Z. Phys. D 40, 78–80 (1997) ZEITSCHRIFTFUR PHYSIK Dc© Springer-Verlag 1997

Fragmentation of Na(NH3)+n cluster ions

C.P. Schulz1, J. Hohndorf1, P. Brockhaus1, I.V. Hertel 1,2

1 Max-Born-Institut, Rudower Chaussee 6, D-12489 Berlin, Germany2 Fachbereich Physik, Freie Universitat Berlin, Arnimallee 14, D-14195 Berlin, Germany

Received: 4 July 1996

Abstract. Photoionization and -fragmentation of Na(NH3)nclusters by 170 fs and 8 ns laser pulses are studied withphoton energies of 2.98 eV to 3.46 eV. In the reflectron time-of-flight mass spectra a strong metastable loss of NH3 isobserved independent of the laser pulse length. From thefragmentation rate constants the internal energy of the clusterions prior to the fragmentation process is determined by anRRK approach.

PACS: 36.40.+d; 33.80.Eh; 82.40.Dm

1 Introduction

Photoionization of clusters is usually accompanied by subse-quent fragmentation of the ionic clusters. On the one hand,this causes a well known identification problem when study-ing neutral clusters. On the other hand, it allows interestinginsights into the dynamics and structure of the ions. It hasbeen argued that fragmentation may be suppressed by fem-tosecond laser pulses. While this is true for the fragmenta-tion through intermediate excited states in the neutral systemthere is, however, no a priori reason to assume this for thefinal ionic channel. The latter fragments on a timescale sig-nificantly longer than the laser pulses involved and so calledmetastable decay occurs even on aµs time scale wherefragmentation channels can be distinguished with relativeease. From the fragmentation rate constants information onthe stability of cluster ions can be obtained and internal en-ergies can be estimated (e.g. [1, 2]). Here we present resultsfor themetastablefragmentation after one-photon ionizationof Na(NH3)n clusters: a model system for solvated metalatoms in polar liquids. Specifically, we address the ques-tion whether the laser pulse length and the photon energyinfluences the observed fragmentation pattern.

2 Experiment

The clusters are produced in a pick-up source [3, 4]. Briefly,a pulsed supersonic beam of neat ammonia (stagnation pres-sure 2 bar) is expanded into an effusive sodium beam. The

neutral cluster beam enters the detection chamber througha 1.5 mm skimmer. Further downstream the clusters areionized with pulsed laser light. The clusters may be ion-ized with laser pulses in two different time domains. ANd:YAG laser (Spectra Physics GCR-270) provides pulsesof 8 ns at 416 nm (2. 98 eV, Raman shifted THG) and355 nm (3. 46 eV, THG). Alternatively, a regenerative am-plifier (Quantronix Model 4810) seeded by a Ti:Sapphire(Spectra Physics Tsunami) is used to produce pulses of170 fs at 410 nm (3.02 eV, SHG).

In the present experiment a reflectron time-of-flight(RETOF) arrangement [5] is used. The photoions are ac-celerated to 1600 eV by two adjustable electric fields in thedirection parallel to the initial neutral cluster beam. Aftera field free 90 cm long drift region the ions are reflectedby a grid-free electrostatic ion reflector which is tilted 2.5◦against the beam axis. Behind a second field free drift region(50 cm) the ions are detected by a standard arrangement oftwo multi-channelplates. The RETOF enables us to discernmetastable fragmentation processes which occur a fewµsafter ionisation when the clusterions are in first field freedrift region. The fragmented ions will arrive at the detectorearlier than the unfragmented parent ions.

3 Results and discussion

The photon energies used allow single photon ionization forNa(NH3)n for n ≥ 4 [4]. Fig. 1 shows two examples of theresulting mass spectra, using the 8 ns and the 170 fs, laserpulses to ionize the clusters, respectively. In both cases twoseries of mass peaks are observed: unfragmented Na(NH3)ncluster ions up ton = 40 (dashed envelope) and fragmentedcluster ions (dotted envelope) which result from a metastablefragmentation process

Na(NH3)+n → Na(NH3)+

n−1 + NH3. (1)

The overall shape of the mass spectrum of the unfrag-mented cluster ions appears to be roughly independent ofthe laser pulse length, except that larger cluster ions seem tobe somewhat more abundant in the spectrum obtained withthe lower photon energy femtosecond laser pulses (Fig. 1b).This points towards a suppression of prompt fragmentation

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Fig. 1. Mass spectra showing metastable fragmentation (dotted line) ofNa(NH3)+

n after photoionization with 355 nm (3.49 eV) 8 ns pulsesa andwith 410 nm (3.02 eV) 170 fs pulsesb

during the ionization, an effect of the energy rather thanthe pulse length as we shall see. In contrast, the lower abun-dance of the Na(NH3)+

16 when ionized with 3.02 eV (410 nm)is a known effect caused by the somewhat higher ionizationpotential of Na(NH3)+

16, which has been attributed to theclosing of the second solvation shell [4] .

After ionization the Na(NH3)+n cluster ions undergo a

slow metastable fragmentation process. The accelerationfields and length of the field free region in our reflectronleads to an observation window for metastable fragmenta-tion between 5 to 25µs for n ∼ 10 up to 10 to 50µs forn ∼ 40. Metastable fragmentation occurs for clusters withn > 5 independent of the laser pulse length and photon en-ergy used. For larger clusters (n > 20) the mass peaks ofthe fragments are even stronger than the peaks of the parentions.

To allow a more quantitative comparison for the differ-ent ionizing laser pulses we have parametrized our data. Forthe sake of simplicity we assume a single exponential decayprocess [6]. The observed relative fragment to parent inten-sity If/Ip can be converted into a metastable rate constantk by the relation [1]:

IfIp

=[exp

(k tff

)− 1]

exp(k tref

)(2)

Fig. 2. Fragmentation rate constants for different photoionization energiesand pulse lengths

where the first factor describes the decay during the traveltime tff in the field free region and the second exponentialcorrects for further fragmentation of the parent ions whilepassing the reflectron (tref ). Figure 2 shows the resultingrate constantsk which increase relatively smoothly towardslarger clusters and appears to decrease somewhat forn >30. Consistently, for all three photons usedn = 16 appearsto be slighly less stable than its neighbors: filling the newthird shell is more efficient than closing the second one.A slightly less bound ionic cluster thus explains the higherionization potential of Na(NH3)16 found earlier. Clearly, thedifferent ionization pulses lead to different rate constants.Inspection of Fig. 2 reveals that the difference can mainlybe attributed to a wavelength effect: The two measurementswith more than 4 orders of magnitude different laser pulselengths (8 ns vs.170 fs) but nearly the same photon energy(2.98 eV vs. 3.02 eV) give rates which differ much lessthan the rates for the two different photon energies at 8 ns(2.98 eV and 3.49 eV). It can be concluded at this point, thatthe metastable fragmentation rate of Na(NH3)+

n cluster ionsdoes not depend on the pulse length but on the photon energyof the ionizing laser (similar to what has been observed forC60 [2]).The energy dependence also shows that we do notdeal with a simple evaporative ensemble [7] which wouldhave lost all memory of its preparation process.

Thus, we may proceed one step further and try to es-timate the rate constants in a simplified model by usingthe Rice-Ramsberger-Kassel (RRK) formula for the micro-canonical decay rate constants [8]:

k(E∗, D) = ν

(E∗ −D

E∗

)s−1

(3)

If we assume a single metastable decay process we canextract the internal energyE∗ of the parent ion from a com-parison of the rate constants from (3) with the experimentalvalues. As a first approximation for the dissociation energywe take the asymptotic valueD = 0.4 eV for the loss ofone ammonia molecule from large Na(NH3)+

n cluster ions[4]. The parameterν denotes a typical vibrational frequencyin the fragmentation coordinate. We use the calculated value(200 cm−1 = 6×1012 s−1) for (NH3)2 from Muguet et al. [9].The number of vibrational degrees of freedoms = 3n − 6

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Fig. 3. Comparison of the internal energyE∗ as calculated from the rateconstants by (3) with the excess energyEexc of the ionization process (4).The open symbolsdenote the different ionization pulses (see Fig. 2). Thesolid line presents a linear fit to the calculated internal energy with a slopeof 62 meV/molecule. Thebroken linesindicateEexc for photon energies3.0 eV (−·−) and 3.49 eV (· · ·) with a cluster size independent temperatureof 200 K

is determined by the number of ammonia moleculesn inthe cluster since one may safely assume the internal vibra-tional modes of NH3 to be frozen. Figure 3 showsE∗ as afunction of the cluster sizen with nearly identical values forall three ionizing laser pulses. The differences seen in therates, Fig. 2, are hardly discernible here due to the exponen-tial nature of (3). The so determined internal energyE∗ofthe parent cluster ion rises linearly with cluster size witha slope of∼ 62 meV per molecule. Thus, it appears thatwe can explain the metastable fragmentation from an aver-age cluster-ion temperature T∼ 242 K. A comparison withthe neutral cluster temperature is not trivial since the lattercannot be determined easily. In earlier experiments we haveestimated it to be∼ 200 K by using the sodium dimer as aprobe for which the vibrational temperature was determinedspectroscopically [3]. Thus, if our line of argumentation iscorrect, only very little additional excess energy is transferedto the nuclear motion of the cluster in the ionization process.

The total excess energyEexc available in the photoion-ization process given by

Eexc (n) = [hν − IP (n)] + (3n− 6)kbT (4)

is also shown in Fig. 3. Herehν denotes the photon energy(we only show curves for 3 eV and 3.49 eV),IP the ion-ization potential, and (3n−6)kbT gives the thermal (van derWaals) vibrational energy of the neutral clusters (kb: Boltz-mann constant). The ionization potentials are known up ton = 20 [4] and for larger n are assumed to decrease with thecluster radius towards the bulk value 1.5 eV [10]. We seethat the available internal energyEexc is drastically higherthanE∗ determined from the metastable decay. To put it dif-ferently: if all the available excitation energy would be leftin the cluster ion the fragementation rates would be orders of

magnitude higher (cf. Eq. 3). Interestingly the energy differ-enceEexc −E∗ shows no size dependence. Within our lineof arguments we attribute this energy difference to kineticenergy of the photoelectron, i.e. photoionization of these sys-tems appears to lead more or less into the ionic ground state.Note, however, that presently we cannot exclude multiplefragmentation - outside our observable time window - dur-ing which the system could loose much of its initial excessenergy. On the other hand, since the rates depend on theionizing photon energy, we cannot treat the system as anevaporative ensemble.

4 Conclusions

Metastable fragmentation is observed in Na(NH3)+n clus-

ters after photoionization with ns and fs laser pulses. Themetastable fragmentation rates do not depend on the laserpulse lengths but increase with photon energy and clus-ter size. Internal energies of the cluster ions prior to themetastable decay have been extracted from the fragmenta-tion rate constants by the RRK model. To fully understandthe dynamics more information is needed on the energy par-titioning between internal excitation and kinetic energy ofthe photoelectron. Clusterion - photoelectron coincidence ex-periments to determine this as function of cluster size are onthe way.

The authors thank Dr. F. Noack for his effort in the femtosecond laserexperiment. This work has been financially supported by the DeutscheForschungsgemeinschaft through Sonderforschungsbereich 337.

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