For the evaluation of the data from the microcalorimeter detector, a medianfilter has been used, which replaces each data point with the median of 4 circumjacent data points. In addition to that, a low pass filter removes frequencies which are significantly higher than 3.5 Hz. Figure 5.1 shows a comparison between the original and the filtered signal, obviously the filtering proce-dure only leads to insignificant changes the signal shape. The dotted line represents the detector signal caused by a laser pulse with the transmission 1 scaled to≈20 % of the original signal. The signal shape, in detail discussed in [193], is influenced by the the heat transfer between sample and the sample holder, the pyroelectric ribbon and in addition by the electronics for filtering and amplification of the signal.
As shown in Figure 5.1, the signal shape for an adsorption experiment is exactly the same when pulsing with a molecular beam and with a Laser. This is true for adsorption processes, in which the surface reaction occurs on a timescale which is short in comparison to the time constant of the detection system. In such cases, any observable that is proportional to the signal intensity as the background substracted peak height or the initial slope [58, 58, 62] of the detector signal can be used for the evaluation, as long as the same procedure is used to evaluate the detector signal during the molecular beam experiment and during the laser calibration procedure. Figure 5.2 shows the evaluated detector signal in V on the left axis versus the pulse number, evaluated with two different procedures for an SCAC measurement of Propylene oxide on a partially covered Pd surface. As the peak height of the detector signal is proportional to the evolved heat on the surface, it can be used to calculate the adsorotion heat: the peak height of the microcalorimetry signal versus the pulse number is represented by black square scatters in Fig. 5.2.
The influence of the low frequency noise in the detector signal on the relative error can be mi-nimized by fitting each pulse to a reference pulse, which is obtained by averaging over several
Abbildung 5.1: The original SCAC detector signal for a typical experiment (black) is shown with the filtered signal (grey). As a comparison, the detector signal during pulsing with the laser beam is shown as a dotted line
laser pulses. The relative scaling factor of this reference pulse, multiplied by the pulse height of the Laser pulse (in V) is shown by the gray circular scatters (shifted relative to the original result by -0.2 V) in Figure 5.2. The standard deviation is reduced by≈30 % when using this method.
In the scope of this work, the adsorption energy is evaluated by determining the peak height of the detector signal.
The detector voltage is proportional to the heat release on the sample [123]. Accordingly, the evaluation of the evolved heat requires the determination of the contact sensitivity of the pyro-electric detector in V/J, which is the proportionality factor between the detector voltage (in V) and the heat release (in J). Therefore, laser pulses of identical length than the molecular beam pulses impinge on the sample to produce a detector voltage. The laser power and the reflectivity of the sample are then used to determine the amount of absorbed energy by the sample per pulse.
With this information, the contact sensitivity of the detector can be determined.
For each microcalorimetry measurement, the incident laser power is determined in situ with a calibrated photodiode. This is necessary due to the laser drift and slight variances in the prism position for each measurement. Figure 5.4a (a) shows a typical photodiode response during an incident laser pulse. The on and off time of the Laser was chosen to be 5 seconds to obtain suf-ficient statistics. The amplitude of 290 mV corresponds to a laser powerPLaserof 13.6µW.
The contact sensitivity of the pyroelectric detector is calibrated for each new contact between ribbon and sample in order to take into account changes in the heat transfer. The incident laser power on the sample is varied by the use of filters with the transmissions 1, 0.285, 0.104, 0.079, 0.068, 0.053. To obtain the absorbed energy during a laser pulse on the sample, the reflectivity,
Abbildung 5.2: Evaluated detector signal obtained via two different evaluation procedures (for details, see text). The gray circular scatters are shifted by -0.2 V with respect to the original signal
which is measured with the procedure described in subsection 4.1.1 has been used. For sup-ported catalysts, it is essential, that the thickness of the oxide layer amounts to≈50 Å, as the reflectivity decreases approximately linearly with the oxide thickness. The heat release during Laser pulsesqcal,Lon the sample is calculated as
qcal,L=PLaser·T·t·(1−α) (5.1)
, whereT is the transmission coefficient of the filter, t is the pulse time andα is the reflectivity.
A plot ofqcal,L, corresponding to a pulse time of 266 ms as a function of the detector voltage of a typical measurement is given in 5.4b (b). Usually, the calibration is performed with 10 laser pulses for each filter. The standard deviation in that example is≈2.7 nJ. As the evolved heat increases linearly with the detector voltage, the heat release for the pulsed molecular beam experiment can be calculated by multiplying the detector signal in V with the sensitivity factor, which is 0.09µJ/V in the this case.
Abbildung 5.3: In situ calibration of the SCAC detector signal: (a) Voltage of the photodiode detector during impingement of the laser beam and (b) correlation between the heat release on the sample and the detector voltage
(a) (b)