Extracting the magic numbers of water clusters from abundance spectra

K.Hansen, Dept. of Physics, Göteborg University

P.Andersson, Dept. of Chemistry, Atmospheric Science, Göteborg University

E.Uggerud, Dept. of Chemistry, Universitetet i Oslo

0 500 1000 1500 2000

Mass (amu)

Inte

nsi

ty

Pure water clustersH+(H2O)n

N=21 (it is ”magic”)

Electrospray source & TOF-MS

Ion product determination

Ion production

(Quadrupole mass filter, off)

(TOF-MS)

(collision cell, empty)Transit time defines cooling time

abundances / smoothened abundance distribution

evaporative activation energies

smoothened ditto

heat capacity/kB

evaporative rate constant frequency factor

Evaporative ensemble abundances:

J.Borggreen et al., Phys.Rev. A 62 (2000) 013202.

Example 1: Na cluster separation energies compared with heat bath evaporation

L. Schweikhard et al. Eur.Phys.J.D 36 (2005) 179

Example 2: Gold clusters in Penning trap compared with modelfree data

0 50 100 150 200 250 300 3500

1

2

3

4

Bulk water heat capacity

Expected temperature offreely evaporating neutral water clusters

Dulong & Petit

Cp

[J/g

K]

T [K]

20 30

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

0 10 20 30 40 50 60 70 80 90 100 110 120

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

(H2O)

NH

+

IN

5422 5423 5424 5425

N

0 10 20 30 40 50 60 70

0.8

1.0

1.2

1.4

N=29N=22

N=21

N=56

(H2O)

NH

+

low heat capacity high heat capacity

rela

tive

D's

N

Conclusions:

’Magic’ appearance of N=21 is due both to enhanced stability of 21 and reduced stability of 22.N=28, 55 appear magic because 29, 56 are anti-magic

Need to understand better:

Thermal propertiesRadiative heating

Z.Shi et al., JCP 99 (1993) 8009

Metastable fragmentation

Radiative heating

Th.Schindler et al., CPL 250 (1996) 301

Z.Shi et al., JCP 99 (1993) 8009