extracting the magic numbers of water clusters from abundance spectra

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