ARTICLE pubs.acs.org/JPCA
Rb and Cs Oligomers in Different Spin Configurations on Helium Nanodroplets Moritz Theisen, Florian Lackner, and Wolfgang E. Ernst* Institute of Experimental Physics, Graz University of Technology, Petersgasse 16, A-8010 Graz, Austria/EU ABSTRACT: The study of small clusters is intended to fill the knowledge gap between single atoms and bulk material. He nanodroplets are an ideal matrix for preparing and investigating clusters in a superfluid environment. Alkali-metal atoms are only bound very weakly to their surface by van der Waals forces. Due to the formation process, high-spin states of alkali-metal clusters on He nanodroplets are favorably observed, which is in contrast to the abundance in other preparation processes. Until now, the prevailing opinion was that stable clusters of the heavy alkali-metal atoms, rubidium (Rb) and cesium (Cs) on He nanodroplets, are limited to 5 and 3 atoms, respectively (Schulz et al., Phys. Rev. Lett. 2004, 92, 13401). Here, we present stable complexes of Rbnþ and Csnþ consisting of up to n = 30 atoms, with the detection of large alkali-metal clusters being strongly enhanced by one-photon ionization. Our results also suggest that we monitored both high-spin and low-spin state clusters created on nanodroplets. The van der Waals bound high-spin alkali-metal clusters should show strong magnetic behavior, while low-spin states are predicted to exhibit metallic characteristics. Alkali-metal clusters prepared in these two configurations appear to be ideal candidates for investigating nanosized particles with ferromagnetic or metallic properties.
’ INTRODUCTION The design and development of nanoscaled materials is of large interest in modern material science. Relatively small clusters of atoms with tailored properties may be created as building blocks for new materials. Moreover, the binding properties, the formation process, and the stability of small atomic clusters are important for the understanding of larger nanoparticles. Alkalimetal (Ak) atoms are ideal candidates for the observation of a property change of the clusters with spin configuration and accordingly different electronic shell structure.1 Recently, sizedependent stable configurations of both low- and high-spin states have been theoretically examined.2 Here, we observe the formation and stability of Akn clusters with n e 30 consisting of rubidium (Rb) and cesium (Cs) atoms on a superfluid helium substrate. So far, clusters of heavy Ak atoms created on He nanodroplets (HeN) had only been discovered for small cluster sizes n e 5 (3) for Rb (Cs).3 Due to the formation process, observation of Ak clusters in high-spin states is strongly favored,4 so the question arises whether larger Rb (Cs) clusters also exist in high-spin states on HeN. On large helium nanodroplets, also, low-spin states of Akn may exist if the deposited binding energy does not destroy the nanodroplet. ’ EXPERIMENTAL SECTION He nanodroplets are generated using our droplet beam apparatus, which has already been described in refs 5 and 6. In brief, HeN are formed in a supersonic expansion of He gas (Air Liquide, Alphagaz 2, grade 6.0) with a stagnation pressure of r 2011 American Chemical Society
40 bar through a 5 μm nozzle. The average size N of the HeN can be steered by varying the nozzle temperature.7 After creation, the nanodroplets are doped by passing through Ak vapor located in a heated pick-up cell, which contains bulk Ak. Rb and Cs samples are loaded into the pick-up cell under an argon gas. The cell is operated at ∼405 K for Rb and ∼390 K for Cs to generate about 12 103 mbar vapor pressure. When several atoms are picked up by one He nanodroplet, Ak clusters are formed in the cold environment. Energy considerations favor the survival of nanodroplets with spin polarized Ak atom clusters, which can be explained by looking at the example of Rb dimers. The binding energy Eb of Rb2 strongly depends on the spin states, with Eb = 3875 cm1 for singlet states and 252 cm1 for triplets.8 Under usual thermal conditions, only the singlet state can be observed due to its higher stability. On the He nanodroplet, the large binding energy when forming low-spin clusters leads to a large release of energy into the nanodroplet. This excess energy causes evaporation of He (∼5 cm1 per He atom7), so that the He nanodroplets shrink in size and can be destroyed. In general, the smaller the binding energy the more probable will be the existence of large Ak clusters on HeN. The pick-up process for Ak clusters itself shows a non-Poissonian behavior,5,9 which is a convolution of the Poissonian pick-up and a spin-dependent component. Special Issue: J. Peter Toennies Festschrift Received: December 23, 2010 Revised: March 31, 2011 Published: April 19, 2011 7005
dx.doi.org/10.1021/jp112223k | J. Phys. Chem. A 2011, 115, 7005–7009
The Journal of Physical Chemistry A
ARTICLE
Table 1. Total Binding Energies for Small Rb and Cs Clusters in Different Spin States Given in cm1a low-spin 1
Table 2. Photon Energies and Estimated Pulse Fluence Applied in the Experiments for the Different Ionization Processes and Ak Clusters
high-spin 1
pulse fluence photon energy (cm1)
cluster
cm
Rb2
3875
1120
250
390
8
Rb
33584
∼0.7
one-photon
Rb3
5321
1580
939
700
14
Cs
32955
∼1.0
one-photon
Cs2 Cs3
3629
1210
267 1144
530 950
8 15
Cs
26535
∼2.6
one- and multiphoton
Cs
11200
∼28.0
multiphoton
Cs3
4597
1650
10
Cs4
7130
2400
10
Cs5
8711
2950
10
Cs6
11227
3700
10
Cs7
16204
4940
10
Cs8
18970
5740
10
Tn (K)
Cs9 Cs10
24753 28149
7140 8070
10 10
12.0
l
l
l
l
13.5
Cs20
63556
17590
evaporated He atoms cm
l
evaporated He atoms ref
l
Ak
ionization process
Table 3. Estimated Survival Chances for He Nanodroplets According to Models (ii) and (iii) after Formation of Csn (Low-Spin) Ni ∼18600 ∼13000
10
a
In addition, the numbers of He atoms is given, which evaporate through the cluster formation process and the Ak atom pick-up process (Rb, ∼170 He atoms per pick-up; Cs, ∼240)13 from HeN, as given in ref 7.
We have summarized available calculated binding energies Eb of Rb and Cs for clusters in high- and low-spin states in Table 1, whereby the binding energies for Rb and Cs are of the same order. Ab initio studies of cesium (Cs), sodium (Na), and potassium (K) clusters in their low-spin states have been published for n e 20 (Na, n e 10; K, n e 20). By trend, the calculated total binding energy divided by the cluster size (“binding energy per atom”) increases with the cluster size.1012 So far, only for Lin and Nan (n = 112) the binding energies in their high-spin states have been derived in a systematic comparison (cf. ref 2). We assume a similar qualitative behavior for Rb and Cs. A time-of-flight (TOF) mass spectrometer (Jordan, angular reflectron time-of-flight D-850) is placed along the beam axis about 35 cm downstream from the pick-up cell, which allows us to detect all ionized molecules at one time. The Akn loaded He nanodroplets are photoionized (Ti:Al2O3 laser, Coherent Indigo, repetition rate 5 kHz, pulse width < 0.5 cm1 for the fundamental wavelength, pulse length ∼ 30 ns, frequency doubling and tripling available). The used photon energies as well as the pulse fluences are given in Table 2. The ionization threshold of bare Rb (Cs) atoms is known as 33691 cm1 (31406 cm1). The one of Cs2 is reported as ∼29200 cm1 from 16,17 and ∼26000 cm1 from the3Σu state. For Cs3 the1Σþ g state an ionization limit of 24690 ( 30 cm1 can be found in ref 16. As the ionization threshold decreases with n down to the bulk value, which is equivalent to the work function (Rb, 2.1 eV; Cs, 1.9 eV),18 the chosen photon energies of frequency doubled and tripled Ti:Al2O3 radiation allow to ionize large clusters by a onephoton process, which is essential for avoiding strong fragmentation. Critical Akn cluster sizes for submersion, as calculated in ref 19, are not reached, so that they tend to reside in a dimple on the surface of the He nanodroplet. Moreover, bulk Cs is not wetted by superfluid He,20 so that immersion into the HeN is unlikely. With our measurement technique, we cannot determine, if larger Ak clusters are wetted or not.
(mJ/cm2)
15.0
∼9000
model Cs3 (%) Cs5 (%) Cs7 (%) Cs9 (%) Cs20 (%) (ii)
∼58
∼11
∼1
99
∼97
∼90
∼42
(ii)
∼35
∼4
