Experimental setup
Production of Pyrene clusters and measurement of their temperature-dependent evaporation rate
S. Zamith
1, Ming-Chao Ji
2, C. Joblin
2, J.-M. L’Hermite
11
LCAR, IRSAMC ; Université de Toulouse; UPS ; CNRS; F-31062 Toulouse, France
2
IRAP, Université de Toulouse; UPS-OMP; CNRS;F-31028 Toulouse, France
Evaporation rates of pure Pyrene clusters
gas aggregation source
thermalization between 25K and ambient temperature with a heat bath.
mass-selection, energy focusing and slowing down, Ek = 10-100 eV
Free flight
products analyzed in TOF mass spectrometer
Source conditions
References
What’s next ?
For more details see ref. [1] and [2]
[1] F. Chirot, S. Zamith, P. Labastie, and J.-M. L’Hermite, Rev. Sci. Instrum. 77, 063108 (2006) [2] I. Braud, S. Zamith, and J.-M. L’Hermite, Rev. Sci. Instrum. 88, 043102 (2017)
Contact: [email protected] , www.lcar.ups-tlse.fr/clusters
Mass spectra
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 0
2000 4000 6000 8000 10000 12000
mass (amu)
0 10 20 30 40
n
Clusters are produced in a gas aggregation source.
Ionization by either :
- a discharge electrode, I 100µA, V 1 kV - an electron gun, I = 2.5 A, V = -100 V
Clusters production
802 804 806 808 810 812 814 816
-2H
mass (amu) Py+4
isotopes
-1H
156 158 160 162 164 166 168 170
T = 166 K T = 121 K
ToF (µs)
T = 217 K
156 158 160 162 164 166 168 170
ToF (µs)
156 158 160 162 164 166 168 170
ToF (µs)
Source allows the production of pure Pyrene clusters or mixed water-Pyrene clusters
Principle of measurement
Temperature controlled clusters are mass-selected and slowed down.
Mass-spectra show the appearance of evaporation products as the cluster temperature is increased.
Evaporations are detected if they take place between the end of the slowing down and the entrance of the reflectron (+ ).
The n-1 peak has a double structure due to evaporations taking place after the second acceleration and before the reflectron ().
156 158 160 162 164 166 168 170
ToF (µs)
I n
I n-1
t is the time of flight in the region + . For n=11 pyrene clusters @ 22eV, t = 160 µs.
110 120 130 140 150 160 170 180 190 200 210 220 0
100 200 300 400 500 600 700 800 900 1000 1100
Evaporation rate W(T) (s-1 )
Temperature (K)
Experimental evaporation rate (Eq. 1) Fit (Eq. 2)
60 80 100 120 140 160 180 200 220 240 0
100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400
Evaporation rate W(T)(s-1 )
Temperature (K)
(Py)+n n
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Observation of dehydrogenation
Above: evaporation rates for all sizes as a function of temperature.
Below: these curves are fitted with Eq. 2 to extract the threshold temperature T0 and the width T:
0 5 10 15 20 25 30 35 40 45
200 220 240 260 280
T 0 (K)
Size n
0 5 10 15 20 25 30 35 40 45
0 20 40 60 80
T (K)
Size n
For n=2-4, no evaporation observed up to 273K. Above n=6, the threshold temperature T0 increases with size. Around n=20, evaporation rates stop evolving with size. W(T) get steeper as the size increases.
• Phase space theory and/or RRKM calculations are needed to interpret our results
• Collision induced dissociation of pyrene clusters could also give threshold energies for dissociation
• Study of mixed water-pyrene clusters
Evolution of the mass spectra with temperature.
Parent cluster Evaporation
)n
(Py
eV 22
@ (Py)
:
Example 11
Extraction of the evaporation rate W(T)
t I
I T I
W
e I
I I
n n
n t T W n
n n
) 1 ln(
) (
) (
1 )
( 1
(Eq. 1)
T T
e T
W T
f ( ) 0 500 ( 0)/
Experimental data are fitted by the empirical function:
(Eq. 2)
0 200 400 600 800 1000 1200 1400
(H2O)n(Py)+2 (H2O)nPy+
masse (uma) Py+
(H2O)nH+
