4.4 eV. Besides the determination of fragmentation patterns and photoabsorption cross sections the monomer evaporation of the excited clusters was observed time resolved on a microsecond to millisecond timescale yielding a monomer separation energy of 1.95(5) eV. In addition, the dissociation energy for Au~-4 could be determined as 1.57(5) eV by time resolved investigation of the sequential decay Au~-5 ~ Au~-4 + Au ~ Au ~-3+ 2 Au.

1. Introduction

Photofragmentation provides a versatile tool for the investigation of physical properties of metal clusters [1-6]. In the recent past a growing number of experiments have employed this method for various investigations. From photoabsorption cross sections collective excitation phenomena were deduced. Detailed investigations allowed the determination of cluster structures (e.g. bi- and triaxial deformations [1-3]). Recently, the temperature dependence of photoabsorption cross sections has been measured [4]. While the electronic transitions of cold clusters are described best by ab initio calculations including the detailed geometric structure, hot clusters may be treated in terms of the jellium model. A different approach for the investigation of intrinsic properties, such as the dissociation energy of the cluster, is the observation of time delayed evaporation of atoms after cluster excitation. After energy

is gained by the absorption of photons, it is distributed into the vibrational modes of the cluster. Given a sufficiently high excitation energy, unimolecular decay may occur. In order to measure the corresponding decay constants, various techniques have been developed. For example, Br~chignac et al. [5] produced an evaporative ensemble of potassium clusters. Sophisticated evaluation methods were applied to extract the dissociation energies from the ratios of monomer and dimer evaporation. Jarrold and co-workers [6] investigated the temporal decay of aluminum cluster ensembles during their flight through a drift tube. The cluster bunch was irradiated collinearly and their fragments were analyzed by a quadrupole mass filter. The longest timescale accessible by this method is limited by the flight time through the ion guide. In contrast, time resolved measurements of the Au~-5 decay presented below allow a direct determination of the decay constants. To our knowledge these are the first time resolved

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C. Walther et a l . / Chemical Physics Letters 256 (1996) 77-82

decay measurements of metal clusters on a timescale up to some ten milliseco-ds. Further treatment of the data employing statistical theories [7] yields the dissociation energies.

2. Experimental apparatus The apparatus has been described recently in detail [8], so only a short overview is given here. Positively charged gold cluster ions are produced by pulsed laser vaporization in a helium buffer gas jet which is expanded adiabatically into the vacuum. The ions are transferred to a Penning trap and captured in flight [9]. The cluster size of interest is selected background free by radial ejection of all unwanted species. By a combination of RF excitation and buffer gas cooling the ions are centered in the middle of the trap [10]. Equilibration with the buffer gas leads to internal cluster energies corresponding to room temperature. The frequency doubled light of a pulsed dye laser (pulse length 10 ns) is guided axially into the trap [11]. In order to control the focus conditions during the measurements, 5% of the light is deflected by a quartz plate and monitored by a beam profile analyzer. After calibration by the use of a laser calorimeter, profile, position and intensity of the laser beam are obtained. Typical values are a profile of 98% gaussian shape with 1.5 mm diameter (FWHM) and pulse energies of up to 2 mJ. After a variable storage duration all charged products are identified by axial ejection out of the trap into a time-of-flight mass spectrometer (TOF-MS) with single-ion detection by a conversion dynode detector.

3. Measurements Fig. 1 shows TOF spectra for the photofragmentation of Au~-5 at 335 nm (3.7 eV) and increasing photon flux density. In order to account for delayed decay processes, clusters were stored for 40 ms after laser excitation. For small laser fluences only Au~-4 and Au~-3 show up whereas at higher values the fragmentation continues down to the trimer. The fragment pattern depends strongly on photon energy. Fifteen values between 2.7 and 4.4 eV have been investigated. At 3.7 eV the Au~-~ and Au~-3 signals

3

4

5

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7 8 9 10 11 12 13 14 15

CLUSTER SIZE

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Fig. 1. Time-of-flight mass spectra of the photofragmentation of size selected Au~5 clusters. Laser wavelength 335 nm, pulse energies as indicated (at a focus diameter of 1.8 ram), storage duration after irradiation 40 ms.

exhibit a similar intensity. At higher photon energy, however, the relative abundance of Au~-3 increases at the expense of Au~-4 due to delayed fragmentation (see below). A pronounced odd-even alternation is found for fragments smaller than Au~3. This behavior is independent of photon energy and due to the enhanced stability of clusters containing an odd number of atoms (i.e. an even number of valence electrons) [12]. After the absorption of one or more photons the clusters cool down by the sequential evaporation of atoms or dimers. While for larger systems only monomer evaporation is observed, odd clusters with n ~< 13 preferentially evaporate dimers [12], leading to an additional enhancement of odd clusters within the decay chain. The decrease of the precursor Au ~ signal and the increase of the fragment signals of Au~-4 and Au~-3 (normalized to the total number of cluster ions) as a function of photon flux at 3.26 eV are shown in Fig. 2. This dependence contains information on the number of photons required for fragmentation [11]. For Au ~ (top) the solid curve corresponds to a one-photon fragmentation. The build-up of Au ~-4 is linear as well whereas Au~3 (bottom) exhibits a quadratic behavior. All smaller fragments show non-linear slopes, indicating that more than one photon is necessary to induce sequential fragmentation. Decreasing the photon energy below 3.05 eV results in a quadratic behavior of all fragments. Hence, the absorption of such an amount of energy is not sufficient to induce an observable dissociation

79

C. Walther et al./ Chemical Physics Letters 256 (1996) 77-82

within 40 ms. For photon energies exceeding 3.7 eV the build-up of Au~-4 as well as that of Au~-3 is linear. The measurements described so far have been performed at a fixed storage duration of 40 ms after the laser pulse. Variation of this period allows the direct observation of delayed dissociation over more than four orders of magnitude from some 10 /~s up to hundreds of ms. The lower limit is given by the time necessary for the ejection of the cluster ions from the trap. All clusters which decay during this period are detected as fragments. Computer simulations have been performed and the experimental data have been compensated for these effects. The upper limit is given by the rate of collisions with residual gas particles and depends on the vacuum conditions. At photon energies between 2.7 and 3.25 eV delayed monomer evaporation of Au~5 is observed. As an example, Fig. 3 (top) shows the temporal behavior of the Au~"4 fragment intensity at 2.9 eV. The normalized fragment intensity F increases exponentially as a function of time: F = 1 - e -'/~,

(1)

while the Au~"5 signal decreases correspondingly at the decay time z. (Au~-4 clusters observed at short storage times are due to the absorption of two photons by the precursor ions and are excluded from the

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STORAGE TIME [/zs] Fig. 3. Relative abundance of cluster fragments as a function of storage duration after laser irradiation for (top) the delayed fragmentation of Au~5 into Au~-4 at 428 nm (2.9 eV) and (bottom) the sequential fragmentation of Au~5 via Au~4 into Au~3 at 326 nm (3.8 eV). The lines are fits of exponentials to the data (see text).

fit.) The decrease of Au~5 and the increase of Au~4 were fitted independently yielding consistant decay constants. For the photon energies investigated, time constants between ~"= 13/~s (at 3.25 eV) and 3.2 ms (2.7 eV) were found (see discussion). For photon energies between 3.25 and 3.65 eV no delayed fragmentation was observed. At photon energies of 3.7 eV and above, however, a new process takes place: delayed sequential evaporation. The primary fragment, Au~-4, evaporates another atom resuiting in Au~-3 (Fig. 3, bottom). The data has been fitted by an exponential in analogy to the decay Au~-5 ~ Au~"4, with the addition of a constant to account for a fast two-photon induced decay. The decay times vary between r = 33 /zs (4.4 eV) and 12.6 ms (3.7 eV). In analogy to the case of Au~5, the increase of the Au~-3 signal was used to confirm the corresponding Au~-4 decay constants. Time dependent processes have been observed for smaller fragments as well. However, the statistical signal variations do not allow one to determine decay constants.

ENERGY/AREA [mJ/cm 2] Fig. 2. Relative abundance of surviving precursor (top; Au+5) and fragment clusters (bottom; Au~-4: II, Au~3: O ) as a function of the energy density of the laser pulse for the photofragmentation at hv = 3.26 eV (380 nm). The lines are fits for one- (full) and two-photon processes (dashed).

4. Results and discussion

The analysis of the data presented above yields the number of photons required for dissociation at a

80

C. Walther et al. / Chemical Physics Letters 256 (1996) 7 7 - 8 2

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PHOTON ENERGY [eV] Fig. 4. Photoabsorption cross sections of Au~-5 as a function of the photon energy.

given photon energy, the cross sections for photoabsorption and, from the time resolved studies, the dissociation energies. As discussed in detail elsewhere [11] the cross sections follow from the study of the fragment yield as a function of laser fluence. In contrast to measurements on sodium clusters where the plasmon energies exceed the monomer separation energy of the cluster by a factor of two or more, the energy of a single photon is not always sufficient to induce fragmentation of a gold cluster on an observable timescale. In order to determine cross sections it is therefore important to know whether the first dissociation step was induced by one or two photons. At energies exceeding 3.2 eV exclusively one-photon processes were found. Due to the finite temperature distribution of the clusters there is no sharp threshold from one to two-photon fragmentation as a function of photon energy but rather a transition region. Hence, a weighted sum of fit functions for both processes was applied to the data. The corresponding ratios can be obtained from time resolved measurements (see below). The resulting cross sections are shown in Fig. 4. They increase for photon energies up to 3.3 eV where a plateau is reached at some 0.4 ,~2. This value is surprisingly small when compared with measurements performed on silver clusters [13]. However, the investigations of Collings et al. [ 14] on Au ~-j and Au ~'s match our data confirming the absolute absorption strength as well as the general shape of the spectra. A local maximum observed at 3.7 eV [14] and assigned to H O M O - L U M O transitions could not be confirmed. However, this may be due to

the difference in the species and methods of the investigation: the photodepletion of cluster-Van der Waals complexes was investigated [ 14] and the influence of the attached Xe atoms may be significant, as recently shown by Knickelbein in the case of neutral copper clusters [15]. Previously, the photoabsorption of Au~- and Au~-~ has been studied [16] and resonances located at 4.0 and 3.7 eV, respectively, were found, in reasonable agreement with the corresponding spectra of silver clusters by Tiggesbaumker et al. [13]. In analogy to their results on Ag l+s one would expect a split resonance due to the non-spherical shape of the open shell system Au ~s, which is not observed. The reason may be the influence of the d-electrons of gold as suggested by Koutecky and co-workers [17]. As mentioned above, the time resolved observation of cluster decay yields information on the dissociation energy. In order to model the underlying statistical process of unimolecular decay, the quantum version of RRK theory developed by Kassel [ 18] was applied where the N-atom cluster is treated as a system of s = 3 N - 6 quantum oscillators and their frequency is approximated by the Debye frequency of the bulk metal v D. Therefore, the number of vibrational quanta is given by n = E / h u D where the internal energy E is the sum of the initial and the photon energy. For a dissociation energy (for evaporation of one atom) of D = m. hv D the rate constant is given by

n!.( n - m + s - 1 ) ! k=g~'o ( n - m ) ! . ( n + s - 1 ) ! '

(2)

where g is the number of decay paths (i.e. the number of surface atoms). The fraction describes the probability of accumulating the m quanta necessary for dissociation in one particular degree of freedom with the n quanta distributed over the s oscillators. Fig. 5 shows the experimental decay times r = 1/k of Au~-5 as a function of photon energy. The full squares represent the values obtained from the decrease of the Au~s signal, the open circles those from the increase of the Au~4 signal. The solid line shows a fit of Eq. 2 to the data. For this fit the clusters are assumed to be at room temperature prior to the laser irradiation. Therefore, only one parameter remains to be fitted: the dissociation energy of Au~5, with a

C. Walther et al./ Chemical Physics Letters 256 (1996) 77-82 104

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PHOTON ENERGY [eV] Fig. 5. Decay time as a function of photon energy as determined by the decrease of the Au~-5 signal (11) and the increase of the Au~4 signal (O). The solid line is a fit to the data and yields a dissociation energy of Au~5 of D,5 = 1.95(5) eV.

value of D15 = 1.95(5) eV. The error represents the change in D~5 if either the vibrational frequency is varied by +50% or the temperature between 270 and 330 K. The sequential time delayed fragmentation Au~-5 - * Au~-4 ~ Au~-3 c a n be described by the same method. To determine the dissociation energy of Au~-4 the dissociation energy of Au~-s must be subtracted from the sum of the initial thermal energy and the photon energy. The fit to the decay times as a function of excitation energy yields a dissociation energy of Au~-4 of O14 = 1.57(5) eV. Earlier CID experiments [12] had indicated the existence of a second dissociation channel for Au~-5, namely the evaporation of dimers. The present photofragmentation experiments, however, show no time correlated decrease in the Au~5 signal and increase in t h e Au~-3 signal (in contrast to the corresponding behavior of Au~- and Au~- [16]). In addition, the build-up of Au~-3 exhibits a nonlinear gradient for photon energies below 3.7 eV, indicating a sequential photoabsorption and decay via Au~-4. If, on the other hand, the energies involved are considered, then dimer evaporation should be an observable decay path: For the production of Au~"3 via a sequential decay, both the monomer evaporation energies of Au~-5 and Au~-4 must be supplied, Dj5 + D j 4 = 3.6 eV. When a preformed dimer is evaporated its dissociation energy of 2.3 eV [19] is gained. This yields a theoretical dissociation energy for dimer evaporation of D D15i m = 3 . 6 - 2 . 3 eV = 1.3 eV. A dissociation

81

energy this low should suppress the formation of the Au~-4 intermediate. In contrast, the experiments show no indication of dimer evaporation. While there is some Au~-3 signal at the lowest photon energies presented, the corresponding decay time for a possible dimer evaporation is expected to exceed 40/zs. In contrast, it was not possible to perform a time resolved measurement of this decay, i.e. it occurred within 10 /xs. It is therefore attributed to a fast sequential decay after the absorption of two photons. The discrepancy resuiting from the measured dissociation energies for the observed monomer evaporation persists: if these values are used to estimate the dimer evaporation energy it yields a lower value. Therefore, on energetic grounds dimer evaporation should be preferred over monomer evaporation. However, the latter is observed, the former not. Apparently, the energy values do not contain the whole story and the discrepancy must presently be left open. There is little theoretical work on the electronic and geometric structures of gold clusters but an extension of the promising ab initio calculations of Ref. [20] to gold could help to gain some further understanding of the dissociation process.

Acknowledgement This work has been supported by the Deutsche Forschungsgemeinschaft (Sch~, 401/7-3), by the M aterialwissenschaftliches Forschungszentrum Mainz and the Graduiertenkolleg 'Physik und Chemie Supramolekulare Systeme' at the Johannes Gutenberg-Universit~tt.

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