DESARROLLO DEL SUJETO: CONSTITUCIÓN

In order to extract the pressure ofC60 inside the scattering cell, measurements of

parison. There have been several measurements of the vapour pressure curves for

C60, with the most recent performed by Popović et al. (1994) and Piacente et al.

(1995). Both papers note that if the sample is impure, there can be significant effects on the specific vapour pressure curves measured.

Popović et al. (1994) measured the vapour pressure curves forC60 at temper-

atures from ∼650 to 780 K using mass spectrometry to analyse the effusing gas from a Knudsen cell. It was noted that as the temperature rose above 700-740 K, the vapour pressure measured was lower than expected and transmission elec- tron microscopy indicated partial decomposition of the C60. This behaviour was

attributed to the presence of solvents in the original sample, and it was found that in the absence of any solvents there was no decomposition. The relationship between pressure, p, and temperature, T, was determined to be

logp= 10.49±0.14− 8276.4±120

T (7.2.1)

where the pressure and temperature are in kPa and K respectively and the num- bers after the ±refer to the uncertainties in the constants.

Piacente et al. (1995) used a torsion effusion method to determine the vapour pressure curves for C60 at temperatures from 730-990 K. The C60 sample was

placed in a cell with effusion holes on opposite side walls. The cell was suspended under vacuum from a torsion wire and moved into the isothermal region of the furnace used for heating. As the C60 gas exited the cell a torque was produced

which was measured directly and is proportional to the C60 pressure. A series of

different cells with different effusion holes were employed for comparison and to accurately determine the constant of proportionality between the pressure and temperature. Piacente et al. (1995) observed similar behaviour to Popović et al. (1994) and noted that the first data runs performed produced higher vapour pressures than those taken later, using the same samples. Investigation of the samples using scanning electron microscopy indicated that the change in vapour pressure was most likely due to initial impurities in the sample, rather than decomposition of the C60 itself. Two equations were determined, the first for

initial samples containing impurities and the second for later samples where the impurities have been removed following heating and cooling. These are

7.2. Experimental Details 137

First Run: logp= 8.28±0.20−9154±150

T (7.2.2)

Second Run: logp= 8.73±0.25−9668±200

T (7.2.3)

where, as before, the pressure and temperature are in kPa and K respectively. Figure 7.2.1 shows equations 7.2.2 and 7.2.3 plotted as a function of temper- ature. In these experiments, a similar phenomenon to those noted in Popović et al. (1994) and Piacente et al. (1995) was observed where later runs produced a lower vapour pressure at equivalent temperatures to earlier runs. The scattering percentage (the percentage of positrons which scatter from a C60 molecule), in

figure 7.2.1 is estimated assuming the total scattering cross section at 6 eV is 106Å (Gianturco and Lucchese, 1999), note that this is from the VECP model, if

the VP CP model is used the scattering percentages are much higher. Based on

these predictions, there could be a difference of∼15◦C in the temperatures from the two models to achieve the same vapour pressure inside the oven.

The differences between the two vapour pressure curves presented make it more challenging to ascertain an absolute magnitude for cross section measure- ments, as the target pressure is part of these calculations. In order to select one of the vapour pressure curves, a number of preliminary measurements were made by altering the oven temperature and measuring the positron scattering percentages (the percentage of positrons which interact with the target). For example, using the vapour pressure curve shown in equation 7.2.1 indicates that the required temperature to produce 1 to 10% scattering is ∼ 200−220◦C, using the lower magnitude VECP model. Preliminary measurements observing the intensity of

the positron beam at different oven temperatures indicated no decrease in beam intensity at these temperatures, corresponding to no target pressure in the oven. However, when the oven temperature was increased to the temperatures predicted by Piacente et al. (1995) a decrease in beam intensity and thus scattering was observed suggesting that in this experiment, the vapour pressures predicted by Piacente et al. (1995) were more applicable. Thus, the model provided by Pi- acente et al. (1995) was chosen for analysis over that of Popović et al. (1994) as it corresponded better to preliminary measurements. All data presented in section 7.3 was analysed using equation 7.2.3, corresponding to later data runs where the relationship between the temperature and pressure was constant.

Temperature (°C)

Pressure (mTorr)

Scatt

ering %

Figure 7.2.1: Relationship between the temperature and vapour pressure using equations 7.2.2 and 7.2.3 (Piacente et al., 1995) and estimation of the scattering percentages at 6 eV, assuming the total scattering cross section is 106Å consistent with Gianturco and Lucchese (1999) and the VECP model. Dotted lines indicate

the temperature required for 10% scattering for the two pressure curves, —equa- tion 7.2.2; - - - equation 7.2.3;– ·· – percentage scattering for equation 7.2.2;– · – percentage scattering for equation 7.2.3